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CE 212Engineering Materials
2009-2010
2
Introduction
The structure of materials can be described on dimensional scales1. The molecular level2. Materials structural level3. The engineering level
3
1. The molecular levelo Smallest scale (atoms, molecules or aggregation
of molecules)o Realm of materials scienceo Particle sizes: 10-7 – 10-3 mmo Examples: crystal structure of metals, cellulose
molecules in timber, calcium silicate hydrates in hardened cement paste, variety of polymers
4
o Atomic models used for description of the forms of physical structure (regular or disordered)
o Chemical and physical factors determine material properties o Chemical composition and/or the rate of chemical
reactions determine material properties such as porosity, strength, durability, etc.
o Mathematical and geometrical models are employed to deduce the way materials behave
5
2. Materials structural level
– Up in size from the molecular level– Material considered as a composite of different
phases Phase IIPhase I
• Concrete • Asphalt
particles such as aggregates distributed in a matrix such as hydrated cement or bitumen
Examples:• Cells in timber • Grains in metals• Concrete• Asphalt • Fiber composites• Masonry - regular composition
Entities within the material structure
Deliberate mixing of disparate parts
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o Particle sizes : 5x10-3 mm (wood cell) - 225 mm (brick length)
o Individual phases of the material can be recognized independently
o More general information can be derived from examination of the individual phases of the material (Multiphase models allow prediction of material behavior)
7
b) State and properties: Structure of material is affected from chemical and physical states of phases. Behavior of material is affected by properties of phases
Three aspects must be considered while formulating the models;
a) Geometry: Particulate or disperse phase scattered or arranged within the matrix or continuous phase model considers shape and size distribution and concentration
c) Interfacial effects: Existence of interfaces between phases may introduce additional modes of behavior. (strength: failure of material being controlled by band strength at an interface)
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3. The engineering levelo Total material is consideredo Material is considered as continuous and homogenous o Average properties for the whole volume of material
bodyo This level is recognized by construction practitioners
9
Technical information on materials used in practice comes from tests on specimens of the total material
Strength and failure tests provide technical informationDeformation tests used in practice Durability tests
Representative cell: minimum volume of the material that represents the entire material systemDimensions for cells: 10-3 mm for metals – 100 mm for concrete –1000 mm for masonryIsotropic material: properties same for all directions – unit cell is a cubeAnisotropic material: properties change with dimensions – unit cell is a parallel pipe
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CONCRETE
11
CONCRETEA composite of mineral particles (aggregates) distributed in a matrix of hardened cement paste (mixture of powder cement and water at the beginning)Versatile, comparatively cheap and energy efficient
Great importance for all types of construction throughout the worldConcrete is fresh and plastic at the beginning (throughout some time after mixing of constituent materials)Final properties of the hardened state of concrete have been gained slowly through timeProperties change with time50-60 % of ultimate strength is developed in 7 days, 80-85 % in 28 daysIncreases in strength have been found in 30 year old concrete
12
History of concrete is very old
Mixtures of lime, sand and gravels have been found in Eastern Europe, in Egypt and in Ancient Greek and Roman timesThis dates from about 5000 BC
History of concrete
Similar materials still known as pozzolona
Romans; first concrete with a hydraulic cement (lime + volcanic ash from near Pozzuoli)
Active silica and alumina in ash reacts chemically with lime
13
Roman structures;
Foundations and columns of aqueducts
Fig. 1: www.wonderquest.com/fountain-octopus-enzyme.htmFig. 2: http://www.artchive.com/artchive/r/roman/roman_colosseum.jpgFig. 3: www.dolceroma.it/images/common/dove/pantheon.jpg
In arches of the Colosseum
In the dome of the Pantheon
14
In 1756, John Smeaton
Mixture of burnt clay bearing limestone & Italian pozzolanafor producing a suitable hydraulic cement to be used in construction of EddystoneLighthouse
Picture ref: http://www.scienceandsociety.co.uk/results.asp?image=10307921Extra info: http://en.wikipedia.org/wiki/Eddystone_Lighthouse
15
In 1790, James ParkerPatented “Roman cement” from calcareous clay burnt in a kiln and ground to a powder
In 1824, Joseph AspdinPatented “portland cement” an artificial mixture of lime and clay bearing materials used in repairs of Thames Tunnel in 1828
In 1890s improvement in kiln technology reduced the cost of Portland cement production. Then widespread production and use started worldwide
16
CONSTITUENT MATERIALS OF CONCRETE
Portland CementsRaw materials; Clay and calcareous stonesSilica from clay + lime from calcareous stone(SiO2) (shale) (CaO) (Chalk or limestone)
Al2O3, Fe2O3, MgO, K2O also exist in clay
17
Chalk + clay reduced to 75μm or less and mixedBlend fed into upper end of inclined long (up to 250m), 6m diameter rotating kiln which is heated to 1500°C at lower end
Cement manufacturing process; simple but involves high temperatures
20°C 250°C 650°C 950°C 1250°C 1500°CDrying Preheating
decomposition of clay minerals
CalciningCaCO3 CaO + CO2
Burning or clinkeringcombination of oxides toproduce calcium silicates,calcium aluminates and calcium aluminoferrites
Raw materials
Clinker
Fuel + air
20°C 250°C 650°C 950°C 1250°C 1500°CDrying Preheating
decomposition of clay minerals
CalciningCaCO3 CaO + CO2
Burning or clinkeringcombination of oxides toproduce calcium silicates,calcium aluminates and calcium aluminoferrites
Raw materials
Clinker
Fuel + air
Fig. The processes taking place in a Portland cement kiln in the wet process
18
At 600°C, CaCO3 in chalk decomposes to give quicklime (CaO) and gaseous CO2
Form as a result of these reactions
Fusion reactions start at 1200°CCalcium silicates, 2CaOSiO2 or 3CaOSiO2Calcium aluminates, 3CaOAl2O3Other oxides act as a flux
Clinker particles (a few mm) emerge from kilnAfter cooling, 3-4% gypsum (CaSO42H2O) is added to clinkerMixture is ground to powder (2-80μm size), (300m2/kg specific surface)
www.cement.org/tech/cct_port_cem_prod_tech.asp
19Diagrammatic representation of the wet process of manufacture of cement
20Diagrammatic representation of the dry process of manufacture of cement
21
Principle oxides in cementCaO (lime):CSiO2 (silica): SAl2O3 (alumina): AFe2O3 (iron oxide): F
Composition;
Four main compounds (phases) formed in fusion process:Tricalcium silicate: 3CaO.SiO2 (C3S)Dicalcium silicate: 2CaO.SiO2 (C2S)Tricalcium aluminate: 3CaOAl2O3 (C3A)Tetracalcium aluminoferrite: 4CaOAl2O3Fe2O3 (C4AF)
22
Each cement grain consists of an intimate mixture of these compounds. Direct chemical analysis is not possible to determinethe amounts. Instead BOGUE formulas are used that were calculated from the results of oxide analysis.
SFASCSC 85.243.172.660.707.4 % 3 −−−−= 3SOS =
SCSSC 32 754.087.2 % −=FAAC 69.165.2 % 3 −=
FAFC 04.3 % 4 =
If A/F > 0.64
SFASCSC 85.286.248.460.707.43 −−−−=SCSSC 32 754.087.2 −=
FAAFCFC 70.11.242 +=+
If A/F ≤ 0.64
23
Compositions of Portland Cements
Oxides (% by wt) Range CaO 60-67 SiO2 17-25 Al2O3 3-8 Fe2O3 0.5 – 6.0
Na2O + K2O 0.2 – 1.3 MgO 0.1 – 4.0
Free CaO 0 – 2 SO3 1 – 3
Principle oxides: CaO & SiO2 ∼ 3 to 1by wt.
Principle oxides: C3S & C2S ∼ 75 – 80 % by wt.
The approximate range of oxide composition that can be expected for Portland Cements
24
Composition of cement depends on quality and proportions of raw materials (limestone and clay)Relatively small variations in oxide composition result in considerable changes in compound composition
Properties of compound cement constituents
MediumHighLowLowHighC4AF
Very slowVery highLowLowFlashC3A
HighLowHighMediumSlowC2S
MediumMediumHighHighMediumC3S
FinalEarly
Sulfate resistanc
e
Heat of hydration
Cementing valueRate ofreaction
25
Typical Portland Cements
Oxides (% by wt) A B C D
CaO 66 67 64 64
SiO2 21 21 22 23
Al2O3 7 5 7 4
Fe2O3 3 3 4 5
Free CaO 1 1 1 1
SO3 2 2 2 2
Potential compound composition (% by wt)
C3S 48 65 31 42
C2S 24 11 40 34
C3A 13 8 12 2
C4AF 9 9 12 15
Typical or average P.C
High-Early Strength P.C
Low Heat P.C
Sulphate Resisting P.C
26
Türk Çimento StandardlarıStandard No İsim Notasyon TS 3441 Portland Çimentosu Klinkeri TS 19 Portland Çimentosu PÇ 32.5
PÇ 42.5 PÇ 52.5
TS 3646 Erken Dayanımı Yüksek Çimento EYÇ 52.5 TS 21 Beyaz Portland Çimentosu BPÇ 32.5
BPÇ 42.5 TS 10157 Sülfatlara Dayanıklı Çimento SDÇ 32.5 TS 10158 Katkılı Çimento KÇ 32.5 TS 26 Traslı Çimento TÇ 32.5 TS 20 Yüksek Fırın Cüruflu Çimento CÇ 32.5
CÇ 42.5 TS 809 Süper Sülfat Çimentosu SSÇ 32.5 TS 640 Uçucu Küllü Çimento UKÇ 32.5 TS 22 Harç Çimentosu HÇ 16 TS 12139 Portland Cüruflu Çimento PCÇ/A
PCÇ/B TS 12140 Portland Kalkerli Çimento PLÇ/A
PLÇ/B TS 12141 Portland Silika Füme Çimento PSFC TS 12142 Kompoze Çimento KZÇ/A
KZÇ/B TS 12143 Portland Kompoze Çimento PKÇ/A
PKÇ/B TS 12144 Puzolanik Çimento PZÇ/A
PZÇ/B TS 23 Çimento Numune Alma Metodları TS 24 Çimentoların Fiziki ve Mekanik Deney
Metodları
TS 687 Çimentonun Kimyevi Analiz Metodlar
27
ENV 197-1’e Göre Çimento İçinde Bulunabilecek Malzemeler(Materials in cement)
Malzeme Kısaltma Sınırlamalar PÇ Klinkeri K C3S+C2S ≥ % 66.7
CaO/SiO2 ≥ 2.0 MgO ≤ % 5
Granüle Yüksek Fırın Cürufu
S Camsı faz miktarı ≥ % 66.7 CaO + SiO2 + MgO ≥ % 66.7 (CaO + MgO)/SiO2 > 1.0
Doğal Puzolan P Reaktif SiO2 ≥ % 25 Endüstriyel Puzolan Q Silisli Uçucu Kül V KK ≤ % 5
Reaktif CaO ≤ % 5 Reaktif SiO2 ≥ % 25
Kireçli Uçucu Kül W % 5 ≤ Reaktif CaO ≤ % 15 Reaktif SiO2 ≥ % 25 KK ≤ % 5 Hacim Genl. < 10mm
% 5 ≤ Reaktif CaO ≤ % 15 Reaktif SiO2 ≥ % 25 KK ≤ % 5 Hacim Genl. < 10mm
Pişirilmiş ŞeyL T σ28 ≥ 25N/mm2
Hacim Genl. < 10mm Kalker L CaCO2 ≥ % 75
Kil miktarı ≤ 1.2 g / 100g Organik Madde Miktarı ≤ % 0.2
Silis Dumanı D Amorf SiO2 ≥ % 85 KK ≤ % 4 Özgül yüzey (BET) ≥ 15m2/g
Minör İlave Bileşen F Kalsiyum Sülfat Katkılar
28
TS EN 197-1 Çimento Tipleri ve Kompozisyonları (Types of cement and compositions)
Çimento Tipi
Adı Notasyon K S D P Q V W T L MİB
I Portland Çimentosu I 95-100 - - - - - - - - 0-5 II/A-S 80-94 6-20 - - - - - - - 0-5 Portland Cüruf Çimentosu II/B-S 65-79 21-35 - - - - - - - 0-5
Portland Silis Dumanı Ç II/A-D 90-94 - 6-10 - - - - - - 0-5 II/A-P 80-94 - - 6-20 - - - - - 0-5 II/B-P 65-79 - - 21-35 - - - - - 0-5 II/A-Q 80-94 - - - 6-20 - - - - 0-5 Portland Puzolan Çimentosu
II/B-Q 65-79 - - - 21-35 - - - - 0-5 II/A-V 80-94 - - - - 6-20 - - - 0-5 II/B-V 65-79 - - - - 21-35 - - - 0-5 II/A-W 80-94 - - - - - 6-20 - - 0-5 Portland Uçucu Kül Çimentosu
II/B-W 65-79 - - - - - 21-35 - - 0-5 II/A-T 80-94 - - - - - - 6-20 0-5 Portland Pişirilmiş Şeyl
Çimentosu II/B-T 65-79 - - - - - - 21-35 0-5 II/A-L 80-94 - - - - - - - 6-20 0-5
Portland Kireçtaşı Çimentosu II/B-L 65-79 - - - - - - - 21-35 0-5 II/A-M 80-94 6-20 0-5
II
Portland Kompoze Çimento II/B-M 65-79 21-35 0-5 III/A 35-64 36-65 - - - - - - - 0-5 III/B 20-34 66-80 - - - - - - - 0-5 III Yüksek Fırın Çimetosu III/C 5-19 81-95 - - - - - - - 0-5 IV/A 65-89 - 11-35 - - - 0-5
IV Puzolanik Çimento IV/B 45-64 - 36-55 - - - 0-5 V/A 40-64 18-30 - 18-30 - - - 0-5
V Kompoze Çimento V/B 20-39 31-50 - 31-50 - - - 0-5
29
EN 197-1 The 27 products in the family of common cements
30
Table cont.
31
Özellik Çimento Tipi Dayanım Sınıfı Sınır Kızdırma Kaybı (%) CEM I
CEM III Bütün Sınıflar ≤ 5
Çözünmeyen Kalıntı (%) CEM I CEM III
Bütün Sınıflar ≤ 5
CEM I CEM II
32.5, 32.5R, 42.5 ≤ 3.5
CEM IV CEM V
42.5R, 52.5, 52.5R
SO3 (%)
CEM III* Bütün Sınıflar
≤ 4.0
Cl (%) Bütün tiplert Bütün Sınıflar ≤ 0.10 * CEM III/C %4.5’e kadar SO3 içerebilir. t CEM III % 0.1’in üstünde Cl- içerebilir. Bu durumda Cl- miktarı belirtilir.
TS EN 197-1 Çimentolarında Aranan Kimyasal Koşullar
32
EN 197-1 Chemical Requirements given as characteristic values
33
TS EN 197-1 Dayanım Sınıfları
Dayanım Sınıfı Basınç Dayanımı Sınırları (N/mm2) 2G 7G 28G 32.5 - ≥ 16 ≥ 32.5, ≤ 52.5 32.5R ≥ 10 - ≥ 32.5, ≤ 52.5 42.5 ≥ 10 - ≥ 42.5, ≤ 62.5 42.5R ≥ 20 - ≥ 42.5, ≤ 62.5 52.5 ≥ 20 - ≥ 52.5 52.5R ≥ 30 - ≥ 52.5 EN 197-1 Mechanical requirements given as characteristics values
34
Hydration(Cement + Water) paste initially fluid mixtureFluidity or consistency remains constant for an initial period after mixing
Final set (max 10 hours after mixing): Mix is completely stiff, hardening and strength gain starts
Initial set (2-4 hours after mixing): Mix starts to stiffer, fluidity is lost at a faster rate
Rate of strength gain is fast for the first 1-2 days . It continues with a decreasing rate in time
35
Rat
e of
hea
t out
put
(J/k
g/se
c)
A
Dormant period
B
C
0.1 1.0 10 100
Time after mixing (hours, log scale)
Rat
e of
hea
t out
put
(J/k
g/se
c)
A
Dormant period
B
C
0.1 1.0 10 100
Time after mixing (hours, log scale)
Hydration reactions are exothermic A: High but very short peak lasts only a few minutes
Dormant period: cement is inactive (2-3 hours)B: Broad peak after final setC: Sharp peak (seldom) after one or two days
36
Heat of HydrationR
ate
of H
eat
Evo
lutio
n
Time
Stage I Rapid Heat Evolution (<15 mins)
Stage II Dormant Period Important for transportation (2-4 hrs)
Stage III Accelerating Stage Begins with initial set (4-8 hrs)
Stage IV Deceleration Stage No longer workable (12-24 hrs)
Stage V Steady State
I
II
III IV
V
nucleationdissolution
hydrolysisC3S reacts
diffusion control
Initial set
Final set
37
VicatVicat apparatusapparatus
Typical Setting Times for Portland Cements
(Gebhardt 1995 and PCA 1996).
38
Vicat Needle Penetration Evolution
39
32
__
32
__
3 3263 HSCACHHSCAC →++
Retarding is provided by addition of gypsum which reacts with C3A to form calcium sulphoaluminate (ettringite)
Hydration compounds involve all four main compounds simultaneously. Processes are extremely complex and not fully understood. Simplified description consider the chemical reactions of each of the compounds individually
1) Initial peak “A” is due to: Rehydration of calcium sulphate hemihydrate
2
____235.02 HSCHHSC →+ (H = H2O)
2) Initial reaction of aluminate phases
633 6 AHCHAC →+
Very rapid reaction of C3A with water results in a flash set in a few minutes
Calcium sulfoaluminate(Ettringite)
Relatively slow reaction. Gypsum added (3-4%) used up to after first 24 hours after mixing. Then C3A hydration reaction taken over and ettringitetransform into monosulphate form ( ). This occurs as peak “C”in cements with C3A>12% 163 HSACC
40
3) C4AF reacts similarly over same time scalesReaction products are similar to C3A products
(has little effect on the overall cement behavior)
4) C3S and C2S react to form bulk of hydrated material after these initial reactions are completed. They are responsible for most of properties of hardened cement
CHHSCHSC 362 3233 +→+Most of the main peak “B” is due to this reaction
CHHSCHSC +→+ 322 342
(Reaction of C3S - faster)
(Reaction of C3S - slower)
41
FLY ASH(cured for 5 years)
42
FLY ASH(cured for 5 years)
43
aggregate
Transition zone Bulk cement paste
Fracture surface of 24 hour old cement paste, showing C-S-H and ettringite
44Typical hydration product development in Portland cement paste
Calcium silicate hydrate (C-S-H) is responsible for strength and other properties
After 1 day CSH dominates,thus; Ca(OH)2 production enhanced
45
Development of strength of pure compounds from Portland cement
46
Increasing temperature accelerates reactions of hydration. Reactions stop completely below -10ºC
Fresh cement and water Initial set
Two or three days old paste
Mature paste
47
1) After mixing fresh cement particles dispersed throughout mix water as single grains or small flocs. Spacing depends on w/c
Microstructural development during HYDRATION of cement
2) During dormant period, ettringite is formed at cement surface as sharp needles or rods.
3) At the end of dormant period, ettringite from adjacent cement particles has began to interfere and C-S-H with a spicular crumpled-foil form has started to appear. Solid layers of foil are a few molecules thick
4) During subsequent hydration, a dense continuous gel of C-S-H is formed between particles, resulting in increasing strength. Also large hexagonal crystals of CH are formed. Some larger pores remain unfilled between grains, and fresh unhydrated cement is left in center of grains.
48
Model – Hydration of cement paste
49
Pore size distribution in 28-day-old hydrated cement paste
50
Structure of hardened cement paste
Residue of unhydrated cement, at center of original grainsHydrates, mainly calcium silicates (C-S-H); also calcium aluminates, sulphoaluminates and ferritesCrystals of calcium hydroxide (calcite)Unfilled spaces between cement grains, called capillary pores
C-S-H occupy about 75 % of volume of HCPC-S-H govern mechanical propertiesC-S-H structure: from poorly crystalline fibers to crumpled sheet-like network of
colloidal scaleExtremely high specific surface: 100-700 m2/g (~ 103 times higher than cement particles)Spaces between C-S-H particles: gel pores: ~ 0.5-5nm ~ 27 % of C-S-H weight
Note: Don’t confuse gel pores with capillary pores (on the average about 2 orders of magnitude larger)
51
Strength of hardened cement paste
Strength arise from Van der Waals forces between hydrate layers
Quantitative estimate of unhydrated cement, hydrated gel and capillary pores was done by Powers in 1950s.
Important futures of his model are:1) Hydration takes place at constant volume
Vcem+water = Vunh.cem+gel+cap.pores
2) Same gel is produced at all stages of hydration regardless of type of cement and water/cement ratioConstants are; a) Chemically combined water: ~23% by wt of cementb) Relative density of gel solids = 2.43c) Relative density of gel+pores = 1.76
52
Gel occupies (including the pores) a space about 1.8 times that of unhydrated cement For too small a space, hydration stops when products grow to fill this space (complete hydration never occurs)For too large a space, 100% hydration doesn’t fill the space (capillary pore)For hcp in water: at w/c = 0.38 → 100% hydration fills space completely and no capillaries form
53
(a) (b)
Composition of hydrated cement paste at the final stage of hydration after prolonged storage a) in water, b) sealed
54
For sealed hcp, self-desiccation occurs at low w/c because of insufficiency of water, hydration stops before it is affected by lack of space for gel. Break-even point for w/cis 0.44.The curves show the final stage (100% hydration) which is rarely achieved. Therefore, hcp contains less cement gel and more unhydrated cement and capillaries than those shown in the figuresUnhydrated cement, not detrimental to strength, results in self-healing property.
55
Water in hardened cement paste
56
Water vapour: in partially filled larger voids
Capillary water:: in capillary pores; bulk water free from attractive forces of solid surfaces. In voids > 50nm (large capilleries)It is free water, and removal does not cause shrinkage In voids < 50nm (small capillaries)capillary tension forces dominate and removal of water may result in shrinkage
Adsorbed water: On solid surfaces under influence of surface attractive forces up to 5 molecular layers (~ thickness of 1.3 nm) Lost on drying to 30% RH and this contributes mainly to shrinkage
Interlayer water: In gel pores < 2.6nm under influence of two surfaces very strongly held. Lost on drying at elevated temperatures and/or to 10% of RH. Causes in considerable shrinkage (Van der Waals forces pull solid surfaces closer together)
Chemically combined water: combined with fresh cement in hydration reactions. Not lost on drying. Heating to very high temperatures evolves this water through decomposition of paste.
57
ADMIXTURES
Chemicals added immediately before or during mixingSignificantly change fresh, early age or hardened properties to advantage Used in small quantities (1-2 % by wt of cement)
PlasticizersWorkability aids; ↑ fluidity or workability of concrete at same w/cWater reducers; ↓ w/c and thus ↑ strength and durability at same workability
58
1) Normal plasticizers; based on lignosulphonates or hydroxycarboxylic acids
2) Superplasticizers; modified lignosulphonates or based on sulphonated melamine or naphtalene formaldehydes– Great increases in workability (flowing concrete) (segregation
occurs if used with high doses of normal plasticizers or high water contents)
– Great decreases in w/c (down to 0.2 at normal fluidity) and thus very large increases in strength (high-strength or high-performance concrete)
59
Plasticizers adsorbe on cement particle surfaces, giving slight negative charges to the surface and thus particles repel each other, breaking up any flocs and causing a better dispersion and wetting of particlesThis results in increased fluidity and slight increase in strength at same w/c ratioPlasticizers may cause retardation of setting time and also may entrain 1-2% air into concrete
60
Accelerators
Increased rate of hardening and enhanced early strength May allow early removal of formwork
May reduce curing time for concrete placed in cold weatherCaCl2 is a popular accelerator. It may cause increased creep and shrinkageProhibited in R.C and P.S.C due to corrosion of steel in presence of chloride ions
61
Typical effects of calcium chloride admixture on (a) setting times, and (b) early strength of concrete
62
Retarders
Delay setting timeCounteracts accelerating effect of hot weather (especially for long transportation distances)Avoids cold joints and discontinuities by controlling setting in large poursSucrase and citric acid and calcium lignosulphonateare examples
63
Air entraining agents
Organic materials which entrain controlled amount of microscopic(less than 0.1mm) bubbles into cement paste of concreteBubbles preserve stability during mixing, transporting, placing,compaction and setting and hardening
Note; entrained air and entrapped air are different
Air entrainment is done for providing freeze-thaw resistance to concreteIn winter time, water in capillary pores expands on freezing resulting in disruptive internal stresses. Successive cycles of freezing and thawing may lead to progressive deterioration. Entrained air, uniformly dispersed in hcp with a spacing factor of not more than 0.2mm, provide a reservoir for water to expandEntrained air volumes of 4-7% by vol. of concrete is required to provide effective protection.
64
Secondary effectsIncrease in workability due to lubricating affect of small air bubbles About 6% decrease in strength for each 1% of air. However, improvement in workability may allow to partially offset the loss in strength by reducing water content and thus w/c ratios
Organic substances reduce surface tension of water and bubbles form during mixingLong chain molecules have hydrophilic and hydrophobic endsThey align themselves radially on surface of air bubble with hydrophilic ends in water and hydrophobic ends in air. Thus they provide air stability
Air bubble
hydrophobic
hydrophilic
65
Cement replacement materials (CRM)Mineral additives that partially replace portland cementCould be by-products from other industries (Economically advantageous)They enhance concrete properties in a variety of ways
Pozzolanic behavior
A pozzolanic material is one which contains active silica (SiO2) and is not cementitious in itself but will, in a finely divided form and in presence ofmoisture, chemically react with calcium hydroxide at ordinary temperaturesto form cementitious compounds
Pozzolanic reaction (Secondary reaction)
S + CH + H → C - S - H
66
1. Fly ash (pulverized fuel ash); ash from pulverized coal used to fire power stations
2. Ground granulated blast furnace slag (ggbs); slag from scum formed in iron smelting in a blast furnace, ground to a powder
3. Condensed silica fume; sometimes called microsilica; very fine particles of silica condensed from waste gases given off in production of silicon metal
4. Natural pozzolans; some volcanic ashes
5. Calcined clay and shale; clay and shale minerals heat treated
Dictionary definitions:Pulverize; to reduce to dust or powder, as by pounding or grindingSmelt ; to melt or fuse (ores) in order to separate the metallic constituents. Husk; the dry external covering of certain fruits or seeds
Types of Cement Replacement Materials
6. Rice husk ash; ash from controlled burning of rice husks after rice grains have been separated
67
Typical composition and properties of cement replacement materials
Fly ash Oxide Low lime High lime
Ggbs Silica fume PC
SiO2 48 40 36 97 20 Al2O3 27 18 9 2 5 Fe2O3 9 8 1 0.1 4 MgO 2 4 11 0.1 1 CaO 3 20 40 - 64 Na2O 1 - - - 0.2 K2O 4 - - - 0.5 Specific gravity, (gr/cm3) 2.1 2.9 2.2 3.15
Particle size (μm) 10-150 3-100 0.01-0.5 0.5-100
Specific Surface (m2/kg) 350 400 15000 350
68
High lime fly ash and ground granulated blast furnace slag are not true pozzolanas. They have certain self-cementing due to high CaOcontent. They may be used at high substitution rates (up to 90%)Low lime fly ash is used at most 40% replacement
Silica fume is used at most 25% replacement (needs superplasticizerto maintain workability)Particles of artificial pozzolanas are smooth surfaced and spherical (Thus they improve workability)
69
Age (days)
Com
pres
sive
Str
engt
h (N
/mm
2 )
70% P.C + 30% F.A
100% P.C
w/c or w/(c+fa) = 0.47
Age (days)
Com
pres
sive
Str
engt
h (N
/mm
2 )
70% P.C + 30% F.A
100% P.C
w/c or w/(c+fa) = 0.47
Pozzolanic reaction and then early strength development is slowWith silica fume, delay is much less due to high surface area and active silica contentAt later ages concretes with cement replacement materials exceed strength of Portland cement only concretesSlower pozzolanic reaction reduces porosity Pozzolanic reaction enhances transition zone between aggregate and cement paste
70
AggregatesDisadvantages of hardened cement paste (hcp); 1. Dimensional instability (high creep and shrinkage)2. High costRemedy to disadvantages; Put aggregates into cement paste → Produce concreteAggregates occupy about 70-80% of total concrete volume
71
Objective;Use as much aggregate as possibleUse largest possible aggregate sizeUse a continuous grading of particle sizes from sand to coarse stones
Thus;Void content of aggregate mixture Amount of hcp required Minimized
Coarse agg.
Fine agg.
72
Concrete composite models
a) Two-phase model for describing deformation behaviorCoarse aggregate dispersed in mortar matrixCoarse and fine aggregate dispersed in hcp matrix
b) Three-phase model for considering cracking and strengthAggregates + hcp + transition or interfacial zone (∼ 50 μm)(cracking and failure starts from interfacial zone, the weakest phase)
73
Types of aggregates
1. According to origin;a) Natural aggregates from natural sand and gravel deposits and
crushed rocks b) Specifically manufactured aggregates such as fly ash pellets,
granulated blast furnace slag
2. According to size;a) Fine aggregate; Particle size from 0 to 4 mm
Ex; natural sand and crushed sand b) Coarse aggregate; Particle size from 4 to 16 or 32 mm
Ex; gravel, crushed limestone
1. According to origin2. Accroding to size3. According to density or specific gravity
Dictionary definition:Pellet; small, rounded or spherical body
74
3. According to density or specific gravity;
a) Normal density aggregates; natural aggregates, Examples; gravels, igneous rocks (basalt, granite), sedimentary rocks (limestone, sandstone)• Mineral composition is not that important • Specific gravities; 2.55 – 2.75 gr/cm3
• Concrete density; 2250 - 2450 kg/m3
• Gravels from deposits in river valleys or shallow coastal watersare directly used after washing and grading, particles are round
• Bulk rock sources (granite, basalt, limestone) require crushing giving angular and sharp particles
Dictionary definitions:Igneous; produced under conditions involving intense heat, as rocks of volcanic origin or rocks crystallized from molten magmaSediment; mineral or organic matter deposited by water, air, or ice.
75
c) Heavyweight aggregates; minerals like barytes to barium sulphate ore and steel shots • To produce high density concrete (3500 to 4500 kg/m3)
(For nuclear radiation shielding)
b) Lightweight aggregates; pumice (a naturally occurring volcanic rock),artificial lightweight aggregates (sintered fly ash, expanded clay or shale, foamed slag)
• To produce lower density concretes (less than 2000 kg/m3)advantages; reduced self-weight, better thermal insulation
• Reduced specific gravity (less than 2.0 gr/cm3) due to voids in particles
• Reduced strength of concrete due to increased porosity
Dictionary definition:Sintering; to form a coherent mass by heating without melting. Barytes; a white or colorless mineral (BaSO4); the main source of bariumOre; a mineral or natural product serving as a source of some nonmetallic substance
76
Grading or particle size distribution (Why do we need that?)
Properties of aggregates
Overall objective– To calculate suitable grading for good workability and stability(continuous grading → low void content)
How much sand?How much crushed
stone?
77
Grading or particle size distribution (Sieve analysis)– Aggregate samples dried, weighed and passed through a stack of
the sieves• Sieve sizes in mm (0.25, 0.50, 1, 2, 4, 8, 16, 31.5)
– Weight of aggregate retained on each sieve measured and converted to percentage retained and then to cumulative
– Then plotted against the sieve size to obtain grading curve
Sieve
SieveShaker
0.25 0.5 1 2 4 8 16 31.50
102030405060708090
100
78
Grading curvesStandards for aggregate define limits inside which the grading curves for coarse and fine aggregate must fall
0.25 0.5 1 2 4 8 16 31.50
102030405060708090
100
0.25 0.5 1 2 4 8 16 31.50
102030405060708090
100
79
1. Limits for Grading (with square opening) Sieve
size 0-8 mm 0-16 mm 0-31.5 mm 0-63 mm (mm) A8 B8 C8 A16 B16 C16 A32 B32 C32 A63 B63 C63 63 100 100 100 31.5 100 100 100 61 80 90 16 100 100 100 62 80 89 46 64 80 8 100 100 100 60 76 88 38 62 77 30 50 70 4 61 74 85 36 56 74 23 47 65 19 38 59 2 36 57 70 21 42 62 14 37 53 11 30 49 1 21 42 57 12 32 49 8 28 42 6 24 39 0.25 5 11 21 3 8 18 2 8 15 2 7 14 2. Limits for Quality (max. %)
Property Fine Aggregate Coarse Aggregate Deleterious Substances
1. Clay lumps 2. Soft particles 3. Coal and Lignites 4. Mud and clay
1.00 -
1.00 3.00
0.25 5.00 1.00 0.50
Sulphate Soundness
1. With Na2SO4 2. With MgSO3
15.00 22.00
18.00 27.00
Abrasion 1. Los Angeles 2. Impact
50 45
Freeze and Thaw (DIN 4226) - 4.00 Organic Impurities: The aggregate may give yellow or lighter color in a 3% solution of, NaOH, but not dark colors
Table. Outline of TS 706 Limits for Concrete Aggregates
80
Example problemDetermine mix proportions of sand and crushed stone such that fineness moduluof mixture will be 4.30.
Sieve size (mm)
Material Passed (%)
0.25 0.50 1 2 4 8 16 31.5
Sand (%) 18 23 28 48 60 90 100 100 Crushed stone (%) 0 0 0 0 5 40 60 100
Fineness modulus; Sum of the cumulative percentages retained on the sieves of the standard series
Fineness modulus ↑ coarser material
81
Fineness Modulus = ------------------------------------------------
For sand = ---------------------------------------
For crushed stone = -----------------------------------
82
Mix proportions by vol: a (for sand), b (for crushed stone)
Using law of simple mixtures; m1a + m2b = mm
a + b = 13.33 a + 5.95 b = 4.30
a = 0.63b = 0.37
Sieve size (mm)Material Passed (%)
0.25 0.50 1 2 4 8 16 31.5
Sand (0.63) 11.3 14.5 17.6 30.2 37.8 56.7 63 63 Crushed stone (0.37) 0 0 0 0 1.9 14.8 22.2 37 Mixture 11 15 18 30 40 72 85 100
83
Limiting grading curves
0.25 0.5 1 2 4 8 16 31.50
102030405060708090
100
84
Grading curves of sand and crushed stone
0102030405060708090
100
0.25 0.5 1 2 4 8 16 31.5
85
Grading curve of the mixture (sand+crushed s.)
0102030405060708090
100
0.25 0.5 1 2 4 8 16 31.5
86
In case of more than two aggregate fractions
1=++ cbam
iiii PcPbPaP =++ 321
mjjjj PcPbPaP =++ 321
Can be extended to as many equation as numberof size fractions which provides full conformity to the desired grading curve
Ideal (desired) grading curvesTS 500 grading curve
⎟⎟⎠
⎞⎜⎜⎝
⎛+=
maxmax
420Dd
Dd
P iii
Fuller parabola
i
ii D
dP 100=
where;Pi = % passing from ith sievedi = opening size of ith sieveDmax= Max particle size (sieve size through which 100% of aggregate passes)
87
Normal weight aggregates contain pores (typically 1-2 % by volume)Particles can absorb and hold water
Completely (oven dry)All pores empty
Air dryPores partially filled
Saturated surface dryAll pores full but no excess water
Saturated or wetexcess water
Field conditions Possible onlyin lab. conditions Field conditions
Absorb some of mix water in freshconcrete
Absorb some of mix water in freshconcrete
No absorbtionand no addition
Add to mix waterin fresh concrete
Aggregate Moisture Conditions
OTHER PROPERTIES OF AGGREGATES1) Porosity and absorbtion
88
Amount of water available for cement hydration, i.e. non-absorbed or free water is of prime importanceTherefore, to ensure the required free water/cement ratio, it isnecessary to allow for the aggregate moisture conditionWhen calculating the amount of mix water;
If aggregate is drier than SSD, extra water must be added If it is wetter, then less mix water is required
2) Elastic properties and strengthElastic properties of aggregates have major influence on elasticproperties of concreteStrength of normal weight aggregates are higher than hcp and do not have major influence on strength of normal strength concreteIn high-strength concrete (greater than 70-80 MPa), strength of aggregates and effect of transition zone between aggregate and hcpbecome seriously important
89
3) Surface characteristics Surface texture have greater influence on the flexural strength than on the compressive strength of the concrete (rougher texture results in a better adhesion)Surface cleanliness is also important for adhesion (surface should be kept clear of the materials such as mud, clay etc.)Better adhesion » stronger interface between aggregate and hcpStronger interface zone » higher mechanical performance
90
Fresh state / early age properties of concrete
Fresh concrete: from time of mixing to end of time concrete surface finished in its final location in the structureOperations: batching, mixing, transporting, placing, compacting, surface finishing
Treatment (curing) of in-placed concrete 6-10 hours after casting (placing) and during first few days of hardening is important
Fresh state properties enormously affect hardened state properties
91
Main properties of fresh concrete during mixing, transporting, placing and compacting
• Fluidity or consistency: capability of being handled and of flowing into formwork and around any reinforcement, with assistance of compacting equipment
• Compactability: air entrapped during mixing and handling should be easily removed by compaction equipment, such as poker vibrators
• Stability or cohesiveness: fresh concrete should remain homogenous and uniform. No segregation of cement paste from aggregates (especially coarse ones)
Fluidity & compactability known as workabilityHigher workability concretes are easier to place and handle but obtaining higher workability by increasing water content decreases strength and durability
92
Compaction of concrete
Finishing of concrete
93
Fill concrete into frustum of a steel cone in three layers
Hand tap concreteIn each layer
Lift cone upDefine slump as downwardMovement of the concrete
Workability measurement methods1. Slump test2. Mini-slump test3. Compacting factor test4. Vebe test5. Flow table test1. Slump test - simplest and crudest test (standardized in ASTM C 143
and EN 12350-2)
94Fig; http://myphliputil.pearsoncmg.com/media/nccer_carpentry_2/module03/fg03_00900.gif
Lift cone up
Define slump as downwardmovement of the concrete
Fig; http://www.arche.psu.edu/thinshells/module%20III/concrete_material_files/image002.gif
95
TrueValid slumpmeasurement0-175 mm
ShearMixes havingtendency to segregate –repeat test
CollapseSlumps greater than175 mm - self-levelingconcrete
Consistency grade Slump (mm) Recommended method of compaction
Stiff, K1 0 - 60 Mechanical compaction like vibration
Plastic, K2 60 – 130 Mechanical or hand compaction (rodding, tampering)
Flowing, K3 130 – 200 Hand compaction or no compaction
Self compacting, K4 ≥ 200 No compaction
96
2. Mini-slump test
• Used for workability testing of cement pastes
• Mini slump cone is a small version of slump cone
• The cone is placed in the center of a piece of glass, paste is cast into cone and then the cone is lifted to measure the average spread of paste.
w/b : 0.2 sp:%2
w/b : 0.22 sp:%2
97
3. Compacting factor test(to distinguish between low slump mixes)
Upper hopper
Lower hopper
Cylinder
approx. 1m
1. Concrete is placed in an upper hopper
2. Dropped into a lower hopper to bring it to a standard state and then allowed to fall into a standard cylinder.
3. The cylinder and concrete weighed (partially compacted weight)
5. The concrete is fully compacted, extra concrete added and then conrete and cylinder weighed again (fully compacted weight)
weight of partially compact concreteCompacting factor = weight of fully compact concrete
98
4. Vebe test
Vebe time is defined as the time taken to complete covering of the underside of the disc with concretecontainer
1. A slump test is performed in a container 2. A clear perspex disc, free to move
vertically, is lowered onto the concrete surface 3. Vibration at a standard rate is applied
99
5. Flow table test(to differentiate between high workability mixes)
1. A conical mould is used to produce a sample of concrete in the centre of a 700 mm square board, hinged along one edge
2. The free edge of the board is lifted against the stop and dropped 15 times
Flow = final diameter of the concrete(mean of two measurements at right angles)
100
Some degree of correlation between the results exist, however the correlation is quite broad since each tests measures the response to different conditions
Correlations between compacting factor, Vebe time and slump
101
A) Behavior of fresh concrete after placing and compacting
From placing to final set, concrete is in a plastic, semi-fluid stateHeavier particles (aggregates) have tendency to move down(SEGREGATION)Mix water has a tendency to move up (BLEEDING)
1. Segregation and Bleeding
EARLY AGE PROPERTIES OF CONCRETE
102
A layer of water (~ 2 % or more of total depth of concrete) accumulates on surface, later this water evaporates or re-absorbed into concrete
BLEEDINGWater-rich pockets
Surfacelaitance
Mix water Cement and
aggregates
Surface laitance; water rich concrete layer hydrating to a weak structure (not good for floor slabs that need to have hard wearing surface)Water-rich pockets; upward migrating water can be trapped under coarser aggregate particles causing loss of strength and local weakening in transition zone
Other effects of bleeding;
103
2. Plastic settlement
Horizontal reinforcing bars may put restraintto overall settlement of concrete
Then plastic settlement cracking can occur
Vertical cracks form along line of the bars, penetrating from surface to bars
Cracks
Reinforcingbars
Plan
Section
104
On an unprotected surface, bleed water evaporates.
If rate of evaporation > rate of bleeding, then surface dries (water content reduces on surface) and plastic shrinkage (drying shrinkage in fresh concrete) will occurRestraint of walls of concrete causes tensile strains in near surface regionFresh concrete has almost zero tensile strength, thus, plastic shrinkage cracking results cracking is in fairly regular “crazing” form
3. Plastic shrinkage
Plastic shrinkage cracking will be increased by greater evaporation rates of the surface water which occurs, i.e. with higher concrete or ambient temperatures, or if the concrete is exposed to wind
105
4) Methods of reducing segregation and bleed and their effects
CAUSES OF BLEEDİNG
Poorly graded aggregate with a lack of fine material with particle size < 300µm
High workability mixes
REMEDIES
1. İncrease sand content2. Air entrain concrete as substitute for fine materials
Provide high workability with superplasticizers rather than
high water contents
Use very fine materials such as silica fume
106
REMEDIES for PLASTIC SETTLEMENT or PLASTIC
SHRINKAGE CRACKS
Revibrate surface region, particularly in large
flat slabs
Apply good curing that stops moisture loss fromsurface as soon as after placing is possible and forfirst few days of hardening
107
B) Curing
Curing methodsSpraying or ponding surface of concrete with waterProtecting exposed surfaces from wind and sun by windbreaks and sunshadesCovering surfaces with wet hessian and/or polythene sheetsApplying a curing membrane, a spray-applied resin seal, to the exposed surface to prevent moisture loss
Curing; proctection of concrete from moisture loss from as soon after placing as possible, and for the first few days of hardening
108
i) Effect of curing temperature
Hydration reactions between cement and water are temperature-dependent and rate of reaction increases with curing temperatureAt early ages rate of strength gain increases with curing temperature (higher temperatures increases rate of reaction, thus more C-S-H and gel is produced at earlier times, achieving a higher gel/space ratio and thus higher strength)At later ages, higher strength are obtained from concrete cured at lower temperatures
(C-S-H gel is more rapidly produced at higher temperature and is lessuniform and hence weaker than produced at lower temperatures)
Standard curing temperature is 22 ± 1 º CHydration proceeds below 0 º C, stop completely at -10 º C
109
ii) Maturity
Cement hydration depends on both time and temperature
10)(T . +Σ= tMaturity → shows correlation with strength
T= -10 ºC is datum lineAt T= -10 ºC, hydration reactions stop, no maturity developedt (hours), T (ºC )Useful in estimating strength of concrete in a structure from strength of laboratory samples cured at different temperatures
110
DEFORMATION OF CONCRETE
Causes of deformation of concrete1) Environmental effects
e.g. moisture movement and heat2) Applied stresses
e.g. short and long term
111
The response of concrete to a compressive stress applied in a drying environment
Load application
Load removal
Elasticstrain
Creepstrain
Elasticrecovery
Creeprecovery
Shrinkagestrain
Stra
in (co
ntra
ction)
t1 t2 Time
112
I → before t1→ net contraction in volume known as shrinkage due todrying, t1→ stress is applied and held constant, t2→ stress is removed(without stress it follows dotted extension beyond t1,differencebetween solid and dotted curves shows effect of loading)
II → on loading → immediate strain response (proportional to stress for low stress level)
III → Compressive strain increases at a decreasing rate, this increase,after allowing for shrinkage represents creep strain
IV → Upon unloading, immediate strain recovery is less than immediate strain on loading.
V → Time-dependent creep recovery
113
1) Drying shrinkage
Loss of capillary water, adsorbed water and interlayer water results in a net volumetric contraction called shrinkageShrinkage is expressed as a linear strain through determination of length change
114
Maximum shrinkage occurs on first drying, considerable part of this is irreversibleThere is a continuous but small swelling of the hcp on continuous immersion in waterAs the strength of the hcp increases → less shrinkage and swellingLow w/c; high degree of hydration; age - decreases porosity
- increases strength- decreases shrinkage
wetting
dryingwetting
wettingdrying
drying reversibleshrinkage
firstdrying
swelling
shrinkage
Initial dryingshrinkage
Time
Swelling with continuous immersion
115
Mechanisms of shrinkage and swelling
1) Capillary tension
Free water surfaces in capillary pores will be in surface tension and upon drying due to drop in ambient vapor pressure free surface becomes more concave and surface tension increases
r R2T )ln(
0 θρ=
PP
Kelvin’s equation
P0: vapor pressure over a plane surfaceT : surface tension of liquidR : gas constant θ : absolute temperatureρ : density of the liquidr : the radius of curvature
d
p0, (r = ∞) p, r
p1, r = d/2
116
Tension within water near meniscus = 2T/r which must be balanced by compressive stresses in surrounding solid.
If evaporation ↑ compressive stresses ↑ shrinkage ↑
Exposing hcp to a steadily decreasing vapor pressure, pores gradually empty starting with the widest first
Higher w/c pastes will shrink mored
p0, (r = ∞) p, r
p1, r = d/2
117
2) Surface tension or surface energy
Surface of both solid and liquid materials is in a state of tensionTo increase surface area, work should be done against this tension forceSurface tension forces induce compressive stresses=2T/r which are important in hcp whose average particle size is very smallAdsorption of water molecules onto the surface of the particles,reduces surface energy and reduces balancing internal compressive stresses leading to an overall volume increase, i.e. swelling
118
3) Disjoining pressure
Water inside gel pores is under influence of surface forces 5 molecules (1.3nm) thick adsorbed water forms on solid surface at saturation which is under pressure from the surface attractive forces. In regions narrower than twice this thickness (~ 2.6nm) interlayer water will be in hindered adsorption resulting in development of swelling or disjoining pressure which is balanced by tension in interparticle bondOn drying, thickness of adsorbed water layer drops and reduces disjoining pressure. This results in an overall shrinkage
Capillary water
Disjoining pressure in region of hinderedadsorptionCapillary
tension
Adsorbed waterlayer
119
4) Movement of interlayer water
Interlayer water has an intimate contact with solid surfaces , to move interlayer water, high energy is neededMovement of interlayer water likely results in significantly higher shrinkage than movement of equal amount of free or adsorbed water
120
Suggested shrinkage mechanismsRelative humidity (%) Source 0 10 20 30 40 50 60 70 80 90 100
<-------------------------------- disjoining pressure -------------------------> Powers (1965) <------------ capillary tension -------->
Ishai (1965) <------surface energy-------> < ------------- capillary tension -----------> Feldman and Sereda (1970) <---interlayer water---> < --capillary tension and surface energy ->
Witmann (1968) <----surface energy---> < -------- disjoining pressure -------------->
Opinion is divided on the relative importance of the above mechanisms and their relative contribution to the total shrinkage
121
1) Drying shrinkage of concrete
Aggregate → dimensionally stableAggregates put restraint to shrinkage deformation of hcp in
concrete
Degree of restraint depends on; aggregate volume concentrationmodulus of elasticity of aggregate
In hcp; unhydrated cement grains also act as a restraint
Drying shrinkage of concrete < drying shrinkage of cement paste
a) Effect of mix constituents and proportions
122
Influence of aggregate content in concrete on the ratio of the shrinkage of concrete to that of neat cement paste
Range for normal concrete
Aggregate content (% by volume)
Shrink
age
ratio
Normal concretes have a shrinkage of 10-30 % of that of neat paste
123
Normal-weight concreteLightweight concrete
Elastic modulus of aggregate (kN/mm2)
Shrink
age
afte
r 1
year
of
drying
(micro
stra
in)
Normal density aggregates have higher stiffness(higher E – modulus)They give more restraint to concreteLightweight aggregate concretes tend to have higher shrinkage
124
Combined effects of aggregate volume ratio and stiffness
npc g
shsh)1( −= εε
concrete ofstrain shrinkage =shcε
hcp ofstrain shrinkage =shpε
content volumeaggregate =g
hcp of ratio poissons =pμaggregates of ratio poissons =aμ
hcp of elasticity of modulus =pEaggregates of elasticity of modulus =aE
7.12.1) 2-(1 2 1
) 1( 3
p
p −=++
−=
a
pa E
En
μμ
μ
125
b) Effect of specimen geometry
Size and shape of concrete specimen influence rate of drying anddegree of restraint from the core, e.g. a member with a large surface area to volume ratio will dry and shrink more rapidlyRestraint from central core of a concrete element which has higher moisture content than the surface puts the surface into tension.Thus, under these tensile stresses, surface cracking may occurC3A and sulphate content of cement affect the shrinkage, also, alkali content and fineness has a significant effect
126
2) Autogenous shrinkage
Continued hydration with an adequate supply leads to slight swelling of cement pasteConversely, with no moisture movement to or from the cement paste, self desiccation leads to removal of water from the capillary pores and autogenous shrinkageIts magnitude is an order of magnitude less than that of drying shrinkageMore pronounced in concretes with low water-to-cement ratios
127
CO2 + H2O → H2CO3
H2CO3 + Ca (OH)2 → CaCO3 + 2H2O
H2O is → weight of paste ↑released shrinkage accompanies,
strength ↑and permeability ↓
Max carbonation at 25-50 % RHIf saturated, H2CO3 can not penetrate concreteIf dry H2CO3 can not form
3) Carbonation shrinkage
Carbonation shrinkage is not a result of loss of water, its cause is chemical
Explanation;Ca(OH)2 is dissolved from stressed region resulting in shrinkage CaCO3 crystallizes in pores reducing permeability and increasing strength
128
Thermal ExpansionCement paste and concrete expand on heating
Thermal expansion of cement pasteCoefficient of thermal expansion of hcp=10 -20 x 10-6 / ºCThe value depends on moisture contentDisturbance of equilibrium between water vapor, free water, freely adsorbed water,water in areas of hindered adsorption and forces between layers of gel solids willdetermine the behavior of cement paste upon being heated.
Thermal expansion coefficient is needed in two main situations;
1. To calculate stresses due to thermal gradients arising from heat of hydration
2. To calculate overall dimensional changes in structures
129
Thermal expansion of concrete
Coefficient of thermal expansion of most rocks = 6 - 10 x 10-6 / ºCTherefore, coefficient of thermal expansion of concrete is less than that of hcpSince aggregate occupies 70- 80 % of concrete volume, effect of humidity is very much reduced, therefore, we assume a constant coefficient of thermal expansion for concrete. This value depends on concrete mix proportions, cement paste content and aggregate type.At temperatures higher than ~ 60 ºC, differential stresses set up by different thermal expansion coefficients of paste and aggregate can lead to internal microcracking
130
Stress-strain behavior
Elasticity; hcp has a near linear compressive stress-strain relationship, modulus of elasticity can be determined from stress-strain data
3c )p-(1 gp EE =
paste of elasticity of modulus =pE 0P when elasticity of modulus c ==gE
porosity apillary cpc =
(represents the modulus of elasticity of the gel)
131
Effect of w/c and age on the elastic modulus of hcpEl
astic
mod
ulus
, (k
N/m
m2 )
10
20
30
0.20 0.30 0.40 0.50 0.60water/cement ratio by weight
37142860
Age(days)
132
Models for concrete behaviorConcrete is a composite multiphase materialElastic behavior depends on elastic properties, relative proportions and geometrical arrangement of; a) unhydrated cement, b) cement gel, c) water, d) fine aggregate, e) coarse aggregate
The model for the concrete behavior requires the following1. The property values for the phases
a. The elastic modulus of the aggregate, Eab. The elastic modulus of the hcp, Epc. The volume concentration of the aggregate, g.
2. A suitable geometrical arrangement of the phases
Several models (unit cubes) are proposed to predict average behavior Assumption for model analysis;1) Applied stress remains uniaxial and compressive throughout the model2) Effects of lateral continuity between layers can be ignored3) Any local bond failure or cracking does not contribute to deformation
133
Hansen’s models;
g
Matrix (hcp) and distributed phase (aggregates) arrangedparallel with direction of loading. Phases undergo same strain
Known as; Parallel-phase modelEqual strain model
Strain compatibility;εc = εa = εp
Equilibrium;Total force = Σ force on each phaseExpressed in terms of stresses and area
σ.1= σa.Va + σp.(1-Va)
Model IMatrix Aggregate
134
Constitutive relations;both phases and concrete are elastic
cc E εσ =c aa E εσ =a pp E εσ =p
Substituting into equilibrium equation)1(E E E ppaacc aa VV −+= εεε
From compatibility equation)1(EEE pac aa VV −+=
135
Matrix (hcp) and distributed phase (aggregates)arranged in series with direction of loading Phases are subjected to same stress
Known as; Series - phase modelEqual stress model
Equilibrium;σc = σ a = σ p
Strains;Total displacement = Σ displacements in each of the phasesexpressed in terms of strainεc = ε a.Va + ε p. (1-Va)
Model II
g
σ
Substituting from constitutive and equilibrium equations and rearranging gives
p
a
a
a
c EV
EV
E)1(1 −
+=
136
Counto’s model;
Aggregate set within hcp complying with volume requirements
Greater resemblance to concreteCombination of Hansen’s two models
)1()1(1
apaa
a
p
a
c VEVEV
EV
E −++
−=
137
Allpaste
Allaggregate
Ec Ea
ModelA
C
B
0 0.5 1.0
Volume concentration of aggregate, Va (g)
Ep
Models A & B give upper and lower boundaries to concrete modulus of elasticity. Model C gives intermediatevalues.
The effect of volume concentration of aggregate on elastic modulus of concrete calculated from simple two-phase models
138
1 5 10
1
2
3
4
5
Model A
C
B
Ea/Ep
E c/E
pPrediction of elastic modulus of concrete (Ec) from the modulus of the cementpaste (Ep) and the aggregate (Ea) for 50 % volume concentration of aggregate
139
Observed stress-strain behavior of concrete
Cementpaste
Concrete
Aggregate
1000 300020000
10
20
30
40
Strain (μs)
Stre
ss,
N/m
2
Stress-strain relationships for cement paste, aggregates and concrete “typical behavior of hcp, aggregate and concrete”
Stress-strain behavior of both aggregates and cement paste is substantially linear almost up to maximum
Composite concrete with intermediate stiffness is markedly non-linear
140
Strain
Stre
ss
Working stress
B
C
A
x0
First cycleSecond cycle
Stress-strain relationships for cement paste, aggregates and concrete “typical short term behavior of concrete”
Unloading/loading cycles show substantial ,but diminishing hysteresis loops.Explanation lies in contribution of microcracking to overall concrete strainsTransition zone is a region of weakness and ever before loading some microcracks occur in this zone. Bleeding, drying and thermal shrinkage determine the number and width of these cracks.As stress level increases, these cracks increase in number, length and width, causing progressively increasing non-linear behavior
141
Elastic modulus of concrete
C
AB
Strain
Stre
ss
Different definitions for elastic modulus,-A: slope of tangent to the curve at any point
(tangent modulus)-B: initial tangent modulus-C: slope of the line between the origin and
a point on the curve (Secant modulus)
According to the testing method,- static modulus of elasticity- dynamic modulus of elasticity
142
n: fundamental resonant frequencyl: length of specimenρ: density of concrete
ρ224 lnEd =
Dynamic modulus of elasticity approximates to the initial tangent modulus (line B).It is higher than secant modulus
• Static modulus of elasticity: secant modulus is calculated from readings of strain at a stress at 40% of ultimate strength. Cylindricalor prismatic specimens are used and loaded longitudinally with a static load
• Dynamic modulus of elasticity: dynamic test is applied to a prismatic specimen and dynamic elastic modulus is calculated as
143
Modulus of elasticity increases with age and decreasing w/c. Thus, increasing compressive strength of concrete results in increased modulus of elasticity
Proposed relations for mean moduli;
cud fE 16.031+=
cuc fE 2.020 +=
(GPa) E ),(MPafcu
Ed= dynamic modulus of elasticityEc= static modulus of elasticityfcu= ultimate compressive strength (28 day strength)
144
for water-saturated cement paste, 0.25 ≤ μ ≤ 0.30 on drying it reduces to 0.2μ is largely is dependent of w/c, age and strength
Anson proposed the relation;n
apc V )1( −= μμfor μp = 0.22, n = 0.42
Poisson’s ratio
μc = poisson ratio of concreteμp = poission ratio of cement pasteVa = volume of aggregates
145
Creep
Magnitude of creep strains is as great or greater than elastic strains on loading. Therefore, they have a significant influence on structural behaviorCreep and shrinkage are interdependent, creep is higher while the concrete is simultaneously drying
146
εsh
(a)
Stra
in
Free shrinkage (no stress)
εbc
(b) Basic creep (stress, no loss of moisture)
(c)Time
Total strainεcr
εdc (drying creep)
εsh
εbc
Total creep stress and drying
εsh: free shrinkage of unloaded concrete in drying condition
εbc: basic creep under loading with no drying (sealed concrete)
εdc: drying creep under loading and drying condition
Total creep strain;εcr= εbc + εdc
Total strain;εtot= εsh + εcr
= εsh + εbc+ εdc
147
Factors effecting creep1. Moisture content before loading: completely dried concrete has very small
creep2. Level of applied stress: creep increases almost linearly with applied stress
up to stress/strength of 0.40 to 0.603. Concrete strength: increasing strength decreases creep4. Temperature: increasing temperature increases creep significantly5. Aggregate volume concentration: aggregate is inert in creep. Hence,
creep of concrete is less than that of cement paste
6. Elastic modulus of aggregate: increasing modulus of aggregate decreases creep of concrete
Neville proposed a relationship:εcr= εcrp (1-Va)n n = 1.7 – 2.1
148
Mechanisms of creep1) Moisture diffusion: applied stress changes internal stresses and upsets the
thermodynamic equilibrium in hcp. Moisture then moves from smaller to larger pores, resulting a. Pressure drop in capillary poresb. Adsorbed water gradually moving from zones of hindered adsorptionc. Interlayer water diffusing slowly out of the gel pores
2) Structural adjustment: stress concentrations in hcp may cause consolidation due toa. Viscous flow with adjacent particles sliding past each otherb. Local bond breakage followed by reconnection nearby
3) Microcracking: defects and cracks existing in concrete before loading propagate and form new cracks and this contributes to creep strains
4) Delayed elastic strain: active creeping component (water in capillary or gel pores) act parallel with elastically deforming component (unhydrated cement particles, gel particles, calcium hydroxide crystalls) Load will be transferred to inert material which then deforms elastically
149
Overall Outline
• Introduction• Concrete• Bituminous materials• Masonry • Polymers and polymer composites• Cement-based fiber composites• Metals• Timber
150
Chapter Outline
• CONCRETE– Cement– Admixtures– Aggregates– Strength and failure of concrete– Durability of concrete– Statistical quality control in the production of
concrete– Property composition relations for concrete and
concrete mix design
151
STRENGTH AND FAİLURE OF CONCRETE1) Strength tests
a) Compressive strength testb) Tensile tests
i. Direct tension testii. Flexural testiii. Splitting tensile test
2) Factors influencing strengtha) Transition zoneb) Water to cement ratio
i. Prediction of concrete strength c) Effect of aged) Effect of humiditye) Effect of aggregate properties, size and volume
concentration3) Cracking and fracture in concrete
152
STRENGTH AND FAILURE OF CONCRETE
Strength of concrete is most important because structural elements must carry imposed loads safelyIn loading test: max stress = strength of specimen
Also, strength is related to: elastic modulus, durability (permeability)Different types of loading in structures result in different modes of failureRelevant strengths: Compressive, tensile, torsional (shear), fatigue, impact, strength under multiaxial loading
Under compressive loading, at max stress, test specimen is still whole (with extensive internal cracking). Complete breakdown subsequently occurs at higher strains and lower stress
http://www.ame-geolab.com/CRIB%20MATERIAL%20PHOTOGRAPHS%20testing%20013.jpg
153
1. Strength tests
a) Compressive strength test: Cubes (100x100x100 mm or 150x150x150 mm)Cylinder: (D= 100mm, H= 200mm or D=150mm, H= 300mm)Steel moulds are used, concrete compacted top surface smoothed
SteelPlatens
1. Cubes or cylinder are placed between steel platens of loading machine
- Cubes are loaded on two parallel smooth steel moulded surfaces- Cylinders are loaded on top and bottom surfaces. Top surface is a trowelled surface.
Therefore it must be capped either with plaster paste or with molten sulfur
2. Load is increased to its ultimate value in a few minutes
154
Cracking patterns during testing of concrete specimens in compression
Cube (H/D=1)
Cylinder (H/D=2)
Cracking pattern of cubes and cylinders show that CUBES are not in uniaxial loadingThis is due to end restraint because of friction between loading platens and concrete surface. This induces lateral tensile stress in both platen and concrete due to Poisson effect. Concrete is in a triaxial stress state with consequent higher strength compared to uniaxial stress caseIN CYLINDERS, there is a centre portion not affected from end restraint. Thus, this central zone is in uniaxial stress state. Therefore, cylinder strength ≅ 0.80 cube strength
Cracking at approx. 45° to axis near ends
Cracking parallel to loads away from ends
155
Height/diameter ratio1 2 3 40
0.8
1.0
1.2
1.4
1.6
1.8
General relationship between height/diameter ration and compressive strength of a concrete cylinder, (Gonnerman, 1925)
Slenderness ratio (h/d) affects strengthAvoid h/d < 1 for test specimensh/d = 1 for cubes, h/d = 2 for cylinders
Height
diameterRelative
st
r.
156
b. Tensile tests
i. Direct tension testResults are not very dependable (due to eccentric loading and failure at grips) not very much used
P
PCross-sectional Area A
fd = P/A
Tensile behavior of concrete can be evaluated by;i. Direct tension testii. Flexural testiii. Splitting tensile test
157
ii) Flexural test
L
P
fb
End area = bxd
-
+
Stress distribution
Modulus of rupture = 2bdPLfb =
Prism specimen(100x100x400mm or 150x150x600mm)
Load is applied at third pointsFailure occurs when flexural tensile crack at bottom of beam propagatesupwards throughout beamModulus of rupture > direct tensile strength
158
iii) Splitting tensile test
compressiontension
Stress distribution
ddlPf s π
2=
Cylinder specimen (φ150mm, height 300mm)Placed on its side in a compression testing machine and loaded across its verticaldiameterStress distribution on a plane of vertical diameter is a near uniform tensionFailure occurs by split or crack along this plane
For the same compressive strength:Modulus of rupture > cylinder splitting > direct tension
159
10 20 30 40 50 60 700Compressive strength (N/mm2)
Modulus of rupture
Cylinder splitting
Direct tension
Ten
sile s
tren
gth
(N/m
m2 )
2
4
6
8
Typical relationships between tensile and compressive strengths of concrete
160
STRENGTH AND FAİLURE OF CONCRETE1) Strength tests
a) Compressive strength testb) Tensile tests
i. Direct tension testii. Flexural testiii. Splitting tensile test
2) Factors influencing strengtha) Transition zoneb) Water to cement ratio
i. Prediction of concrete strength c) Effect of aged) Effect of humiditye) Effect of aggregate properties, size and volume
concentration3) Cracking and fracture in concrete
161
Aggregate
hcp
Transition zonearound aggregate particles
Use 3-phase model when considering strength of concrete
Transition zone (~50μm wide) is the weakest phase, cracking and failureinitiate in this zoneDue to drying shrinkage, cracks are present before loadingAs loading increases in compression or tension, cracks in this zone start propagating into hcp, resulting in paths through concrete
Typical crack path through normal strength concrete
2. FACTORS INFLUENCING STRENGTH
a) Transition Zone
162
b) Water/cement ratio:Strength of concrete depends on;
Strength of hcpStrength of aggregatesStrength of transition zone
Strength of hcp is governed byPorosity (indirectly depending on w/c)degree of hydration
Strength of transition zone is dependent on;w/c
163
Abrams; 1/
2c W C
kfk
=when w/c (by wt)k1 & k2 empirical constants depending onage, curing regime, type of cement, entrained air, test method, aggregate typeand size
Graf; 2( / )cc
cG
ffK W C
=where fcc = compressive strength of cementKG = empirical constant related to testing condition KG = 4-8, average = 6
i) Prediction of Concrete Strength
Various relationships are given by several researchers
164
Feret;
2)(vwc
cKf Fc ++=
when c, w, v → volumes of cement, water, and airvoids in 1cm3 of concrete
KF = empirical constant depending on age, type andamount of cement
KF = 80 – 300 MPafor 7 days strength KF = 150 MPafor 28 days strength KF = 180 MPa
Bolomey;
`)( kvW
CKf Bc −+
=
where c, w → weights of cement and waterv → volume of air voids
KB = empirical constant depending on age, type and amount of cement
KB = 7 – 35 MPafor 7 days strength KB = 15 MPafor 28 days strength KB = 19 MPak`=0.5
165
Mix design methods use one of these formulas or curves or tables based on these equations to estimate w/c required for a given strengthTo achieve a homogenous, cohesive concrete without significant segregation w/c < 10.4 < w/c < 0.7 → produce concretes of normal to medium strength (20-50 MPa)w/c ≈ 0.20 – 0.30 → high or very high strength concretes are produced (70 - 150 MPa)
By using superplasticizer to achieve adequate workabilityBy incorporating silica fume at 5-10 % cement replacement to improve properties of transition zone while also increasing strength of cement paste to a limited degreeBy selecting aggregate having high inherent strength and good bondcharacteristics
With this approach, at w/c ≤ 0.26, compressive strengths > 130 MPa are achieved and placed with conventional mixing, transporting and compaction methods, but with extreme care given to high standards of site practice and supervision and quality control
166
c) Effect of ageHydration reaction continues in time with decreasing rateThus even after years, in presence of moisture, there will be some strength increase
d) Effect of humidityCuring in water results in higher strength compared to air curing Moisture all through the life of concrete provides higher strengths
167
e) Effect of aggregate properties, size and volume concentration
Aggregate strength becomes important in high strength concretes. WHY??
Increasing volumetric proportion of aggregate in the mix, at a constant w/c, produce a relatively small increase in concrete strength. There is a maximum limit to aggregate content (~ 80) for practical concretes
Larger maximum aggregate size reduces strength at lower w/c. In high strength concretes, Dmax is limited to 8 or 16 mm. WHY???
With some carbonate and siliceous aggregates, structure and chemistry of the transition zone is influenced by aggregate mineralogy and surface conditions. Increased surface roughness improves bonding due to mechanical interlocking. Thus concretes with crushed rock aggregates have about 15 % higher strengths than concretes with smooth gravel aggregates
168
3. CRACKING AND FRACTURE IN CONCRETE
Stage 4
Stage 3
Stage 2
Stage 1
Stre
ss (% u
ltim
ate)
30
50
75
100
Strain
Stress – strain behavior of concrete under compressive loading: a) from Glucklich (1965); b) From Newman (1966).
Transverse
Longitudinal
Volumetric
169
STRENGTH AND FAİLURE OF CONCRETE1) Strength tests
a) Compressive strength testb) Tensile tests
i. Direct tension testii. Flexural testiii. Splitting tensile test
2) Factors influencing strengtha) Transition zoneb) Water to cement ratio
i. Prediction of concrete strength c) Effect of aged) Effect of humiditye) Effect of aggregate properties, size and volume
concentration3) Cracking and fracture in concrete
170
Stage 1: Below 30% of ultimate load transition zone cracks remain stable, stress-strain curve is approximately linear
Stage 2: Between 30-50% of ultimate load. Cracks increase in length, width and number. However, they still remain stable, non-linearity is observed.
Stage 3: Above 50% of ultimate load, cracks start to spread into matrix, towards 75% of ultimate load, cracks become unstable and curve further deviates from linearity
Stage 4: Unstable crack growth and propagation is frequent, leading to high strains. Transverse strains start increasing faster than longitudinal strains resulting in an overall increase in volume
171
Strain
Stre
ss
MortarConcrete
hcpCoarse aggregate
Typical stress-strain characteristics of aggregate, hardened cement paste, mortar and concrete under compressive loading
172
Chapter Outline
• CONCRETE– Cement– Admixtures– Aggregates– Strength and failure of concrete– Durability of concrete– Statistical quality control in the production of
concrete– Property composition relations for concrete and
concrete mix design
173
Durability of concreteOutline• Transport mechanisms through concrete• Flow processes• Degradation of concrete
1) Durability against freezing and thawing2) Durability against chemical action3) Durability against very high temperatures
• Durability of steel in concrete– Principles of corrosion of steel in concrete– Two stages of corrosion damage
• Carbonation induced corrosion• Chloride induced corrosion
174
Durability of concrete
Durability: ability of a material to remain serviceable for at least the required lifetime of the structure
Concrete is not inherently of high durabilityDegradation of concrete arises from;
Environment to which concrete is exposedInternal causes within concrete
Most important factor is the rate at which moisture, air or other aggressive agents can penetrate the concrete
175
Transport mechanisms through concrete
Porous impermeable material Porous permeable material
High porosity, low permeability Low porosity, high permeability
Hcp and concrete contains pores of varying types and sizes. Rate of flow (permeability) depends on not only porosity but the degree of continuity of pores andtheir size
176
Flow process depend on degree of saturation of hcp or concrete
At very low humidities: moisture is in vapor state and adsorbed on dry surfaces of the paste
with increasing humidity; adsorption is completed. Flow is taking place through thepore as a direct vapor movement due to pressure gradient
Vapour flow Liquid flow Adsorbed phase
177
At humidity sufficient for water condensation:vapor flow is enclosed through a shorter path
At increasing humidity: condensed water zone extend and flow is augmented by transfer in adsorbed layer
178
At increasing humidity: liquid flow under pressuregradient in incompletely saturated state
Finally at high humidities: liquid flow under pressure gradient in completely saturated state
179
Flow processes
Movement of a fluid under a pressure differential – i.e. permeationMovement of ions, atoms or molecules under a concentration gradient, i.e. diffusionCapillary attraction of a liquid into empty or partially empty pores, i.e. sorption
180
Flow processes
1) Flow or movement of a fluid under a pressure differential
xhKux ∂
∂−=Darcy’s law
For flow in x direction, ux = mean flow velocity
xh
∂∂
= rate of increase in pressure head in x-direction
K = coefficient of permeability (m/sec)
181
2) Movement of ions, atoms or molecules under concentration gradient, process of diffusion governed by Fick’s law
xcDP
∂∂
−=
For flow in x direction, P = transfer rate of substance per unit area normal to x-direction
gradiention concentrat=∂∂xc
D=diffusivity (m2/sec)
182
3) Adsorption and absorption of a liquid into empty or partially empty pores by capillary attraction
2/1Stx =
where, x = depth of penetrationS= sorptivity (mm/sec1/2)t = time (sec)
183
Exposure zonePrimary transport mechanisms
Atmospheric Gas diffusionWater vapour diffusion
Splash
Tidal
Submerged
Water vapour diffusion ionic diffusion
Water vapour diffusionWater absorption ionic diffusion
Ionic diffusionWater permeability
Primary transport mechanisms in the various exposure zones of a concrete offshore structure
184
Coef
ficien
t of
per
mea
bilit
y
10-12
10-10
10-8
10-6
0 10 20 30
Age (days)
The effect of hydration on the permeability of cement paste (w/c =0.7)
Capillary porosity (%)
Coef
ficien
t of
per
mea
bilit
y (x
10-1
3 m/s
ec)
0 10 20 30 40
1
2
3
4
5
67
8
910
The relationship between permeability and capillary porosity of hardened cement paste
Permeability; measure of the ability of a material to transmit fluids
185
0.3 0.5 0.7Water/cement ratio
Perm
eabi
lity
(x 1
0-13
m/s
ec)
2
4
6
8
10
The relationship between permeability and water/cement ratio of mature cement paste (% 93 hydrated)
186
Degradation of Concrete1) Durability against freezing and thawing
(important in cold climates)
Damage occurs due to;freezing and thawing cycles of water in capillary pores and entrapped air voids of the cement pastewater in the pores of aggregates may also freeze and affect the durabilityof concrete against frost actionwater in the gel pores is adsorbed on CSH surfaces and does not freezeuntil temperature drops to about -78 ºC. After capillary water has frozen, it posses a lower thermodynamic energy than still-liquid gel water, whichtherefore tends to migrate to supplement capillary water and thus increasedisruption
Water – cement ratio is a controlling factor of durability of concrete against freezing and thawing cycles because its magnitude determines the amount and size of the capillary pores in the cement paste. For this reason, water-cement ratio is limited in specifications for durability of concrete against frost action
187
Max. values of w/c for durability against frost action
Climatic condition Severe Mild
In air In water In air In water Types of structures
Fresh Salty Fresh Salty Thin elements 0.49 0.44 0.40 0.53 0.49 0.40 Medium size elements 0.53 0.49 0.44 - 0.53 0.44 Exterior faces of mass concrete (large elements)
0.58 0.48 0.44 - 0.53 0.44
188
Air-entrainment into concrete increases the durability of concrete against freezing and thawing. Optimum percentage of air-entrainment is about 4-6 % by volume. Air entrainment is achieved through some chemical admixtures and thus use of air-entraining admixtures has been a rule for concretes subjected tosevere climatic actionFreezing of the water in the pores of the aggregates may also damage concrete via damaging aggregate particles.Durability of aggregate against frost action depends upon its pore characteristics and saturation degree. Indirect or direct tests of freezing and thawing are used to measure durability of aggregates
189
The effect of air entrainment and water/cement ratio on the frost resistance of concrete moist-cured for 28 days (US Bureau of Reclamation, 1955)
Air entrained
Not air entrained
Water/cement ratio0.4 0.5 0.6 0.7 0.8
1000
2000
3000
4000
Num
ber o
f fre
eze-
thaw
cyc
les
for
25%
wei
ght l
oss
190
Therefore, to make concrete durable1) Cement paste should be made durable
(↓ w/c, air-entrain, pozzolanic materials)2) Use durable aggregates3) Apply good curing to concrete
Concrete should be tested for durability against frost actionPrismatic specimens are subjected to 300 cycles of freezing at -17 ºC and thawing at + 4 ºC, a cycle completed in 4 hours. Specimens are tested non-destructively from time to time by measuring dynamic modulus of elasticity through resonance frequency testing. Decrease in Ed with respect to control concrete indicates the degree of durability against frost action
191
Freeze – thaw cabin
192
Concrete in industrial buildingsConcrete containing contaminated aggregatesConcrete in contact with sulphate soils or sulphate ground watermay be subjected to the harmful effects of some chemical compounds
2) Durability against chemical action
Degree of action of certain chemicals in concreteDegree of Action Chemical Weak Strong Very Strong
pH value 6.5 – 5.5 5.5 – 4.5 < 4.5 CO2 in water, mg/lt 15 – 30 30 – 60 > 60 NH4 in water, mg/lt 15 – 30 30 – 60 > 60 Mg+2 in water, mg/lt 100 – 300 300 – 1500 > 1500 SO4
-2 in water, mg/lt 200 – 600 600 – 3000 > 3000 SO4
-2 in soils, mg/kg 2000 - 5000 > 5000 -
193
Sulphates can react with hydrated aluminate phases in hardened cement paste to produce ettringite through reaction;
3 18 3 32. . 2CH 2S 12H C A3CSHC ACS H + + + →
OHCaSOOCaOAlOHSOOHCaOHCaSOOCaOAl 24322322432 3233122)(2183 →+++
This is an expansive reaction, causing disruption
NaOHOHCaSOOHOHCaSONa 222)( 242242 +→++
224224 )(22)( OHMgOHCaSOOHOHCaMgSO +→++
Reactions of other forms of sulphates can occur with CH in hcp forming gypsumagain with an increase in volume;
GypsumSeverity of attack depends on the type of sulphateDamage of attack by MgSO4 > Damage of attack by Na2SO4 > Damage of attack by CaSO4
194
Deterioration due to sulphates decreases with;
⇓ C3A contentHigher cement content and lower water-cement ratio. WHY??Cement replacement materials decreases permeability and improvesresistance to sulphates
195
To resist the weak action;Concrete properly designed and well consolidated and curedw/c should be less than 0.60
To resist strong action;Concrete should be properly designed and well consolidatedw/c should be less than 0.50Lateral dimension of the structural elements should be enlarged to ensure some protective coverSpecial cements & cement replacement materials should be used
To resist very strong action; In addition to all above, protect the concrete by applying a protective cover
196
3) Durability against very high temperatures
Concrete maybe subjected to very high temperatures above 1000ºC in the case of;
Fire of buildingsPavements of air fieldFurnaces or chimneys of industrial buildings
Rate of degradation depends on;Maximum temperature Concrete constituents Size of element
197
For the cement paste
a) at 105ºC, capillary and gel water has been lost and shrinkage occurs
b) at 250-300ºC, aluminous and ferrous constituents of cement loose crystal water
c) at 400-700ºC, siliceous constituents loose crystal water and shrink meanwhile Ca(OH)2 looses its water and converts to CaO. After cooling, if it is wetted, it again converts to Ca(OH)2 and expand. These may result in severe cracking in hcp.
198
For the aggregates
a) Aggregates expand very mildly with increasing temperatureb) At 550ºC, some siliceous aggregates may expand excessively
due to a change in crystal structurec) At 900ºC, limestone aggregates may be calcined (separated
from CO2)d) Basalt, blast furnace slag, crushed firebrick usually don’t
change their volume extensively up to and well above 1000ºC
199
To produce a concrete durable against medium temperatures (500-600ºC)A proper design must be achieved Use limestone aggregates
To produce a concrete durable against higher temperatures (900-1100ºC)A refractory concrete should be designedStabilizing agent (shamotte earth, powdered silica, etc.) should be used in place of some fine aggregateUse basalt, blast furnace slag or crushed firebrick
On the other hand, aluminous cement is very profitable for refractory concrete
200
Durability of steel in concreteSteel exist in concrete for reinforcing the concrete to compensate for weakness of concrete under tensile and shear stressesSound concrete provides excellent protective medium for steel.If protection is broken, steel is left vulnerable to corrosion. Corrosion products, being expansive, caused cracking and/or spallingof concrete, exposing steel to more rapid corrosion
http://cce.oregonstate.edu/structural/images/corrosion_1.JPG
201
Principles of corrosion of steel in concrete
Metal
Oxygenatedwater
O2Fe2O3.H2O
Fe(OH)2
Fe++ 2OH-
1/2O2
H2O
Cathodicarea
Anodicarea
Fe2e-
Spacial arrangement of corrosion reactions of iron in moist air or oxygenated water
Anode reaction;2Fe → 2Fe++ +4e-
Cathode reaction;4e + O2 + 2H2O → 4OH -at some distance from surface2Fe++ + 4OH -→ 2Fe (OH)2 (Ferrous hydroxide (black rust))Followed by4Fe(OH)2 + O2 → 2Fe2O3H2O + H2O (Ferric hydroxide (red rust))
202
Electrolyte is the pore water in contact with steelNormally highly alkaline (pH = 12-13) due to Ca(OH)2 from the cement hydration and the small amounts of Na2O and K2O in cementIn such a solution, anodic reaction gives out Fe3O4 instead of Fe++. Fe3O4 is deposited at metal surface as tightly adherent thin film and stops any further corrosion (Steel is said to be passive)
Concrete passivation may be broken by;A loss of alkalinity by carbonationChloride ions
203
Different forms of damage from steel corrosion
cracking spalling
lamination corner effects
204
Two stages of corrosion damage
Level of corrosion tocause concrete cracking
t0 t1age
Corr
osio
n
t0 = time for depassivating agents to reach steel and initiate corrosion t1 = time for corrosion to reach critical levels, sufficient to crack concrete
205
Carbonation induced corrosion
Carbonation first occurs on the surface and progresses inwardsReaction is a diffusion controlled process
2/1 tkx = k = constantx = carbonation deptht = time
High quality, well cured concrete is subjected to limited carbonation(only 20 – 30mm after years)
206
Chloride induced corrosion
Common sources of corrosionCalcium chloride (an accelerating admixture)Contamination in aggregatesSea water for coastal and marine structuresRoad deicing salts (on bridge decks)
First two enter into concrete during mixing and steel may never be passivated (t0=0)Last two have to penetrate the concrete cover sufficiently to depassivate the steel (t0=finTransport mechanisms are permeability, diffusivity, sorptivity
207
If protection against corrosion can not be guranteed, then Use corrosion inhibiting admixture (calcium nitrate)Use corrosion resistant stainless steel bars or epoxy coated bars Cover concrete surface by a protectorApply cathodic protection
208
Chapter Outline
• CONCRETE– Cement– Admixtures– Aggregates– Strength and failure of concrete– Durability of concrete– Statistical quality control in the production of
concrete– Property composition relations for concrete and
concrete mix design
209
Statistical quality control in the production of concreteOUTLİNE
• TS 500• TS EN 206-1• Conformity criterion
210
Statistical quality control in the production of concrete
Concrete quality will show variation due to factors such as;variations in concrete making materials, production and measuring methods and human,Compressive strength of concrete, which is an excellent measure of not only the mechanical strength, but also some other properties of concrete, is taken as a basis for investigating the quality of the concrete
When a concrete structure is designed, the designer will first decide about the compressive strength of the concrete. This strength shall be selected among the values given in the standards of that country, taking into account
Level of importance of structureMaterialsEquipmentProduction facilities
TS 500: Requirements for design and construction of reinforced concrete structures → revised on Feb., 2000, April 2002
TS 11222: Ready - Mixed concrete → Revised on Feb., 2001
TS EN 206-1: Concrete, Ready - Mixed concrete → obligatory standardis adopted to accord with EU countries (March, 2004)
Problem; Different conformity conditions and rules are defined by abovestandards
(abrogated after acceptance of TS EN 206 – 1)
212
TS 500 (2000) (Design principles for reinforced concrete buildings)Presents concrete design strengths (fcd) as;C14, C16, C20, C25, C30, C35, C40, C45, C50
C14 28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm).
TS500 specifies that these strength values are lower limits withconfidence degree of 90%.
That is, only 10% of the specimens taken from the produced concrete may have compressive strengths below these design strengths
TS 500 strength requirements
213
fcafcd
t σ
Considering test variation in conrete strength obeys normal probabilistic distributionlaw, the standard deviation (σ) for the concrete production plant should be known for this purpose
Calculation of aim strength (given by TS 500 (1984))(not included in TS 500 (2000))
TS 500 strength requirements
214
TS 500 (1984)
Confidence limits in normal distribution
Confidence parameter
(t)
Degree of risk (r)
Degree of reliability
(1-r) 0.00 0.50 0.50 0.67 0.25 0.75 1.00 0.16 0.84 1.28 0.10 0.90 1.65 0.05 0.95 1.96 0.025 0.975 2.33 0.01 0.99 3.00 0.001 0.999
Risk is P (fc ≤ fcd)Reliability is P (fc > fcd)
Then required average strength (aimed or targeted strength) to be used in mix designcalculations is given for 90% confidence as:fca = fcd + 1.28 σ
fcd = design strengthfca = aim strength
TS 500 strength requirements
215
If standard deviation (σ) is not known, then aimed strength is obtained by increasing the design strength by Δf as given
for BS14 & BS16 → Δf = 4MPafor BS18 - BS30 → Δf = 6MPafor BS35 - BS50 → Δf = 8MPa
Ex; for a design strength (fcd) of BS25, strength to be aimed in mix design is calculated as: fca = 25+6 = 31MPa
TS 500 strength requirements
216
TS 500 specifies to take3 test specimen for 1 unit (1 unit = 100m3 of concrete or 450m2 of area )
3 specimens → 1 group3 groups → 1 party P (G1, G2, G3)
Compressive strengths of these specimens at 28 days after standard curing are evaluated statistically and arithmetical mean (fcm) is determined for the sample space.
Acceptance or rejection of concrete is made on two basis:Mean value (fcm) and minimum value (fcmin) are determined
for acceptance fcm ≥ fcd + 1.0 (MPa) (mean value of each party)fcmin ≥ fcd – 3.0 (MPa) (Minimum arithmetical mean calculated from
groups of each party)
TS 500 strength requirements
217
TS EN 206-1 strength requirements
TS EN 206-1Concrete design strengths C8-C100 (16 concrete classes are defined)Strengths are given as C20/25
C 20 / 25 28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm).
28 day compressive strengths of concrete cube specimens in MPa (a=15cm).
Lightweight concrete design strengths LC8 – LC80 (14 concrete classes are defined)
LC 8 / 9 28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm).
28 day compressive strengths of concrete cube specimens in MPa (a=15cm).
218
Minimum rate of sampling for assessing conformityMinimum rate of sampling
Subsequent to first 50m3 of production Production
First 50m3 of production Concrete with production
control certification Concrete without production control certification
Initial (until at least 35 test results are obtained)
3 samples 1/200 m3 or 2/production week
Continuous b (when at least 35 results are available)
1/400 m3 or 1/production week
1/150 m3 or
1/production day
a Sampling shall be distributed throughout the production and should not be more than 1 sample within each 25 m3 b Where the standard deviation of the last 15 test results exceeds 1,37σ, the sampling rate shall be increased to that required for initial production for the next 35 test results
a
219
Conformity criteria for compressive strength
Criterion 1 Criterion 2 Production Number “n” of test results for compressive strength in the group Mean of “n” results
(fcm) N/mm2 Any individual test result (fci) N/mm2
Initial 3 ≥ fck + 4 ≥ fck - 4 Continuous 15 ≥ fck + 1.48 σ ≥ fck - 4
TS EN 206-1
220
TS EN 206-1 performance requirements
Two definitons are given for concrete;1) Designed concrete; concrete for which the required properties
and additional characteristics are specified to the producer who is responsible for providing a concrete conforming to the required properties and additional characteristics
2) Prescribed concrete; concrete for which the composition of the concrete and the constituent materials to be used are specified to the producer who is responsible for providing a concrete with the specified composition
221
TS 500 vs. TS EN 206-1
TS EN 206-1 is more specific by means of durability regulations than TS 500. Limit values and conformity criterions are given for properties other than strength
The number of samples to be tested is smaller in TS EN 206-1. This can be a disadvantage.
Ref; Selçuk Türkel, Kamile Tosun, The evaluation of TS EN 206-1, TS 500 and TS 11222 standards from the view point of concrete
222
Chapter Outline
• CONCRETE– Cement– Admixtures– Aggregates– Strength and failure of concrete– Durability of concrete– Statistical quality control in the production of
concrete– Property composition relations for concrete and
concrete mix design
223
Property composition relations for concrete mix designOUTLINE
• Workability• Concrete mix design calculations
a) Preliminary designb) Trial batch production and measurement
224
Property composition relations
Required propertiesWorkabilityCompressive strengthDeformation (elastic, creep, shrinkage, thermal)PermeabilityDurability against freezing & thawing, chemical attack, fire
225
THE MIX DESIGN PROCESS
226
Workability
Empirical equation that relate the mix water to workability
(10 )W mα= −
where, W = amount of mixing water, dm3/m3
α = coefficient depending on consistency & type of aggregate m = fineness modulus of the aggregate mixture
αave
474336-40Fluid403731-33Plastic373328-30Stiff
Sea sand & crushed stone
Natural sand & crushed stone
Natural sand & gravel
Consistency grade
αave
474336-40Fluid403731-33Plastic373328-30Stiff
Sea sand & crushed stone
Natural sand & crushed stone
Natural sand & gravel
Consistency grade
227
Concrete mix design calculations
a) Preliminary designGeometrical compatibility
3 31 1000a c w
A C Wa c w v v m dmδ δ δ
+ + + = + + + = =
228
2)(vwc
cKf Fc ++=
Bolomey
Feret
Graf
Abrams CWc kkf /
2
1=
2)/( CWKff
G
ccc =
`)( kvW
CKf Bc −+
=
Strength requirement;
(taken by weight)
(taken by volume)
(taken by weight)
(taken by weight)
229
Constraint on air content;
Assumption for volume of entrapped air voids∼2-3 % for stiff consistency∼1-2 % for plastic consistency∼0-1 % for fluid consistencySolve for C, A, W, V where C+A+W = Δth
230
b) Trial batch production and measurement
Preliminary design → first approximation to actual valuesTrial batch produced based on proportions obtained from preliminary design. Only water requirement is adjusted till required workability is obtainedThis is done by adding mix water incrementally to the mixture of dry ingredients (cement + aggregates) in the mixer
Workability is tested by slump test throughout incremental addition of mix water. Unit weight of the trial batch is measured (Δm). Strength specimen (cylinders or cubes) are cast from the trial batch with desired workability. Specimens are tested at 28 days to check whether strength requirement is satisfied or not. If not mix proportions are altered.
231
SummaryDetermine w, c, a, v considering given requirementsCalculate theoretical weight (Δth)Cast trial batchMeasure actual unit weight (Δac)Calculate actual proportions using Δth, Δac
232
Example (Concrete mix design)Given: Portland cement (fcc = 41MPa, δc=3,15kg/m3)Aggregate : mixture of sand & sandy gravel
Sieve size (mm) 0.25 0.50 1 2 4 8 16 δ Sand 8 23 45 60 100 100 100 2.60 Sandy - gravel 1 3 7 12 20 55 100 2.65
Desired properties:Fresh concrete: plastic consistencyHardened concrete: BS16
Restrictions:For the aggregate mixture, m=4.07Assume v=1%Cmin=300kg/m3
233
Overall Outline
• Introduction• Concrete• Bituminous materials• Masonry • Polymers and polymer composites• Cement-based fiber composites• Metals• Timber
234
Chapter OutlineBITUMINOUS MATERIALS
– Introduction– Sources of bitumen– Chemistry and Molecular structure– Types of bitumen– Aggregates– Strength and Failure
• Modes of breakdown• Evaluation of road condition
– Viscosity of bitumen– Factors affecting deformation of bituminous mixes– Property composition relations for bituminous mixes
235
BITUMINOUS MATERIALS
Bituminous materials include all materials consisting of aggregate bound with either bitumen or tar. Mineral dust called “filler” is also used. Bituminous materials are used in highway engineering to construct flexible pavements.
Factors determining the type of bituminous mixturesBitumen contentBitumen gradeAggregate gradingAggregate size
picture: www.saocl.com
236
SOURCES of BITUMEN
1) Natural deposits (types of deposit range from almost pure bitumen to bitumen-impregnated rocks and bituminuous sands with only a few per cent bitumen)
• Rock asphalt (Porous limestone or sandstone impregnated with bitumen with 10% content, “Val de Travers”, Switzerland, “Tar sands” of North America)
• Lake asphalt (Bitumen lake with mineral matter dispersed throughout the bitumen, Trinidad lake (55 % bitumen, 35 % mineral matter, 10 % organic matter)
2) Refinery bitumen
237
2) Refinery bitumen: residual material left after the fractional distillation of crude oil
Petrol Kerosene Diesel oil Lubricating oil Bitumen
Cutback bitumen
Penetration gradebitumen
Fluxing/blending
Crude petroleum
Products of distillation
Fig. Preparation of refinery bitumens
238
Chemistry and molecular structure
Bitumen is a complex colloidal system of hydrocarbons which is soluble in trichloroethyleneConstituents of bitumen areCarbenes; fraction insoluble in carbon tetrachlorideAsphaltenes; fraction insoluble in heptaneMaltenes; fraction soluble in heptane
239
Types of bitumen
1) Penetration grade bitumen: refinery bitumen with a range of viscosities Penetration test classifies the bitumen according to hardness. For road bitumen penetration grades is from 15 to 450.Softening point, on the other hand, specifies the viscosity
2) Oxidized bitumens: air is introduced into bitumen under pressureOxygen(from air) reacts with some compounds to result in more asphaltene with higher molecular weight. Thus, hardness of bitumen is increased with reduced ductility and temperature dependence. Mostly used for industrial applications such as roofing and pipe coatings
240
3) Cutbacks: penetration grade bitumens for which the viscosity is temporarily reduced by dilation in a volatile oil. After application, oilevaporates and bitumen turns to its former viscosity
Curing time definesSlow curingMedium curingRapid curing
Cutbacks
241
4) Emulsions: two-phase system made up of two immiscible liquids, bitumen being dispersed as fine globules in water. Emulsifier provides dispersal of bitumen globules. Emulsifier in a hydrocarbon chain with cationic and anionic functional group. Hydrocarbon chain has affinity for bitumen where as ionic part has affinity for water. Each droplet carries a like charge, depending on charge of ionic part of emulsifier. Cationic emulsions are positively charged. Anionic emulsions are negatively charged. Cationic emulsions aid adhesion of bitumen on negatively charged aggregate surfaces.
Attraction of particleto substrate
WaterPhase(continuous Phase)
Bitumendroplet(dispersed phase)
Repulsion betweenparticles(like charges repel)
Surfaceactive agent
Opposite chargesattractAdhesion
of bitumento aggregatesurface
242
Aggregates
Aggregates make up about 92 % of bituminous materialsCoarse aggregates → retained on 2.36mm sieve, Fine aggregates → passes 2.36mm sieve but is retained on 75µm sieve. Filler → passes 75µm sieve
Open textured aggregate mixtures: Grading is continuous and provides a dense packing of particles. Strength and resistance to deformation are largely determined by aggregate grading with bitumen acting as adhesiveDense graded aggregate mixtures: Aggregate grading is still important but properties are determined largely by the matrix of fines and bitumen
Aggregate particles must have sufficient strength for surfacing materials, they must be resistant to abrasion and polishing. Shape and surface texture are also important.
243
Strength and Failure
Purpose of the road is to distribute applied load from traffic to a level which the underlying subgrade can bear
There are two forms of failure:Road surface may deteriorate through breakdown of surface material so that skidding resistance dropsRoad structure deteriorates – gradual and develops with the continued application of wheel loads
244
Modes of breakdown
Bituminousmaterial
Granularmaterial
Subgradesoil
rutdepth
Permanent deformation
Nearside wheeltrack
Case I
movingwheelload
Asphaltlayer
Granularlayer
Subgrade
εz , critical vertical subgrade strains
εt , critical tensile strain
fatigue crack
Fatigue cracking and critical strains
Case II
245
Two modes of breakdown
Case I - Permanent deformation occurs in wheel tracks. Rutting is associated with deformations in all layers of pavement. It islinked to loss of support from subgrade soil.Case II - Cracking appears along wheel tracks, cracking is caused by tensile strain developed in bound layers as each wheelload passes. It is a fatigue failure.
246
Evaluation of road condition
If failure can be defined, life of a road can be determined provided that loading can be assessed and performance of the materials evaluated. Complete collapse of roads are not encountered. Therefore, failure of roads must be identified in terms of serviceability and/or repairability.
It is accepted that if cracking is visible at surface, road is regarded as being at critical condition or as having failed.For no visible cracking, if rut depth reaches 20mm, road is regarded as having failed
247
Viscosity of bitumenViscosity of a liquid is the property that retards flow so that when a force is applied to the liquid, the higher the viscosity, the slower will be the movement of the liquid. The viscosity of bitumen is dependent upon both its chemical composition and its structure.
There are two most common measures of viscosity
Softening point: Temperature at which a bitumen reaches a specified level of viscosity
Standard tar viscometer: (used to measure the viscosity of tars) time taken for 50ml of the tar to run out of a cup through a standard orifice
224
248
Another test to evaluate viscosityPenetration test: commonly applied to bitumen for material
characterization. The test measures the hardness of bitumen which is related to viscosity. It measures depth to which a needle penetrates a sample of bitumen under a load of 100 gr over a period of 5 sec at a temperature of 25ºC
Bitumen is viscoelastic; therefore, the penetration will depend on the elastic deformation and viscosityBitumens are thermoplastic materials so that they soften as the temperature rises but become hard again when the temperature drops. Susceptibility of bitumen to temperature is determined from the penetration value and softening point temperature
Viscoelasticity describes materials that exhibit both viscous and elastic characteristics when undergoing plastic deformation
249
Factors affecting deformation of bituminous mixes1) Bituminous viscosity: when a stress is applied to a bituminous material,
both the aggregate particles and the bitumen will be subjected to the stress. Aggregate particles, being hard and stiff, while undergo negligible strain, whereas the bitumen, being soft, will undergo considerable strain. Deformation is associated with movement in the bitumen and the extent of the movement will depend on its viscosity.
2) Aggregate: bituminous mixtures utilizing a continuously graded –aggregate rely mainly on aggregate particle interlock for their resistance to deformation. Thus, grading and particle shape of aggregate are major factors governing deformation. Characteristics of fine aggregate are important in gap-graded materials. WHY??
3) Temperature: permanent strain increases with temperature due to the reduction in viscosity and stiffness of bitumen.
250
Property composition relations for bituminous mixes
McKesson – Frickstad (California) formula
P = 0.015a + 0.03b+ 0.17cwhere, a = % of aggregate retained on No 10 (2mm) sieve
b = % of aggregate remaining between No 10 (2mm) and No 200 (0.075mm) sieves
c = % of aggregate passing No 200 (0.075mm) sieveP = % of bitumen by weight of the bituminous mixture
In this equation, amount of bitumen is related to the surface area of the aggregate groups.
251
In lab., some tests have been done to determine the optimum bitumen content
Compression testStability test (Marshall test, Hubbard-Field test, Hveem test)Triaxial test
252
Required properties are 1) minimum stability of 225kg (2207N), 2)a maximum flow of 0.5cm, 3) air voids (3-5%) and 4) degree of saturation of aggregate voids with bitumen (75-85%).
Test is repeated with mixes of different bitumen contents and bitumen content fulfilling all requirements is determined as OPTIMUM BITUMEN CONTENT
Marshall test (most common)
ObjectiveTo determine an optimum binder content from a consideration of mix strength (stability), mix density, mix deformability (flow)
Cylindrical samples of 10cm diameter and 6.25cm height specimen is placed between the crescent shaped testing heads of testing machine and the force is applied from the side surfaces, subjecting the sample to an internal shearing towards the stress free plane surfaces. Loading speed is 5cm/min. Resultant maximum load and the corresponding deformations are measured
Asphaltsample
253
5 6 7 8Bitumen Content (%)
Unitweight
Airvoids
Marshallstability
Flow
In compacted bituminous mixture
in water)(weight - air)in (weight airin weight
=d
∑+=
i iagg
iagg
bit
bitm PP
G
)()(
100
δδ
100xG
dGV
m
m −=
Existing unit weight
Theoretical max, unit weight
Air void content
Analysis of mix design data from the Marshall test
254
Example
Properties and mix proportions of materials used for a bituminous mixture are as follows
Following data are obtained from a lab. specimen made using this mix:weight in air = 111.95 grweight in water = 61.16 gr
Material Density (δ) Mix proportions (P) Asphalt 1.04 kg/dm3 10 % by wt Crushed stone 2.70 kg/dm3 90 % by wt
255
Calculate unit weight of the lab. specimenCalculate the theoretical maximum unit wt. of the mixCalculate volume percentage of voids in the lab. specimenThis mix is placed on the road and consolidated under a roller. After consolidation a core sample is taken and the unit wt. of this core sample is determined to be 2.13kg/dm3. The specified degree of compaction for this pavement being at least 95% of the compaction of a laboratory specimen, indicate if this bituminous pavement is acceptable?
256
Solution:a) d =
b) Gm =
c) v =
d) specified unit weight =
257
Overall Outline
• Introduction• Concrete• Bituminous materials• Masonry • Polymers and polymer composites• Cement-based fiber composites• Metals• Timber
258
Brickwork and Blockwork(Masonry)
Masonry;One of the oldest building materialUsed by mankind for more than 6000 yearsAncient civilizations of Middle East, Greeks and Romans used masonryMany of mudbrick work has been lost. However, stone structures such as Egyptian pyramids, Greek temples and many structures from fired clay bricks have survived for thousands of yearsThe Romans used both fired clay bricks and hydraulic mortar
259
These are four main techniques for achieving stable masonry
a) Dry stone walls: Irregularly shaped laminar pieces are placed by hand in an interlocking mass
b) Ashlar: Medium to large blocks are made to a few sizes and assembled to a basic grid pattern either without or with mortar having very thin joints
c) Normal brickwork: Small to medium units of different sizes are assembled to a basic grid pattern and mortar is used as a packing material
d) Random rubble walls: Irregularly shaped and sized pieces are bonded together with adherent mortar
260
(a) (b)
(c) (d)
261
MaterialsSand: particles with sizes from about 10mm diameter down to 75μm
diameter obtained from riverbeds, sea beaches, older deposits from alluvial and glacial action. Most common sands are based on silica (SiO2)
Sand should be free of clay particlesMortar sand should have particles smaller than 5mm, Good range of particle size distribution is also needed (for good packing)Naturally occurring sands should be sieved and washedVery flaky and vary absorbent sand particles are not desired
262
Aggregates: Natural aggregates, sintered fly ash pellets, expanded clay and foamed slag
Binders: Binds mixtures of sands, aggregates, fillers to make mortar for masonry
Organic plasticizers: For improving plasticity or workability of mortars. These
plasticizers also entrain air as small bubblesLatexadditives: Synthetic copolymer plastics may be produced in the form
of a “latex”, a finely divided dispersion of the plastic in water usually stabilized by a surfactant such as a synthetic detergent. Solid content is ~ 50% of dispersion. They increase adhesion of mortar to all substrate. They increase tensile strength and durability
263
Mortar
Mortars are composed of cementing material, sand and water. There are three types depending on type of binder:
1) Cement mortar 2) Mixed mortar 3) Lime mortar
The sand to be used in mortar must be free of mud, clay, organic matter. It must have a good grading in size range of 0 - 4mm.
The composition of the mortar is chosen in such a way to obtain a good workability. For this purpose, all the voids of the aggregate must be filled with the paste of the cementing material and all the sand particles must be surrounded by a film of this paste. Existence of smaller particles in the sand is advantageous because; they fill the voids between larger particles and reduce the need for cementing material
If the water content of the paste is high, then the strength of the mortar will be lowered, if the cementing material is in excessive, the mortar is said to be too rich, and it tends to crack due to shrinkage. Taking all these factors into account, certain mortar compositions have been accepted as a result of many years of experience.
264
Types and Mix Proportions of Masonry Mortars (TS 2848)
Volume Proportions of Mix Class of Mortar
Type No
Type of Mortar Sand Cement Masonry
cement Lime putty
PowderLime
Minimum compressive strength (MPa)
A - Cement 3 1 - - - 15 1 Cement 4 1 - - - 2 Mixed 4 1 ½ - - 3 Mixed 4 1 - - ½ B 4 Mixed 4 1 - - 1
15
1 Mixed 7-9 1 2 - - 2 Cement 5 1 - - - C 3 Mixed 5 1 - 1 -
5
1 Mixed 6-8 1 - 2 - 2 Mixed 6-8 1 - - 3 D 3 Mixed 2-3 - 1 - -
2
E - Lime 3 - - 1 - 0.5
The last column shows the expected minimum compressive strength of the mortar at the age of 28 days
265
Natural stones;Rocks from magnetic, volcanic, sedimentary or metamorphic origin make up the natural building units for masonry. Magnetic and volcanic origin stones, being crystalline, give good quality materials. Examples are: granite, diorite, rhyolite, basalt, andesite, trechite, etc. However, care should be taken for mica, feldspar and other nondurable minerals contained in these rocks
Sedimentary rocks are not crystalline but layered. They may contain voids and fossils. Examples are: conglomerate, breccia, sandstone, shale, limestone, chert. Sandstones and limestones make good stones for masonry.
Metamorphic rocks are those who experienced a change in their structure. Good quality stones are gneiss, quartzite and marble. However, slate and shale are of poor quality.
241
266
Properties of Natural Stones
Type of Stone Comp. Str. (MPa) Unit wt. (kg/dm3)
Water absorption (%) by wt.
Basalt 90 - 165 3.35 0.5 – 2.0 Granite 75 - 145 2.65 0.6 Andesite, Diorite 55 - 75 2.56 0.2 Dense limestone 55 - 102 2.70 0.2 – 2.0 Travertene 32 - 97 2.5 – 2.60 0.4 – 2.4
267
Fired Clay Bricks
Bricks are made by forming the unit from moist clay by pressing,extrusion or casting followed by drying and firing (burning) to a temperature usually in the range of 850-1300ºCDuring firing process complex chemical changes occur and clay particles are bonded together by sintering (transfer of ions between particles at points where they touch) or by partial melting to a glass. Most important properties of bricks are compressive strength, size tolerances and unit weight. A low unit weight is desired toreduce the dead load and to provide heat and sound insulation
268
Definitions Dimension (mm) Tolerances for dimensions (mm)
Normal brick (NT) 190x190x50 50 ± 2 Modular brick (MT) 190x90x85 85 ± 3, 90 ± 3 Brick block (BT) 190 ± 3 or 190 - 2
Comp. Str. Type of Brick Unit wt (max) (kg/dm3) Mean
(MPa) Min (MPa)
λ
Cmhkcal
Absorption max (%)
Solid Brick (DOT/20) Solid Brick (DOT/12) Solid Brick (DOT/8)
1.80 1.80 1.80
20 12 8
16 9.5 6.5
0.75 -
Brick with vertical holes (DDT/15) Brick with vertical holes (DDT/10) Brick with vertical holes (DDT/6)
1.20 1.20 1.20
15 10 6
12 8 4.5
0.34 -
Brick with horizontal holes (YDT/3.6) Brick with horizontal holes (YDT/2.4)
0.8 0.50
3.6 2.4
3 2 0.32 -
Solid clinker brick (DOK/30) Clinker brick with holes (DEK/30)
1.80 1.80
30 30
25 25 0.75 8
Requirements for Factory made Bricks (TS 705)
269
Concrete Units
Used in building partition in houses since 1920sConcrete unit is poured into a mould and vibrated and demoulded after setting. It is a labor demanding and slow processConcrete is filled into a mould (die) and a dynamic presshead compacts the concrete into the die. Then, green concrete is ejected on to a conveyer system and taken away to cure either in air or after in streamThus solid and hollow bricks and blocks are produced either in dense concrete or as a porous open structure by using gap-graded aggregates and not compacting fullyBlock sizes are 200x200x40 (mm), 150x200x400 (mm)They contain usually two large vertical holes. TS 406 classifies them into strength levels of 2.5, 5, 7.5, 10 and 15 MPa. Maximum unit weight is not to exceed 1.60 kg/dm3
270
Masonry constructionBasic method of construction; units are laid one on top of another in such a way that they form an interlocking mass in at least the two horizontal dimensionsMortar needs to be firmly dry for dense low absorption units. While high absorption units need a sloppy wet mortarWalls and columns are built by laying out a plan at foundation level and masonry rises up layer by layerThe foundation layers are horizontal It is essential to maintain the verticalityThickness of mortar joints must be kept constantJoint color and shape influence the appearance
271
Reinforced and post-stressed masonry forms
(a)
(a)Bed joint reinforced(b)Reinforced pocket walls
(b)
Reinforced pocket wall
Pockets formed as built, shuttered then filled with concrete to bond in the reinforcement
272
(c)Grouted cavity(d)Quetta bond
Bars either bonded in with mortar as the brickwork is rasied or grouted in with concrete in lifts
Vertical or horizontal or both rebar reinforcement
Ties at 5/m2
Concrete grouted cavityWidth = 2 x cover +bar
(c) (d)
273
Stretcher or half bond
Soldier course
Quarter bond
Half brick bonds
Bond (interlocking) between units is achieved asStretcher bond or half bondThird quarter bondSoldier course
Bond patterns
274
Typical block/brick cavity wall
275
Flat shear ties at 5/m2
Mortar filledcollar joint < 25 mm
Collar-jointed brick wall
276
Header
CloserStretchera) English b) Flemish garden wall
c) Heading d) Rat-trap
Bonded wall types
277
Structural behavior
Unreinforced masonry is; good at resisting compression forces, moderate to bad at resisting shear forces very poor when subjected to direct tension
However, reinforced masonry is good also at resisting tension forces
Any masonry under compressive stress also resists bending since the compressive prestress in the wall must be overcome before any tensile strain can occur
Most of small masonry structures are still designed using experience-based design rules. Strength of masonry elements are predicted from strength and/or other characteristics of materials used in masonry construction
Then check is made against worst loading conditions obtained from past data. A factor of safety is applied to allow for statistical uncertainty in the material characteristics and loads. Relatively high (3-5) safety factors are used due to high variability and brittle failure mode
278
Forces on walls
Lateral(flexural)
Shear(in plane)
Compressive
Shear(normal)
Tensile
279
Generated by tensionstresses in the bedjoints
Disturbing force e.g. due to wind load
Restoring force
Generated by massacting at centre of gravity
Gravity mechanism Flexural resistanceCrack
Mechanisms for resisting bending forces in masonry cantilevers
280
Compressive loadingMasonry is most effective under compressive loadIf a load or force is put on a wall at a point, it would logically spread or toward from the point of application in a stretcher bonded wall since each unit is supported by the two units below it.
Such a compressive force causes elastic shortening (strain) of the masonry. As a result of Poisson’s ratio effects, a tension strain and hence a stress is generated normal to the applied stress.
281
Stability: slender structures
For masonry wall and columns, if ratio of height to thickness (h/d = slenderness ratio) is small, then the strength will depend largely on the strength of the constituent materials. Other factors that affect the strength of masonry are • Degree of workmanship• Thickness of joints• Type of arrangement of bricks
190 9010
9010
190
211 brick length column
282
Some empirical formulas for masonry design for h/d ≤ 10
Brocker formula
23max unitmortar σσσ =
e
dh
mortarunit
++
⎟⎠⎞
⎜⎝⎛ +
=)(316
104
max
σσ
σ
Graf formulaσ : in kgf/cm2
e : + 10 for good workmanship0 for medium workmanship
- 5 for poor workmanship
283
Comp. strength of stone (MPa) Type of wall Type of mortar 20 30 50 80 120
Walls with irregularly shaped natural stones
Lime Mixed Cement
0.2 0.2 0.3
0.2 0.3 0.5
0.3 0.5 0.6
0.4 0.7 1.0
0.6 0.9 1.2
Walls with natural Stones with horizontal joints
Lime Mixed Cement
0.3 0.5 0.6
0.4 0.7 1.0
0.6 0.9 1.2
0.8 1.2 1.6
1.0 1.6 2.2
Walls with stones with more than 3-plane faces
Lime Mixed Cement
0.4 0.7 1.0
0.6 0.9 1.2
0.8 1.2 1.6
1.0 1.6 2.2
1.6 2.2 3.0
Walls with matural stone blocks
Lime Mixed Cement
0.8 1.2 1.6
1.0 1.6 2.2
1.6 2.2 3.0
2.2 3.0 4.0
3.0 4.0 5.0
Comp. strength of bricks (MPa) 2.5 5 7.5-10 15 25 35
Brick Walls
Lime Mixed Cement
0.3 0.5 0.6
0.4 0.7 1.0
0.6 0.9 1.2
0.8 1.2 1.6
1.0 1.6 2.2
_ 2.2 3.0
Table a - Allowable Stresses for Masonry (DIN 1053)
Allowable stresses in this table are acceptable for h/d ≤ 10. If 10 < h/d ≤ 20, reduced allowable stresses for slender walls should be used(Table b). Slender walls with h/d > 20 are not permitted for masonry.
284
Linear interpolation for intermediate values permitted
Allowable stress of unslender wall (MPa) h/d 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.6 2.2 3.0 4.0 5.0
12 - - 0.3 0.4 0.5 0.6 0.6 0.7 0.8 1.1 1.5 2.0 3.0 4.0 14 - - - 0.3 0.3 0.4 0.4 0.5 0.6 0.8 1.0 1.4 2.2 3.0 16 - - - - - 0.3 0.3 0.3 0.4 0.6 0.7 1.0 1.4 2.2 18 - - - - - - - - 0.3 0.4 0.5 0.7 1.0 1.4 20 - - - - - - - - - - 0.3 0.5 0.7 1.0
Table b - Reduced allowable stresses for slender masonry, in N/mm2
Allowable stressesfor slenderwalls h/d > 10
285
Movements and elastic moduli of masonry materials
Masonry component material
Thermal expansion coefficient α (per ºC x 10-6)
Reversible moisture movement (%)
Irreversible moisture movement (%)
Modulus of elasticity E (N/mm2)
Granite 8 - 10 - - 20 – 60 Limestone 3 - 4 0.01 - 10 – 80 Marble 4 - 6 - - 35 Sandstone 7 – 12 0.07 - 3 – 80 Slate 9 – 11 - - 10 – 35 Mortar 10 – 13 0.02 – 0.06 - 0.04 – 0.1 20 – 35 Dense concrete brick/blockwork
6 - 12
0.02 – 0.04
- 0.02 – 0.06
10 – 25
LWAC blockwork 8 - 12 0.03 – 0.06 - 0.02 – 0.06 4 – 16 AAC blockwork 8 0.02 – 0.03 - 0.05 – 0.09 3 – 8 Calcil brickwork 8 – 14 0.01 – 0.05 - 0.01 – 0.04 14 – 18 Clay brickwork 5 - 8 0.02 0.02 – 0.10 4 - 26
286
Example;A masonry wall of 190 mm thickness (1 brick length) and 3.04 m height will be built of solid bricks DOT/12 according to TS705 and mixed mortar C-3 according to TS2348. Determine the maximum load that could be permitted on a 1m length of this wall. Solution;
101619304
dh ratio sSlendernes >===
from Table a σall= 0.9 MPa (for σbrick = 7.5 – 10 for mixed mortar)
from Table b σall= 0.3 MPa (for σall = 0.9 MPa for unslender wall and h/d=16)
Mixed load on 1m length of wall
mkNmmNAP all /57)119.0(/103.0 226max =×××=⋅= σ
287
Overall Outline
• Introduction• Concrete• Bituminous materials• Masonry • Polymers and polymer composites• Cement-based fiber composites• Metals• Timber
288
Polymers and polymer compositesPolymeric materialsproduced by combining a large number of small molecular units (monomers) by polymerization to form long chain molecules
1) Thermoplastic polymersLong chain molecules held together by weak Van der Waals forces cycle of softening by heating and hardening by cooling can be repeated almost indefinitely. However, with each cycle, the material tends to become more brittle2) Thermosetting polymersEpoxies and phenolics are principle examples cross linking exists between chains3) Foamed polymersTwo-phase system of gas dispersed in solid polymer, produced by adding a blowing agent to molten resin. Gas is released and causes polymer to expand. Small gas cells increase the volume of resin many times. They can be either thermoplastic or thermosetting. 4) ElastomersLong-chain polymer molecules made of coiled and twisted chains. Flexible material that undergoes very large deformations. Volcanization provides rigidity and hardness.
289
Fibers for polymer compositesLoad transfer from matrix to fibers results in high-strength, high-modulus material. Glass, carbon, boron fibers are examples of amorphous and crystalline fibers used in polymeric matrices.
1) Glass fibersManufactured by drawing molten glassfrom an electric furnace through platinum bushings at high speed. Filaments cool from liquid state at about 1200ºC to room temperature in 10-5 seconds.
Winding
Sizing process
Platinum bushing
Strands
204filaments
Moltenglass
290
Four types of glass are used for fibers- E-glass of low alkali content (most used one in composites in
construction industry)- A-glass of high alkali content (previously used in aircraft
industry)- Z-glass developed for reinforcing cement-based materials
because of its high resistance to alkali attack - S2-glass fiber used for extra high strength and high modulus
applications in aerospace
291
2) Carbon fibersPreviously produced from pitch obtained by destructive distillation of coalSynthetic fiber polyacrylonitrile is spun and concurrently stretched so that molecular chains are aligned parallel to the fiber axis. Fiber is then heated under tension to 250ºC in oxygen environment where it gains strength. Carbonization starts when polymer is heated in inert atmosphere. Higher stiffness is obtained through greater heat energy given to carbon filament
3) Aramid fibers (Tradename - Kevlar)Most successful commercial organic fiber developed by DuPont Comp. Poly-parabenzamide fibers E=130GPa2 forms of Kevlar fiber
Kevlar 29: high strength and intermediate modulusKevlar 49: high modulus and same strength as Kevlar 29 (preferred for high-performance composite materials)
292
Mechanical properties
Polymer propertiesMechanical properties are highly dependent on network of molecular units and on the lengths of cross-link chains. Curing process determines the length of cross-linked chains. Composites are heat cured to maximize mechanical properties
Mechanical properties of thermoplastics arise from properties ofmonomer units and high molecular weight
293
Mechanical properties of common thermosetting and thermoplastic polymers
Specific weight
Ultimate tensile strength (GPa)
Modulus of elasticity in tension, GPa
Coefficient of linear expansion 10-6/ºC
Thermosetting Polyester 1.28 45 – 90 2.5 – 4.0 100 – 110 Epoxy 1.30 90 – 110 3.0 – 7.0 45 – 65 Phenolic 1.35 - 1.75 45 – 59 5.5 – 8.3 30 – 45 Thermoplastics Polyvinylchloride (PVC)
1.37 58.0 2.4 – 2.8 50
Acrylonitrile butadiene styrene (ABS)
1.05 17 – 62 0.69 – 2.82 60 – 130
Nylon 1.13 – 1.15 48 – 83 1.03 – 2.76 80 – 150 Polyethylene (high-density)
0.96 30 - 35 1.10 – 1.30 120
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Fiber propertiesComposite materials have high specific strength and high specific stiffnessachieved by the use of low-density fibers with high strength and modulusvalues
Specific weight
Ultimate tensile strength (GPa)
Modulus of elasticity in tension, (GPa)
Carbon fiber Type I 1.92 2.00 345 Type II 1.75 2.41 241 Type III 1.70 2.21 200 E – Glass 2.55 2.40 72.4 S2 – Glass fiber 2.47 4.6 88.0 Kevlar fibers 29 1.44 2.65 64 49 1.45 2.65 127
295
Polymer composites
Mechanical properties of polymers can be greatly enhanced by incorporating fillers and/or fibers into resin formulations
For structural applications, such composites should 1) Consist of two or more phases2) Be manufactured by combining separate phases such that
dispersion of one material in the other achieves optimum properties of the resulting material
3) Have enhanced properties composed with those of individual components
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In fiber-reinforced polymer materials, primary phase (the fiber) uses plastic flow of secondary phase (the polymer) to transfer load to fiber. This results in a high-strength, high-modulus composite.High strength and high modulus properties of fibers are associated with very fine fibers with diameters of 7-15μm. Fibers are usually brittle.Polymers may be ductile or brittle and generally have low strength and stiffnessBy combining two components a bulk material is produced with a strength and stiffness dependent on fiber volume fraction and fiber orientation.Interface between fiber and matrix plays a major role in physical and mechanical properties of composite materialLoad is transferred from fiber to fiber through the interface and the matrix
297
Assumptions made for the analysis of composite materials
The matrix and the fiber behave as elastic materialsBond between fiber and matrix is perfect and consequently there will be no strain discontinuity across the interface Material adjacent to fiber has the same properties as the material in bulk formFibers are arranged in a regular or repeating array
Properties of interface region are very important for fracture toughness of the materialWeak interface → low strength and stiffness → high resistance to fracture (ductile)Strong interface → high strength and stiffness → low fracture toughness (very brittle)
298
Elastic properties of continuous unidirectional laminae
1
E11, σ11
E22, σ22
2
3
Basic laminate
299
1) Longitudinal stiffness
The orthotropic layer has three mutually perpendicular plane of property symmetry, characterized elastically by four independent elastic constants.
E11= modulus of elasticity along fiber directionE22= modulus of elasticity in the transverse directionν12 = Poisson’s ratio i.e. strains produced in direction 2 when specimen is
loaded in direction 1.G12= longitudinal shear modulusν21 = Poisson’s ratio, i.e. obtained from E11 ν21 = E22 ν12
If line of action of a tensile or compressive force is applied parallel to fibers of a unidirectional lamina, εm = εf provided bond is perfectAs both fiber and matrix behave elastically thenσf = Ef εf and σm = Em εm where εf = εm
300
As Ef > Em, stress in fiber must be greater than stress in matrixThus fibers carry major part of applied loadComposite load Pc = Pm + Pfσc Ac = σm Am + σf Af
σc = σm Vm + σf Vfwhere A = area of the phaseV = volume fraction of the phaseVc = volume of composite = 1for perfect bondεc = εm = εfFrom above equationEc εc = Em εc Vm = Ef εc VfEc = Em Vm = Ef VfEc = E11 = Em (1-Vf) + EfVf (law of mixtures)
1σ11
2
3
1σ11
2
3
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2) Transverse stiffness
Applied load transverse to fibers acts equally on fiber and matrix; therefore,σf = σmεf = σ22/Ef and εm = σ22/Emε22 = Vf εf + Vm εm orε22 = Vf σ22/Ef + Vm σ22/Em (substitute σ22= E22ε22)E22 = Ef Em / [ Ef (1-Vf) + EmVf ]to take into account of Poisson contraction effectsE22 = Em`Ef / [ Ef (1-Vf) + Em`Vf ]where Em`= Em / (1-νm
2)
1
σ22
2
3
1
σ22
2
3
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Elastic properties of short-fiber composite materials
As aspect ratio (l/d) decreases, effect of fiber length becomes more significant When a composite containing uniaxially aligned discontinuous fibers is stressed in tension parallel to the fiber direction there is a portion at the end of each finite fiber length and in surrounding matrix where stress and strain fields are modified by discontinuityEfficiency of the fiber to stiffen and to reinforce the matrix decreases as the fiber length decreases
303
Diagrammatic representation of the deformation field around a discontinuous fiber embedded in a matrix
Uniform load
Deformationof matrix
Shear stress at interfaceTensile stress in fiber
Fiber
ResinUniform load
lc/2 lc/2
σf,max
The critical transfer length over which fiber stress is decreasedfrom maximum to zero at the end of fiber is referred to as half the critical length of fiber
304
To achieve the maximum fiber stress, fiber length must be equal to or greater than the critical value lcReinforcing efficiency of short fibers is less than that for long fibers. Orientation of short fibers in a lamina is random and therefore lamina is assumed to be isotropic on a macro scale.Rule of mixture for long-parallel fiber case is modified by inclusion of a fiber orientation distribution factor n
Ec = E11 = ηEf Vf + Em Vmη = 0.375 for a randomly oriented fiber array
= 1.0 for unidirectional laminae when tested parallel to fiber= 0 for unidirectional laminae when tested perpendicular to fiber= 0.5 for a bidirectional fiber array
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Failure of the fiber composite
σfu
σ
σfσcu
σc
σmuσ′mσm
matrix
composite
fiber
ε εfu ε
σfu
σmu
σ′m
0 Vf
Fiber contro
lled
σ cu= σ fu V f+
σ′ m (1-V f)
σcu = σmu (1-Vf)
matrix controlled
σ
1VminVcr
mfu
mmucrV
σσσσ
′−′−
=mmufu
mmuVσσσ
σσ′−+
′−=min
306
- If failure is due to breaking of fibers (fiber controlled, Vf > Vmin, then σcu > σmu) σcu = σ′m Vm + σfu Vf = σ′m (1-Vf) + σfu Vf
- If failure is due to matrix flow (matrix controlled, Vf << Vmin, then σcu < σmu) σcu = σmu (1-Vf)
when Vf < Vmin, fiber content in the composite is so small that fibers break before ultimate strength of matrix is reached; fibers act as if there are voids in place of them and in fact decrease strength of matrix by a factor equal to (1-Vf).
when Vf > Vcr, fibers become effective in increasing the strength of composite above that of matrix. From practical point of view,there is no meaning to use a fiber ratio less than this criticalvalue.
307
For discontinuous fibers
my
fuc
rl
τσ
=my
fuc
dl
τσ2
=or andcll
=α
for which tensile strength in the fiber will be reached at the same forceas the yield strength of matrix on the interface
If α > 1, then matrix will yield first before fiber breaks in tension
308
Average value of tensile stress in the fiber is found as
)11(α
βσσ −−=
−
ff factor efficiencyfiber ==
−
ησσ
f
f
where β is related to stress distribution on fiber. β= 0.5 for fully plastic stress distribution β = 1.0 for fully elastic distribution
309
Strength of composite for discontinuous parallel fibers
)1(')11( fmffucu VVd
−+−
−= σα
βσσ for Vf > Vmin
If α > 5 and β = 0.5 then 9.0≥cu
cud
σσ
If fibers are not too short, their discontinuity may be neglected
310
ExampleProperties of matrix and fiber materials for a glass fiber reinforced composite with continuous fibers of 30% volume fraction are as follows
E (GPa) σfu (MPa) εu Polymer matrix 2 200 0.10 (yield) Glass fibers 140 2800 0.02 a) Determine the modulus of elasticity of the composite for tension
parallel to fibersb) Assuming linear elastic behavior of fibers until failure and elastic-plastic
behavior of matrix, determine ultimate tensile strength of compositec) What minimum fiber volume ratio should be used to obtain a reinforcing
effect
311
Durability of polymer composites
Polymer composites change with time and most significant factorsareElevated temperaturesFireMoisture Adverse chemical environmentsNatural weathering when exposed to sun’s ultra-violet radiation
312
TemperatureFluctuating temperatures have a greater deterioration effect on GRP. Difference in coefficients of thermal expansion of glass and resin may cause debonding.Exposed to high temperatures a discoloration of the resin may occur composite becoming yellow. Both polyster and epoxy show this effect. As a result of exposure to high temperatures, the composite becomes brittle.
FireA composite material must meet appropriate standards of fire performanceSome mineral filler, calcium carbonate can improve mechanical properties. Aluminum trihydrate and antimony trioxide are used as fillers to enable flame-retardant properties
313
MoistureCross-linked polymers absorb water which may cause a decrease in strength and modulus of elasticity. Absorption of water by polyesters and epoxies leads to swelling of the laminate.Water will also cause some surface flaws on fibers and reduce strength, long-term water absorption may cause weakening of the bond between fiber and polymer
314
Natural weathering causes some deterioration of GRP composites. Sunlight degrades both polyester and epoxy resins. As a result of discoloration, loss of light transmission occur. UV absorbers and stabilizers are added to resin formulations. A rise of temperature accelerates chemical reaction and hence degradation.Weathering can affect mechanical properties of GRP composites through surface debonding. Because weathering is a surface effect, thickness of laminate becomes important. 3mm thick laminate shows 12-20% reduction in flexural strength after 15 years exposure, while 10mm laminate shows only ~ 3% reduction after 50 years exposure
Weather
315
Use of polymers and polymer composites
Use of polymers and polymer composites in construction industry falls into three categories;
Non - load - bearingSemi - load – bearingLoad - bearing
Unreinforced polymers
Fiber – reinforced polymers
316
Selection of most appropriate resin should be done considering particular end use since every material does not possess all thefollowing characteristics
High light transmissionInfinite texture possibilitiesMinimum maintenance requirementsInfinite design possibilitiesResistance to water and corrosionHigh specific strengthHigh impact resistance
317
Disadvantages of polymers in construction areHigh cost of materials. However, low density and reduced foundation size make them competitiveLow stiffness and strength (use in composites)Poor scratch resistanceDegradation under UV light (stabilizers used)Low resistance to fire and high temperatures (additives used)
Non-load bearing thermoplastic polymers such as polyethylene have been used to manufacture pipes for transportation of water, oil and gas
318
Geotextiles• Main fibers used in geotextiles are polyethylene, polypropylene,
polyester and polyamide. Fibers are the load-bearing elements in geotextiles and framing technique determines the structure and physical and mechanical characteristics of the system
Geomembranes• Manufactured in impermeable sheet form from thermoplastic
polymers or bituminous materials either reinforced or unreinforced. Matrix can be reinforced by textiles.
Geo-linear elements• Long, slender strips or bars consisting of a unidirectional filament
fiber core made from a polyester, aramid or glass fiber in a polymer sheath of a low-density polyethylene or a resin. Fiber provides strength and deformation characteristics and matrix protects fiber and provides bonding with the soil
Thermoplastic polymers known as geosynthetics are used in soil environment in five categories
319
Geogrids• Grid-like structures of thermoplastic polymer material.
They form a quasi-composite system in conjunction with soil grid structure in fiber and soil is assumed to be the matrıx
• Two forms– Cross-laid strips– Punched thermoplastic polymer sheets
Geocomposites• Hybrid systems. Two or more different types of
thermoplastic polymer systems. They form a drainage passage along the water course with polymer core as drainage channel and geotextile skin as the filter
320
Marine applicationsDominant materials for pleasure craft. Effective in replacing wood
Truck and automobile systemsSports – car bodies and truck cabsStrength, stiffness, toughness, corrosion resistance and high-quality finish are physical and mechanical properties that must be satisfied. However, economy is the crucial consideration
Aircraft and space applicationsUsed for making components in aircraft industryThe fin of European airbus is a component made from a sandwich construction with carbon fiber/epoxy resin face material. The Westland helicopter rotor blades are made from carbon fiber composite material
321
Pipes and tanks for chemicalsCritical consideration is the corrosive resistance and ease of fabrication of complex shapes
Civil engineering structuresApplications have started with GRP structures
A dome structure erected in 1968 in Benghazi, LibyaRoof structure in Dubai Airport built in 19721970-1980 prestigious buildings in UK(Morpeth School, Mondial House, Covent Garden Flower Market etc.)
These buildings are built as a composite system, with either steel or reinforced concrete structural system and GRP composite as load-bearing infill panelsIn 1970, a classroom system, using only GRP, by folding flat plates into a folded plate system so that stiffness is provided by the structural shape.In 1980’s more ambitious structural elements were produced
322
FRP radar dome (25m diameter, factory made FRP sandwich panels are bolted together on site)
Mondial house (clad with FRP panels)
FRP modular classroom
323
MATERİAL Relative density
Diameter thickness ratio (microns)
Length (mm)
E (GPa)
Tens. Str. (MPa)
Failure strain (%)
Volume in composite (%)
Mortar matrix 1.8-2.0 300-5000 - 10-30 1-10 0.01-0.05 85-97
Concrete matrix 1.8-2.4 10000-20000 - 20-40 1-4 0.01-0.02 97-99.5
Asbestos 2.55 0.02-30 5-40 164 200-1800 2-3 5-15
Carbon 1.16-1.95 7-18 3-cont. 30-390 600-2700 0.5-2.4 3-5
Glass 2.7 12.5 10-50 70 600-2500 3.6 3-7
Polyethylene HDPE filament 0.96 900 3-5 5 200 - 2-4 High modulus 0.96 20-50 Cont. 10-30 > 400 > 4 5-10 Polypropylene (Monofilament) 0.91 20-100 5-20 4 - - 0.1-0.2
Polyvinyl alcohol (PVA) 1-3 3-8 2-6 12-40 700-1500 - 2-3
Steel 7.86 100-600 10-60 200 700-2000 3-5 0.3-2.0
Performance is controlled byvol. fraction of fibersproperties of fibers and matrixbond between the two
324
Table shows that elongations at break of all fibers are two or three orders of magnitude greater than strain at failure of matrix. Hence matrix cracks before fiber strength is approached
Most organic fibers have modulus of elasticity less than five times that of the matrix. Low modulus fibers are used in situations where matrix is expected to be uncracked. Large Poisson’s ratio of these fibers may cause debonding and pull-out, woven meshes or networks of fibers are necessary for efficient composites.
Steel fibers of varying cross-sections or bond ends provide anchorage; glass fiber bundles may be penetrated with cement hydration products to give effective bondingDmax of mortars or concrete affect efficient fiber distributionConcrete with a Dmax = 10mm is preferred. Dmax > 20mm is never allowed. To avoid shrinkage, at least 50% by volume of inert filler
(aggregate, fly ash, or limestone dust) should be used
325
Structure of fiber-matrix interface
Properties of fiber reinforced cementitious materials depends on microstructure of interface
Interface is initially water-filled zone which develops to a microstructure
different than bulk matrix
There are 3 layers in interface;a) Very thin (less than one micron) Ca(OH)2 rich rather
discontinuous, directly in contact with fiberb) Massive Ca(OH)2 layerc) Porous zone (up to 40 microns) consisting of CSH and some
ettringite
326
Structure and post-cracking composite theory
In hardened cement-based composites, fibers bridging the cracks contribute to the increase in strength, failure strain and toughness. Limited volume fraction of low modulus fibers do not contribute to the modulus of elasticity of the composite
327
εεfuεmu
matrix
σmu
σcu
σfu
0
σcu
σfu
σmu
0 Vmin Vcr Vf 1.0 Vf
fiber Fibercontrolled
σ′fVf
σfuVf
compositeσ′f
σmu (1 – Vf) + σ′f Vf
σcu
σ cu =
σ fu V f
σcu = σmu (1-Vf) + σ′ fVf
(uncracked)σcu = σ′f Vfcracked)
σ′f = σmu
σ′f < σmu
328
Modulus of elasticity of the composite1) Where the matrix and fibers behave elastically
Ec = Ef Vf + Em (1-Vf)
2) After the point where the matrix starts cracking
ff
fc V
dd
E .⎟⎟⎠
⎞⎜⎜⎝
⎛=
εσ
329
Ultimate tensile strength1) Matrix fails and later composite carries stress at a decreased
level. This mode of failure is called a “matrix controlled” failure. This occurs under the condition that Vf < Vcr
σcu = σmu (1-Vf) + σ’f Vf (uncracked)σcu = σ’f Vf (cracked)σ’f = stress in the fiber when matrix cracks (ε = εmu)
2) Matrix fails and later composite carries stress at an increased level. This mode of failure is called “fiber controlled” failure. This occurs under the condition that Vf > Vcr, σcu = σfu VfThen the critical volume fraction for fibers is found as
fmufu
mucrV
'σσσσ
−+=
330
On the other hand, there is a fiber volume ratio that determines the strengthening effect of fibers in the composite
fu
muVσσ
=min
It is observed that Vcr is a function of σ’fIf σ’f = σmu, then Vcr = Vmin
If σ’f < σmu, then Vcr < Vmin
331
When discontinuous fibers are used there is a critical length for fibers so that critical bond shear stress along the fiber-matrix interface does not lead to bond failure, that is, fibers will break rather than pull-out
⎟⎟⎠
⎞⎜⎜⎝
⎛−
=φφνσ
φυ tantan
lntan ncrL fu
cr
where, r = radius of fiberυ = Poisson’s ratio of fiberc = cohesionφ = Angle of fraction between matrix and fibern = normal pressure exerted on the surface of fiber due to
shrinkage of matrix
332
Determination of critical length of discontinuous fibers is donethrough experimentation
When discontinuous fibers of short length are used (L < Lcr)Then we may have a third mode of failure which is due to reaching the bond strength at the fiber-matrix interface. In this case the strength of the composite is determined not by fiber strength but the adhesion between the fiber and matrix
333
ExampleIt is planned to reinforce a concrete matrix of tensile strength σmu = 8 MPa with polymer fibers to increase its tensile strength. Fiber diameter is 0.25mm. Fiber tensile strength is 80 MPa and fiber-matrix bond strength is 0.2 MPa. Assuming constant bond shear stress distribution along the interface, calculate critical fiber length If discontinuous parallel fibers of 40mm length are used and a composite tensile strength of 11MPa is required, what fiber volume ratio will be necessary? In this case what will be the failure mode?If the moduli of elasticity of concrete and fibers are 20GPa and 50GPa, respectively, plot stress-strain diagram of composite in part b)For a continuous use of above fibers, and for the purpose of obtaining a reinforcing effect of the fibers for any volume ratio, what should be the lower limit of fiber modulus of elasticity.Calculate critical fiber ratio for value of Ef in c
334
Overall Outline
• Introduction• Concrete• Bituminous materials• Masonry • Polymers and polymer composites• Cement-based fiber composites• Metals• Timber
335
Metals, their differences and uses
Metals in Civil Engineering
Ferrous(iron + carbon) Non-ferrous
Cast iron(C > 1.7 %)
Steel(C < 1.5 %)
Structural steel(C ≈ 0.25 %) Aluminium Copper
336
1) Cast irons
Major consumption in pipes and fittingsIn civil engineering for tunnel segments and mine shaft tubingCarbon content is greater than 2 %. Hard and brittle.
2) SteelSteel is obtained through carbon reducing operations. Carbon steel is an alloy of iron and carbon. Amount of carbon within the lattice determines the properties of the steel. Alloys containing less than 0.008 % carbon are classed as irons. Steel has a carbon content less than 2.0 %
C ≤ 0.25 % ⇒ mild steel, low carbon steel (structural steel is in this category)0.3 % ≤ C ≤ 0.6 % ⇒ medium carbon steel, carbon steelC > 0.6 % ⇒ high carbon steel
Normally, Mn and S are added to steel during productionIf elements other than Mn and Si are added ⇒ alloy steelsIf elements like Cr and Ni are added ⇒ stainless steels
337
2.1 Structural steelFour grades of structural steelsGrade Min.ten.st.(MPa)
40 40043 43050 50055 550
All structural steels are readily weldable. For higher grades care is needed in welding
2.2 Heat treated steelsProperties of steel (C > 0.3 %) can be varied by heat treatment
Heating to high temperatureFast cooling by quenching in oil or waterFollowed by reheating to about 650ºC (tempering)
Fast cooling produces ⇒ hard, brittle microstructure (known as martensite)Used only for cutting tools, no use in structural engineering
338
2.3 Stainless steelsFerrow alloys: containing at least 12% Cr and some Ni and Mo. Chromiumproduces a stable passive oxide film.
Three basic types (grouped according to metallurgical structure):1. Martensitic: 13% Cr, low carbon, hard, used for cutlery, unweldable2. Ferritic: 13% Cr, low carbon, unweldable3. Austenitic: 18% Cr and 8% Ni, low carbon, most resistant to pitting type of corrosion, weldable
339
3. Aluminium and alloys
Used both structurally and decoratively in cladding, roofing, window frames, window and door furniture. Lightness of aluminium is an advantage in structures with high self-weight / live load. Such as roofs, footbridges and long span structuresHigh durability of aluminium makes it usable in polluted and coastal areasHigh cost limits its useHigh initial cost maybe offset by reduced maintenanceStructural sections are produced by extrusion
Eal = 70GPa & Est = 210 GPa ⇒ Aluminium deflects more under same loadHowever, specific moduli (E/ρ) is comparable(E/ρ)al = 20, (E/ρ)st = 29
340
4. Copper and alloysUsed in applications where high thermal and electrical conductivity needed such as domestic water services, heating, sanitation4.1 Brasses; copper – zinc alloys ⇒ enhanced strength and corrosion resistance4.2 Bronzes; copper – tin alloys ⇒ high corrosion resistance
341
Design for minimum weight
To find what is the least amount of material needed to carry design loads
P Consider a cantilever beam of length L
End deflection isEI
PL3
3
=δ
4
3
3ExPL
=δ , x is a unit of length
P = Applied loadL = length of beam E = modulus of elasticityI = second moment of area
342
2/1
2/15
3 EPLW ρ
δ ⎟⎟⎠
⎞⎜⎜⎝
⎛=
for a given set of design condition (P, L, δ), W is minimized by minimizing 2/1Eρ
W = ρAL = ρLx2weight is
A = area of the cross-sectionρ = density of the material of which the beam is made
22
24
LWxρ
=
343
Materials in decreasing order of efficiency in bending
25.60Copper21.82Concrete17.17Steel12.86Titanium11.25GFRP10.0Aluminium4.55Timber Timber is oustanding
Then comes Al and GFRPThese compete in applications where weight iscostly (aircraft, sailing dinghies, racing cars, small-scale building, squash rackets, golf clubs, tables and chairs)
344
Overall Outline
• Introduction• Concrete• Bituminous materials• Masonry • Polymers and polymer composites• Cement-based fiber composites• Metals• Timber
345
TIMBERTimber has been one of the basic materials of construction since the earliest days of human kind. Today it has been largely superseded by concrete and steel. However, the use of timber remains extensive.
Wood – A Nature Made Composite
Wood contains; 60 % cellulose (C6H10O5)n28 % lignine (C41H36O6)12 % pectine (C6H8O6)n & some others
346
Internal structure of wood is made up of groups of long and slender cells, called “grain”, that look like a group of thin pipes. The walls of the cells are made up of “ligno-cellulose” and they are bonded to each other with “pecto-cellulose”. The length of the cells is between 1-6 mm and their diameter is about 1/100 of their length
Depending on internal structure, the trees may be separated into three groups;
1) Softwoods without any vessels or pores for the sap like pine, fir, spruce, cedar and cypress
2) Hardwoods with diffused vessels or pores like maple, beech, poplar 3) Hardwoods with ring-porous structure like oak, ash etc.
Even though chemical composition is the same, differences in internal structure result in differences of properties. Unit weights vary between 0,398 – 0,720 kg/dm3 and flexural strengths between 65 – 110 MPa.
347
Type of tree Unit weight (kg/dm3)
Compressivestr. (MPa)
Flexural str. (MPa)
Poplar (kavak) 0,398 41 69 Fir (köknar) 0,431 36 71 Spruce (ladin) 0,436 31 69 Yellow pine (sarıçam) 0,515 38 65 Cedar (sedir) 0,523 45 77 Black pine (karaçam) 0,568 48 110 Red pine (kızılçam) 0,577 38 73 Oak (meşe) 0,690 55 94 Beech (kayın) 0,720 53 105
Table 1. Mechanical properties of trees in Turkey in air-dried conditionand parallel to grains
Even though chemical composition is the same, differences in internal structure result in differences of properties.
348
Largest properties are obtained in the direction parallel to grain and in dry condition which is the condition in which wood is usually utilizedBeing an organic material, wood is affected by the moisture in the atmosphere, by the ultraviolet rays of sun and deteriorated by parasites such as fungus or worms. Wood has to be protected by coatings and poisonous preservativesIts anisotropical nature and the limitations of size make its use difficult. To correct those deficiencies, artificial wood products like plywood, fiberboard, etc. are manufactured and used to a great extend
Properties of timber change with- direction- moisture content
349
102030405060708090
10 20 30 40 50
100
60 70 80 w (%)
Comp.Str.(MPa)
Composite strength – moisture relationfor soft - woods with unit wt = 0.420 kg/dm3
350
3rd class 2nd class 1st class Strength (MPa) Pine Oak Pine Oak Pine Oak Tensile // - - 8.5 10.0 10.5 11.0 Compressive // 6.0 7.0 8.5 10.0 11.0 12.0 Compressive ⊥ 2.0 3.0 2.0 3.0 2.0 3.0
Timber specification (TS 647)Allowable stresses
Modulus of elasticityPine: // 10 GPa
⊥ 0.3 GPaOak: // 12.5 GPa
⊥ 0.6 GPa
Recommended