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8/3/2019 Concrete Technology Workshop 2010 Lecture 1a - Materials
1/6
Professor Mark Alexander, Dr. Hans Beushausen
Concrete Materials and Structural Integrity Research Group
University of Cape Town
Workshop, May 2010
Concrete Technology forStructural Engineers
07:30 - 08:30 Registration
08:30 -09:30 Introduction; Different types of cements, hydration and early ageproperty development of concrete; Concrete admixtures
09:30- 10:30 Compressive strength of concrete; Tensile and flexural strength
10:30 -11:00 Tea break
11:00 -12:00 Behaviour under load; Deformation principles; Elastic properties
12:00 - 13:00 Shrinkage and creep
13:00 - 14:00 Lunch break
14:00 - 14:30 Concrete deterioration mechanisms
14:30 - 15:00 Concrete durability
15:00 -15:30 Tea break
15:30 -16:00 Special concretes: SCC, HSC
16:00 -16:30 Sponsors presentations on concrete technology
16:30 Discussion and cl osu re
Concrete:Constituent Materialsand Properties
Professor Mark Alexander
Dr. Hans Beushausen
Concrete Technology for Structural EngineersMay 2010
Concrete Technology for Structural Engineers
Workshop, May 2010
Contents
Constituent materials for structural concrete
Cements cement/binder types
portland cements
hydration
microstructure
SA cement types
Aggregates not dealt withexplicitly here
Admixtures brief- further lecture
Concrete Technology for Structural Engineers
Workshop, May 2010
CEMENTING MATERIALS
(Binders)
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Hydraulic Cements/Binders
Cements or binders which, when mixed with water, set orharden in air or water by a process of hydration, forming
compounds which are volumetrically stable, durable, andincrease in strength with age.
Basic constituents are oxides of Ca, Si, Al, Fe
Ca0/Si02 ratio 2,6 3,6, typically 2,8
Implies excess of calcium in the system
Concrete Technology for Structural Engineers
Workshop, May 2010
Most common example: Portland Cement
Table : Composition of Portland cement clinker (FromFulton9)
Oxide % by mass
CaO 63 69
SiO2 19 24
Al2O3 4 7
Fe2O3 1 - 6
MgO 0.5 3.6
Na2O + 0.658 K2O 0.2 0.8
Formation of hydrated calcium silicates
Concrete Technology for Structural Engineers
Workshop, May 2010
Binders which, when mixed with water, will harden very
slowly (generally too slowly for engineering purposes),and therefore require an activator to accelerate thehydration.
Comprise same basic oxides as hydraulic binders, but indifferent proportions.
Ca0/Si02 ratio 0,92 1,05, typically 1,02 therefore adeficiency of calcium to form calcium silicates
Latent Hydraulic Binders
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Workshop, May 2010
Most common example: ground granulated blast furnaceslag or GGBS
Also Corex slag (GGCS) (W. Cape)
Table : Chemical composition of South African GGBS (FromFulton9)
Oxide % by mass
SiO2 34 40
CaO 32 37
Al2O3 11 16
MgO 10 13
FeO 0.3 0.6
MnO 0.7 1.2
K2O 0.8 1.3
S 1.0 1.7
TiO2 0.7 1.4
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Materials which are siliceous or alumino-siliceous and in
themselves possess little or no cementitious properties, butcan react with lime in t he presence of water to form stable
hydrated cementitious compounds.
Examples: Volcanic ashes and earths; calcined shales andclays; fly ash (FA); condensed silica fume (CSF).
In common use in concrete in SA: FA (CSF rarely used).
Pozzolanic Materials
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For F A: Ca0/Si02 ratio
0,09 to 0,13, but can vary widely.For CSF: Ca0/Si02 ratio 0,01, very low Ca0 content.
Table : Chemical composition of South African FA (ex Matla, Lethabo& Kendal)and CSF (FromFulton9)
In South Africa, we often refer to slag, fly ash and C SF collectively asCement extenders.They are also called Supplementary cementitious materials
Oxide % by mass
FA CSF
SiO2 48 55 92 96
Al2O3 28 34 1.0 1.5
CaO 4 7 0.3 0.6
Fe2O3 2 4 1.0 1.6
MgO 1 2 0.6 - 0.8
Na2O + 0.658 K2O 1 - 2 0.8 1.3
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PORTLAND CEMENTS
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Workshop, May 2010
PC manufactured in a large rotary kiln h ightemperature process (1400-1450 C).
Raw materials mainly limestone, shale/clay -milled toform raw meal, which is then calcined to produceclinker.
Raw meal:oxides of Ca, Si, A and Fe,(compositions given previously.)
Clinker: milled with smallamount of gypsum tomake Portland Cement.
Manufacture
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Workshop, May 2010
Cement chemistryshorthand notation
C = CaO S = SiO2 A = Al2O3 F = Fe2O3
Primary clinker compounds are
C3S Tricalcium silicate
C2S Dicalcium silicate
C3A Tricalcalcium aluminate
C4AF Tetracalcium aluminoferrite
E.g. C3S = 3CaO.SiO2
Concrete Technology for Structural Engineers
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Compound composition of S.A. Portland cements
Table : Compound composition of South African CEM I cements FromFulton9
Compound Formula Abbrev. % by massin cement
Tricalciumsilicate
3CaO.SiO2 C3S 60 73
Dicalciumsilicate
2CaO.SiO2 C2S 8 30
Tricalciumaluminate
3CaO.Al2O3 C3A 5 12
Tetracalciumalumino-ferrite
4CaO.Al2O3.Fe2O3 C4AF 8 16
Magnesia MgO M 1.9 3.2
Gypsum Raw material - 4.4 6.7
Free lime CaO - 0.2 2.5
Concrete Technology for Structural Engineers
Workshop, May 2010
Hydration of Portland Cements
Hydration is the formation of a compound by the
combination of water with some other substance (i.ereaction with water), in this case wit h clinker minerals.
The main strength forming compounds in PC are thecalcium silicates (C3S & C2S).
The primary reaction products are calcium silicate
hydratesand calcium hydroxide.
Thus:
CS + H CSH
Calcium silicate water calcium silicatehydrates
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Workshop, May 2010
Rates of strength gain
of various clinker minerals
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Workshop, May 2010
Example (C3S hydration)
2C3S + 6H C3S2H3 + 3CH (calcium hydroxide).
CH is calcium hydroxide, CaOH2
Together with the metal alkalis (Na and K), these giveconcrete its high alkalinity (pH = 12.5 - 13.0)
- essential for durability of embedded reinforcing steel
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Early-agedevelopment ofhydrationproducts andmicro-structure
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Influence of main hydration components on concrete properties
Compon-ent
Strength Deformations Durability
CSH Provides cohesiveand adhesiveproperties of conc.
Gel pores influenceshrinkage and creepthrough water loss.
Gel insoluble. Generallylow permeability.
CalciumHydroxide
Reduces porosity,but may causecleavage andstrength reduction.
Dimensionallystable.Restrains CSHdeformations.
Blocks capillary pores andlowers permeability.Leached by water. Attack-ed by acids. Carbonates.
Ettringite Not significant.Reduces totalporosity.
Minor e ffect. Ettring ite ( i f formed fromsulphate attack) isexpansive.
Unhydrated
cementSignificant only inlow-porosity pastes.
Restrains CSHdeformations
Renewed hydration givesautogenous healing ofinternal cracking.
CapillaryPores
Capillary porosity isa major factorinfluencing strength.
Fine pores and gelpores contribute toshrinkage and creep.
Porosity influencespermeability and diffusivity.Large pores increasepermeability.
Concrete Technology for Structural Engineers
Workshop, May 2010
Slag (GGBS) and Fly Ash (FA)
Recall: hydration of PC produces excess CaOH2and analkaline pore solution
Cement extenders require an alkaline environment toinduce hydration (Activators)
Hydration of Cement Extenders
Concrete Technology for Structural Engineers
Workshop, May 2010
GGBS: Once activated, hydrates to form CSH but does notconsume CaOH2
FA: Activated by alkalis, then hydrates by consuming excessCaOH2 (from hydration of PC) to f orm CSH.
In general, hydration products produced by GGBS and FA
are similar to those produced by hydration of PC.
However, extended cement concretes have lower CaOH2contents - generally an advantage.
Hydration of Cement Extenders
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Workshop, May 2010
w/b 0.60
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 20 40 60
Age (days)
fc,slagc
oncrete
/fc
,PC
GGBS
GGCS
GGAS
E.g. (1) Strength performance of different slag concretes
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E.g. (2) w/c required for a Grad 30 concrete as function of differentbinders
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Heat of Hydration Hydration of Portland cements and cementextenders produces substantial amounts of heat (heat of hydration)
Graph below shows rate of heat evolution in early stagesStage 1: Early rapid heat evolution mainly C 3A Stage II: Induction or dormant period
Stage III: Initial set 2-4h, followed by accel. period to max. heat rate (4-8h) C3S hydration
Stage IV: Reaction slows Stage V: steady state
0.1 1 10 100
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Heat of Hydration (contd)
Total heats of hydration of PC and
other binders
CEM I - Range for SA CEM I:
270-320 kJ/kg
Proportion of the above for typical
blended binders
50% GGBS: 60%
30% FA: 55%
5% CSF: 90%
But note: there is large variability in theabove need for specific testing incritical cases
Concrete Technology for Structural Engineers
Workshop, May 2010
Heat rate curves for typical SA cement blends inconcrete (From Ballim et al)
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Total heat curves for SA cement blends in concrete low heat PC clinker (From Ballim et al)
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Workshop, May 2010
Heat rate curves for GGBS, FA, CSF cement blends (From Ballim et al)
GGBS
CSF
FA
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Workshop, May 2010
Powers Model for HydrationHas the advantage that it can predict porosity which is key tounderstanding behaviour of PC concretes
Schematics of PowersModel
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Features of Powers Model
The formation of the rigid cement gel (i.e. CSH ) is always the sameregardless of the stage of hydration, type of PC, w/c.
Characteristic feature of CSH is porositygel poresand capillary pores.
Gel has a characteristic porosity of 28% (i.e. gel pores very tiny, 0.001-0.003 m).
Capillary pores are much larger (0.01 1.0 m). Remnants of originalwater-filled space.
The combined water (water of hydration) is a constant proportionby mass ofthe cement with which it combines
wn/c = constant = 0.23
Densities: unhydrated cement = 3.14 - 3.17 g/cm3; gel solids= 2.51 g/cm3 ;gel including pores= 1.80 g/cm3
The gel expansion factorr elates the volume of gel (including pores) to the
volume of unhydrated cement from which it is formed 2,2
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Workshop, May 2010
Schematic of volume relationships duringhydration (after Addis)
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Limits of Hydration
1. The Condition of Limiting Space water-cured concrete.
2. The Condition of Limiting Water sealed concrete (e.g. large members,mass concrete).
These lead to a critical w/c of approx. 0.40 (range 0.36 -0.42)
Below this critical w/c: capillary porosity is minimised
Above this critical w/c: the system increasingly is governed by capillaryporosity
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Workshop, May 2010
The Condition of Limiting Space (water-cured concrete) gives:
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Concrete Technology for Structural Engineers
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Role of capillary pores
Capillary porosity is a major factor influencing strength
Fine pores and gel pores contribute to shrinkage and
creep
Porosity influences permeability and diffusivity. Largepores increase permeability
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This is the degree to which the cement has hydrated at
any given time - critically influenced by curing.
Curing is the combination of temperature, moisture, andtime effects coupled with the type of binder (e.g. fast orslow hydration)
In essence, time and temperature are interchangeable(see later lecture on concrete strength the MaturityConcept)
This means that at lower curing temperatures, oneneeds to cure for longer
This is also true for blended cements due to their slower
hydration characteristics
Concept of degree of hydration
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Degree of hydration(contd) Temperature of curing is also important influences rate
of hydration and therefore rate of strength development
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Workshop, May 2010
EXTENDED CEMENTS/BINDERS
Ground granulated blastfurnace slag GGBS
Fly Ash FA
Condensed Silica Fume CSF
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Extender Effects Suitability for use in
Mass
concrete
Marine
exposure
ASR
GGBS
Fresh concrete
May improve workability
Slightly retards setting
Hardened concrete
Slower strength development
Improved long term strength
Reduced permeability
Prevents or retards ASRBinds chlorides and reduces
chloride ingress
Lower heat of hydration rate
High
GGBS
contents
> 50%)
help
reduce risk
of thermal
cracking.
GGBS
particularly
suited to
marine
conditions;
provides
substantial
resistance to
chlorideingress and
controls
reinf.
corrosion.
Requires >
40% GGBS
content to
control
potential
ASR for
susceptibleaggregate
types.
Table: Effects of extenders on properties of concrete (from Table1.5, Fulton 9).
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Table: Effects of extenders on properties of concrete (fromTable 1.5, Fulton 9).
Exten-
der
Effects Suitability for use in
Mass
concrete
Marine
exposure
ASR
FA
Fresh concrete
Improves workability and reduces
water content
Slightly retards setting
Hardened concrete
Slower strength development
Improved long term strength
Refines pore structure, reducespermeability
Prevents or retards ASR Binds
chlorides and reduces chloride
ingress
Lower heat of hydration rate
FA content of
30%
significantly
reduces risk
of thermalcracking.
Fly Ash
content of
30%
enhances
resistance
to chloride
ingress and
reinf.
corrosion
due to
chlorides.
Requires
FA
content of
> 20% to
control
potential
ASR forsuscep.
agg.
types.
Concrete Technology for Structural Engineers
Workshop, May 2010
Table: Effects of extenders on properties of concrete (fromTable1.5, Fulton9).
Extender Effects Suitability for use in
Mass
concrete
Marine
exposure
ASR
CSF
Fresh concrete
Reduces workability
Increase cohesiveness
Significantly reduces
bleeding
Hardened concrete
Increases strength
Reduces permeability
Substantially r efines pore
structure
Not
suitable
for use in
mass
concrete.
CSF significantly
reduces physical
permeability, but
does not bind
chlorides
effectively.
Nevertheless canimprove
resistance to
chloride ingress.
Requires
CSF
content of
> 15% to
control
potential
ASR forsusceptible
aggregate
types.
Concrete Technology for Structural Engineers
Workshop, May 2010
In general, cement extenders:
o Reduce cost of concrete
o Enhance durability of concrete
o Reduce industrial waste
o Improve physical microstructure and chemicalresistance of concrete
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SA CEMENT TYPES
SA cements are manufactured according to SANS50197-1
The Table that follows gives Common cements i.e cements for concrete that are based onPortland cement technology
The next slide interprets the symbols given in theTable
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Workshop, May 2010
(a) Type
of Cement
(b) Clinker
content
category
(c) Proportion and type of
extender in cements
(d) 28-day
strength class
CEM
Denotes a
common
cement, i.e.
for concrete
I -> 95%
clinker
II -may
contain up to
35% extender
(except CSF)
IIImay
contain > 35%
GGBS
In CEM II:
A6-20%
extender
B21-35%
extender
In CEM III:
A36-65%
extender
B66-80%
extender
C81-95%
extender
In CEM II:
2nd capital
letter indicates
type of
extender:
DCSF
L Limestone
SGGBS
V or W - FA
Number indicates 28
day strength class.
Lower boundary of a
windowfor strength
32.5 32.5R
42.5 42.5R
52.5 52.5R
R denotes high
early strength cement
Cement Notation(a) CEM (b) I, II, III (c) A, B, C (d) 32.5 42.5 52.5 R
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Concrete Technology for Structural Engineers
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Concrete Technology for Structural Engineers
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Important tounderstand thatconcrete can be madeto most strengthgrades with most of thecommon cements irrespective of thestrengthen class of thecement (within certainlimits)
The controlling factorwill be w/c ratio in themix design
Strength class 52.5
Strength class 42.5
Strength class 32.5
Strength vs w/c for a rangeof SA binder types
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SA CEMENT TYPES various cement companies
CementType
Afrisam Lafarge NPC PPC
CEM I 52.5 N - - 42.5 N/R52.5 N
CEM I I A -M(V -L) 42.5NA-M (L) 42.5 NB-L 32.5 N
A-V 52.5 NA-M(V-L) 42.5 NA-V 42.5 N
A-L 32.5 RB-S 42.5 NB-L 32.5 N
A-L 32.5 RB-M (L-S) 32.5 RB-V 32.5 RB-L 32.5 R
CEM IIIA - - A 32.5 N -
CEM IV - A-V 32.5 RB-V 32.5 R
- -
CEM V A (S-V) 32.5 N - - Corex slag in bulk(not available atpresent)
BULK OF CEMENTSLIE HERE
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Thank you and questions
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