Concrete Technology Workshop 2010 Lecture 1a - Materials

8/3/2019 Concrete Technology Workshop 2010 Lecture 1a - Materials

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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


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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


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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


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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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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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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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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


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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


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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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Rates of strength gain

of various clinker minerals


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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.


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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


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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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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


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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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Heat rate curves for GGBS, FA, CSF cement blends (From Ballim et al)

GGBS

CSF

FA


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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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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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The Condition of Limiting Space (water-cured concrete) gives:


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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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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.


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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.


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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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(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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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


Workshop, May 2010

Thank you and questions

Documents

Concrete Technology Workshop 2010 Lecture 1a - Materials