4 engineering ggbs,pfa concrete durability

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Buildings Practice Facilities Plants/PetrochemicalsChapter 4

For Concrete Durability- Supplementary Cementing MaterialsProduction of Every Ton of Portland Cement an Essential constituent of concrete, releases about

one Ton of Carbon dioxide (CO2) into atmosphere!!!

1 Partial replacement of cement with SCMs reduces greenhouse gas (GHG) emissions proportionately, resulting in a more “green” concrete through reduced energy consumption, which is required to produce cement with avoidance of process emissions, related to limestone calcinations.

2 Additional benefits include minimization of waste disposal by using such materials for land filling with these industrial byproducts, causing lesser pressure on natural resources, such as reduction in limestone consumption, used for production of cement by replacement of part of cement with SCMs, while SCMs are used judiciously, improved concrete properties and durability can be achieved.

3 Also, required for sustainability of available energy for future generations to come.4 Sustainable development demands that state of art technology should be developed in

such a way that current requirements are met, while preservation of energy should also, be left available to be used by future generation, without difficulty or causing unbalancing to natural equilibrium of energies.

5 It is also, true that yet, a thorough knowledge/technology requires development to understand certainly, all aspects of producing durable concrete.

6 What is understood by article meaning of term durability being how far hardened concrete can serve a structure keeping intact all its inherited properties at production time, while accepting effects imparted by material hysteresis including its further adverse impacts on reinforcement or other material constituents/ingredients

7 Supervision & Quality Requirement Assurance is also, necessary on concrete production from plant or site mixer to site locations, which part has been missing or undervalued in many areas of globe.

8 Thorough knowledge of concrete durability & other properties, require good understanding by personnel working on concrete.

9 In particular, qualified site supervisors, semi qualified site supervisors, site consultants & site foreman, must be trained to understand as well as, apply quality demands to achieve durable status of concrete.

10 In fact, concrete technology has widened its scope to unlimited levels, yet it is distant from clear understanding by common applicants, who should be responsible for concrete application resulting in deliveries of substandard concrete, even in areas, where good skilled supervisors are available.

11 It is also, suggested that all engineers should strive to have on front experience to understand concrete technologies, in addition to having understanding of office & laboratory activities.

Supplementary Cementing Materials (SCMs) 1 Supplementary cementing materials are those, which when used with Portland cement,

contribute to properties of hardened concrete through hydraulic or pozzolonic activity or by both.

2 Typical examples are PFA/fly ash, Ground Granulated Blast Furnace Slag (GGBFS), silica fume and natural Pozzolonas.

3 SCMs are well developed materials & used properly, while mixing concrete. 4 Hardly, a site is left where SCMs are not used.

of 20 2012 Int. P Eng Suraj Singh

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For Concrete Durability- Supplementary Cementing MaterialsFly Ash

1 Fly ash is finely divided residue resulting from combustion of pulverized coal that is carried from combustion chamber of a furnace by exhaust gases.

2 For reference specifications for use of fly ash in concrete are given in CSA A3001, which classifies fly ash as F, CI, or CH by its calcium oxide content.

3 Type F refers to fly ash with CaO content less than 8%, Type CI has a CaO content ranging from 8 to 20% and Type CH fly ash has a CaO content greater than 20%.

4 Higher CaO content generally, denotes higher self cementing properties.

Chemical composition requirementsFor reference

1 CSA A 3000-03 standard permits a maximum of 5% SO3 content for Type F fly ash and

a maximum loss on ignition (LOI) of 8%. maximum SO3 and LOI contents for both

Types CI and CH is 5% and 6%, respectively. 2 SO3 content has been reported to influence to some degree early age compressive

strength of mortar and concrete specimens. 3 Higher SO3 content, higher is resultant early age strength.

4 However, a maximum limit on SO3 is considered necessary in order to avoid an excess

sulphate content in hardened concrete, which may contribute to disruptive expansion due to internal sulphate attack.

5 LOI is determined by mass loss of fly ashes heated at a temperature of 750 ± 25°C, unburned carbon is largest component of LOI.

6 Water required for workability of concretes is influenced by carbon content of fly ashes, while shape of carbon particles (porosity), higher carbon content of a fly ash, more water is needed produce a paste of normal consistency.

7 Also, dosage of air entraining admixtures for fly ash concrete to achieve a certain air content increases with an increasing carbon content in fly ash used (carbon absorbs organic admixtures, such as air entraining agents).

8 Water demand and dosage of air entraining admixtures both increase with increasing porosity of carbon particles.

Physical properties and requirements 1 Physical properties of fly ash vary over a wide range. 2 Specific gravity (SG), for instance, ranges from a low value of ~1.90 in some fly ashes to

a high value of ~3.00 for iron rich fly ash. 3 SG values of around 2.2 being however, common. 4 By comparison, specific gravity of portland cement is ~3.15. 5 Fineness is one of primary physical characteristics of fly ash that relates to its pozzolonic

activity. 6 It is well known that particles larger than 45microns, show little or no reactivity under

normal hydration conditions. 7 It has been reported that pozzolonic activity is directly proportional to amount of particles

finer than 10microns fineness of fly ash ranges from less than 2% retained to more than 30% retained on 45m sieve.


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For Concrete Durability- Supplementary Cementing Materials

Mineralogical composition 1 Mineralogical analysis of fly ashes typically shows 50-90% of non crystalline particles or

glass and some crystalline phases. 2 Reactivity of fly ashes is predominately related to non crystalline phase or glass. 3 Higher calcium fly ashes (e.g. Type CH) contains significant quantities of reactive

crystalline phases that influence properties of fly ash in concrete.

Quality control 1 Fly ash from a given power plant may vary with time, depending on many factors, such

as changes in burning conditions or source and composition of coal. 2 Variations in fly ash properties may affect performance of concrete. 3 Fly ash properties that are most likely to affect its performance in concrete are fineness,

particle shape, glass content and composition, LOI, autoclave expansion, SO3, CaO, and

alkali contents. 4 Variability of fly ash colour should also, be monitored for architectural concrete

applications. 5 Changes in fly ash colour can also, indicate changes in carbon content or power plant

burning conditions, which may affect performance of fly ash in air entrained concrete.

Ground, granulated blast-furnace slag (GGBFS) 1 Ground, granulated blast furnace slag (GGBFS) is a nonmetallic product consisting

essentially of silicates and aluminosilicates of calcium and other bases, developed in a molten condition simultaneously, with iron in a blast furnace followed by water chilling rapidly to form glassy granular particles and then ground to cement fineness or finer.

2 In concrete, GGBFS reacts with Portland cement to form cementitious products. 3 Slag possesses both cementing and pozzolonic properties.

Chemical composition requirements 1 Compared to fly ash, GGBFS is usually rich in calcium and magnesium oxides. 2 For similar reasons as mentioned above for fly ash, CSA A3000-04 limits SO3 and

sulphide sulphur contents in GGBFS to 4.0 and 2.5%, respectively.

Physical properties and requirements 1 Unlike fly ash, GGBFS is ground to a desired particle size or surface area, depending on

degree of activation required as well as, economic considerations. 2 It is reported that slag particles < 10 μm contribute to early strength development (up to

28-day), particles in 10-45 μm range continue to hydrate beyond 28 days and contribute to later age strength, while particles above 45 μm generally, show little or no activity.

3 Typically, in order to obtain satisfactory strength development in concrete, Blaine surface

area of GGBFS ranges between 4000 and 6000 cm2/g. 4 In order to avoid disruptive expansion of concrete containing GGBFS, due to MgO

content, CSA A3000-03 limits maximum autoclave expansion to 0.8%.

Mineralogical composition


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For Concrete Durability- Supplementary Cementing Materials1 Mineralogical analyses of GGBFS samples show glass contents ranging from 80 to

100%. 2 As with fly ash, reactivity of GGBFS is strongly dependent on glass content.

Blended hydraulic cements 1 Blended hydraulic cements can include various proportions of one (binary), two (ternary)

or three (quaternary) SCMs such as fly ash, GGBFS and silica fume. 2 These cements can be used with additional fly ash and slag added at concrete batch plant.

Aggregates 1 There are many reports indicating that alkali aggregate reactions can be mitigated by

proper use of fly ash or GGBFS. 2 Proper amount of these SCMs to control alkali aggregate reactions in concrete shall

depend on reactivity of aggregate and should be determined through a testing program.

Chemical admixtures 1 Chemical admixtures used in concrete incorporating SCMs, should conform to codal

requirements. 2 In determining quantities of admixtures to use, SCM is usually added to mass of cement.

Air entraining agents 3 In general, dosage of air entraining admixtures required for concrete to achieve certain air

% content increases with increasing fly ash content, increasing fineness and loss on ignition value of fly ash.

4 It also, marginally increases in some cases with increasing slag content and fineness.

Water reducers and super-plasticizers (High-Range Water Reducers HRWR) 1 In general, fly ash increases workability of a given mix while, water content can be

reduced to achieve a given workability. 2 For GGBFS, water reduction depends strongly on fineness. 3 Fineness can be controlled in case of GGBS, while it cannot be controlled in case of fly

ash.4 No compatibility issues have been found between fly ashes/slag and commonly used ypes

of super plasticizers. 5 However, compatibility tests are recommended, especially for SCMs concrete with a low

water to cementitious materials ratio (W/CM), typically lower than 0.35.

Accelerators 1 Use of low calcium fly ash and to some extent, high calcium fly ash and GGBFS

generally, decrease early age strength of concrete particularly, in cold weather conditions compared to a normal Portland cement concrete with similar workability and similar 28 day compressive strength.

2 Accelerators can be used to partially, compensate for this early age strength reduction.3 However, calcium chloride is not recommended as an accelerator for concrete with high

volumes of fly ash or for any reinforced or pre stressed concrete.


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Concrete Design Considerations - Effects of SCMs on Concrete Properties 1 Following description in general, gives a summary of effect of fly ash and GGBFS on

main properties of concrete.

Fly ash 1 Properties of fresh concrete as well as, its mechanical properties and durability shall be

influenced by inclusion of fly ash as replacement for Portland cement. 2 An extent to which, these properties shall be affected, would depend on nature and

proportion of fly ash used. 3 Outlines of manner in which fly ash shall affect properties of concrete, in general.

Properties of fresh concrete

Slump and workability 1 It is generally, known that partial replacement of Portland cement by fly ash in concrete,

reduces water requirement to obtain a given consistency or increases workability and slump for a given water content, compared to that of concrete without fly ash.

2 This phenomenon is generally, attributed to spherical shape and smooth surface of fly ash particles, as opposed to angular cement particles.

3 Particularly, in manufacture of precast concrete, workability improvement can result in elements with sharp and distinctive corners, edges and with a better surface appearance.

Bleeding 1 Bleeding of fly ash concrete depends on manner, in which fly ash is used. 2 When fly ash is used as a direct replacement for cement with no reduction in water

content, bleed water of fly ash concrete generally, increases. 3 However, when a reduction of water due to use of fly ash, as a replacement of cement in

concrete is made to maintain similar workability, bleed water of resulting fly ash concrete is generally, lower than that of concrete made without fly ash.

4 High volume fly ash concrete at low unit water content does not bleed. 5 This generally creates a problem for finishers of flatwork surfaces, who are used to work

with more bleed water at surface during finishing. 6 As with some concrete particularly, with a low water to cementitious material ratio, care

is required to prevent plastic shrinkage cracking at surface immediately, after placing concrete by following required measures on hot weather concreting.

7 Plastic cracking occurs when rate of evaporation at concrete surface exceeds rate at which, bleed water replenishes water at surface.

Autogenous temperature rise 1 Use of low calcium fly ash as partial replacement of Portland cement in concrete shall

generally, contribute to reducing temperature rise in concrete, compared to Portland cement concrete.

2 This is too important an application in massive block concrete to reduce potential for cracking associated with excessive thermal gradients.

3 High calcium fly ashes, depending on total alkali content may increase temperature rise.


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Setting time 1 For similar 28 day compressive strength and workability, setting time of fly ash concrete

may be longer than normal Portland cement concrete for a given combination of cement and chemical admixtures.

2 This may influence schedule for finishing horizontal surfaces, particularly, at high levels of replacement (>~30%) and/or in cool weather.

3 In this case, using a dosage of water reducer in lower limit of range proposed by manufacturer, can contribute at decreasing to some extent, initial setting time of fly ash concrete.

4 However, this should not be conducted at expense of strength development and durability of concrete.

5 Still, in hot weather conditions, extended setting time can be beneficial. 6 Concrete accelerators may be used to offset increase in setting time, when using fly ash.

Mechanical properties

Strength development 1 Strength development of fly ash concrete is strongly affected by type of fly ash and

curing temperature. 2 Use of low calcium fly ashes generally, decreases compressive strength of concrete at

early ages (up to 28 days) and increases it at later ages due to pozzolonic reaction of fly ash, when compared to Portland cement concrete with similar 28 day compressive strength.

3 On other hand, use of high calcium content has a marginal effect on strength development.

4 In cool weather, low temperature generally, slows down chemical reaction between cement and water and therefore, strength development of concrete.

5 For fly ash concrete, this effect is more pronounced due to reduced Portland cement in mixture and greater dependence of pozzolonic reaction on temperature.

6 However, concrete with fly ash can be proportioned to achieve similar 1day strength as Portland cement concrete mixture by judicious proportioning of mixture.

7 This usually requires a reduction in water to cementitious materials ratio (W/CM), reduction being greater with higher levels of SCMs.

8 It should be noted that reduction in strength might not be as pronounced in precast concrete, where heat curing is used.

9 Reduction in early age strength can also be partially compensated for, by incorporating silica fume with fly ash to produce a ternary blended cement or by using suitable accelerators.

Young’s modulus of elasticity 1 Since modulus of elasticity of concrete is related to its compressive strength in general,

effect of fly ash on elastic modulus of concrete is similar to effect of fly ash on strength development.


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For Concrete Durability- Supplementary Cementing Materials2 Therefore, elastic modulus of fly ash concrete is generally lower at early ages and higher

at later ages than that of concrete made without fly ash and having similar 28 day compressive strength.

Drying shrinkage 1 Effect of fly ash on drying shrinkage is highly dependent on how concrete is

proportioned. 2 If full advantage is taken of reduced water demand and unit water content is reduced and,

if W/CM is also, reduced to achieve strength parity at 28 days, fly ash concrete shall have significantly, reduced shrinkage compared to Portland cement concrete.

3 Impact of fly ash on drying shrinkage also, depends on maturity of concrete, when drying commences.

4 If drying starts at one day, fly ash concrete may shrink more. 5 Hence importance of proper curing of fly ash concretes is envisaged.

Creep 1 Effect of fly ash on creep is mainly related to impact that fly ash has on ultimate strength

of concrete. 2 Since, fly ash increases ultimate strength of concrete due to pozzolonic reaction, creep of

fly ash concrete is generally, lower than that of a Portland cement concrete with similar 28 day compressive strength.

3 However, if fly ash concrete is loaded at an early age, creep may be higher.

Durability characteristics

Corrosion resistance 1 Incorporation of fly ash within concrete results in finer pores in hydrated cement paste

leading to a decrease in permeability and chloride ingress rates. 2 Properly proportioned fly ash concrete subjected to adequate curing should in general, be

less permeable at later ages than a corresponding Portland cement concrete (having similar 28d strength) resulting in better corrosion protection for reinforcing steel.

3 Fly ashes shall generally, increase chloride binding leading to further improving resistance to rapid chloride penetration.

Resistance to freezing and thawing 1 Resistance to freezing and thawing cycling of concrete is not affected by use of fly ash.2 This property is a direct function of air void spacing factor of concrete that is obtained by

proper use of air entraining admixtures. 3 However, fly ash concrete must have adequate strength prior to exposure to freezing as is

case for normal Portland cement concrete.

Resistance to deicing salt scaling 1 Laboratory test data indicates that fly ash concrete containing more than about 25 % fly

ash is less resistant to deicing salt scaling than Portland cement concrete. 2 However, some field data have shown acceptable performance of concrete incorporating

more than 40 to 50% of fly ash


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Sulphate resistance 1 In general, low calcium fly ashes have been found to increase sulphate resistance of

concrete. 2 However, this may or may not be applicable for high calcium fly ashes.

Alkali Silica reactions (ASR) 1 In general, inclusion of fly ash can mitigate expansion caused by alkali silica reactions in

concrete. 2 However, amount of fly ash to be used for controlling alkali silica reactions depends on

type of reactive aggregate, exposure conditions, alkali content of concrete, type of fly ash and water to cementing materials ratio of mixture.

3 Published data indicate that percent replacement of cement by low calcium fly ash required to mitigate ASR, may range from 25 to 35%.

4 For high calcium fly ashes, there is some indication that effective replacement levels may be much higher than those for low calcium ashes.

Carbonation 1 Use of fly ash decreases permeability of concrete inhibiting easy penetration of carbon

dioxide into concrete. 2 However, it also, reduces calcium hydroxide content in concrete due to pozzolonic

reaction consequently, showing an increased propensity for carbonation. 3 Also, fly ash concrete usually takes longer to reach same level of strength as concrete

made without fly ash. 4 Therefore, not properly cured fly ash concrete may carbonate more than Portland cement

concrete particularly, at higher replacement levels. 5 When carbonation is likely to be an issue, concrete with high levels of SCM requires

extended curing and/or reductions in W/CM.

Durability in marine environment 1 Permeability is considered a major significant factor affecting durability of concrete in

seawater. 2 It is evident that fly ash has good potential to improve concrete durability in a marine

environment provided, it is well cured.

GGBFS (Slag) 1 Properties of fresh concrete as well as, its mechanical properties and durability shall be

influenced by inclusion of slag as replacement for Portland cement. 2 An extent to which, these properties can be affected, depends on percentage of slag used

and its fineness. 3 Description outlines manner in which, replacement of Portland cement by slag affects

properties of concrete. Properties of fresh concrete

1 Slump and workability 2 GGBFS improves workability and cohesion of concrete, but greater improvement is

obtained with higher GGBFS contents.


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For Concrete Durability- Supplementary Cementing Materials3 High value of fine GGBFS does not improve workability of concrete.

Bleeding1 Use of slag does not have a significant influence on bleeding of concrete.

Autogenous temperature rise 1 Use of slag may reduce, Autogenous temperature rise in concrete elements as well as,

associated risk of thermal stress and cracking if, sufficiently large percentages of slag being used as a partial replacement for Portland cement to at least 50% provided, slag is not ground to a very high fineness: >~6000 Blaine).

2 Higher levels of replacement (>65%) may be required for concrete in warm weather condition.

Setting time 1 Setting time of slag concrete may be longer compared to Portland cement concrete

particularly, in cold weather. 2 Slower set shall depend on reactivity of slag and its percentage used. 3 This may influence schedule for finishing flatwork surfaces particularly, for higher

volume replacements, such as more than 40%. 4 Due to its slightly slower strength development, slag concrete is more sensitive than

conventional concrete to cold weather conditions for concrete placing. 5 This may further slow setting time of slag concrete. 6 In warm weather, setting times are similar to those of Portland cement concrete.


Strength development 1 In general, concrete containing GGBFS gains strength more slowly, tending to have

lower strength at early ages and equal or higher strength at later ages compared to that of Portland cement concrete of similar 28day compressive strength.

2 However, at an equivalent replacement level effect is comparatively, less than that for most fly ashes.

3 In hot weather, strength gain can be as high as /higher than Portland cement concrete. 4 Slightly slower strength development and resulting lower early age strengths of slag

concrete might be a problem for form removal in some cases, when high percentages of slag are used particularly, in cold weather conditions.

5 As for fly ash is concerned, this can be overcome by a judicious proportioning of concrete mixture, such as reducing W/CM or adding silica fume to produce a ternary blend.

Drying shrinkage 1 There is no significant difference in shrinkage characteristics of concrete with and

without GGBFS as part of cementitious materials, if paste content is same. 2 Also, advantage may be taken of improved workability and associated water reduction

achievable with GGBFS.


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For Concrete Durability- Supplementary Cementing MaterialsCreep

1 GGBFS appears to reduce creep due to increase of ultimate strength and elastic modulus of concrete with GGBFS.

2 However, if concrete is loaded at early ages, creep might be higher than that of Portland cement concrete due to lower gain of early age strength of concrete with GGBFS.

Durability characteristics

Corrosion resistance 1 Inclusion of GGBFS in concrete increases its resistance to chloride ion penetration

particularly, at later ages. 2 It also, improves chloride binding, which is very advantageous for protecting reinforcing

steel from corrosion for which requirement, slag concrete should be well cured as applicable for fly ash concrete.

Resistance to freezing and thawing 1 Resistance to freezing and thawing cycling of concrete is not affected by use of GGBFS.2 This property is a direct function of air void spacing factor of concrete that is obtained by

proper use of air entraining admixtures. 3 However, concrete must have adequate strength prior to exposure to freezing as is case

for normal Portland cement concrete.

Resistance to deicing salt scaling 1 As for fly ash concrete, laboratory data indicates that slag concrete is slightly less

resistant to deicing salt scaling than Portland cement concrete, which limits percentage of slag that can be recommended in concrete flatwork exposed to deicing salts to less than 50% although, there are some contradictory results on this issue.

Sulphate resistance 1 Concrete containing GGBFS dosages greater than 35% by mass of cementitious material,

has demonstrated an improvement in resistance to sulphate attack. 2 For equivalent performance to Type HS cement, slag levels of 35 to 65% may be required

depending on Al2O3 content of slag.

3 Increasing slag levels would be required with increasing Al2O3 content in slag.

Resistance to alkali silica reaction 1 Inclusion of slag in adequate percentages (usually more than 35%) can be used to

mitigate expansion caused by alkali silica reaction in concrete.

Carbonation 1 As for fly ash concrete, concrete with GGBFS, if not properly cured, may carbonate more

than Portland cement concrete.

Durability in marine environments 2 Permeability is considered major factor affecting durability of concrete in seawater.


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For Concrete Durability- Supplementary Cementing Materials3 Therefore, it is evident that GGBFS has potential to improve concrete durability in

marine environment particularly, given slag performance in chloride environments.

Table gives a summary of general effects of fly ash and GGBFS on concrete propertiesSummary of General Effects of Fly Ash and GGBFS on Concrete Properties (Comparison

with Portland cement concrete with similar 28day compressive strength) Fly Ash

GGBFS Comments

Fresh properties

Water demand ↘ ↘~ 1 Fly ash-water reduction decreases with increasing fineness and

carbon content of fly ash. 2 GGBFS-does not have a strong effect on water demand.

Workability ↗ ↗~

1 Fly ash-spherical particle shape of fly ash assists in improving workability.

2 GGBFS-does not have a strong effect on slump, but increases pumpability.

Bleeding ↘ ~

1 Fly ash-bleeding and segregation are in general reduced and pumpability is improved.

2 However, low bleed water may increase risk of plastic shrinkage cracking.

3 GGBFS-does not have a strong effect on bleeding.

Setting times ↗ ↗

1 Fly ash-longer setting times compared to normal concrete, which may affect finishing schedule.

2 Cold weather conditions may further slow setting times. 3 GGBFS-its effect on setting times is less than that of fly ash.

Autogenous temperature rise ↘ ↘

1 Fly ash-generally reduces risk of thermal stress and cracking (especially type F and CI).

2 GGBFS-may reduce risk of thermal cracking if at least 50% is used and Blaine fineness is lower than 6000 cm2/g and if at least 65% is used in warm weather.


Compressive strength ↘ ↗

↘ ↗

1 Fly ash-Decreases mechanical properties at early ages (especially at 1d and in cold weather).

2 Long-term mechanical properties, such as compressive and flexural strengths and modulus of elasticity of fly ash concrete are typically superior to those of Portland cement concrete of similar 28day compressive strength.

3 GGBFS-similar behavior to fly ash concrete, except that slag concrete has higher early age mechanical properties and lower long term mechanical properties compared to fly ash concrete with similar contents.

Flexural strength ↘ ↗

↘ ↗

Modulus of elasticity ↘ ↗

↘ ↗

Drying shrinkage ~↘ ~ 1 Fly ash-long term drying shrinkage and creep of fly ash concrete

shall be similar to or lower than that of Portland cement concrete of similar 28day compressive strength.

2 GGBFS-appears to reduce creep and has no significant effect on drying shrinkage.

Creep ~↘ ~↘

Durability

Permeability ↘ ↘ 1 Fly ash-reduces water and chloride ion permeability, especially at

later ages if well cured. 2 GGBFS-similar to fly ash

Corrosion resistance ↗ ↗ 1 Fly ash-increases protection of reinforcing steel from corrosion if

well cured. 2 GGBFS-similar to fly ash

Sulphate resistance ↗ ↗

1 Fly ash-use of low calcium fly ash (CSA Class F and Cl with CaO content < 20%) increases resistance to sulphate attack.

2 Fly ashes-with more than 20% CaO should be investigated for sulphate resistance (rarely used).

3 GGBFS: content required should be investigated (usually more than 35% is required)

Concrete Mixture ProportionsProcedure indicated below for selection of mixture proportions used for Portland cement concrete is also, applicable to concrete incorporating inclusion of either fly ash or slag with some modifications. of 20 2012 Int. P Eng Suraj Singh

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For Concrete Durability- Supplementary Cementing Materials1 Estimation of mixing water and selection of air content2 Selection of water to cementitious materials ratio (W/CM) to meet durability and strength

parameters3 Calculation of cement content4 Calculation of coarse aggregate content5 Calculation of fine aggregate content6 Selection of admixture dosage rates7 Adjustment for aggregate moisture8 Trial batch adjustments

Above steps may be dictated depending on method used for incorporation of fly ash/GGBFS in concrete.

Simple replacement method 1 Consists of a direct replacement of a portion of Portland cement by fly ash or GGBFS on

one for one basis, either by volume or by mass, which mainly consists of modifying an existing Portland cement mix to include fly ash or GGBFS, without other adjustments.

2 Concrete designed with this method, usually has lower performance compared to that of concrete made with Portland cement only.

Modified replacement method 1 Consists of developing fly ash/GGBFS concrete mixtures with similar workability and

similar compressive strength to that of Portland cement concrete at a specified age.2 In general, such concretes have a higher total weight of cementitious materials and lower

W/CM than that of Portland cement concrete.

Estimation of mixing water 1 As mentioned earlier, use of fly ash in concrete generally, reduces water demand required

to achieve a certain level of workability, while use of GGBFS does not significantly, affect water demand due to fact that fineness can be controlled by keeping to determined limit, degree of granulation.

2 Therefore, for each type of fly ash and fly ash content and same criteria for GGBFS to some extent, data should be developed to replace values for different slumps and nominal maximum sizes of aggregates.

3 Mixing water is also, dependent on sand gradation. 4 Unit water content of concrete mixture designed for long term durability should be as low

as practical. 5 Consideration should always be given to using water reducing admixtures.6 Considerations should also be allowed for constructability of concrete placement.

Selection of W/CM 1 Selection of W/CM is related to durability requirements and to specified compressive

strength, usually at 28 days. 2 Use of fly ash and GGBFS affects strength development of concrete and consequently,

relationship between compressive strength and W/CM. 3 Trial batches can be made in order to develop this relationship.


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For Concrete Durability- Supplementary Cementing Materials4 Design of laboratory programme would preferably, use a factorial design optimization

method that provides with a minimum of well planned trial batches, possibility to explore a large range of compositions.

5 Relationships shall also, depend on type of fly ash/GGBFS, type of cement used, curing conditions & curing procedures.

6 In general, to achieve similar, early age strength up to 28 days, as for a Portland cement concrete, fly ash and slag concrete might require lower W/CM.

7 Given equation respecting fly ash and slag contents may be satisfied FA/40 + S/45 > 1, maximum W/CM of concrete should meet defined requirements except, when concrete is exposed to freezing and thawing, in which case required values should be reduced by 0.05.

8 Also, for reinforced concrete elements exposed to moisture and air, with depths of cover less than 50 mm, W/CM should not be greater than 0.40 for HVSCM1 (FA/40 + S/45 > 1) and not greater than 0.45 for HVSCM2 (FA/30 + S/35 > 1).

Maximum W/CM requirements for different

CSA Classes of exposure, and different SCMs contents

Class of exposure*

Not HVSCM (High Volume SCM) concrete

HVSCM2† exposed to freeze thaw cycles

HVSCM1‡ exposed to freeze thaw cycles

C-XL C-1 C-2 F-1 F-2 A-1 A-2 A-3 S-1 S-2 S-3

0.37 0.40 0.45 0.50 0.55 0.40 0.45 0.50 0.40 0.45 0.50

0.37 0.40 0.45 0.50 0.55 0.40 0.45 0.50 0.40 0.45 0.50

0.32 0.35 0.40 0.45 0.50 0.35 0.40 0.45 0.35 0.40 0.45

Exposure lasses as defined by CSA A23.1 Class of exposure* Definition

C-XL

1 Structurally reinforced concrete exposed to chlorides or other severe environments with or without freezing and thawing conditions, with higher durability performance expectations than C-1, A-1 or S-1 classes.

C-1 1 Structurally reinforced concrete exposed to chlorides with or without freezing and thawing conditions.

C-2 1 Non structurally reinforced (i.e. plain) concrete exposed to chlorides and freezing and thawing.

C-3 1 Continuously submerged concrete exposed to chlorides, but not to freezing and thawing.

C-4 1 Non structurally reinforced concrete exposed to chlorides, but not to freezing and thawing

F-1 1 Concrete exposed to freezing and thawing in a saturated condition, but not to chlorides.

F-2 1 Concrete in an unsaturated condition exposed to freezing and thawing, but not to chlorides.

N 1 Concrete exposed neither to chlorides nor to freezing and thawing.

A-1

1 Structurally reinforced concrete exposed to severe manure and/or silage gases with or without freeze thaw exposure.

2 Concrete exposed to vapour above municipal sewage or industrial effluent, where hydrogen sulphide gas may be generated.

A-2 1 Structurally reinforced concrete exposed to moderate

severe manure and/or silage gases and liquids with or without freeze thaw exposure.


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A-3

1 Structurally reinforced concrete exposed to moderate severe manure and/or silage gases and liquids with or without freeze thaw exposure in a continuously submerged condition.

2 Concrete continuously submerged in municipal or industrial effluents.

A-4 1 Non structurally reinforced concrete exposed to

moderate manure and/or silage gases and liquids without freeze thaw exposure.

S-1 1 Concrete subjected to very severe sulphate exposure. S-2 1 Concrete subjected to severe sulphate exposure. S-3 2 Concrete subjected to moderate sulphate exposure.

Fly ash and GGBFS contents

Properties of fly ash and slag i.e. reactivity, influence on water demand in concrete, influence on dosage of admixtures and variability of material shall relate to following considerations.

1 Required service life of structure and type of exposure to which, concrete is subjected. 2 Curing temperature, whether hot or cold weather. 3 Type of structural element (vertical/horizontal). 4 Usually, suspended horizontal elements require higher early age strength for formwork

removal as well as, all horizontal elements require more care in finishing. 5 Other factors to be considered before determining specific percentages of supplementary

cementing materials to be used in concrete in a specific region includea Availability of SCMs. b Current percentage of fly ash or slag commonly used in local concrete

operations/applications. c Experience of concrete producers and contractors with use of SCMs in concrete. d Introduction of concrete incorporating percentages of fly ash or slag higher than those

used in common concrete practice in a specific region, may require training of personnel in local construction industry including cement suppliers, concrete producers, contractors and testing laboratories.

All types of concrete applications

For all types of concrete applications, it is recommended to use: 1 In cold weather (air temperature is at or below 5°C) 2 Minimum 15% of SCMs (fly ash or slag or a mixture of both) 3 In hot weather (air temperature is at or above 27°C 4 Minimum 25% of SCMs (fly ash or slag or a mixture of both) 5 Note: Understood that for some types of applications and types of SCM, it is possible

while, in some cases, it is required to use much higher SCMs percentages in concrete than above minimums, but for some other applications and types of SCMs, even use of 15% might represent a challenge.

6 However, objective of recommended minimum percentages is to increase average use of SCMs in cement and concrete from 10% to 20% in order to reduce GHG emissions per

m3 of concrete produced and also, to increase durability and service life of concrete thus promoting sustainability.


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For Concrete Durability- Supplementary Cementing MaterialsMassive concrete structures for which consideration is given to temperature rise caused by hydration of Portland cement

1 Inclusion of high volumes of fly ashes and slags in concrete is an effective way to significantly, reduce heat of hydration and consequently, reduce risk of thermal cracking.

2 Recommended to use of as much fly ash and slag, as possible in concrete for massive type of application.

3 In order to significantly reduce heat of hydration, it is recommended to use:

In cold weather1 Minimum 40% of fly ash - Minimum 50% of fly ash

In hot weather: 1 Minimum 50% of fly ash - Minimum 65% of fly ash 2 However, it should be noted that for some fly ashes and slag, even use of 65% would not

be enough to reduce risk of thermal cracking. 3 In this case, concrete mixtures should be evaluated for this particular property.

Concrete exposed to a sulphate environment 1 SCMs contents to be used to produce a concrete resistant to sulphate attack, depends on

type of SCM used and type of sulphate exposure (moderate or severe).

Regardless of weather conditions, it is recommended to use: 1 Minimum 20% of fly ash 2 Minimum 35 to 55% of slag (minimum being increased with an increased sulphate

exposure severity and increased Al2O3 content in slag).

3 It is also, possible to produce a sulphate resistant concrete by using ternary blended cements.

Concrete made with reactive aggregates 1 Recommended practice suggests an approach based on a risk analysis to select minimum

% of either fly ash or slag to control alkali silica reaction in concrete. 2 Safe SCMs content depends on reactivity level of aggregate, type of structure, exposure

conditions and expected service life. 3 Regardless of weather conditions, it is recommended to use: a Minimum 25% to 35% of fly ash b Minimum 35% to 50% of slag 1 Also possible to produce a concrete resistant to ASR by using ternary blended cements.

Structurally reinforced concrete exposed to chlorides with or without freezing and thawing cycling

1 Most effective way to produce concrete resistant to chloride ion penetration is by using SCMs and a low W/CM.

2 In order to produce a concrete with a coulombs value for chloride ion penetrability less than 1500 (being an unit used for measurement of Rapid Chloride Penetration) into hardened concrete within 56 days, it is recommended to use regardless of weather conditions


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For Concrete Durability- Supplementary Cementing Materialsa Minimum 30% of SCMs (fly ash or slag or a mixture of both). b Use of ternary blends with silica fume can also, produce concrete with coulombs values

less than above maximum. 1 To achieve above coulombs value, concrete must be properly proportioned,

correctly/properly laid, vibrated & well cured.

Hand finishing concrete flatwork exposed to a combination of deicing salts and freezing and thawing cycles

1 Resistance to deicing salt scaling of concrete mixtures including either fly ash or slag, still remains highly controversial.

2 Indeed, numerous laboratory test data based on test procedures have indicated that concrete mixtures including more than about either 20% fly ash or 25% slag, often perform unsatisfactorily, when exposed to freezing and thawing cycles in presence of deicing salts.

3 On other hand, several reported cases of concrete structures including significant amounts of fly ash indicate that such concrete performed well, when exposed to deicing salts in field.

4 Based on actual data, it is recommended to use

In cold weather: 1 Maximum 25% of fly ash - 35% of fly ash

In hot weather: 1 Maximum 35% of fly ash -Maximum 50% of fly ash

Production & Placing Concrete

Production Effect of SCMs use on technical requirements, such as equipment and operation (sequence of mixing) of ready mix concrete plants

1 Production of concrete including SCMs should meet requirements for production and delivery of concrete.

2 However, following indications should be taken into account particularly, when using high volumes of SCMs (more than 30% fly ash or 35% slag).

3 In general, fly ash has a relative bulk density much lower than that of cement (2.00 to >2.60 v/s 3.15 for Portland cement and ~2.9 for GGBFS).

4 Therefore, silo and weighing scale volumes may require redesign to accommodate larger volumes of powder (particularly, with high volumes of SCMs) or be prepared to deal with possibility of double or even, triple batching to make a load of concrete.

5 As all cementing materials are pneumatically, loaded into silos by either dropping or auguring to weighing mechanism, there appears a lot of potential dust and again, due to lightness of some products, air pressures should be adjusted accordingly, in silos as well as, in weigh hoppers to prevent false weights.

6 It is preferred, but not always possible to auger fly ash as it has a propensity to flow past gates.

7 A slight incline to augers is also, preferable.


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For Concrete Durability- Supplementary Cementing Materials8 Fact is that now, all concrete batching plants are well equipped with all computerised

advanced arrangements & there, would be no issue as far as, production & delivery of some regular quantity in line to conformance to code provisions as well as, specification requirements are concerned.

9 Purpose of foregoing explanation was to have a review of past provisions & general arrangements.

10 Proper planning is required ahead of pouring to receive concrete trucks within specified limits to allow adequate time of transport & placing as well as, finishing.

Placing, finishing and stripping of concrete 1 Placing, finishing and formwork removal of concrete incorporating SCMs,

Placing 1 Use of fly ash and slag generally, increases workability, while decreases segregation and

bleeding of concrete making concrete easier to place and fill forms leading to relative convenience to pump to place, to consolidate and finish.

2 Ternary blends of cement, fly ash/slag and silica fume with super plasticizers are very cohesive mixtures that tend to present marginal placing difficulties particularly, by bucket/skip and crane.

3 Concrete made with high volumes of SCMs more than 30% fly ash or 35% slag shall often be supplied with a slump of 150 to 200 mm using super plasticizers and is very workable, when slump is low, but very cohesive.

4 Only a minimum of vibration is required to consolidate such concrete. 5 In construction of high wall sections, external vibration shall mobilize concrete in lower

lifts, for which formwork must be designed for a full liquid head. 6 Internal vibrators are acceptable however, concrete must not be over vibrated to avoid

segregation 7 Significant issue herein erupts about well design formwork, so as to receive concrete with

full pumping force, without causing any side openings. 8 Concrete rapid pouring operations on fast track projects require strong formwork for a

suitable constructability.

Finishing

Concrete with moderate levels of SCMs 15 to 25% 1 It is a general view of finishers that use of moderate levels of SCMs makes finishing of

concrete slabs easier due to increase of workability and volume of paste produced. 2 In fact, some finishers complain if SCM is not used in concrete.

Concrete with high volumes of SCMs >30 to 35% and low W/CM 1 Concrete with low W/CM with including high volumes of SCMs can present problems

with finishing. 2 Problems would reveal in most cases due to impact resulting from reduced water content

in mix releasing less bleed water available to condition surface of fresh concrete. 3 Effects of low bleed water can be mitigated if diligent attention is made to control

amount of surface drying on fresh concrete.


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For Concrete Durability- Supplementary Cementing Materials4 By attentive care and by utilizing either a fog spray, evaporative inhibitor or mid range

water reducer compatible with fly ash system can keep workability for long allowing surface water sheen replenished, while shortage of moisture presence causing finishing difficulties, can be either eliminated or at least significantly, reduced.

5 Use of high volumes of SCMs in concrete also, increases setting time, acting as a positive attribute in warm weather, allowing delays finishing operations within certain specific limits.

6 Alternatively, set non chloride accelerators can be used to compensate for delays in setting time.

Slabs exposed to a combination of chlorides and freezing and thawing cycles a Regardless of SCMs content in concrete, following description provides some guidance

on finishing in order improve scaling resistance of concrete: 1 Wait until bleeding is stopped, before final finishing operations 2 Keep surface damp, but not wet between initial strike off and final finish 3 Use minimal working of surface during finishing 4 Avoid use of steel trowels, wherever possible 5 Use wood trowel/float 6 Apply curing after final finish by membrane or keeping moist 7 Curing compound was found to increase scaling resistance of fly ash concrete 8 If it is not possible to allow 1month of “maturing” before first freeze or salt application,

use a minimum amount of SCM.

Stripping 1 Form removal/stripping/de shuttering being an operation contingent upon attaining a

predefined in situ compressive strength for respective element. 2 In cases of vertical elements, a value of 6 to 10 MPa is usually required prior to stripping

forms. 3 Concrete easily achieves this stripping strength at 1day or even, earlier with high volumes

of SCMs. 4 For suspended slabs, a higher strength is required typically 70 – 75% of design 28day

strength. 5 Concrete with moderate levels of SCMs up to 25% easily achieves required stripping

strength at early age. 6 For concrete with high volumes of SCMs > 30 to 35%, a judicious proportioning of

concrete mixture is required to achieve a required stripping strength in a timely manner.7 This usually requires a reduction in W/CM, incorporation of silica fume or use of some

non chloride accelerators particularly, in cool weather. 8 Structural designers may permit stripping time earlier for suspended slabs, if properly re

shored during stripping. 9 To be kept in consideration that generally, specification do require allowance of removal

of plywood or plates prior to stripping time, provided props or vertical supporting system members are kept intact, until full stripping time is over & even, in some case, upper slabs are laid.


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For Concrete Durability- Supplementary Cementing MaterialsCuring objectives

Certain criteria should be met by curing 1 Satisfactory moisture content requires to be maintained, so that hydration of cementitious

material continues long enough, so as to have achieved required strength, durability and impermeability, while shrinkage induced cracking is minimized.

2 Concrete ambient temperatures affect rate of hydration and ultimate strength of concrete.3 Suitable minimum and maximum temperature limits are necessary to be controlled. 4 Temperature differentials within concrete, between surface of concrete and ambient

temperature need be controlled, so that deleterious thermal cracking is eliminated.5 Permissible temperature gradients are dependent on size and geometry of concrete

section. 6 As deliberated, use of fly ash and slag generally, reduce risk of thermal cracking.7 On many massive concrete blocks nowadays, a new concept of Thermal curing is

adopted, which requires no allowance of evaporation of water used for concrete mixing.8 Complete curing is effected by heat generated from within massive block designed for

Thick as well as Massive foundations or other structures. 9 This method utilizes heat generated due to Thermal gradient & same energy is consumed

for excellent curing of concrete within block. 10 On many major foundations on major plant facilities, such activity of curing is effected

by including GGBS concrete to a cement replacement between 55 to 70 % maintaining concrete temperature below 24 degrees C.

11 Combination of membrane curing & water curing is also, very effective on other elements that require, immediate application of water based curing compound, followed by common procedural curing, so that concrete temperate may be maintained within defined limits.

Allowable curing regimes (CSA A23.1) Curing regime

Name Description

1 Basic 3 d at ≥ 10°C or for a time necessary to attain 40% of specified strength.

2 Additional 7 d at ≥ 10°C and for a time necessary to attain 70% of specified strength…

3 Extended

A wet curing period of 7 d. curing types allowed are ponding, continuous sprinkling, absorptive mat or fabric kept continuously wet…

1 For formed components, leaving forms in place, until above criteria are met, complies with intent of basic and additional curing regimes.

2 If forms are removed earlier, concrete components should be wrapped in plastic or coated uniformly, in line to manufacturer's recommended, either rate of application with a high quality membrane curing compound or be kept wet by application of water through hessian/soaker hoses or fog sprays.

3 For flatwork maintenance, a moist condition is needed, not only to promote hydration, but also, to reduce drying shrinkage that could lead to excessive cracking.

4 Moisture can be applied either by fog sprays or by covering with presoaked burlap or proprietary mats that hold moisture.

5 Covering soaked hessians with polyethylene sheets can reduce water loss from concrete.


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For Concrete Durability- Supplementary Cementing Materials6 Weather conditions requiring special protection of flatwork to avoid develop cracking

quickly. 7 Proper planning should undergo before pouring concreting starts, so that materials and

equipment should be available on site, whenever required to be used. 8 There is no difference between curing a Portland cement concrete and a concrete with

SCMs contents less than 30 to 35%. 9 For concrete with higher volumes of SCMs, curing regime becomes more stringent. 10 Although there is no difference in specified curing for conventional concrete and SCM

concrete with SCM contents less than 30 to 35%, SCM concrete must be cured as specified/required, for it is less forgiving than conventional concrete.

11 It is also, suggested that designer or engineer responsible for project, should specify on drawing about degree of required curing arrangements to produce specific concrete

12 It was experienced on one project, where even concrete spacers procured from proprietary vendor, while these too were tested for Alkali Silica Reaction as well as, for Rapid Chloride Penetration.

Conclusion: 1 Concrete should be placed using SCMs to achieve better production, better placement,

better curing, better protection from plastic cracking, better site controls as well as, to achieve durability, which is mainly affected by reduction of Rapid Chloride penetration impacts.

2 Advance arrangements required for suitable forms & receiving concrete deliveries. 3 Do not permit any lapse on any activity during concrete from trial design to finishing on

site. 4 Cure in line with agreed method statements.

References: Use of Fly Ash and Slag in Concrete: A Best Practice Guide / January 2005Personal field experience instances


4 engineering ggbs,pfa concrete durability

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