Sab 2112 - L6 Admixtures In Concrete

ADMIXTURES IN CONCRETE 1. Chemical

admixtures 2. Pozzolanic admixtures

Admixtures A material added to plastic concrete or mortar, other than water, aggregates, cement, and fibre TO change one or more properties of fresh or hardened concrete

When to use admixture? When the desired modification of properties of fresh or hardened concrete cannot be achieved by changes in the composition of the mix proportion

Example High-strength concrete Concrete grade 80 Cement = 450kg/m3 Water/Cement ratio = 0.35 Coarse Aggregate = 1120 kg/m3 Fine Aggregate = 980 kg/m3 WHAT WILL BE THE PROBLEM?

Form of Admixtures Liquid Flake Powder

Chemical Admixture

CHEMICAL ADMIXTURES Chemical added in very small amount or quantity to the mixture to develop special properties in fresh or hardened concrete

1) 2) 3)

4) 5)

Accelerating admixture Retarding admixture Water-reducing/highrange water-reducing admixture Air-entraining admixture others

Accelerating admixture To accelerate the setting and early strength development of concrete, particularly in cold weather applications E.g. calcium chloride (CaCl2)

Early removal of formwork Compressive strength at 3 days at least 25% higher than concrete without admixture The amount of CaCl2 should be limited to 2% by weight of cement

Cont. Large dosage may cause severe corrosion and loss of strength at later ages CaCl2 is not recommended in hot weather concreting and for pre-stressed concrete and may be ineffective at temperatures below freezing because concrete can freeze before it reaches the initial set.

Problems Associated with Accelerator Increase the heat of hydration Found to aggravate corrosion of reinforcing and pre-stressing steel Increase creep and shrinkage

Retarding Admixture Added to concrete to delay its setting and hardening, especially in hot weather applications (temperature greater than 32 °C) Hot weather increases the rate of hardening which makes it difficult to place and finish the plastic concrete Recommended in large structures and difficult situations such as large piers and foundations, to delay the initial set as well as to keep concrete workable throughout the entire placing period (eliminate cold joint)

Cont. Sugar in any form is a retarder (coke?) The use of retarder may reduce the strength at early age

Water-Reducing Admixture Used to reduce the quantity of mixing water required, which increase the compressive strength, and to produce concrete of desired consistency or high slump. Can reduce water required by 12% or more Its effect is due to increased dispersion of cement particles, causing a reduction in the viscosity of the concrete.

Pumping Concrete

High-Range Water Reducer Also called Superplasticizer Added in small dosage with mixing water (typically 0.2% by weight of cement) to increase slump Flowing concrete – slump greater than 180 mm Dosage rate may vary from brand to brand for different types of cement.

Without SP

With SP

Cont. By using superplasticizer, it is possible to have w/c ratio in the range of 0.3 to 0.45 which would be impossible otherwise. Addition of superplasticizer increases cost by 5% BUT savings in labour can be as high as 33%

Effect of Superplasticizer Increase in workability Increase in compressive strength Eliminates segregation Allow good dispersion of cement particles in water

Superplasticizer – how it works?

Concrete with Superplasticizer Has smooth surface Much less likely to chip and spall Has fewer plastic shrinkage cracks Higher compressive strength because of rapid gain in strength Early removal of formwork

Less shrinkage Low permeability Less time to place and finish the concrete An accelerating curing period

Air-Entraining Admixture Added to entrain air in concrete Air entrainment is a process by which a large number of microscopic air bubbles are dispersed throughout the concrete Diameter of air bubbles in the range of 20 to 200 µm

Air Bubbles

Advantages It causes the disruption of the continuity of capillary pores result in reduction in permeability and reduces internal stresses caused by expansion of the pore water on freezing. Foaming agents are added in small amount (typically about 0.05% by weight of cement) during mixing and can be used to entrain 3 to 10% air.

Cont. The use of air-entraining admixture can: – Improve workability – Reduce bleeding – Reduce segregation

However: – Lower compressive strength

Chemical Admixture Description

Applications

Water-reducing Admixture

To get dense concrete, to improve workability

Retarding admixture

To delay setting and hardening, hot weather concreting, large structures

Accelerating admixture

To accelerate setting and early strength development, cold weather concreting

Air-entraining admixture To improve durability and workability

POZZOLANIC / MINERAL ADMIXTURES

Mineral Admixtures Are natural pozzolanic materials or industrial by-products that are used in concrete to replace part of cement It is known as supplementary cementing material Are generally added in relatively large quantities in comparison with chemical admixtures Contains reactive silica (SiO2)

Pozzolans Are siliceous or siliceous and aluminous materials which in themselves possess little or no cementitious property BUT in finely divided form and in the presence of moisture, these materials can react with calcium hydroxide of concrete at ordinary temperatures to form compounds of cementitious properties. This reaction is called POZZOLANIC REACTION

Reactions Hydration process: Cement + H2O → C-S-H gel + Ca(OH)2 Pozzolanic reaction: SiO2 + Ca(OH)2 → C-S-H gel

Effect of Pozolanic Admixture

Example of Pozzolans Ground Granulated Blastfurnace Slag Pulverised Fuel Ash Silica Fume Rice Husk Ash Palm Oil Fuel Ash

Method of Replacement Replacing part of the cement content (by weight of cement) Can be added durning mixing process or grind together with cement (pozzolan cement)

Effect of pozzolanic admixtures Lower early strength Lower heat of hydration Higher ultimate strength Less permeable Reduce cost Increase workability (some)

Slag Is a non-metallic product, consisting essentially of silicates and aluminates of calcium bases Not only pozzolanic but also cementitious It is develop in a molten condition simultaneously with iron in a blastfurnace Is the glassy granular material formed when molten blastfurnace slag is rapidly chilled as by immersion in water Amount replacement : up to 80%

Slag

Fly Ash (PFA) Is a by-product of burning powdered coal in electric generating power plant Generally finer than Portland cement and consists mainly of small spheres of glass involving silica, alumina, and ferric oxide Amount of replacement : 20 to 70% Two types: – Class F Fly Ash – low calcium fly ash. Has no cementitious properties but can react with calcium hydroxide – Class C Fly Ash – high-calcium fly ash. Has cementitious properties in addition to pozzolanic properties

Fly Ash

Microstructure of Fly Ash

Silica Fume (SF) Is a by-product of the electric arc furnaces in the silicon metal and ferrosilicon alloy industries Consist of non-crystalline silica (85 to 90% silicon oxide) Very fine particles – less than 0.1um (100 times finer than cement) Highly pozzolanic and very reactive Amount replacement : 10 to 20%

Rice Husk Ash (RHA) Is an agricultural fly ash which is obtained from burning and grinding of rice husk Contains silica Amount replacement : 10 to 30%

Palm Oil Fuel Ash (POFA) Obtained from the burning of palm oil clinker Amount replacement : 10 to 30%

Factors Affecting the Suitability and Performance of Admixtures in Concrete

Type of cement Mix proportions and method of mixing Temperature and time when the admixture is added into the mix Chemical composition of the admixture

Monitoring and Precaution Selection of material Trial mix Short-term and long-term effect Eliminate overdosing – on site Manufacturer’s advice

Thank You

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