Concrete, Bricks and Blocks and Stones: ADMIXTURES

Admixtures are chemical materials in the form of powder or fluids. they are added to the concrete before or during mixing. It can give it certain characteristics, for example change its fresh, early age or hardened state of the concrete. They are normally supplied as aqueous solutions of the chemical for convenient of dispensing and dispersion through the concrete during mixing. The popularity and use have increased considerably in recent year. Difference countries used difference rate of the admixture, example UK is about 12% of all concrete.

Admixtures usually being classified according to their mode of action rather than by their chemical constituent. For example, the European standard BS EN934 includes requirements for:

water- reducing/ plasticising admixtures
high- range water- reducing/ superplasticising admixtures
set and hardening accelerating admixtures
set retarding admixtures
air- entraining admixtures
water- resisting admixtures
water- retaining admixtures
set-retarding/ water- reducing/ superplasticising admixtures
set- accelerating/ water- reducing/ plasticising admixtures

The last three requirements are admixtures with a combination of actions.

There are five distinct type need to consider due to they make up together more than 80% of the total quantities used in concrete:

plasticisers
superplasticisers
accelerators
retarders
air- entraining agents

PLASTICISERS

Plasticisers also called workability aids or dispersants are additives which increase the fluidity, workability or plasticity of a cement paste or concrete. In order to produce stronger concrete, less water is added (without "starving" the mix), which makes the concrete mixture less workable and dofficult to mix, nessitating the use of plasticizers, water reducers, superplasticisers or dispersants.

PICTURE: Plasticisers

Plasticisers or water-reducers, if a constant workability or fluidity is required then the water content can now be reduced, thus leading to a lower water:cement ratio and increased strength.

PICTURE: Concrete Plasticisers with thixotropic action for concrete zero-slump and low-slump concrete (C0/C1 acc. To EN206). For example,raiklway sleepers, slatted floor elements, manholes, rings, cones etc.

VIDEO: Plasticisers in caulks and sealants

SUPERPLASTICISERS

Superplasticisers also known as high range water reducers are more powerful than plasticiser. They used to achieve increase in fluidity and workability of a much greater magnitude than obtainable with plasticisers. The addition to concrete allows the reduction of the water to cement ratio, not affecting the workability of the mixture and enables the production of self- consolidating concrete and high performance concrete. This effect drastically improves the performance of the hardening fresh paste. Indeed the strength of concrete increase whenever the amount of water used for the mix decreases.

VIDEO: Using superplasticisers to increase workability without decreasing strength

VIDEO: Superplasticisers

PICTURE: cement absorb differences quantities of superplasticisers

PICTUCE: schematic picture of polycarboxylate superplasticisers and its effect on cement

ACCELERATORS

Accelerators is used to increase the time of hardening of cement paste:

enhancing early strenth gain
reduced the curing time
setting time

Thus, this allows concrete to be placed in winter and no need worry about the frost damage. Typical materials sed for acceleration are Calcium Nitrate Ca(NO3)2 and Sodium Nitrate NaNO3. calcium chloride CaCl2 has been very popular, but calcium chloride was historically very common used as it was effective and readily available.

PICTURE: compressive strength of accelerators

RETARDERS

Retarders are difference with the accelerators. It delay the setting time of mix. Here are the examples of retarders used:

counteracting the accelerating effect of hot weather, if the concrete has to be transported over a long time or distance.
controlling the set in large pours, wehere concreting may take several hours to achieve concurrent setting, thus avoiding cold joits and uniform strength development.

The mode of action of retarders involves modification of the formation of the early hydration product, including the portlandite crystals. As with other admixtures, temperatures, mix proportions, fineness and composition of the cement and time of addition of the admixtures all affect the degree of retardation, and it therefore difficult to generalise.

PICTURE: concrete surface retarders

PICTURE: portland cement with retarders

AIR ENTRAINING AGENTS

Air entraining agents are organic materials which, when into the concrete by the addition to the mix of an air entrating agent, entrain a controlled quantity of air in the form of tiny air bubbles in concrete. The diameter od the bubbles are generally in the range 0.02 - 1mm, with an average distance between them of about 0.2mm. They are sufficiently stable to be uncanged during the placing, compaction, setting and hardening of the concrete.

The primary effect of air entrainment is to increase the durability of the hardened concrete, especially in climates subject to freeze- thaw which will otherwise lead to progressive deterioration of the concrete or cement.

The secondary purpose is to increase the workability of the concrete while in a plastic state.

Air entrainment has two important secondary effects:

increase in workability of mix while in a paste state; with the air bubbles act like small ball-bearings
increase in porosity results in a drop in a streength, this must be taken into account in mix design but the improvement in workability means that loss can at least be partially offset by reducing the water:cement ratio.