Accelerator Chemical
HTPB 95 can be used either to increase the rate of stiffening/setting of the concrete or to increase the rate of hardening and early strength gain to allow earlier demoulding and handling. Most accelerators primarily achieve one rather than both of these functions. Accelerators are most effective at low temperature. It is a prime use of set accelerators is in the control of the setting time of concrete floor slabs in cold weather when an extension of set delays finishing and power towelling operations. Set accelerators are a very effective way of controlling the setting time of such concretes, even those containing cement replacements.
Accelerators are also used to reduce the risk of damage by freezing when concreting in cold weather and to allow the earlier removal of formwork. The exposed faces of struck concrete must still be protected and properly cured. At normal temperatures, a technically better way of enhancing early strength is to use a high range water reducer/superplasticizer. A large reduction (greater than 15%) in the water-cement ratio can more than double compressive strength at ages less than 24 hours. Accelerators can be used in conjunction with superplasticizers where very early age strength is required, especially at lower temperatures. Other applications where accelerating admixtures are used are in urgent concrete repairs and in sea defence work, to ensure early stiffening of concrete in the tidal zone.
Accelerating admixtures increase the rate of hydration of tricalcium silicate (C3S) and tricalcium aluminates (C3A) phases of the cement, thereby providing earlier heat evolution and strength development. It should be noted that accelerators do not depress the freezing point of water significantly and should not be referred to as anti-freeze admixtures. The accelerating effect of both set and strength accelerating admixtures is most pronounced at lower temperatures of the order to 5-10oC. Liquid admixtures are generally considered easier to dispense accurately and are more readily dispersible evenly through the mix.
Accelerating admixtures can be used with all types of Portland cement including those covered by EN 197-1 and with other binders blended with CEM 1 cement at the mixer. It can achieve 1-day strength gains of up to 100% higher than the equivalent plain concrete mix. However, the cost of achieving this with a chloride-free accelerator The setting time of concrete containing an accelerator will be shorter than that of the equivalent plain concrete containing no accelerator. The acceleration is typically 1 to 2 hours for a set accelerator but less than one hour for hardening accelerators. This will be affected by accelerator type, dose and ambient temperature. It is stipulated that for set accelerators, a mix containing the admixture must have an initial setting time at 5°C of 60% or less than that of a mix without the accelerator. It has the ability to break down the natural passivity of steel provided by the alkalinity of concrete and thereby encourages the corrosion of reinforcement or other embedded steel. The corrosive influence of calcium chloride and its ability to increase shrinkage make it a potential hazard to long-term durability in reinforced concrete and its use should be avoided for this application.
Accelerators increase the rate of heat release and may, therefore, give a greater temperature rise than the equivalent plain concrete mix. The total heat of hydration is unaffected. Accelerating admixtures do not normally have an adverse effect on bleeding. The bleeding capacity of most concretes is related to setting time. Cold weather will prolong setting of concretes and so exacerbate the potential for concretes to bleed. Hence, accelerators may reduce bleeding purely as a result of reduced concrete setting time.
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