Janat Almamari - Questions

First of all let us understand what an accelerator is. An accelerator is a chemical admixture which increases the rate of hydration of concrete; which results in rapid gain of strength in concrete. Some of the examples of accelerators are- Calcium chloride, Flourosilicates, silicates, tri-ethanol amine etc. The mechanism it follows is that the admixture combines with C3A and form hydrocomplex compounds which activates C₃A and increases the rate of hydration.

Applications of accelerating admixture:

1. Cold weather concreting– In cold climatic regions, for example in countries like Russia, Serbia etc, temperatures are very low and close to or even less than zero. Since hydration is a chemical reaction, the rate of the reaction significantly reduces with the fall of temperature; it will take a long time for the concrete to harden. This slow rate of hardening (i.e. gain of strength) results in higher stripping time of formwork and hence prolonged construction period making the project economically non feasible. Further there is a problem of frost attack. Accelerators prove to be a good remedy for this problem and helps in increasing the rate of hydration.
2. Rigid Pavement construction– We remember the devastating effects of Uttarakhand floods in 2013 which washed away many homes including roads. For getting help in such disasters, it requires quick resumption of road transportation. Hence, roads have to be built at a faster pace and with quick gain of strength. Accelerating admixture again proves to be useful in this scenario.
3. Prefabricated structures– Lets take example of prefabricated structure like railway sleepers. Suppose it is provided at every 1 m spacing and 500 km railway tracks has to be built. Total no of sleepers required=500 x 10³ /1= 5 lakhs sleepers. Now a manufacturer is having 1000 moulds only. If he allows 28 days strength gaining process i.e. 1000 sleepers in approx. 1 month time. Therefore time required for 5 lakh sleepers=5 x 10⁵ /1000=500 months= almost 42 years. Usually duration of project will be max 5 years.That’s why here also accelerating admixture proves useful and reduces the production time and enables to quickly reuse the same mould again and again.