Effect of retempering with retarding admixture on properties of concrete subjected to prolonged mixing

Abstract

In situations like delivery of concrete from central mixing plant, in road construction, in constructing lengthy tunnels, in transportation of concrete by manual labor, in hilly terrain long hauling of concrete is required. Loss of workability and undue stiffening of concrete may take place at the time of placing on actual work site¹. In such situations engineers at site, many a time reject the concrete partially set and unduly stiffened due to the time elapsed between mixing and placing. Mixed concrete is a costly material and it cannot be wasted without any regard to cost. It is required to see whether such a stiffened concrete could be used on work without undue harm with use of retarding admixtures. The process of remixing of concrete, if necessary, with addition of just the required quantity of water is known as ‘retempering’ of concrete¹. Sometimes, a small quantity of extra cement is also added while retempering. In the site sometimes the concrete has to wait for some time to enter in the formwork after it is mixed. This may be due to some break down in the conveyance or quarrel between the labors. In such situations the concrete looses its plasticity. But since the quantity is enormous², such concrete cannot be wasted. In such situations addition of small quantity of cement and water along with retarding admixture can bring back the plasticity to concrete. Thus retempering becomes important in such odd situations². In this paper an attempt is made to study the strength characteristics of concrete containing retarding admixture at retempering time of 15min upto 90 min. The tests are conducted to evaluate the strength characteristics of concrete like compressive strength, tensile strength, flexural strength for different retempering times.