Re-tempering of prolonged mixed concrete with chemical admixtures

Abstract

The practice of retempering onsite is frequently performed to increase slump beyond typical specification's limits in order to cope with the need for expediting the casting operations and reducing the consolidation effort. The adverse effects of elevated temperatures on the properties of the fresh concrete, namely reduced strength gains and increased slump loss are briefly discussed. Also, the possible use of chemical admixtures to overcome these adverse effects is examined. In this study, the effects of prolonged mixing and retempering processes on the properties of fresh and hardened concrete, such as temperature, slump loss, and strength development were investigated. Thirteen different concrete mixtures having the same initial slump were produced in a laboratory mixer. The effects of one superplasticizing and one retarding admixtures on the consistency of fresh concrete, were studied at 19°C in concrete mixes subjected to prolonged mixing for up to 150 minutes. This is done so as to simulate the prolonged agitation of ready-mixed concrete in truck mixers. Concrete samples were taken at different mixing times to estimate the effects of prolonged mixing on the properties of fresh concrete. The effect of such mixing and retempering on compressive strength of concrete, at the ages of 3, 7, and 28 days, was also studied. It was found that the admixtures in question accelerated the rate of slump loss at all ages, while concrete compressive strength linearly increased with mixing time for the retempered admixtures. The rate of slump loss of the concrete mixes varied according to the dosage of admixture used and also the time of retempering.