Efeitos da correção do abatimento do concreto dosado em central em suas propriedades mecânicas

Abstract

In the ready-mixed concrete plants, it is usual not to add all the water of the concrete trace already in the plant, being part of this addition carried out in the building site, to correct the workability. Factors such as ambient temperature, relative humidity, aggregate conditions and agitation of the concrete mixer are mainly responsible for the reduction of workability, as they can cause evaporation of part of the kneading water. The procedure regulated by the standard NBR 7212 (ABNT, 2012) only allows the addition of water up to the limit specified by the concrete trace. In practice, this addition of water often does not follow strict control, depending on the driver's experience. This situation can cause problems in concrete performance, if the water/cement factor exceeds the stipulated for the concrete trace, being able to cause loss of the resistance and homogeneity of the mixture. The superplasticizer admixture appears as an alternative for the retempering, reducing the harmful effects to the concrete. Thus, the present work aims to compare the addition of water with the addition of a Mid Range admixture to regain the workability of the concrete, verifying the impacts on its mechanical characteristics within and beyond the limit time stipulated by NBR 7212 (ABNT, 2012). For this purpose, a slump loss and retempering test was performed, simulating the transportation and delivery process of the ready-mix concrete, for a duration of 4 hours, noting the amount of water or admixture added, environmental factors and compressive strength. The results showed little variation in the slump retention between the methods and a smaller variation in the resistance in concrete whose retempering process was carried out with admixture. It was also possible to verify that the time and the retempering agent are significant in the compressive strength. Based on the study carried out, it is possible to use the concrete even after the limit time stipulated by the Brazilian standard, for the specific materials and environmental conditions of the study cited above, if admixtures are used, and it reinforces the recommendation of the standard of not exceeding the water/cement ratio stipulated by the concrete trace.