Análise das propriedades do concreto quando utilizado após início de pega: tempo de utilização de concreto estrutural vencido

Abstract

The present research work consisted in the analysis of the main properties of the concrete used after the start of the handle, in which the use of overdressed structural concrete was evaluated. The changes in mechanical strength, modulus of elasticity, incorporated air content, specific mass, plasticity and electrical resistivity of the concrete at the defined dosages were identified in eight hours of mixing. NBR 7212 (ABNT, 2012) recommends that the transportation of concrete by concrete mixer trucks is less than 90 minutes after the addition of the first water in the mixture, and the launching and densification of the concrete must be carried out in less than 150 minutes. In this context, the use of non-depleted concrete for four types of Portland cement was evaluated by means of experimental tests, in which a common dosage was used for CPII E 32, CPIII 40 RS, CP IV 32 RS and CPV ARI. The start and finish times of each cement were measured and four (4) test bodies were molded for each dosage, every 60 minutes after addition of water, ending the tests in 480 minutes, which gave the conditions to evaluate the influence and the behavior of each type of cement in the studied properties. As a result, the maximum time of concrete utilization after initial addition of water in the mix was identified for each cement type, as well as the main changes in the properties studied, and it may be concluded that the concrete can be used after the time recommended by the standard with no significant losses of compressive strength in the evaluated conditions, but it can be observed that at each hour of mixing there was a reduction of the electrical resistivity of the concrete, which showed that the concrete was more susceptible to the chemical attacks in prolonged mixtures. No correlation was found between the modulus of elasticity and the compressive strength, however, at each hour of mixing, a large loss of abatement, decrease in density and specific mass of the concrete can be observed. Finally, there were no significant changes in the temperature of the concrete during the mixing time and the results of the incorporated air content were inconclusive.