Determinação do Tempo Zero em Microconcretos de Alta Resist̂encia contendo Poĺımeros Superabsorventes usando a Técnica do Ultrassom

The use of high-strength concrete (HSC) is increasing in the construction industry due to its numerous advantages such as durability and high resistance. However, HSC has lower water/cement ratio, and also a denser microstructure than conventional concrete, it is subjected to a pronounced effect of autogenous shrinkage, which begins from the moment at which the mixture changes from liquid state to the solid state. One technique that has recently been used by researchers to reduce or even eliminate the phenomenon of autogenous shrinkage is the addition of superabsorbent polymers (SAP) to the concrete, promoting water supply inside the concrete during the hydration process of the cement paste. Therefore, this paper deals to determine the exact moment that occurs the transition suspension-solid through the propagation of ultrasonic waves technique. The terminology 'time-zero' or ' threshold percolation' to designate the moment that occurs the transition of cement paste from fluid to solid state is used. Thus, for the ratio w/c = 0.3 were determined four times zero of High-Strength Fine-Grained Concretes: reference and content of 0.1%, 0.2% and 0.3% of SAP; also for each ratio w/c = 0.35 and w/c = 0.40, was made the test in two mixtures: reference and 0.2% of SAP. The results in the eight High-Strength Fine-Grained Concretes studied in this research showed evidence that the addition of superabsorbent polymer seems to have no a significant effect at time-zero, and analyze the effects of low water/cement ratio in the transition from fluid to solid of High-Strength Fine-Grained Concretes. The research also contributed to ratify the effectiveness of ultrasound technique and to experimentally determine the autogenous shrinkage.