Investigação da relação entre os parâmetros de projeto das estruturas de concreto armado visando à durabilidade

Abstract

The amount of studies that focus on concrete structures durability has been increasing because of the new Brazilian Standard regarding to Performance and its life cycle criteria for buildings, added to the perception of early lifespan loss of buildings. There are definitions in technical standards and scientific literature about project parameters of concrete structures, as water/cement, cement consumption, compressive strength and concrete covering. However, up to this date in Brazil, no relations have been established between these characteristics in a way to grant more flexibility for the professionals when specifying parameters of structures focusing on durability. Besides this gap, there is a necessity to consider the aggressiveness classes established by agents which are deleterious to structures. Within this context, this study aims to define the relations between the parameters of different concretes compositions and how they differentiate due to the environmental aggressiveness classes, focusing on the necessary covering to ensure durability. Thus, the study of concretes according to Standard specifications was proposed, besides checking the attack front of structures through theoretical life cycle prediction models, and the conduction of accelerated tests of carbonation and salt spray. The test periods that were defined represent each one of the aggressiveness classes of ABNT NBR 6118, guided by the theoretical models of deterioration of structures. After this, each of the four concrete mixes was submitted to different aggressiveness. Thereby, it was possible to determine the impact in the coating of the reinforcement for each aggressiveness class, whether milder or more severe. Each mix was evaluated for physical and mechanical characteristics. It was found out that the concrete cover necessary for the steel bars varies linearly with the compressive strength resistance of the mixes, indicating the possibility to consider more than one resistance class of concrete and coating within different aggressiveness classes, as occurs in some international technical standards of dimensioning of structures. It is noteworthy that the difference of concrete cover necessary in relation to aggressiveness classes becomes rougher when concretes with compressive strength higher than 30 MPa are used. Another observation is that that the attack for chloride ions is more severe than carbonation, determining, in most verifications, the minimum thickness to ensure the expected durability. Lastly, the statistical analysis pointed out a significant influence of the mix in the carbonation front and between the mixes 1 and 4 for salt spray.