افزایش ویژگی های مکانیکی و دوام بتن متراکم بازیافتی Enhancing mechanical and durability properties of recycled aggregate concrete

1. Introduction According to the World Commission on Environment and Development (WCED), ‘‘sustainable development is a development that meets the needs of the present without compromising the ability of future generations to meet their own needs” [1]. When considering the total amount of natural resources in a large scale, in Europe for example, the exploitation of resources might not be as significant as in a smaller scale, in Cyprus for instance, where the increased exploitation may reduce the resources to a crucial level, due to country’s small size, compromising the needs of the following generations. There is a significant research activity regarding the mechanical performance of recycled aggregate concrete (RAC) which shows that the strength of RAC is adequate for use as structural concrete. On the contrary, the replacement percentage of natural aggregates with recycled aggregates and the durability properties of RAC are still under investigation, since a wide variability in the results is reported [2]. In fact, the durability properties are still under examination and so the RCA are mainly used for secondary level construction activities, as road base and landfilling materials [3,4]. The microstructure of RAC is much more complicated than that of conventional concrete since it includes two kinds of interfacial transition zones, one between the RCA and the new mortar and a second one between the RCA and the adhered mortar. The old mortar includes many micro-cracks, formed during RCA production, and has high porosity [5], thus it becomes the weakest link in RAC and its strength is the upper limit of the strength of concrete. As the mortar-aggregate bond strength increases, the concrete strength also increases [6,7]. RAC is weaker than Natural Aggregate (NA) concrete and its failure occurs through the aggregates themselves (including the old interfacial transition zone (ITZ)) instead of the new ITZ [8,9] similarly to the high strength concrete [3,5]. The presence of the adhered mortar is significant since recycled aggregates consist of 65–70% by volume of natural coarse aggregate and 30–35% by volume of old cement paste [7]. In order to enhance the properties of RCA, it is fundamental to elaborate a treatment method that is capable to remove the adhered mortar at such level that diminishes the negative effects. In general, RCA are of lower quality than NA. RCA have higher water absorption values and lower densities. The water absorption of RCA ranges from 3 to 15% [3,7]. The presence of the mortar lowers the density values of the recycled aggregates (2200–2400 kg/ m3 ) [7]. Furthermore, the resistance to fragmentation is lower and their texture is more porous, rough and irregular due to crushing of the old concrete. The lower quality aggregates have an immediate effect on the quality of the hardened concrete. The increase of the replacement ratio of RCA decreases the compressive strength of RAC. Generally, the compressive strength can be 10–25% lower than conventional for 100% replacement. Etxeberria et al. reported losses of 20–25% for 100% replacement, maintaining the same effective w/c ratio (0.50) and the same cement content [8].