استقامت دانه های بازیافتی بتن: راه امن برای توسعه پایدار

Fine and coarse recycled aggregates recovered from demolished masonry and concrete structures were utilized in the manufacture of new concrete mixtures. Three properties of these new concretes were analyzed: water absorption, total pores volume, and carbonation. The recycled concrete families were created by replacing parts of the natural aggregates forming families of concrete with 0%, 20%, 50%, and 100% of aggregates from recycled sources. The usual comparison between mixtures by comparison between behaviors of concrete families. This research shows that the mix design nomogram (MDN) is a new and useful tool that allows the researchers to compare properties and behaviors of different concretes. The results show that the family concrete with the highest pore volume and with the same compressive strength of 20, 30, and 40 MPa (2900, 4350, and 5800 psi) did not always correspond to the concrete family with the highest degree of carbonation. This experiment also showed that some compositional characteristics of concrete could have more influence on the durability that the traditional physical aspects.

Every aggressive agent present in the environment surrounding a concrete structure can percolate, diffuse, and penetrate across the pores of the concrete according to transport mechanism laws. Water absorption by immersion and total pore volume are considered better indicators in evaluating the potential durability of concrete than capillary absorption, which only occurs under special circumstances in which the concrete is not saturated and is in the presence of water. Furthermore, indirect ways of evaluating concrete durability, such as porosity studies, can be insufficient for measuring the effectiveness of behavior of the concrete in the presence of aggressive external agents. This research could demonstrate that knowing and weighing the influence of the type of concrete, and its composition in recycled aggregate is more important than its physical characteristics in the performance analysis of recycled concretes. Wirquin et al. [1], in 2000, reported that a study of water absorption in recycled aggregate concretes showed that the processes of water absorption in recycled aggregate and in natural aggregate concretes are similar and obey the same laws. In addition, Mehta and Monteiro [2] reported that the water, as a primary agent, is able to create and degrade natural and artificial materials, as concrete. It is also a central factor behind for most of the problems regarding concrete durability, as water works as a transport vehicle for aggressive ions and as a cause of chemical processes causing physical and mechanical degradation of concrete structures. Water, ions, and gas penetrating the concrete also can change the concrete degradation kinetics during the structure service life. This investigation shows that it is possible to evaluate the influence of recycled aggregates to the depth of carbonation of concrete and that CO2 gas penetration depends on the cement's composition, porosity, and aggregate mineral composition (i.e., chemicals aspects). The use of mix design nomogram (MDN) introduced by Helene and Monteiro [3] allows the researchers to make a correct and relevant comparison between the different concrete families, adopting the same ascending reference compressive concrete strength like 20, 30, and 40 MPa (2900, 4350, and 5800 psi), instead of the usual poor comparison between individual mixture results.

Under the conditions of this research, the following can be concluded: • Concrete made with recycled aggregates (20%, 50%, and 100% replacement) from old masonry or from old concrete can have the same fresh workability and can achieve the same compressive strength of concrete made by natural aggregates in the range of 20–40 MPa at 28 days. • Minimum water absorption and total pore volume for the recycled aggregates concrete were observed at 20% replacement when using CRCA and CRMA. These can be explained by grade aggregate compacting factors and the ideal pozzolanic proportions. For all other conditions performed in this investigation, when water absorption and total pore volume are increased, the replacement of recycled aggregate also increases. • In the recycled aggregates concrete performed in this study, the carbonation depth decreased when the replacement was 20% or 50%. For CRMA concrete family, this better behavior also occurred when the replacement was 100%. This behavior shows that carbonation depth depends strongly on the chemical composition of the concrete and not only on the physical aspects. • When the natural aggregate is replaced by 20% of the recycled aggregates from old concrete or old masonry, the resulting recycled concrete will likely present same, and sometimes better, behavior than the reference concrete made with natural aggregates in terms of the properties studied in this investigation. This fact justifies the efforts to use these concretes, which can contribute to the preservation of the environment and can achieve the same final performance with probably less cost than ordinary concretes.