Claudio Javier Zega

Use of recycled fine aggregate in concretes with durable requirements

The use of construction waste materials as aggregates for concrete production is highly attractive compared to the use of non-renewable natural resources, promoting environmental protection and allowing the development of a new raw material. Several countries have recommendations for the use of recycled coarse aggregate in structural concrete, whereas the use of the fine fraction is limited because it may produce significant changes in some properties of concrete. However, during the last decade the use of recycled fine aggregates (RFA) has achieved a great international interest, mainly because of economic implications related to the shortage of natural sands suitable for the production of concrete, besides to allow an integral use of this type of waste. In this study, the durable behaviour of structural concretes made with different percentage of RFA (0%, 20%, and 30%) is evaluated. Different properties related to the durability of concretes such as absorption, sorptivity, water penetration under pressure, and carbonation are determined. In addition, the results of compressive strength, static modulus of elasticity and drying shrinkage are presented. The obtained results indicate that the recycled concretes have a suitable resistant and durable behaviour, according to the limits indicated by different international codes for structural concrete.

Recycled Concretes Made with Waste Ready-Mix Concrete as Coarse Aggregate

Using waste materials for new products is a global trend undergoing rapid development. Recycling materials allows for a more efficient life cycle and contributes to environmental protection. In the construction field, this trend has gained importance because of the shortage of natural resources and because of environmental problems caused by storing building-demolition wastes. This situation has led to the search for new applications for these wastes, and their use as aggregates in concrete is an interesting alternative. In this paper, some characteristics of recycled coarse aggregates obtained by crushing waste ready-mix concrete, as well as the mechanical and durability properties of recycled concretes made by using 25, 50, and 75% of these aggregates, are presented. Recycled concretes show lower compressive strength than conventional concrete for the higher strength level, whereas the durability properties of the two are similar.

Estimation of Compressive Strength of Recycled Concretes with the Ultrasonic Method

In the present paper the errors in compressive strength estimation when the ultrasonic method is applied to conventional and recycled concretes are evaluated. Tests were performed on laboratory prototypes made with conventional concretes containing natural coarse aggregates and on recycled concretes containing 75% of recycled coarse aggregates (crushed concretes) by volume. In both types of concrete the strength was estimated by employing previously obtained correlation equations between the ultrasonic pulse velocity and compressive strength measured on concretes made with the same materials. The results from both types of concrete indicate that the errors in the estimated strength can be considered technologically acceptable.

Effect of Entrained Air on Recycled Concrete Prasoperties

Air entrainment is a good practice to improve the durability of concrete exposed to cycles of freezing and thawing. For a given admixture dosage, the air content will depend on mix proportions and the characteristics of the materials employed. The use of crushed waste concretes (recycled coarse aggregates), which have different characteristics and properties compared to natural coarse aggregates, could modify the air content and behavior of recycled concretes. In this paper, some fresh and hardened state properties of entrained air recycled concretes were evaluated and compared with those of entrained air conventional concrete. Recycled concretes were elaborated with 50, 75, and 100% replacement by volume of natural coarse aggregates by crushed waste concretes. The results indicate that the recycled concretes show a satisfactory mechanical and durable behavior, similar to or better than that of conventional one.