Mitigation of the negative effects of recycled aggregate water absorption in concrete technology

Abstract

The widespread use of natural aggregates in construction activities, together with the global population increase, gave rise to a depletion of this natural resource and to progressive increase of its transport distances. On the other hand, the construction and demolition wastes (C&DW) resulting from the construction activities are often deposited in landfills and city outskirts, causing environmental and social problems, such as erosion, deforestation, water contamination and human conflicts. The reuse of C&DW in concrete preparation would be a good solution for both problems. Recycled aggregates show, however, high water absorption due to porosity. At saturation, water flows from the inside to the engaging cement paste matrix and at dryness the opposite process occurs. This water flow breaks the aggregate-cement paste bonds and increases the W/C ratio in the interfacial transition zone, this degrades the fresh and hardened concrete properties.In this work a staged mixing method based on the aggregate water absorption over time was developed. A staged mixing procedure was optimized to regulate the water flow and manufacture concrete, using recycled aggregates, with levels of workability, strength and shrinkage equivalent to those of conventional concrete. The physical, mechanical and geometrical properties of the aggregates were related to the properties of concrete in its fresh and hardened state. Three types of commercial recycled aggregates were evaluated. Two types of natural aggregates were also studied for comparison purposes.