Waste generation from the production of ready-mixed concrete.

In Brazil, ready-mixed concrete (RMC) suppliers are adopting management strategies to reduce waste generation. This is because of the increasing scarcity of adequate places for the deposition of waste, which raises the associated costs. Therefore, it is necessary to know the actual volume production of residues in order to optimize the implementation of the recycling. The waste produced in RMC plants are mainly in the form of leftover concrete and adhered to the internal part of the drum of the mixer truck. The present study analyzed the data and produced an inventory of the amount of waste generated by of one of the largest Brazilian concrete companies. In parallel, an experimental study was performed in the real conditions of an RMC plant in order to measure the influence of concrete characteristics on the volume of concrete adhered to the drum. The results showed that about 3% of all the produced volume returns to the plant, comprising approximately 1.6% adhered concrete and 1.4% leftover concrete. The study proved that the volume of concrete adhered concrete depends on the cement consumption and the slump. The volume of concrete loaded and the region where the RMC plant is located do not influence those results. The inventory showed that the main cause of concrete leftovers is associated to over order and corresponds to 58% of this volume.

Evaluation of concrete recycling system efficiency for ready-mix concrete plants.

The volume of waste generated annually in concrete plants is quite large and has important environmental and economic consequences. The use of fresh concrete recyclers is an interesting way for the reuse of aggregates and water in new concrete production. This paper presents a study carried out for over one year by one of the largest ready-mix concrete producers in Brazil. This study focused on the evaluation of two recyclers with distinct material separation systems, herein referred to as drum-type and rotary sieve-type equipment. They were evaluated through characterization and monitoring test programs to verify the behaviour of recovered materials (aggregates, water, and slurry). The applicability of the recovered materials (water and aggregates) was also evaluated in the laboratory and at an industrial scale. The results obtained with the two types of recyclers used were equivalent and showed no significant differences. The only exception was in terms of workability. The drum-type recycler generated fewer cases that required increased pumping pressure. The analysis concluded that the use of untreated slurry is unfeasible because of its intense negative effects on the strength and workability of concrete. The reclaimed water, pre-treated to ensure that its density is less than 1.03g/cm(3), can be used on an industrial scale without causing any harm to the concrete. The use of recovered aggregates consequently induces an increase in water demand and cement consumption to ensure the workability conditions of concrete that is proportional to the concrete strength level. Therefore, the viability of their use is restricted to concretes with characteristic strengths lower than 25MPa.