Tests show promise for recycled aggregate use in concrete

At IST, University of Lisbon, as well as in other research centers around the world, various experiments regarding the possibility of using recycled concrete to produce new concrete have been performed in recent years. Recently, the first full-scale experiments on recycled concrete structures were made and one of the tests was rather ambitious: the structures were subjected to horizontal forces until they collapsed in order to evaluate their seismic capacity. The behavior of the structures was accurately predicted with common engineering methods and the capacity of the three recycled concrete structures was the same as the capacity of a conventional concrete structure also tested. Such promising results show that concrete recycling is a feasible and ecological solution that the concrete industry should consider adopting.

In recent decades, a global effort has been directed towards sustainability and the concrete industry is no exception. The use of recycled aggregates on new concrete seems a solution that could minimize the ecological footprint of this industry, enabling reductions of the amount of landfill disposal, quarry mining, and fossil emissions due to transportation. Recycled aggregates are obtained by crushing concrete elements, in the same way that natural aggregates are produced by crushing stone taken from quarries. However recycled aggregates have some different properties that need to be accounted for when producing recycled aggregates concrete. The objective of this experiment was to study the possibility of using recycled aggregates produced from high quality concrete rejects in concrete structures designed according to common engineering practice and using current calculation procedures.

Recycled aggregates are made of crushed stone and attached mortar. This mortar has high deformability, water absorption, and roughness properties that have to be considered when designing concrete mixes. This change of properties draws suspicions from the construction agents towards recycled aggregates incorporation. These suspicions are a hindrance towards a large-scale use of this eco-friendly material, despite the increasing scientific knowledge regarding this material showing its applicability. In this context, the objective of the research project conducted at IST was to evaluate if the incorporation of recycled aggregates only had to be considered at the material and mechanical level of if the structural performance was affected in a way that this change in material and mechanical properties would not represent accurately. If the changes in behavior were predicted solely due to the commonly tested material and mechanical properties, a huge boost in confidence could be generated from this investigation since most of the studies on recycled concrete are in respect of these kind of parameters. In recent years, some experiments have been made in structural elements and scaled-down two-dimensional frame structures. The innovative aspects of the tests conducted by the research team of IST (full-scale testing of three-dimensional structures) not only have an added impact due to the magnitude of the experimental campaign per se, but also address the possible occurrence of phenomena that scaled-down specimens would disregard due to scale effects. Additionally, this was the first monotonic destructive test, an experimental technique used to assess seismic behavior, made on recycled concrete structures.

The behavior of the structures was adequately predicted: the consideration of the effect of recycled aggregate incorporation on the material and mechanical properties was enough to predict the structural response and the design standards provided a seismic-resistant structural solution. A fundamental aspect in the seismic response of a structure is its deformation capacity, which was not only adequate, complying with international standards, but also predicted by design calculations and independent of recycled aggregate incorporation. These destructive tests and other studies made on the same experimental campaign imply that, from a structural point-of-view, the construction sector can have confidence in the use of this material.

The research can be found in a paper titled “Horizontal Destructive Load Tests on Full-Scale Recycled Aggregates Concrete Structures”, published by American Concrete Institute (ACI) Structural Journal.

The research project included not only the seismic behavior experiments, but also dynamic characterization and vertical load tests on the structures. Other material and mechanical tests were made on the structures, as well as on specimens produced in the laboratory facilities at IST. The mechanical and durability long-term behavior of the recycled concrete structures was analyzed and it was concluded that the use of high quality recycled products on concrete is no hindrance to a good performance. A research of this magnitude could not be possible without partnerships: Opway, a construction company, FCT (the Portuguese Foundation for Science and Technology) and the Portuguese Programme for Research and Development Associated with Public Contracts, were the sponsors that made these experiments possible.

Further studies concerning recycled aggregate incorporation on concrete are ongoing in IST’s Department of Civil Engineering, Architecture and Georresources and CERIS Research Centre.