Cement treated recycled crushed concrete and masonry aggregates for pavements

Abstract

This research is focusing on the characterization of the mechanical and deformation properties of cement treated mixtures made of recycled concrete and masonry aggregates (CTMiGr) in relation to their mixture variables. An extensive laboratory investigation was carried out, in which the mechanical properties of CTMiGr and the deformation characteristics relevant to shrinkage crack susceptibility were evaluated. The main aim of this research is to develop models which allow the structural properties of CTMiGr to be estimated from its mixture composition. These models are then used to develop a mixture optimization tool for CTMiGr taking into account the requirements that have to be set to the material in structural pavement designs. To realize the research objective, firstly a series of tests were conducted on CTMiGr mixtures which varied in composition. The test program comprised of measuring compression, indirect tension and deformation properties of CTMiGr mixtures. The recycled construction and demolition materials used in this study were recycled masonry aggregate (RMA) and recycled concrete aggregate (RCA) which are used for unbound granular bases/sub-bases in the Netherlands. For a good understanding of the influence of the mixture variables on the properties of CTMiGr, four important mixture variables (ratio of amount of RMA to RCA by mass, cement content, degree of compaction and curing time) were selected to be taken into account in an elaborate experimental program. The experimental results gave insight into the influences of the different mixture variables on the structural properties of CTMiGr. They showed that the mechanical properties and deformation behavior of CTMiGr depend on the mixture proportioning of CTMiGr. It was possible to develop accurate models to estimate the structural properties of CTMiGr from the mixture variables. It is noteworthy that the RMA content in CTMiGr strongly determines its mechanical and deformation properties. Due to the presence of the low-strength RMA, failure of CTMiGr originates either through the RMA particles or in cracks and discontinuities in the internal structure (the matrix) or in the bonding layer between aggregates and matrix. This will depend on the mixture variables. Numerical work using a lattice model further demonstrated that if the tensile strength of RMA is higher than 1.0 MPa, its contribution to the strength of CTMiGr becomes less important. Finally, by implementing a comprehensive analysis of the structural properties of CTMiGr, some guidelines have been given for its mixture optimization. In this way the mixture design of CTMiGr can be optimally related to the characteristics of the designed pavement structure. If in a pavement the vertical compressive stresses at the top of the CTMiGr layer are low, crushing that might occur at the top of the CTMiGr layer is not an issue. In that case it is preferred to design the CTMiGr mixture by lowering the cement content, enhancing the degree of compaction and increasing the RMA content. On the other hand, when the vertical compressive stresses at the top of the CTMiGr layer are high, it is recommended to decrease the RMA content as well as to adjust the cement content and the degree of compaction. In all cases increasing the degree of compaction is beneficial.