Numerical Evaluation of Square Reinforced Concrete Slabs Incorporating High Volumes of Recycled Materials

Concrete remains the most widely used building material in the world, due to its locally available constituent materials, versatility of form and shape, functionality, and durability. However, due to the processes involved in the manufacturing of cement, the sustainability of concrete both globally and in local communities, is often called into question. Cement production accounts for approximately 7 percent of CO2 emissions globally and, as the resources required for concrete production (e.g., aggregates, sand, potable water, etc.) continue to diminish, this poses a series problem for the long-term viability for the concrete construction industry. A three-pronged solution to this problem is therefore required which focuses on reducing, reusing, and recycling our concrete infrastructure. As urban populations are only expected to increase in the coming decades, concrete structures will continue to be constructed. This research is aimed at investigating the flexural response of two-way slabs produced using low-carbon concrete (LCC) containing recycled and secondary materials. Given that concrete floor slabs account for the largest portion of concrete in a typical building, the use of LCC in slabs has the potential to have a significant reduction in the structure’s overall CO2 footprint. The primary objective of this research is to examine the effects of replacing portland cement with high volumes of slag, fine and coarse recycled concrete aggregates (RCAs). To address the main research objective, a three-phase experimental program was completed. The first phase involved the material characterization of the various constituent materials including cement, slag, natural sand, natural coarse limestone aggregates, and fine and coarse RCAs. The second phase focused on concrete mixture design development. An experimental matrix included the batching and testing of several low carbon concrete mixtures consisting of one target strength class (30 MPa) and combinations of up to 100% replacement of natural limestone and sand with fine and coarse RCAs and up to 50% replacement of portland cement with ground granulated blast furnace slag. The next phase of experimental testing program was then completed to help better understand the individual and combined effects of the various constituent materials on fresh (e.g., slump, density, air content) and hardened concrete properties (e.g., compressive and splitting tensile strength) of low carbon concrete. Based on the top performing concrete mixtures, the third phase incorporated their measured material properties into a finite element model. Four two-way slabs (one control slab and three slabs containing different LCC mixtures) were then analyzed to evaluate and compare the maximum flexural capacity, deformational characteristics, and crack patterns. The obtained flexural and deformation response of the slabs were then compared with results obtained from the yield line analysis, Response 2000 sectional analysis which is based off the moment capacity and results were compared to CSA empirical code equations. Findings showed that firstly, the predicted pattern of failure from the finite element analysis was in concordance with that of the yield line analysis, secondly an inverse relationship between density LCC and the replacement ratio of recycled/secondary materials was obtained, and thirdly, that 50% of the numerical deflection values were in alignment with ACI crossing beam method. In terms of flexural capacity, relatively low decreases in load of 4.6%, 8.7%, and 9.8%, were computed for LC-C, LC-CF, LC-CFS, respectively. These results are promising for demonstrating the feasibility of utilizing LCC incorporating high volumes of recycled and secondary materials in two-way slab systems.