Recycled Fine and Coarse Aggregates' Contributions to the Fracture Energy and Mechanical Properties of Concrete.

This paper investigates the fracture mechanical properties of concrete, using crushed concrete aggregates (CCA) and granulated blast furnace slag (GGBS) for partial cement replacement. CCAs made from prefabricated concrete replace 100% of the fine and coarse fractions in concrete recipes with w/c ratios of 0.42 and 0.48. Two pre-treatment methods, mechanical pre-processing (MPCCA) and accelerated carbonation (CO2CCA), are investigated for quality improvements in CCA. The resulting aggregates show an increased density, contributing to an increase in the concrete's compressive strength. The novelty of this paper is the superposition of the effects of the composite parts of concrete, the aggregate and the cement mortar, and their contributions to concrete fracture. Investigations are directed toward the influence of fine aggregates on mortar samples and the influence of the combination of coarse and fine aggregates on concrete samples. The physical and mechanical properties of the aggregates are correlated with mortar and concrete fracture properties. The results show that CCA concrete achieves 70% of the fracture energy values of concrete containing natural aggregates, and this value increases to 80% for GGBS mixes. At lower w/c ratios, MPCCA and CO2CCA concretes show similar fracture energies. CO2CCA fine aggregates are the most effective at strengthening the mortar phase, showing ductile concrete behavior at a w/c ratio of 0.48. MPCCA aggregates contribute to higher compressive strengths for w/c ratios of 0.42 and 0.48. Thus, mechanical pre-processing can be improved to produce CCA, which contributes to more ductile concrete behavior.

Improving Recycled Aggregate Quality by Mechanical Pre-Processing.

Concrete with crushed concrete aggregates (CCA) shows lesser compressive strength than reference concrete with natural aggregates. The goal of this study is to improve the strength of structural concrete with 53% and 100% CCA replacements without increasing the cement content. Thus, improvements in CCA quality are induced by combining mechanical and pre-soaking pre-processing techniques. Mechanical pre-processing by rotating drum is separately pursued on fine and coarse CCA for 10 and 15 min respectively. Results show, adhered mortar content and CCA water absorption reduces as pre-processing duration increases. Pre-processing influences CCA particle grading, flakiness index, shape index, void-content, unit-weight and density, jointly seen as packing density, which increases with pre-processing duration. Water amount to pre-soak CCA before concrete mixing is stable despite grading modifications, due to reduced water absorption resulting from mechanical pre-processing. Compressive strength and workability for pre-processed CCA50 and CCA100 concrete are comparable to reference concrete and show similar trends of improvement with packing density. Packing density markedly shows the quality improvements induced by pre-processing on CCA, maybe considered as one of the quality assessment indexes for CCA. Packing density should be investigated for other recipes to see the stability of the trend with workability and compressive strength.