Workability of ready-mix concrete made with industrial recycled aggregates

Abstract

The state-of-the-art on recycled aggregate concrete is comprehensive but lacks research aimed at the production of ready-mix concrete. For instance, the workability of recycled aggregate concrete, which is paramount for ready-mix production, is scarcely studied to date, and the recycled aggregates and concrete compositions typically used in research do not represent those expected in industrial environments. To address these shortcomings, this paper: (i) describes the production process of ready-mix concrete along with challenges and preferences for ready-mix recycled aggregate concrete production; (ii) reviews research on the workability of recycled aggregate concrete; (iii) evaluates the workability of four ready-mix concrete compositions made in the laboratory with several incorporation ratios of two types of coarse recycled aggregates (concrete waste and mixed construction and demolition waste).To ensure representativity, the compositions and raw materials used were provided by a ready-mix concrete plant, and the recycled aggregates were produced by a CDW plant. The experiments were planned after visiting three ready-mix plants, and all options regarding experimental planning were agreed upon with the plants. The main findings of the paper are that: (i) specific requirements of ready-mix concrete producers (e.g., workability loss over time) are not accounted for in the state-of-the-art and are a limiting factor to maximize the incorporation ratio of recycled aggregates; (ii) recycled aggregates produced from mixed construction and demolition waste should be avoided because of significant cement increase (larger than 2.5%) and workability loss over time (relative slump losses over 30% already at 15 min after mixing), even at small incorporation ratios (5%–10% of the total aggregates); (iii) partial incorporation ratios of recycled concrete aggregates produced in industrial environments (therefore, partly composed of unintended constituents and fines) of up to 20% of the total (fine + coarse) aggregates may be feasible, but a maximum incorporation ratio of 10% is recommended; (iv) the proposed incorporation ratio corresponds to replacing half of the aggregates of a size fraction used in compositions of the ready-mix concrete plant, meaning that the plant only needs to install a single additional bin and feeder.