Abstract
This study intends to evaluate high and low-strength concrete mixes made with high volume of fly ash (FA) and recycled concrete aggregates (RCA) from both a mechanical and economic point of view. For this purpose, the mechanical characteristics of concrete, namely compressive strength (fcm), splitting tensile strength (fctm), and modulus of elasticity (Ecm) were correlated with the cost of 1 m3 of concrete mixes, taking into account the most common scenarios (e.g., cost of the raw materials, transportation between supplier and concrete plant, and mixing procedure) in the centre of Portugal. The results show that the incorporation of FA and RCA are detrimental to the mechanical properties of concrete. Ecm is predominantly influenced by RCA, and “fcm” and “fctm” are mainly controlled by FA incorporation. However, after a given age, the rate of the strength development (fcm, fctm and Ecm) of RCA concrete containing FA significantly accelerates over time relative to the reference concrete (without FA and RCA) and to the mixes made with either RCA or FA. Furthermore, the cost of concrete does not significantly change by incorporating RCA. The use of superplasticizer (SP) significantly increases the cost of concrete. However, the higher cost of concrete due to the use of SP can be offset by replacing cement with FA. Regarding the optimization process, concrete mixes with the lowest cost may not necessarily be the optimum choice regarding cost efficiency. In fact, the mechanical properties of concrete also need to be considered to aid the decision on the optimal concrete mix. Finally, the results show that the optimum mixes in terms of cost and mechanical characteristics are mostly the ones made with simultaneous incorporation of RCA, FA, and SP, rather than with their individual incorporation.
Highlights
Concrete can be considered as the most demanded product in the construction industry
The results show that the mechanical properties of concrete mixes made with and without SP decreased by increasing the replacement ratios of OPC and NA with fly ash (FA) and recycled concrete aggregates (RCA), respectively
The differences between the mechanical behaviour of the reference concrete and “RCA concrete containing FA” significantly decreased over time. This is due to the pozzolanic reaction between the SiO2 of FA and the Ca(OH)2 of RCA, which accelerates the rate of concrete strength development over time, relative to OPC concrete
Summary
Concrete can be considered as the most demanded product in the construction industry. Provided a given durability is guaranteed, the cost and strength characteristics of concrete are essential in the choice of the mix. For the same strength class, apart from taxes, the cost of concrete depends on the economy of the country, namely the cost of labour of transportation, and the type and availability of the materials (Figure 1). It may not be reliable to classify concrete based on a.
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