Concrete with fine and coarse recycled aggregates: E-modulus evolution, compressive strength and non-destructive testing at early ages
Introduction
The European Union has established a new strategy on circular economy, in which a more clever use and management of wastes is intended. Indeed, EU-issued documentation [1], [2] claims that improvements in eco-design, prevention and reuse of wastes could produce a net saving of up to 600.000 million euros per year for the European Companies. Similar pathways are being paved throughout the world in view of the recent Paris climate agreement [3]. In fact, developing ways to transform wastes in by-products, has a significant importance from the environmental and economical points of view. A relevant branch of this type of approach is the valorization of construction and wastes, using them as materials for manufacturing concrete and other construction materials [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. In such context, the use of concrete wastes as recycled aggregates is an interesting option [3], [6], [14]. There is a significant number of studies on the influence of coarse recycled aggregate on the mechanical and physical properties of concrete [9], [15], [16], [17], [18] and it is known that its use produces a decrease of density, compressive strength and E-modulus in concrete, as compared to identical mixtures based on natural aggregates. Nevertheless, some standards like EHE-08 [19], consider that the use of coarse aggregate substitution percentages under 20% does not induce any significant effect on concrete properties. Therefore, it allows the use of recycled aggregates to produce structural concrete within such limits, without requiring any additional studies (as compared to those required for conventional concrete with natural aggregates).
There are studies on the influence of coarse recycled concrete aggregate on the relationship between compressive strength and E-modulus, which is a very important matter in view of the prescriptive aspects of nowadays regulations, being centered on defining concrete classification of mechanical performance with basis on compressive strength only. For example, in the work of Kakizaki et al. [20] and Katz [21] a correction was proposed. The correction uses one variable: density. Other authors proposed to take into account two factors: density and percentage of substitution of coarse recycled concrete aggregate [22]. Genetic programing, model tree and/or artificial neural networks have been used in order to predict certain properties of concrete in the last few years [15], [17], [23], [24]. Some researchers have proposed formulas to estimate the compressive strength and/or E-Modulus in concretes with coarse recycled aggregates [15], [23], [24], [25], [26]. A recent study [17] proposed new formulations (analytical and based on genetic programing) for estimating the compressive strength using a combination of non-destructive tests and other factors related to the curing conditions and composition of the eco-concrete. In that study several types of eco-concrete were investigated, including concretes with different replacement percentages of fine and coarse recycled aggregate. The proposed formulas are very accurate but they estimate compressive strength only; the E-Modulus has not yet been studied with such framework.
The studies about the influence of the fine recycled aggregate on the physical and mechanical properties of concrete are scarce, but there is some literature on the subject [14], [27], [28], [29]. These studies conclude that for low percentages of substitution of fine recycled aggregate (equal or less than 25%) the decrease in compressive strength is not significant and, in some cases, even a slight increase is observed. However, another particular study by Khatib [27] observed an important decrease of the compressive strength (24%) with 25% of replacement. Also, in that particular study [27], for percentages higher than 25%, there is agreement that an important decrease of compressive strength is to be expected. For E-Modulus, with a replacement of 30%, the variation is small. With a 100% of replacement of fine aggregate, the decrease is important, close to 20% [27]. Note that, nowadays, the use of fine recycled aggregate of concrete is not allowed for structural concrete in several standards [19], [30], [31], [32].
In comparison with the use of coarse or fine recycled aggregate alone, the combined use of fine and coarse aggregate produces a higher economic and energetic saving in the concrete production process because the sieving for separating fractions is not necessary and all of the produced recycled material is used, which means that there is no generation of a new by-waste.
There are some studies on concretes with the combined use of fine and coarse recycled aggregates [6], [14], [17], [33], [34], [35], [36], [37], [38], [39]. Some of these studies [14], [17], [34], [35] suggest that the use of coarse recycled aggregate has a higher impact on the compressive strength and on E-modulus than the use of fine recycled aggregate. A recent paper studied the influence of replacement of fine and coarse recycled aggregate on the evolution of compressive strength and its relation with ultrasonic pulse velocity depending on the curing temperature [14]. None of the contributions found in the literature has yet focused on the influence of the replacement on the evolution of the E-Modulus at early ages of concrete beams, neither on its relationship with several UPV tests and compressive strength. In addition, there are several studies relating the electrical conductivity with the mechanical properties of concrete [40], [41], [42]. It was found only one recent paper [41] about the influence of the coarse recycled aggregate on the electrical conductivity (at 56 days). No studies about the influence of the recycled aggregate on the electrical conductivity at early ages were found. No studies about the influence of the partial replacement of the recycled aggregates on the relationship between E-Modulus and electrical conductivity at early ages were found.
The aim of the research reported herein is precisely to close such research gaps and contribute to a better understanding of the influence of fine and coarse recycled aggregate on the mechanical properties of concrete at early ages and in order to advance towards more permissive and more sustainable standards, without incurring structural risk.
Section snippets
Materials
The recycled aggregate used in this research originates from decommissioned precast concrete sleepers with compressive strength higher than 30 MPa. After an adequate treatment (crushed and removed impurities with magnetic separation) a recycled aggregate with 0/12 mm fraction (RA-0/12) was obtained. This is the same recycled aggregate used in a recent study [14] dedicated to the study of the influence of the partial replacement of this recycled aggregates on the relationship between compressive
Influence of the recycled aggregates in the evolution of E-Modulus at early ages
Fig. 2 shows the evolution of E-Modulus at early ages for concretes with different replacement percentages of natural aggregate by recycled aggregate. Two beams were tested for each concrete. The differences between the results of the two beams of each concrete are negligible (less than 1%), for that reason, the averaged curve of each is presented. The evolution of E-Modulus of the four concretes is similar during the first hours and no influence of recycled aggregate is observed. However, from
Conclusions
From the first 12 h, the influence of the fine and coarse recycled aggregate starts to be noted; the higher the amount of recycled aggregate is, the lower the E-Modulus. From the first 24 h, the loss of E-Modulus due to the use of recycled aggregate has certain proportionality with the amount of recycled aggregate replacement.
The replacement of natural aggregate by recycled aggregate has a clear influence on the relationship between E-Modulus and the NDT used: UPV and internal electrical
Conflict of interest
None.
Acknowledgments
This study was developed with the support of the project of Program FEDER-INNTERCONECTA ITC-20113055 “Development of value adding technologies for RCDs for innovative applications”, convened by the Center for Industrial Technological Development (CDTI, for its initials in Spanish), dependent on the Ministry of Economy and Competitiveness and co-funded by the Technological Fund – FEDER Funds. Funding provided by the Portuguese Foundation for Science and Technology (FCT) to the Research Project
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