A comparative study of the mechanical and life cycle assessment of high-content fly ash and recycled aggregates concrete

https://doi.org/10.1016/j.jobe.2020.101173Get rights and content

Highlights

  • Effect of fly ash and recycled aggregates on the environmental impacts and mechanical performance of concrete.

  • Global warming, energy consumption, strength and modulus of elasticity of concrete.

  • Relationship between mechanical behavior and environmental impact of concrete.

Abstract

In this paper, an extensive experimental work is made to understand the mechanical behaviors, environmental impacts (EI) and resources use of concrete mixtures containing high amounts of fly ash (FA) and/or recycled aggregates (RA). Then, the results of this study are compared with previous studies, and also non-expected results and missing information from those studies are highlighted. For that purpose, the Life Cycle Assessment (LCA) methodology was used to determine the most influential factors, resources use (e.g. non-renewable energy consumption - PE-NRe) and EI (e.g. potential global warming - GWP) for probable scenarios in the center of Portugal, according to EN 15804 and ISO 14040. In addition, the mechanical behavior (compressive and tensile strength, and modulus of elasticity) are also considered. The results show that the use of RA negatively affects all the mechanical parameters of concrete when the binder content is kept constant. The use of FA also affects the mechanical performance of concrete except for the modulus of elasticity. In addition, according to the results of this study, the GWP and PE-NRe of concrete mixtures seem not to be considerably affected by the incorporation of RA. However, in previous studies, the GWP and PE-NRe of RA concrete strongly depends on the transportation scenario. The LCA of concrete significantly decreases with the use of FA, regardless of the transportation scenario. According to the relationship made based on the results of this study (same LCA scenario) and previous studies (different LCA scenarios), there are no clear relationships between the mechanical behaviors, and GWP and PE-NRe of concrete mixtures with and without “RA and FA” because the incorporation of each of the non-traditional materials differently affects each characteristic of concrete and LCA results may be different for other approaches or assumed scenarios in life cycle inventory modelling. Thus, an optimization method is needed to find the optimum concrete in terms of mechanical performance, EI and resources use point of view.

Introduction

After water, concrete is one of the most demanded materials worldwide. Therefore, it has a significant influence on environmental impacts (EI) and many alternative materials such as construction demolition wastes [[1], [2], [3], [4], [5], [6], [7], [8], [9]] and/or cementitious materials [[10], [11], [12], [13], [14]] have been proposed to be used instead of traditional materials in order to decrease the EI of concrete.

Generally, non-renewable energy consumption (PE-NRe) and potential global warming (GWP) are the most influential factors on the EI of any product [15]. One way to decrease them is by using non-conventional materials. Therefore, this study focuses on the effect of the incorporation ratio of both “construction demolition wastes”, such as fine recycled aggregates (FRA) and coarse recycled aggregates (CRA), and/or “cementitious materials”, such as fly ash (FA) on the GWP and PE-NRe of concrete. In addition, it is well-known that the compressive strength (fcm), splitting tensile strength (fctm) and modulus of elasticity (Ecm) are some of the most important factors to evaluate the mechanical behavior of concrete. Therefore, this study pays particular attention to the effect of FA and recycled aggregates (RA) on the fcm, fctm and Ecm of concrete.

An extensive literature review and experimental work were performed to understand the EI and mechanical behaviors of concrete containing high amounts of FA and/or FA. The reason for focusing on both aspects is that they are both considered important factors to identify a sustainable concrete [16,17]. In other words, both characteristics need to be considered to aid the selection of the optimum concrete, namely in a green scenario [18]. For example, by comparison with low-strength concrete, high-strength concrete structural elements need smaller cross-section, and this directly changes the total volume of concrete needed to build the building [19] and its total EI as a consequence. On the other hand, a high cement content or superplasticizer (SP) need to be used to obtain high-strength concrete. Previous studies concluded that cement is one of the essential contributors to the total CO2 emissions of concrete [[20], [21], [22]]. Thus, to understand the effect of any non-traditional material (e.g. FRA, CRA and FA), the best concrete from one point of view only (e.g. EI, resources use and mechanical performance) may not be a good solution in terms of sustainability. Therefore, a specific method like CONCRETop (A multi-criteria decision method for concrete optimization - [18]) may be required to choose the optimum concrete.

This study provides a comprehensive synthesis of previous studies on the effect of FA and/or RA on mechanical behavior and EI of concrete. The results show that the individual influence of non-traditional products on concrete has been comprehensively studied from different points of view, e.g. mechanical behavior and EI [[23], [24], [25], [26], [27]]. However, most of the researchers studied the effect of high incorporation ratio of RA and FA from one viewpoint only (fcm or EI) of either low- or high-strength concrete, and there is still a gap between the mentioned aspects as a result of the indirect relationship between them, especially when both non-traditional products are used in concrete. Thus, the second part of this study (experimental work) is to fulfil the mentioned gap, namely compare the individual and combined effects of both FA and RA in terms of most mechanical properties (fcm, fctm, Ecm) or EI of high- and low-strength concrete.

Section snippets

Materials and methods

As stated above, the objective of current study is to analyze the influence of different incorporation ratios of RA and/or FA on the EI and mechanical properties of concrete. Therefore, an experimental campaign was made to understand the mechanical characteristics (e.g. fcm, fctm and Ecm) of concrete at different ages (0.2–12 months). Concerning the EI (e.g. GWP and PE-NRe), the Simapro software was adopted to calculate the data obtained from EPD reports, site specific data, and other

Results and discussion

In this section, the individual and joint effects of high quantities of RA and FA on the fcm (§3.1), fctm (§3.2), Ecm (§3.3) and EI (energy consumption and global warming - §3.4) of concrete are shown based on both the extensive experimental work (Table 3) and compared with the literature.

Conclusions

The conclusions of the present study made according to the results of this study and previous studies cited in Table 4 and Fig. 3, Fig. 5, Fig. 7, Fig. 9, Fig. 13, Fig. 14, Fig. 16, Fig. 17, Fig. 26, Fig. 27, Fig. 29, Fig. 30, to understand the effect of the incorporation ratio of fly ash (FA) and recycled aggregates (RA) on the environmental impact (EI) and mechanical performance of concrete. The following outcomes can be drawn from the results.

Knowledge

We thank the support provided by the Portuguese Foundation for Science (FCT) and Technology, and CERIS, IST - University of Lisbon.

Declaration of competing interest

None.

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