Full length articleRSM-based optimized mix design of recycled aggregate concrete containing supplementary cementitious materials based on waste generation and global warming potential
Introduction
The construction industry has a critical role with respect to economy; however, it is not an environmentally friendly industry. In the last century, irreparable damages to human habitation have been imposed by this industry. Also, it generates a large amount of waste and carbon dioxide and consumes a huge amount of resources (Bravo et al., 2015; Sun et al., 2020). Concrete is the most widely used material in this industry. It is estimated that the annual rate of concrete consumption is one cubic meter per person in the world (Ramezanianpour 2014). The concrete industry is regarded as a major consumer of natural aggregate resources as aggregates typically account for 70–80% of the concrete volume (Verian et al., 2018). Furthermore, natural aggregates resources are unrenewable and the mining of aggregate can cause ecological imbalances, such as water pollution by suspended solid matter and erosion in the river banks (De Brito and Saikia 2012).
Also, Construction and Demolition waste (C&DW) comprises one of the largest portions of solid wastes in the world (Yazdanbakhsh and Lagouin 2019), such that it accounts for 31% of the total waste (830 million tons) produced in the European Union annually (Fischer et al., 2009). C&DW consists of different components, the main one being concrete waste that comprises about 50% of C&DW (Hendriks and Pietersen 2000). This means that approximately 15% of the total solid waste generated in the world is concrete waste (Hendriks and Pietersen 2000; Fischer et al., 2009). Therefore, C&DW management in general and concrete waste management in particular play a critical role in achieving a sustainable solid waste management. In order to prioritize the options of waste management, a hierarchy has been developed as the "Waste Management Hierarchy". This hierarchy is categorized into 6 levels from low to high environmental impact: source reduction (reduce), reuse, recycle, compost, incineration, and landfill. Among these, source reduction (reduce), reuse, and recycling, generally referred to as "3R strategy", are the best options to reach sustainability in waste management (Haupt et al., 2017).
The first R (reduce or source reduction) is an efficient tool in waste management. Source reduction not only minimizes waste generation, but also reduces raw materials consumption. One of the methods of source reduction is increasing the service life of products. Waste generation and raw materials consumption can be decreased by increasing the service life of the products. This would also reduce its environmental impacts indirectly.
In addition to reduction, reusing and recycling C&DW are efficient methods to achieve sustainability in C&D waste management. In order to apply “reuse” in the concrete industry, C&DW can be used as building materials. Any attempt to reuse concrete waste will play a key role towards achieving a close-loop life cycle in the concrete industry. One of the best ways in this regard is the reuse of concrete waste as aggregate in production of new concrete (Guo et al., 2018; Munir et al., 2019). By applying this strategy, not only a huge portion of C&DW would be reused, but also consumption of natural aggregate resources could be minimized.
In recent decades, many researchers have studied the properties of recycled concrete aggregate (RCA). According to the past research, it can be said that the mechanical properties such as compressive strength, splitting tensile strength, and flexural strength of RCA concrete decreases with increase in the RCA content (McNeil and Kang 2013; Silva et al., 2015). Also, research have generally shown that using RCA in concrete has a negative impact on concrete's durability. In fact, by increase in the RCA content, increase was observed in the chloride migration coefficient, total water absorption, and capillary water absorption of concrete (Thomas et al., 2018; Bravo et al., 2015; Kapoor et al., 2016).
Besides consumption of natural resources and waste generation, the concrete industry consumes a large amount of cement. Life cycle assessment of concrete shows that cement production accounts for approximately 88% of the total CO2 emissions of concrete production (Marinković et al., 2010). Carbon dioxide is one of the main greenhouses gasses, and production of one ton of cement leads to emission of 866 Kg of CO2 (Celik et al., 2015). In fact, the cement industry is responsible for 7% of world's total CO2 emission (Yang et al., 2015). The environmental impacts of cement and concrete were calculated in several researches (Marinković et al., 2010; Yang et al., 2015; Kim et al., 2016; Smith and Durham 2016; Kurda et al., 2018; Huang et al., 2019 Kurda et al., 2018b)
A direct method to decrease CO2 emissions of concrete production is reducing the cement consumption by using supplementary cementitious materials (SCMs) in concrete. In the last decade, research on the environmental impacts of using SCMs in concrete has been emerging, and several SCMs including zeolite (Valipour et al., 2014), GGBFS, silica fume (Yang et al., 2015), and fly ash (Teixeira et al., 2016) have been studied.
According to the results of previous studies, use of SCMs such as silica fume and GGBFS reduces CO2 emissions of concrete. Also, use of SCMs in optimal percentage improves the durability of concrete (Ramezanianpour 2014; Khan and Siddique 2011). Therefore, substituting optimal amounts of SCMs with cement not only reduces greenhouse gas emissions (direct way), but also helps in reducing emissions via increasing the durability of concrete (indirect way).
Overall, from a holistic point of view, the meaning of sustainable concrete based on waste management hierarchy (3R strategy) is aiming at a “high-quality concrete”. In this case, waste generation and raw materials consumption are reduced, which in turn alleviates the environmental impacts observed throughout the life cycle. For this purpose, using both RCA and SCMs in concrete could be considered as an effective strategy to make concrete more environmentally friendly. In addition, the negative effect of using recycled aggregate in concrete on its mechanical and durability properties could be offset by using SCMS.
Previous studies have often studied the mechanical properties and durability of concrete containing recycled aggregates and SCMs without analyzing the environmental impact. Also, the environmental impacts of SCMs and RCA concretes with plain concrete have often been studied without considering the mechanical properties and service life on the results. Nevertheless, from a technical point of view, the performance of concrete is related to its mechanical properties, which is a function of its compressive strengths. Therefore, comparing the engineering properties and environmental factors of concrete with different compressive strengths is not flawless. Also, when comparing the environmental impacts of two types of concrete with similar performances, ignoring the service life in functional unit of these two products could end in misleading results. As such, a comprehensive experimental and analytical study is required to study the effect of recycled aggregates and SCMs on the mechanical properties and service life of concrete. In addition, it is essential that optimum mix designs for concrete be obtained considering not only the aforementioned aspects, but also the environmental impacts and sustainability potential.
This research aimed at two main targets: first was developing a model able to predict the compressive strength and chloride migration coefficient (Dnssm) of concrete by using response surface method (RSM) and calculation of the service life using the fib model, GWP and sustainability potential of mixture designs. Next, optimization technique was used to obtain the optimal mix design for concretes in different classes of compressive strength while maximizing the service life and minimizing the GWP. Also, an environmental comparison (GWP, NA consumption, waste generation and recycling potential) between plain concrete and the optimal mix design was performed for concretes in the same compressive strength class as well equal service life. Finally, a comparison was performed to examine the effect of considering the service life of concrete when assessing the environmental impact
For RSM modeling, three variation factors were considered. Concrete waste was used as recycled coarse aggregate (RCA). In addition, two industrial wastes (GGBFS and silica fume) were used as cement replacement materials at levels up to 40% and 10%, respectively. Rapid chloride migration test (RCMT) at 28 and 90 days (chloride migration coefficient) and compressive strength at 7, 28, and 90 days, GWP, service life and sustainability potential were considered as responses of the model.
Section snippets
Materials
In this study, A Type II Portland cement conforming to ASTM C 150–7 (ASTM C 150-07 2008) was used. Ground-granulated blast-furnace slag (GGBFS) and silica fume (SF) were also used as SCMs. The chemical and physical composition and X-ray diffraction (XRD) analysis of binder materials were presented in Table A1 and Fig A1 respectively. Natural and recycled concrete aggregate (RCA) were used as coarse aggregate. The source of RCA was waste concrete samples from a concrete laboratory, crushed by
Model fitting and modification of equations
In this study, five responses, including 7-day, 28-day and 90-day compressive strength, 28-day and 90- day Dnssm, were considered for the CCD model. The results of responses for 20 mixture designs are presented in Table A4.
In analysis of variation (ANOVA), several statistical criteria were investigated including p-value and regression coefficient, which describes the direction of the relationship between a predictor and the response variable. In this study, a maximum amount of p-value of 0.1
Conclusions
In this study, first, the mechanical and durability properties of concrete containing recycled aggregate, GGBFS, and silica fume were modeled using RSM. Also, the GWP and service life of each concrete mixture were calculated. Next, optimization technique was used to achieve the optimal mixture designs in 6 strength-based classes of concrete mixtures via minimizing GWP and maximizing service life. The following conclusions can be drawn from this study:
1. Up to 20% replacement level, GGBFS had a
CRediT authorship contribution statement
Alireza Habibi: Methodology, Investigation, Formal analysis, Resources, Writing - original draft. Amir Mohammad Ramezanianpour: Conceptualization, Methodology, Writing - review & editing, Supervision. Mahdi Mahdikhani: Conceptualization, Methodology, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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