Utilizing recycled aggregate concrete in sustainable construction for a required compressive strength ratio
Introduction
There have been constant efforts to build a harmonious environment in the world, which is characterized by material, energy, production, economic, and environmental efficiencies, and a circular economy (Zhang et al., 2019a), so that its economy can increase steadily maintaining a harmonic balance with the environment. Since sustainable and eco-friendly constructions are needed for such an environment, many governments have started to pay a closer attention to the excessive exploitation of the ores from mines because the mining process can cause serious dust pollution to the environment. After some relevant policies were put forward by the government, many construction enterprises cannot afford sufficient gravel (main raw material of concrete) for their projects because the gravel price is higher than before. On the other hand, the amount of construction and demolition waste (CDW) in China showed an increasing trend in the last 15 years from 8.8 × 109 kg in 2000 to 3.9 × 1012 kg in 2015 due to the massive urban renewal and urbanization (Zhang et al., 2019a), and this figure increased to around 8.50 × 1011 kg and 5.3 × 1011 kg in the European Union and United States, respectively (Villoria Sáez and Osmani, 2019). However, only a part of this CDW was recycled during this period, and the remaining waste endangered the environment, i.e. deteriorating the landfills (Akhtar and Sarmah, 2018; Munir et al., 2018; Tam et al., 2016; Xiao et al., 2018; Zhang et al., 2018). Therefore, using the waste concrete in CDW to produce recycled coarse aggregate (RCA) and then to replace natural gravel is a promising solution, which can not only address the urgent aggregate demand for those building enterprises, but also minimize the CDW pollution to the environment.
The construction industries build better infrastructures for the countries and then help the government to develop the economy. However, the construction industry also endangers the environment (Cao et al., 2019), i.e., releasing about 1.25–3.25 × 1012 kg of CO2 gas annually due to the excessive consumption of cement (Li et al., 2018) and then depleting the ozone layer (Obla, 2009; Siddique et al., 2018). For a harmonious environment, the construction industry should be made sustainable through maximizing the pollution reduction, which can be achieved by effectively reusing the waste concrete as building materials (Zhang et al., 2019a). Therefore, how to expand the application of recycled aggregate in actual construction projects has become a research hotspot in the last decade (Akhtar and Sarmah, 2018; Tam et al., 2016; Xiao et al., 2018). Many effective techniques have been proposed and they can be classified into three categories: (1) improving the mixing approach to enhance the property of recycled aggregate concrete (RAC) with RCA (Li et al., 2012; Rajhans et al., 2019); (2) adding various admixtures including pozzolanic materials and water reducing materials (Kapoor et al., 2016; Tittarelli et al., 2014); and (3) strengthening the RCA through removing the old mortar (OM) (Katz, 2004; Tam et al., 2007) or improving its quality (Kou and Poon, 2010; Li et al., 2009; Pan et al., 2017; Xuan et al., 2016, 2017). Although these studies are very valuable and have achieved excellent results that can improve the property of RAC, they cannot be widely applied in actual projects in a short term because these methods normally include several stages that last a few days or need some expansive and special strengthening materials and equipment. The amount of concrete reaches approximately 2.5 × 1012 kg annually, therefore, the engineers in the construction industry are unwilling to use these complex, time-consuming, and uneconomical techniques after considering the economic and time costs of their projects. Therefore, the construction industry still faces a big obstacle to successfully move to the path of sustainable development.
Concrete suppliers normally design the mix proportions according to the requirements of construction companies based on two main concrete properties, the slump of fresh concrete and the compressive strength of hardened concrete. Compared with the natural aggregate concrete (NAC) prepared by natural coarse aggregate (NCA), whose mortar is regarded as homogeneous and this assumption has been applied by many researchers (Li et al., 2020; Liew and Akbar, 2020; Liew et al., 2019; Liu and Zhang, 2019; Yin and Zhang, 2019), the RAC is more heterogeneous because it has two kinds of mortars (Mi et al., 2019a; Xuan et al., 2016, 2017): (1) the OM adhered to the aggregate in RCA; and (2) the new mortar (NM) added during the mix preparation. In addition, the OM includes some cracks that show significant influences on the property of composites (Zhang et al., 2020). Therefore, the property of RAC changes dramatically if the feature of RCA changes a lot. Some results showed that the slump (Aslani et al., 2018; Li et al., 2019) and mechanical property (Alexandridou et al., 2018; Guo et al., 2018; Li et al., 2019; Muduli and Mukharjee, 2019; Rodríguez et al., 2016) of RAC are weaker than those of NAC, whereas some results concluded an opposite trend (Ajdukiewicz and Kliszczewicz, 2002; Kou and Poon, 2015; Pan et al., 2017). Due to this, some concrete-based corporations and construction companies doubted how to select the proper original concrete (OC) to prepare RCA and then to produce RAC according to their demands. Although some results showed that the quality of RCA has an important impact on the performance of RAC, and the quality of RCA relies heavily on the feature of OC (Verian et al., 2018), there is still a lack of a direct relationship between the feature of OC and the property of RAC for the engineers in concrete industry to use according to the requirements of the slump and compressive strength in actual projects. Most importantly, the CDW treatment companies normally collect the waste concretes from different sites and then merge them directly for actual use, which makes the recycled aggregate more heterogeneous and then becomes an additional concern for the construction companies to effectively recycle the CDW.
The increase of CO2 concentration in the air is an urgent problem for the environment (Cloete et al., 2019), and it also impairs the concrete structures. Carbonation, a chemical reaction where CO2 gas first reacts with the hydration products in concrete, i.e. Ca(OH)2 and calcium silicate hydrate, and then creates CaCO3, is mainly responsible for the destruction of the passivating film that protects steel (Behfarnia and Rostami, 2017; Lu et al., 2018; Mi et al., 2019b, 2020; Shen et al., 2019). Therefore, the increase of CO2 concentration may accelerate the carbonation in reinforced-concrete structures. This means that improving the carbonation resistance of RAC is a key solution to promote the effective utilization of recycled aggregate. Some investigations (Evangelista and De Brito, 2010; Medjigbodo et al., 2018; Silva et al., 2015) have demonstrated that the carbonation of RAC is more serious than that of NAC if the selected OC is weaker than the designed RAC. There are many types of OCs because different kinds of structures in old buildings normally use different types of concretes. Therefore, the engineers of concrete companies also care about how to improve the carbonation resistance of RAC by selecting a proper OC and then preparing RCA, thereby increasing the recycling of CDW.
The microstructure of mortar, including pore structures, the pH values in pore solutions, and Ca(OH)2 and CaCO3 contents, is mainly responsible for the macroscopic property of concrete, i.e., the compressive strength and durability (Mi et al., 2019a, 2019b). Although some studies haves pointed out that the RCA brings the OM into the concrete and then impairs the concrete property (Kou and Poon, 2010; Li et al., 2009; Xuan et al., 2016, 2017), few works separately investigated the OM and NM in RAC and then used their microstructure results to analyze the mechanical performance and durability of RAC, which can help the engineers acquiring a better understanding of RAC and then guide future research in order to improve the utilization of RCA.
The above-mentioned analysis reveals that it is necessary to put forward a parameter for the engineers to directly assess the relationship between the properties of OC and RAC for helping construction companies select an economical and efficient method to recycle the OC in CDW in a short term for sustainable development goals, and also reveals that there will be more buildings using RAC in the following decades, but these buildings will be under more serious CO2 attack.
To this end, three types of OCs from a laboratory were first crushed as three kinds of artificial RCAs, and then, these RCAs were separately used to prepare three types of RACs. The compressive strength ratio () between OC and RAC was first put forward to relate the OC with RAC. After that, the influence of on the slump, compressive strength, and carbonation depth of RAC and on the microstructures of the NMs and OMs before and after carbonation were separately investigated. The objectives of this study are: (1) to propose to directly relate the property of OC with that of RAC; (2) to evaluate the influences of on the slump, compressive strength and carbonation depth of RAC; (3) to establish the mathematical relationships between the value and the values of slump, compressive strength, carbonation depth, and carbonation zone width; (4) to assess the influences of on the microstructures of NMs and OMs in the RACs before and after carbonation; (5) to explore whether changing the can reduce the inhomogeneity of mortar microstructure in carbonated RAC; (6) to put forward a new sustainable construction design reflecting the effect.
This study is significant for the application of RAC in the real word. The proposed here could first remind the CDW treatment companies of classifying the waste concretes from different sites, rather than mixing them and then directly utilizing them in actual projects, which makes the recycled aggregate more heterogeneous, and then, it becomes an additional concern for the construction companies to effectively recycle CDW. Hopefully, the engineers in concrete companies can choose proper OC to prepare RAC according to the slump and strength needs, thereby building a bridge between construction companies and concrete suppliers to expand the application and carbonation resistance of RAC in actual projects and then to effectively recycle the waste concrete in CDW. This could minimize the CDW amount in a short term and could reduce the exploitation of the ores from mines due to the replacement of NCA by RCA and the reduced market demand of natural gravel. Furthermore, this can eliminate the worries of construction companies about the economic and time costs when they plan to reuse the OC in CDW. Finally, this research can help the construction industry to successfully convert itself into a sustainable one and then help the government developing the economy with a lower environmental cost. This study is also significant for the academic research of RAC. This research separately investigated the microstructures of NMs and OMs in the RACs prepared by different kinds of OCs. The microstructural difference between the NMs and OMs could remind the readers of paying closer attention to the mortar inhomogeneity of RAC when they plan to theoretically calculate the carbonation depth value of RAC and the carbonation service lives of RAC structures, rather than fully drawing upon the experience on NAC structures, whose mortar is regarded as homogeneous.
Section snippets
Calculation of compressive strength ratio
The proposed here is defined as the ratio between the compressive strength values of OC and RAC, and can be expressed aswhere is the compressive strength value of OC, and is the compressive strength value of RAC. can be directly obtained by testing the 100 mm × 100 mm × 100 mm specimens according to the requirements in the Chinese standard (MOHURD, 2019). can be calculated by testing the 100 mm × 100 mm × 100 mm specimens based on the requirements in a Chinese
Aggregate
The river sand classified as medium sand was served as the natural fine aggregate (NFA), and its fineness modulus was 2.5. Gravel was used as the NCA, and its size range was 5 mm–25 mm. The NFA and NCA were first used to prepare OC. This OC was then crushed using a jaw crusher after it was cured for 28 days. After that, the crushed products with the size range of 5–25 mm were used as the RCA. This artificial RCA was finally mixed with the NFA and other materials for preparing the RAC. The
Expanding application by adjusting compressive strength ratio
The slump of fresh concrete and the compressive strength of hardened concrete are two key properties that affect the decisions of the engineers in construction companies whether they will choose the concrete. Therefore, the influences of the value on the slump and compressive strength of RAC were studied and will be discussed in the following sections to offer a practical tool for the engineers to use in actual projects and then to improve the application of RAC.
Sustainable construction design reflecting compressive strength ratio effect
CDW treatment companies normally receive the waste concretes or OCs from different sites and then directly mix them for actual use after crushing. This makes the recycled aggregate more heterogeneous which becomes an additional concern for the construction companies to effectively recycle the CDW. Therefore, a new sustainable construction design reflecting the effect is developed, and its schematic diagram is given in Fig. 16. The first step is to classify the OCs from different old
Conclusions
This study explored the influences of the compressive strength ratio between original concrete and recycled aggregate concrete (RAC) on the slump, compressive strength, and carbonation resistance of RAC, and the results based on the conducted experiments are summarized as follows:
- 1.
Increasing the compressive strength ratio from 0.69 to 1.13 could improve the slump value of RAC from 50 mm to 65 mm because less water is absorbed by recycled aggregate due to the decline of its water absorption ratio
CRediT authorship contribution statement
Renjie Mi: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing - original draft. Ganghua Pan: Conceptualization, Methodology, Supervision, Resources. K.M. Liew: Conceptualization, Methodology, Supervision, Resources, Writing - review & editing. Tong Kuang: Conceptualization, Methodology, Investigation, Data curation.
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.
Acknowledgments
The financial support from the Scientific Research Foundation of Graduate School of Southeast University (YBPY1918), Fundamental Research Funds for the Central Universities, Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0081), and National Key Research and Development Program of China (2017YFC0703100) are acknowledged.
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