1. Introduction
In recent days, the boom in urban development and industrialization has resulted in the high consumption of natural aggregates apart from other materials. And also, due to demolition of the second largest consumption material (concrete) by humans, leads to the generation of construction and demolition waste (C&DW), which is to be relocated, reused, or recycled. The constant population growth and modernization of regions imply a daily increase in the generation of concrete waste [
1]. Researchers estimated that aggregate consumption, the global consumption of aggregate for construction, is expected to reach 62.9 billion metric tonnes by the end of 2024, up from 43.3 billion metric tonnes in 2016, in terms of volume. The damage to ecosystems could be reversed with the classification of waste and residues, therefore, the aggregates used for construction, preparing concrete or other types of mixtures, would have a more sustainable purpose. For a country, one of the main investment sectors across the industries available is the construction sector [
2]. Concrete is one of the factors that determines the level of development of countries, not only for the construction of new civil/architectural works but also playing an important role in the repair, retrofitting, and/or reconstruction of existing works.
According to data from the European Statistical Office, Eurostat, each European citizen produces an average of 2000 kg of waste per year, excluding mining waste (including the latter, this figure would exceed 5000 kg/person/year) [
3]. Of this waste pool, more than one third corresponds to the construction sector. For this reason, the study of the use of this demolition waste has been reinforced by researchers around the world, and even countries such as Italy and Denmark are developing standards and principles related to its processing [
4,
5,
6]. There are few countries (such as Denmark and Germany) which have achieved reuse of demolition waste in higher percentages than 80% [
7]. On the other hand, other nations have reported percentages lower than 10% [
8]. This is the reason why the recycling issue is still in force. The use of recycled aggregates (RA) not only provides the solution for its deposit in landfills but also the preservation of natural resources resulting from the extraction of natural aggregates. The global average worldwide waste generation in construction and demolition was equivalent to 1.68 kg/capita/day in 2018.
Self-compacting concrete (SCC) is used to facilitate proper filling and structural performance of constrained and/or reinforced areas [
9,
10]. This material possesses different characteristics than traditional concrete, for example, in terms of strength. The idea of this material was first introduced in 1992 by Okamura [
11] and has gained space over time in the construction industry [
10,
12], due to the ease of placement in hard-to-reach areas with less effort and time [
13] The advantages of this type of material include technological, social and economic advantages; however, its cost is 2 to 3 times higher than that of conventional concrete due to the high demand for cementation materials and chemical admixtures (super plasticizers or water reducers).
A decrease in the carbon footprint for concrete could be achieved, effectively reducing carbon dioxide (CO
2) emissions [
14]. The use of RA in concrete leads to a 20% decrease in CO2 emissions approximately and preservation of natural resource by aggregate extraction as 60% [
15]. A good part of global industries are working to be more sustainable, such as the construction sector [
16]. Local and state administrations, the scientific community, and the general public are increasingly aware of the depletion of natural resources and their deterioration for sustainable development. As a result of this development, the need to recycle waste arises, in which resources become part of the circular economy and are preserved from generation to generation [
17,
18]. Society as a whole has realized the need to combine economic development with sustainability and environmental protection. In recent years, some studies on the use of these wastes with concrete have been published, however, they are minimal. Most of the studies use recycled coarse aggregates in the manufacture of concrete [
19,
20,
21,
22], the least use recycled fine aggregates [
23,
24,
25,
26], other studies use aggregates of different nature [
27,
28,
29,
30], although most are aggregates from concrete waste [
31,
32,
33,
34]. Other studies focus on the manufacture of mortars [
35,
36,
37,
38].
In this context, the European Strategy establishes the following objectives for waste policy in all its member states: (a) reduction of waste generated; (b) increase in recycling and reuse; (c) limitation of incineration; (d) limitation of the use of landfills [
39]. With these premises, the need for waste management policies that reduce environmental and health impacts and improve the efficiency of available resources is clear. The long-term goal is to turn the world into a recycling society, avoiding waste and using waste as a resource wherever possible. The goal is to achieve much higher levels of recycling and minimize the extraction of additional natural resources. From this it can be understood that nowadays people across the globe are aware of reusing waste by recycling it. This has a significant impact not only on the environmental side, but also through using this waste, the cost of concrete itself is reduced apart from enhancing the concrete properties.
There are so many parameters in determining the SCC properties, in which the influenced parameter is design parameters [
40]. Most of the properties of concretes are determined by the proper proportion of ingredients used in the mix. If there is a slight variation in the proportion of ingredients used, there is a drastic variation in fresh and hardened properties being reported in literature [
40]. Some of the proportion commonly used by researchers is water to cement ratio, water to binder ratio, coarse aggregate to fine aggregate ratio, total aggregate to cement ratio, water to solid ratio, etc., [
41]. Apart from these proportions, several ingredients directly influence the properties of concrete also reported in the literature [
42]. These proportions and percentage or mass of ingredients used for the mix are known as design parameters [
40]. Without considering mixing conditions and environmental conditions, these design parameters have a greater impact on concrete properties.
Due to the ease of casting and testing in actual site requirements, most construction sites are casting cubical or cylindrical specimens to determine the compressive strength and split tensile strength [
43]. Concrete is good in compression and most of the hardened properties (elasticity and durability properties) are related to compressive strength. This is for ease of calculation of other properties through knowing the compressive strength. Concrete is weak in tension but when it is used as a flexural member, some part of concrete is subjected to tension. Hence, it is necessary to study the concrete properties concerning tension; one such experiment accepted widely by researchers and academicians is split tensile strength [
44]. Irrespective of the size of specimens, these two geometrical shapes are accepted widely. These strengths are determined by researchers at various ages of curing period or various curing conditions but the most acceptable method of curing is normal curing condition, and the curing period is 28 days. Because at actual practice, 28 days curing will obtain almost 90% of strength at normal curing condition [
45]. For simplification purposes, some studies in the literature use the strength ratio to determine other properties which may be either mechanical or durability properties [
44]. The ultimate strain value in uniaxial tension is expressed in terms of the strength ratio [
46]. The material constants defining the failure envelope are related to the strength ratio [
47]. Hence, it becomes important to identify the effect of strength ratio on various grades of concrete with variation in proportion, as is also reported in the literature [
44].
By utilizing waste material from the construction sector, without harming the environment (usage of natural aggregates) in larger proportions, natural aggregates should be replaced by RA. However, some international standards allow usage of RA in SCC up-to a certain limit and they more often discuss fresh concrete properties only. Most hardened properties of concrete depend on strength (compressive and split tensile strength) properties at any age. These mechanical properties mostly depend on the design parameters as discussed earlier [
40]. Many researchers have reviewed SCC with recycled aggregate, which is available, but have not studied the effect of the design parameters (more than three parameters) on two strength parameters simultaneously. As of the author´s knowledge, this review based on different grades of concrete and on the ingredient’s proportions is the first of its kind. Hence, this review is based on the impact of the design parameters on the strength ratio of SCC with RA.
2. Review Methodology
2.1. Search Strategies
The review methodology used in this review is reported in
Figure 1. Reports on ingredients and mechanical properties of SCC from recent literature were searched and all other articles were omitted. More focus was given to SCC with recycled aggregates (both RCA and RFA), and were selected for further processes. Admixtures are added to SCC with several benefits like reducing the cost of the mix, enhancing the fresh and hardened properties, and increasing the homogeneity of the mix. Therefore, SCC with recycled articles and admixtures (mineral and chemical—both) were considered important for selecting articles. The design parameters were derived from the ingredients used in the mix for a given volume. Articles with both mechanical properties and design parameters were selected for this important review.
2.2. Data Extraction
From available literature on SCC with recycled aggregate, the design parameters were estimated using mix design (based on various methods). The collected literature should contain the SCC mix design as various ingredient weights in a given volume to satisfy the SCC properties (fresh and hardened). Since most of the literature did not concentrate on the powder particle in RFA or the natural fine aggregate, it was neglected for this study. Binder content included the cement and mineral admixtures i.e., binder material to bind aggregates together. W/C ratio, W/B ratio, TA/C ratio, FA/CA ratio, SP (kg/m3), W/S ratio (%), % of RFA, % of RCA, the fresh density of mix, compressive load area and split tensile strength load area were different design parameters considered for the current study. Total aggregates are the sum of fine aggregate, coarse aggregate and recycled aggregates, from which is given total aggregates to cement ratio. Overall solid contents in the mix like aggregates, recycled aggregates, cement and binders from which the water to solid ratio were estimated. The articles without mixed proportions, or any one of the required ingredients were omitted from the review process.
Based upon compressive strength grade of SCC with recycled aggregate, six families were divided. The parameters like the different international codes or standards for specimen testing, the shape of the specimen, curing condition of specimens, and size of specimens were not taken into account, while considering the 28 days compressive strength of control or reference SCC mix. Initially, the families were classified based on the control or reference SCC mix and 28 days compressive strength. The 28 days compressive strength for control or reference mix lies in the range of 70 MPa to 80 MPa, grouped as a family I. Similarly, family II, family III, family IV, family V and family VI consisting of 28 days compressive strength and lie in the range of 60 MPa to 70 MPa, 50 MPa to 60 MPa, 40 MPa to 50 MPa, 30 MPa to 40 MPa and 20 MPa to 30 MPa. With respect to compressive strength in each family, the corresponding split tensile strength is also tabulated. Concerning compressive strength, the parameters like international codes or standards for specimen testing, shape of the specimen, curing condition of specimens and size of specimens are not considered. In absence of any one of the strengths at 28 days, the article itself is omitted for the review purpose.
3. Design Parameters from Literature
The family I consist of three articles which consist of control mix and compressive strength lying between 70 MPa to 80 MPa—their values are tabulated in
Table 1 along with split tensile strength in the range of 2.50 MPa to 4.46 MPa. Gesoglu et al., 2015 [
48] used nine different mix proportions as shown in
Table 1. The lower the W/C or W/B ratio, the higher the compressive strength is observed from family I. TA/C ratio lies in the range of 3.3 to 5.2 for high strength mix. FA/CA ratio lies in the range of 1.7 to 2.5 and SP available in the mix is in the range of 2 kg to 7 kg. Replacement of natural aggregates with 100% of recycled aggregates results in higher compressive strength. The lower the W/S ratio, the higher the compressive strength, the higher the W/S ratio, the lower the compressive strength, as observed from
Table 1. The highest W/S ratio is observed from Sadeghi-Nik., et al. 2019 [
49]. W/S ratio is in the range of 5.1% to 8.4% is observed for a higher strength mix. Gesoglu et al. 2015. [
48], observed that although there was higher compressive strength, there was no corresponding increase in split tensile strength.
For family II, there are four articles with compressive strength at 28 days for control or reference SCC in the range of 60 MPa to 70 MPa, as detected in literature. Compared to
Table 1, the compressive strength for family II in
Table 2 is decreased due to an increase in W/C and W/B ratio. For family II, the TA/C ratio lies in the range of 4.0 to 5.7 and increased compared to the family I. FA/CA ratio for family II is in the range of 2.1 to 3.8 which is also increased compared to family I. SP used for family II is in the range of 1.8 to 6.6 which is decreased compared to the family I. Compared to family I, the W/S ratio for family II lies in the range of 3.4% to 8.1%. The literature considered for family II has 100% replacement for their natural aggregates by recycled aggregates. Family II consists of split tensile strength at 28 days in the range of 2.20 MPa to 5.30 Mpa.
For family III, the reference or control SCC compressive strength at 28 days is in the range of 50 MPa to 60 MPa and the design parameters are tabulated in
Table 3. Ten articles with control SCC compressive strength in the range of 50 MPa to 60 MPa along with split tensile strength in the range of 0.96 MPa to 5.50 are tabulated in
Table 3. Compared to families I and II, an increase in W/C and W/B ratio is observed which results in a decrease in compressive strength. FA/CA ratio lies in the range of 1.5 to 6.1 for family III, which is higher than family I and II. Higher TA/C ratio results in a higher W/C ratio as reported by Aslani et al., 2018 [
54] and Guo et al., 2020 [
55], effectively leading to a higher W/S ratio in percentage. Aslani et al., 2018 [
54] also used a larger proportion of binder material in the mix to increase in SP content and W/B ratio. Apart from research by Aslani et al., 2018 [
54] there is shown to be some relationship between design parameters and strength. Increase in W/S ratio when compared to Family I and II are due to more quantity of available liquid in the system.
Family IV consists of ten articles from the literature, with reference mix compressive strength at 28 days in the range of 40 MPa to 50 MPa and their corresponding split tensile strength in the range of 1.80 MPa to 5.17 MPa, along with design parameters as tabulated in
Table 4. Due to a decrease in compressive strength for family IV, there is a decrease in W/C and W/B ratio for Family I, Family II and Family III observed. Except for Kou et al., 2009, there is a decrease in usage of SP when compared to Family I, II and III. An increase in W/C or W/C ratio counteracts a decrease in usage of SP. An increase in W/S when associated with other previous families is due to the availability of more water in the system which results in a decrease in strength. TA/C lies in the range of 3.3 to 7.8 and FA/CA lies in the range of 1.5 to 3.6, when compared to other families, these two ratios are decreased. Manzi et al., 2017 [
62] and Nili et al., 2019 [
63], replaced natural fine aggregate with recycled fine aggregate and simultaneously natural coarse aggregate with recycled coarse aggregate.
Family V consists of eight articles from literature with reference SCC compressive strength at 28 days lying in the range of 30 MPa to 40 MPa and corresponding split tensile strength lying in the range of 2.28 MPa to 4.12 MPa at 28 days as tabulated in
Table 5. Apart from Aslani et al., 2018 [
54] and Babalola et al., 2020 [
67], there are no drastic increases shown in W/C or W/B ratio when compared to other literature in Family V. Compared to previous families, there is an increase in W/C and W/B ratio observed for family V. Requirement of SP is reduced to 1.90 kg to 6.10 kg for family V when compared to other families due to increase in water content of the mix. Due to an increase in water content, there is an increase in the W/S ratio for family V observed. An increase in TA/C and FA/CA ratio is observed for family V, when compared to all other families. Bahrami et al., 2020 [
12], Sun et al., 2020 [
68] and Surendar et al., 2021 [
69] used a constant TA/C ratio, FA/CA ratio and W/S ratio in percentage and reported a slight variation in strength requirement for minimum modification in aggregates. Several authors tried 100% replacement for natural aggregate and achieved the minimum requirement of strength.
Three articles from literature consist of reference mix compressive strength of SCC in the range of 20 MPa to 30 MPa, constituent Family VI and their corresponding split tensile strength in the range of 1.61 MPa to 3.07 MPa, along with design parameters as tabulated in
Table 6. W/C and W/B ratio is found to be high among families for family VI result in lower strength properties. Lowest TA/C and FA/CA are found to be high among the families, which results in higher water requirements for this family. Due to increasing the fresh concrete properties, there is an increase in SP content observed for family VI. Due to the increase in water content and SP content for family VI, there is an increase in the W/S ratio observed for family VI.
Relationship between Compressive Strength and Split Tensile Strength to Compressive Strength Ratio for Different SCC Grades
The relationship between the ratio of split tensile strength to compressive strength and compressive strength of SCC recycled concrete for various strength grades are shown in
Figure 2. It is also observed that concerning different concrete compressive strength grades, there is not an identical relationship between strength ratio and compressive strength. A higher split tensile strength for a medium-strength grade is observed when compared to the high and low strength grade of SCC with recycled aggregate.
For the family I, Wang et al., 2020 [
50] observed the split tensile strength to compressive strength ratio as highest in the range of 0.065 to 0.1, as observed from
Figure 2a. Among the family I, Wang et al., 2020 [
50] possess higher split tensile strength, which results in a higher strength ratio. The remaining authors have a strength ratio in the range of 0.045 to 0.065 due to medium split tensile strength. Except for Wang et al., 2020 [
50] and Sadeghi-Nik et al., 2019 [
49], the strength ratio decreases with an increase in compressive strength observed whereas for the remaining authors the strength ratio increases with increases in compressive strength. The strength ratio converges at a point of 0.055, when compressive strength greater than 75 MPa observed.
With the decrease in compressive strength grade for family II, there is a decrease in strength ratio observed from
Figure 2b. Fiol et al., 2018 [
52] observed that there is a drop in ratio with an increase in compressive strength. Out of seven works of literature, results from four works show that there is an increase in strength ratio with a decrease in compressive strength, and the remaining three works show vice versa results. The range of strength ratio lies between 0.048 to 0.084 for compressive strength of range 73 MPa to 30 MPa.
The family III, strength ratio majorly lies between the ranges of 0.050 to 0.093 for compressive strength of range 71 MPa to 14 MPa observed from
Figure 2c except for the Grdic et al., 2010 [
59] which shows a higher strength ratio. Grdic et al., 2010 [
59], has higher split tensile strength and medium compressive strength which results in a higher strength ratio. Most of the authors observed that with a decrease in compressive strength there is a decrease in strength ratio observed. Aslani et al., 2018 [
54], Uygunoglu et al., 2014 [
57] and Tang et al., 2016 [
60] had higher split tensile strength than control or reference mix for their first replacement of natural aggregate by recycled aggregate which resulted in an increase in strength ratio compared to other literature.
For the family IV, the strength ratio lies between the range of 0.049 to 0.105 with compressive strength lying in the range of 57 MPa to 27 MPa as observed from
Figure 2d. It is observed that with a decrease in compressive strength, the strength ratio slowly increases because of the lower compressive strength value with the same split tensile strength value. Nili et al., 2019 [
63] and Singh et al., 2014 [
1] show the higher split tensile strength than the control mix, resulting in a higher strength ratio. Even though there was a sudden drop in the compressive strength found in certain literature, there is no such drop of strength observed for split tensile strength.
The strength ratio lies in the range of 0.059 to 0.115 with compressive strength lying in the range of 25 MPa to 48 MPa for the family V, as observed from
Figure 2e. A unique feature of assembling strength ratio in the range of 0.092 to 0.1 is observed in this family when compared to all others. Several articles of literature with strength ratio increases with a decrease in compressive strength and the vice-versa cases are similar in the literature observed. Surendar et al., 2021 [
69] show a higher strength ratio without any admixtures resulting in lower usage of SP.
For family VI, the strength ratio lies in the range of 0.054 to 0.122 with compressive strength lying in the range of 22 MPa to 42 MPa as observed in
Figure 2f. Aslani et al., 2018 [
54] show a higher strength ratio due to the presence of three admixtures, resulting in densely packed materials contribute to higher strength. Aslani et al., 2018., did not replace the natural aggregate with recycled aggregate more than 50% which resulted in higher strength values. The remaining articles show a strength ratio lying in the range of 0.054 to 0.089.
In general, the strength ratio decreases with increasing compressive strength (irrespective of the grade of concrete) at a decreasing rate as observed and already reported in literature [
44]. It can be explained as: the increasing rate of split tensile strength occurs at a much smaller proportion compared to the increasing rate of compressive strength. These results are in agreement with findings in the literature [
44]. It is also observed that the strength ratio is 0.050 to 0.120 for the lower grade of SCC [Family V and VI] and increased to the range of 0.050 to 0.150 for a medium grade of SCC [Family III and IV]. For a higher grade of SCC [Family I and II], the strength ratio is decreased in the range of 0.045 to 0.110. These findings agree with those results observed in literature [
44].