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An evaluation of the recycled aggregate characteristics and the recycled aggregate concrete mechanical properties
Construction and Building Materials 240 (2020) 117978
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Review
An evaluation of the recycled aggregate characteristics and the recycled aggregate concrete mechanical properties Guoliang Bai a,b, Chao Zhu a, Chao Liu a,⇑, Biao Liu a a b
School of Civil Engineering, Xi’an University of Architecture & Technology, Xi’an 710055, China Key Lab of Structural Engineering and Earthquake Resistance, Ministry of Education (XAUAT), Xi’an 710055, China
h i g h l i g h t s The quantitative relationships between the content of old mortar and the quality of recycled aggregate were obtained. The effects of the recycled aggregate proportion on the mechanical properties of concrete were analyzed. A method for improving the performance of recycled aggregates suitable for engineering applications was proposed. Some suggestions for further popularization and application of recycled concrete were put forward.
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Article history: Received 6 July 2019 Received in revised form 13 November 2019 Accepted 28 December 2019
Keywords: Recycled aggregate concrete Attached mortar Mechanical properties Aggregate reinforcement
a b s t r a c t The difference between the properties of recycled aggregate (RA) and natural aggregate (NA) are caused due to the old mortar attached to RA. In this review, the quantitative relationships were represented about the content of old attached mortar and the performance of RA at the material level. In term of the component level, the influence of replacement ratio of RA on the mechanical properties of concrete was summarized. Finally, some researches have focused on ways to improve the properties of aggregate. The results show that the method of stripping old mortar to improve the performance of recycled aggregates was not the only way to promote the application of recycled aggregates. However, by evaluating some simple and economical methods such as controlling the water-cement ratio, adjusting the aggregate moisture content and the different mixing method can improve the performance of recycled concrete in order to meet the requirement of concrete quality. Three aspects of the performance prediction, application range and reinforcement method for RAs were illustrated in the article. It was believe that the results could promote the accurate application for RAs with different quality in engineering, thus the application range of RAs was expanded. Ó 2019 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The characteristic of recycled concrete aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Correlation between attached mortar content and aggregates particle size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Correlation between attached mortar content and water absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Correlation between attached mortar content and saturated surface dry density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Correlation between attached mortar content and Los Angeles abrasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Correlation between attached mortar content and sulphate content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The effect of the recycled aggregate proportion on the mechanical properties of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. The compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. The split tensile strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. The flexural strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. The shrinkage and creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. E-mail address: [email protected] (C. Liu). https://doi.org/10.1016/j.conbuildmat.2019.117978 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
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4.
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Performance enhancement of recycled aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Separation of adhered mortar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Strengthening of adhered mortar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction In the last decades, a large-scale reconstruction of old urban areas has been carried out in China, the dramatic increase in the amount of construction and demolition waste (CDW, about 30%– 40% of municipal solid waste) [1,2]. In addition, China consumes about 40% of the world’s cement and steel each year due to a mass of fresh construction. The extensive type exploitation for a long time has caused a significant shortage of natural resources in China [3,4]. The exploitation of natural resources was prohibited widely by the government. The most scientific and effective approach to deal with CDW at the present stage was to crush, wash, sift and classify to obtain RA, and then partially or completely replace the natural aggregate (NA) with RA for preparation of concrete. A series of standards and procedures have been published to facilitate the carry out of this action [5,6]. The category of RA will be determined by structural form of the demolished construction and the resource production technology. RAs can be divided into recycled concrete aggregate, recycled brick aggregate, and recycled mixed aggregate as shown in Fig. 1. Meanwhile, the miscellaneous materials such as wood blocks, paper scraps, plastics and other pollutants are also contained in RAs [7–9]. At present, compared with other recycled coarse aggregate types, recycled concrete aggregate is the most extensive implemented RA in construction, and organizations in many countries have developed specifications [10–12]. In this paper, the performance and application of recycled concrete aggregate were focused on.
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The old mortar adhered on the surface of aggregate was the most fundamental difference between RA and NA as shown in Fig. 2 [13,14]. The amount of mortar attached to the aggregate surface will be determined by the processing technology and quantity during the production of RAs. According to Nagataki [15], the more the crushing process, the less bonding mortar and the higher the density of the RA. However, the density of recycled fines will decrease due to excessive crushing process. The processing stage should not be too much or too little for high quality RA, otherwise the aggregates cannot be used for certain applications [16]. The presence of adhering old mortar results in the water absorption, crushing index, and Los Angeles wear coefficient of RA were higher than original aggregate [17–19], resulting in the performance of recycled aggregate concrete (RAC) is weaker than natural aggregate concrete (NAC). Recycled concrete aggregates were commonly used in road bases and some secondary roads in China at present stage. The application in the terms of building structure was merely in the low-rise demonstration house. It can be said that the promotion and application of RAC was lagging behind in China. In this review, The characteristic of RAs was evaluated by quantitatively analyzed between attached mortar content and key indicators of RAs, such as density, water absorption, Los Angeles wear coefficient and sulfate content at the material level. In term of the component level, the effects of the proportion of RA on the mechanical properties of RAC were analyzed, and the factors of water-cement ratio and aggregate moisture content were also considered. In addition, the reinforcement techniques of RAs were
Fig. 1. Main categories of recycled coarse aggregate.
Fig. 2. Schematic diagram of recycled concrete coarse aggregate.
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discussed. The results of the review can be used to forecast the performance of RA, so as to provide reference for its accurate application in engineering. Finally, several optimized approaches were recommended to expand the application scope of RAs and guarantee the sustainable development of the construction industry. 2. The characteristic of recycled concrete aggregates 2.1. Correlation between attached mortar content and aggregates particle size The particle size of RA varies with the production process [20,21]. Several researchers [22–25] observed that there was a certain relationship between the particle size of RA and the content of the attached mortar. The results extracted from literature [22,26– 30] are shown in Fig. 3. From the plot it is evident that the attached mortar content increase with decrease in the aggregate size. This behavior may be attributed to the processing techniques. The amount of adhesive mortar on the recycled coarse aggregate was reduced at the beginning of mechanical processing. However, as it gradually crumbles the cement mortar accumulates in smallsized RA [31]. This is also the reason why the density of recycled fine aggregate is generally lower than that of recycled coarse aggregate. The 95% confidence interval of statistics indicates that in a batch of aggregate with the same aggregate particle size, the probability of the average value of the attached mortar content falling into the dark red portion is over than 95%. The 95% prediction interval represents the range of the predicted bound mortar content for a given aggregate size. The addition of confidence interval promotes the application of this relation in practical engineering. 2.2. Correlation between attached mortar content and water absorption The water absorption was a decisive index affecting the performance of recycled aggregate. Literature shows that RA has a higher water absorption capacity than raw aggregate caused by the pore structure of bonded mortar [13,32]. Similar observations was carried out by Hansen et al. [22] who found that the water absorption of RA was 2.3–4.6 times higher than that of NA regardless of the strength of original concrete [22,24]. Several studies show that the high absorbency of RA leads to the reduction of RAC workability, which can be compensated by pre-soaking. However, excessive water absorption of aggregate will adversely affect the strength of
Fig. 3. Relationship between attached mortar content and fraction size.
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concrete. Therefore, some codes prohibit the use of RA with a water absorption rate higher than 7–10% in structural concrete [13,34]. Brito et al. [35] investigated the effect of multiple recycling on the concrete performance and noted that the water absorption rate of RA enhanced with the number of recycling increased due to the progressively augment of adhered mortar content. However, in consideration of the research was executed in fewer cycles, it is impossible to determine whether this upward trend will continue indefinitely or whether the absorption will gradually approach a certain value. The correlation between adhered mortar content and water absorption of RA are shown in Fig. 4. The 95% confidence interval and prediction interval of the correlation are given in the figure. It indicates that in a batch of aggregate with the same attached mortar content, the probability of the average value of the water absorption falling into the dark red portion is over than 95%. The 95% prediction interval can provide a reference for the water absorption index prediction of a single individual. 2.3. Correlation between attached mortar content and saturated surface dry density The saturated surface dry density was another decisive factor to determine aggregate quality. The saturated surface dry density of aggregate must be taken into account when considering its water absorption in the process of mixing and stirring [36]. Researchers have found that the saturated surface dry density of RA has nothing to do with the grade of original concrete [22], but the watercement ratio of original concrete and the amount of aggregate adhesion mortar [37]. Additionally, Debieb et al. [38] reported that the proportion of more than 20% recycled fine aggregate will result in a large loss of the machinability of RAC, which has a significant relationship with the surface dry density of RA. The relationship between adhered mortar content and saturated surface dry density in the collected literature is shown in Fig. 5. The plot presents that the saturated surface dry density of RA decreases with the augment of the attached mortar content. Consistent with the foregoing, the 95% confidence interval and prediction interval can be used as an auxiliary to improve the accuracy of predicting the saturated surface dry density. 2.4. Correlation between attached mortar content and Los Angeles abrasion At present, RA was principally used in road engineering [39–44]. The wear resistance has been investigated by several
Fig. 4. Relationship between attached mortar content and absorption.
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sulfate content of aggregate was particularly meaningful as the expansion of concrete will be caused by excessive sulfate content [45]. Previous studies have noted that sulfate content in the RA was higher than NA due to the presence of sulfate in adhered old mortar [46]. Fig. 7 shows that the sulfate content increases with the increase of content of attached mortar. The 95% confidence intervals and prediction intervals provide references for the accurate application of the obtained relations. In addition, Spanish standards require aggregates to contain less than 0.8% sulfate, while China sets a 1% limit. 3. The effect of the recycled aggregate proportion on the mechanical properties of concrete 3.1. The compressive strength
Fig. 5. Relationship between attached mortar content and saturated surface dry density.
Fig. 6. Relationship between attached mortar content and Los Angeles abrasion.
studies. Fig. 6 illustrates the relationship between the content of attached mortar and the Los Angeles abrasion for literature: the higher content of adhesion mortar, the greater Los Angeles abrasion was. shown in the figure, the higher dispersion of the data may be caused by the quality difference of the RA. The 95% confidence interval of statistics indicates that in a batch of aggregate with the same content of adhered mortar, the probability of the average value of the Los Angeles abrasion falling into the dark red part is over than 95%. Some studies have reported that the wear coefficient in Los Angeles increase gradually with the decrease of the particle size of RA due to the smaller the aggregate particle size was, the more worn away parts of the stones were. In addition, the relevant national norms on aggregate Los Angeles wear coefficient also made provisions. Spanish standards for structural concrete stipulate that Los Angeles aggregate wear factor should be less than 40%. This index should be less than 50% for structural concrete and for pavement concrete should not be more than 40% in China.
The compressive strength is the most effective characteristic of concrete, which affects the mechanics, durability and other properties of concrete [47–50,63]. This review shows a consensus that the compressive strength of concrete decrease as the replacement rate of RA increase, irrespective of aggregate type and quality, as observed in all publications (except a RA generated from a high grade/quality source concrete and incorporated in a new low grade/quality concrete mixture). The results also show that the increase of the content of RA was unfavorable to the compressive strength of concrete. However, there are a few cases in which the incorporation of a small proportion of recycled aggregate can increase the strength of concrete [37,51]. This special case was largely attributed to the good control of the grading of RA [47]. Fig. 8 (a) presents the relative compressive strength of concrete with different replacement ratio of RA, regardless of type and quality. There is a clear trend that the incorporation of RCA leads to lower compressive strengths when compared to that of the control concrete. A 95% confidence interval was included in Fig. 8. As indicated, there is a 95% probability that the RAC specimens containing 100% coarse RCA could have about 0.766 times lower compressive strengths than that of corresponding NAC specimen. However, in a few cases RAC exhibited similar or even slightly greater strength. Since this was observed mostly for specimens containing RCA, it is possible that the bond strength in the interface transition zone (ITZ) between the old attached mortar and new cement paste may be improved for some reasons. The boxplot obtained by exploratory analysis of data with SSPS software is shown in Fig. 8(b), the outlier and extremes in the data were evident in the figure. It can be observed from the quartile distance
2.5. Correlation between attached mortar content and sulphate content Generally, the sulfate as main component of gypsum was abundant in RA because of its high content of gypsum. So controlling the
Fig. 7. Relationship between attached mortar content and sulphate content.
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Fig. 8. Relationship between replacement ratio of recycled coarse aggregate and relative compressive strength (a) and exploratory analysis box plot (b).
that the higher the replacement ratio of RA, the greater the dispersion of data, which was probably caused by the difference in the properties of RA. In addition, the compressive strength of RAC decreases with the increase of water-cement ratio, which was similar to ordinary concrete [52,53]. However, the compressive strength of RAC varies greatly at the lower water-cement ratio [53]. Khan [52] concluded that the water-cement ratio of RAC should be 0.05–0.1 less than NAC to achieve the similar compressive strength value, which was attributed to the low water utilization ratio of the ITZs between old adhered mortar and aggregate. In terms of water absorption rate, with the increase of water absorption rate, the intensity of RAC decrease under normal circumstances [37]. However, Ryu [54] mentioned in the article that the decrease of compressive strength with the increase of water absorption rate was not obvious. He believed that the quality of the ITZ generated on the aggregate would affect the compressive strength and not only depend on the water absorption. 3.2. The split tensile strength The indirect method was usually applied to calculate the tensile strength of concrete. The splitting tensile test was considered as the most commonly used evaluation method. Bairagi et al. [58]
reported that the split tensile strength of RAC was 6%, 10% and 40% less than that of NAC when 25%, 50% and 100% RA replacement ratio were used, respectively. Similar results were noted for specimen that the split tensile strength decreases with the increase of RA replacement ratio [37,47,50,51]. On the contrary, many literatures show that the tensile strength of RAC was the same or even exceeds the tensile strength of virgin aggregate concrete when the replacement ratio of RA was as high as 30% [51,55,56]. The relationship between the incorporation ratio of RA and relative splitting tensile strength is shown in Fig. 9(a). It can be observed that with the increase of the RA replacement ratio, the concrete shows a slight trend of decrease in the relative tensile strength of split. The upper and lower 95% confidence intervals are also included in the figure. The 95% upper and lower confidence levels were defined by UCL and LCL. It suggested that there had a 95% probability of the relative split tensile strength of concrete with 100% incorporation of RCA was 0.832 times lower than that of virgin aggregate concrete. As can be clearly found from the boxplot of Fig. 9(b), there are many abnormal points in this group of data. The results of the interquartile range show that the dispersion of the data increases slightly with the increase of the replacement rate of the RA, indicating that the difference in aggregate quality has a limited effect on the tensile strength of the concrete.
Fig. 9. Relationship between replacement ratio of recycled coarse aggregate and relative split tensile strength (a) and exploratory analysis box plot (b).
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In addition, the split tensile strength was reported to be affected by curing conditions. Fonseca et al. [51] shows that for the RAC with 50% RA content, the concrete split tensile strength under open environment and water immersion conditions were higher than laboratory culture conditions. However, the tensile strength of concrete under different curing conditions was similar when the specimen with 100% replacement ratio was investigated. At the same time, literature observed that the concrete split tensile strength increase with the decrease of water-cement ratio, which was quite encouraging [57]. 3.3. The flexural strength Some researchers noted that the flexural strength of concrete decrease gradually with the increase of the proportion of RA [33,58–61]. Bairagi et al. [58] reported that the flexural strength of RAC with 25%, 50% and 100% replacement ratio of RA were 6%, 13% and 26% lower than that of NAC respectively. This behavior was attributed to poor bonding quality between the old adhered mortar and the new cement paste. Oliveira et al. [62] believes that the flexural strength of RAC was lower than that of NAC regardless of RA moisture conditions. However, when saturated RA was used, the loss of flexural strength was more significant [65]. The relationship between the replacement ratio of RCA and relative flexural strength is shown in Fig. 10(a). It can be observed from the plot that with the increase of the incorporation of RA, the decrease of the relative flexural strength of concrete is limited. The 95% LCL illustrated that there is a 95% probability that the flexural strength of RAC with 100% RCA content is about loss below 16.8% when compared with NAC. You can obtain from the boxplot of Fig. 10 (b) that the median value of the relative flexural strength of concrete with different proportion of RA was equal basically. However, the relative flexural strength value was more discrete under the high replacement ratio of RA. Furthermore, the flexural strength decrease with the increase of water-binder ratio. Concrete with higher flexural strength can be prepared by using RA with lower water absorption [65].
the reduction in concrete volume due to the loss of internal moisture. It has been investigated by many scholars that the shrinkage of RAC increases with the replacement ratio of RA increases [32,50,77–79]. This situation was due to the fact that a larger proportion of water is required for the preparation of RAC. Matias et al. [77] reported that the pre-soaking of RA for 24 h results in an increase in dry shrinkage as it would results in an increase in entrained water in the RAC. It shows that moisture situation of RA also affects the drying shrinkage of RAC. However, drying shrinkage can be reduced by adding mineral admixture to RAC [50,80]. Domingo-Cabo et al. [79] noticed that RAC with a lower RA replacement ratio (20%) showed similar shrinkage to NAC at 28 days. However, after 6 months, the shrinkage of the 20%, 50%, and 100% replacement RAC was 4%, 12%, and 70% higher than that of NAC, respectively. Other studies have shown that improved mixing process can also reduce the shrinkage of RAC [78]. The variation of drying shrinkage of RAC with respect to time investigated by researchers is present in Fig. 11. The variation tends to flatten out over time, indicating that the shrinkage of RAC decreases over time.
3.4. The shrinkage and creep Drying shrinkage and creep were two parameters of deformation for concrete. Creep was the increase in deformation of concrete under a sustained load, while dry shrinkage was caused by
Fig. 11. Shrinkage rate of RAC for different experimental researches.
Fig. 10. Relationship between replacement ratio of recycled coarse aggregate and relative flexural strength (a) and exploratory analysis box plot (b).
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4. Performance enhancement of recycled aggregate The performance improvement of RA was carried out through the treatment of attached mortar, which was mainly divided into the separation of old mortar and the strengthening of old mortar. The following two aspects will be stated. 4.1. Separation of adhered mortar Montgomery [64] removes the adhesive mortar in the particles through ball mill to improve the quality of aggregate. Later, hot grinding appeared. Tateyashiki. et al and Ma. et al. [65,66] obtained high-quality RAC by heating then grinding. However, the performance of the RA was reduced due to the generation of microcracks during the grinding process. Katz [67] used ultrasonic water cleaning to remove the attached mortar. This method was suitable for the separation of mortar with poor adhesion, but not for mortar with strong adhesion. Tam et al. [68] adopted acid solution immersion to remove the attached mortar on the aggregate surface. This way will increase the acid content in RA with high cost and low feasibility. 4.2. Strengthening of adhered mortar Kou et al. [69] claimed that the mechanical properties of RAs can be promoted by treating with a PVA polymer solution. Wan et al. [70] treated RCA with 1% PVA solution and found that the
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compressive and workability of concrete were heightened. Silicon-based waterproof polymers have been proved to significantly improve aggregate quality [71,72]. In general, PVA and silicon-based waterproof polymers can effectively enhance the quality of recycled aggregate and the workability of RAC, but they cannot improve or even have a negative effect on the compressive strength of concrete. According to previous research reports, Tam et al. [73] reported a two-stage mixing approach (TSMA) to process the regeneration aggregate, in TSMA, recycled aggregates, cement and half of the water were mixed in advance. TSMA could improve the microstructure of old mortar and ITZ by producing a thin layer of cement slurry on RA. As shown in Fig. 12, the compressive strength of RAC can be improved effectively by this process. Another improved hybrid methods were proposed by Tam et al. [74,75], and the modified TSMA are drawn in Figs. 13 and 14. It was found that the modified TSMA was encouraging for improving concrete compressive strength. Liang et al. [76] suggested mortar mixing approach (MMA) and sand enveloped mixing approach (SEMA) to enhance the performance of RAC. The specimens were produced with 100% RA, which was surface pretreated 7 days before mixing. The mixing techniques are shown in Figs. 15 and 16. The TSMA seems to be a rather effective method for the analysis and comparison of the above methods for improving the performance of RA due to its cost-effectiveness and operability. Meanwhile, it providing excellent results for all mechanical experiments, this technology can be widely applied during the practical engineering.
Fig. 12. Two stage mixing approach (TSMA).
Fig. 13. Modified TSMA1.
Fig. 14. Modified TSMA2.
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Fig. 15. Mortar mixing approach.
Fig. 16. Sand enveloped mixing approach.
5. Conclusion In this paper, the effect of the adhered mortar content on the aggregate performance at material level, the influence of the incorporation of recycled aggregate on the concrete mechanical performance at component level, and the performance improvement technology of recycled aggregate were summarized and analyzed. The following conclusions could be drawn. It was inevitable that RA will adhere to old mortar on the surface due to the production technology. The presence of these adhering mortars was the essential reason why the quality of RA was weaker than that of NA. Quantitative analysis was carried out between the old mortar content and density, water absorption, wear coefficient, and sulfate content of several aggregate key indicators. These data can be used to establish the demand for RAs for different applications. The loss of compressive strength of concrete was more sensitive to the incorporation of RA than the tensile strength and flexural strength of concrete. In addition, the water-cement ratio and the water absorption rate of the aggregate were also significant effects on the quality of the concrete. This result can provide a reference for the application of RA. For example, different preparation techniques can be employed for components of different forms of stress in the construction process. In terms of RAC reinforcement, the TSMA method seems to be a fairly worthwhile method due to cost-effectiveness and operability. There are many quantitative studies on the content of attached mortar and aggregate properties, but the relationship between the adhered mortar content and the mechanical properties concrete mostly stays in qualitative research. Therefore, more quantitative experiments should be carried out on the content of old mortar and the mechanical properties of RAC, and the quantitative relationship between the content of adhered mortar and the mechanical properties of RAC should be established, so that to achieve the pre-judgment of the performance of RAC in terms of different application requirements. The performance of RAC was inferior to that of NAC. The pursuit of special techniques to strip old adhered mortar to improve the property of RAC does not seem to be a practical means.
However, the concrete can be improved within a certain range by some simple and low-cost approach such as adjusting water-cement ratio, aggregate water content, mixing method and admixture. The performance of RAC can be adjusted to suit the quality requirements of specific engineering pairs of concrete. For example, it can be applied to some road projects, or some minor parts of the main structure to promote the application range of RAs, so as to ensure the sustainable development of the construction industry.
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. Acknowledgements The authors would like to acknowledge the National Natural Science Foundation of China (Grant NO. 51878546), the Natural Science Basic Research Program of Shaanxi Province (Grant NO. 2019JM-597), the Innovative Talent Promotion Plan of Shaanxi Province (Grant NO. 2018KJXX-056), the Key Research and Development Projects of Shaanxi Province (Grant NO. 2018ZDCXLSF-03-03-02) and the Science and Technology Innovation Base of Shaanxi Province (Grant NO. 2017KTPT-19) for financial support. References [1] J.J. Xiao, W.G. Li, Y.H. Fan, X. Huang, An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (2012) 364– 383, https://doi.org/10.1016/j.conbuildmat.2011.12.074. [2] C. Liu, H.W. Liu, C. Zhu, G.L. Bai, On the mechanism of internal temperature and humidity response of recycled aggregate concrete based on the recycled aggregate porous interface, Cem. Concr. Compos. 103 (2019) 22–35, https:// doi.org/10.1016/j.cemconcomp.2019.04.016. [3] Y. Wang, S.H. Zhang, D.T. Niu, L. Su, D.M. Luo, Strength and chloride ion distribution brought by aggregate of basalt fiber reinforced coral aggregate concrete, Constr. Build. Mater. 234 (2020) 117390, https://doi.org/10.1016/ j.conbuildmat.2019.117390. [4] Y.Y. Wang, J.J. Huang, D.J. Wang, Y.F. Liu, Z.J. Zhao, J.P. Liu, Experimental study on hygrothermal characteristics of coral sand aggregate concrete and aerated concrete under different humidity and temperature conditions, Constr. Build. Mater. 230 (2020) 1–17, https://doi.org/10.1016/j.conbuildmat.2019.117034.
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Review
An evaluation of the recycled aggregate characteristics and the recycled aggregate concrete mechanical properties Guoliang Bai a,b, Chao Zhu a, Chao Liu a,⇑, Biao Liu a a b
School of Civil Engineering, Xi’an University of Architecture & Technology, Xi’an 710055, China Key Lab of Structural Engineering and Earthquake Resistance, Ministry of Education (XAUAT), Xi’an 710055, China
h i g h l i g h t s The quantitative relationships between the content of old mortar and the quality of recycled aggregate were obtained. The effects of the recycled aggregate proportion on the mechanical properties of concrete were analyzed. A method for improving the performance of recycled aggregates suitable for engineering applications was proposed. Some suggestions for further popularization and application of recycled concrete were put forward.
a r t i c l e
i n f o
Article history: Received 6 July 2019 Received in revised form 13 November 2019 Accepted 28 December 2019
Keywords: Recycled aggregate concrete Attached mortar Mechanical properties Aggregate reinforcement
a b s t r a c t The difference between the properties of recycled aggregate (RA) and natural aggregate (NA) are caused due to the old mortar attached to RA. In this review, the quantitative relationships were represented about the content of old attached mortar and the performance of RA at the material level. In term of the component level, the influence of replacement ratio of RA on the mechanical properties of concrete was summarized. Finally, some researches have focused on ways to improve the properties of aggregate. The results show that the method of stripping old mortar to improve the performance of recycled aggregates was not the only way to promote the application of recycled aggregates. However, by evaluating some simple and economical methods such as controlling the water-cement ratio, adjusting the aggregate moisture content and the different mixing method can improve the performance of recycled concrete in order to meet the requirement of concrete quality. Three aspects of the performance prediction, application range and reinforcement method for RAs were illustrated in the article. It was believe that the results could promote the accurate application for RAs with different quality in engineering, thus the application range of RAs was expanded. Ó 2019 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The characteristic of recycled concrete aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Correlation between attached mortar content and aggregates particle size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Correlation between attached mortar content and water absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Correlation between attached mortar content and saturated surface dry density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Correlation between attached mortar content and Los Angeles abrasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Correlation between attached mortar content and sulphate content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The effect of the recycled aggregate proportion on the mechanical properties of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. The compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. The split tensile strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. The flexural strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. The shrinkage and creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. E-mail address: [email protected] (C. Liu). https://doi.org/10.1016/j.conbuildmat.2019.117978 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
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4.
5.
Performance enhancement of recycled aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Separation of adhered mortar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Strengthening of adhered mortar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction In the last decades, a large-scale reconstruction of old urban areas has been carried out in China, the dramatic increase in the amount of construction and demolition waste (CDW, about 30%– 40% of municipal solid waste) [1,2]. In addition, China consumes about 40% of the world’s cement and steel each year due to a mass of fresh construction. The extensive type exploitation for a long time has caused a significant shortage of natural resources in China [3,4]. The exploitation of natural resources was prohibited widely by the government. The most scientific and effective approach to deal with CDW at the present stage was to crush, wash, sift and classify to obtain RA, and then partially or completely replace the natural aggregate (NA) with RA for preparation of concrete. A series of standards and procedures have been published to facilitate the carry out of this action [5,6]. The category of RA will be determined by structural form of the demolished construction and the resource production technology. RAs can be divided into recycled concrete aggregate, recycled brick aggregate, and recycled mixed aggregate as shown in Fig. 1. Meanwhile, the miscellaneous materials such as wood blocks, paper scraps, plastics and other pollutants are also contained in RAs [7–9]. At present, compared with other recycled coarse aggregate types, recycled concrete aggregate is the most extensive implemented RA in construction, and organizations in many countries have developed specifications [10–12]. In this paper, the performance and application of recycled concrete aggregate were focused on.
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The old mortar adhered on the surface of aggregate was the most fundamental difference between RA and NA as shown in Fig. 2 [13,14]. The amount of mortar attached to the aggregate surface will be determined by the processing technology and quantity during the production of RAs. According to Nagataki [15], the more the crushing process, the less bonding mortar and the higher the density of the RA. However, the density of recycled fines will decrease due to excessive crushing process. The processing stage should not be too much or too little for high quality RA, otherwise the aggregates cannot be used for certain applications [16]. The presence of adhering old mortar results in the water absorption, crushing index, and Los Angeles wear coefficient of RA were higher than original aggregate [17–19], resulting in the performance of recycled aggregate concrete (RAC) is weaker than natural aggregate concrete (NAC). Recycled concrete aggregates were commonly used in road bases and some secondary roads in China at present stage. The application in the terms of building structure was merely in the low-rise demonstration house. It can be said that the promotion and application of RAC was lagging behind in China. In this review, The characteristic of RAs was evaluated by quantitatively analyzed between attached mortar content and key indicators of RAs, such as density, water absorption, Los Angeles wear coefficient and sulfate content at the material level. In term of the component level, the effects of the proportion of RA on the mechanical properties of RAC were analyzed, and the factors of water-cement ratio and aggregate moisture content were also considered. In addition, the reinforcement techniques of RAs were
Fig. 1. Main categories of recycled coarse aggregate.
Fig. 2. Schematic diagram of recycled concrete coarse aggregate.
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discussed. The results of the review can be used to forecast the performance of RA, so as to provide reference for its accurate application in engineering. Finally, several optimized approaches were recommended to expand the application scope of RAs and guarantee the sustainable development of the construction industry. 2. The characteristic of recycled concrete aggregates 2.1. Correlation between attached mortar content and aggregates particle size The particle size of RA varies with the production process [20,21]. Several researchers [22–25] observed that there was a certain relationship between the particle size of RA and the content of the attached mortar. The results extracted from literature [22,26– 30] are shown in Fig. 3. From the plot it is evident that the attached mortar content increase with decrease in the aggregate size. This behavior may be attributed to the processing techniques. The amount of adhesive mortar on the recycled coarse aggregate was reduced at the beginning of mechanical processing. However, as it gradually crumbles the cement mortar accumulates in smallsized RA [31]. This is also the reason why the density of recycled fine aggregate is generally lower than that of recycled coarse aggregate. The 95% confidence interval of statistics indicates that in a batch of aggregate with the same aggregate particle size, the probability of the average value of the attached mortar content falling into the dark red portion is over than 95%. The 95% prediction interval represents the range of the predicted bound mortar content for a given aggregate size. The addition of confidence interval promotes the application of this relation in practical engineering. 2.2. Correlation between attached mortar content and water absorption The water absorption was a decisive index affecting the performance of recycled aggregate. Literature shows that RA has a higher water absorption capacity than raw aggregate caused by the pore structure of bonded mortar [13,32]. Similar observations was carried out by Hansen et al. [22] who found that the water absorption of RA was 2.3–4.6 times higher than that of NA regardless of the strength of original concrete [22,24]. Several studies show that the high absorbency of RA leads to the reduction of RAC workability, which can be compensated by pre-soaking. However, excessive water absorption of aggregate will adversely affect the strength of
Fig. 3. Relationship between attached mortar content and fraction size.
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concrete. Therefore, some codes prohibit the use of RA with a water absorption rate higher than 7–10% in structural concrete [13,34]. Brito et al. [35] investigated the effect of multiple recycling on the concrete performance and noted that the water absorption rate of RA enhanced with the number of recycling increased due to the progressively augment of adhered mortar content. However, in consideration of the research was executed in fewer cycles, it is impossible to determine whether this upward trend will continue indefinitely or whether the absorption will gradually approach a certain value. The correlation between adhered mortar content and water absorption of RA are shown in Fig. 4. The 95% confidence interval and prediction interval of the correlation are given in the figure. It indicates that in a batch of aggregate with the same attached mortar content, the probability of the average value of the water absorption falling into the dark red portion is over than 95%. The 95% prediction interval can provide a reference for the water absorption index prediction of a single individual. 2.3. Correlation between attached mortar content and saturated surface dry density The saturated surface dry density was another decisive factor to determine aggregate quality. The saturated surface dry density of aggregate must be taken into account when considering its water absorption in the process of mixing and stirring [36]. Researchers have found that the saturated surface dry density of RA has nothing to do with the grade of original concrete [22], but the watercement ratio of original concrete and the amount of aggregate adhesion mortar [37]. Additionally, Debieb et al. [38] reported that the proportion of more than 20% recycled fine aggregate will result in a large loss of the machinability of RAC, which has a significant relationship with the surface dry density of RA. The relationship between adhered mortar content and saturated surface dry density in the collected literature is shown in Fig. 5. The plot presents that the saturated surface dry density of RA decreases with the augment of the attached mortar content. Consistent with the foregoing, the 95% confidence interval and prediction interval can be used as an auxiliary to improve the accuracy of predicting the saturated surface dry density. 2.4. Correlation between attached mortar content and Los Angeles abrasion At present, RA was principally used in road engineering [39–44]. The wear resistance has been investigated by several
Fig. 4. Relationship between attached mortar content and absorption.
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sulfate content of aggregate was particularly meaningful as the expansion of concrete will be caused by excessive sulfate content [45]. Previous studies have noted that sulfate content in the RA was higher than NA due to the presence of sulfate in adhered old mortar [46]. Fig. 7 shows that the sulfate content increases with the increase of content of attached mortar. The 95% confidence intervals and prediction intervals provide references for the accurate application of the obtained relations. In addition, Spanish standards require aggregates to contain less than 0.8% sulfate, while China sets a 1% limit. 3. The effect of the recycled aggregate proportion on the mechanical properties of concrete 3.1. The compressive strength
Fig. 5. Relationship between attached mortar content and saturated surface dry density.
Fig. 6. Relationship between attached mortar content and Los Angeles abrasion.
studies. Fig. 6 illustrates the relationship between the content of attached mortar and the Los Angeles abrasion for literature: the higher content of adhesion mortar, the greater Los Angeles abrasion was. shown in the figure, the higher dispersion of the data may be caused by the quality difference of the RA. The 95% confidence interval of statistics indicates that in a batch of aggregate with the same content of adhered mortar, the probability of the average value of the Los Angeles abrasion falling into the dark red part is over than 95%. Some studies have reported that the wear coefficient in Los Angeles increase gradually with the decrease of the particle size of RA due to the smaller the aggregate particle size was, the more worn away parts of the stones were. In addition, the relevant national norms on aggregate Los Angeles wear coefficient also made provisions. Spanish standards for structural concrete stipulate that Los Angeles aggregate wear factor should be less than 40%. This index should be less than 50% for structural concrete and for pavement concrete should not be more than 40% in China.
The compressive strength is the most effective characteristic of concrete, which affects the mechanics, durability and other properties of concrete [47–50,63]. This review shows a consensus that the compressive strength of concrete decrease as the replacement rate of RA increase, irrespective of aggregate type and quality, as observed in all publications (except a RA generated from a high grade/quality source concrete and incorporated in a new low grade/quality concrete mixture). The results also show that the increase of the content of RA was unfavorable to the compressive strength of concrete. However, there are a few cases in which the incorporation of a small proportion of recycled aggregate can increase the strength of concrete [37,51]. This special case was largely attributed to the good control of the grading of RA [47]. Fig. 8 (a) presents the relative compressive strength of concrete with different replacement ratio of RA, regardless of type and quality. There is a clear trend that the incorporation of RCA leads to lower compressive strengths when compared to that of the control concrete. A 95% confidence interval was included in Fig. 8. As indicated, there is a 95% probability that the RAC specimens containing 100% coarse RCA could have about 0.766 times lower compressive strengths than that of corresponding NAC specimen. However, in a few cases RAC exhibited similar or even slightly greater strength. Since this was observed mostly for specimens containing RCA, it is possible that the bond strength in the interface transition zone (ITZ) between the old attached mortar and new cement paste may be improved for some reasons. The boxplot obtained by exploratory analysis of data with SSPS software is shown in Fig. 8(b), the outlier and extremes in the data were evident in the figure. It can be observed from the quartile distance
2.5. Correlation between attached mortar content and sulphate content Generally, the sulfate as main component of gypsum was abundant in RA because of its high content of gypsum. So controlling the
Fig. 7. Relationship between attached mortar content and sulphate content.
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Fig. 8. Relationship between replacement ratio of recycled coarse aggregate and relative compressive strength (a) and exploratory analysis box plot (b).
that the higher the replacement ratio of RA, the greater the dispersion of data, which was probably caused by the difference in the properties of RA. In addition, the compressive strength of RAC decreases with the increase of water-cement ratio, which was similar to ordinary concrete [52,53]. However, the compressive strength of RAC varies greatly at the lower water-cement ratio [53]. Khan [52] concluded that the water-cement ratio of RAC should be 0.05–0.1 less than NAC to achieve the similar compressive strength value, which was attributed to the low water utilization ratio of the ITZs between old adhered mortar and aggregate. In terms of water absorption rate, with the increase of water absorption rate, the intensity of RAC decrease under normal circumstances [37]. However, Ryu [54] mentioned in the article that the decrease of compressive strength with the increase of water absorption rate was not obvious. He believed that the quality of the ITZ generated on the aggregate would affect the compressive strength and not only depend on the water absorption. 3.2. The split tensile strength The indirect method was usually applied to calculate the tensile strength of concrete. The splitting tensile test was considered as the most commonly used evaluation method. Bairagi et al. [58]
reported that the split tensile strength of RAC was 6%, 10% and 40% less than that of NAC when 25%, 50% and 100% RA replacement ratio were used, respectively. Similar results were noted for specimen that the split tensile strength decreases with the increase of RA replacement ratio [37,47,50,51]. On the contrary, many literatures show that the tensile strength of RAC was the same or even exceeds the tensile strength of virgin aggregate concrete when the replacement ratio of RA was as high as 30% [51,55,56]. The relationship between the incorporation ratio of RA and relative splitting tensile strength is shown in Fig. 9(a). It can be observed that with the increase of the RA replacement ratio, the concrete shows a slight trend of decrease in the relative tensile strength of split. The upper and lower 95% confidence intervals are also included in the figure. The 95% upper and lower confidence levels were defined by UCL and LCL. It suggested that there had a 95% probability of the relative split tensile strength of concrete with 100% incorporation of RCA was 0.832 times lower than that of virgin aggregate concrete. As can be clearly found from the boxplot of Fig. 9(b), there are many abnormal points in this group of data. The results of the interquartile range show that the dispersion of the data increases slightly with the increase of the replacement rate of the RA, indicating that the difference in aggregate quality has a limited effect on the tensile strength of the concrete.
Fig. 9. Relationship between replacement ratio of recycled coarse aggregate and relative split tensile strength (a) and exploratory analysis box plot (b).
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In addition, the split tensile strength was reported to be affected by curing conditions. Fonseca et al. [51] shows that for the RAC with 50% RA content, the concrete split tensile strength under open environment and water immersion conditions were higher than laboratory culture conditions. However, the tensile strength of concrete under different curing conditions was similar when the specimen with 100% replacement ratio was investigated. At the same time, literature observed that the concrete split tensile strength increase with the decrease of water-cement ratio, which was quite encouraging [57]. 3.3. The flexural strength Some researchers noted that the flexural strength of concrete decrease gradually with the increase of the proportion of RA [33,58–61]. Bairagi et al. [58] reported that the flexural strength of RAC with 25%, 50% and 100% replacement ratio of RA were 6%, 13% and 26% lower than that of NAC respectively. This behavior was attributed to poor bonding quality between the old adhered mortar and the new cement paste. Oliveira et al. [62] believes that the flexural strength of RAC was lower than that of NAC regardless of RA moisture conditions. However, when saturated RA was used, the loss of flexural strength was more significant [65]. The relationship between the replacement ratio of RCA and relative flexural strength is shown in Fig. 10(a). It can be observed from the plot that with the increase of the incorporation of RA, the decrease of the relative flexural strength of concrete is limited. The 95% LCL illustrated that there is a 95% probability that the flexural strength of RAC with 100% RCA content is about loss below 16.8% when compared with NAC. You can obtain from the boxplot of Fig. 10 (b) that the median value of the relative flexural strength of concrete with different proportion of RA was equal basically. However, the relative flexural strength value was more discrete under the high replacement ratio of RA. Furthermore, the flexural strength decrease with the increase of water-binder ratio. Concrete with higher flexural strength can be prepared by using RA with lower water absorption [65].
the reduction in concrete volume due to the loss of internal moisture. It has been investigated by many scholars that the shrinkage of RAC increases with the replacement ratio of RA increases [32,50,77–79]. This situation was due to the fact that a larger proportion of water is required for the preparation of RAC. Matias et al. [77] reported that the pre-soaking of RA for 24 h results in an increase in dry shrinkage as it would results in an increase in entrained water in the RAC. It shows that moisture situation of RA also affects the drying shrinkage of RAC. However, drying shrinkage can be reduced by adding mineral admixture to RAC [50,80]. Domingo-Cabo et al. [79] noticed that RAC with a lower RA replacement ratio (20%) showed similar shrinkage to NAC at 28 days. However, after 6 months, the shrinkage of the 20%, 50%, and 100% replacement RAC was 4%, 12%, and 70% higher than that of NAC, respectively. Other studies have shown that improved mixing process can also reduce the shrinkage of RAC [78]. The variation of drying shrinkage of RAC with respect to time investigated by researchers is present in Fig. 11. The variation tends to flatten out over time, indicating that the shrinkage of RAC decreases over time.
3.4. The shrinkage and creep Drying shrinkage and creep were two parameters of deformation for concrete. Creep was the increase in deformation of concrete under a sustained load, while dry shrinkage was caused by
Fig. 11. Shrinkage rate of RAC for different experimental researches.
Fig. 10. Relationship between replacement ratio of recycled coarse aggregate and relative flexural strength (a) and exploratory analysis box plot (b).
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4. Performance enhancement of recycled aggregate The performance improvement of RA was carried out through the treatment of attached mortar, which was mainly divided into the separation of old mortar and the strengthening of old mortar. The following two aspects will be stated. 4.1. Separation of adhered mortar Montgomery [64] removes the adhesive mortar in the particles through ball mill to improve the quality of aggregate. Later, hot grinding appeared. Tateyashiki. et al and Ma. et al. [65,66] obtained high-quality RAC by heating then grinding. However, the performance of the RA was reduced due to the generation of microcracks during the grinding process. Katz [67] used ultrasonic water cleaning to remove the attached mortar. This method was suitable for the separation of mortar with poor adhesion, but not for mortar with strong adhesion. Tam et al. [68] adopted acid solution immersion to remove the attached mortar on the aggregate surface. This way will increase the acid content in RA with high cost and low feasibility. 4.2. Strengthening of adhered mortar Kou et al. [69] claimed that the mechanical properties of RAs can be promoted by treating with a PVA polymer solution. Wan et al. [70] treated RCA with 1% PVA solution and found that the
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compressive and workability of concrete were heightened. Silicon-based waterproof polymers have been proved to significantly improve aggregate quality [71,72]. In general, PVA and silicon-based waterproof polymers can effectively enhance the quality of recycled aggregate and the workability of RAC, but they cannot improve or even have a negative effect on the compressive strength of concrete. According to previous research reports, Tam et al. [73] reported a two-stage mixing approach (TSMA) to process the regeneration aggregate, in TSMA, recycled aggregates, cement and half of the water were mixed in advance. TSMA could improve the microstructure of old mortar and ITZ by producing a thin layer of cement slurry on RA. As shown in Fig. 12, the compressive strength of RAC can be improved effectively by this process. Another improved hybrid methods were proposed by Tam et al. [74,75], and the modified TSMA are drawn in Figs. 13 and 14. It was found that the modified TSMA was encouraging for improving concrete compressive strength. Liang et al. [76] suggested mortar mixing approach (MMA) and sand enveloped mixing approach (SEMA) to enhance the performance of RAC. The specimens were produced with 100% RA, which was surface pretreated 7 days before mixing. The mixing techniques are shown in Figs. 15 and 16. The TSMA seems to be a rather effective method for the analysis and comparison of the above methods for improving the performance of RA due to its cost-effectiveness and operability. Meanwhile, it providing excellent results for all mechanical experiments, this technology can be widely applied during the practical engineering.
Fig. 12. Two stage mixing approach (TSMA).
Fig. 13. Modified TSMA1.
Fig. 14. Modified TSMA2.
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Fig. 15. Mortar mixing approach.
Fig. 16. Sand enveloped mixing approach.
5. Conclusion In this paper, the effect of the adhered mortar content on the aggregate performance at material level, the influence of the incorporation of recycled aggregate on the concrete mechanical performance at component level, and the performance improvement technology of recycled aggregate were summarized and analyzed. The following conclusions could be drawn. It was inevitable that RA will adhere to old mortar on the surface due to the production technology. The presence of these adhering mortars was the essential reason why the quality of RA was weaker than that of NA. Quantitative analysis was carried out between the old mortar content and density, water absorption, wear coefficient, and sulfate content of several aggregate key indicators. These data can be used to establish the demand for RAs for different applications. The loss of compressive strength of concrete was more sensitive to the incorporation of RA than the tensile strength and flexural strength of concrete. In addition, the water-cement ratio and the water absorption rate of the aggregate were also significant effects on the quality of the concrete. This result can provide a reference for the application of RA. For example, different preparation techniques can be employed for components of different forms of stress in the construction process. In terms of RAC reinforcement, the TSMA method seems to be a fairly worthwhile method due to cost-effectiveness and operability. There are many quantitative studies on the content of attached mortar and aggregate properties, but the relationship between the adhered mortar content and the mechanical properties concrete mostly stays in qualitative research. Therefore, more quantitative experiments should be carried out on the content of old mortar and the mechanical properties of RAC, and the quantitative relationship between the content of adhered mortar and the mechanical properties of RAC should be established, so that to achieve the pre-judgment of the performance of RAC in terms of different application requirements. The performance of RAC was inferior to that of NAC. The pursuit of special techniques to strip old adhered mortar to improve the property of RAC does not seem to be a practical means.
However, the concrete can be improved within a certain range by some simple and low-cost approach such as adjusting water-cement ratio, aggregate water content, mixing method and admixture. The performance of RAC can be adjusted to suit the quality requirements of specific engineering pairs of concrete. For example, it can be applied to some road projects, or some minor parts of the main structure to promote the application range of RAs, so as to ensure the sustainable development of the construction industry.
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. Acknowledgements The authors would like to acknowledge the National Natural Science Foundation of China (Grant NO. 51878546), the Natural Science Basic Research Program of Shaanxi Province (Grant NO. 2019JM-597), the Innovative Talent Promotion Plan of Shaanxi Province (Grant NO. 2018KJXX-056), the Key Research and Development Projects of Shaanxi Province (Grant NO. 2018ZDCXLSF-03-03-02) and the Science and Technology Innovation Base of Shaanxi Province (Grant NO. 2017KTPT-19) for financial support. References [1] J.J. Xiao, W.G. Li, Y.H. Fan, X. Huang, An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (2012) 364– 383, https://doi.org/10.1016/j.conbuildmat.2011.12.074. [2] C. Liu, H.W. Liu, C. Zhu, G.L. Bai, On the mechanism of internal temperature and humidity response of recycled aggregate concrete based on the recycled aggregate porous interface, Cem. Concr. Compos. 103 (2019) 22–35, https:// doi.org/10.1016/j.cemconcomp.2019.04.016. [3] Y. Wang, S.H. Zhang, D.T. Niu, L. Su, D.M. Luo, Strength and chloride ion distribution brought by aggregate of basalt fiber reinforced coral aggregate concrete, Constr. Build. Mater. 234 (2020) 117390, https://doi.org/10.1016/ j.conbuildmat.2019.117390. [4] Y.Y. Wang, J.J. Huang, D.J. Wang, Y.F. Liu, Z.J. Zhao, J.P. Liu, Experimental study on hygrothermal characteristics of coral sand aggregate concrete and aerated concrete under different humidity and temperature conditions, Constr. Build. Mater. 230 (2020) 1–17, https://doi.org/10.1016/j.conbuildmat.2019.117034.
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