Mechanical and durability properties of sustainable self-compacting concrete with recycled concrete aggregate and fly ash, slag and silica fume

Construction and Building Materials 231 (2020) 117115

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Mechanical and durability properties of sustainable self-compacting concrete with recycled concrete aggregate and fly ash, slag and silica fume Zhanggen Guo a,⇑, Tao Jiang a, Jing Zhang a, Xiangkun Kong a, Chen Chen a,c, Dawn E. Lehman b a b c

School of Civil Engineering, Nanjing Tech University, Nanjing 211800, China Department of Civil and Environmental Engineering, University of Washington, Seattle 98195-2700, USA JiangSu HongJi energy saving new technology Co., LTD, China

h i g h l i g h t s
Sustainable SCC made with recycled concrete aggregate and fly ash, slag, silica fume.
Using 50% and 100% RCA declined the compressive and splitting tensile strength.
Using a combination of fly ash, slag, silica fume improved the mechanical properties.
Shrinkage of RA-SCC increases with an increase in RCA content.
SCC with high volumes of SCM had higher resistance to freezing and thawing.

a r t i c l e

i n f o

Article history: Received 25 January 2019 Received in revised form 17 September 2019 Accepted 28 September 2019

Keywords: Self-compacting concrete (SCC) Recycled aggregate concrete (RAC) Supplementary cementitious material (SCM) Mechanical properties Durability

a b s t r a c t This research aims to maximize the content of supplementary cementitious material (SCM) and recycled concrete aggregate (RCA) in self-compacting concrete (SCC) by using a combination of fly ash, slag and silica fume. A sustainable SCC was proposed by substantially substituting natural aggregates with RCA and cement with SCM. A total of 23 mixes, including binary, ternary and quaternary mixes were prepared. Binary mixes were prepared with fly ash and ternary mixes were prepared with fly ash and slag. Quaternary mixes were blended with fly ash, slag, silica fume. The mechanical and durability properties were studied. The effect of RCA and SCM was investigated as well as using a combination of fly ash, slag and/or silica fume. The test results indicate that the proposed combination of fly ash, slag and silica fume can compensate for the detrimental effect of RCA and significantly improve the mechanical and durability properties of SCC with RCA, thus optimize the sustainability performance of SCC by minimizing cement and natural resources content. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, concrete is the most widely used construction material in China. According to China concrete & Cement-based Products Association, 5.51 billion tons of concrete were produced in 2017, which consumed 5 billion tons of natural aggregates (i.e., limestone and river sand) and released 0.83 billion tons of CO2, resulting in serious environmental problems [1]. CO2 emitted from the production and transportation of concrete is about 10% of the total man-made CO2 in the environment. The carbon footprint related to concrete production is therefore considerably high [2]. ⇑ Corresponding author at: School of Civil Engineering, Nanjing Tech University, 30 Puzhu Road(S), Nanjing, 211800, China. E-mail addresses: [email protected], [email protected] (Z. Guo). https://doi.org/10.1016/j.conbuildmat.2019.117115 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

On the other hand, construction and demolition (C&D) wastes have been increased rapidly in China over the past ten years. In 2015, there were approximately 3.9 billion tons of C&D wastes generated in China, resulting in serious environmental problem [3]. Therefore, the sustainable development of concrete in terms of reducing CO2 emissions and C&D wastes, and conservation of non-renewable natural resources is an imperative of Chinese contemporary concrete industry and has attracted widespread research interest around the world over the last several decades [4]. Recycling C&D wastes as recycled aggregate to produce RAC can limit the consumption of non-renewable natural resources and minimize waste and its associated releases, which will contribute to both environmental and resource preservations. It is thus an effective method to foster sustainable development of concrete industry [5]. The mechanical and durability properties of RAC,

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the structural performances of RAC elements and structures have been extensively studied around the world over the last twenty years [6–10]. Majority of previous studies indicated that the adhered mortars on RCAs lead to a higher porosity and water absorption capacity as well as lower strength of the RAC, compared to conventional concrete. Recently, several enhancement methods, including mechanical grinding, heat grinding, pre-soaking in water, carbonation treatment and pozzolan slurry, were proposed to improve the mechanical and durability properties of RCAs [5]. Nevertheless, a lot of programs have revealed that RAC is feasible for application in practical engineering and some applications of RAC in building structures have been successfully conducted in China [11,12]. In addition, substitution of ordinary Portland cement (OPC) with SCMs in the production of concrete can decline carbon emissions, thus improve the green footprint in the concrete production [13]. Several SCMs, such as fly ash (FA), ground granulated blast furnace slag (SL) and silica fume (SF) are waste materials from industrial processes, which generally have hydraulic and/or pozzolanic properties. A considerable number of programs have been carried out to investigate the influence of replacing OPC with SCMs on the fresh, mechanical and durability properties of the concrete. It was concluded that when SCMs were used at optimal levels, they can significantly enhance the fresh and hardened state properties of the concrete [14,15]. However, concrete incorporating various SCMs generally have slower strength development compared to non-SCM concrete, which is mainly attributed to the reason that pozzolanic reaction is much slower than hydraulic reaction [3,14]. During the past two decades, SCMs have been successfully and widely used in the concrete production around the world [16–18]. The successful application of SCMs in concrete production can not only reduce the waste materials and their associated environmental impacts, but also minimize OPC and its associated CO2 releases, thus consist of environmental and economic benefits towards the sustainability goal of the concrete industry [14]. More recently, several studies have been conducted to study the possibility of using RCAs to produce SCC. Pereira-de-Oliveira et al. [19] and Grdic et al. [20] investigated the influence of RCAs on the fresh and hardened state properties of SCC and concluded that the incorporation of RCAs slightly jeopardized the workability and strength of the SCC mixes. It is viable to replace natural coarse aggregates (NCAs) by RCAs to produce SCC. The effect of RCAs on the compressive and shear strength of high-strength SCC was assessed by Fakitsas et al. [21] and it was concluded that SCC mixes made with various RCAs have superior compressive strength. Kebaïli et al. [22] found that substituting NCAs with RCAs impaired the self-compacting ability of SCC which is mainly attributed to the angular shape and rough surface texture of RCAs. Kou and Poon [23] investigated the fresh, hardened and durability properties of SCC incorporating recycled coarse and fine aggregates (RFAs) and concluded that both RCAs and RFAs can be successfully used to produce SCC. Gesoglu et al. [24] studied the failure mechanism and the drying shrinkage of SCC incorporating RCAs and/or RFAs and found that the incorporation of RCAs and/or RFAs increased the drying shrinkage of SCC. Carro-Lópeza [25] studied the rheology of SCC with RFAs and concluded that SCC mixes made with RFAs have suitable passing and filling ability, superior compressive strength, with an RFA replacement ratio up to 20%. Khodair and Bommareddy [26] tested twenty SCC mixtures with various FA, SL and RCAs and concluded that the compressive and tensile strength of recycled aggregate SCC (RA-SCC) mixes decreased with increasing RCA replacement ratio. In addition, partially replacing cement with SCMs reduced the compressive strength, but enhanced the resistance to chloride permeability of SCC mixes. These previous investigations have indicated that RCAs can be successfully used to replace natural aggregate to produce SCC. A

sustainable and green SCC mixes is proposed in this paper by replacing natural aggregate with RCAs and OPC with SCMs. The replacement of OPC with industrial waste materials and natural aggregate with C&D wastes in concrete can not only effectively decrease the CO2 emissions and the carbon footprint of concrete but also conserve non-renewable natural resources. In addition, the waste disposal sites created by the demolition of old structures and industrial by-products can be reduced as well as their associated environmental impacts.

2. Research objective This research aims to maximize the replacement level of SCMs and RCAs in SCC with the objective of achieving superior fresh state and comparable mechanical properties, thus optimize the sustainability aspect of SCC mixes by minimizing OPC and non-renewable natural resources content. A total of 23 SCC mixes, including binary mixes (cement and FA), ternary mixes (cement and FA, SL) and quaternary mixes (cement and FA, SL, SF) were prepared by substantially replacing NCAs with RCAs and OPC with SCMs. The workability of SCC mixtures incorporating high volumes of RCAs and SCMs has been studied in a preceding paper [27]. The test results indicated that RCAs slightly impaired the workability of RASCC mixes, due to the adhered mortars, which increased the friction between aggregate particles and the harshness of SCC mixes. In addition, the surface pores of RCAs absorbed a small amount of cement paste, resulting in a decrease in the flowability of the SCC mixes. Nevertheless, RA-SCC with high volumes of SCMs achieved good filling and passing ability, adequate cohesion and maintained sufficient resistance to segregation as well as satisfied the requirements prescribed by European SCC specification [28] for workability. This paper investigated the mechanical and durability properties of RA-SCC with considerable content of RCAs (75%, 100%) and high volumes of SCMs (50% and 75%). The effects of RCAs replacement of NCAs and the replacement of OPC with SCMs with different proportions on the mechanical and durability characteristics of SCC were assessed. In particular, the influence of using a combination of FA, SL and/or SF was studied in detail.

3. Experimental program 3.1. Material The materials used in this test were comprised of water, cement, sand, NCAs, RCAs, SCMs and chemical admixture. The locally available natural river sand was used as fine aggregate. The apparent density, water absorption in saturated surface dry (SSD) condition and fineness modulus of river sand is 2610 kg/m3, 1.1% and 2.43, respectively, which were measured according to Chinese code (JGJ 52-2006) [29]. Crushed limestone obtained from a local supplier was used as NCA, with a nominal maximum size of 20 mm. The RCAs were obtained from the tested concrete components at our structural laboratory with an original compressive strength of 30 MPa. Firstly, the tested concrete components were crushed by drilling machine and the reinforcements were removed. The aggregates were then crushed by a jaw crusher. At last, the RCAs were obtained by sieving with the maximum and minimum particle size of 20 mm and 5 mm, respectively. The gradation curves of natural and recycled coarse aggregate were determined and are displayed in Fig. 1, indicating RCAs exhibited a continuous granulometric curve and fulfilled the requirements prescribed by Chinese code (JGJ 52-2006) [29]. Furthermore, the physical and mechanical properties of NCAs and RCAs were measured and are illustrated in Table 1. The FA, SL, SF and OPC are commercially available in China. OPC with a 28d nominal compressive strength of 42.5 MPa and SCMs (FA, SL and SF) were used as binding materials. The chemical analysis and physical properties of cement and mineral admixtures were determined and are summarized in Table 2. To achieve high workability, high-range water-reducing admixture (HRWRA) is usually added into SCC mixtures. A polycarboxylate superplasticizer (SP) with a specific gravity between 1.010 and 1.120 reported by the manufacturer was used as the chemical admixture. The maximum suggested dosage of the polycarboxylate

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to each mix to represent the RCA and SCM replacement level and SCM combination type, which was coded A (B, C)-x-y-1(2,3) (x = RCA replacement ratio; y = SCM replacement ratio, 1 = binary mixes, 2 = ternary mixes, 3 = quaternary mixes). A0-0 is the control specimen and did not contain either RCAs and SCMs. The additional amount of free water as shown in Table 3 was associated with the higher water absorption ability of RCAs and was calculated according to the SSD conditions. The total binder content is 520 kg/m3 and was kept constant. The superplasticizer amount is increased as the RCA content increased. The rougher surface texture and porosity of RCAs are mainly responsible for the increase in superplasticizer demand of RA-SCC mixes. 3.3. Casting, curing and testing

Fig. 1. The fraction gradation curve of natural and recycled coarse aggregates.

Table 1 The properties of coarse aggregates. Properties

NCA

RCA

Dry bulk density (kg/m3) Surface dry specific density (kg/m3) Void fraction (%) Water absorption (%) fragmentation coefficient

1465 2698 49.5 0.5 1.7

1387 2594 46.0 4.9 30

superplasticizer provided by its manufacturer is 1.43% of cement (by weight). The SP used in this test was determined based on Chinese code GB50119-2013 ‘‘Technical specification for application of concrete admixture” [30] and similar workability.

3.2. Concrete mixtures The mix design of RA-SCC mixes was carried out based on EFNARC (2005) [28] and Chinese code JGJ/T 283-2012 ‘‘Technical specification for application of selfcompacting concrete” [31], Chinese code DG/TJ08-2018-2007 ‘‘Technical code on the application recycled concrete” [32] as well as in conjunction with the method proposed by Rajhans et al. [33]. This research focuses on high replacement percentages for RCAs and SCMs as well as replacing OPC by a combination of FA, SL and SF. SCC with different combinations of varying percentages of RCA and different binding materials were prepared. The replacement ratio of RCA is 50 and 100% (by weight) as well as the replacement level of SCM is 50 and 75% (by weight). A total of 23 mixes, including control mixes, binary mixes (cement and FA), ternary mixes (cement and FA, SL) and quaternary mixes (cement and FA, SL, SF) were developed. In addition, to get a wide range of calcium oxide (silicon dioxide + aluminum oxide), SCC mixes with different contents of FA, SL and SF were produced. Binary mixes were developed by the addition of 50% or 75% FA. Ternary mixes were prepared by the addition of the same content of FA and SL (i.e., 25% or 37.5%). Quaternary mixes were produced by the addition of FA, SL and SF, with the same proportion of FA, SL and SF. The mix proportions and design parameters of all mixtures are summarized in Table 3. The mixes were divided into three series, which were named A, B and C, based on their water to binder (w/b) ratio: 0.35, 0.40, 0.45. A designation was given

A rotary drum mixer was used to mix the proportioned materials. The mixing sequence included three steps: first, RCAs, natural coarse and fine aggregate, and binder materials (SCMs and OPC) were added into the mixer and dry mixed for about 2 min. Second, 90% of the water was placed into the mixer and mixed for another 2 min. At last, superplasticizer and last 10% of the water was mixed and then added into the mixtures and mixed for 3 min until a homogeneous mixture was obtained. The procedure of mixing was repeated for all mixtures. All concrete specimens were cast in steel molds without compaction and vibration. After casting, according to Chinese code (GB/T 50081-2002) [34], all concrete specimens were kept in a room with a temperature of (20 ± 2) °C and a relative humidity of 95% for 24 h before being demolded and were then cured in a room with standard condition (i.e., an average temperature of (20 ± 2) °C and a relative humidity of 95%) until the day of testing. The mechanical properties (i.e., cube compressive strength, axial compressive strength and splitting tensile strength) of hardened SCC mixes were determined according to Chinese code-Standard for Test Method of Mechanical Properties on Ordinary Concrete [34]. For each SCC mixture, three 150 mm cubes were produced to determine cube compressive strength and three prisms with a dimension of 150 mm  150 mm  300 mm were prepared to measure axial compressive strength. Furthermore, three 150 mm cubes were used to determine the splitting tensile strength. All mechanical properties were carried out on hardened concrete mixes at the age of 28 days. Drying shrinkage tests were implemented according to Chinese code-Standard for test methods of long-term performance and durability of ordinary Concrete [35]. For each SCC specimen, three prisms with a dimension of 75 mm  75 mm  285 mm were prepared to measure the drying shrinkage. After removing the concrete specimens from the standard curing chamber, the initial length of each SCC specimen was determined. The prisms were then kept in an environmental condition with a temperature of (20 ± 2) °C and a relative humidity of (60 ± 5) % during the entire measurement program. The drying shrinkage measurements were carried out at 1, 3, 7, 14, 28, 45, 60, 90 and 120 days. Two steel discs were placed on each of the longitudinal faces of the prisms to measure the drying shrinkage. The 28-day freeze-thaw test of hardened SCC mixtures was carried out in accordance with Chinese code [35]. For each SCC specimen, three 100 mm cubes were tested for 25 and 50 cycles, respectively. In a single cycle, the SCC mixtures were frozen in air from 20 °C to 18 °C beyond 4 h and were then thawed in water from 18 °C to 20 °C beyond 4 h. The changes in weight and cube compressive strength of each SCC mixture were calculated after 25 and 50 freeze-thaw cycles and the average value was calculated too.

4. Results and discussions 4.1. Mechanical properties A summary of the hardened concrete test results, including cube compressive strength, tensile splitting strength and axial compressive strength of all SCC mixes is given in Table 4.

Table 2 Physical properties and chemical analysis of OPC, fly ash, slag, silica fume. Compounds

Cement

Fly ash

Slag

Silica fume

SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 Loss on ignition specific surface area (m2/kg) Specific gravity (g/m3)

20.73 4.78 3.87 64.73 2.05 0.50 0.10 2.47 1.10 410 3.12

52.79 49.47 8.58 5.61 0.90 1.37 0.66 0.62 3.51 3000 2.43

31.76 14.82 1.58 44.01 5.63 0.39 0.34 1.47 2.27 520 2.25

95.48 0.40 0.03 0.44 0.40 0.25 0.32 0.42 0.90 20,000 2.08

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Table 3 Concrete mixtures composition. Mix series

Mix code

OPC

FA

SL

SF

A

0-0 50-0 50-50-1 50-50-2 50-50-3 50-75-1 50-75-2 50-75-3 100-0 100-50-1 100-50-2 100-50-3 100-75-1 100-75-2 100-75-3

520 520 260 260 260 130 130 130 520 260 260 260 130 130 130

0 0 260 130 104 390 195 156 0 260 130 104 390 195 156

0 0 0 130 104 0 195 156 0 0 130 104 0 195 156

100-0 100-75-1 100-75-2 100-75-3

455 113.7 113.7 113.7

0 341.25 170.63 136.5

100-0 100-75-1 100-75-2 100-75-3

404 101 101 101

0 303 151.5 121.2

w/b = 0.35

B w/b = 0.40

C w/b = 0.45

Binder

Water

NCA

RCA

Sand

Superplasticizer

Additional water

0 0 0 0 52 0 0 78 0 0 0 52 0 0 78

182 182 182 182 182 182 182 182 182 182 182 182 182 182 182

867 433 433 433 433 433 433 433 0 0 0 0 0 0 0

0 416.4 416.4 416.4 416.4 416.4 416.4 416.4 832 832 832 832 832 832 832

785 785 785 785 785 785 785 785 785 785 785 785 785 785 785

4.02 4.16 5.72 5.43 6.76 5.94 4.68 6.80 4.72 4.78 5.67 6.86 7.40 7.28 7.57

0 24.7 24.7 24.7 24.7 24.7 24.7 24.7 32.9 32.9 32.9 32.9 32.9 32.9 32.9

0 0 170.6 136.5

0 0 0 68.3

182 182 182 182

0 0 0 0

832 832 832 832

785 785 785 785

3.19 4.01 1.37 3.19

32.9 32.9 32.9 32.9

0 0 151.5 121.2

0 0 0 60.6

182 182 182 182

0 0 0 0

832 832 832 832

785 785 785 785

2.83 3.20 1.21 3.63

32.9 32.9 32.9 32.9

Table 4 Cube and axial compressive strength, tensile splitting strength of all SCC mixes. Mix No.

fcu,28 (MPa)

Mix No.

fcu,28 (MPa)

Mix No

fcu,28 (MPa)

A-0-0 A-50-0 A-50-50-1 A-50-50-2 A-50-50-3 A-50-75-1 A-50-75-2 A-50-75-3

53.45 46.54 18.04 33.51 40.31 7.17 19.66 35.54

B-100-0 B-100-75-1 B-100-75-2 B-100-75-3 – – – –

29.81 13.89 18.35 26.23 – – – –

C-100-0 C-100-75-1 C-100-75-2 C-100-75-3 – – – –

19.75 9.63 12.07 18.86 – – – –

Mix No.

fcu,28 (MPa)

fts,28 (MPa)

fcp,28 (MPa)

A-0-0 A-50-0 A-100-0 A-100-50-1 A-100-50-2 A-100-50-3 A-100-75-1 A-100-75-2 A-100-75-3

53.45 46.54 43.89 21.00 38.38 49.44 13.64 30.84 42.75

4.39 3.49 3.32 1.57 2.75 3.66 0.96 1.67 2.89

39.34 35.88 31.04 15.50 26.21 37.89 10.45 22.77 33.96

It is obvious from Table 4 that the addition of RCAs and SCMs influenced the mechanical properties of SCC mixes. The hardened test results are presented along with their graphical figures and analysis. In addition, a comparative analysis of SCC prepared with various RCAs and SCMs is also presented. In particular, the influences of RCA replacement level and using a combination of FA, SL and/ or SF were analyzed in detail. 4.1.1. Cube compressive strength The 28-day cube compressive strength shown in Table 4 is the average value of three SCC mixtures each time. Fig. 2 illustrates the effect of w/b ratio on the cube compressive strength of SCC specimens, where cube compressive strength is plotted against w/b ratio under different RCA and SCM replacement level. It is clearly evident from Fig. 2 that the 28-day cube compressive strength of SCC mixtures decreased with an increase in the w/b ratio, which is in good agreement with the theoretically expected

performance. Furthermore, as also can be seen from Fig. 2, the reduction in cube compressive strength with the increase of w/b ratio for non-SCM SCC mixes is less than that of SCC mixes made with SCMs, indicating that SCMs are more sensitive to the w/b ratio. The influence of RCA replacement percentage on the cube compressive strength of RA-SCC is shown in Fig. 3, where compressive strength is plotted against RCA replacement ratio under different SCM replacement levels. It is evident from Fig. 3 that the cube compressive strength of non-SCM RA-SCC mixtures declined with the RCA replacement ratio increasing. The compressive strength of specimen A-50-0 (50% RCA) and A-100-0 (100% RCA) was 12.9% and 17.9% less than that of the control specimen A-0-0, respectively, which is consistent with the conclusion presented by Etxeberria et al. [36] that a reduction of 20%~25% was obtained for RASCC mixes. This is mainly due to the weaker interfacial zone and porosity as well as higher water absorption of RCAs, compared to

Z. Guo et al. / Construction and Building Materials 231 (2020) 117115

Fig. 2. Influence of w/b ratio on the cube compressive strength.

5

Fig. 4. Influence of SCM content on the cube compressive strength in series A.

Fig. 3. Effect of RCA replacement ratio on the cube compressive strength in series A.

74.5% less than that of control specimen A-100-0, respectively, indicating that substantially replacing OPC with FA in SCC resulted in considerably lower cube compressive strength. The same trend was not observed for quaternary mixes. However, it should be noted that quaternary RA-SCC mixes prepared with FA, SL, SF have comparable compressive strength to control normal SCC. The cube compressive strength of specimen A-100-50-3 (50% SCM) is 12.6% higher than that of the control specimen A-100-0, which indicates that replacing OPC by a combination of FA, SL and SF resulted in an increase in the compressive strength. This is mainly attributed to the pozzolanic reaction and synergy between these SCMs with different particle sizes. The relationship between compressive strength and SCM combination type is demonstrated in Fig. 5. It is evident from Fig. 5 that at a given SCM replacement level, replacing the cement by a combination of FA, SL and/or SF has consistently increased the compressive strength of RA-SCC [38,39]. Using 50% or 75% FA resulted in the lowest compressive strength in comparison with other mixes for the cases of 50 and 75% RCA. In comparison with binary mixes, the cube compressive strength of ternary and

natural coarse aggregate. In addition, it is interesting to find that the compressive strength of RA-SCC mixes with various SCMs increased slightly with an increase in the RCA content as shown in Fig. 3, which is in contrast with the results obtained for nonSCM RA-SCC mixes. As mentioned above, more polycarboxylate superplasticizer was added into RA-SCC mixes with higher RCA replacement ratio (Table 3), which is mainly responsible for the increase of the cube compressive strength. This is consistent with the conclusion obtained by Pereira et al. [37] that superplasticizer can effectively improve the mechanical properties of RAC. The influence of SCM content on the cube compressive strength is depicted in Fig. 4, where compressive strength is plotted against SCM content under different RCA replacement levels and different SCM combination type. It is obvious from Fig. 4 that for binary and ternary mixes, an increase of SCM replacement with OPC is associated with a reduction in cube compressive strength, which is mainly attributed to the limited CaO of FA, resulting in the delayed hydraulic reaction. In addition, the reduction in cube compressive strength with the increase of SCM content for ternary mixes is less than that of binary mixes with FA. Due to the hydraulic activity and slag activity index of SL, the addition of SL increases the compressive strength of concrete. The cube compressive strength of specimen A-100-50-1 (50% FA) and A-100-75-1 (75% FA) is 60.7% and

Fig. 5. Relationship between SCM combination type and cube compressive strength.

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quaternary RA-SCC mixes increased by 83% to 174% and 123% to 496%, respectively. According to the test results, the conclusion can be drawn that using a combination of FA, SL and/or SF can compensate for the detrimental effect of RCAs and increase the cube compressive strength of RA-SCC mixes. This is mainly attributed to several reasons: firstly, several previous programs have concluded that SL has hydraulic activity with some activators (cement) in addition to certain pozzolanic activity [40]. Secondly, SF with a high reactivity can react with calcium hydroxide (Ca(OH)2) to form calcium silicate hydrate (C-S-H) and densified silica fume with a fine particle can fill the surface pores of recycled concrete (RC) and the voids between aggregate particles, thus enhance the compactness of the structure, resulting in an increase in the compressive strength of RA-SCC mixes. At last, the remarkable synergistic effect between FA, SL and SF can lead to a higher packing density and denser microstructure, thus contributing to the increase of the compressive strength of RA-SCC. Celik et al. [41] also obtained a similar synergistic effect in SCC mixture made with blended cement containing FA and limestone powder. According to the test results, quaternary RA-SCC mixes with 100% RCA and 75% SCM and a w/b of 0.35 achieved a 28-day cube compressive strength over 35 MPa, which is acceptable for certain engineering applications (i.e., conventional concrete structures (beams, columns, slabs)). Thus, it can be concluded that a less environmentally burdensome product can be developed with more cement replacement by using a combination of FA, SL and SF and more natural aggregate replacement by RCA. It is well known that the compressive strength of concrete is highly related to water to cement (W/C) ratio and generally can be calculated as follows [42]: ðW=CÞ

f c ¼ k1 =k2

ð1Þ

In an earlier work, the authors proposed a similar formula to calculate the compressive strength of concrete made with recycled coarse and fine aggregate, as a function of W/C ratio [43]. ðW=CÞ

f cu ¼ k1 =k2

 ð1  k3  W RCA  RRCA  k4  W RFA  RRFA Þ

ð2Þ

where WRCA and WRFA is the water absorption of recycled coarse and fine aggregate, respectively. RRCA and RRFA is the replacement percentage of recycled coarse and fine aggregate, respectively. k1, k2, k3 and k4 are empirical constants and were obtained by nonlinear regression using test results. Khodair and Bommareddy [26] proposed a linear regression model to predict the compressive strength of SCC made with RCAs and high volume of FA, and SL.

f cu ¼ A0 þ A1  RRCA þ A2  FA þ A3  SL þ A4  Age

As shown in Eq. (5), RCA and FA, SL had a negative effect on the compressive strength of SCC mixes, which is consistent with the results obtained by Khodair and Bommareddy [26]. The cube compressive strength of RA-SCC calculated by Eq. (5) and the comparison with test results are illustrated in Fig. 6. It is apparent that there is a good agreement between predictions and test results. The relationship between test results and calculations is almost linear. 4.1.2. Axial compressive strength The 28-day axial compressive strength of RA-SCC mixes in series A (w/b = 0.35) with various SCMs are summarized in Table 4. As expected, the axial compressive strength of RA-SCC reduced with the increase of RCA replacement ratio as shown in Table 4. Compared to control normal SCC, the axial compressive strength of A50-0 (50% RCA) and A-100-0 (100% RCA) declined by 8.8% and 20.1%, respectively. The relationship between axial compressive strength and SCM content is illustrated in Fig. 7. The axial compressive strength of ternary and binary RA-SCC decreased as SCM content increases as shown in Fig. 7. Compared to specimen A-100-0, the reduction in axial compressive strength for binary and ternary mixes ranges from 50.1% to 197%, 15.6% to 26.6%, respectively. In addition, it should be noted that quaternary mixes achieved comparable axial compressive strength to replicate control SCC mixes as shown in Fig. 7, which is similar to the cube compressive strength. The quaternary mixes A-100-50-3 and A-100-75-3 show axial compressive strength increases of 9.4–22.1% in relation to replicate non-SCM specimen A-100-0, indicating that using a combination of FA, SL and SF compensated for the adverse effect of RCAs and significantly increased the axial compressive strength of RA-SCC. The reason is the same as discussed in the case of cube compressive strength. The axial compressive strength to cube compressive strength ratio (f cp =f cu ) of RA-SCC was calculated and is displayed in Table 5. The f cp =f cu of RA-SCC mixes ranges from 0.68 to 0.79 as shown in Table 5 and there is no distinct difference between normal SCC and RA-SCC mixes with various SCMs, which reveals that RCAs and SCMs have nearly no influence on thef cp =f cu . 4.1.3. Splitting tensile strength The 28-day splitting tensile strength of RA-SCC mixes in series A (w/b = 0.35) with various SCMs is presented in Table 4. Each presented value is the average of three measurements. The splitting tensile strength decreased with an increase in the RCA replacement ratio as shown in Table 4. The splitting tensile strength of A-50-0

ð3Þ

where A0, A1, A2, A3 and A4 are empirical constants. Based on Eqs. (2) and (3), an empirical formula was proposed in this paper to predict the cube compressive strength (f cu ) of SCC incorporating RCAs and FA, SL, SF.

f cu;28 ¼

k1 w=b

k2

 ðA0 þ A1  RRCA þ A2  FA þ A3  SL þ A4  SFÞ

ð4Þ

where RRCA , FA, SL, SF is the RCA, fly ash, slag, silica fume replacement ratio, respectively. A0, A1, A2, A3, A4, k1 and k2 are constants and were obtained by nonlinear regression analysis using test results, which is 53.45, 0.02, 0.59, 0.11, 0.57, 17.15, 3772.9 respectively. The correlation coefficient R2 = 0.93. Therefore, the compressive strength of RA-SCC with SCMs can be calculated as follows:

f cu;28 ¼

17:15

 ð53:45  0:02  RRCA  0:59  FA  0:11  SL 3772:9w=b þ 0:57  SFÞ

ð5Þ

Fig. 6. Comparison of predicted 28-day cube compressive strength and test results Note: A-50-50, A-50-75, A/B/C-100-75 including binary, ternary and quaternary mixes.

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Z. Guo et al. / Construction and Building Materials 231 (2020) 117115

Fig. 7. Effect of SCM content on the axial compressive strength in series A.

(50% RCA) and A-100-0 (100% RCA) is less than that of normal SCC A-0-0 by 20.5% and 24.4%, respectively. The effect of SCM content on the splitting tensile strength is demonstrated in Fig. 8. The splitting tensile strength of ternary and binary mixes reduced with the SCM content increasing as shown in Fig. 8. Ternary and binary mixes show strength reduction of 17.2–89% in relation to replicate non-SCM SCC A-100-0. Additionally, it should be noted that a considerable increase in splitting tensile strength is observed for quaternary mixes, indicating that using a combination of FA, SL and SF significantly enhance the splitting tensile strength of RA-SCC. The splitting tensile strength of quaternary mixes A-100-50-3 is increased by 10.2%, when compared to non-SCM mixes A-100-0. Over all, the effect of RCAs and SCMs on the splitting tensile strength is similar to that on the cube compressive strength and axial compressive strength. The reason is the same as discussed in the case of cube compressive strength. The splitting tensile strength to cube compressive strength ratio (f ts =f cu ) of RA-SCC mixtures was calculated and summarized in Table 5. As can be seen from Table 5, the f ts =f cu of RA-SCC ranges from 0.054 to 0.076. Additionally, the ratios of RA-SCC are less than that of control normal SCC, which indicates that although the addition of RCAs in SCC reduced both cube compressive and splitting tensile strength, the decrease in splitting tensile strength was more pronounced than that in cube compressive strength, which is mainly due to the weak bond between mortar and aggregate and lower quality and porosity of RCAs. The similar result was obtained in the research conducted by Khodair and Bommareddy [26]. 4.1.4. Relationship between different strength indexes As mentioned above, the ratios of axial compressive strength to cube compressive strength and splitting tensile strength to cube

Fig. 8. Relationship between splitting tensile strength and SCM replacement ratio.

compressive strength of RA-SCC mixtures are similar to those of normal SCC. Chinese code-Design of concrete structures [44] proposed an equation to calculate the relationship between cube compressive strength and corresponding axial compressive strength of conventional concrete:

f cp ¼ 0:76f cm

ð6Þ

As shown in Table 5, the ratio of measured axial compressive strength to corresponding cube compressive strength of all RASCC ranges from 0.68 to 0.79, with an average of 0.745, which is just 0.015 less than the value of conventional concrete (0.76). Therefore, the formula proposed to calculate relationship between cube compressive strength and corresponding axial compressive strength for normal SCC can still be used for RA-SCC, which is in accordance with the conclusion obtained by Lotfy and Al-Fayez [45]. For conventional concrete, Chinese code [44] and ACI Committee 318 [46] proposed a similar equation to calculate the relationship between splitting tensile strength and corresponding cube compressive strength, respectively: 3=4

f ts;150 ¼ 0:19f cu

ð7Þ

1=2

f ts ¼ 0:49f c

ð8Þ

where f ts , f c is the splitting tensile strength and compressive strength of cylinder specimen. The formulas suggested by Chinese code [44] and ACI Committee 318 [46] both indicated that the splitting tensile strength of concrete mixes is highly related to compressive strength and can be calculated as follows: b

f ts ¼ af cu

ð9Þ

Table 5 The comparison between calculated data and measured value. Mix No.

Measured value f ts

Eq. (10) f ts

Error

Measured value f cp

predicted f cp

Error

f ts =f cu

f cp =f cu

A-0-0 A-50-0 A-100-0 A-100-50-1 A-100-50-2 A-100-50-3 A-100-75-1 A-100-75-2 A-100-75-3

4.39 3.49 3.32 1.57 2.75 3.66 0.96 1.67 2.89

4.14 3.48 3.23 1.28 2.73 3.75 0.74 2.07 3.13

5.69% 0.3% 2.7% 18.5% 0.7% 2.4% 22.9% 19.3% 8.3%

39.34 35.88 31.04 15.50 26.21 37.89 10.45 22.77 33.96

40.62 35.37 33.36 15.96 29.17 37.57 10.37 23.44 32.49

3.3% 1.4% 7.5% 3.0% 11.3% 0.8% 0.8% 2.9% 4.3%

0.082 0.075 0.076 0.075 0.072 0.074 0.070 0.054 0.068

0.74 0.77 0.71 0.74 0.68 0.77 0.77 0.74 0.79

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Z. Guo et al. / Construction and Building Materials 231 (2020) 117115

The relationship between splitting tensile strength and cube compressive strength of RA-SCC mixtures is proposed in this paper. Eq. (9) was used to fit the relationship between splitting tensile strength and cube compressive strength for RA-SCC mixtures. The coefficient a and b were considered as the main variables and obtained by nonlinear regression analysis using test results obtained in this test is 0.028 and 1.26, respectively. The corresponding correlation coefficient R2 = 0.96. Thus, the relationship between splitting tensile strength and cube compressive strength of RA-SCC mixtures with various SCMs can be expressed as follows: 1:26

f ts ¼ 0:028f cu

ð10Þ

The predictions using Eq. (10) and the comparison with test results obtained in this test and other research [26,47–49] are demonstrated in Fig. 9. There is a good agreement between predictions and experimental results, as shown in Fig. 9. However, there are limited studies to investigate the sustainable recycled concrete with FA, SL and SF, resulting in limited data. This model will be checked and improved by test results obtained from open literatures in the future. 4.2. Durability performances 4.2.1. Resistance to freezing and thawing Extensive previous research [50] has indicated that the resistance to freezing and thawing of RAC is inferior to that of normal concrete. The freeze-thaw resistance in terms of percent change in weight and cube compressive strength of each RA-SCC mixture

Fig. 9. Comparison between calculated and measured values of splitting tensile strength.

was investigated in this paper. Table 6 summarizes the percent change in weight and cube compressive strength of all RA-SCC mixtures after 25 and 50 freeze-thaw cycles. As shown in Table 6, the compressive strength of all RA-SCC mixes decreased with the increased number of freeze-thaw cycles. In addition, the replacement of natural aggregate to RCAs resulted in a lower residual compressive strength compared to control normal specimen, which is attributed to the higher water absorption ability and porosity of RCAs compared to natural aggregate. RCAs with additional mortars usually absorb a lot of water and the absorbed water develops internal stress upon freezing. When the internal stress exceeds the tensile strength of aggregate, microcracks will be formed inside the concrete. In addition, the compressive strength of RA-SCC binary mixes declined slightly with the SCM content increasing. After 50 cycles, the highest compressive strength loss was found for RA-SCC binary mixes incorporating 75% FA and 100% RCA, namely 29.14%, which did not satisfy the requirement prescribed by Chinese code [35] for strength loss (i.e., 25%). Nevertheless, the strength of ternary and quaternary mixes with 100% RCA reduced by 18.86% and 12.32% after 50 cycles, respectively, which both fulfill the requirement and less than that of binary mixes, indicating that RA-SCC made with FA, SL and/or SF exhibited favorable durability performance and met the requirement prescribed by Chinese code [35]. At last, it is interesting to find that quaternary RA-SCC mixes have almost the same strength loss as the control normal SCC. The reason is the same as discussed in the case of compressive strength that densified silica fume with a fine particle filled the surface pores and the voids between aggregate particles and the synergetic effect between these different particle size SCMs was created, resulting in a compact structure and high strength. Similar test results have been obtained in other studies in which cement was replaced by a combination of SF and FA [49] and FS and SL [51]. As can be seen from Table 6, the weight of normal concrete decreased after 25 cycles, whilst the weight of RA-SCC mixes increased slightly. The increase in weight of RA-SCC mixes is probably due to the reason that at early ages, RCAs absorbed significant amounts of water. The weight of the absorbed water is higher than that of spalled concrete, resulting in an increase in the concrete weight. However, after 50 freeze-thaw cycles, the weight of normal and RA-SCC mixtures both reduced, which is mainly attributed to the reason that RCAs were saturated with water, no more water could be absorbed. The weight of the absorbed water is less than that of spalled concrete. In addition, the weight loss of RA-SCC mixes with SCMs is less than that of non-SCM mixes. Theoretically, the weight loss is related to local surface ‘‘pop-outs”. The pop-out effect usually results from the expansion of saturated aggregate near the surface and the consequent disintegration of the surrounding cement paste [52]. At last, as shown in Table 6, the weight loss of all RA-SCC mixes, which is usually less than 0.5%, is almost neglected and meets the requirement of the maximum acceptable limit of weight loss (i.e., 5%) specified by Chinese code [35].

Table 6 Loss of weight and strength after 25 and 50 freezing and thawing cycles. Mixture notation

Weight loss

Strength loss

25 cycles

A-0-0 A-100-0 A-100-50-1 A-100-75-1 A-100-75-2 A-100-75-3

50 cycles

25 cycles

50 cycles

m1 (g)

m2 (g)

Dm25

m1 (g)

m2 (g)

Dm50

f 1 (MPa)

f 2 (MPa)

Df 25

f 1 (MPa)

f 2 (MPa)

Df 50

2295.1 2305.7 2187.5 2085.0 2220.7 2211.0

2291.7 2316.9 2203.7 2089.3 2220.1 2209.5

0.15% 0.49% 0.74% 0.21% 0.03% 0.07%

2314.1 2312.1 2123.9 1980.1 2173.7 2160.0

2306.9 2305.3 2133.8 1980.1 2170.2 2156.4

0.31% 0.29% 0.47% 0% 0.16% 0.17%

53.34 52.97 27.44 19.87 37.78 49.84

51.28 50.34 24.20 17.39 35.06 46.96

3.86% 4.97% 11.81% 12.48% 7.20% 5.78%

63.47 63.17 26.80 24.26 40.73 61.84

56.17 57.95 20.40 17.79 33.05 54.22

11.5% 8.26% 23.88% 29.14% 18.86% 12.32%

Z. Guo et al. / Construction and Building Materials 231 (2020) 117115

4.2.2. Drying shrinkage The drying shrinkage results, which were measured at 1, 3, 7, 14, 28, 45, 60, 90, 120 days, for all RA-SCC mixtures, are presented in Fig. 10. Each presented value is the average of three measurements. As can be seen from Fig. 10, the drying shrinkage of all RA-SCC mixtures increased with time and majority of shrinkage (i.e., 75% of maximum shrinkage) took place during the first 28 days. The shrinkage then slowed down considerably. After 90 days, the drying shrinkage was almost constant and tends to be stable. The effect of RCA content on the drying shrinkage of RA-SCC mixes is demonstrated in Fig. 11, where drying shrinkage is plotted against RCA replacement ratio at different days. There is a systematic increase in drying shrinkage as the RCA replacement percentage increases as shown in Fig. 11. The total shrinkage strain measured at 120 days using prisms made from RA-SCC with 50%, 100% RCA is 2.94% and 12.13% higher than that of the control specimen. This is mainly attributed to the higher porosity and water absorption of RCAs generated by the additional old cement paste bonded to the recycled aggregate. Most of the previous studies have obtained the same results that the addition of RCAs increased the drying shrinkage of recycled aggregate concrete [26,43].

Fig. 10. The drying shrinkage of RA-SCC mixes.

9

Fig. 12. Effect of SCM content on drying shrinkage.

The effect of SCM content on the drying shrinkage of SCC mixes is plotted in Fig. 12, where drying shrinkage is plotted against SCM replacement ratio at different days and different SCM combination type. It is obvious that the drying shrinkage of RA-SCC mixes prepared with SCMs (binary, ternary and quaternary mixes) is less than that of mixtures made with high content Portland cement. The influence of FA, SL and SF on the drying shrinkage of normal concrete and recycled aggregate concrete has been studied by several investigations [2,7,26]. These previous programs indicated that replacing OPC with SCMs (FA, SL and SF) reduced the creep and shrinkage of concrete mixes, which is mainly attributed to the reason that the adoption of pozzolanic materials decreased the content of cement used, resulting in a reduction in the heat of hydration. The result obtained in this test is consistent with the previous research [53]. In addition, Khodair and Bommareddy [26] found that replacing cement with 50% FA has the most considerable influence in reducing the drying shrinkage strain. The same trend of free shrinkage was also obtained in this test for the specimens containing 50% FA. The magnitude of 120-day drying shrinkage of RA-SCC mixes with 50% SCM is the least as shown in Fig. 12. The reduction in the drying shrinkage of RA-SCC mixes with 50% SCM ranges from 1.4% to 13.5%, compared to control non-SCM specimen. 5. Conclusions This paper presents an experimental research to study the use of RCAs and SCMs as replacement of natural aggregate and cement in the production of SCC. A total of 23 SCC mixes incorporating RCAs (50% and 100%) and high volumes of SCMs (50%, 75%) were prepared. The mechanical and durability properties of RA-SCC were investigated. The test results and analysis stated in this research allow the following conclusions to be drawn:

Fig. 11. Effect of RCA content on drying shrinkage.

1. The cube and axial compressive strength, splitting tensile strength of RA-SCC mixes reduced as the RCA replacement ratio increase, indicating the adverse effect of the addition of RCAs. 2. The use of 50% and 75% FA decreased the compressive strength of RA-SCC. Nevertheless, quaternary RA-SCC with 20% FA, 20% SL, 10% SF or 30% FA, 30% SL, 15% SF achieved comparable strength compared to normal SCC, indicating that using a combination of FA, SL and SF compensated for the adverse effect of RCAs and improve the mechanical properties of RA-SCC mixes.

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Z. Guo et al. / Construction and Building Materials 231 (2020) 117115

3. Based on nonlinear regression using test data, a formula for predicting the cube compressive strength of RA-SCC mixes with high volumes of SCMs is proposed as well as equations to calculate the relationship between cube and axial compressive strength and splitting tensile strength. 4. The addition of RCAs impaired the durability performances of SCC mixes. An increase in RCA replacement percentage is associated with an increase in drying shrinkage and a reduction in freezing and thawing resistance. However, quaternary RA-SCC mixes exhibited comparable durability properties compared to the control normal SCC. 5. Quaternary RA-SCC with high volumes of SCMs exhibited superior mechanical and durability performance as well as resulted in lower cement content without compromising mechanical and durability properties. Thus, the content of RCAs and SCMs can be maximized by using a combination of FA, SL and SF, which will improve the sustainability aspect of RA-SCC by minimizing cement and non-renewable natural resources content.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research is funded by Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. SJCX19_0229), Key laboratory of concrete and pre-stressed concrete structure of Ministry of Education (Grant No. CPCSME2017-04) and National Natural Science Foundation of China (Grant No. 50708045). The authors wish to gratefully acknowledge the support of these organizations for this research. References [1] Y.Y. Hu, China gravel industry in transition——talk on the future development trend of the industry, China Build. Mater. 07 (2018) 36–38 (in Chinese). [2] K. Kuder, D. Lehman, J. Berman, G. Hannesson, R. Shogren, Mechanical properties of self consolidating concrete blended with high volumes of fly ash and slag, Constr. Build. Mater. 34 (2012) 285–295, https://doi.org/10.1016/ j.conbuildmat. 2012.02.034. [3] M. H. Fu, Investigation on modifications and applications of recycled fine aggregate prepared from demolition concrete, Southeast university, Nanjing, China, (2016), pp. 2-4 (in Chinese). [4] G. Long, Y. Gao, Y. Xie, Designing more sustainable and greener selfcompacting concrete, Constr. Build. Mater. 84 (2015) 301–306, https://doi. org/10.1016/j. conbuildmat.2015.02.072. [5] C.J. Shi, Y.K. Li, J.K. Zhang, W.G. Li, L.L. Chong, Z.B. Xie, Performance enhancement of recycled concrete aggregate - a review, J. Cleaner Prod. 112 (2016) 466–472, https://doi.org/10.1016/j.jclepro.2015.08.057. [6] B. Wu, Z. Li, Mechanical properties of compound concrete containing demolished concrete lumps after freeze-thaw cycles, Constr. Build. Mater. 155 (2017) 187–199, https://doi.org/10.1016/j.conbuildmat.2017.07.150. [7] M. Amario, C.S. Rangel, M. Pepe, R.D. Toledo, Optimization of normal and high strength recycled aggregate concrete mixtures by using packing model, Cem. Concr. Compos. 84 (2017) 83–92, https://doi.org/10.1016/j. cemconcomp.2017.08.016. [8] X. Shan, J. Zhou, V.W.-C. Chang, E.-H. Yang, Life cycle assessment of adoption of local recycled aggregate and green concrete in Singapore perspective, J. Cleaner Prod. 164 (2017) 918–926, https://doi.org/10.1016/j. jclepro.2017.07.015. [9] C.M. Mah, T. Fujiwara, C.S. Ho, Life cycle assessment and life cycle costing toward eco-efficiency concrete waste management in Malaysia, J. Cleaner Prod. 172 (2018) 3415–3427, https://doi.org/10.1016/j.jclepro.2017.11.200. [10] F.L. Gayarre, J.G. Perez, C.L.-C. Perez, M.S. Lopez, A.L. Martínez, Life cycle assessment for concrete kerbs manufactured with recycled aggregate, J. Cleaner Prod. 113 (2016) 41–53, https://doi.org/10.1016/j.jclepro.2015.11.093. [11] C.S. Poon, S.C. Kou, L. Lam, Influence of recycled aggregate on slump and bleeding of fresh concrete, Mater. Struct. 40 (2007) 981–988, https://doi.org/ 10.1617/s11527-006-9192-y.

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