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Damage behavior of the multiple ITZs in recycled aggregate concrete subjected to aggressive ion environment
Construction and Building Materials 245 (2020) 118419
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Damage behavior of the multiple ITZs in recycled aggregate concrete subjected to aggressive ion environment Gongbing Yue a, Zhiming Ma b,⇑, Miao Liu b, Chaofeng Liang b,c, Guangzhong Ba c a
College of Civil Engineering and Architecture, Qingdao Agricultural University, Qingdao 266109, China College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China c Department of Structural Engineering, Tongji University, Shanghai 200092, China b
h i g h l i g h t s Quantifying the properties of multiple ITZs in RAC subjected to aggressive ion environment. Investigating the effects of old and new concrete strength on the properties of multiple ITZs in RAC. Establishing the relationship between the micro-hardness of multiple ITZs and exposure days.
a r t i c l e
i n f o
Article history: Received 30 July 2019 Received in revised form 5 January 2020 Accepted 12 February 2020
Keywords: Recycled aggregate concrete (RAC) Multiple ITZs Sulfate attack Chloride attack
a b s t r a c t Recycled aggregate concrete (RAC) is inevitably subjected to various aggressive environments; however, the damage behavior of multiple interfacial transition zones (ITZs) in RAC under such environments has received little consideration in previous studies. Therefore, this paper was developed to quantify the influence of sulfate and chloride attack on the properties of multiple ITZs in RAC. A modeled RAC sample that contained multiple ITZs was first prepared, and the micro-hardness and micro-structure of multiple ITZs undergoing sulfate and chloride attack were determined. The results showed that the ITZOA-NM between old aggregate and new mortar was the weak point of RAC when the new concrete strength was lower than the old concrete strength; however, the ITZOA-OM between old aggregate and old mortar was the weakness of RAC when the new concrete strength was higher than the old concrete strength. A binomial relationship between ITZs properties and sulfate attack duration was observed, and the microhardness of multiple ITZs decreased linearly with increasing chloride attack duration. The degradation model for the micro-hardness of multiple ITZs was similar to that for the compressive strength of RAC subjected to an aggressive ion environment. The properties of ITZOA-NM were more sensitive to aggressive ion attack compared with that of other ITZs. Increasing the strength of new concrete is an effective method of improving the properties of multiple ITZs in RAC. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction A large amount of construction and demolition wastes (CDW) are being produced with the rapid onset of global urbanization [1,2]. For example, in China, the output of CDW reached approximately 1.8 billion tones in 2018 [3]. However, a series of environmental and social problems were brought about by the improper disposal of CDW that contained hazardous substances [4,5]; therefore, recycling technology has developed over the past twenty years to reduce the amount of such CDW. Previous studies have reported that waste concrete and bricks, which account for more than 80% of CDW by weight, could be recycled into recycled con⇑ Corresponding author. Dr. Zhiming Ma in Yangzhou University, China E-mail address: [email protected] (Z. Ma). https://doi.org/10.1016/j.conbuildmat.2020.118419 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
crete aggregate (RCA) as the second building materials for use in recycled aggregate concrete (RAC) [6,7]. The micro-characteristics [8], durability performance [9], mechanical properties [10] and structural behavior [11] of RAC have been investigated systematically by worldwide scholars, and the results highlight that incorporating RCA has an adverse impact on the properties of prepared RAC. Because the existence of adhered old mortar on RCA, the interfacial transition zones (ITZs) in RAC is more complex than that in plain concrete, and the existence of multiple ITZs in RAC is the leading cause of diminished properties in RAC [12,13]. Generally, RAC is composed of old aggregate, old mortar, new aggregate and new mortar, and there are four types of ITZs contained in RAC, including the ITZOA-OM between old aggregate and old mortar, the ITZOA-NM between old aggregate and new mortar, the ITZOM-NM between old mortar and
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new mortar, and the ITZNA-NM between new aggregate and new mortar; moreover, the properties of ITZOA-NM are similar to those of ITZNA-NM [14]. Investigating the influence of the performance of multiple ITZs on the properties of RAC is necessary to better understand the degradation mechanism of RAC. Exposure to an aggressive ion environment causes the durability and mechanical properties of concrete to decrease because of a series of physical and chemical reactions. Concrete subjected to sulfate attack leads to concrete expansion, cracking, spalling and eventual failure with the production of insoluble and expansive salt (such as Ettringite and Gypsum et al.) [15–17]. The previous studies reported that incorporating RCA increased the degradation rate of concrete durability, which was due to the inferior properties of adhered old mortar and multiple ITZs of RAC [18,19]; moreover, concrete with recycled fine aggregate may suffer more serious sulfate attack compared with that with RCA. However, incorporating supplementary cementing materials can improve the sulfate attack resistance of oridinary concrete and RAC [20–22]. Sun et al. [23] reported that the existence of ITZ had an adverse impact on the properties of concrete subjected to sulfate ion attack. Properties of multiple ITZs in RAC are more complicated than those in plain concrete, and one can expect that the effects of multiple ITZs on the properties of RAC are more significant compared with those of plain concrete exposed to sulfate ion attack. However, the properties of multiple ITZs in RAC exposed to a sulfate ion attack have received little consideration in previous studies. Chloride ion penetration is the leading reason for steel corrosion in reinforced concrete, and the incorporation of RCA further increases chloride ion penetration [24–26]. Some researchers have reported that chloride permeability of concrete with carbonated RAC was lower than that with untreated RCA [27–29], and the incorporation of nanomaterial and supplementary cementing materials decreased the chloride permeability of RAC [30,31]. Previous findings showed that the properties of old ITZ had an adverse impact on the chloride diffusion of RAC; as such, Xiao et al. [32] and Ying et al. [14] reported that the chloride diffusion coefficient increased with old ITZ thickness increasing. However, studies on the properties of multiple ITZs in RAC subjected to chloride ion attack have received little consideration from previous studies, especially for the microhardness of multiple ITZs in RAC with chloride ion attack. Exposure to a marine environment or saline-alkali soil condition inevitably exposes RAC to sulfate ion attack or chloride ion attack. Although previous studies have investigated the properties of RAC under such aggressive ion environment, the related researches were mainly focused on the macro-properties of RAC, as such, mechanical properties and durability performance. The investigations on the damage behavior of multiple ITZs in RAC were quite limited, and a quantified evaluation was lacking. Therefore, this paper investigated the degradation behavior of multiple ITZs in RAC subjected to an aggressive ion environment. The micro-hardness and micro-structure of multiple ITZs in RAC were determined after aggressive ion attack, and the effects of old and new concrete strength on the properties of multiple ITZs in RAC were investigated. Finally, this paper established the relationship between the properties of multiple ITZs and the exposure days of aggressive ion attack. The authors hope the findings in this paper are helpful to the further investigation and application of RAC exposed to an aggressive ion environment.
2. Materials and experimental details 2.1. Mixture proportions and modeled RAC preparation Previous studies found that the existence of multiple ITZs was the leading reason for the diminished properties of RAC compared
with NAC [33,34]. Fig. 1(a) shows a typical image of RCA and prepared concrete. The RAC was generally composed of old aggregate, old mortar, new aggregate and new mortar, and four types of ITZs were mainly contained in RAC, including ITZOM-NM, ITZOA-OM, ITZOA-NM and ITZNA-NM; besides, the properties of ITZOA-NM were almost the same as those of ITZNA-NM [14]. Therefore, this paper mainly focused on the properties of ITZOM-NM, ITZOA-OM and ITZOA-NM in RAC exposed to aggressive ion environment. Considering that the properties of multiple ITZs in RAC were difficult to observe and quantify, this paper prepared a modeled RAC sample to quantify the impacts of multiple ITZs on the properties of RAC under aggressive ion environment. Referring to related investigations on the modeled RAC [35,36], an updated modeled RAC sample was used in determining the properties of multiple ITZs in RAC exposed to aggressive ion environment. Fresh new mortar sieved from new concrete mixture was used in the preparation of modeled RAC, which is an effective and common method in evaluating the properties of modeled RAC; in this case, the negative effect of new aggregate on the testing precision was reduced. Fig. 1(b) shows the method of preparing a modeled RAC sample with multiple ITZs. Considering that the strength of waste concrete derived from CDW was mostly around C40 in practical construction engineering, the core sample of old concrete was first drilled from a concrete beam with an original strength of C40. The old aggregate, old mortar and ITZOA-OM could be observed from the core sample. The core sample of old concrete was subsequently mixed with the new mortar which was sieved from new concrete mixture with strength of C30, C40 and C50 to prepare a modeled RAC sample. Table 1 shows the mix proportion of new concrete used in preparing modeled RAC, and the fineness modulus of sand was 2.6. Furthermore, such modeled RAC sample with multiple ITZs can also be used to investigate the ITZ properties of RAC with a demolished concrete block or lump [37]. First, the old concrete sample with a size of 75 100 mm was first fixed in a mold with a size of 100 100 100 mm. Subsequently, the new concrete mixture was prepared in terms of Table 1, and then separating the new aggregate from the new concrete mixture to obtain fresh mortar mixture. After thatm, such fresh mortar mixture was poured into the mold to prepare modeled RAC sample with a size of 100 100 100 mm, and the modeled RAC sample after hardening was placed in a standard curing room (T = 20 ± 2 °C, RH 95%) for 28d. The effects of old and new concrete strength on the properties of multiple ITZs in RAC were also considered in this study. The RACOC40-NC30 sample represents that the strength class of old concrete and new concrete is respectively C40 and C30, as well as manifests that the old concrete strength is higher than the new concrete strength. The RACOC40-NC40 sample represents that the old concrete strength (C40) is the same as the new concrete strength (C40). The RACOC40-NC50 sample represents that the old concrete strength (C40) is lower than the new concrete strength (C50). After preparing, curing, cutting and polishing the modeled RAC sample, the ITZOM-NM, ITZOA-OM and ITZOA-NM can be observed from the curing interface of the modeled RAC sample, as shown in Fig. 1(c).
2.2. Sulfate and chloride attack test Sulfate attack test was conducted according to the ‘‘Standard for test methods of long-term performance and durability of ordinary concrete (GB/T50082-2009)” [38]. After 28d curing, the modeled RAC samples were placed in a 5% Na2SO4 solution for 0 d, 30 d, 60 d and 90 d of sulfate exposure. When reaching each exposure day, some of the RAC samples were pulled from the Na2SO4 solution and then dried completely in an air-dry oven, and the properties of multiple ITZs in the RAC were determined.
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Fig. 1. Preparing modeled RAC sample with multiple ITZs.
Table 1 Mixture proportions of new concrete in preparing modeled RAC, kg/m3. Concrete with various strengths
Cement
Sand
Aggregate
C30 C40 C50
300 375 467
780 750 713
1170 1125 1070
The chloride permeability of concrete can be determined by a long-term immersion test [39,40] and rapid chloride permeability test (RCPT) [41]. This paper adopted the method of solution immersion to investigate the properties of multiple ITZs in RAC exposed to a chloride attack environment, and the RAC samples after 28 d of curing were placed in a 3.5% NaCl solution for 0 d, 30 d, 60 d and 90 d. After each exposure day, the RAC samples were removed from the NaCl solution and dried in an air-dry oven, and then the microhardness and micro-structure were determined to quantify the properties of multiple ITZs in RAC subjected to chloride attack. 2.3. Micro-hardness test and multiple ITZ analysis Micro-hardness is closely related to the properties of hydration products and erosion products of concrete, and the Vickers hardness tester is frequently used to determine micro-hardness [42–44]. The micro-hardness of multiple ITZs in RAC after aggressive ion attack was determined by a Vickers hardness tester. Because there was a great difference in the micro-hardness of the aggregate, old mortar and new mortar, the higher and lower loads both had an adverse impact on the measuring accuracy; thus, the applied load used in the micro-hardness determination was 50 g, which was obtained from a number of repeated tests. The micro-hardness and width of various ITZs in RAC can be determined in this way. For making the testing results of micro-hardness more accurate, the standard area values were obtained by a box figure, and Fig. 2(d) shows the point selection and accuracy control for the testing data. 3. Results and discussion 3.1. Properties of multiple ITZs in RAC without aggressive ion attack Figs. 3–5 show the micro-hardness of multiple ITZs in RAC without aggressive ion attack. The micro-hardness of the aggregate
(approximately 325 MPa) was much higher than that of the ITZs, and the old mortar and new mortar had stable micro-hardness values. However, a low micro-hardness and large fluctuation could be observed for the ITZs, which was attributed to the inferior properties of ITZs with loose structures [45]. Comparing the results in Figs. 3–5, the micro-hardness of ITZOA-NM did not obviously change with increasing new concrete strength; however, the microhardness of ITZOA-NM and ITZOM-NM both increased with the increasing strength of new concrete. The width of ITZs was 60–80 lm, and a similar result was also obtained by Li et al. [11,13]. Because a high concrete strength frequently resulted in a high density and a low porosity, the micro-hardness of ITZOA-NM and ITZOM-NM were both improved with the increase of new concrete strength. Moreover, the old mortar on RCA absorbed the free water contained in the new mortar, which reduced the effective water-to-cement ratio (w/c) around the ITZON-NM and its micro-hardness was improved. Comparing the micro-hardness of ITZOA-NM and ITZOM-NM, the impact of new concrete strength on the micro-hardness of ITZOA-NM was more significant than that of ITZOM-NM. For example, the lowest micro-hardness of ITZOA-NM with C50 new concrete was approximately 42.0% and 101.6% higher than that with C40 and C30 new concrete, and the results become 26.9% and 42.2% for ITZOM-NM. As shown in Figs. 3 and 4, when the old concrete strength was not lower than the new concrete strength, the lowest microhardness of ITZOM-NM and ITZOA-OM was 33.9% and 69.4% higher, respectively, than that of ITZOA-NM in RACOC40-NC30 sample, and the results were 5.7% and 18.2% for RACOC40-NC40 that represented the old concrete strength was similar to the new concrete strength. In this case, ITZOA-NM was the weak point of RAC. This is because the boundary effect of old aggregate results in an increase in the free water content around old aggregate, and more pores are formed around ITZOA-NM with the evaporation of free water, which reduces the properties of ITZOA-NM. It can be concluded that the properties of adhered old mortar have less influence on the properties of RAC, and the adhered old mortar may not be removed from RCA. However, as shown in Fig. 5, it can be concluded that the ITZOA-OM was the weak point of RAC when the old concrete strength was lower than the new concrete strength. Therefore, when using low-quality RCA to prepare high-strength RAC, the
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Fig. 2. Point selection and accuracy control for the micro-hardness test.
Fig. 3. Multiple ITZs in RACOC40-NC30 subjected to normal environment.
Fig. 4. Multiple ITZs in RACOC40-NC40 subjected to normal environment.
existance of adhered old mortar had an adverse impact on the properties of RAC, and the adhered old mortar on RCA should be removed as much as possible.
Fig. 6(a) shows SEM images of multiple ITZs in RACOC40-NC40. The crack number and width of ITZOA-OM were lower than those of ITZOA-NM and ITZOM-NM, possibly because the adhered old mortar has a
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Fig. 5. Multiple ITZs in RACOC40-NC50 subjected to normal environment.
denser structure and a lower porosity after a long time of hydration. Based on the SEM images of ITZOA-NM, many cracks with widths of 2–3 lm was observed, and Ca(OH)2 and C-S-H existed on ITZOA-NM. The density of ITZOM-NM was higher than that of new mortar, and abundant hydration products could be observed on it, which was because the old mortar with high absorption reduced the effective w/c ratio around ITZOM-NM and thus its properties were improved. Fig. 6(b) further shows SEM images of the paste in concrete with various strengths. The total hydration products increased with the increase of new concrete strength, and the pores and cracks can be well filled with the hydration products; thus, the density of the paste increased with increasing strength. Furthermore, the compressive strength of RAC with various qualities of RCA was determined, and the results are shown in Table 2. When the content of binding materials were totalled 300 kg/m3, the RAC with high-quality RCA and low-quality RCA was 2.3% and 8.4% lower than that of NAC. It can be concluded that the impact of RCA quality on the properties of RAC was not obvious when using RCA to prepare low-strength RAC. This may be because
the strength of RAC was determined by ITZOA-NM, which can be obtained in Figs. 3 and 4. However, when total binding materials in the RAC were totalled 500 kg/m3, the RAC with high-quality RCA and low-quality RCA was 5.4% and 30.1% lower, respectively, than that of NAC. The effect of RCA quality on the properties of high-strength RAC was significant, and a significant decrease in the compressive strength can be observed with the addition of low-quality RCA. This may be because the ITZOA-OM became the weak point of RAC when using RCA to prepare high-strength RAC, which can be obtained from Fig. 5. As shown in Fig. 7 [46–49], a similar conclusion can be obtained from the relative references, which further proved that improving the quality of RCA was beneficial to the properties of RAC. 3.2. Properties of multiple ITZs in RAC subjected to sulfate attack The failure of RAC exposed to sulfate attack mainly contributes to the performance degradation of ITZs [18]. Fig. 8(a) shows the micro-hardness of ITZOA-OM in RAC subjected to sulfate attack. After
Fig. 6. SEM images of the multiple ITZs and the pastes with various strengths.
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Table 2 Compressive strength of RAC with various qualities of RCA (28d). Total binding materials (kg/m3)
300 500
Types of coarse aggregate (WA presents the water absorption, %) NCA (WA = 0.6%)
High-quality RCA (WA = 2.1%)
Low-quality RCA (WA = 7.2%)
39.1 59.1
38.2 55.9
35.8 41.3
tion, the degradation rate of ITZOA-NM decreased with the increase of new concrete strength; for example, the lowest micro-hardness of ITZOA-NM in RACOC40-NC30, RACOC40-NC40 and RACOC40-NC50, after 90 d of sulfate attack was 19.0%, 15.9% and 8.8% lower, respectively, than that without sulfate attack. Fig. 10 shows the microhardness and SEM images of ITZOM-NM, and a similar conclusion can be obtained.
Note: WA presents the water absorption and the RCA replacements are 100%.
3.3. Relationship between the properties of multiple ITZs and sulfate attack days
0, 30, 60 and 90 d of sulfate attack, the lowest micro-hardness of ITZOA-OM was 83.0, 88.6, 75.3 and 72.7 MPa, and the ITZOA-OM width was 72.0, 70.5, 78.0 and 82.5 lm, respectively. The micro-hardness of ITZOA-OM slightly improved after a short days of sulfate attack, whereas the micro-hardness decreased with a further increase of sulfate attack days. Fig. 8(b) shows the SEM images of ITZOA-OM after various days of sulfate attack. A denser structure can be observed for ITZOA-OM after 30 d of sulfate attack compared with that without sulfate attack. However, abundant corrosion products were produced on the ITZOA-OM after 90 d of sulfate attack, which produced more pores on ITZOA-OM and thus decreased its microhardness. Fig. 9 shows the micro-hardness and SEM images of ITZOA-NM in RAC after various sulfate attack days. For the ITZOA-NM in RACOC40NC30, the lowest micro-hardness of ITZOA-NM increased by 17.5%, and the width of ITZOA-NM decreased by 9 lm after 30 d of sulfate attack compared with that without sulfate attack. This may be because the expansion production of ettringite and AFt filled the micro pores of ITZOA-NM and increased its density; thus, the micro-hardness of ITZOA-NM was improved after a short days of sulfate attack. Fig. 9(d-e) further shows the SEM images of ITZOA-NM after 0 and 30 d of sulfate attack, and the ITZOA-NM after 30 d of sulfate attack was denser than that without sulfate attack along with the formation of expansive corrosion. However, for the ITZOA-NM in RAC after 60 and 90 d of sulfate attack, the width of ITZOA-NM increased to 88 lm and 96 lm, and the lowest micro-hardness of ITZOA-NM decreased by 11.1% and 19.0% compared with that without sulfate attack, which indicated that the properties of ITZOA-NM decreased after prolonged sulfate attack. This may be because the prolonged sulfate attack resulted in the formation of massive expansion production around ITZOANM and the formed pores reduced its properties [50,51]. Fig. 9(f) shows SEM images of ITZOA-NM after 90 d of sulfate attack, and expansive corrosion products (such as Aft and CaSO4) could be observed on it. Comparing the results in Fig. 9(a–c), the microhardness of ITZOA-NM in high-strength RAC is higher than that in the low-strength RAC after the same sulfate attack days. In addi-
The ITZs with the lowest micro-hardness was the weak point of RAC, and it was easy to reach failure with the application of a sulfate attack. After collecting and analyzing the data in Figs. 8–10, Fig. 11(a–c) shows the relationship between the lowest microhardness of multiple ITZs and sulfate attack days, and a binomial relation exists between them. A short duration of sulfate attack was beneficial to the strength of multiple ITZs in the RAC, whereas a long duration of sulfate attack decreased the strength of multiple ITZs. In addition, the specific equation was also described in this figure, where the MHOA-OM, MHOA-NM and MHOM-NM were the lowest micro-hardness of ITZOA-OM, ITZOA-NM and ITZOM-NM, in MPa; D presented the exposure days of sulfate attack, in days. The strength of ITZOA-NM was more sensitive to the sulfate attack compared with that of the other ITZs, and it was significantly impacted by the new concrete strength of RAC. Comparing the results in Fig. 11(a–c), the ITZOA-NM possessed the lowest micro-hardness compared with the other ITZs in RACOC40-NC30 after the same sulfate attack days, which indicated that the ITZOA-NM was more prone to fail when using the high-quality RCA to prepare low-strength RAC. However, the ITZOA-OM had the lowest microhardness when compared with the other ITZs in RACOC40-NC50 after the same sulfate attack days, and this highlighted that the ITZOA-OM was more prone to fail when using low-quality RCA to prepare high-strength RAC; in this case, the adhered old mortar on RCA should be strengthened or removed as much as possible. In particular, the performance degradation of multiple ITZs in RAC with the application of sulfate attack was higher than that without sulfate attack, and thus the adverse effect of multiple ITZs on the properties of RAC exposed to sulfate attack should be carefully considered. Fig. 11(d) collected the compressive strength of NAC and RAC after various sulfate attack days [52,53], and the changing trend for the compressive strength of RAC was similar to that for the micro-hardness of multiple ITZs in RAC, which indicated that the findings in this paper could well explain the deterioration mechanism of RAC exposed to sulfate attack. When attributed to the existence of multiple ITZs in RAC, the sulfate attack resistance of RAC was lower than that of NAC.
Fig. 7. Related references on the strength of concrete with various qualities of RCA [46–49].
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Fig. 8. Micro-hardness and SEM images of ITZOA-OM after sulfate ion attack days.
Fig. 9. Micro-hardness and micro-structure of ITZOA-NM in RAC after various sulfate attack days.
3.4. Multiple ITZ properties of RAC exposed to chloride attack Exposure to a marine environment inevitably subjects concrete to chloride attack; Yang [54] investigated the strength of plain concrete exposed to chloride attack, and the findings showed that the fatigue life decreased by 14.9–19.0% compared with that without chloride attack. This section investigated the effect of chloride attack on the properties of multiple ITZs, which was helpful to better understand the performance degradation of RAC exposed to chloride attack. Fig. 12(a) shows the micro-hardness of ITZOA-OM in RAC after various chloride attack days. The micro-hardness decreased with increasing exposure days of chloride attack; for example, the lowest micro-hardness of ITZOA-OM in RAC after 30 d, 60 d and 90 d of chloride attack was 2.4%, 5.4% and 10.5% lower, respectively, than that without chloride attack. Furthermore, the width of ITZOA-OM increased with increasing chloride attack days; for example, the widths of ITZOA-OM after 0 d, 30 d, 60 d and 90 d
of chloride penetration were 67 lm, 73 lm, 76 lm and 80 lm, respectively. The high porosity of the ITZs provided more paths for chloride penetration, and Friedel salt and soluble CaCl2 were produced along with the reaction between the chloride ion and calcium aluminate hydrate, which reduced the hardness and density of the ITZs [55–57]. The chloride ion also reacted with Ca(OH)2 that was gathered around the ITZs, which resulted in the formation of expansive salt CaCl2Ca(OH)2H2O and more cracks were formed on the ITZs. Meanwhile, the C-S-H decomposed with decreasing OH content and PH value, which further weakened the properties of the ITZs; therefore, imposed damage on ITZs increased with increasing days of chloride attack. Fig. 12(b) shows SEM images of ITZOA-OM in RAC after 0 d and 90 d of chloride attack. The micro-structure of ITZOA-OM after 90 d of chloride attack was looser than that without chloride attack, and part of the paste around ITZOA-OM happened to spalling.
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Fig. 10. Micro-hardness and micro-structure of ITZOM-NM with various sulfate attack days.
Fig. 11. Effects of RCA quality and sulfate attack days on the properties of multiple ITZs.
Fig. 13 shows the micro-hardness and micro-structure of ITZOA-NM in RAC after various chloride attack days. The microhardness of ITZOA-NM decreased and the width of ITZOA-NM increased with increasing chloride attack days; for example, the lowest micro-hardness of ITZOA-NM after 30 d, 60 d and 90 d of chloride attack were 7.4%, 11.2% and 17.2% lower, respectively, than that without chloride attack, and the width of ITZOA-NM after 0 d, 30 d, 60 d and 90 d of chloride attack were 88 lm, 92 lm,
95 lm and 97 lm, respectively. Compared with the results in Fig. 13(a–c), the micro-hardness of ITZOA-NM improved with the increase of new concrete strength after suffering the same number of chloride attack days. This may be because the high strength of new concrete resulted in better properties of ITZOA-NM and improved its resistance to chloride penetration. Fig. 13(d-e) presents SEM images of ITZOA-NM with and without chloride attack. The content of C-S-H and Ca(OH)2 decreased with the production
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Fig. 12. Micro-hardness and micro-structure of ITZOA-OM with various chloride attack days.
Fig. 13. Micro-hardness and micro-structure of ITZOA-NM after various chloride attack days.
of insoluble and expansive salt (3CaOAl2O3CaCl2H2O), which resulted in the formation of new pores and cracks on the ITZOA-NM in RAC. Fig. 14 shows the micro-hardness and micro-structure of ITZOMNM in RAC after various numbers of chloride attack days. The increasing number of exposure days resulted in a reduction in the micro hardness of ITZOM-NM properties, and there was an obvious increase in the width of ITZOM-NM with the increase of chloride attack days. As shown in Fig. 14(c), the lowest micro-hardness of ITZOM-NM after 30 d, 60 d and 90 d of chloride penetration decreased by 6.0%, 10.9% and 12.6%, respectively, compared with that without chloride attack. When subjected to chloride attack, the micro-hardness of ITZOM-NM in RAC with a high strength of new concrete was higher than that with a low strength of new concrete. After 90 d of chloride attack, the lowest micro-hardness of ITZOM-NM was 66.3 MPa, 73.0 MPa and 108.3 MPa when the new concrete strengths were C30, C40 and C50. Therefore, improving the new concrete strength is an effective way of enhancing the properties of ITZOM-NM in RAC when exposed to a chloride attack environment.
3.5. Relationship between the ITZ properties and chloride exposure days Fig. 15 shows the relationship between the micro-hardness of multiple ITZs and chloride attack days, and the results highlight that the micro-hardness decreases linearly with the increase of chloride attack days. The detailed equation is also described in this figure, where the MHOA-OM, MHOA-NM and MHOM-NM were, respectively, the lowest micro-hardness of ITZOA-OM, ITZOA-NM and ITZOM-NM in RAC, in MPa; D presented the chloride attack time, in days. Compared with the results in Fig. 15(a–c), the ITZOA-NM was the weak point of RACOC40-NC30 when the old concrete strength was higher than the new concrete strength; in this case, the adverse effect of adhered old mortar on RCA was not obvious on the properties of prepared RAC exposed to chloride attack. However, the ITZOA-NM and ITZOA-OM were both weak points of RACOC40-NC50 when the old concrete strength was lower than the new concrete strength; in this case, the existence of adhered old mortar on RCA had an adverse impact on the properties of RAC.
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Fig. 14. Micro-hardness and micro-structure of ITZOM-NM after various chloride attack days.
Fig. 15. Relationship between the properties of multiple ITZs and the exposure days of chloride attack.
Because of the aggregate and mortar being two types of materials, the ITZ between the aggregate and mortar was more prone to fail compared with the ITZOM-NM; moreover, increasing the new concrete strength is an effective way of improving the micro-hardness of ITZOA-NM and ITZOM-NM in RAC exposed to chloride attack. Although this study investigated the degradation behavior of multiple ITZs in RAC subjected to aggressive ion environments, there are still some shortcomings that should be further studied in the future. For example, the modeled RAC sample was used in this paper to quantify the properties of multiple ITZs in RAC, and thus the properties of multiple ITZs in normal RAC need to be investigated. Moreover, the relationship between the macroproperties of RAC and the micro-properties of multiple ITZs should be further established. In addition, the RAC is inevitably subjected to the loading condition, and the properties of multiple ITZs in RAC with the coupling effects of aggressive ion attack and applied loading should be also investigated.
4. Conclusions This paper investigated the properties of multiple ITZs in RAC exposed to sulfate ion and chloride ion attack. Based on the results and discussions above, the following conclusions can be obtained.
(1) When the strength of old concrete used in preparing RCA was higher than that of new concrete, the ITZOA-NM had the lowest micro-hardness compared with the other ITZs; in this case, the adverse effect of adhered old mortar on the properties of RAC was less, and the adhered old mortar may not be removed from RCA. However when the old concrete strength was lower than the new concrete strength, the ITZOA-OM was the weak point of RAC, and the adhered old mortar on RCA had significant adverse impact on the properties of RAC; in this case, the adhered old mortar should be removed from RCA as much as possible. (2) There was a slight increase in the micro-hardness of ITZs after suffering a short duration of sulfate attack, whereas the micro-hardness of ITZs decreased with the further increase of sulfate attack days. A binomial relationship existed between the micro-hardness of multiple ITZs and sulfate ion attack duration. The properties of ITZOA-NM were more sensitive to sulfate ion attack compared with those of other ITZs, and increasing the strength of new concrete was helpful to enhance the properties of ITZOA-NM in RAC exposed to sulfate ion attack. (3) Chloride penetration led to a decrease in the micro-hardness of ITZs in RAC, and the micro-hardness of ITZs decreased linearly with the increase of chloride ion attack days. The
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ITZOA-NM was the weak point in RACOC40-NC30 which represented the old concrete strength was higher than the new concrete strength. However, the ITZOA-NM and ITZOA-OM were both the weak points in RACOC40-NC50 which represented the old concrete strength was lower than the new concrete strength. For RAC subjected to chloride ion attack, the properties of ITZs between the aggregate and mortar should receive sufficient attention. CRediT authorship contribution statement Gongbing Yue: Methodology, Writing - review & editing. Zhiming Ma: Supervision, Writing - review & editing. Miao Liu: Validation, Formal analysis. Chaofeng Liang: Writing - review & editing. Guangzhong Ba: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors gratefully acknowledge the funding by the National Natural Science Foundation of China (51578297, 51708319) and China Postdoctoral Science Foundation (2019M651579) References [1] J.Z. Xiao, Z.M. Ma, T.B. Sui, A. Akbarnezhad, Z.H. Duan, Mechanical properties of concrete mixed with recycled powder produced from construction and demolition waste, J. Clean. Prod. 188 (2018) 720–731. [2] P. Rajhans, S.K. Panda, S. Nayak, Sustainable self compacting concrete from C&D waste by improving the microstructures of concrete ITZ, Constr. Build. Mater. 163 (2018) 557–570. [3] Z.M. Ma, W. Li, H.X. Wu, C.W. Cao, Chloride permeability of concrete mixed with activity recycled powder obtained from C&D waste, Constr. Build. Mater. 199 (2019) 652–663. [4] Z.M. Ma, M. Liu, Z.H. Duan, Effects of active waste powder obtained from C&D waste on the microproperties and water permeability of concrete, J. Clean. Prod. (2020) 120518. [5] D. Yu, H. Duan, Q. Song, X. Li, H. Zhang, H. Zhang, J. Wang, Characterizing the environmental impact of metals in construction and demolition waste, Environ. Sci. Pollut. Res. 25 (14) (2018) 13823–13832. [6] V.W. Tam, M. Soomro, A.C.J. Evangelista, A review of recycled aggregate in concrete applications (2000–2017), Constr. Build. Mater. 172 (2018) 272–292. [7] C.F. Liang, B.H. Pan, Z.M. Ma, Z.H. He, Z.H. Duan, Utilization of CO2 curing to enhance the properties of recycled aggregate and prepared concrete: a review, Cem. Concr. Compos. 105 (2020) 103446. [8] I.S. Del Bosque, W. Zhu, T. Howind, A. Matías, M.S. De Rojas, C. Medina, Properties of interfacial transition zones (ITZs) in concrete containing recycled mixed aggregate, Cem. Concr. Compos. 81 (2017) 25–34. [9] H. Guo, C. Shi, X. Guan, J. Zhu, Y. Ding, T.C. Ling, Y. Wang, Durability of recycled aggregate concrete–a review, Cem. Concr. Compos. 89 (2018) 251–259. [10] M. Ahmadi, S. Farzin, A. Hassani, M. Motamedi, Mechanical properties of the concrete containing recycled fibers and aggregates, Constr. Build. Mater. 144 (2017) 392–398. [11] W.G. Li, J.Z. Xiao, C.J. Shi, C.S. Poon, Structural behaviour of composite members with recycled aggregate concrete-an overview, Adv. Struct. Eng. 18 (6) (2015) 919–938. [12] J.Z. Xiao, W.G. Li, Z. Sun, D.A. Lange, S.P. Shah, Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation, Cem. Concr. Compos. 37 (2013) 276–292. [13] W.G. Li, J.Z. Xiao, Z.H. Sun, S. Kawashima, S.P. Shah, Interfacial transition zones in recycled aggregate concrete with different mixing approaches, Constr. Build. Mater. 35 (2012) 1045–1055. [14] J. Ying, J. Xiao, L. Shen, M.A. Bradford, Five-phase composite sphere model for chloride diffusivity prediction of recycled aggregate concrete, Mag. Concr. Res. 65 (9) (2013) 573–588. [15] T. Ikumi, I. Segura, Numerical assessment of external sulfate attack in concrete structures. A review, Cem. Concr. Res. 121 (2019) 91–105. [16] J. Yao, J. Chen, C. Lu, Entropy evolution during crack propagation in concrete under sulfate attack, Constr. Build. Mater. 209 (2019) 492–498. [17] S. Gao, J. Jin, G. Hu, L. Qi, Experimental investigation of the interface bond properties between SHCC and concrete under sulfate attack, Constr. Build. Mater. 217 (2019) 651–663.
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Damage behavior of the multiple ITZs in recycled aggregate concrete subjected to aggressive ion environment Gongbing Yue a, Zhiming Ma b,⇑, Miao Liu b, Chaofeng Liang b,c, Guangzhong Ba c a
College of Civil Engineering and Architecture, Qingdao Agricultural University, Qingdao 266109, China College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China c Department of Structural Engineering, Tongji University, Shanghai 200092, China b
h i g h l i g h t s Quantifying the properties of multiple ITZs in RAC subjected to aggressive ion environment. Investigating the effects of old and new concrete strength on the properties of multiple ITZs in RAC. Establishing the relationship between the micro-hardness of multiple ITZs and exposure days.
a r t i c l e
i n f o
Article history: Received 30 July 2019 Received in revised form 5 January 2020 Accepted 12 February 2020
Keywords: Recycled aggregate concrete (RAC) Multiple ITZs Sulfate attack Chloride attack
a b s t r a c t Recycled aggregate concrete (RAC) is inevitably subjected to various aggressive environments; however, the damage behavior of multiple interfacial transition zones (ITZs) in RAC under such environments has received little consideration in previous studies. Therefore, this paper was developed to quantify the influence of sulfate and chloride attack on the properties of multiple ITZs in RAC. A modeled RAC sample that contained multiple ITZs was first prepared, and the micro-hardness and micro-structure of multiple ITZs undergoing sulfate and chloride attack were determined. The results showed that the ITZOA-NM between old aggregate and new mortar was the weak point of RAC when the new concrete strength was lower than the old concrete strength; however, the ITZOA-OM between old aggregate and old mortar was the weakness of RAC when the new concrete strength was higher than the old concrete strength. A binomial relationship between ITZs properties and sulfate attack duration was observed, and the microhardness of multiple ITZs decreased linearly with increasing chloride attack duration. The degradation model for the micro-hardness of multiple ITZs was similar to that for the compressive strength of RAC subjected to an aggressive ion environment. The properties of ITZOA-NM were more sensitive to aggressive ion attack compared with that of other ITZs. Increasing the strength of new concrete is an effective method of improving the properties of multiple ITZs in RAC. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction A large amount of construction and demolition wastes (CDW) are being produced with the rapid onset of global urbanization [1,2]. For example, in China, the output of CDW reached approximately 1.8 billion tones in 2018 [3]. However, a series of environmental and social problems were brought about by the improper disposal of CDW that contained hazardous substances [4,5]; therefore, recycling technology has developed over the past twenty years to reduce the amount of such CDW. Previous studies have reported that waste concrete and bricks, which account for more than 80% of CDW by weight, could be recycled into recycled con⇑ Corresponding author. Dr. Zhiming Ma in Yangzhou University, China E-mail address: [email protected] (Z. Ma). https://doi.org/10.1016/j.conbuildmat.2020.118419 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
crete aggregate (RCA) as the second building materials for use in recycled aggregate concrete (RAC) [6,7]. The micro-characteristics [8], durability performance [9], mechanical properties [10] and structural behavior [11] of RAC have been investigated systematically by worldwide scholars, and the results highlight that incorporating RCA has an adverse impact on the properties of prepared RAC. Because the existence of adhered old mortar on RCA, the interfacial transition zones (ITZs) in RAC is more complex than that in plain concrete, and the existence of multiple ITZs in RAC is the leading cause of diminished properties in RAC [12,13]. Generally, RAC is composed of old aggregate, old mortar, new aggregate and new mortar, and there are four types of ITZs contained in RAC, including the ITZOA-OM between old aggregate and old mortar, the ITZOA-NM between old aggregate and new mortar, the ITZOM-NM between old mortar and
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new mortar, and the ITZNA-NM between new aggregate and new mortar; moreover, the properties of ITZOA-NM are similar to those of ITZNA-NM [14]. Investigating the influence of the performance of multiple ITZs on the properties of RAC is necessary to better understand the degradation mechanism of RAC. Exposure to an aggressive ion environment causes the durability and mechanical properties of concrete to decrease because of a series of physical and chemical reactions. Concrete subjected to sulfate attack leads to concrete expansion, cracking, spalling and eventual failure with the production of insoluble and expansive salt (such as Ettringite and Gypsum et al.) [15–17]. The previous studies reported that incorporating RCA increased the degradation rate of concrete durability, which was due to the inferior properties of adhered old mortar and multiple ITZs of RAC [18,19]; moreover, concrete with recycled fine aggregate may suffer more serious sulfate attack compared with that with RCA. However, incorporating supplementary cementing materials can improve the sulfate attack resistance of oridinary concrete and RAC [20–22]. Sun et al. [23] reported that the existence of ITZ had an adverse impact on the properties of concrete subjected to sulfate ion attack. Properties of multiple ITZs in RAC are more complicated than those in plain concrete, and one can expect that the effects of multiple ITZs on the properties of RAC are more significant compared with those of plain concrete exposed to sulfate ion attack. However, the properties of multiple ITZs in RAC exposed to a sulfate ion attack have received little consideration in previous studies. Chloride ion penetration is the leading reason for steel corrosion in reinforced concrete, and the incorporation of RCA further increases chloride ion penetration [24–26]. Some researchers have reported that chloride permeability of concrete with carbonated RAC was lower than that with untreated RCA [27–29], and the incorporation of nanomaterial and supplementary cementing materials decreased the chloride permeability of RAC [30,31]. Previous findings showed that the properties of old ITZ had an adverse impact on the chloride diffusion of RAC; as such, Xiao et al. [32] and Ying et al. [14] reported that the chloride diffusion coefficient increased with old ITZ thickness increasing. However, studies on the properties of multiple ITZs in RAC subjected to chloride ion attack have received little consideration from previous studies, especially for the microhardness of multiple ITZs in RAC with chloride ion attack. Exposure to a marine environment or saline-alkali soil condition inevitably exposes RAC to sulfate ion attack or chloride ion attack. Although previous studies have investigated the properties of RAC under such aggressive ion environment, the related researches were mainly focused on the macro-properties of RAC, as such, mechanical properties and durability performance. The investigations on the damage behavior of multiple ITZs in RAC were quite limited, and a quantified evaluation was lacking. Therefore, this paper investigated the degradation behavior of multiple ITZs in RAC subjected to an aggressive ion environment. The micro-hardness and micro-structure of multiple ITZs in RAC were determined after aggressive ion attack, and the effects of old and new concrete strength on the properties of multiple ITZs in RAC were investigated. Finally, this paper established the relationship between the properties of multiple ITZs and the exposure days of aggressive ion attack. The authors hope the findings in this paper are helpful to the further investigation and application of RAC exposed to an aggressive ion environment.
2. Materials and experimental details 2.1. Mixture proportions and modeled RAC preparation Previous studies found that the existence of multiple ITZs was the leading reason for the diminished properties of RAC compared
with NAC [33,34]. Fig. 1(a) shows a typical image of RCA and prepared concrete. The RAC was generally composed of old aggregate, old mortar, new aggregate and new mortar, and four types of ITZs were mainly contained in RAC, including ITZOM-NM, ITZOA-OM, ITZOA-NM and ITZNA-NM; besides, the properties of ITZOA-NM were almost the same as those of ITZNA-NM [14]. Therefore, this paper mainly focused on the properties of ITZOM-NM, ITZOA-OM and ITZOA-NM in RAC exposed to aggressive ion environment. Considering that the properties of multiple ITZs in RAC were difficult to observe and quantify, this paper prepared a modeled RAC sample to quantify the impacts of multiple ITZs on the properties of RAC under aggressive ion environment. Referring to related investigations on the modeled RAC [35,36], an updated modeled RAC sample was used in determining the properties of multiple ITZs in RAC exposed to aggressive ion environment. Fresh new mortar sieved from new concrete mixture was used in the preparation of modeled RAC, which is an effective and common method in evaluating the properties of modeled RAC; in this case, the negative effect of new aggregate on the testing precision was reduced. Fig. 1(b) shows the method of preparing a modeled RAC sample with multiple ITZs. Considering that the strength of waste concrete derived from CDW was mostly around C40 in practical construction engineering, the core sample of old concrete was first drilled from a concrete beam with an original strength of C40. The old aggregate, old mortar and ITZOA-OM could be observed from the core sample. The core sample of old concrete was subsequently mixed with the new mortar which was sieved from new concrete mixture with strength of C30, C40 and C50 to prepare a modeled RAC sample. Table 1 shows the mix proportion of new concrete used in preparing modeled RAC, and the fineness modulus of sand was 2.6. Furthermore, such modeled RAC sample with multiple ITZs can also be used to investigate the ITZ properties of RAC with a demolished concrete block or lump [37]. First, the old concrete sample with a size of 75 100 mm was first fixed in a mold with a size of 100 100 100 mm. Subsequently, the new concrete mixture was prepared in terms of Table 1, and then separating the new aggregate from the new concrete mixture to obtain fresh mortar mixture. After thatm, such fresh mortar mixture was poured into the mold to prepare modeled RAC sample with a size of 100 100 100 mm, and the modeled RAC sample after hardening was placed in a standard curing room (T = 20 ± 2 °C, RH 95%) for 28d. The effects of old and new concrete strength on the properties of multiple ITZs in RAC were also considered in this study. The RACOC40-NC30 sample represents that the strength class of old concrete and new concrete is respectively C40 and C30, as well as manifests that the old concrete strength is higher than the new concrete strength. The RACOC40-NC40 sample represents that the old concrete strength (C40) is the same as the new concrete strength (C40). The RACOC40-NC50 sample represents that the old concrete strength (C40) is lower than the new concrete strength (C50). After preparing, curing, cutting and polishing the modeled RAC sample, the ITZOM-NM, ITZOA-OM and ITZOA-NM can be observed from the curing interface of the modeled RAC sample, as shown in Fig. 1(c).
2.2. Sulfate and chloride attack test Sulfate attack test was conducted according to the ‘‘Standard for test methods of long-term performance and durability of ordinary concrete (GB/T50082-2009)” [38]. After 28d curing, the modeled RAC samples were placed in a 5% Na2SO4 solution for 0 d, 30 d, 60 d and 90 d of sulfate exposure. When reaching each exposure day, some of the RAC samples were pulled from the Na2SO4 solution and then dried completely in an air-dry oven, and the properties of multiple ITZs in the RAC were determined.
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Fig. 1. Preparing modeled RAC sample with multiple ITZs.
Table 1 Mixture proportions of new concrete in preparing modeled RAC, kg/m3. Concrete with various strengths
Cement
Sand
Aggregate
C30 C40 C50
300 375 467
780 750 713
1170 1125 1070
The chloride permeability of concrete can be determined by a long-term immersion test [39,40] and rapid chloride permeability test (RCPT) [41]. This paper adopted the method of solution immersion to investigate the properties of multiple ITZs in RAC exposed to a chloride attack environment, and the RAC samples after 28 d of curing were placed in a 3.5% NaCl solution for 0 d, 30 d, 60 d and 90 d. After each exposure day, the RAC samples were removed from the NaCl solution and dried in an air-dry oven, and then the microhardness and micro-structure were determined to quantify the properties of multiple ITZs in RAC subjected to chloride attack. 2.3. Micro-hardness test and multiple ITZ analysis Micro-hardness is closely related to the properties of hydration products and erosion products of concrete, and the Vickers hardness tester is frequently used to determine micro-hardness [42–44]. The micro-hardness of multiple ITZs in RAC after aggressive ion attack was determined by a Vickers hardness tester. Because there was a great difference in the micro-hardness of the aggregate, old mortar and new mortar, the higher and lower loads both had an adverse impact on the measuring accuracy; thus, the applied load used in the micro-hardness determination was 50 g, which was obtained from a number of repeated tests. The micro-hardness and width of various ITZs in RAC can be determined in this way. For making the testing results of micro-hardness more accurate, the standard area values were obtained by a box figure, and Fig. 2(d) shows the point selection and accuracy control for the testing data. 3. Results and discussion 3.1. Properties of multiple ITZs in RAC without aggressive ion attack Figs. 3–5 show the micro-hardness of multiple ITZs in RAC without aggressive ion attack. The micro-hardness of the aggregate
(approximately 325 MPa) was much higher than that of the ITZs, and the old mortar and new mortar had stable micro-hardness values. However, a low micro-hardness and large fluctuation could be observed for the ITZs, which was attributed to the inferior properties of ITZs with loose structures [45]. Comparing the results in Figs. 3–5, the micro-hardness of ITZOA-NM did not obviously change with increasing new concrete strength; however, the microhardness of ITZOA-NM and ITZOM-NM both increased with the increasing strength of new concrete. The width of ITZs was 60–80 lm, and a similar result was also obtained by Li et al. [11,13]. Because a high concrete strength frequently resulted in a high density and a low porosity, the micro-hardness of ITZOA-NM and ITZOM-NM were both improved with the increase of new concrete strength. Moreover, the old mortar on RCA absorbed the free water contained in the new mortar, which reduced the effective water-to-cement ratio (w/c) around the ITZON-NM and its micro-hardness was improved. Comparing the micro-hardness of ITZOA-NM and ITZOM-NM, the impact of new concrete strength on the micro-hardness of ITZOA-NM was more significant than that of ITZOM-NM. For example, the lowest micro-hardness of ITZOA-NM with C50 new concrete was approximately 42.0% and 101.6% higher than that with C40 and C30 new concrete, and the results become 26.9% and 42.2% for ITZOM-NM. As shown in Figs. 3 and 4, when the old concrete strength was not lower than the new concrete strength, the lowest microhardness of ITZOM-NM and ITZOA-OM was 33.9% and 69.4% higher, respectively, than that of ITZOA-NM in RACOC40-NC30 sample, and the results were 5.7% and 18.2% for RACOC40-NC40 that represented the old concrete strength was similar to the new concrete strength. In this case, ITZOA-NM was the weak point of RAC. This is because the boundary effect of old aggregate results in an increase in the free water content around old aggregate, and more pores are formed around ITZOA-NM with the evaporation of free water, which reduces the properties of ITZOA-NM. It can be concluded that the properties of adhered old mortar have less influence on the properties of RAC, and the adhered old mortar may not be removed from RCA. However, as shown in Fig. 5, it can be concluded that the ITZOA-OM was the weak point of RAC when the old concrete strength was lower than the new concrete strength. Therefore, when using low-quality RCA to prepare high-strength RAC, the
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Fig. 2. Point selection and accuracy control for the micro-hardness test.
Fig. 3. Multiple ITZs in RACOC40-NC30 subjected to normal environment.
Fig. 4. Multiple ITZs in RACOC40-NC40 subjected to normal environment.
existance of adhered old mortar had an adverse impact on the properties of RAC, and the adhered old mortar on RCA should be removed as much as possible.
Fig. 6(a) shows SEM images of multiple ITZs in RACOC40-NC40. The crack number and width of ITZOA-OM were lower than those of ITZOA-NM and ITZOM-NM, possibly because the adhered old mortar has a
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Fig. 5. Multiple ITZs in RACOC40-NC50 subjected to normal environment.
denser structure and a lower porosity after a long time of hydration. Based on the SEM images of ITZOA-NM, many cracks with widths of 2–3 lm was observed, and Ca(OH)2 and C-S-H existed on ITZOA-NM. The density of ITZOM-NM was higher than that of new mortar, and abundant hydration products could be observed on it, which was because the old mortar with high absorption reduced the effective w/c ratio around ITZOM-NM and thus its properties were improved. Fig. 6(b) further shows SEM images of the paste in concrete with various strengths. The total hydration products increased with the increase of new concrete strength, and the pores and cracks can be well filled with the hydration products; thus, the density of the paste increased with increasing strength. Furthermore, the compressive strength of RAC with various qualities of RCA was determined, and the results are shown in Table 2. When the content of binding materials were totalled 300 kg/m3, the RAC with high-quality RCA and low-quality RCA was 2.3% and 8.4% lower than that of NAC. It can be concluded that the impact of RCA quality on the properties of RAC was not obvious when using RCA to prepare low-strength RAC. This may be because
the strength of RAC was determined by ITZOA-NM, which can be obtained in Figs. 3 and 4. However, when total binding materials in the RAC were totalled 500 kg/m3, the RAC with high-quality RCA and low-quality RCA was 5.4% and 30.1% lower, respectively, than that of NAC. The effect of RCA quality on the properties of high-strength RAC was significant, and a significant decrease in the compressive strength can be observed with the addition of low-quality RCA. This may be because the ITZOA-OM became the weak point of RAC when using RCA to prepare high-strength RAC, which can be obtained from Fig. 5. As shown in Fig. 7 [46–49], a similar conclusion can be obtained from the relative references, which further proved that improving the quality of RCA was beneficial to the properties of RAC. 3.2. Properties of multiple ITZs in RAC subjected to sulfate attack The failure of RAC exposed to sulfate attack mainly contributes to the performance degradation of ITZs [18]. Fig. 8(a) shows the micro-hardness of ITZOA-OM in RAC subjected to sulfate attack. After
Fig. 6. SEM images of the multiple ITZs and the pastes with various strengths.
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Table 2 Compressive strength of RAC with various qualities of RCA (28d). Total binding materials (kg/m3)
300 500
Types of coarse aggregate (WA presents the water absorption, %) NCA (WA = 0.6%)
High-quality RCA (WA = 2.1%)
Low-quality RCA (WA = 7.2%)
39.1 59.1
38.2 55.9
35.8 41.3
tion, the degradation rate of ITZOA-NM decreased with the increase of new concrete strength; for example, the lowest micro-hardness of ITZOA-NM in RACOC40-NC30, RACOC40-NC40 and RACOC40-NC50, after 90 d of sulfate attack was 19.0%, 15.9% and 8.8% lower, respectively, than that without sulfate attack. Fig. 10 shows the microhardness and SEM images of ITZOM-NM, and a similar conclusion can be obtained.
Note: WA presents the water absorption and the RCA replacements are 100%.
3.3. Relationship between the properties of multiple ITZs and sulfate attack days
0, 30, 60 and 90 d of sulfate attack, the lowest micro-hardness of ITZOA-OM was 83.0, 88.6, 75.3 and 72.7 MPa, and the ITZOA-OM width was 72.0, 70.5, 78.0 and 82.5 lm, respectively. The micro-hardness of ITZOA-OM slightly improved after a short days of sulfate attack, whereas the micro-hardness decreased with a further increase of sulfate attack days. Fig. 8(b) shows the SEM images of ITZOA-OM after various days of sulfate attack. A denser structure can be observed for ITZOA-OM after 30 d of sulfate attack compared with that without sulfate attack. However, abundant corrosion products were produced on the ITZOA-OM after 90 d of sulfate attack, which produced more pores on ITZOA-OM and thus decreased its microhardness. Fig. 9 shows the micro-hardness and SEM images of ITZOA-NM in RAC after various sulfate attack days. For the ITZOA-NM in RACOC40NC30, the lowest micro-hardness of ITZOA-NM increased by 17.5%, and the width of ITZOA-NM decreased by 9 lm after 30 d of sulfate attack compared with that without sulfate attack. This may be because the expansion production of ettringite and AFt filled the micro pores of ITZOA-NM and increased its density; thus, the micro-hardness of ITZOA-NM was improved after a short days of sulfate attack. Fig. 9(d-e) further shows the SEM images of ITZOA-NM after 0 and 30 d of sulfate attack, and the ITZOA-NM after 30 d of sulfate attack was denser than that without sulfate attack along with the formation of expansive corrosion. However, for the ITZOA-NM in RAC after 60 and 90 d of sulfate attack, the width of ITZOA-NM increased to 88 lm and 96 lm, and the lowest micro-hardness of ITZOA-NM decreased by 11.1% and 19.0% compared with that without sulfate attack, which indicated that the properties of ITZOA-NM decreased after prolonged sulfate attack. This may be because the prolonged sulfate attack resulted in the formation of massive expansion production around ITZOANM and the formed pores reduced its properties [50,51]. Fig. 9(f) shows SEM images of ITZOA-NM after 90 d of sulfate attack, and expansive corrosion products (such as Aft and CaSO4) could be observed on it. Comparing the results in Fig. 9(a–c), the microhardness of ITZOA-NM in high-strength RAC is higher than that in the low-strength RAC after the same sulfate attack days. In addi-
The ITZs with the lowest micro-hardness was the weak point of RAC, and it was easy to reach failure with the application of a sulfate attack. After collecting and analyzing the data in Figs. 8–10, Fig. 11(a–c) shows the relationship between the lowest microhardness of multiple ITZs and sulfate attack days, and a binomial relation exists between them. A short duration of sulfate attack was beneficial to the strength of multiple ITZs in the RAC, whereas a long duration of sulfate attack decreased the strength of multiple ITZs. In addition, the specific equation was also described in this figure, where the MHOA-OM, MHOA-NM and MHOM-NM were the lowest micro-hardness of ITZOA-OM, ITZOA-NM and ITZOM-NM, in MPa; D presented the exposure days of sulfate attack, in days. The strength of ITZOA-NM was more sensitive to the sulfate attack compared with that of the other ITZs, and it was significantly impacted by the new concrete strength of RAC. Comparing the results in Fig. 11(a–c), the ITZOA-NM possessed the lowest micro-hardness compared with the other ITZs in RACOC40-NC30 after the same sulfate attack days, which indicated that the ITZOA-NM was more prone to fail when using the high-quality RCA to prepare low-strength RAC. However, the ITZOA-OM had the lowest microhardness when compared with the other ITZs in RACOC40-NC50 after the same sulfate attack days, and this highlighted that the ITZOA-OM was more prone to fail when using low-quality RCA to prepare high-strength RAC; in this case, the adhered old mortar on RCA should be strengthened or removed as much as possible. In particular, the performance degradation of multiple ITZs in RAC with the application of sulfate attack was higher than that without sulfate attack, and thus the adverse effect of multiple ITZs on the properties of RAC exposed to sulfate attack should be carefully considered. Fig. 11(d) collected the compressive strength of NAC and RAC after various sulfate attack days [52,53], and the changing trend for the compressive strength of RAC was similar to that for the micro-hardness of multiple ITZs in RAC, which indicated that the findings in this paper could well explain the deterioration mechanism of RAC exposed to sulfate attack. When attributed to the existence of multiple ITZs in RAC, the sulfate attack resistance of RAC was lower than that of NAC.
Fig. 7. Related references on the strength of concrete with various qualities of RCA [46–49].
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Fig. 8. Micro-hardness and SEM images of ITZOA-OM after sulfate ion attack days.
Fig. 9. Micro-hardness and micro-structure of ITZOA-NM in RAC after various sulfate attack days.
3.4. Multiple ITZ properties of RAC exposed to chloride attack Exposure to a marine environment inevitably subjects concrete to chloride attack; Yang [54] investigated the strength of plain concrete exposed to chloride attack, and the findings showed that the fatigue life decreased by 14.9–19.0% compared with that without chloride attack. This section investigated the effect of chloride attack on the properties of multiple ITZs, which was helpful to better understand the performance degradation of RAC exposed to chloride attack. Fig. 12(a) shows the micro-hardness of ITZOA-OM in RAC after various chloride attack days. The micro-hardness decreased with increasing exposure days of chloride attack; for example, the lowest micro-hardness of ITZOA-OM in RAC after 30 d, 60 d and 90 d of chloride attack was 2.4%, 5.4% and 10.5% lower, respectively, than that without chloride attack. Furthermore, the width of ITZOA-OM increased with increasing chloride attack days; for example, the widths of ITZOA-OM after 0 d, 30 d, 60 d and 90 d
of chloride penetration were 67 lm, 73 lm, 76 lm and 80 lm, respectively. The high porosity of the ITZs provided more paths for chloride penetration, and Friedel salt and soluble CaCl2 were produced along with the reaction between the chloride ion and calcium aluminate hydrate, which reduced the hardness and density of the ITZs [55–57]. The chloride ion also reacted with Ca(OH)2 that was gathered around the ITZs, which resulted in the formation of expansive salt CaCl2Ca(OH)2H2O and more cracks were formed on the ITZs. Meanwhile, the C-S-H decomposed with decreasing OH content and PH value, which further weakened the properties of the ITZs; therefore, imposed damage on ITZs increased with increasing days of chloride attack. Fig. 12(b) shows SEM images of ITZOA-OM in RAC after 0 d and 90 d of chloride attack. The micro-structure of ITZOA-OM after 90 d of chloride attack was looser than that without chloride attack, and part of the paste around ITZOA-OM happened to spalling.
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Fig. 10. Micro-hardness and micro-structure of ITZOM-NM with various sulfate attack days.
Fig. 11. Effects of RCA quality and sulfate attack days on the properties of multiple ITZs.
Fig. 13 shows the micro-hardness and micro-structure of ITZOA-NM in RAC after various chloride attack days. The microhardness of ITZOA-NM decreased and the width of ITZOA-NM increased with increasing chloride attack days; for example, the lowest micro-hardness of ITZOA-NM after 30 d, 60 d and 90 d of chloride attack were 7.4%, 11.2% and 17.2% lower, respectively, than that without chloride attack, and the width of ITZOA-NM after 0 d, 30 d, 60 d and 90 d of chloride attack were 88 lm, 92 lm,
95 lm and 97 lm, respectively. Compared with the results in Fig. 13(a–c), the micro-hardness of ITZOA-NM improved with the increase of new concrete strength after suffering the same number of chloride attack days. This may be because the high strength of new concrete resulted in better properties of ITZOA-NM and improved its resistance to chloride penetration. Fig. 13(d-e) presents SEM images of ITZOA-NM with and without chloride attack. The content of C-S-H and Ca(OH)2 decreased with the production
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Fig. 12. Micro-hardness and micro-structure of ITZOA-OM with various chloride attack days.
Fig. 13. Micro-hardness and micro-structure of ITZOA-NM after various chloride attack days.
of insoluble and expansive salt (3CaOAl2O3CaCl2H2O), which resulted in the formation of new pores and cracks on the ITZOA-NM in RAC. Fig. 14 shows the micro-hardness and micro-structure of ITZOMNM in RAC after various numbers of chloride attack days. The increasing number of exposure days resulted in a reduction in the micro hardness of ITZOM-NM properties, and there was an obvious increase in the width of ITZOM-NM with the increase of chloride attack days. As shown in Fig. 14(c), the lowest micro-hardness of ITZOM-NM after 30 d, 60 d and 90 d of chloride penetration decreased by 6.0%, 10.9% and 12.6%, respectively, compared with that without chloride attack. When subjected to chloride attack, the micro-hardness of ITZOM-NM in RAC with a high strength of new concrete was higher than that with a low strength of new concrete. After 90 d of chloride attack, the lowest micro-hardness of ITZOM-NM was 66.3 MPa, 73.0 MPa and 108.3 MPa when the new concrete strengths were C30, C40 and C50. Therefore, improving the new concrete strength is an effective way of enhancing the properties of ITZOM-NM in RAC when exposed to a chloride attack environment.
3.5. Relationship between the ITZ properties and chloride exposure days Fig. 15 shows the relationship between the micro-hardness of multiple ITZs and chloride attack days, and the results highlight that the micro-hardness decreases linearly with the increase of chloride attack days. The detailed equation is also described in this figure, where the MHOA-OM, MHOA-NM and MHOM-NM were, respectively, the lowest micro-hardness of ITZOA-OM, ITZOA-NM and ITZOM-NM in RAC, in MPa; D presented the chloride attack time, in days. Compared with the results in Fig. 15(a–c), the ITZOA-NM was the weak point of RACOC40-NC30 when the old concrete strength was higher than the new concrete strength; in this case, the adverse effect of adhered old mortar on RCA was not obvious on the properties of prepared RAC exposed to chloride attack. However, the ITZOA-NM and ITZOA-OM were both weak points of RACOC40-NC50 when the old concrete strength was lower than the new concrete strength; in this case, the existence of adhered old mortar on RCA had an adverse impact on the properties of RAC.
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Fig. 14. Micro-hardness and micro-structure of ITZOM-NM after various chloride attack days.
Fig. 15. Relationship between the properties of multiple ITZs and the exposure days of chloride attack.
Because of the aggregate and mortar being two types of materials, the ITZ between the aggregate and mortar was more prone to fail compared with the ITZOM-NM; moreover, increasing the new concrete strength is an effective way of improving the micro-hardness of ITZOA-NM and ITZOM-NM in RAC exposed to chloride attack. Although this study investigated the degradation behavior of multiple ITZs in RAC subjected to aggressive ion environments, there are still some shortcomings that should be further studied in the future. For example, the modeled RAC sample was used in this paper to quantify the properties of multiple ITZs in RAC, and thus the properties of multiple ITZs in normal RAC need to be investigated. Moreover, the relationship between the macroproperties of RAC and the micro-properties of multiple ITZs should be further established. In addition, the RAC is inevitably subjected to the loading condition, and the properties of multiple ITZs in RAC with the coupling effects of aggressive ion attack and applied loading should be also investigated.
4. Conclusions This paper investigated the properties of multiple ITZs in RAC exposed to sulfate ion and chloride ion attack. Based on the results and discussions above, the following conclusions can be obtained.
(1) When the strength of old concrete used in preparing RCA was higher than that of new concrete, the ITZOA-NM had the lowest micro-hardness compared with the other ITZs; in this case, the adverse effect of adhered old mortar on the properties of RAC was less, and the adhered old mortar may not be removed from RCA. However when the old concrete strength was lower than the new concrete strength, the ITZOA-OM was the weak point of RAC, and the adhered old mortar on RCA had significant adverse impact on the properties of RAC; in this case, the adhered old mortar should be removed from RCA as much as possible. (2) There was a slight increase in the micro-hardness of ITZs after suffering a short duration of sulfate attack, whereas the micro-hardness of ITZs decreased with the further increase of sulfate attack days. A binomial relationship existed between the micro-hardness of multiple ITZs and sulfate ion attack duration. The properties of ITZOA-NM were more sensitive to sulfate ion attack compared with those of other ITZs, and increasing the strength of new concrete was helpful to enhance the properties of ITZOA-NM in RAC exposed to sulfate ion attack. (3) Chloride penetration led to a decrease in the micro-hardness of ITZs in RAC, and the micro-hardness of ITZs decreased linearly with the increase of chloride ion attack days. The
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ITZOA-NM was the weak point in RACOC40-NC30 which represented the old concrete strength was higher than the new concrete strength. However, the ITZOA-NM and ITZOA-OM were both the weak points in RACOC40-NC50 which represented the old concrete strength was lower than the new concrete strength. For RAC subjected to chloride ion attack, the properties of ITZs between the aggregate and mortar should receive sufficient attention. CRediT authorship contribution statement Gongbing Yue: Methodology, Writing - review & editing. Zhiming Ma: Supervision, Writing - review & editing. Miao Liu: Validation, Formal analysis. Chaofeng Liang: Writing - review & editing. Guangzhong Ba: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors gratefully acknowledge the funding by the National Natural Science Foundation of China (51578297, 51708319) and China Postdoctoral Science Foundation (2019M651579) References [1] J.Z. Xiao, Z.M. Ma, T.B. Sui, A. Akbarnezhad, Z.H. Duan, Mechanical properties of concrete mixed with recycled powder produced from construction and demolition waste, J. Clean. Prod. 188 (2018) 720–731. [2] P. Rajhans, S.K. Panda, S. Nayak, Sustainable self compacting concrete from C&D waste by improving the microstructures of concrete ITZ, Constr. Build. Mater. 163 (2018) 557–570. [3] Z.M. Ma, W. Li, H.X. Wu, C.W. Cao, Chloride permeability of concrete mixed with activity recycled powder obtained from C&D waste, Constr. Build. Mater. 199 (2019) 652–663. [4] Z.M. Ma, M. Liu, Z.H. Duan, Effects of active waste powder obtained from C&D waste on the microproperties and water permeability of concrete, J. Clean. Prod. (2020) 120518. [5] D. Yu, H. Duan, Q. Song, X. Li, H. Zhang, H. Zhang, J. Wang, Characterizing the environmental impact of metals in construction and demolition waste, Environ. Sci. Pollut. Res. 25 (14) (2018) 13823–13832. [6] V.W. Tam, M. Soomro, A.C.J. Evangelista, A review of recycled aggregate in concrete applications (2000–2017), Constr. Build. Mater. 172 (2018) 272–292. [7] C.F. Liang, B.H. Pan, Z.M. Ma, Z.H. He, Z.H. Duan, Utilization of CO2 curing to enhance the properties of recycled aggregate and prepared concrete: a review, Cem. Concr. Compos. 105 (2020) 103446. [8] I.S. Del Bosque, W. Zhu, T. Howind, A. Matías, M.S. De Rojas, C. Medina, Properties of interfacial transition zones (ITZs) in concrete containing recycled mixed aggregate, Cem. Concr. Compos. 81 (2017) 25–34. [9] H. Guo, C. Shi, X. Guan, J. Zhu, Y. Ding, T.C. Ling, Y. Wang, Durability of recycled aggregate concrete–a review, Cem. Concr. Compos. 89 (2018) 251–259. [10] M. Ahmadi, S. Farzin, A. Hassani, M. Motamedi, Mechanical properties of the concrete containing recycled fibers and aggregates, Constr. Build. Mater. 144 (2017) 392–398. [11] W.G. Li, J.Z. Xiao, C.J. Shi, C.S. Poon, Structural behaviour of composite members with recycled aggregate concrete-an overview, Adv. Struct. Eng. 18 (6) (2015) 919–938. [12] J.Z. Xiao, W.G. Li, Z. Sun, D.A. Lange, S.P. Shah, Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation, Cem. Concr. Compos. 37 (2013) 276–292. [13] W.G. Li, J.Z. Xiao, Z.H. Sun, S. Kawashima, S.P. Shah, Interfacial transition zones in recycled aggregate concrete with different mixing approaches, Constr. Build. Mater. 35 (2012) 1045–1055. [14] J. Ying, J. Xiao, L. Shen, M.A. Bradford, Five-phase composite sphere model for chloride diffusivity prediction of recycled aggregate concrete, Mag. Concr. Res. 65 (9) (2013) 573–588. [15] T. Ikumi, I. Segura, Numerical assessment of external sulfate attack in concrete structures. A review, Cem. Concr. Res. 121 (2019) 91–105. [16] J. Yao, J. Chen, C. Lu, Entropy evolution during crack propagation in concrete under sulfate attack, Constr. Build. Mater. 209 (2019) 492–498. [17] S. Gao, J. Jin, G. Hu, L. Qi, Experimental investigation of the interface bond properties between SHCC and concrete under sulfate attack, Constr. Build. Mater. 217 (2019) 651–663.
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