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Improvement in corrosion resistance of recycled aggregate concrete by nano silica suspension modification on recycled aggregates
Cement and Concrete Composites 106 (2020) 103476
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Cement and Concrete Composites journal homepage: http://www.elsevier.com/locate/cemconcomp
Improvement in corrosion resistance of recycled aggregate concrete by nano silica suspension modification on recycled aggregates Weilai Zeng a, b, Yuxi Zhao a, Haibing Zheng b, Chi sun Poon b, * a b
Institute of Structural Engineering, Zhejiang University, Hangzhou, 310058, Zhejiang, PR China Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
A R T I C L E I N F O
A B S T R A C T
Keywords: Recycled aggregate concrete Nano-modification Corrosion Cracks Interfacial transition zone
This work modified recycled aggregates (RAs) by soaking them in a nano-silica (NS) suspension and evaluated the modification effects on recycled aggregate concrete (RAC). Firstly, different soaking times were studied based on the surface microhardness and the penetration depth of NS particles in the RA. Secondly, mechanical prop erties (e.g., compressive strength and microhardness of interfacial transition zones) and durability properties (e. g., steel corrosion and corrosion-induced cracking) were tested. The 1-h soaking modification was selected as the optimal soaking time. The nano-modification improved the protection of steel and the resistance to corrosioninduced cracking, which was believed to be related to the improvement of the ITZ properties according to the microhardness test results.
1. Introduction In order to recycle construction and demolition wastes, and to conserve natural sand and stone resources, recycled concrete aggregates (RAs) produced from construction and demolition waste have been used to replace natural coarse aggregate for the production of new concrete, recycled aggregate concrete (RAC). RAs generally exhibit poorer prop erties [1–3], for example, higher water absorption and lower apparent density, which are caused by the presence of old mortar attached. Many studies showed that RAC normally exhibited inferior mechanical and durability properties, when the replacement of RAs was 100% [1,4–9]. Recently, a range of modification methods have been proposed to improve the properties of RA and RAC, which can be classified into two approaches: one is to remove the old mortar from RAs, e.g., mechanical rubbing [10], microwave heating [11,12], ultrasonic cleaning [13] and acid cleaning [14,15]; the other is to enhance the properties of the old mortar of RAs, e.g., accelerated CO2 carbonation [16], microbial induced carbonation precipitation [17,18] and adding nano materials in the mixture [19–21]. The nano material modification technology had been previously used to improve the properties of natural aggregate concrete [22,23], and the mechanism of modification was revealed. It was believed that nano particles not only accelerated cement hydration but also filled the pores in the concrete [24,25].
With the development of nano material technology, nano materials have become cheaper especially in China. For example, the price of the nano-silica (NS) suspension (30% nano particle by weight) used in this study was 4800 Chinese Yuan (US$680) per ton. Moreover, the con sumption of nano particles is very small (about 4 kg in 1 m3 concrete) during the modification process as demonstrated by some previous works [26–28]. Therefore, it is expected that the cost of using nano material modification technology would not be excessive, at about 60 Chinese Yuan (US$9) per cubic meter of concrete and it is believed that the use of nano material modification technology is feasible in practical applications. Based on that, nano material modification technology has been applied to the enhancement of RAC. Bibhuti et al. [26] combined the use of colloidal NS and RAs in concrete and found that the compressive strength and the density increased, while the water absorption and the voids volume decreased. Li et al. [27] studied the dynamic behaviors of RAC incorporated with NS under impact loading. The results showed that nano-modified RACs exhibited higher quasi-static and dynamic compressive strengths, and lower dynamic increase factor values compared to the control RAC. They [28] also examined the micro structures and porosity of the interfacial transition zone (ITZ) in the nano-modified RAC by using scanning electron microscopy and mercury intrusion porosimetry. The results showed that NS improved the microstructure properties and enhanced the mechanical strength of
* Corresponding author. E-mail address: [email protected] (C. Poon). https://doi.org/10.1016/j.cemconcomp.2019.103476 Received 8 April 2019; Received in revised form 5 November 2019; Accepted 19 November 2019 Available online 22 November 2019 0958-9465/© 2019 Elsevier Ltd. All rights reserved.
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2. Materials
Table 1 Mix proportion of mortar. Mass ratio
Cement
Water
Natural river sand
417 kg
208 kg
833 kg
2.1. Recycled coarse aggregate Two types of RAs were used in this study. To find the optimal soaking time, laboratory prepared mortar cubes with sizes of 20 � 20 � 20 mm were prepared as model RAs. The mix proportion of the mortar is listed in Table 1. 14-d steam curing at 60 � C was applied to these mortar cubes before the NS modification. Moreover, an industrial sourced RAs ranging from 10 to 20 mm were provided by a construction and de molition waste recycling plant, used to prepare RAC to study the effect of nano-modification on the mechanical and durability properties of RAC. The residual mortar content of the industrial sourced RAs was 18%. The properties of the collected RA are shown in Table 2.
Table 2 Properties of industrial sourced coarse aggregate before and after NS modification. Unmodified RAs Modified RAs
Water absorption
Apparent density
8.22 � 0.22% 7.38 � 0.28%
2566 � 46 kg/m3 2588 � 29 kg/m3
2.2. Other materials Table 3 Concrete mixtures. No.
Cement
Water
Natural river sand
Unmodified RA
Modified RA
CRAC (control) MRAC (modified)
350 kg
192.5 kg 192.5 kg
760 kg
980 kg
-
760 kg
-
980 kg
350 kg
In this study, ASTM Type I ordinary Portland cement with a grade of 52.5 MPa was used. The fine aggregate was natural river sand. The NS suspension was a water-based colloidal NS suspension, purchased from a company in Shaoxing, China. The content of NS particle was 30% by weight, and the size of the NS particle was from 8 to 15 nm. The pH value of the NS suspension was about 10.5. 2.3. Mixtures of concrete
RAC. Moreover the porosity of RAC was obviously reduced by the NS incorporation. Besides, adding NS into the mixture of RAC, presoaking RAs in the NS suspensions was another nano-modification approach. Shaikh et al. [29] compared the effect of presoaking RAs in a NS suspension and direct mixing NS in the concrete on the properties of RAC. It was found that RAC prepared with the presoaked RA had a lower volume of permeable voids, water sorptivity and chloride ion penetration. Zhang et al. [30] modified RAs by using two cement slurries containing nanomaterials and examined the modification effects on microstructure and macro properties of RAC. It was found that the elastic modulus of new ITZs between the old and new cement mortars was enhanced, proving the improvement of the properties of both the RA and RAC on the macro scale. But in these previous works, only one soaking duration was used with no explanation provided on the choice of such. Therefore it is one of interest to optimize the soaking time to improve the modification efficiency. Moreover structural concretes are always incorporated with steel bars and usually suffer from the attack of aggressive ions (such as chloride ions) which may induce the steel corrosion. Corrosion of the steel bars will produce rust, inducing expansive pressure and causing cracks [31–35]. Previous studies have shown that higher risks of steel corrosion are associated with RAC [36] due to the poorer durability properties of RAC, but few have assessed the effect of RA modification on the corrosion behavior. Therefore it is another of interest to study the effect of nano-modification of RA on steel corrosion and the corrosion-induced cracking.
To investigate the effect of the colloidal NS suspension soaking modification, two groups of concrete were prepared i.e. the concrete with unmodified RAs (CRAC) and modified RAs (MRAC) respectively, as listed in Table 3. 3. Modification methods For the optimal soaking time study, different soaking times were designed as follows: 0 s, 6 s, 2.5 min, 1 h, and 24 h. Before modification, the content of NS particles in the suspension was adjusted to 15% by weight. For each soaking time group, two mortar cubes samples were prepared. One was immersed in the NS suspension for the surface microhardness test. The other was sealed with silica gel except for the top surface so that only this surface was exposed. Then the mortar was immersed in the NS suspension for the penetration depth test. After the NS modification, the modified mortar cubes were dried in air for 7 d following by immersion in absolute ethyl alcohol for 24 h to prevent further cement hydration. Finally, these mortar cubes were oven dried at 60 � C until constant weight and kept in a vacuum chamber for the subsequent testing. For the modification effect study, the industrial sourced RAs were immersed in the NS suspension (15% NS particle by weight) for the
Fig. 2. Indent areas and the corresponding indent matrices for NS penetration depth test.
Fig. 1. Schematic diagram of sample processing for NS penetration depth test. 2
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Fig. 3. Sample preparation process of ITZ microhardness test.
Fig. 4. Indent areas and corresponding indent matrices for ITZ microhard ness test.
optimal soaking time determined by the above model mortar test. Then the modified RAs were taken out from the NS suspension and air dried for 7 d before they were used to cast the RAC. The water absorption and apparent density of the industrial sourced RAs before and after NS modification are listed in Table 2.
Fig. 6. Frequency distributions of microhardness values on NS modified sur faces with different soaking times.
The microhardness test was conducted with a digital Vickers microhardness tester (HVX-1000A, China) equipped with 40 measure ment lens and 10 objective lens. Measurements were carried out using a 10 g load and 10 s loading time. A series of 50 indentations were con ducted on each specimen to obtain a large amount of data for statistical analysis.
4. Test methods 4.1. Optimal soaking time study Based on the mechanism of nano-modification, the microhardness of test surface and the penetration depth of NS particles in the NS modified model mortar can reflect the optimal time required for the nanomodification.
4.1.2. Penetration depth test For the NS penetration depth test, the mortar cube specimens were cut into two halves and the cutting surfaces was chosen as test surface as shown in Fig. 1. Then the test surfaces were grinded and polished similar to the surface microhardness test. The equipment and the measurement method used were the same as those described in Section 4.1.1. The indent areas were designed as a 19 � 0.9 mm rectangular grid and a 20 � 10 matrix (in total 200 indents) was used for the grid indents, as shown in Fig. 2.
4.1.1. Surface microhardness test For the microhardness test, one of the treated surfaces of the mortar cubes specimens was chosen as the test surface. The test surfaces were ground and polished by a polishing equipment (Buehler AutoMet 250) with grits of P180, P400, P1200 and MetaDi supreme diamond of 9 μm, 3 μm, 0.5 μm successively. Next, these specimens were cleaned with an ultrasonic cleaner and dried at 60 � C until their weight were stable. Finally these specimens were stored in a vacuum chamber for the further tests.
Fig. 5. Corrosion-induced crack propagation observation: (a) specimens; (b) set up for inducing corrosion damage. 3
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Fig. 9. Penetration depths with different soaking times.
Fig. 7. Average microhardness values of modified surfaces at different soak ing times.
concrete. Both ends of the steel bars were treated with epoxy resin to prevent corrosion at those locations. They served as electrodes in the cubes. The side and bottom surfaces of each cube were treated with a waterproof paint to ensure that chloride ion mainly penetrated through the top surface. A wire was tied to one end of each steel bars to connect to the electrochemical workstation. The design and preparation of the samples was based on a previous work by the authors [36]. This method was used to ensure that the corrosion could be monitored continuously without damaging the specimens. At the curing age of 35 d, all the specimens were subjected to 22 wetting and drying cycles to accelerate the corrosion of steel. Every cycle was consisted of a 3-day wetting period and a 4-day drying period. The specimens were submerged in a 3.5% NaCl solution during the wetting period. During the drying period, the specimens were exposed to air in the laboratory at 25 � C. On the last day of each wetting-drying cycle, the electrochemical workstation was connected to the cubic specimens to obtain the polar ization curve of the working steel bar. Then the Echem Analyst software was used to fit a Tafel curve with the date to determine the values of the corrosion current density icorr and open circuit potential E.
4.2. Test on the RAC sample 4.2.1. ITZ microhardness test To reveal the effect of the NS modification on the micro mechanical properties, three 20 � 20 � 10 mm concrete slices were cut from a 100 mm concrete cube at the age of 28 d from each group of concrete as shown in Fig. 3. Then these slices were immersed in absolute ethyl alcohol for 24 h, to stop the cement hydration. After that these slices were oven dried at 60 � C for 1 d and were embedded in an epoxy resin with a 20 mm height � 30 mm diameter cylindrical rubber mould. Next, these specimens were polished, cleaned and dried, using the same methods mentioned in Section 4.1.1. Finally these specimens were stored in a vacuum chamber before further testing. The microhardness equipment used and the measurement method were the same as mentioned in Section 4.4.1. To obtain the micro hardness of the ITZs, the indent area was designed as a 300 � 250 μm rectangular grid with an 11 � 6 matrix, as shown in Fig. 4. The 2-D distribution maps of each indent area were drawn with the ContourColour fill function of Origin. In the 2-D distribution maps, different colors represent the different microhardness ranges. Based on the microhardness distribution maps, the left and right boundaries of the ITZs were determined. The microhardness value of each indentation within the boundaries were collected and calculated with statistical analysis. For each type of ITZ and mortar, three indent areas were tested.
4.2.3. Corrosion-induced cracks observation To investigate the NS modification effect on corrosion-induced cracking, three 80 mm-diameter and 150 mm-height cylindrical rein forced concrete specimens with a 12 mm-diameter plain steel bar was used for the observation of corrosion-induced cracks as shown in Fig. 5
4.2.2. Steel corrosion rate test To obtain effect of NS modification on the steel corrosion of RAC, three 100 mm cubic reinforced concrete specimens were prepared for the steel corrosion rate study. Two S275J0 (EN 10025-2: 2004) hot rolled steel round bars with a diameter of 12 mm were embedded in the
Fig. 8. Microhardness values at different depths.
Fig. 10. Picture showing three types of ITZs. 4
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Fig. 11. Microhardness distribution maps within the indent area: (a) ITZ1-CRAC; (b) ITZ1-MRAC; (c) ITZ2- CRAC; (d) ITZ2-MRAC; (e) ITZ3-CRAC; (f) ITZ3-MRAC.
(a). Both ends of specimens and the exposed steel bars were coated with epoxy resin to avoid corrosion of the end of steel. One end of steel bar was connected a DC power supply. The current density applied was 200 μA/cm2 [36]. The other end of the steel bar was additionally covered with a protective paint to avoid the leaking of corrosion products to the NaCl solution. At the curing age of 35 d, 12 wetting and drying cycles coupled with DC power were applied to the cylindrical specimens to accelerate the steel corrosion. All the specimens were wrapped in stainless steel nets with sponge material filling the space between the specimens and the nets as shown in Fig. 5(b). Each cycle consisted a 1-day wetting period
and a 2-day drying period. During the wetting period, the sides of the specimens and the wrapping sponge material were saturated with 3.5% NaCl solution, coupled with the direct current. During the drying period, the DC power was switched off, and the specimens were removed from the sponge and exposed to air. On the last day of each two wetting-drying cycle, the propagation of corrosion-induced cracks on the surfaces was recorded by a digital camera. With the function of AutoCAD, the length of each crack was determined. The widths of the cracks were measured with a PTS-C10 intelligent crack measuring instrument produced by a company in Wuhan, China. 5
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soaking time was, the more lately the microhardness decreased over the depth. The black dashed line at 42.82 HV was drawn in Fig. 8 to represent the mean microhardness value of the untreated surfaces. It can be seen that the microhardness value at inner part of the sample was lower than that of the untreated surface. It might be because the dis tribution of aggregate particles in the boundary layers was different from that at the center, which is also known as the wall effect [37]. The intersection points of the black dashed line and the microhardness-distance curves can be regarded as the penetration depths of NS modification with a rough estimate. The NS penetration depths were obtained with the function ‘Screen Reader’ of Origin as shown in Fig. 9. From Fig. 9, the penetration depth increased with increasing soaking time, and the penetration depth with1-hour soaking was 76% of that with 24-h soaking. Combined with the results in Section 5.1.1, it was believed that 1-h soaking was sufficient, which was also applied to modify the industrial sourced RAs.
Fig. 12. Microhardness of three types of ITZs.
5. Result and discussion
5.2. Modification of RAC with industrial sourced RA
5.1. Optimal soaking time for NS modification
5.2.1. ITZ’s microhardness Based on the Chinese Standard (GB/T 50081-2002), three 100 mm concrete cubes were tested. The compressive strength of the RAC at the age of 28 d was improved from 49.3 � 2.5 to 56.7 � 2.8 MPa after NS modification. There usually are three types of ITZs in the RAC as shown in Fig. 10, including ITZ1 between the virgin natural aggregate and the new mortar, ITZ2 between the old mortar and the new mortar, and ITZ3 between the virgin natural aggregate and the old mortar. For each type of ITZ, three indent areas were selected and tested and the distribution maps of the microhardness values were obtained. Very similar results were obtained for the three areas and the representative distribution map for each type of ITZ is shown in Fig. 11. In Fig. 11, the boundaries of ITZ1, ITZ2 and ITZ3 were plotted with a red line according to the color change in the map because the micro hardness values of the ITZ were lower. It can be seen that the width of ITZ3 was much narrower than that of ITZ1 and ITZ2, because ITZ3 had been existed for a long time so that it mechanical property might have been improved with the progress of hydration. Moreover, the width of ITZ2 was decreased from 90 to 60 μm as shown in Fig. 11(c) and (d) after the NS modification. Fig. 12 shows the mean values and the standard deviations of the microhardness of three types of ITZs before and after the NS modification. For each type of ITZ, three indent areas were measured. According to Fig. 12, before NS modification, the microhardness of ITZ2 was the lowest. After the modification, the microhardness of ITZ2 increased significantly. A light increase was also observed for ITZ1. However the microhardness values of ITZ3 were similar before and after
5.1.1. Surface microhardness test Fig. 6 shows the frequency distributions of microhardness on the NS modified surfaces with different soaking times. It can be seen in Fig. 6 that the frequency of small microhardness values (from 20 to 30 HV) decreased obviously for all the mortar cubes with increasing soaking time. The frequency distribution curves showed that the most dominant frequency peak shifted to the right as the soaking time increased, e.g., the major peak of specimens without modification fell in the interval of 40–50 HV, while for specimens with 24 h-soaking modification the major peak fell in the interval of 50–60 HV. Moreover, the average microhardness values of the modified sur faces with different soaking times are shown in Fig. 7. From Fig. 7, the microhardness of the modified surfaces increased from 42.82 to 54.12 HV after 24-h soaking. This was because more NS particles would penetrate into the mortar cubes and modified the sur faces. A sharp increase of the microhardness value was found within the first hour of soaking. Therefore 1-h soaking time is believed to be suf ficient to enhance the microhardness of the modified surfaces. 5.1.2. Penetration depth test To study the penetration depth of NS particles in the specimens, for each soaking time group the microhardness at the same depth away from treated surface were determined to obtain the mean values as shown in Fig. 8. It can be found in Fig. 8 that the microhardness value decreased with increasing depth, and finally became stable. Moreover the longer the
Fig. 13. CRAC’s and MRAC’s: (a) corrosion current density; (b) open circuit potential. 6
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Fig. 14. Propagation path of external corrosion-induced cracks of (a) CRAC; (b) MRAC.
the modification. The surface of the old mortar was enhanced after soaking in the NS suspension, which helped improving the properties of ITZ2 between the old mortar and the new mortar. For ITZ1, some NS particles were absorbed on the surface of the virgin natural aggregate, which were released to the new mortar during the concrete casting process. That might improve the properties of ITZ1 between the virgin natural aggregate and the new mortar. For ITZ3, it was difficult for the NS particle to penetrate into to such a great depth, rendering the effect of modification not noticeable.
amounts of steel corrosion product ρ was determined by integrating the corrosion current density icorr with respect to time [36]. While in this study, icorr was measured only on the last day of each drying and wetting cycle, which was not able to represent the variation in icorr over the whole drying-wetting cycle. Therefore the relative steel corrosion Rρ was introduced as: PM MRAC PM MRAC ρMRAC m¼1 icorr;m � tm m¼1 icorr;m Rρ ¼ ¼ PM CRAC ¼ PM CRAC ; (1) ρCRAC i � t m m¼1 corr;m m¼1 icorr;m
5.2.2. Steel corrosion rate Fig. 13 shows the corrosion current density and open circuit poten tial of the two concrete mixtures during the 22-weeks drying and wet ting cycles. From Fig. 13, it can be seen that for both CRAC and MRAC, the corrosion current density increased with increasing drying and wetting cycles, while an opposite trend was observed with the open circuit po tential. It means that, with the increasing drying and wetting cycles, the steels in CRAC and MRAC were more likely to corrode. This was because harmful materials such as H2O and O2 penetrated to surface of steel and reacted with iron ions, with the effect of drying and wetting cycle. Be sides Cl as a catalyst would promote the corrosion of steel bars and reduce the resistance of concrete. Therefore the increasing chloride ion ingression accelerated the corrosion of steel. Fig. 13 also shows that MRAC exhibited a lower steel corrosion current density than CRAC. In Fig. 13(a), the blue dash line at 0.1 μA/ cm2, reflects that the steel was considered to be corroded [38]. It can be found that the steel in CRAC might start to corrode at the 14th wetting-drying cycle, which was earlier than the steel in MRAC. After the NS modification, the ITZs within the RAC became denser which made it harder for the harmful materials such as Cl , H2O and O2 to penetrate through the concrete to the steel. To have the better analysis on the steel corrosion, the approximate
where M was the number of drying and wetting cycles, (in this study M CRAC ¼ 22); iMRAC corr;m and icorr;m was the corrosion current density of MRAC and CRAC respectively at the last day of the mth drying and wetting cycle; tm was the time of the mth drying and wetting cycle (in this study tm ¼ 7 d). The mean value of Rρ for CRAC was 1 obviously. While the mean value of Rρ for MRAC decreased to 0.59 after the NS modification. Therefore modification by soaking RAs in NS suspension could signifi cantly improve the RAC’s ability of protecting steel from corrosion. 5.2.3. Corrosion-induced cracks propagations Fig. 14 shows the propagation path of corrosion-induced cracks on the side surface of CRAC and MRAC at the 12th wetting-drying cycle. As shown in Fig. 15, there were two cracks on the side surface of the CRAC; while for MRAC there was only one crack. Because MRAC had the stronger resistance to steel corrosion as mentioned in Section 5.2.2, the steel in CRAC would form more corrosion products. Moreover, as MRAC showed better mechanical properties as discussed in Section 5.2.1, the expansion caused by the corrosion products would induce cracking more easily in CRAC. The areas of corrosion-induced cracks at different damage cycles were calculated and are shown in Fig. 15. In Fig. 15, both the cracks areas of CRAC and MRAC increased over the damage cycles. Especially at the later damage cycles the areas of cracks formed showed a rapid increase. It also can be found that at the 8th damage cycle, corrosion-induced cracks started to appear on the side surface of CRAC. But for MRAC, cracks started appearing on the side surface at the 10th damage cycle. Besides, the crack areas of CRAC was larger than that of MRAC. With the direct current, the chloride ion migrated to the surface of the steel in CRAC earlier, which accelerated the corrosion rate of steel. Additionally the relatively poorer mechanical properties of CRAC resulted in earlier cracking, which made chloride ion migrate into concrete more easily. 6. Conclusions In this study, RAs were modified by soaking in the NS suspension with the determined efficient soaking time, and the modification effects are summarized as follows: (1) The increasing trends in the treated surface microhardness and the penetration depths of NS particles proved that soaking RAs in
Fig. 15. Areas of corrosion-induced cracks of CRAC and MRAC. 7
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NS suspension for 1 h could achieve high efficiency in improving the properties of model RAs. (2) With the modification by 1-h soaking in the NS suspension, enhanced mechanical properties were achieved: the compressive strength improved obviously; the width of ITZ between the old mortar and the new mortar decreased, and the microhardness of ITZ increased. (3) After NS modifications, RAC exhibited improved durability properties, including stronger protection of steel from corrosion and less corrosion-induced cracks. It was also related to the improvement of ITZ at the micro level.
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Declaration of competing interest I declare there is no conflict of interest re the publication of the paper. Acknowledgements Financial supports from the National Natural Science Foundation of China (51578489) and The Hong Kong Polytechnic University are gratefully acknowledged. References [1] N. Kisku, H. Joshi, M. Ansari, et al., A critical review and assessment for usage of recycled aggregate as sustainable construction material, Constr. Build. Mater. 131 (2017) 721–740. [2] T.C. Hansen, Recycling of Demolished Concrete and Masonry, CRC Press, 2014. [3] M. Martín-Morales, M. Zamorano, A. Ruiz-Moyano, et al., Characterization of recycled aggregates construction and demolition waste for concrete production following the Spanish Structural Concrete Code EHE-08, Constr. Build. Mater. 25 (2) (2011) 742–748. [4] J. Xiao, W. Li, Y. Fan, et al., An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (2012) 364–383. [5] M. Etxeberria, E. V� azquez, A. Marí, et al., Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete, Cement Concr. Res. 37 (5) (2007) 735–742. [6] Q. Liu, J. Xiao, Z. Sun, Experimental study on the failure mechanism of recycled concrete, Cement Concr. Res. 41 (10) (2011) 1050–1057. [7] N. Otsuki, S. Miyazato, W. Yodsudjai, Influence of recycled aggregate on interfacial transition zone, strength, chloride penetration and carbonation of concrete, J. Mater. Civ. Eng. 15 (5) (2003) 443–451. [8] K. Shicong, C.S. Poon, Compressive strength, pore size distribution and chlorideion penetration of recycled aggregate concrete incorporating class-F fly ash, J. Wuhan Univ. Technol. 21 (4) (2006) 130–136. [9] Hongru Zhang, Yuxi Zhao, Integrated interface parameters of recycled aggregate concrete, Constr. Build. Mater. 101 (2015) 861–877. [10] H. Shima, R. Matsuhashi, Y. Yoshida, et al., Life cycle analysis of high quality recycled aggregate produced by heating and rubbing method, IEEJ Trans. Electron. Inf. Syst. 123 (10) (2003) 1680–1687. � F. Bourgeois, et al., Assessment of a microwave-assisted recycling [11] K. Bru, S. TouzE, process for the recovery of high-quality aggregates from concrete waste, Int. J. Miner. Process. 126 (2) (2014) 90–98. [12] A. Akbarnezhad, K.C.G. Ong, M.H. Zhang, et al., Microwave-assisted beneficiation of recycled concrete aggregates, Constr. Build. Mater. 25 (8) (2011) 3469–3479.
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Cement and Concrete Composites journal homepage: http://www.elsevier.com/locate/cemconcomp
Improvement in corrosion resistance of recycled aggregate concrete by nano silica suspension modification on recycled aggregates Weilai Zeng a, b, Yuxi Zhao a, Haibing Zheng b, Chi sun Poon b, * a b
Institute of Structural Engineering, Zhejiang University, Hangzhou, 310058, Zhejiang, PR China Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
A R T I C L E I N F O
A B S T R A C T
Keywords: Recycled aggregate concrete Nano-modification Corrosion Cracks Interfacial transition zone
This work modified recycled aggregates (RAs) by soaking them in a nano-silica (NS) suspension and evaluated the modification effects on recycled aggregate concrete (RAC). Firstly, different soaking times were studied based on the surface microhardness and the penetration depth of NS particles in the RA. Secondly, mechanical prop erties (e.g., compressive strength and microhardness of interfacial transition zones) and durability properties (e. g., steel corrosion and corrosion-induced cracking) were tested. The 1-h soaking modification was selected as the optimal soaking time. The nano-modification improved the protection of steel and the resistance to corrosioninduced cracking, which was believed to be related to the improvement of the ITZ properties according to the microhardness test results.
1. Introduction In order to recycle construction and demolition wastes, and to conserve natural sand and stone resources, recycled concrete aggregates (RAs) produced from construction and demolition waste have been used to replace natural coarse aggregate for the production of new concrete, recycled aggregate concrete (RAC). RAs generally exhibit poorer prop erties [1–3], for example, higher water absorption and lower apparent density, which are caused by the presence of old mortar attached. Many studies showed that RAC normally exhibited inferior mechanical and durability properties, when the replacement of RAs was 100% [1,4–9]. Recently, a range of modification methods have been proposed to improve the properties of RA and RAC, which can be classified into two approaches: one is to remove the old mortar from RAs, e.g., mechanical rubbing [10], microwave heating [11,12], ultrasonic cleaning [13] and acid cleaning [14,15]; the other is to enhance the properties of the old mortar of RAs, e.g., accelerated CO2 carbonation [16], microbial induced carbonation precipitation [17,18] and adding nano materials in the mixture [19–21]. The nano material modification technology had been previously used to improve the properties of natural aggregate concrete [22,23], and the mechanism of modification was revealed. It was believed that nano particles not only accelerated cement hydration but also filled the pores in the concrete [24,25].
With the development of nano material technology, nano materials have become cheaper especially in China. For example, the price of the nano-silica (NS) suspension (30% nano particle by weight) used in this study was 4800 Chinese Yuan (US$680) per ton. Moreover, the con sumption of nano particles is very small (about 4 kg in 1 m3 concrete) during the modification process as demonstrated by some previous works [26–28]. Therefore, it is expected that the cost of using nano material modification technology would not be excessive, at about 60 Chinese Yuan (US$9) per cubic meter of concrete and it is believed that the use of nano material modification technology is feasible in practical applications. Based on that, nano material modification technology has been applied to the enhancement of RAC. Bibhuti et al. [26] combined the use of colloidal NS and RAs in concrete and found that the compressive strength and the density increased, while the water absorption and the voids volume decreased. Li et al. [27] studied the dynamic behaviors of RAC incorporated with NS under impact loading. The results showed that nano-modified RACs exhibited higher quasi-static and dynamic compressive strengths, and lower dynamic increase factor values compared to the control RAC. They [28] also examined the micro structures and porosity of the interfacial transition zone (ITZ) in the nano-modified RAC by using scanning electron microscopy and mercury intrusion porosimetry. The results showed that NS improved the microstructure properties and enhanced the mechanical strength of
* Corresponding author. E-mail address: [email protected] (C. Poon). https://doi.org/10.1016/j.cemconcomp.2019.103476 Received 8 April 2019; Received in revised form 5 November 2019; Accepted 19 November 2019 Available online 22 November 2019 0958-9465/© 2019 Elsevier Ltd. All rights reserved.
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2. Materials
Table 1 Mix proportion of mortar. Mass ratio
Cement
Water
Natural river sand
417 kg
208 kg
833 kg
2.1. Recycled coarse aggregate Two types of RAs were used in this study. To find the optimal soaking time, laboratory prepared mortar cubes with sizes of 20 � 20 � 20 mm were prepared as model RAs. The mix proportion of the mortar is listed in Table 1. 14-d steam curing at 60 � C was applied to these mortar cubes before the NS modification. Moreover, an industrial sourced RAs ranging from 10 to 20 mm were provided by a construction and de molition waste recycling plant, used to prepare RAC to study the effect of nano-modification on the mechanical and durability properties of RAC. The residual mortar content of the industrial sourced RAs was 18%. The properties of the collected RA are shown in Table 2.
Table 2 Properties of industrial sourced coarse aggregate before and after NS modification. Unmodified RAs Modified RAs
Water absorption
Apparent density
8.22 � 0.22% 7.38 � 0.28%
2566 � 46 kg/m3 2588 � 29 kg/m3
2.2. Other materials Table 3 Concrete mixtures. No.
Cement
Water
Natural river sand
Unmodified RA
Modified RA
CRAC (control) MRAC (modified)
350 kg
192.5 kg 192.5 kg
760 kg
980 kg
-
760 kg
-
980 kg
350 kg
In this study, ASTM Type I ordinary Portland cement with a grade of 52.5 MPa was used. The fine aggregate was natural river sand. The NS suspension was a water-based colloidal NS suspension, purchased from a company in Shaoxing, China. The content of NS particle was 30% by weight, and the size of the NS particle was from 8 to 15 nm. The pH value of the NS suspension was about 10.5. 2.3. Mixtures of concrete
RAC. Moreover the porosity of RAC was obviously reduced by the NS incorporation. Besides, adding NS into the mixture of RAC, presoaking RAs in the NS suspensions was another nano-modification approach. Shaikh et al. [29] compared the effect of presoaking RAs in a NS suspension and direct mixing NS in the concrete on the properties of RAC. It was found that RAC prepared with the presoaked RA had a lower volume of permeable voids, water sorptivity and chloride ion penetration. Zhang et al. [30] modified RAs by using two cement slurries containing nanomaterials and examined the modification effects on microstructure and macro properties of RAC. It was found that the elastic modulus of new ITZs between the old and new cement mortars was enhanced, proving the improvement of the properties of both the RA and RAC on the macro scale. But in these previous works, only one soaking duration was used with no explanation provided on the choice of such. Therefore it is one of interest to optimize the soaking time to improve the modification efficiency. Moreover structural concretes are always incorporated with steel bars and usually suffer from the attack of aggressive ions (such as chloride ions) which may induce the steel corrosion. Corrosion of the steel bars will produce rust, inducing expansive pressure and causing cracks [31–35]. Previous studies have shown that higher risks of steel corrosion are associated with RAC [36] due to the poorer durability properties of RAC, but few have assessed the effect of RA modification on the corrosion behavior. Therefore it is another of interest to study the effect of nano-modification of RA on steel corrosion and the corrosion-induced cracking.
To investigate the effect of the colloidal NS suspension soaking modification, two groups of concrete were prepared i.e. the concrete with unmodified RAs (CRAC) and modified RAs (MRAC) respectively, as listed in Table 3. 3. Modification methods For the optimal soaking time study, different soaking times were designed as follows: 0 s, 6 s, 2.5 min, 1 h, and 24 h. Before modification, the content of NS particles in the suspension was adjusted to 15% by weight. For each soaking time group, two mortar cubes samples were prepared. One was immersed in the NS suspension for the surface microhardness test. The other was sealed with silica gel except for the top surface so that only this surface was exposed. Then the mortar was immersed in the NS suspension for the penetration depth test. After the NS modification, the modified mortar cubes were dried in air for 7 d following by immersion in absolute ethyl alcohol for 24 h to prevent further cement hydration. Finally, these mortar cubes were oven dried at 60 � C until constant weight and kept in a vacuum chamber for the subsequent testing. For the modification effect study, the industrial sourced RAs were immersed in the NS suspension (15% NS particle by weight) for the
Fig. 2. Indent areas and the corresponding indent matrices for NS penetration depth test.
Fig. 1. Schematic diagram of sample processing for NS penetration depth test. 2
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Fig. 3. Sample preparation process of ITZ microhardness test.
Fig. 4. Indent areas and corresponding indent matrices for ITZ microhard ness test.
optimal soaking time determined by the above model mortar test. Then the modified RAs were taken out from the NS suspension and air dried for 7 d before they were used to cast the RAC. The water absorption and apparent density of the industrial sourced RAs before and after NS modification are listed in Table 2.
Fig. 6. Frequency distributions of microhardness values on NS modified sur faces with different soaking times.
The microhardness test was conducted with a digital Vickers microhardness tester (HVX-1000A, China) equipped with 40 measure ment lens and 10 objective lens. Measurements were carried out using a 10 g load and 10 s loading time. A series of 50 indentations were con ducted on each specimen to obtain a large amount of data for statistical analysis.
4. Test methods 4.1. Optimal soaking time study Based on the mechanism of nano-modification, the microhardness of test surface and the penetration depth of NS particles in the NS modified model mortar can reflect the optimal time required for the nanomodification.
4.1.2. Penetration depth test For the NS penetration depth test, the mortar cube specimens were cut into two halves and the cutting surfaces was chosen as test surface as shown in Fig. 1. Then the test surfaces were grinded and polished similar to the surface microhardness test. The equipment and the measurement method used were the same as those described in Section 4.1.1. The indent areas were designed as a 19 � 0.9 mm rectangular grid and a 20 � 10 matrix (in total 200 indents) was used for the grid indents, as shown in Fig. 2.
4.1.1. Surface microhardness test For the microhardness test, one of the treated surfaces of the mortar cubes specimens was chosen as the test surface. The test surfaces were ground and polished by a polishing equipment (Buehler AutoMet 250) with grits of P180, P400, P1200 and MetaDi supreme diamond of 9 μm, 3 μm, 0.5 μm successively. Next, these specimens were cleaned with an ultrasonic cleaner and dried at 60 � C until their weight were stable. Finally these specimens were stored in a vacuum chamber for the further tests.
Fig. 5. Corrosion-induced crack propagation observation: (a) specimens; (b) set up for inducing corrosion damage. 3
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Fig. 9. Penetration depths with different soaking times.
Fig. 7. Average microhardness values of modified surfaces at different soak ing times.
concrete. Both ends of the steel bars were treated with epoxy resin to prevent corrosion at those locations. They served as electrodes in the cubes. The side and bottom surfaces of each cube were treated with a waterproof paint to ensure that chloride ion mainly penetrated through the top surface. A wire was tied to one end of each steel bars to connect to the electrochemical workstation. The design and preparation of the samples was based on a previous work by the authors [36]. This method was used to ensure that the corrosion could be monitored continuously without damaging the specimens. At the curing age of 35 d, all the specimens were subjected to 22 wetting and drying cycles to accelerate the corrosion of steel. Every cycle was consisted of a 3-day wetting period and a 4-day drying period. The specimens were submerged in a 3.5% NaCl solution during the wetting period. During the drying period, the specimens were exposed to air in the laboratory at 25 � C. On the last day of each wetting-drying cycle, the electrochemical workstation was connected to the cubic specimens to obtain the polar ization curve of the working steel bar. Then the Echem Analyst software was used to fit a Tafel curve with the date to determine the values of the corrosion current density icorr and open circuit potential E.
4.2. Test on the RAC sample 4.2.1. ITZ microhardness test To reveal the effect of the NS modification on the micro mechanical properties, three 20 � 20 � 10 mm concrete slices were cut from a 100 mm concrete cube at the age of 28 d from each group of concrete as shown in Fig. 3. Then these slices were immersed in absolute ethyl alcohol for 24 h, to stop the cement hydration. After that these slices were oven dried at 60 � C for 1 d and were embedded in an epoxy resin with a 20 mm height � 30 mm diameter cylindrical rubber mould. Next, these specimens were polished, cleaned and dried, using the same methods mentioned in Section 4.1.1. Finally these specimens were stored in a vacuum chamber before further testing. The microhardness equipment used and the measurement method were the same as mentioned in Section 4.4.1. To obtain the micro hardness of the ITZs, the indent area was designed as a 300 � 250 μm rectangular grid with an 11 � 6 matrix, as shown in Fig. 4. The 2-D distribution maps of each indent area were drawn with the ContourColour fill function of Origin. In the 2-D distribution maps, different colors represent the different microhardness ranges. Based on the microhardness distribution maps, the left and right boundaries of the ITZs were determined. The microhardness value of each indentation within the boundaries were collected and calculated with statistical analysis. For each type of ITZ and mortar, three indent areas were tested.
4.2.3. Corrosion-induced cracks observation To investigate the NS modification effect on corrosion-induced cracking, three 80 mm-diameter and 150 mm-height cylindrical rein forced concrete specimens with a 12 mm-diameter plain steel bar was used for the observation of corrosion-induced cracks as shown in Fig. 5
4.2.2. Steel corrosion rate test To obtain effect of NS modification on the steel corrosion of RAC, three 100 mm cubic reinforced concrete specimens were prepared for the steel corrosion rate study. Two S275J0 (EN 10025-2: 2004) hot rolled steel round bars with a diameter of 12 mm were embedded in the
Fig. 8. Microhardness values at different depths.
Fig. 10. Picture showing three types of ITZs. 4
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Fig. 11. Microhardness distribution maps within the indent area: (a) ITZ1-CRAC; (b) ITZ1-MRAC; (c) ITZ2- CRAC; (d) ITZ2-MRAC; (e) ITZ3-CRAC; (f) ITZ3-MRAC.
(a). Both ends of specimens and the exposed steel bars were coated with epoxy resin to avoid corrosion of the end of steel. One end of steel bar was connected a DC power supply. The current density applied was 200 μA/cm2 [36]. The other end of the steel bar was additionally covered with a protective paint to avoid the leaking of corrosion products to the NaCl solution. At the curing age of 35 d, 12 wetting and drying cycles coupled with DC power were applied to the cylindrical specimens to accelerate the steel corrosion. All the specimens were wrapped in stainless steel nets with sponge material filling the space between the specimens and the nets as shown in Fig. 5(b). Each cycle consisted a 1-day wetting period
and a 2-day drying period. During the wetting period, the sides of the specimens and the wrapping sponge material were saturated with 3.5% NaCl solution, coupled with the direct current. During the drying period, the DC power was switched off, and the specimens were removed from the sponge and exposed to air. On the last day of each two wetting-drying cycle, the propagation of corrosion-induced cracks on the surfaces was recorded by a digital camera. With the function of AutoCAD, the length of each crack was determined. The widths of the cracks were measured with a PTS-C10 intelligent crack measuring instrument produced by a company in Wuhan, China. 5
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soaking time was, the more lately the microhardness decreased over the depth. The black dashed line at 42.82 HV was drawn in Fig. 8 to represent the mean microhardness value of the untreated surfaces. It can be seen that the microhardness value at inner part of the sample was lower than that of the untreated surface. It might be because the dis tribution of aggregate particles in the boundary layers was different from that at the center, which is also known as the wall effect [37]. The intersection points of the black dashed line and the microhardness-distance curves can be regarded as the penetration depths of NS modification with a rough estimate. The NS penetration depths were obtained with the function ‘Screen Reader’ of Origin as shown in Fig. 9. From Fig. 9, the penetration depth increased with increasing soaking time, and the penetration depth with1-hour soaking was 76% of that with 24-h soaking. Combined with the results in Section 5.1.1, it was believed that 1-h soaking was sufficient, which was also applied to modify the industrial sourced RAs.
Fig. 12. Microhardness of three types of ITZs.
5. Result and discussion
5.2. Modification of RAC with industrial sourced RA
5.1. Optimal soaking time for NS modification
5.2.1. ITZ’s microhardness Based on the Chinese Standard (GB/T 50081-2002), three 100 mm concrete cubes were tested. The compressive strength of the RAC at the age of 28 d was improved from 49.3 � 2.5 to 56.7 � 2.8 MPa after NS modification. There usually are three types of ITZs in the RAC as shown in Fig. 10, including ITZ1 between the virgin natural aggregate and the new mortar, ITZ2 between the old mortar and the new mortar, and ITZ3 between the virgin natural aggregate and the old mortar. For each type of ITZ, three indent areas were selected and tested and the distribution maps of the microhardness values were obtained. Very similar results were obtained for the three areas and the representative distribution map for each type of ITZ is shown in Fig. 11. In Fig. 11, the boundaries of ITZ1, ITZ2 and ITZ3 were plotted with a red line according to the color change in the map because the micro hardness values of the ITZ were lower. It can be seen that the width of ITZ3 was much narrower than that of ITZ1 and ITZ2, because ITZ3 had been existed for a long time so that it mechanical property might have been improved with the progress of hydration. Moreover, the width of ITZ2 was decreased from 90 to 60 μm as shown in Fig. 11(c) and (d) after the NS modification. Fig. 12 shows the mean values and the standard deviations of the microhardness of three types of ITZs before and after the NS modification. For each type of ITZ, three indent areas were measured. According to Fig. 12, before NS modification, the microhardness of ITZ2 was the lowest. After the modification, the microhardness of ITZ2 increased significantly. A light increase was also observed for ITZ1. However the microhardness values of ITZ3 were similar before and after
5.1.1. Surface microhardness test Fig. 6 shows the frequency distributions of microhardness on the NS modified surfaces with different soaking times. It can be seen in Fig. 6 that the frequency of small microhardness values (from 20 to 30 HV) decreased obviously for all the mortar cubes with increasing soaking time. The frequency distribution curves showed that the most dominant frequency peak shifted to the right as the soaking time increased, e.g., the major peak of specimens without modification fell in the interval of 40–50 HV, while for specimens with 24 h-soaking modification the major peak fell in the interval of 50–60 HV. Moreover, the average microhardness values of the modified sur faces with different soaking times are shown in Fig. 7. From Fig. 7, the microhardness of the modified surfaces increased from 42.82 to 54.12 HV after 24-h soaking. This was because more NS particles would penetrate into the mortar cubes and modified the sur faces. A sharp increase of the microhardness value was found within the first hour of soaking. Therefore 1-h soaking time is believed to be suf ficient to enhance the microhardness of the modified surfaces. 5.1.2. Penetration depth test To study the penetration depth of NS particles in the specimens, for each soaking time group the microhardness at the same depth away from treated surface were determined to obtain the mean values as shown in Fig. 8. It can be found in Fig. 8 that the microhardness value decreased with increasing depth, and finally became stable. Moreover the longer the
Fig. 13. CRAC’s and MRAC’s: (a) corrosion current density; (b) open circuit potential. 6
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Fig. 14. Propagation path of external corrosion-induced cracks of (a) CRAC; (b) MRAC.
the modification. The surface of the old mortar was enhanced after soaking in the NS suspension, which helped improving the properties of ITZ2 between the old mortar and the new mortar. For ITZ1, some NS particles were absorbed on the surface of the virgin natural aggregate, which were released to the new mortar during the concrete casting process. That might improve the properties of ITZ1 between the virgin natural aggregate and the new mortar. For ITZ3, it was difficult for the NS particle to penetrate into to such a great depth, rendering the effect of modification not noticeable.
amounts of steel corrosion product ρ was determined by integrating the corrosion current density icorr with respect to time [36]. While in this study, icorr was measured only on the last day of each drying and wetting cycle, which was not able to represent the variation in icorr over the whole drying-wetting cycle. Therefore the relative steel corrosion Rρ was introduced as: PM MRAC PM MRAC ρMRAC m¼1 icorr;m � tm m¼1 icorr;m Rρ ¼ ¼ PM CRAC ¼ PM CRAC ; (1) ρCRAC i � t m m¼1 corr;m m¼1 icorr;m
5.2.2. Steel corrosion rate Fig. 13 shows the corrosion current density and open circuit poten tial of the two concrete mixtures during the 22-weeks drying and wet ting cycles. From Fig. 13, it can be seen that for both CRAC and MRAC, the corrosion current density increased with increasing drying and wetting cycles, while an opposite trend was observed with the open circuit po tential. It means that, with the increasing drying and wetting cycles, the steels in CRAC and MRAC were more likely to corrode. This was because harmful materials such as H2O and O2 penetrated to surface of steel and reacted with iron ions, with the effect of drying and wetting cycle. Be sides Cl as a catalyst would promote the corrosion of steel bars and reduce the resistance of concrete. Therefore the increasing chloride ion ingression accelerated the corrosion of steel. Fig. 13 also shows that MRAC exhibited a lower steel corrosion current density than CRAC. In Fig. 13(a), the blue dash line at 0.1 μA/ cm2, reflects that the steel was considered to be corroded [38]. It can be found that the steel in CRAC might start to corrode at the 14th wetting-drying cycle, which was earlier than the steel in MRAC. After the NS modification, the ITZs within the RAC became denser which made it harder for the harmful materials such as Cl , H2O and O2 to penetrate through the concrete to the steel. To have the better analysis on the steel corrosion, the approximate
where M was the number of drying and wetting cycles, (in this study M CRAC ¼ 22); iMRAC corr;m and icorr;m was the corrosion current density of MRAC and CRAC respectively at the last day of the mth drying and wetting cycle; tm was the time of the mth drying and wetting cycle (in this study tm ¼ 7 d). The mean value of Rρ for CRAC was 1 obviously. While the mean value of Rρ for MRAC decreased to 0.59 after the NS modification. Therefore modification by soaking RAs in NS suspension could signifi cantly improve the RAC’s ability of protecting steel from corrosion. 5.2.3. Corrosion-induced cracks propagations Fig. 14 shows the propagation path of corrosion-induced cracks on the side surface of CRAC and MRAC at the 12th wetting-drying cycle. As shown in Fig. 15, there were two cracks on the side surface of the CRAC; while for MRAC there was only one crack. Because MRAC had the stronger resistance to steel corrosion as mentioned in Section 5.2.2, the steel in CRAC would form more corrosion products. Moreover, as MRAC showed better mechanical properties as discussed in Section 5.2.1, the expansion caused by the corrosion products would induce cracking more easily in CRAC. The areas of corrosion-induced cracks at different damage cycles were calculated and are shown in Fig. 15. In Fig. 15, both the cracks areas of CRAC and MRAC increased over the damage cycles. Especially at the later damage cycles the areas of cracks formed showed a rapid increase. It also can be found that at the 8th damage cycle, corrosion-induced cracks started to appear on the side surface of CRAC. But for MRAC, cracks started appearing on the side surface at the 10th damage cycle. Besides, the crack areas of CRAC was larger than that of MRAC. With the direct current, the chloride ion migrated to the surface of the steel in CRAC earlier, which accelerated the corrosion rate of steel. Additionally the relatively poorer mechanical properties of CRAC resulted in earlier cracking, which made chloride ion migrate into concrete more easily. 6. Conclusions In this study, RAs were modified by soaking in the NS suspension with the determined efficient soaking time, and the modification effects are summarized as follows: (1) The increasing trends in the treated surface microhardness and the penetration depths of NS particles proved that soaking RAs in
Fig. 15. Areas of corrosion-induced cracks of CRAC and MRAC. 7
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NS suspension for 1 h could achieve high efficiency in improving the properties of model RAs. (2) With the modification by 1-h soaking in the NS suspension, enhanced mechanical properties were achieved: the compressive strength improved obviously; the width of ITZ between the old mortar and the new mortar decreased, and the microhardness of ITZ increased. (3) After NS modifications, RAC exhibited improved durability properties, including stronger protection of steel from corrosion and less corrosion-induced cracks. It was also related to the improvement of ITZ at the micro level.
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