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Carbonation Resistance of Reinforced Concrete under Bending Load

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Carbonation Resistance of Reinforced Concrete under Bending Load
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   Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64 714 Carbonation Resistance of Reinforced Concrete under Bending Load  Yan Liu Tianjin University, Key Laboratory of the Ministry of Education on Binhai Civil Engineering Structure and Security, Tianjin 300072, China Hebei Construction Group Corporation Limited, Baoding 071000, China College of Urban and Rural Construction, Agricultural University of Hebei, Baoding 071001, China liuyan7521@126.com  Jun Ren College of Urban and Rural Construction, Agricultural University of Hebei, Baoding 071001, China 1375733149@qq.com Zhongxian Li Tianjin University, Key Laboratory of the Ministry of Education on Binhai Civil Engineering Structure and Security, Tianjin 300072, China    zxli@tju.edu.cn Qiuli Gao Hebei Construction Group Corporation Limited, Baoding 071000, China    Gao8089@163.com Shengli Zhao College of Urban and Rural Construction, Agricultural University of Hebei, Baoding 071001, China    zhaovictory@163.com  A  BSTRACT .   Fly ash has been used more and more often to take the place of cement as the admixture of concrete in the construction of concrete buildings. However, with the increase of the carbon dioxide (CO 2  ) concentration in the atmosphere, carbonization damage has become an essential factor affecting the durability of fly ash concrete. Here a long-term bending load device was developed to explore how the pouring surface and the bending load affect the carbonization resistance of reinforced concrete under rapid carbonization. In addition, the relationship between the bending-tension and bending-compression loads with respect to the carbonization damage of test blocks was also investigated. Due to the differences in the concrete compactness, the carbonization depth of the pouring surface was Citation: Liu, Y., Ren, J., Li, Z.X., Gao, Q., Zhao, S.L., Carbonization resistance of reinforced concrete under bending load, Frattura ed Integrità Strutturale, 49 (2019) 714-724. Received: 24.04.2019  Accepted: 13.06.2019 Published: 01.07.2019 Copyright:  © 2019 This is an open access article under the terms of the CC-BY 4.0,    Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64   715 found to be greater than that of the bottom at the same position. To a certain extent, with the increasing bending-load stress, different carbonization resistances were observed in the bending-tension zone and the bending-compression zone of the concrete test blocks. Meanwhile, to study the relationship between the carbonization damages in the bending-tension zone and the bending-compression zone of concrete test blocks, a carbonization influence coefficient of bending tension-compression load was proposed,  which provides a convenient and scientific guidance for the detection and evaluation of concrete carbonization damages in practical engineering. K  EYWORDS .  Reinforced concrete; Bending load; Pouring surface; Carbonization depth; Carbonization influence coefficient of bending tension-compression load.  which permits unrestricted use, distribution, and reproduction in any medium, provided the srcinal author and source are credited. I NTRODUCTION   he replacement of cement with fly ash as the admixture of concrete can not only reduce cement consumption but also effectively improve various properties of concrete [1-3]. Therefore, fly ash has been used more and more  widely in concrete buildings. On the other hand, due to the deterioration of the natural environment and the increase of the CO 2  concentration in the atmosphere, carbonization damage has become one of the most significant factors affecting the durability of fly ash concrete. However, the effect of load on the carbonization resistance of fly ash concrete was rarely discussed [4-10]. In practical engineering, the structure of a building is subject to the combined action of both load and environment, especially the former, so studying the environment alone cannot fully solve the carbonization problem in practical engineering [11-15]. Most of the building components such as beams and slabs are mainly subjected to the bending load in concrete structures. So the carbonization analysis under bending loads is still the focus of the durability study on fly ash concrete. However, the current research conclusions cannot be directly compared as they were obtained from different experimental platforms. What is more, most of the research focused on plain concrete, without considering the influence of steel bars on its carbonization, which cannot provide very helpful reference for the actual engineering, as studies [16-18] have shown that steel bars make the concrete more compact on the setting and hardening process of the concrete, which inhibits the carbonization damage of the concrete. This paper performed a rapid carbonization test to investigate how the bending load affects the properties of reinforced concrete blocks with  varying fly ash contents. Then it analyzes the effects of the pouring surface and the bending tension and compression loads on the carbonization resistance of concrete and the relationship between the tension and compression loads with respect to their effects on the carbonization resistance of concrete. Furthermore, it proposed a carbonization influence coefficient of bending tension-compression load with respect to reinforced concrete. In summary, this study can provide convenient and scientific guidance for the detection and evaluation of carbonization damages in concrete structures in practical engineering. E XPERIMENT    Experimental materials .O 42.5 ordinary Portland cement and Grade II fly ash (type F) were used in the experiment. River sand was used as fine aggregate, with a fineness modulus of 2.74 and good gradation. The continuously graded crushed stone  with a particle size of 5-20mm was adopted as coarse aggregate. In this experiment, polycarboxylates high-performance water reducing admixture was adopted. The strength of the concrete blocks was C30, and four fly ash contents (the mass ratio of fly ash) were selected, namely, 0%, 20%, 30% and 40%. The concrete mix proportions are listed in Tab. 1. The test blocks were divided into several groups, each of which contains 51 blocks. In each group, 3 blocks were subjected to the measurement of ultimate bending load, and 48 underwent the carbonization test.  T P   Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64 716 Number Fly ash content Cement (Kg) Fly ash (Kg) Sand (Kg) Stone (Kg) Sand rate Water (Kg)  W/B Water-reducer A 0% 373.63 0 682.871114.1638%179.340.48 0.40%B 20% 298.90 74.73 682.871114.1638% 179.340.48 0.46% C 30% 261.54 112.09 682.871114.1638% 179.340.48 0.52% D 40% 224.18 149.45 682.871114.1638%179.340.48 0.56%    Table 1: Concrete mix proportions   Experimental method Preparation of test blocks: 100mm×100mm×300mm rectangular concrete test blocks were used in the experiment. The reinforcement diagram of a test block is shown in Fig. 1. Figure 1: Reinforcement diagram of the reinforced concrete block Load device: A long-term bending-load device was developed in the experiment by reference to the other load devices [16-17, 19-24]. As shown in Fig. 2, the device consists of two screws with a diameter of 12mm, eight nuts, four shims and a steel column support. If two torque wrenches are used to twist the nuts at the same time and with the same speed to produce a tightening force, the screw will bear tension, forming pressure on both sides of the block, thereby exerting bending load on the test block. The bending tension load generated on the side of the block away from the support is defined as the tension surface, while the bending compression load generated on the side of the block near the support is called the compression surface. In this experiment, the compression load is directly applied on the pouring surface. The bending load stress of the block can be adjusted by using the torque wrench to apply the torque. This loading strategy is highly reliable, as evidenced by its extensive application in the research of concrete durability under bending stress. Figure 2: Long-term bending-load device   Experimental procedure  The three-point bend loading test was first performed on the concrete blocks using a universal testing machine. The ultimate bending load F   (i.e. the pulling force of the screws at both ends of each block) of the three blocks in each mix ratio were measured separately on the universal testing machine. To ensure the reliability of measured results, the average  value was taken as the final F   value of reinforced concrete under this mix ratio. The corresponding F   was 37.88 kN, 33.16 kN, 34.18 kN and 32.04 kN when the fly ash content of the test blocks was 0%, 20%, 30% and 40%. Each group of test blocks were loaded with 0%, 20%, 40% and 60% of the ultimate bending load based on the formula T  = K  • F  • D  [25] (  T   is the torque magnitude; K   is the torque coefficient, which was set to 0.19; and D   is the screw diameter,    Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64   717  which was set to 12mm). The carbonization experiments were carried out on the reinforced concrete test blocks with four mix proportions under long-term bending load.  The carbonization test was carried out in the standard fast carbonation box in the laboratory. The environmental parameters in the carbonization box include CO 2  concentration of (20 + 3) %, relative humidity of (70 + 5) %, and temperature of (20 + 2) ºC. The concrete blocks were carbonized for three different periods, namely, 7, 14 and 28 days. During the test, the concrete blocks were put into the carbonization box and carbonized to the corresponding period.  After that, they were taken out for the measurement of carbonation depth. Specifically, each test block was split along the mid-span direction, and phenolphthalein, 1% w/v in alcohol, was dripped evenly on the block surface with a rubber head dropper. The carbonized part of the block did not change color, while the non-carbonized part became purple red.  According to the change of cross-section color, the carbonation depths at 9 points were measured within 60mm in the middle of tension zone and the compression zone, and the average value was taken as the final carbonization depth of the area.  To prevent the stress loss of the long-term load device, two preventive measures were taken in this experiment: (1) The pre-tension was carried out before the bending load was applied. First, a torque which was 1.05-1.1 times the corresponding stress was applied to the test block for 2-5min, and then the load was removed and the corresponding torque was applied. (2) During the carbonization process, the applied torque was measured every 7 days and any reduction of torque would be compensated. R  ESULTS AND DISCUSSION  Effect of the pouring surface on the carbonization resistance of concrete n the experiment, the carbonization depths of each block were measured from four longitudinal sides, and it was found that the carbonization depths were different from different sides, showing the different carbonization resistances of the concrete. Through analysis, it was found that the carbonization resistances of the pouring surface (top surface) and the non-pouring surface (bottom surface) were different. Some literatures [26-30] have found that the segregation of concrete during vibration lead to the unevenness of the block from the top to the bottom, and that the pouring surface of the concrete in actual construction has a certain effect on the carbonization resistance of concrete. To compare the difference in the carbonization performance between the pouring surface and the non-pouring surface, the carbonization influence coefficient of the pouring surface, denoted as K  , is defined as follows:  pb   X K  X    (1)  where  X   p  is the carbonization depth of the pouring surface (top surface) while  X  b   is the carbonization depth of the non-pouring surface (bottom surface).  With the data in Tab. 2, the variation curve of the carbonization influence coefficient K   of the pouring surface with different fly ash contents can be determined (as shown in Fig. 3).  As shown in Fig. 3, K   decreased as the carbonization period increased. In the first 14 days, the decreasing rate was relatively fast, while in the subsequent 14 days, the declining trend became relatively gentle. In addition, the value of K   was always greater than 1. This shows that the carbonization speed of the pouring surface was greater than that of the bottom surface under the same mix proportion. As both the top and the bottom were reinforced in the same way, the difference in the carbonization depth might be caused by the different compactness of the top and the bottom surfaces of the concrete. Therefore, the cross sections of the concrete test blocks were selected and the tops and the bottoms of the cross sections were observed, as shown in Fig. 4.  As indicated in Fig. 4, the top of the cross section showed less coarse aggregate and denser pores, compared to the bottom of the test block. In addition, there was a thin layer of mortar at the top of the block. Due to the gravity and  vibration when the block was poured and vibrated, the cohesion between the cement paste and the aggregate was not enough to resist against the external vertical downward force acting on the aggregate, so the heavy aggregate would sink and the light cement slurry would float. As a result, there was an increase in the coarse aggregate and a decrease in the cement paste from top to bottom along the cross section of the block. The water-to-cement ratio at the top became larger and that at the bottom smaller. There were more capillary pores and larger cracks at the top in comparison with those at I   Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64 718 the bottom. This suggests that the compactness at the top is worse than that at the bottom. Through measurement of the electric flux at the tops and bottoms of other test blocks, it was also noticed that the electric flux at the top was much larger than that at the bottom [26], further proving the widespread phenomenon of higher permeability at the tops of the concrete blocks after setting and hardening. As a result, CO 2  can more easily and rapidly enter and erode the inner part of the block from the top. Furthermore, the carbonization depth at the top was greater than that at the bottom, so the coefficient K  was greater than 1. However, when the carbonization depth reached a certain value, the closer the central carbonization zone of the cross section was, the smaller the difference in the concrete compactness would be. Thus, the carbonization depths at the top and bottom would be closer, making the curve change more gently. Fly ash content CoefficientCarbonization depth/ mm 7d 14d 28d 0  X   p   4.74 5.48 7.94  X  b    4.50 5.34 7.93 K 1.05 1.03 1.00 20%  X   p   7.13 8.71 9.58  X  b    5.40 7.20 8.26 K 1.321.211.16 30%  X   p   7.61 8.76 9.98  X  b    5.90 7.30 8.60 K 1.291.20 1.1640%  X   p   8.00 9.25 10.63  X  b    6.20 7.84 9.75 K 1.29 1.18 1.09  Table 2: Carbonization depths of the pouring and bottom surfaces of concrete/mm Figure 3: Curve of the carbonization influence coefficient K of the pouring surface at different fly ash contents Figure 4: Comparison between the top and bottom surfaces of the concrete test blocks  As shown in Fig. 3, the K   values of the blocks without fly ash were always lower than those of the blocks with fly ash.  When the blocks are vibrated, the fluidity and segregation of the cement paste will increase under the action of shaking
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