A STUDY ON RELATIONSHIP BETWEEN CURING PROCESS AND RESISTIVITY OF SURFICAL CONCRETE

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1 A STUDY ON RELATIONSHIP BETWEEN CURING PROCESS AND RESISTIVITY OF SURFICAL CONCRETE Wei Chen (1), Meili Li(1,2), Lin Zhang (1), Jueshi Qian (1), Yan Pei (1) (1) College of Material Science and Engineering, Chongqing University, Chongqing, , China (2) Henan Building Research Institute Co., Ltd., Zhengzhou, , China Abstract: The process of curing has remarkable influence on the surface performance of concrete. By embedding electrodes at different depths from the surface of concrete, and testing the resistivity of concrete in different curing process in immersion state under water, the resistivity changes were analyzed and discussed. It is shown that there is significant relationship between curing process and the resistivity changes of surface concrete along with ages. Also, there is remarkable correlation between curing process and the resistivity changes of surface concrete in immersion state under water, especially, high w/c ratio concrete. It is considered that the curing process can be real-time monitored by comparing both the changing rate of concrete surface resistivity at stand curing condition and the concrete surface resistivity at the practical application, as well as evaluating the concrete curing efficiency. Keywords: resistivity, curing process, surface, concrete 1. INTRODUCTION Curing process, especially early age curing temperature and humidity has great influence on concrete performance, for example the hydration, microstructure, strength development and durability. The influence is more significant on concrete surface structure [1-3] : cracking at early age is always closely related to curing. And the performance of surface concrete obviously affects its durability. It is very effective to improve concrete durability through the selection of an appropriate curing regime, by which the temperature and humidity during hydration and hardening process can be strictly controlled [4]. The influence-depth of curing process on concrete surface structure is affected by the water-to-binder ratio, external environment conditions, and mineral admixture types, etc [5]. Cather [6] believed that the curing affecting zone is the depth region in which internal humidity of concrete are influenced by external environment conditions. Shuhui Dong et al. [7] found that the transportation length of water in deeper concrete is longer and thus the deeper the concrete, the smaller the influence of the external environment humidity is. Dong-Woo Ryu [8] studied the influence of environmental conditions on the relative humidity and relative CHEN Wei: Doctoral Student; vigerguke@tom.com Corresponding author: QIAN Jueshi, Professor, qianjueshi@163.com Funded by the National Natural Science Foundation of China (No ) Page 1

2 moisture content of the inside concrete. It revealed that the only obvious changes of relative humidity and moisture content inside the concrete, which were introduced by the changes of external temperature and humidity occurred on concrete surface area. The aforementioned studies showed that the influence of curing on concrete (mainly the influence on concrete moisture content) at different depth is different. Although concrete is a poor electrical-conductor or even an electrical-insulator in fully dry condition, it becomes conductive once it contains some water: the concrete resistance changes along with the moisture content. Consequently, the moisture content of concrete can be reflected by measuring the concrete resistivity [9]. For normal concrete, the first 7d curing is very important. Thus in this work, the concrete resistivity-changing characteristics, under different curing regimes, at different concrete depth in the first 7d curing were studied. For the 3d and 7d old concrete, under water-immersion condition, the surficial resistivity was measured at different water-immersion time and depth. Based on these characteristics, the relationship between curing process and concrete surficial resistivity, also, the probability of using resistivity-measuring technique to monitor and evaluate concrete curing process was analyzed. 2. EXPERIMENTAL 2.1 Materials Materials: The cement used was Lafarge 42.5 ordinary Portland cement, and its chemical composition was SiO %, Al 2 O %, Fe 2 O %, CaO 58.99%, MgO 2.53%, and LOI 3.08%. The fine aggregate was natural sand. Its fineness modulus was 3.1 and the apparent density was 2680kg/m 3.The coarse aggregate was natural limestone with a particle size range of 5~20mm. Sika CR poly carboxylic acid water reducing agent was used to get a workable fresh concrete. The water reducing ratio was 32%, and the dosage was 0.8%. Electrodes: The electrodes were made of band copper with the width and length of 0.6cm and 35cm, respectively. Experiment: cm plastic rectangular molds were used for concrete casting; concrete resistance was measured by Fluke's 8808A multi-meter; the 1000W-tungsten lights and fans were used for providing the environmental light and wind conditions. 2.2 Method Two concrete mix proportions, designated as C30 and C60, are shown in Table 1. In order to eliminate the size effect of concrete specimen, cm concrete specimens were cast. Concrete was cured within mold to ensure that curing can just affect surface of concrete, and Vaseline was used to seal the gap between concrete surface and the mold. Three different curing conditions were applied to concrete specimens: (1) air curing; (2) water curing; (3) dry curing: a 1000W-tungsten light was hang on the top of concrete sample and fans was also placed beside concrete, adjusting the height of light to make the RH and temperature in the range of 80%-90% and C, accordingly. During the curing period, the indoor relative humidity ranged from 62% to 71%, and temperature ranged from 20 to 30 C. The electrodes were embedded in model before concrete casting. Schematic diagram of sample for concrete electrode placement is shown in Fig 1. Holes were drilled in the longitudinal direction of plastic mold, and then copper electrodes were embedded into the holes. After which, concrete was cast. The locations of electrodes were 1, 2, 3, 4, 5cm from Page 2

3 the surface of concrete. Table 1 Mix proportions of concrete Specimens Cement/ kg/m 3 Sand/ kg/m 3 Crushed stone/ kg/m 3 Water/ kg/m 3 Water reducing agent /kg/m 3 f c, 28 d /MPa C C mode Surface of concrete 40cm 5cm 3cm 1cm 5cm 5cm 15cm electrodes 2cm 4cm 30cm Fig.1 Schematic diagram of specimen for concrete electrode placement 3 RESULTS AND DISCUSSION 3.1 Resistivity of concrete on different curing conditions Due to the influence of curing, the temperature and humidity of concrete change greatly from the surface to the inner part. This will affect the microstructure of concrete, thus the concrete resistivity will be affected. The resistivity variation of specimens at different depths cured under different conditions for 7d is shown in Fig.2. It can be seen in Fig.2, for both C30 and C60 concrete, with the growth of age, different curing conditions have similar effects on the concrete resistivity development. In dry curing conditions, the resistivity of each depth increased with time. The same trend was seen in air curing condition, but the growth was more modest. In water curing condition, no significant change was seen. The resistivity of the two type concrete at different depth was analyzed and the difference was significant. For C30 concrete, in dry condition, from the depth 1 to 5cm, the resistivity increased rapidly with age. The nearer of the electrode to the concrete surface, the more great of the increase it showed. The same trend was also seen in air-cured concrete. In contrast, for C60 concrete, the growth of resistivity with age was much smaller, while resistivity changes at different depths were almost the same. In the process of early age hydration and hardening, the internal humidity and temperature of concrete change greatly [10]. It is reported [11-12] that the ambient temperature would affect the early age temperature stress of concrete. A higher ambient temperature will not only raise the temperature of concrete itself, but accelerate the hydration of concrete. Therefore, a high temperature stress can be formed and concrete cracking is going to occur [13]. In addition, the environment humidity with which concrete contact will also affect the inside concrete Page 3

4 humidity, particularly the water exchange between environment and concrete [14-15]. The concrete temperature and humidity at different depth will affect the hydration and hardening process of concrete. It has been proved that the hydration and hardening process of concrete has a close relationship with the resistivity of early-age concrete [16-17]. Age 7d 3d Water Air Dry 1d 2h Depth/cm Resistivity/KO. cm 7d 3d Age Water Air Dry 1d 2h Depth/cm (a) C30 (b) C60 Fig.2 Concrete resistivity variation vs. depth under different curing conditions Resistivity/KO. cm Dry curing leads to a rapid water evaporation. Meanwhile, concrete disseminates faster under high temperatures due to its rapid hydration, which results in a high porosity. In addition, the cracks introduced by temperature stress will lead to a reduction of effective conductive media in concrete. Therefore, the concrete resistivity in a dry curing condition exhibited a much higher increase. For the high w/c concrete (C30), it had a higher moisture content. Its resistivity increased dramatically when a large quantity of water was consumed. The smaller the depth of the concrete, the faster water evaporates and the shaper the resistivity increases. For the low w/c concrete (C60), there would be the same trend in dry curing condition, yet a dense structure of the cement paste increased the contact area of conductive medium. Thus its resistivity increase rate is much smaller. At the same time, the resistivity difference at different depth is not significant. In air curing conditions, the temperature and humidity differences between external environment and internal concrete were small, thus the increase of resistivity of C30 and C60 with ages were slow. Comparatively, the resistivity of high w/c concrete (C30) increased more than low w/c concrete (C60). The resistivity growth rates at 1 and 2 cm are greater than those at 3, 4, 5 cm. In water curing conditions, there was no moisture gradient between internal and external environment of concrete, thus the resistivity increase was not obvious. 3.2 Relationship between curing and resistivity of concrete in water curing condition Based on the results above, the resistivity of surface layer concrete was greatly influenced by curing in the early age. The influence maybe caused by the difference of humidity and porosity of concrete. So water migration rate from surface to internal part is different when concrete was water-cured. In this paper, C30 and C60 concretes were cured at different curing conditions for 3d and 7d. Then the samples were immersed in water. The resistivity variation with water immersion time was investigated. The results are shown in Fig. 3 and Fig. 4. Page 4

It can be seen in Fig.3 the resistivity of C30 concrete under water after different curing conditions changes greatly, and the resistivity at every depth exhibits great difference too.

5 It can be seen in Fig.3 the resistivity of C30 concrete under water after different curing conditions changes greatly, and the resistivity at every depth exhibits great difference too. After concrete was cured in dry or air condition, once it was immersed in water the resistivity decreased. Moreover, with the increase of depth, the onset point of resistivity reduction, which corresponds to the gradual water penetration into internal concrete, was postponed. However, the water-cured concrete did not show any big resistivity change. That was due to its full water-saturation. Comparatively, the resistivity of dry-curing concrete, decreased more than that of air cured concrete, and resistivity decrease cured in dry and air condition after 7 days was greater than those after 3 days of curing. It can be seen in Fig. 4 that the resistivity of water-immersed C60 concrete did not change significantly after different curing methods. After 3 days of dry and air curing, the resistivity at 1 and 2 cm decreases after a long time of water-immersion and the decreasing point of the 2cm point was later than that of 1cm point. There was no change at depth of 3, 4 and 5cm. For concrete, after 7d dry and air curing, and immersed in water for a long time, its resistivity only at the depth of 1 cm decreased. Resistivity of water-cured concrete does not change at any depth either. With the water consumption, its internal structure after cement hydration became dense. The porous structure and its moisture content are important factors affecting the resistivity of concrete. C30 concrete had a high W/C ratio, and in dry curing condition, it had a higher moisture loss, thus the structure was more porous and the relative humidity was lower. When immersed into water, it absorbed water quickly, which lead to a rapid decrease of resistivity, also the more pores and lower relative humidity near the surface of concrete, the greater degree of resistivity reduced. C30 concrete lost less water in air curing, so its resistivity decreased less after water immersion. For C60 concrete, it had a low W/C ratio, and its structure was very dense after hardening. It was difficult for water penetrating into concrete even in dry condition and the water was only able to affect the resistance of concrete at the depth of 1cm. After water curing, the internal humidity of concrete was saturated, therefore the resistivity of both C30 and C60 concrete at different depths were unchanged when immersed in water. Generally speaking, with the extension of water immersion time, the resistivity reduction magnitude of concrete was in the order of dry curing, air curing and water curing. (a) Pre-cured 3 days Page 5

under water after different curing conditions 4

6 (b) Pre-cured 7 days Fig.3 The variation of resistivity of C30 concrete under water after different curing conditions (a) Pre-cured 3 days (b) Pre-cured 7 days Fig.4 The variation of resistivity of C30 concrete under water after different curing conditions 4 EFFECTIVENESS OF MEASURING CONCRETE RESISTIVITY TO MONITOR ITS CURING Page 6

7 It can be seen from the results of section 3.1, there was an apparent correlation between the curing method and resistivity of concrete at different depths. On the condition of poor curing (dry curing), the resistivity of concrete showed a rapid increase, especially at the depth of 1, 2cm from surface of concrete. However, it showed no apparent change when concrete was water-cured. Therefore, whether concrete has been well cured can be monitored through the resistivity change at different depth from concrete surface. The concrete resistivity change was different between cured after 7 days and the initial. The growth of resistivity change at different depths, shown in Fig. 5 has been analyzed. From section 3.2, we know that after cured in different conditions, concrete s resistivity decreases gradually at the depths from 1cm to 5cm when its surface was immersed in water, moreover, the decrease ranges were inconsistent when the curing condition are different. Though 24 hours water immersion, concrete s resistivity change at different depths has been analyzed, shown in Fig.6. Changes of resistivity / K cm C30 Dry C30 Air C30 Water C60dry C60 Air C60 Water Depth / cm Fig.5 the changes of concrete in 7d different curing conditions Changes of resistivity / K cm C30 Dry C30 Air C30 Water C60 Dry C60 Air C60 Water change of resistivity /k cm Depth/cm Depth / cm Fig. 6 the resistivity changes of concrete in 24h water immersion As shown in Fig.5, under the experimental condition in this paper, resistivity change of different concrete was not consistent with curing process. For high W/C ratio C30 concrete cured in the dry condition after 7d, the resistivity changed at depth of 1cm is 300kΩ cm, whereas 10kΩ cm to low W/C ratio C60 concrete. Furthermore, the resistivity change of C30 concrete at the depth of 1, 2cm is greater than that of 3, 4, 5cm, whereas the resistivity change of C60 concrete at different depths appears little different. As shown in Fig.6, after 24 hours immersion in water and cured in dry conditions for 7d, resistivity change of C30 concrete is 350kΩ cm at depths of 1cm and 10kΩ cm at depths of 5cm.However, under the same curing condition to C60, its resistivity change is 4kΩ cm at depth of 1cm and 0.2kΩ cm at 5cm. The resistivity of concrete is relevant with its type. The method using resistivity change to evaluate the curing effect must be based on comparison with the homogeneous concrete, considering the difference of concrete types, material composition, mix proportion and other factors. Therefore, in order to monitor curing effect of practical concrete projects, concrete specimens with the same mix proportion are necessary. A part of the specimens are cured in the same condition with practical concrete project, and the other are cured in the standard conditions. The resistivity changing under standard curing conditions contrast with those in practical state, the variation of which can be used to monitor curing process and evaluate the curing effect. Page 7

8 5 CONCLUSIONS The process of curing has remarkable influence on the surface performance of concrete. There is significant relationship between curing process and the resistivity changes of surface concrete. Also, there is remarkable correlation between curing process and the resistivity changes of surface concrete in immersion state under water. The curing process can be real-time monitored by comparing the changing rate of concrete surface resistivity at stand curing condition with the concrete surface resistivity at the practical application. Furthermore, the concrete curing efficiency can be evaluated by testing the resistivity changes of surface concrete in immersion state under water. REFERENCES [1] Austin S A. Air permeability versus sorptivity: Effects of field curing on cover concrete after one year of field exposure[j]. Magazine of Concrete Research, 2000, 52(1): [2] Alizadeh R, Ghods P. Chini M. Effect of curing conditions on the service Life design of RC structures in the Persian gulf region[j]. Journal of Materials in Civil Engineering, 2008, 20(1): 2-8. [3] Austin S A,Robins P J. Influence of early curing on the sub-surface permeability and strength of silica fume concrete[j]. Magazine of Concrete Research, 1997, 49(178): [4] Li Kefei,Chen Zhaoyuan. Specification and recommendation of concrete cover in durability design[j]. Journal of Southeast University (Natural Science Edition), 2006, 36(2):23-26 [5] Dan Y, Dan Z, Seongcheol C, et al. Literature review of curing in Portland cement concrete pavement[m]. Austin: The United States of America, 2006: [6] Cather R. How to get better curing[j]. Concrete, The journal of the Concrete Society, 1994, 26(5): [7] Dong Shuhui, Ge Yong Analysis of relative humidity change in low water-cement ratio concrete[j]. Journal of Wuhan University of Technology, 2009, 31(7):84-87 [8] Dong-Woo Ryu, Jeong-Won Ko, Takafumi Noguchi. Effects of simulated environmental conditions on the internal relative humidity and relative moisture content distribution of exposed concrete[j]. Cement and Concrete Composites, 2011, 33(1): [9] Meili Li, Shanshan Xu, Jueshi Qian, et al. Measurement of concrete resistivity for assessment of the curing efficiency of high performance concrete[j]. Journal of Zhengzhou University (Engineering Science), 2009, 30(1): [10] Huang Yu, Qi Kun, Zhang Jun. Development of internal humidity in concrete at early age[j]. Journal of Tsinghua University (Science &Technology), 2007, 47(3): [11] Sule M,Breugel K V. The effect of reinforcement on early-age cracking due to autogenously shrinkage and thermal effects[j].cement and Concrete Composites, 2004,26(5): [12] Burkan I O, Ghani R A. Finite element modeling of coupled heat transfer,moisture transport and carbonation processes in concrete structures[j].cement and Concrete Composites,2004,26(1): [13] Wang Jiachun, Yan Peiyu. Analysis of early-age thermal stress in concrete structure[j]. Journal of Southeast University (Natural Science Edition), 2005, 35(1):15-18 [14] Wang Xinyou, Jiang Zhengwu. Review on the mechanism and model of moisture transfer in concrete[j]. Journal of Building Materials, 2002, 5(1):66-71 [15] Zhang Jun, Hou Dongwei. Calculation of moisture diffusion coefficient in early age concrete from interior humidity tests[j]. J Tsinghua Univ (Sci &Tech),2008,48(12): [16] Wei Xiaosheng, Xiao Lianzhen. Study on hydration of Portland cement using an electrical resistivity method[j]. Journal of the Chinese Ceramic Society, 2004, 32(1):34-38 [17] Li Z, Wei X, Li W. Preliminary interpretation of Portland cement hydration process using resistivity measurements[j].aci Material Journal, 2003,100(3): Page 8

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