Experimental study on early cracking sensitivity of lightweight aggregate concrete
Introduction
Early cracking is one of the main quality problems of concrete, especially for high-strength concrete [1]. High-strength concrete usually has significant temperature deformation [2] and self-desiccation [3], [4] due to the use of low water/cement ratio (w/c) and fine supplementary cementitious materials [5], which are the main causes of early cracking [4]. In order to avoid such early cracking risk of concrete, it is necessary to prevent the decrease in internal relative humidity [6]. Although external water curing is normally adopted, the cracking of high-strength concrete still exists because of the slow penetration of water in concrete [7], [8].
Ceramsite with high water absorption capacity is an efficient material to reduce the cracking potential of concrete in its early ages [6], [9]. Water in saturated lightweight aggregates can be supplied to cement paste [10], [11] to prevent the decrease in internal relative humidity. ICC2003-RILEM TC [12] defined this behavior as internal curing. Concrete with internal curing exhibited smaller autogenous shrinkage and creep than the normal concrete [13], [14]. Desorption of ceramsite in concrete can strengthen the interfacial structure between ceramsite and cement paste matrix [4], [15], thus substantially increasing the potential robustness of materials in early ages with respect to temperature changes [16], [17]. Partial replacement of aggregates by lightweight aggregate can also significantly reduce self-drying and delay cracking [5], [18]. Meanwhile, some valuable researches also show that the single incorporation of expansive agent can mitigate the risk of cracking induced by shrinkage [19].
However, different types of ceramsite have different moisture contents and water absorption/desorption processes [20], which will lead to different curing degrees and different cracking behaviors at early ages. The combined effects of desorption of lightweight aggregate and addition of expansive agent are also unclear. In this study, 30 groups of mixing proportions were used in splitting tensile test and plate cracking test to quantify the effects of moisture content, replacement ratio of aggregates and expansive agent on the early age cracking behavior of concrete. Temperature-stress experiments were also conducted to explore the development patterns of temperature and restraining stress of concrete with different mixing proportions at early ages. The effects of different ceramsite, replacement rates of aggregates and expansive agent on the cracking sensitivity of lightweight aggregate concrete, as well as the effect of moisture content of aggregates in concrete (1 m3) on tensile stress (72 h) due to restraint were discussed.
Section snippets
Materials
Three kinds of ceramsite used in this study were Nantong ceramsite (namely, N fly ash-clay ceramsite, as shown in Fig. 1), Yichang ceramsite (namely, Y shale ceramsite, as shown in Fig. 2), and Zhengzhou ceramsite (namely, Z clay ceramsite, as shown in Fig. 3). All aggregates were prewetted for 24 h before the test. Portland cement (P·II 42.5R) from Guangzhou Zhujiang Cement Co. Ltd. and Polycarboxylate superplasticizer (SP) were adopted. The expansive agent used in the experiment was the SY-G
Basic properties of aggregates
The compressive cylindrical strength, apparent density, packing density, and water absorption rate for three aggregates (N, Y, and Z) are shown in Table 4. It can be seen that the compressive cylindrical strengths of ceramsite N and Y are in the vicinity of 8 MPa, while ceramsite Z has a much lower strength 3.91 MPa compared to ceramsite N and Y; the apparent and the packing density are not so much different for the three kinds of ceramsite. The water absorption rate represents another trend that
Effects of different ceramsite on the cracking sensitivity of lightweight aggregate concrete
Cracking behaviors of concrete with various mixtures investigated in this study were closely related to the moisture content of ceramsite. The ceramsite with higher moisture content inhibited cracking more efficiently (see Fig. 11, Fig. 12, Fig. 13, Fig. 14). For the expansive agent-free groups (N1, Y1, Z1 and N3), with the increase in the moisture content of ceramsite, the total cracking area gradually decreased in the plate test and the second zero-stress temperature, as well as the early
Summary and conclusions
The cracking sensitivity of LWAC at early ages is susceptible to the moisture content of ceramsite, the replacement rate of aggregates and the use of expansion agent. According to the present experimental investigation and the analysis of the experimental data, the following conclusions can be drawn:
The prewetted ceramsite can significantly reduce the cracking risk of concrete at early ages. And the moisture content of aggregates had a relatively high influence on early cracking. The plastic
Acknowledgements
The authors gratefully acknowledge the financial support provided by the joint funds of the National Natural Science Foundation of China and Guangdong Province (U1301241), International Cooperation and Exchange of the National Natural Science Foundation of China (No. 51520105012); the General Program of National Natural Science Foundation of China (No. 51478272); the Science and Technology Foundation for Basic Research Plan of Shenzhen City (JCYJ20140418182819159, JCYJ20160422095146121), and
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