| Cement-based composites for engineering use are a new type of short fibre reinforced cement-based composites with super-strength and toughness.The difference between ECC material and ordinary concrete material is that its ultimate tensile strain energy can reach more than 3%.At the same time,ECC material has obvious strain hardening characteristics and multi crack characteristics after hardening,and it can effectively control the width of cracks.Because of the good affinity between PVA and cement,ECC materials are usually prepared with PVA fibers.Because the cost of fibers needed to prepare ECC materials is relatively high,ECC is not used alone or ECC is mostly used to reinforce structures.According to the concept of functionally graded concrete put forward by Wang Xinggang of Southeast University,ECC material and ordinary concrete were superimposed together in this paper.In this way,the bonding performance between ECC material and ordinary concrete interface becomes very important.At the same time,in order to speed up the construction progress in practical projects,concrete structures are often in service at the early stage.In order to ensure the stability of concrete structures in the early stage of service,it is necessary to have a thorough understanding of the law of early age development of interfacial bonding properties of concrete.In this paper,a series of studies have been earmed out with the support of the National Natural Science Foundation of China(51308489).The main research work of this paper is as follows:(1)The early mechanical properties of ECC materials were studied by cubic compression test and four-point bending test,and the formula of early compressive strength of ECC materials was proposed.At the same time,the early ultimate tensile strain of ECC materials was deduced by four-point bending test.(2)Through the splitting tensile test of ECC-concrete specimens,the effects of age,concrete strength grade and interface treatment on ECC-concrete were investigated.The experimental results show that the splitting tensile strength of ECC-concrete increases with the increase of age.Base on interface not being processed,the splitting tensile strength of ECC-concrete is reduced when the steel wire mesh is laid,and the splitting strength is increased when the interface groove is treated.The splitting strength of ECC-concretc naterial is increased greatly when the strength grade of concrete is C40.(3)Based on the double-sided shear test of ECC-concrete specimens,the effects of age,concrete strength grade and interface treatnent on the interlayer shear strength of ECC-concrete were studied.The experimental results show that the growth rates of the interlayer shear strength of ECC-concrete are different in different age periods.Based on the non-treatment method of interface,the shear performance of concrete interface can be improved when the interface is grooved,and the shear performance of concrete interface can be reduced when the interface is treated with fine wire mesh.The shear strength of ECC-concrete interface increases roughly with the increase of concrete strength.(4)Scanning electron microscopy(SEM)was used to study the microstructural characteristics of the interface between ECC and ECC-concrete.The experimental results show that the microcosmic space of ECC material develops densely in the early age stage,and there are hardly any holes and cracks in it.The hydration products in the interface of ECC-concrete are well bonded,and the microcosmic space of the interface area becomes more compact with the increase of age.The main hydration products of ECC-concrete interface bonding zone are approximately the same under various concrete strength grades.When the interface is treated with fine wire mesh,there are many holes and cracks in the interface of ECC-concrete.Without special treatment of the interface and groove treatment of the interface,the tightness of the interface of ECC-concrete increases with the increase of the strength grade of concrete. |
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