Study On The Fatigue Damage Characterization And Capillary Imbibition Of Concrete

Concrete plays an increasingly important role in the national economy and social life for the rapid process of urbanization in recent years.Concrete durability is significantly affected by water transport because many of the most important damage processes take place in concrete with water,so it is important to study the water transport mechanism to determine the durability of concrete.However,concrete always serves with different kinds of load,hence the damage caused by the load is an important factor affecting the concrete water transport and ion erosion.Therefore it's very necessary to explore the damage mechanism of concrete and the its impact on the water transport.Based on technology of digital image correlation(DIC),this paper firstly analyzes the crack initiation mechanism of concrete under unilateral splitting and fatigue splitting.Then,transmission X-ray radiography(TXR)was used to achieve the in-situ,continuous and visual monitor on water transport of the concrete slices under no stress.Finally,X-ray tomography(XCT)was used to achieve the in-situ,continuous and visual monitor on water transport of the damaged cube concrete,and the influence of the damage on the water absorption of the concrete was analyzed.The main results of this work are as follows:(1)Based on the observation of optical microscope and DIC cloud image,it is believed that concrete crack fracture by unilateral splitting decreases along the direction of cracking,while the concrete crack fracture by fatigue splitting is larger in the middle than that at both ends.At the same time,the DIC strain cloud map can well reflect the evolution of cracks in concrete by unilateral splitting or fatigue splitting,including crack initiation time,length and spatial distribution.The evolution process of a single cycle of fatigue load is given to reveal the mechanism of fatigue damage accumulation.Combined with the analysis of key points and line area on the concrete surface,we can accurately detect the local damage development.The approach of DIC has advantages over the traditional mechanics test in revealing the evolution process and mechanism of crack initiation and propagation on the basis of global material deformation.(2)Pure water in concrete is difficult to be distinguished clearly by TXR technology,Adding CsCl to water can enhance the contrast of TXR image,TXR technology combined with CsCl enhancing technology can be used to continuously observe the imbibition front in situ.The results show that the CsCl solution with concentration of 6%-15%can be used as a liquid reagent for TXR to study the capillary imbibition of concrete.CsCl intrusion into cement matrix will also enhance its electron scattering ability and improve the contrast of BSE image.TXR combined with SEM/BSE/EDS technology provides a new method to characterize the transport behavior of concrete(3)CsCl solution with concentration of 15%was used as the XCT test eikonogen for capillary imbibition of concrete.A method based on gray threshold which could eliminate the artificial influence was introduced to calculate the imbibition height.The results shows the relation of the water absorption height and radication of time is very consistent with the L-W mechanism,and basically follows the ?h-t0.25 mechanism.The water absorption characteristic of damaged concrete XCT image is different from that of non-destructive concrete,concrete damage like cracks make concrete obtain strong suction and provide more water transport channels,which significantly improve the rate of water absorption of concrete.