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Analysis Of Compressive Stress-induced Cracks In Concrete


In order to study the causes of arch foot cracking, a multiscale numerical simulation method was used to establish the finite element model of Xizha Bridge during the construction stage of the Xiaoqing River restoration project in Jinan by using Midas Civil, and the internal forces under adverse conditions were extracted. On this basis, Abaqus was used to establish the local model of arch foot, and the plastic damage model parameters were introduced to conduct stress analysis. The results show that the anchorage stress of prestressed steel bundle is too high. On the one hand, the stress component produced by the bending of the prestressed steel bundle can squeeze the concrete inside the bending angle, and on the other hand, it will stretch the concrete outside the bending angle, resulting in concrete cracking. There is a tendency of relative displacement between arch rib and arch foot, and the interface surface of arch foot and arch rib is pulled by the displacement of arch rib, resulting in cracking. Arch foot inner bend produces a certain tensile stress, and if this place is not paid enough attention to, insufficient reinforcement will produce large cracks. Finally, it is suggested that concrete cracking can be avoided by arranging enough reinforcement bars under anchor and sealing reinforcement bars, encrypting steel mesh, arranging shear studs, and extending insertion depth.




analysis of compressive stress-induced cracks in concrete



The stress-strain formula of concrete under uniaxial compression:where is the representative value of uniaxial tensile strength of concrete, MPa; is damage evolution parameters of uniaxial concrete under tension; is the parameters of descending section of uniaxial concrete under tensile state; is the peak tensile strain of uniaxial concrete tensile strength; is the representative value of uniaxial compressive strength of concrete, MPa; is the peak compressive strain of uniaxial concrete compressive strength; is the parameters of descending section of uniaxial concrete under compression; is the damage evolution parameters of uniaxial concrete under compression.


In order to facilitate the observation of the stress cloud diagram of the key parts, the larger principal compressive stress and the principal compressive stress caused by the prestressed anchorage are hidden in the analysis, and the elements with excessive stress caused by the anchorage are removed. As shown in Figure 5, it can be found that in working condition 10, the maximum principal tensile stress occurs in three places: the first is the central position of the lower part of the arch foot, the second is the interface between the arch rib insertion arch foot and the arch foot concrete, and the third is the upper edge of the arch foot.


For the large main tensile stress in the second place, referring to the engineering drawings, it is found that, as shown in Figure 6, the prestressed steel bundle in the lower part has a large vertical bending, and this vertical bending causes a part of the prestressed component to form a vertical bending. The resultant force below pulls the concrete, so the arch foot is easy to form a large main tensile stress in the place where the prestressed steel bundle is bent, resulting in possible cracks.


It is found that there is a larger principal compressive stress in the blue-green region shown in Figure 7, which is similar to the larger principal tensile stress in the second place in this section. The reason for the larger principal compressive stress of concrete in this area is the same as that of the larger principal tensile stress of some concrete in the previous section, which is caused by the bending of prestressed steel beams.


The previous results show that although the main compressive stress and main tensile stress are relatively large in concrete, the plastic damage is mainly tensile plastic damage, and the possibility of cracks in arch foot caused by tensile cracking is much greater than that caused by crushing.


Figure 13 shows the cloud diagram of the main compressive stress at the arch foot without anchorage in working condition 17. The maximum principal compressive stress of working condition 17 is smaller than that of working condition 10 and working condition 12, which is due to the greater axial force of arch rib that brings greater horizontal force and reduces the compressive stress of concrete in tie beam direction caused by prestress. But at the same time, the prestressed steel beam has to bear greater stress, which brings greater tensile and compressive stress to the anchorage end concrete.


Figures 14 and 15 show the plastic damage cloud of concrete. The plastic damage position of condition 17 has no change compared with condition 10 and condition 12, but it has a certain development in the anchorage end of prestressed steel beam, which confirms the analysis of 3.4.2.


Combined with Midas Civil and Abaqus software, through multiscale numerical simulation analysis, according to the stress cloud diagram, the local stress of the arch foot caused by prestress and external load is shown. The causes of arch foot cracking of concrete filled steel tube arch bridge are studied, including(1)Prestressed steel anchor end stress is too large which will lead to concrete at the end of the bear huge stress(2)The stress component caused by the bending of the prestressed steel strand makes the concrete to be stretched and squeezed, resulting in concrete cracking(3)The junction of the arch foot and arch rib is cracked due to the relative displacement trend(4)There is a certain tensile stress at the inner bending of the arch foot, which will produce large cracks


The results of an experimental investigation on the microcracking of high-performance concrete subjected to biaxial tension-compression stresses are presented. Short-term static tests and microcracking mapping were performed on 12.5 cm square by 1.25 cm thick plates. Strain controlled tests were executed in a biaxial testing machine constructed at the University of Texas. The primary variables studied were the deformations and the ultimate stress level at each stress ratio as well as the microcracking patterns and total crack lengths. For the microcracking study, the plates, after straining, were impregnated by an epoxy and then examined under a microscope. Microcracks were classified into simple and combined cracks, since this distinction allows for a much better representation of the microcracking process. A simple crack is either a bond or mortar crack where a combined crack contains both of these. For all stress ratios tested, the stress-strain behavior was directly related to the internal microcracking pattern. In all cases, the failure was directly related to the formation and propagation of the combined cracks.


Since the beginning of last century, research has shown the existence of microcracks in concrete. However, only since the 1960's have these microcracks been observed, characterized and measured. The development of fracture mechanics models, during the last 30 years, enabled the structure of concrete to be taken into consideration. This fact has led to the increasingly application of fracture mechanics in the design of concrete elements2. In spite of this, the theory of fracture mechanics in concrete is not yet as mature as continuum theories3, such as elasticity and viscoelasticity. This is in part due to the limited understanding of the formation and propagation of microcracks in concrete.


Several methods have been used to study the microcracking of concrete, including acoustic emission5, microscope technique with dye6, and computerized tomography analysis7. According to Nemati et al.8, most techniques have limited capability in representing the geometry and state of microcracks as they exist under load specially when concrete is subjected to tensile stresses. Since in this study, concrete was not only subjected to tensile stresses but also to a biaxial state of stresses, the selected technique consisted of microscope examination of strained specimens after the injection of a blue dye epoxy under pressure (0.15 MPa) 9. The epoxy used was composed of a blue dye, a resin (Shell R 828), ether and a hardener. The injection process consisted of pouring the epoxy into a tray with several specimens so that it flowed up and around each specimen. The tray was then put into a chamber and the pressure was applied. The impregnation required about 2 h.


Typical stress-strain curves at different principal stress ratio tested are shown in Fig. 1. It is interesting to note that only for the case of uniaxial compression, the stress-strain curve deviates significantly from linearity at high levles of straining. This behavior is directly related to the observed internal microcracking process and will be explained in the next section. The analysis of the plots also indicate that the ultimate compressive strength under biaxial tension-compression is significantly less than the uniaxial compressive strength


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