Study On Fracturing Characteristics And Mechanism Of High-Pressure Water Jet Impacting Concrete

The concrete crushing technology is very important in the projects of the maintenance repair of concrete structures.The traditional methods such as vibration hammers,pneumatic picks and explosive,will cause more cost,new cracks and low repair quality.On a contrary,the high-pressure water jet impacting the concrete is not only more effective but protecting environment.It is working in the projects of both the bridge widening and the pavement renovation.Because the mechanism of concrete crushing is not clearly,the high-pressure water jet technology development is slow in the field of the concrete crushing.In order to improve the application level of the high-pressure water jet technology,the quality and efficiency of the high-pressure water jet fracturing the concrete,this paper investigated the characteristic and mechanism of crack propagation of the concrete under the highpressure water jet impacting by tests and numerical simulation.Conclusions are as follows.(1)The high-speed camera method was used to study the crushing process of the high-pressure water jet vertical impacting concrete.The results show that the reflected water jet generated during the impact process has different states before and after the concrete is crushed.The direction angle of reflected water jet has a minimum value of 31°.The degree of chemistry is related to the degree of disturbance.The high-pressure water jet impacting concrete can be divided into two stages: the water jet splitting concrete and stress wave fracturing concrete.Based on the illustrated method,the crack propagation displacement,crack propagation angle,crack propagation speed and the crack propagation acceleration are used to quantify the characteristics of the surface crack propagation.The discontinuous release of energy causes the main crack and the secondary crack I to exhibit an extended,stagnant peri-cycle evolution,in which the main crack period gradually increases with its increasing displacement.While the secondary crack I only undergoes one cycle.Due to the selectivity of crack propagation,the secondary crack II is almost in a stagnation period after the evolution in a short period of time.(2)By the CT scanning technology,it detected the meso fracture state of the concrete in all directions under the water jet impacting,and illustrated the mechanical process of the "V" shaped taper crushing core,surface cracks and circumferential cracks in concrete,based on the CT scanning results and the liquid-solid collision theory.Then it investigated the failure mechanism of the concrete by the water jet impacting in cracking,expanding,and rupturing stages,and revealed the five typical cracks propagation characteristics and mechanisms.Furthermore,the CT scan images were processed through the histogram equalization and the threshold segmentation method,and a damage characterization method was proposed based on the image gray value.And then it studied the CT scale damage distribution characteristic in concrete by the water jet impacting,and quantitatively pointed out that there is obvious damage in the interfacial transition zone of the unbroken area in the concrete by the water jet impacting,and the damage becomes more serious with an increase in the distance to the crack.(3)In order to study the mechanism of the water jet fracturing in concrete,it established a numerical model of the water jet fracturing in concrete based on Smooth Particle Hydrodynamics(SPH).In addition,based on the image processing technology,the degree of concrete fragmentation was defined to characterize the crushing state of the concrete under the water jet impacting quantitatively.The results show that the water hammer effect causes the concrete fragmentation on the liquid-solid contact boundaries,and the shear stress drives the cracks to propagate toward the water jet axis direction,resulting in the bowl-shaped crushing pit.The conical cracks are induced by both the shear stress on the liquid-solid contact boundaries and tensile stress.The shear stress on the liquid-solid contact boundaries produces the vertical cracks.The spall cracks are induced by the tensile stress generated by the superimposed stress waves reflecting on the free boundaries and the pre-crack.The failure evolution of the concrete attenuates with difference changing rates along the water jet axis direction.The fragmentation degree of concrete attenuated from fast to slow with axial direction of the jet,and the attenuation degree of energy was reflected synchronously.