Study On Dynamic Mechanical Properties And Crack Propagation Of Concrete Water Jet-Breaking

With the development of society,the maintenance of concrete pavement becomes more and more important,and concrete crushing is one of the important work in the maintenance of concrete pavement.At present,the traditional crushing technology is widely used in breaking concrete,with the disadvantages of heavy workload and easy to cause secondary damage.Water jet crushing concrete technology has obvious advantages of high accuracy,low damage to the original structure and non-contact cutting,which plays an important role in road maintenance,post-disaster reconstruction,emergency rescue and other projects.Nevertheless,the water jet impact involves high pump pressure,large specific energy consumption,small effective crushing range of the jet,and poor local crushing precision.Especially,for the emergency removal of the concrete structures after the disaster or stress sensitive area,the crushing accuracy needs to be improved.The key to solve these problems lies in the general destructive law and cracking mechanism of concrete materials under high-speed liquid impact.To improve the repair level of concrete structures and the efficiency of emergency demolition,this paper explored the mechanical properties and internal crack propagation characteristics of concrete under water jet impact by experimental methods and numerical simulation analysis.The main conclusions are as follows:(1)Based on the digital image correlation(DIC)technology,this paper analyzed the dynamic strain distribution characteristics on the surface of concrete under different water jet pressures,and results indicate that with the water jet impact,the concrete surface would appear approximate circular strain areas,which can be divided into strain concentration area,strain transition area and strain weak action area.With the water jet pressure of 200 MPa and 240 MPa,there was not only the strain concentration area occurred in the liquid-solid contact area,but also the strain concentration area appeared in the original defects or pores of the concrete due to the effect of the internal shock wave of the concrete,and these strain concentration areas were easy to be destructed.By the strain data filtration based on fourier transform technique,it can be found that the strain data in three strain regions under water jet presure of 160 MPa hadn’t significant difference,and the strain curves of all regions were irregular due to the uneven impact of water jet.The strain curves of all regions under water jet presure of 200 MPa and 240 MPa didn’t appear coincidence,and the strain values in strain concentration area were always greater than those in strain transition area and strain weak action area,indicating that the greater the jet pressure,the larger the internal stress wave intensity of concrete was.Besides,according to the strain nephogram and the displacement curve of a typical point in the strain concentration area,this paper predicted the macroscopic failure of the concrete surface,and it is found that the distribution pattern in the strain concentration area determined the propagation path of the surface crack of concrete.(2)By the smoothed particle hydrodynamics(SPH)method and finite element method(FEM),and verified in the numerical simulation result by the internal crack morphology comparison of computed tomography(CT)test.The simulation results were consistent with the crack morphology in CT test,which proved that the meso-numerical simulation of water jet impact concrete simulated in this paper were reliable.(3)Based on the established numerical model of concrete impacted by water jet,the stress effect of concrete and crack initiation mechanism in the whole process were revealed.Results indicate that the stress waves propagated slowly along the axial direction of the jet and quickly along the radial direction of the jet due to existence of meso-structures.Then,a crater was generated under the strong water hammer pressure of water jet,and due to the existence of aggregate,the repetitious superposition of stress waves caused the irregular shape of the crater.In the intense action zone of water jet,the crack at protruding part of aggregate firstly appeared and then connected to the crater or the other protruding part of the adjacent aggregate,forming the connecting cracks between aggregate and aggregate.And when the crack tips met aggregate at a large angle,the energy of concrete could be fully released to aggregate,causing the crack penetrating aggregate.In the weak action zone of water jet,the decreases of stresses in concrete induced that cracks preferred to propagate along aggregate.Besides,with the sustained impact of water jet,the crater gradually enlarged,and internal cracks propagated to the boundary of concrete to form flank cracks,finally,the concrete took place the whole debacle with the expansions and fusions of the internal cracks and the flank cracks.According to the Image processing technology,the damage levels of concrete in each stage were analyzed,and it is found that the internal damage of concrete in axial and radial directions decreased sharply with the increase of distance,and due to the existence of weak interface phase,the damage distal and boundary areas of concrete also appeared.By the comprehensive damage factor,the concrete could be divided into severe damage area,high damage area,medium damage area and micro damage area.