| At present,the drilling and blasting method is usually used in the construction of mining and tunnels.The strength and stability of the surrounding rock near the site are reduced by the disturbance from the impact loading induced by the blasting operation.Thus,concrete should be immediately sprayed on the surface of surrounding rock to reduce the weathering situation.Furthermore,sprayed concrete can provide flexible support resistance for the surrounding rock,preventing a large deformation in the surrounding rock.In order to improve the efficiency and construction process,the next blasting operation is often conducted after the spraying concrete operation finished in a short interval time,the fresh concrete near the excavation site doesn't reach the stable age and the strength is far less than the designed strength.Seismic wave,air shock and flying stones are formed in the blasting operation,and conduct impacting effects on the newly sprayed concrete and surrounding rock.The cracking and spalling occur frequently in sprayed concrete subjected to disturbance during the repeatedly blasting excavation construction.Furthermore,the energy stored in surrounding rock releases rapidly induced by the blasting impact load,resulting in the geological disasters such as rockburst,rock bursts,mine earthquake,which poses a great threat to the life of workers and the safety and efficiency of constrction.Research indicates that the mechanical properties of concrete and rock under dynamic loading condition are different from that in the static load condition.However,most of the previous research concerned mainly on the dynamic mechansm properties of single mecium for concrete or rock individually.The dynamic mechanism properties of the double medium,near the adhesive surface formed by the early age sprayed concrete and surrounding rock,are different from that of single medium,but little literature have been reported about this issue on the double medium of sprayed concrete and surrounding rock.Therefore,it is essential to conduct the research on dynamic mechanical properties of the double medium near the adhesive surface formed by the early age sprayed concrete and surrounding rock to make up the deficiency of the previous research and provides some engineering guidance for the site.At present,drop hammer impact test machine,light gas guns,explosive plane wave generator and split Hopkinson pressure bar?SHPB?are the dynamic loading test devices that have been used widely.Split Hopkinson pressure bar has been used widely in the research on the dynamic mechanism properties of concrete and rock,and fruitful achievements have been achieved.Furthermore,split Hopkinson apparatus is widely used in the research on the dynamic characteristics of various materials such as metal,rock,concrete,rubber and ceramics as the improvement of the apparatus and technology,and its application is still expanding.In this paper,laboratory tests and numerical simulation are adopted to study the dynamic mechanism properties and the features of damage and failure of sprayed concrete-surrounding rock combined bodies based on the experimental techniques,including split Hopkinson pressure bar,computed tomography and scanning electron microscope.The half-sine waveform loading created by the spindle shaped striker is used as loading waveform,and the specimen length in SHPB tests is selected based on the stress equilibrium results of the specimens with different lengths.The mechanical properties,wave characteristics,damage characteristics and failure characteristics of the sprayed concrete-surrounding rock combined bodies under impact loading are studied systematically based on a large number of test data,obtained with the SHPB system of75 mm in diameter,and the following conclusions are drawn:?1?The mechanical property tests for sprayed concrete-surrounding rock combined bodies in static load condition have been carried out to analyze the evolution process from damage to failure.Both uniaxial compression strength of the combined bodies and the tensile strength of the bonding surface increase as the age of concrete in the static load condition,but the peak displacement of the combined bodies decreases.The failure process of the combined body in the uniaxial compression test can be divided into three stages.In the initial stage,there exist the tension type cracks in the concrete and several fragments around the specimen,and several cracks go through the bonding surface in the axial direction.In the mid-stage,the tensile failure surfaces extend into the concrete,creating the bonding fragments and X-type failure surfaces in specimen.In the last stage,concrete produces a lot of fragments and shear failure surfaces are produced in the red sandstone.The tensile failure of the adhesive surface results from the decreasing of the adhesive force in the tensile test,and fracture surface is relatively smooth.?2?SHPB impact compression tests for sprayed concrete-surrounding rock combined bodies have been carried out to study the dynamic properties,failure laws and the fragment average size and fractal dimension.Furthermore,the relationship between the strain rate and fragment quality has been revealed in the study.There is a strain rate dependence for the dynamic compressive strength,dynamic strength increasing factors,peak strain of the combined bodies under impact loading.Both the dynamic compressive strength and dynamic strength increasing factors present the increasing tendency as the growth of strain rate,but the peak strain increases firstly and decreases later as the growth of strain rate.The damage degree of the specimens aggravates as the strain rate growth from 45s-1 to 100s-1.All of the fragment quality,fine fragment number and fractal dimension increase as the growth of strain rate.The average size of fragments increases firstly,then decrease.There is exponential function relationship between the dynamic compression strength and factual dimension.The failure mode of the combined bodies can be divided into three type,including I-type,?-type and?-type.I-type is that the tension type cracks appear in the concrete and axial cracks around the specimen go through the bonding surface.?-type is that the specimens produce several bonding fragments containing the double conical surface or the single conical surface,and tensile failure surfaces appear in the rock.?-type is that concrete is crushed into lots of fragments,and several tensile failure surfaces appear in the rock,producing a great mass of bonding fragments.?3?The cyclical impact compression tests for sprayed concrete-surrounding rock combined bodies have been carried out to study the.change characteristics of the stress-strain curves as the impact times.And it obtains the conclusion,which has important guiding significance for the field blasting construction,according to the relationship between the accumulated dissipation energy and damage of the specimens.The mechanical properties of early age concrete have great influence on the dynamic mechanical characteristics of the combined bodies.The mechanical characteristics of the combined bodies at the age of 3d present the ductility at the first impact.The brittleness of the combined bodies enhances as the growth of age,which changes to ductility gradually in the process of cyclical impact tests.When the impact pressure is 0.52MPa,the axial strain decreases firstly and increases later but the elastic modulus increases firstly and decreases later with the increase of impact times.When the impact pressure increases to 0.56MPa,the mechanical characteristics weaken,making the peak strain increasing and elastic modulus decreasing.The accumulative dissipated energy of the combined bodies in 0.54MPa impact pressure condition is more than that in 0.56MPa impact pressure condition,but the accumulative damage of the specimens in 0.54MPa impact pressure condition is worse than that in 0.56MPa impact pressure condition,presenting a negative correlation between the accumulative damage and accumulative dissipated energy.As can be seen from the result,the lower impact energy condition is beneficial for the combined bodies to resist the multiple disturbances induced by external impact loading,which has a important guide significance for the site construction.In the blasting construction of roadway and tunnels,we can reduce the explosive amount appropriately and increase the blasting times to reduce the damage in early age concrete and surrounding rock,which is beneficial to the stability of the supporting structure formed by the sprayed concrete and surrounding rock.?4?Based on the test waveform results obtained from the SHPB tests of the combined bodies,the influence rules of the impact velocity,age and impact times on the stress wave transmission coefficient have been studied.The experimental results show that the transmission coefficient will decreases as the growth of impact velocity or impact times.Furthermore,it increases as the increase of concrete age under the same experimental velocity.?5?The microscopic mechanics experiment of the combined bodies,relying on CT and SEM microscopic test technologies,has been carried out to study the microcosmic damage laws in combined bodies and dynamic fracture morphological characteristics in rock.Results show that the crack propagation path of concrete under impact loading is mainly affected by two factors,including the guide effect of interface transition zone and the crack resistance effect of aggregate.There are four kinds of crack propagation modes in concrete,including propagation in interface transition zone,propagation in mortar matrix,aggregate fracture and crack-arrest by aggregate.The damage degree in the combined bodies increases as the growth of cyclical impact times.Then the CT value of images decreases,but the fractal dimension of crack increases gradually.Meanwhile,the distribution interval of histogram curves decreases,but the peak value presents a increas-ing tendency.The dynamic fracture mode in rock is mainly tensile failure in the impact compression tests proved by the SEM images,and the fracture morphology includes transgranular fracture and intergranular fracture. |
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