Experimental Study On Impact Resistance Of Concrete After Freeze-thaw Damage

Concrete structures such as cross-sea platforms and bridges,which are built in the cold region of high latitude,will encounter the occasional impact of ships and other floating objects,and will also face extreme environmental erosion during their service life.Freeze-thaw cycles,as a type of environmental erosion in cold areas,have an important impact on the long-term safety of concrete structures.At present,the research on the impact resistance of concrete structures of the world is mainly based on the normal environment.Most of the relevant researches focus on the general change trend summary of the experimental phenomena,and fail to explain the degradation law of the impact performance indexes of concrete caused by freezethaw damage factors from a theoretical perspective.Moreover,there is a lack of elaboration on the related failure mechanism at the microscopic level.Therefore,the research on the impact resistance of concrete structures in the context of freeze-thaw cycles has practical significance and engineering application value.Based on the above problems,this paper mainly studied the influence of freeze-thaw damage on the impact mechanical properties,deformation characteristics and strain rate effect of concrete.A variety of freezing-thawing damage degrees were designed in the experiment,and the dynamic load was applied to the concrete by the dynamic loading device.The main research contents of this paper are as follows:Firstly,the dynamic load in the range of medium and low strain rate was applied to the freeze-thaw damaged concrete by the electro-hydraulic servo test system.The effects of different freeze-thaw cycles and different loading rates on the uniaxial dynamic compressive strength,stress-strain curve,elastic modulus,peak strain and strain energy density of concrete were investigated.In addition,the evaluation index of freeze-thaw damage of concrete was analyzed.Secondly,the split hopkinson pressure bar(SHPB)test system was used to apply impact load in the range of high strain rate to freeze-thaw damaged concrete.The stress wave after reducing the effect of high frequency oscillation by waveform shaping technology was analyzed and the stress was balanced.With the help of the "three wave formula",the stress-strain curves of each group of concrete were obtained,and the influence of freeze-thaw damage on the mechanical and deformation properties of concrete under high strain rate was discussed.Then,based on the relevant research results in the field of dynamic size effect of concrete,we explored the "critical strain rate" of freeze-thaw damaged concrete in the multi-strain rate range,and analyzed the distribution law of DIF value.By introducing "strain rate effect correction function" and "freeze-thaw degradation function",a concrete compressive strength calculation formula was established which considered the effect of dynamic augmentation and freeze-thaw damage.Based on the theory of continuous damage and the fact that the crack distribution of rock material conforms to Weibull distribution,the dynamic constitutive model of freeze-thaw damaged concrete was established.Finally,according to the aggregate gradation theory and the Fuller gradation curve plane formula,the aggregate random placement program was prepared from the microscopic level using MATLAB to generate two-dimensinal numerical concrete.Then the dynamic impact loading process of freeze-thaw damaged concrete was simulated based on ABAQUS finite element analysis software.The accuracy and applicability of the two-dimensional random aggregate model were verified by comparing the numerical model with the experimental results.