Experimental Study On Micro Crack Detection Of Concrete Based On Ultrasonic Nonlinear Modulation

Concrete is a widely used building material in infrastructure construction.It has the advantages of high strength,simple operation technology and low price.However,due to the characteristics of concrete itself and the limitations of construction technology,micro-cracks and other micro-damage exist in the early stage of forming,which seriously affects the durability and fatigue resistance of concrete,therefore,it is necessary to detect and evaluate the concrete micro cracks,so as to take necessary measures to control its development.There are many methods for concrete damage detection and evaluation.The traditional ultrasonic detection method is usually based on the linear wave theory to determine the larger damage,which has great limitations for the detection o f microcracks.Because the change of concrete microcracks has great influence on material nonlinearity,it is of great significance to use this characteristic to detect and evaluate concrete microcracks.At present,the method of concrete microcrack detection is mainly based on the high-order harmonic method of non-linear coefficients,but this method is highly influenced by the environment and has low sensitivity.Based on the ultrasonic nonlinear modulation method to detect metal micro-cracks,this paper proposes a ultrasonic nonlinear modulation method to detect concrete micro-cracks,and the feasibility and universality of the method for detecting micro cracks in concrete are demonstrated by corresponding experiments.Based on the theory of nonlinear ultrasonic wave,according to the method of solving the ultrasonic dynamic equation when single-frequency and double-frequency signals are used as excitation signals,the solution of the equation when broadband signals pass through solid materials is studied.On the basis of this solution,the coupled solution of wave equation is solved when single-frequency signal and broadband signal act on solid materials simultaneously,the generation mechanism of modulated signals in ultrasonic nonlinear modulation is analyzed.In the case of neglecting high order and hysteretic nonlinearity,the concept of sideband peak counting method and damage index is proposed.And the characteristic of damage index increasing with material nonlinearity is analyzed theoretically,which is consistent with the experimental results.The microcracks were prepared by accelerated alkali aggregate reaction and the expansion rates of different age specimens were measured,it is found that the expansion rate increases with the increase of curin g age.And after SEM quantitative analysis,the microcracks increase with the increase of alkali aggregate reaction time,it is proved that the theory of ultrasonic nonlinear modulation is effective in evaluating concrete micro-cracks,which lays a foundation for testing the micro-cracks of concrete specimens.The experimental study on the detection of microcracks in cement mortar specimens proves the feasibility of detecting microcracks in cement mortar specimens by edge-band peak counting method.Through the test of different percussion force on Lossless and lossy cement mortar specimens,it is found that the damage index curves corresponding to different relative energy levels tend to coincide and the peak value approaches 0.The peak value of damage inde x does not change with the increase of relative energy levels,it is shown that under the experimental conditions,the percussion force has no effect on the results of microcrack detection.The cumulative micro-damage and persistent micro-damage of cement mortar specimens were detected,and it was found that the damage index peaked with the change of threshold value.However,the damage index curves of the specimens without accelerated alkali-aggregate reaction tend to coincide and the peak value approaches 0,and the peak value of the damage index does not change with the increase of age.But the damage index curve of the accelerated alkali-aggregate reaction specimens increases with age,and the peak value of the damage index increases nonlinearly with age,it shows that the damage index increases with the development of micro cracks in cement mortar specimens.The detection of microcracks in concrete specimens with coarse aggregate further verifies the high sensitivity and universality of the ultrasonic nonlinear modulation method for detecting microcracks in concrete.Through the detection of cumulative micro-damage and persistent micro-damage of concrete,it is found that the damage index of specimens peaks with the change of threshold value.However,the peak values of the damage index of the non-accelerated alkali-aggregate reaction specimens at different ages were close to 0 and did not change with age.And the peak value of damage index accelerated by alkali aggregate reaction increased significantly with the increase of age,it shows that for concrete specimens,the damage index also increases with the increase of micro cracks.Furthermore,it is found that the peak value of damage index of sustained micro-damage specimens is larger than that of cumulative micro-damage specimens at the same curing age,this is due to osmosis,the effective alkali-aggregate reaction time of cumulative micro-damage is less than that of sustained micro-damage,which shows the high sensitivity of this method.By testing the micro cracks of concrete with different water cement ratio,it is found that when the curing age is the same,the damage index curves of concr ete specimens with different water cement ratios increase from 0 to peak value,then decrease to 0,and the peak value increases with the increase of water cement ratio,it shows that the larger the water cement ratio of concrete,the greater the influence of alkali-aggregate reaction on the development of microcracks,and the easier microcracks are formed.