Analysis Of Impact Echo On Tendon Ducts Grouting Quality Based On Empirical Modal Decomposition

Prestressed concrete bridges,due to their mature construction technology and low cost advantages,occupy a dominant position in the domestic bridge construction industry.However,the tendon ducts grouting work is hidden,and the quality of grouting cannot be visually observed by conventional means after grouting.Therefore,problems such as rust and failure of prestressed steel strands due to incomplete grouting have emerged,which pose a huge safety hazard to bridge structures.Finding and developing an effective non-destructive testing technology for the quality of prestressed duct grouting is the key to solving this problem.Among a wide range of non-destructive testing techniques,the impact-echo method,with its strong penetration ability,good frequency spectrum characteristics and mature theoretical basis,has become the preferred method.However,there are still many problems in practical applications.Based on the technical status at home and abroad,this study conducted research on the impact-echo method for detecting the quality of tendon ducts grouting,including the following aspects:Firstly,a survey was conducted on the current research status of the impact-echo method for detecting the quality of tendon duct grouting at home and abroad.The deficiencies of existing research were analyzed,and two research directions were proposed: modeling methods considering the microscale of concrete and finding more reasonable signal processing methods.The theoretical basis of the impact-echo method,namely the theory of elastic waves,was briefly introduced.The relevant principles and usage methods of the impact-echo method were discussed from three steps: generating the impact signals,collecting the response signals and analyzing the signals.Secondly,a two-dimensional numerical model was established using the ABAQUS/Explicit program to perform finite element analysis on the impact-echo detection method.Based on the diameter distribution probability of the Fuller grading curve in two dimensions,a random aggregate model is generated.The comparative analysis shows that the microstructure has little effect on the frequency peak position in the response signal but only affects the amplitude,and both modeling methods are feasible.Using the microscopic model for modeling and analysis of the impact-echo method,the signal characteristics of the impact-echo method were studied under three classic working conditions: full grouting,defects in the tendon ducts,and completely empty tendon ducts.The half-wave loss phenomenon of the steel strand and the frequency drift phenomenon of the plate thickness caused by defects were verified.It was also found that the influence of the waveguide material on the detection results was small due to the thin wall thickness of the waveguide.Then,the modeling and analysis of the commonly encountered double-layer tendon ducts working condition and the tilted bottom plate working condition in actual engineering were conducted.The results showed that when defects existed in both the upper and lower layers of the tendon duct,the defects in the lower layer were often undetectable,and additional testing with a modified impact plane was needed.The tilting of the bottom plate had little effect on the response signal.Finally,the influence of parameters such as the impact position,the diameter of the tendon ducts,and the depth of burial on the impact-echo method was studied.The results showed that the position of the impact point relative to the centerline of the tendon duct had a significant impact on the detection results,and multiple testing points should be taken near the estimated centerline of the tendon duct to avoid misjudgment.The increase of the defect diameter in the tendon duct would exacerbate the plate thickness frequency drift phenomenon and could be used as an auxiliary means to judge defects.When the ratio of defect diameter to plate thickness was greater than 0.2 and the ratio of defect depth to plate thickness was greater than 0.3,the defect detection effect was better,and the characteristic frequency could be clearly observed in the spectrum.To extract characteristic signals more intuitively,the Complete Ensemble Empirical Mode Decomposition with Adaptive Noise(CEEMDAN)method was proposed.Theoretical analysis and numerical experiments shows that the CEEMDAN method can effectively solve the problem of mode mixing in the empirical mode decomposition process and avoid the noise residue problem encountered in the ensemble empirical mode decomposition method.Therefore,it is an ideal signal processing method for the impact-echo method.Finally,three model specimens were manufactured for on-site testing,namely a singlelayer duct concrete plate,a double-layer duct concrete plate,and an uneven thickness concrete plate.The test results shows that the impulse echo method can detect grouting defects in prestressed concrete boreholes,and is effective for single-layer boreholes and inclined bottom plates,but needs improvement for double-layer borehole detection.Consistent with finite element analysis results,the use of the CEEMDAN method to process the test response signals shows good noise reduction and clearly visible characteristic frequencies in the decomposed modal components,preliminary demonstrating the effectiveness of this method.Based on the results of the finite element analysis and experimental results,a preliminary discussion on the future development of using the impulse echo method to detect grouting quality in pre-stressed concrete boreholes was conducted.