An Investigation On Concrete Cracking Monitoring Based On Digital Image Correlation And Its Safety Evaluation

Concrete crack detection is one of the basic issues received much attention in the safety of building structures.An investigation on concrete cracking monitoring based on digital image correlation and its safety evaluation is presented in this work by means of the digital image correlation recently developed in the field of computer image technology,in which the test object is a concrete beam under the bending.It includes four aspects of contents:design of the experimental platform for digital image correlation method,optimization of the analyzed digital signal,test of observing the cracking of the concrete beams during their deformation and safety of the surface crack propagation on the concrete,in detail,as follows.(1)A scheme to develop the experimental platform available for the digital image correlation measurement is proposed.This scheme stresses on three basic technical points,high-quality of the analyzed image,reliable description ofthe correlation coefficient function and applicable approaches matching with the conventional treatments.Considering actual conditionsof the laboratory,atechnical implementation route is formed and as a restriction,the inexpensive industrial CCD camera is chosen.(2)A method of optimizing the analyzed digital image signal is proposed to match with the signal acquired by the common industrial CCD camera.Aiming at the requirement of the optimization of signals in the image correlation measurement,it introduces the two-dimensional empirical mode decomposition technique into the processing of digital images,by which a noised image is decomposed into finite components at different levels from high to low frequency.These components may be filtered by their contributions to the image at different levels.A sum of the filtered components gives a new image to reach the goal of removing noise from the acquired image.Furthermore,by using the grid calibration method,the CCD lens can be effectively rectified to increase the quality of the adopted image.The rectification of lens may satisfactorily meet the requirement of optimizing the analyzed digital image signal in the digital image correlation measurements.(3)Test of concrete cracking monitoringis conductedon the bending concrete beam by digital image correlation method.Resultsin the observed displacement and strain fields showthat the digital image correlation method provides an effective tool for monitoring deformationphenomena of concrete surface cracking.The displacement components can accurately capture information details related to concrete cracking during loading and the high strain areasdetermined by the strain component describe geometrical patterns and locations of the observed cracks.(4)An approach to identifying the crack propagation on the concrete surface by digital image is proposed.On basis of comparison of the experimental results from test of concrete based on the digital image correlation method,a theoretical relationshipthrough the observed strain componentsis derived fromthe linear simplifiedstrength criterion of concreteby introducing the equivalent strain description.The theoretical results are in good agreement with the experimental observations.This workprovides a specific method for monitoring the safety of concrete structures.The obtained results in the laboratory show that the presented method has agood value of applications.