The leading cause of concrete cracking is the generation of internal stress generated by the volume change under restrained conditions, which exceed the ultimate value of strength. In particular, the restrained stress generated by temperature change is significant, particularly for large-volume concrete. This study proposed a new restrained method to separate the thermal stress, autogenous shrinkage stress, and drying shrinkage stress from total restrained stress. Unlike traditional restrained methods, Invar steel’s characteristic low coefficient of linear expansion was used to generate the thermal stress in this method. To investigate the feasibility of this method, two ordinary cement concrete with 0.30 and 0.40 water-to-binder (w/b) ratios and a ultra high performance concrete (UHPC) with 0.21 w/b ratio were tested. The total stress obtained by the accumulation of the three separated stresses were compared with the experimental total stress. The discrepancy between the direct value and the accumulated value of total stress was within 20 %, indicating that the restrained stress separation method proposed in this study produced viable results. Overall, the new restrained method proposed in this study enabled to test and separate the concrete restrained stresses easily and accurately, which was meaningful for evaluating the risk of concrete cracking in practical engineering.

The digital image method of monitoring structural displacement is receiving more attention today, especially in non-contact structure health monitoring. Some obvious advantages of this method, such as economy and convenience, were shown while it was used to monitor the deformation of the bridge structure during the service period. The image processing technology was used to extract structural deformation feature information from surveillance video images containing structural displacement in order to realize a new non-contact online monitoring method in this paper. The influence of different imaging distances and angles on the conversion coefficient (η) that converts the pixel coordinates to the actual displacement was first studied experimentally. Then, the measuring and tracking of bridge structural displacement based on surveillance video images was investigated by laboratory-scale experiments under idealized conditions. The results showed that the video imaging accuracy can be affected by changes in the relative position of the imaging device and measured structure, which is embodied in the change in η (actual size of individual pixel) on the structured image. The increase in distance between the measured structure and the monitoring equipment will have a significant effect on the change in the η value. The value of η varies linearly with the change in shooting distance. The value of η will be affected by the changes in shooting angle. The millimeter-level online monitoring of the structure displacement can be realized using images based on surveillance video images. The feasibility of measuring and tracking structural displacement based on surveillance video images was confirmed by a laboratory-scale experiment.

基于监控影像和数字图像处理技术,提出了一种表征结构位置特征点位移变化的方法.首先在结构上安置圆形标靶,用模板匹配法检测圆形标靶的初始图像坐标区域,裁剪出感兴趣区域(ROI);然后用大津法确定该区域的二值化阈值,提取标靶轮廓点坐标,使用最小二乘法将轮廓拟合成椭圆,获得椭圆中心坐标及长短轴尺寸,圆形标靶直径与椭圆长短轴的比值为转换系数η;椭圆中心坐标变化量和η的积为结构实际对应坐标方向的位移量.实验表明:在水平与竖直方向的位移加载实验中,该方法与高精度位移计对比的相对误差均＜3％,可以实现mm级位移监测,精准表征结构对应特征点位移变化.

The structural displacement is one of the important indicators to describe the health state of the bridge during service, and the digital image method monitoring structural displacement was paid more attention. In this paper, the image processing technology was used to extract structural deformation feature information by the surveillance cameras images containing structural deformation. A new non-contact measurement method of structural displacement using surveillance camera and digital image processing technology was proposed, and it was confirmed by experiments that the full-time monitoring of tracking structural displacement can be to realize by using surveillance cameras images. Comparing with two various calibration methods, the virtual circle center method based on circular object was selected in this study and it was suitable for surveillance camera calibration. Moreover, A non-contact, real-time, high-precision and convenient displacement monitoring system software would be designed, and this on-line monitoring method of structural displacement can meet to millimeter level.