Quantification Analysis Of Concrete Creep Under Ring-shape Restraint And Quantitative Evaluation Of Concrete Cracks With Digital Image Technology

With the wide use of concrete in construction engineering, early-age cracking of concrete structures has become increasingly common and severe. Early-age cracking of concrete will lead to degradation of concrete's performance, and will weaken its durability. Thus, cracking resistance of concrete is attracting more and more attention in recent years, as well as the evaluation of existing cracks on the surface of concrete.In this paper, the development of three kinds of basic mechanical indexes, such as compressive strength, elastic modulus and flexural strength, and autogenous shrinkage with time were studied. The influence of mineral admixtures was also analyzed in this research. By means of a four-point bending test, the initial cracking tensile strain and ultimate tensile strain of concrete were identified and their growths with age were studied.Based on the analytical solution of concrete ring in restrained conditions, with the strain obtained from the contact surface of steel ring in a concrete test, the development of restrained shrinkage and actual inner stress of concrete were calculated. The stress relaxation was taken as the difference between the actual inner stress calculated from analytical solution and the theoretical stress obtained from the analysis of concrete ring under restrained shrinking which ignores the stress relaxation. The total deformation of concrete ring under restrained conditions can be seen as the superposition of free shrinkage, elastic deformation and creep deformation. Considering the deformation compatibility between the inner boundary of concrete ring and outer boundary of steel ring, the creep deformation can be solved. With this method, this research conducted an investigation on the influence of mineral admixtures on creep and stress relaxation of concrete.The subjective factors hiding behind the measurement of length and width of cracks on concrete are the fatal defects of traditional methods, so as the low accuracy and efficiency. Based on digital image technology, a new graphical method was put forward to extract the parameters of concrete cracks. An algorithm which was derived from fractal theory was proposed and programmed with MATLAB to calculate the characteristic parameters of concrete cracks, such as area, fractal dimension, length and width. The flat slab concrete cracking test showed that the graphical extraction method of cracks had the advantages of high precision, convenience and quickness. The algorithm also performed accurately. This research provided not only an effective method to describe the irregular nature of early-age concrete cracks with Fractal theory, but also a powerful tool for the quantitative evaluation of cracks in concrete.