Research On Cementitious Materials Restrained Shrinkage Test Methods

There are many reasons that can lead to concrete cracks, which can be divided into loading cracks, subordinate loading cracking and non-loading cracks. The shrinkage of concrete is the main reason in the non-load leading cracks. Concrete shrinkage is unharmful in freedom situation. Actually, the concrete structures are all restrained which lead to the concrete occurring cracking. The cracks of cement-based materials can reduce the durability of the concrete, and even affect the integrity of the concrete structure.There are a number of test methods developed to measure the early shrinkage of concrete, while the restrained ring test(RRTM) is widely referred, The RRTM enjoys many merits: methodology is straightforward, test apparatus is simple and so on. There are three major problems in RRTM:(1) specimens are axisymmetric in geometry, which leads to cracking location taking place in a random fashion;(2) Usually, the thickness of the specimen is 30mm~35mm, and the maximum size of coarse aggregate is 15 mm which is not really reflect the shrinkage performance of concrete;(3) According to reference, the maximum specimen wall on mortar test is 75 mm, If the concrete specimen wall is 75 mm as well, it not cracks because the cracking sensitive is very low. In order to solve those problems, restrained squared eccentric ring test method and restrained eccentric ring test method are proposed in this paper. Mechanical analysis and test are used to verify the feasibility and accuracy of the above methods, and statistics analysis is applied to static the distribution of the cracking location and the cracking time, experiment is applied to test the crack sensitivity of restrained eccentric ring test method(RSERSTM) and Weiss theory model is employed to estimate the cracking stress of RSERSTM. In order to test the cracking time of the cement-based materials simply and accurately, a new time detecting method is proposed. Conclusions are as follows:(1) Restrained eccentric ring test method(RSERSTM) on paste and mortar shrinkage test. The purpose of this test method is to alter the traditional test method which contains an outside circle constraint to the shape of a square one and to offset the inner circle iron ingot constraint along middle one of the two opposite lines of the mode to make an eccentric circle constraint. And the size of the specimen is 170mm×170mm×35mm, and further experiment results shows that, the cracking time of the RERTM specimens were early than RRTM, and 98% of the cracking location offset distance is less than 15.35 mm. Therefore, it can be concluded that the restrained eccentric ring test method can fixe the cracking location effectively, and reduce the experiment period, and the cracking sensitive is improved. According to comparison the cracking develop route of RSERSTM and RRTM, the cracking of RRTM begins at the inner circumference and the RSERSTM begins at the outer circumference, those results verified the finit element analysis conclusion is correct, and it is also verified the cracking sensitive of the RSERSTM is higher than RRTM.(2) The squared eccentric ring shrinkage test method(RSERSTM) on concrete shrinkage test. The principle of concrete test method is the same to cement paste and mortar, and the difference is the size of specimen. In order to actually reflect the shrinkage cracking performance of concrete, the maximum size of the coarse aggregate is chosen to be 25 mm, the size of the specimen is 600mm×600mm×75mm, and the narrowest section of specimen is 75mm×75mm. The numerical analysis shows that a anticipated cracking location occurs at the narrowest section; the experiment results show that the RSERSTM specimen with 0.5 water to cement cracks in two weeks, and the cracking location offset distance is little, while the specimen with 75 mm thickness concrete wall is no cracking. The experiment is applied to verified the cracking sensitive of the RSERSTM, taking the concrete with 0.5 water to cement for example, while the specimen still cracked under all sealed condition. Weiss’ s cracking stress formulation is recommended to calculate the stress distribution of the squared eccentric ring restrained specimen. According to the formulation advocated by Weiss, the cracking stress estimation of concrete restrained eccentric ring specimen is 3.7MPa~4.2MPa.(3) The restrained eccentric ring test method(RERTM) on concrete shrinkage test. The outer boundary of specimen is circular. The diameter of specimen is 600 mm and the height is 75 mm. The diameter of the inner steel ring is 300 mm, and the inner steel ring is shifting to the outer border to ensure the distance between the outside edge of the inner steel ring and the mould frame is 75 mm. The cracking stress of the restrained eccentric restrained specimen is calculated as well. The experiment result shows that the RERTM can fix the cracking location effective, and the cracking sensitivity is improved as well, but the effect is worse than RSERSTM, but better than RRTM.In order to test concrete cracking time directly, a new device is advocated in the paper. The device is consisted by an electrical materials without strength, wield, battery and clock. The electrical material is film on the proposed cracking location. After the electrical material is dry, all of the composite was formed a circuit. The specimens are watched 12 h once, if the specimens cracked, the clock stopped and the finger direction is the cracking time. The method in this paper is simple and the experiment operation is low, the effect is better for paste and mortar test.