Experimental Study on Mechanical Properties of Concrete at Super-Early Age.

Few studies have reported the cohesion and friction angle of concrete at a super early age. However, these two mechanical parameters are necessary to study the influence of engineering vibration on super-early-age concrete. In view of this, the mechanical properties of the super-early age-concrete are investigated in this work by direct shear testing. Firstly, the shear strength of the super-early-age concrete is measured by the direct shear experiment under different normal pressures at different times. Secondly, the cohesion and friction angle of the super early age concrete are calculated according to the Mohr-Coulomb criterion of failure. To overcome the great discreteness and randomness in the measured data, a new robust regression analysis algorithm is presented to replace the traditional regression analysis method to obtain more reliable and reasonable mechanical parameters. According to the experimental and theoretical analysis results, it is found that the friction angles of the super early age concrete are located in the interval of [50°, 70°]. The cohesion of the concrete is about 78.7 kPa at the initial setting state and about 190.9 kPa at the final setting state, respectively. It has been shown that the cohesion of the concrete at a super-early age tends to increase rapidly with time. The method and test results of this work can be used as a reference for relevant engineering practice. Specifically, the proposed regression method can be extended to the data analysis of other mechanical parameters of concrete, as well as other brittle materials such as rock. The test results of early concrete cohesion and friction angle can be used to analyze the adverse effects of vibration on newly cast concrete members in pile driving and blasting engineering.