Effects of Mixing Proportions and Sample Age on Mechanical Properties of Plastic Concrete; An Experimental Study

Plastic concrete, essentially composed of aggregates, cement, water and bentonite is used for creating an impermeable barrier (cut-off wall or cut-off trench) under the dams constructed on the high permeable foundation. Because of the triaxial condition of the stress on cut-off wall (vertical load from the dam body weight and confining pressure from the surrounding soil) its behavior in triaxial stress condition is very important. The available data on the triaxial behavior of plastic concrete is rare. Therefore the investigations mechanical behavior in this condition will be interesting and with the practical uses. For this purpose simple compression tests as well as triaxial compression tests were done on the samples of 10 centimeters diameter and 20 centimeters of heights with the confining pressures of 0, 300 and 500 kPa. The effect of mixing proportions and sample ages on mechanical characteristics are investigated. From the obtained results it can be concluded that an increase in cement content can increase the shear strength as well as the elastic modulus. Accepting a frictional criterion for the shear strength of plastic concrete an increase in cement content increases the cohesion coefficient but reduces the internal friction angle. Increasing the bentonite content reduces the shear strength as well as the shear parameters (C and f) and deformability and reducing the elastic modulus. The effect of confining pressure is also investigated. It is founded that the increasing in confining pressure increases the compressive strength as well as the deformability. Increasing the age of the sample, also increase the shear strength as well the elastic modulus and the cohesion coefficient but it reduces the internal friction angle. Finally, from the obtained results an empirical relation between the axial stress, confining pressure and the compressive strength is proposed.

Keywords:

Plastic concrete , Triaxial test , Bentonite , Mechanical characteristics