Finite element evaluation of the boundary conditions for biaxial testing of high strength concrete

This paper represents a numerical investigation, using a non linear finite element approach, to evaluate the different load application platens used in the biaxial testing of concrete. Three methods are evaluated numerically: the dry ordinary solid steel testing platens, the brush platens, and the friction reducing Teflon sheets. The effect of confinement on the displacement field in addition to the stress distribution in the loading direction are presented and discussed. The shear stresses induced in the specimen are evaluated for both uniaxial and biaxial loading conditions. Finally, the buckling capacity of the brush platens is examined. The finite element results indicate that the brush platens provide the most homogeneous displacement fields. The displacement fields are close to those obtained without lateral confinement. In addition, the shear stresses induced in the specimen are the lowest for the brush platens. The current study was used to design brush bearing platens for biaxial testing of high strength concrete. A brush platen with rod dimensions of 5×5 mm cross-section and 75 mm height can be safely used in testing high strength concrete with compressive strength up to 100 MPa with a reasonable factor of safety against buckling of the brush rods.