Bond of cement grouted reinforcing bars under constant radial pressure

A ribbed rebar or rock bolt, grouted with Portland cement is the most common type of reinforcement in geomechanical projects such as tunnels, rock slopes and foundations. Due to the frictional nature of the bond slip the normal stress acting on the rebar is the most important parameter controlling the bond capacity of the reinforcement. The more the confining stress, the higher would be the mobilized load bearing capacity of the system. To quantify this effect, series of laboratory tests were designed to study the effect of confining pressure on the bond capacity. A modified triaxial Hoek cell (usually used for testing the strength of rock samples under radial confining pressure) was used to facilitate application of a “constant radial confining pressure” to the grouted sample while pulling the bolt axially through the cement annulus. During test, axial load and displacement of the bolt as well as the radial dilation of the grout was recorded and stored in computer using a data acquisition system. The results show a non-linear relation between the increase of bond capacity and confining pressure. The radial dilation is quantified also as a function of confining pressure. A peak-residual behaviour is also a characteristic of these results which shows the importance of limiting the deformation of rock blocks to avoid entering into the post peak range of the reinforcement with low values of bond.