The effect of very low bearing pressure on the behavior of cast-in anchors

The effect of bearing pressure on the behavior of cast-in anchors is addressed presenting the results of an experimental campaign on cast-in anchors with different head type and same embedment depth. In particular, three cast-in anchor solutions were tested with the anchor-head ranging from relatively small head (high bearing pressure) to very large head-size (low bearing pressure). For the small head-size only, the anchor is installed using high strength grouting mortar after the hardening of the base concrete material. The concrete base member was lightly reinforced. Anchors were tested under axial force and different mechanical response (load-displacement) are observed. Failure modes change depending on the anchor’s type. The force transfer mechanism might migrate from pure concrete cone formation to structural collapse of the concrete base member. In some cases, the cone surfaces can be clearly recognized despite of the presence of a splitting crack. In other tests a plate failure was obtained, characterized by the presence large triangular segments between cracks radially arranged. This aspect is strictly related to the bearing pressure at the anchor-head location. Indeed, a hydrostatic stress-strain field is developed with different gradients according to the head-size. Small head-size leads to an increasing of the bearing pressure with severe crushing of concrete and consequent reduction of the expected load capacity. Furthermore, the observed failure mechanism suggests that the crack pattern propagation is unaffected by the presence of grouting mortar.