Compressive strength of micropile-to-grout connections

Strengthening foundations with micropiles is progressively being used, due to the major advantages that this technique presents. Nevertheless, the influence of some relevant parameters in the overall behavior of the retrofitted foundations has not yet been studied. Generally, micropiles are installed in holes drilled through the existing RC footing, which are then filled with grout. The efficiency of the load transfer mechanism depends on the bond strength of both the micropile–grout and the concrete–grout interfaces.This paper describes an experimental study performed to specifically study the influence of the following parameters on the bond strength between micropile–grout interface: hole diameter; embedment length of the micropile; and level of confinement of the grout mass. Thirty micropile–grout specimens were submitted to monotonic push-off tests, until failure. Bond strength was found to increase with a decrease of the hole diameter and with an increase of the confinement level.

► The micropile/grout interface is here in described.
► After the peak load, there is a sudden decrease on the connection capacity, followed by a ductile response.
► The connection capacity is first controlled by chemical adhesion at the insert/grout interface.
► After debonding, the connection capacity is controlled by friction, being proportional to the radial confining.