Grout Pad

Due to the difficulty of constructing a smooth and level concrete surface on the connection foundation (further complicated by the presence of anchor rods), grout is typically installed under the base plate to ensure full bearing contact for the effective transfer of bearing loads. Although grout is installed for most base plate connections, it has received limited research attention. As a rule of thumb, grout is required to have a specified compressive strength of at least twice the specified strength of the concrete in the foundation (Fisher & Kloiber, 2006). The thickness of the grout pad generally depends on construction methods and the as-built elevation of the foundation surface, but is typically one to two inches thick (Fisher & Kloiber, 2006). Since the grout pad is usually unreinforced and unconfined, the grout pad thickness should be as short as possible. Observations of past earthquakes have indicated that relatively thick grout pads may lead to catastrophic damage. Common failure modes of the grout pad include:–

o   Grout Bearing Failure – As with the concrete foundation, the grout pad may fail due to compressive bearing stresses from the base plate. However, there is virtually no confinement of the grout and the grout pad is typically very thin. Thus, the mechanisms of grout bearing failure and associated strength capacities may be different than the general mechanisms of concrete bearing failure.

o   Local Grout Crushing – As with the concrete foundation, the grout may locally crush under the application of very large bearing stresses.

o   Grout Spalling – Reinforcement is generally not provided for the grout pad; thus, the grout is susceptible to spalling damage, especially under cyclic flexural loading of the base plate . Spalling damage may be reduced or prevented by installing wire mesh or fiber reinforcement.

Grout Damage in Shear – The gout pad, which is typically installed by simply placing workable grout over a cured concrete surface, is weakly bonded to the concrete foundation (due to the cold joint) and shear failure between the concrete and grout is possible, especially under the action of the anchor rod bearing mechanism for shear transfer.