Does a patch applied to the vertical surface of a structural member (for example, a bridge pier) carry loads? I've heard that patches of this type serve no true structural purpose and are simply applied to protect the rebar.
We spoke to consultant Russell Fling and he cited a number of factors that influence whether the patch will carry loads: location and depth of the patch, shrinkage and modulus of elasticity of the patching material, and thermal and structural movement of the column. The patch will immediately carry the live loads applied to the structure for example, when a truck crosses the bridge. Unless the patch expands, however, the patch will not carry dead loads at first. When the structure spalls, the stresses in the structural member are redistributed to compensate for the loss of cross-sectional area. Patching the area will not immediately redistribute the dead loads. Whether the patch ever carries the dead loads of the structure (and how much load it carries) depends on the movement of the structure over time, shrinkage of the patch, and the depth of the patch. If the movement of the structural member due to creep and fatigue is greater than the amount of movement due to shrinkage of the patch, the patch will begin to carry loads. Higher modulus materials will carry more of the load than lower modulus materials. How much load the patch will carry depends greatly on how deep the patch is. If the patch does not extend into the rebar cage, it will probably never carry much of the dead load and its purpose will primarily be to provide coverage for the rebar.