Q.: We're required to remove, clean up and replace gunite that we recently applied to cover a concrete wall surface that was spalling. We had faithfully followed specifications and took special care with the bushhammering of the old surface to be sure that it was all thoroughly roughened. Much of the gunite came off within a month after it was applied. We have not had this trouble before and we wonder if you have any idea of what might be wrong.

A.: Bushhammering should not be done to prepare a surface for guniting. Although it does roughen the base, or substrate, bushhammering inhibits bond because it powders both the cement paste and rock and drives the fine particles into the surface pores. These particles prevent the gunite itself from entering the pores and bonding thoroughly to the old concrete, or substrate. This leaves a weakened plane between the new material and the old and encourages irregularly shaped slabs of the new material to come off later.

If bushhammering was required by the specifications, you should get the specifications changed before applying new gunite. The surface should be prepared by chipping off the gunite you applied and then sandblasting to remove the powder produced by bushhammering. The substrate should then be thoroughly moistened to prevent it from soaking up water from your new shotcrete.