Q.: A warehouse floor has suffered spalling at joints that contain a metal keyway. What caused the spalling?

A.: Without knowing more it is impossible to say, but spalling does occur frequently at keyed joints that are subjected to hard-wheel traffic. Although this can happen to any keyed joint it seems to occur more frequently in joints formed by metal keyways that are left in place (that is, in control joints, since keyways are removed from construction joints before concreting the adjoining slab). There can be any of several causes:

  • Keyways of any kind cause a small vertical gap between the male and female portions when the joint opens. The pounding in this gap from repeated impact of traffic can cause spalling.
  • The slab may be too thin. The small amount of concrete above the keyway in a 4- or 5-inch-thick slab offers less resistance to spalling than in a thicker slab. If keyways are to be used in such circumstances it might be well to thicken the edge to 6 inches and use a keyway made for a 6-inch-thick slab.
  • The trouble can be caused by surface subsidence during bleeding of a mix that is too wet. The finished surface of most of the slab is lower than the top of the metal keyway. Even though the finisher trowels the nearby surface up to the level of the top of the keyway the joint presents a high ridge to the impact of oncoming traffic.