Foundation and grout failures are quite commonplace in industry today. As our infrastructure ages, some say that these failures are inevitable. However poor base installation practices often reduce the life of our equipment. Here are some examples.
Failure to Remove Laitance
A common grout installation error is failure to remove the laitance from the foundation prior to installation and leveling of the sole plate. Concrete is a mixture of sand, cement and aggregate. The aggregate provides the strength of the material. When concrete is poured, the aggregate tends to sink, leaving a surface layer that consists mostly of mortar with little inherent strength. Laitance is the term used to describe that layer. Often, after pouring the base, the surface is “roughed up” a little to provide a good bonding surface for the grout. It is essential that that laitance is completely removed and aggregate is exposed before pouring the grout. Failure to do this will inevitably lead to premature failure of the base/grout system.
Poor Base Leveling Techniques
A common practice today is to use hard shims placed under or close to the sole plate anchor bolts to level the base prior to grouting. This practice inevitably reduces the life of the base/grout system. For grout to function reliably it needs to be placed in compression. When hard shims are used, upon torquing the anchor bolts, the shims are placed in compression and not the grout. This leads to premature failure of the grout.
A better technique is to use independent leveling screws to level the sole plate. These screws are protected from the grout with duct tape, foam sleeves or a release agent. The grout is then poured as usual. Once the grout has cured, the leveling bolts are removed (and their sole plate holes plugged) before torquing down the anchor bolts. This places the grout into compression. As illustrated in figure 2 Sleeves should be used on the anchor bolts to ensure they are tightened over the correct effective length.
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