Concrete Q & A: Anchoring into Housekeeping Pads
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There are many cases where an anchor bolt (cast-in-place [CIP], expansion, undercut, or adhesive) is to be placed in concrete that has a cold joint. A common example occurs when a piece of equipment is installed on a thin (3 to 6 in. [76 to 152 mm]) housekeeping pad. The equipment needs to be anchored for seismic loads, and calculations typically indicate the anchor embedment must be greater than the thickness of the pad. The typical sequence of construction is to place the slab, place the equipment pad at some later date, and install the anchors in the hardened concrete. Whenever possible, I have required that the anchor embedment be measured from the top of the slab, not from the top of the pad. This should be conservative, but is not always possible. Questions: 1) What does the joint do to the capacity of the anchors if the embedment is measured from the top of the pad and the anchor crosses through the joint? 2) Can the embedment be measured from the top of the pad and still give the strength capacity for the anchor? 3) If the slab surface is roughened and is in a saturated surface dry condition when the pad is placed, will that be sufficient to make the pad act monolithically with the underlying slab and allow development of the full capacity of the anchor? 4) If a mechanical anchor is used, how close to the cold joint can the expansion elements be allowed? 5) Would an adhesive anchor have an advantage over a CIP or mechanical anchor because the failure cone is coming from the bond length rather than from a point at the bottom of the embedment?