Q: I’m installing 1/2-inch-diameter screw anchors in concrete footings. How long does the concrete need to cure before I can drill and drive the anchors? And are there concrete mixes that have a faster curing time?
A: Bill Palmer, an engineer and the editor-in-chief of Concrete Construction, a sister publication of JLC, responds: Screw anchors (such as those available from Simpson Strong-Tie or ITW Red Head) are a great addition to the more common concrete anchors on the market today and have the same pull-out strength as expansion anchors or adhesive anchors. Screw anchors start with a smooth hole, but these innovative products create a bond by cutting threads into the concrete with high-strength serrations.
Anchor manufacturers rate the pull-out strength of screw anchors in tension and shear based on specified concrete strength. Most concrete achieves its rated strength in 28 days, although in reality the concrete usually achieves its specified strength more quickly than that. In most parts of North America, 3,000 psi is the minimum recommended compressive strength for footings. Assuming you are using 3,000-psi concrete for the footings, you should be safe installing the anchors after 28 days in most circumstances.
But there are a few things that can greatly alter the “normal” rate of strength gain: concrete temperature and the use of fly ash or admixtures. Concrete gains its strength through a chemical reaction called hydration that depends heavily on the temperature of the concrete. Below 50°F, concrete typically gains strength much more slowly. Adding fly ash to the mix can also make concrete gain strength more slowly.
If you need your footings to harden more rapidly, accelerating admixtures can make that happen. The most common of these is calcium chloride—although it may not be a good choice because it can also cause the anchor bolts to corrode. Non-chloride accelerators tend to be more expensive (and less effective) and should be matched to the specific cement being used. Rely on your concrete producer for this.
If you want to install the anchors before 28 days have passed, and want to be sure the rated strength has been achieved, you can test the concrete footing with a Schmidt hammer. However, this expensive tool is not something that the average contractor is likely to be carrying in his tool
