Friction Anchors

For light- and medium-duty applications, friction anchors tend to be the least expensive. Concrete screws. Concrete screws are typically available in 3/16- and 1/4-inch-diameters for light-duty applications or 3/8-inch-diameters for heavier-duty applications. Screw lengths range from 1-1/4 to 6 inches, but usually with only enough thread for embedments up to 2-1/2 inches. Most manufacturers recommend drilling a pilot hole 1/4 inch longer than the screw to allow room for displaced concrete or mortar to collect. Concrete screws have hardened-steel threads, which cut a thread in the sides of a predrilled hole. If you’ve used concrete screws, you’ve probably stripped out a fair number of holes. That's because the tolerance between the screw shank and the thread diameter is very tight, so the pilot hole must be of an exact diameter: Too big, and the threads won’t bite deep enough; too small, and the shank either pulverizes the sides of the hole or jams. Depending on how far you were able to drive a concrete screw before it jams, you'll either bend it or strip the head. To ensure a perfectly matched pilot hole, several concrete screw manufacturers package "tolerance-matched" bits with their concrete screws. You’ll pay more for Tapcons (ITW Buildex), Tappers (Powers-Rawl), or Scru-n-Taps (Star Fasteners) compared with generic brands sold in home centers, but we’ve noticed fewer stripped holes using tolerance-matched bits. All three manufacturers also offer drivers that slip over the drill bit, so you can drill the pilot and immediately drive the screw without changing bits.

Concrete screws.The tolerance between the screw shank and the pilot hole is critical when using concrete screws. Several manufacturers provide "tolerance matched" carbide drill bits for their screws, but even with these, it’s easy to strip the holes if the base material isn’t up to specs. In poor base materials, try drilling the smallest hole possible. Concrete screws work well for temporary installations, such as attaching formwork to existing foundations. But they are not the best choice for applications that must be removed, then reinstalled. In theory, a concrete screw can be unscrewed and reseated. But you lose considerable holding power, even if you’re lucky enough to reseat the threads. Mostly, you wind up stripping the concrete. Because of the narrow tolerance between the hole and the threads, concrete screws require a solid base material, such as a sound poured concrete wall or slab. In brick or block, manufacturers recommend installing the screw in the mortar joint for optimum holding power. This will work best in a new brick or block wall with consistent mortar joints, but on older structures that have been repointed, the joints won’t always be stable. If you're having trouble with poor base materials, it may help to drill a smaller pilot hole or use the next larger diameter screw. PAFs. Powder-actuated fasteners (PAFs) are one of the most commonly used concrete anchors in the trade, but not the strongest. They are certainly the fastest way to fasten to concrete, provided you drive the pin into a sound base material. In weak base materials, like soft concrete or old mortar, they are likely to pull out, especially if you’re trying to take a bow out of a board. As common as they are, and as careful as manufacturers have been about training and licensing, PAFs are often misused. Operators tend to use the fastening tool like a nail gun, yet concrete cannot sustain a close network of stresses. Keep pins at least 3 inches from each other and from the free edge of concrete. Each shot exerts a tremendous shock load, so unless you’re firing into good, dense concrete or mortar, the impact of a PAF often causes the base material to crumble or spall. To avoid overstressing the base material, manufacturers recommend using the lowest load possible. This means you have to work up from the lowest power load until you find the one that just seats the pin with the proper amount of embedment. This rule may be one of the most overlooked in the field; most operators tend to go with whatever loads they have on hand. Hammer-in. There are several distinct kinds of hammer-ins, although they are typically lumped into one group. Many light-duty (non-structural) hammer-ins look like rivets. After predrilling, you insert the plug and hammer home the pre-assembled pin, which spreads the sides of the plug. These fasteners are available in zinc, nylon, and Zamac (an alloy). Other light-duty hammer-ins, such as Star Fastener’s Strikr, look just like a PAF that you pound in with a small sledgehammer. In masonry and green or lightweight concrete, you don’t need a pilot hole if you use the manufacturer’s installation tool. This tool resembles a "pea shooter" — a pounding rod that slides inside a metal tube. The tube holds the pin straight, and it gives you more to hold onto as you work. In dense concrete, however, it’s hard work, and even with the driving tool you don’t always get a good seat for the pin. A slightly undersized pilot hole often works better. One of the strongest hammer-ins is the crook-style pin, typified by the Rawl Spike. This pin has a curved shaft which compresses as it is pounded into a pilot hole, exerting force at three points on the sides of the hole. Ramset/Redhead recently introduced two hammer-ins — the King-Con and Redi-Drive — that have holding values comparable to the Spike. The King-Con is made chiefly for attaching 1-by or 2-by furring strips or sleepers to concrete. It has a wide spiral flange that’s slightly larger than the pilot hole, while the shaft is slightly smaller. This makes the King-Con very quick to pound in, but requires a tolerance-matched bit.

	Hammer-ins.Two of the latest light- and medium-duty concrete anchors include Redhead’s King-Con (top) and Redi-Drive (bottom). King-Cons have been developed for attaching strapping and 2x-stock. Redi-Drives, which have the highest holding values among hammer-in fasteners, have been targeted to the electrical trades. Both require precise pilot holes, but can be installed with a 3-pound sledge.

The Redi-Drive has a straight shank that’s slightly larger than the hole, so it provides continuous holding along the entire length of the hole. As a result, Redi-Drives have one of the highest pullout values among hammer-in fasteners. But this rigorous holding power also makes Redi-Drives more difficult to pound in, requiring about twice as many hammer blows to seat as the King-Cons. Redi-Drives have been targeted for commercial electrical and hvac installations, and are sized to fit through predrilled holes in standard fixtures (the King-Con’s spiral is too wide for this application).