There are many ways to attach structural members and fixtures to concrete, and the choice of anchoring system depends on a variety of factors. Sometimes, anchors are positioned, and then embedded as the concrete is poured. More often, though, anchors are installed in hardened concrete and provide either a mechanical or adhesive means of attachment. This article will touch on both methods, but with an emphasis on installing mechanical anchors.
Generally, and certainly in applications where an anchor carries a structural load, the architect or engineer of record will specify the type, size, spacing, and often the brand name of the anchor. Concrete anchors are subject to building code approval, including any seismic requirements, which the designer will have taken into account. It’s possible for a contractor to submit an alternative anchor for consideration, but you can’t substitute a different product without approval.
In nonstructural applications, such as hanging a light fixture, HVAC duct, or handrail, the anchors may or may not be specified. The construction documents may call for a particular anchor or instruct the contractor to follow the fixture manufacturer’s recommendation. In the absence of specific requirements, a contractor can choose what type of anchor to use, as long as it has sufficient capacity for the load.
Adhesive anchors derive their resistance to tension loads through adhesion or bond. A two-part adhesive is pumped into a hole drilled into concrete, then a threaded rod or other anchor element is inserted in the hole. As it cures, the adhesive forms a chemical bond between the concrete and the anchor. Depending on the particular system and the jobsite conditions, the curing process can range from a few to as many as 24 hours.
Adhesive anchors offer certain advantages: When properly installed, they provide excellent holding strength, and they can be spaced more closely together than mechanical anchors. They also can be used in wet conditions or even under water. On the other hand, they also have their drawbacks: The drilled holes must be cleaned out very carefully or bond strength can suffer, and the long cure times can cause inconvenient delays.
Mechanical anchors can be divided into two main categories: screw-type and wedge-type anchors. The screw-type is simpler; threads on the anchor itself engage with the concrete walls of a pilot hole to form a mechanical bond. Different anchor head configurations serve different functions. In some cases, the anchor head itself secures the member or fixture to the concrete.
In others, the anchor head is designed to accept a threaded rod that holds the attached item. Wedge-type anchors depend on a two-step process. The anchor is driven into a pilot hole, then torqued down, causing the anchor’s expansion sleeve at the bottom of the hole to spread and grip the concrete.