Construction Concerns: Anchors in Concrete and Masonry

Article and photos by Gregory Havel

When we need to attach something to wood framing in a building, we use nails or screws. These are simple and easily understood. A nail and a screw may have similar shear strength, but a screw will hold more reliably under tension and vibration.

It is not so simple when we need to attach something to concrete, masonry, or even a plastered wall or ceiling. Holes must be drilled in these materials for the fasteners, and the materials are hard enough that the fasteners may not grip well. The solution developed hundreds of years ago was to fill the hole in the masonry or concrete with a plug of a material that would hold the fastener, and at the same time expand and grip the inside of the hole.

Photo 1. Click to enlarge

Wood was the earliest material used as anchors in masonry, to accept nails, pegs, or screws. Sometimes a hole would be drilled in the masonry after the mortar was set (or a hole could be left in a mortar joint) and a wood plug would be whittled to fit and pounded into the hole. Sometimes, pieces of wood shingle were laid between bricks in the mortar joints. Photo1 shows a piece of rough-sawn 2 × 4 laid up in the wall to replace an entire wythe of bricks and used to attach firring strips for wood lath and plaster; and a piece of 1 × 8 laid into the wall to support the window jamb. Some of these wood anchors have been in place for more than a hundred years and are still in good condition–if the wood was dry when it was installed, and if it has not been exposed to water or insect damage. If the wood anchor has rot or insect damage, it will loosen and fail quickly. If it is exposed to fire, it will burn. If a nailer that replaced a wythe of brick is exposed to fire, it will burn and create a weak plane in the wall–like the notch we cut in a tree to direct where it falls. Although wood anchors are no longer used in construction, many older buildings were assembled using them, and the condition of any individual wood plug or nailer is usually unknown until it fails.

Photo 1. Click to enlarge

Lead anchors (photo 2, A, B) were developed for use with bolts and screws. Lead is corrosion-resistant and will not rot. Since it is ductile and malleable, it expands easily to grip firmly the inside of the hole, and conforms tightly to the threads on the screw or bolt, but can deform and loosen over time because of stress or vibration. Since the melting point of lead is 621oF (327oC), a lead anchor will melt in a fire and release whatever it is holding. Although lead anchors of several types are still available, they are little used today, because of their tendency to loosen; their low melting point; and lead's toxicity. However, millions of them that were installed years ago and are still doing their job. They used to be available in most of the styles and sizes shown in photo 2.

ZAMAC anchors (photo 2 C-J) look like traditional lead anchors, but are lighter in weight and die-cast from an alloy of Zinc, with small amounts of Aluminum, Magnesium, And Copper (hence the name) added to change the properties of the metal. These anchors are available in dozens of styles and sizes, and are available from any hardware or industrial supply store.

Plastic anchors (photo 2 K) were developed in the 1950s for use with screws and nails, for holding light loads in concrete, masonry, plaster, and drywall board. These are corrosion-resistant and will not rot and are available in many sizes. They are also inexpensive and work well for the purposes for which they were designed. Since they are easy to use, they are often used to hold greater weights than their design allows. In a fire, they will soften, melt, or burn and release whatever they are holding. Millions of these anchors support electrical boxes, conduit straps, light fixtures, picture frames, towel bars, and nearly any kind of hardware that can be found in residential, commercial, or industrial buildings.

Photo 3. Click to enlarge

Steel anchors are available in many sizes and varieties for different purposes (photo 3). Most steel anchors are plated with cadmium or zinc (and formerly lead) for corrosion resistance. These steel anchors provide the greatest strength and are the least likely to loosen from stress or vibration. With a melting point of 1,800-2,600oF (982-1427oC), they are the least likely to release their loads in a fire. Some types of steel anchors are available in stainless steel for use in corrosive environments.

Epoxy and cement compounds are also used to set anchor bolts in concrete and masonry. A hole is drilled to the proper size and cleaned to remove all of the dust and debris. The bolt is inserted to the proper depth. A two-part epoxy compound is mixed and injected into the space between the hole and the anchor bolt from a double syringe or caulk gun. In another method, the bolt-setting cement (hydraulic cement) is mixed with water and poured into the hole around the anchor bolt. The epoxy compound or bolt cement bonds to both the steel and the concrete or masonry and expands as it sets.

Each type and style of anchor works best for specific applications. Some types are intended only for dead loads; some loosen from vibration; others are intended only for shear loads, not tension loads; and some should not be used to support overhead loads.

Most contractors and craftsmen are knowledgeable enough to use the proper type and size of anchor for its intended application. Unfortunately, many do-it-yourself people and store clerks are guided by more appearance and packaging than by practicality and engineering, and are unlikely to do much research before making a selection.

For more information on today's anchoring systems for use in concrete and masonry, Internet search for:

These anchoring systems do not include the anchor bolts used to attach structural steel or heavy machinery to concrete foundations. These are usually embedded in the concrete foundation when it is poured, and are a topic for another time.