Anchor Bolts Resource


Expansion Anchors Are Fastening Hardware

They are most often used to secure items to hard materials, like concrete, brick, stone, or block. They come in many sizes and styles, with the two main categories being pre-expanded and un-expanded. The strength of the base material and the weight of the connecting object will influence the choice of anchor.

Pre-expanded anchors, like the spike or split drive, are appropriate for small loads. Spike anchors are available in 1/4”, 1/2”, 3/16”, and 3/8” sizes and have multiple head styles. A curve built into the shaft of the anchor applies force against the base material once the pre-expanded anchor is driven into the drilled hole. This type of anchor does not require any other action to set its expansive force. It works for light loads on solid concrete, brick, block, or stone. 

Split drive anchors fit into 1/4” diameter holes. They have a flare near the end that exerts force once inserted into the hole. They are only appropriate for use in solid concrete. 

For medium to heavy loads, un-expanded anchors become necessary. These expansion anchors are designed to expand when a threaded bolt is inserted and tightened in a clockwise direction. Building contractors have several types of construction expansion anchors to choose from. Wedge anchors, sleeve anchors, strike anchors, drop-in anchors, hammer drive anchors, single expansion anchors, and double expansion anchors fall into the un-expanded category. 

Single expansion anchors are designed with a single expansion nut that pushes against the base material as the bolt is screwed into place. They are appropriate for medium loads being fastened to hard and dense material like concrete or stone. 

Double expansion anchors have two expansion nuts on both ends. The expansion nuts shift and press against the base material as the bolt threads into the anchor. Because of the distributed force of two nuts, double expansion anchors are needed when the base material might be porous or have inconsistent hardness. Manufacturers provide load ratings for fastener products to guide selection, but the durability of connection ultimately relies on the strength of the base material. 

Exposure to water or other corrosive materials influences fastener choice as well. Many expansion anchors are made from zinc-coated carbon steel. These perform best in dry locations. Wet locations will require expansion anchors that are made from stainless steel or hot dipped galvanized steel. 

Expansion anchors make it possible for builders and homeowners to hang shelving, pictures, or stair rails on concrete or brick walls. They also make it possible to build finished interior walls against concrete basement walls, attach ledger boards for floor or deck joists, and more. Building supply and hardware stores stock all types of concrete fasteners. They can be purchased online as well. Manufacturers also sell concrete anchor bolts directly to consumers. Jobs involving heavy loads might warrant direct communication with a fastener manufacturer to ensure selection of appropriate expansion anchors for the job. 

How Are Expansion Anchors Installed?

Holes must be drilled to accept the anchors. When working on hard materials, like concrete, a hammer drill fitted with a masonry bit is needed. This type of drill hammers the bit against the hard material as it also rotates the bit. After drilling the hole deep enough to hold the anchor, the hole must be cleaned of all debris. The expansion anchor is then tapped into the hole with a hammer. 

How Far Apart Do Expansion Anchors Need To Be Spaced?

Multiply the diameter of expansion anchors by 10 to calculate spacing. For example, 1/2” anchors must have a minimum spacing of 5” apart. When approaching an exposed edge of the base material, builders cannot place an anchor any closer than 5 diameter widths. Proper spacing is crucial because the pressure exerted by the anchors could cause cracking if they were too close together. 

How Deep Should Concrete Anchor Holes Be Drilled?

Best practices call for drilling holes 1/2” deeper than the length of the anchor. The extra space provides an area to collect any debris that fell too far inside to remove.

Is It Better To Anchor In Brick Or Mortar? 

Brick usually provides a harder and stronger base than mortar but the weight of the object to be attached and the condition of the brick matters as well. Old brick that appears crumbly could crack when expansion anchors are inserted. When the brick is fragile, drilling into mortar would be the better choice. 

Can Anchor Bolts Be Removed?

Un-expanded expansion anchors allow for the removal of the screw or bolt, but the anchor will remain permanently embedded in the material. The defunct installation can then be patched over and made flush to cover up any embedded component or holes.