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How Concrete Fasteners Work

Saturday, May 23, 2009
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Fastening to concrete is unique compared to other fastening applications such as fastening two pieces of metal together by using a screw or a bolt and nut. Concrete anchors are much more difficult to install and use. The concept of fastening something to a solid base material is completely different than for almost any other type of fastening application. Concrete is the most widely used base material in the world for the last 2000 years and probably will remain so for the next 2000 years due to its simplicity, strength, versatility and the abundance of the ingredients used to make it.

The process with which we fasten to concrete is simple:

  1. Make a hole in the concrete
  2. Put something in the hole to take up the space created
  3. Expand the material in the hole via a nail, screw or setting tool

We are making a hole with a certain amount of volume and then inserting more material into the hole. This increased volume of material pushes against the interior wall of the hole and creates friction. This friction is how most mechanical concrete anchors obtain their holding values.  All mechanical type concrete anchors work on the same basic principle- Drill a specific sized hole, insert the anchor and expand the anchor larger than the hole in order to make it difficult for the anchor to be pulled out of the hole. Concrete anchors are designed to be inserted into a hole in concrete and not come out.

Wedge Anchors

Wedge anchors are two-piece concrete anchors that are assembled into one unit. The steel rod (made from carbon steel or stainless steel) is threaded on one end and the opposite end starts out slightly smaller in diameter and tapers out to the full diameter of the rod. A clip is then permanently attached to this end of the rod. The wedge anchor is inserted into a hole in concrete until the threads are below the surface of the concrete. The nut and washer are placed on the threads and tightened until finger tight. Using a wrench, the nut is then turned, which pulls the anchor up to wedge the clip between the stud and the wall of the concrete. When drilling a hole in concrete for a wedge anchor, the hole size is equal to the anchor diameter size.

Sleeve Anchors

The sleeve anchor is made of four different parts. The stud (which is threaded and flared or cone-shaped at one end), the sleeve and the nut and washer. The expander sleeve is assembled over the stud with the nut and washer threaded on the opposite side of the cone-shaped end. The sleeve anchor is inserted into a hole drilled in the base material (either concrete, brick or block). The nut is turned, which pulls the stud up through the expander sleeve, expanding it up against the inside wall of the base material. The hole size to be drilled into the concrete for a sleeve anchor is equal to the diameter of the anchor being used.

Concrete Screws

Concrete screws are different than all the rest of the anchors because they do not use expansion to derive their holding values. Concrete screws are a special threaded screw, with hardened notched threads and high-low threads. The notches and the high-low threads help to eliminate concrete shavings from the hole as the screw taps threads into the base material. The hole size for concrete screws is smaller than the diameter of the screw. A 3/16” screw requires a 5/32” hole and a 1/4” screw requires a 3/16” hole. The concrete screw is inserted into the hole and turned either by hand or by a rotation drill until the concrete screw is tight against the fixture being fastened.

Drop-In Anchor

Drop-in anchors are a female anchor designed to be placed in concrete and then to have a threaded rod or bolt inserted. The drop-in anchor is made up of two parts: the expansion shield (made from zinc plated carbon or stainless steel) and a case hardened expander plug that is cone-shaped and made from zinc plated carbon or stainless steel. One end of the shield is tapered, with four cut slots that run a portion of its length. The surface of this end may be smooth or knurled while the other end will be smooth. The expander plug is placed at the end of the anchor that has the four slots, and the other end is threaded. The anchor is set by placing the anchor into a hole in concrete and by setting the expander plug using a setting tool. Each diameter of the drop-in anchor has a specific setting tool. This setting tool is a steel rod with one end being necked down. The necked down portion of the setting tool is inserted into the drop-in anchor and pounded with a hammer until the lip of the anchor meets the lip of the setting tool. This action pushes the expander plug down into the drop-in anchor expanding the anchor where the four cuts are. As with all female type anchors, the size of the designated size of the anchor refers to the bolt size that goes into the anchor; the hole size is larger than the anchor size.

Machine Screw Anchor

Machine screw anchors are a female type anchor into which a threaded item is placed. Machine screw anchors are made up of two parts, the internally threaded cone, and the sleeve. The sleeve is placed over the threaded cone and inserted into a hole drilled into the base material of concrete, brick or block, threaded cone first. The machine screw anchor is set by the sleeve being pushed over the expander sleeve wedging the sleeve between the expander sleeve and the inside wall of the concrete. The anchor is properly set when the lip of the setting tool meets the lip of the anchor. Each diameter machine screw anchor has a specific setting tool that is designated by the diameter of the anchor being used. The machine screw anchor size is designated by the inside diameter of the bolt to be used with the anchor, the hole size required is larger than the anchor size being used.

Strike Anchor

Strike anchors are for use in solid concrete and are considered an impact expansion type of anchor. The strike anchor is made up of four parts: the body that is made from carbon steel with an interior hole the entire length of the anchor, a drive pin that is hardened, and a nut and washer plated in a yellow zinc. The body of the anchor is threaded on one end with the other end having four slots cut a portion of the length - the surface of this part of the body has ribs around the circumference. The length of the drive pin that is hardened must equal the length of the anchor and is placed inside the interior hole of the anchor body. The anchor is set by placing anchor into a predrilled hole in concrete with the nut and washer attached. The anchor must be tapped lightly until the nut and washer are tight against the base material or fixture being fastened down. The hardened pin is then driven into the anchor until the head of the pin meets the end of the anchor body, which will provide for the proper setting. As the pin is pushed into the anchor, the anchor is expanded. The hole that is needed to be drilled for the strike anchor is the same diameter as the diameter of the anchor being used.

Hammer Drive Anchor

Hammer drive anchors are made from a Zamac material that is strong and malleable. Hammer drive anchors are a light duty concrete anchor, made up of two parts the body and the zinc plated steel pin. The body of the hammer drive anchor is split from the bottom up for most of its length, with a mushroom head. The anchor body is hollowed out that runs through the head thickness and down into the shank for the entire length. The steel pin is what expands the anchor; it is made of high carbon steel, with a small head on one end and with the other end pointed. The anchor is set by hammering the steel pin into the anchor body. As the nail pushes through the anchor body, the split part expands to push against the interior wall of the hole in the base material. The hole diameter to be drilled for the hammer drive anchor is equal to the diameter of the anchor being used.

Split Drive Anchor

Split drive anchors are made from carbon steel that is heat-treated through, hardened and zinc plated. Split drive anchors are one piece with either a flat countersunk head or a round head. At the base of the anchor, opposite sides of the head the shank is sheared into two pre-expanded parts. These two halves are compressed when the anchor is driven into a predrilled hole in concrete. These two halves continually try to get back to their original shape, pushing against the inside wall of the hole. The split drive anchor requires a hole size that is equal to the diameter of the anchor being used.

Lag Shield

Lag shields are made up of two parts that are assembled into one piece. The lag shield is made from a Zamac material, which is a zinc alloy that is rust-resistant. The inside of the lag shield has internal threads designed to accept lag screw threads and its threads are tapered and run the length of the anchor. The outside body of the anchor has ribs that run the majority of the length of the anchor. The lag shield is set by inserting a lag screw into the anchor. Turning the lag screw into the tapered threads expands the two halves of the anchor and pushes against the base material. Lag shield anchors are designated by the diameter of the bolt that goes into the anchor. It is important that the hole size to be drilled is larger than the designated anchor size.

For many years, creating a nice consistent diameter hole in concrete was painstakingly difficult. This was until the invention of the electric hammer drill. Before the electric hammer, drill holes were made by using what was called a star drill. The star drill was a steel bar with one end shaped like a large X (the drilling end) the other end of a stud (the striking end). The drilling end was placed onto the concrete, the striking end was hit with a heavy hammer, and after each blow, with the hammer, the operator would turn the stud slightly, over time a hole could be drilled into the concrete. Before the star drill, other methods were used to create holes in concrete, using chisels and punches.

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Always use personal protective equipmentAs with any anchoring project, it is important to keep safety in mind and follow instructions carefully. Always remember to wear safety goggles, handle all tools with extra care and follow all technical specifications. This article is meant to serve only as a basic explanation of concrete fasteners. Always refer to manufacturer's instructions or consult a contracting expert during any anchoring project.

Article Written By:
Mike Pistorino, Vice-President Operations





Concrete Fastening Systems, Inc. has over 40 years of experience selling concrete fasteners. We can ship out one box or a whole pallet of concrete anchors. Our products are of the highest quality... "your satisfaction is guaranteed". We ship all orders the same day the order is received.

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