The terms "concrete anchor" and "concrete fastener" are generally used interchangeably although by definition, they are not the same. A concrete anchor will work better in some applications while at other times concrete fasteners should be used to complete the job. Before starting any project, be certain to choose the one that will finish the job in the most effective and safe manner possible.

The term concrete anchor refers to a heavy duty type of anchor that is usually of a larger diameter. These diameters include 5/8", 3/4", 7/8", 1" and 1-1/4". A concrete fastener is used in lighter duty applications and are available in diameters including 3/16", 1/4", 3/8" and 1/2". Whether you call them a concrete anchor or concrete fastener is not important. The distinction is made here simply to explain the difference between the two. Throughout this article, the terms will be used interchangeably.

The term concrete anchor can also refer to an anchor that is to be used in a base material of concrete, brick or block. Because many of these anchors can be used in brick and block, they are sometimes described as masonry anchors. There are three types of concrete fasteners that can only be used in solid concrete. These are the wedge anchor, drop-in anchor and the strike anchor. The wedge and strike anchors are both male, or stud, type anchors and the drop-in anchor is a female type anchor. These concrete anchors will be the focus of this article.

The Concrete Wedge Anchor

Types of Wedge Anchors

Concrete wedge anchors are made from zinc-plated carbon steel, hot-dipped galvanized carbon steel, 303 stainless steel or 316 stainless steel. The choice of plating or type of steel is dependent upon the corrosive resistance required for each specific application. Zinc plated carbon steel offers some rust resistance, hot-dipped galvanized anchors are excellent rust resistors while the 303 and 316 stainless steel fasteners provide superior rust resistance.

Wedge Anchor Uses

Zinc plated wedge anchors are for indoor use or applications where the anchor will not be subjected to the elements. The hot-dipped galvanized anchor is primarily used outdoors in mild climates where the anchor will be subjected to water. Stainless steel wedge anchors are used in areas where the anchor will be submerged in water or exposed to other caustic materials.

Determining the Right Length of Wedge Anchors

The wedge anchor comes in 9 different diameters ranging from 1/4" to 1-1/4". The length of the anchor required is determined by adding the thickness of the material being fastened, the minimum embedment for the diameter of anchor being used and the height of the nut and washer. The height of the nut and washer is roughly equal to the diameter of the wedge anchor itself. For example, the nut and washer for a 5/8" anchor is approximately 5/8" high. Below is a chart indicating the minimum embedment for each diameter of wedge anchor.

So with this information, to determine the length of wedge anchor required for a 5/8" diameter wedge anchor fastening a 2" thick fixture, one would use the following formula:

2" (thickness of material) + 2-3/4" (minimum embedment) + 5/8" (space for nut & washer) = 5-3/8" minimum anchor length

Based on the lengths available, one would use a 5/8" x 6" wedge anchor.

Drilling a Hole for a Concrete Wedge Anchor

The diameter of the hole to be drilled in the concrete is the same diameter as the wedge anchor being used. The hole should always be drilled using a hammer drill and carbide-tipped drill bit. Always be careful when handling any power tools and wear appropriate safety gear such as goggles. Before inserting the anchor into the hole, clear the hole of all debris. Carefully hammer the wedge anchor into the hole to the desired depth. To protect the threads of the wedge anchor, place the washer and thread the nut a couple of turns before hammering in the wedge anchor. Once in place, the anchor is set into the concrete by tightening the nut. This pulls the anchor up, wedging the clip between the steel of the wedge anchor and the concrete. It is recommended that the nut be tightened utilizing a torque wrench to ensure that it is tightened to the required torque value.

The Concrete Strike Anchor

Strike Anchor Types

The strike anchor is the only other male anchor that is designed for use in solid concrete only. Strike Anchors are manufactured from carbon steel with yellow zinc plating. Unlike concrete wedge anchors, the strike anchor is not available in hot-dipped galvanized or stainless steel.

Sizing of Strike Anchors

Strike anchors are available in six different diameters ranging from 1/4" to 3/4". The length of the anchor required is determined by adding the thickness of the material being fastened, the minimum embedment for the diameter of anchor being used and the height of the nut and washer. The height of the nut and washer is about equal to the diameter of the nut itself. For example, the nut and washer for a 5/8" diameter nut is approximately 5/8" high. Below is a chart outlining the minimum embedment for each strike anchor diameter:

Example of how to correctly determine the length of the strike anchor needed for attaching a 1-1/2" thick fixture using a 1/2" diameter strike anchor:

1-1/2" (thickness of material) + 2" (minimum embedment) + 1/2" (space for nut/washer)= 4" minimum anchor length

Based on the lengths available, the correct size for the above application would be a 1/2" x 4-3/4" strike anchor.

Drilling Holes for Concrete Strike Anchors

The appropriate diameter of the hole to be drilled in the concrete is the same diameter as the anchor being used. The hole should always be drilled using a hammer drill and a carbide-tipped drill bit. Before inserting the anchor into the hole, clear the hole of all debris. Then, with the nut, washer and set-pin in place, insert the strike anchor into the hole. The strike anchor is set by driving the pin down through the hollowed out center stud thus spreading the anchor at the opposite end of the threads. Using a properly sized hammer, set the pin with several sharp and square strikes on the head of the pin until it is flush with the top of the anchor. The anchor is properly set when the head of the pin meets the stud.

Should I use a Wedge or Strike Anchor?

Determining whether to use a concrete wedge anchor or strike anchor is simply a matter of personal preference. Both anchors have essentially the same holding values. However, the wedge anchor's pull out and shear values are a little better and more consistent than that of the strike anchor. Wedge anchors are also more popular merely due to the availability of a larger variety of diameters, lengths and materials. A strike anchor is preferred when the application requires all the anchors to protrude from the concrete in equal lengths, resulting in a clean and consistent finished look. This can be difficult to accomplish with the wedge anchor.

The Drop-In Anchor

Types of Concrete Drop-In Anchors

The drop in anchor is the only female type anchor for use in solid concrete only. Drop-in anchors are manufactured from carbon steel that is zinc plated and are also available in 304 or 316 stainless steel. Making the choice between carbon zinc and stainless steel depends entirely upon the end use and degree of rust resistance required. Zinc plated carbon steel offers some basic rust resistance, 304 stainless steel affords better rust resistance and 316 stainless steel provides superior rust resistance.

Drop-In Anchor Diameters

There are five diameters of drop-in anchors available- 1/4", 3/8", 1/2", 5/8" and 3/4". The diameter of the drop-in anchor refers to the inside diameter or the diameter of the bolt/threaded rod that is inserted into it. Each diameter is only available in one length and the minimum embedment for each anchor diameter is equal to the length of the anchor.

Hole Size for Drop-In Anchors

The diameter of the hole to be drilled in the concrete is equal to the outside diameter of the anchor. The hole should always be drilled using a hammer drill and carbide-tipped drill bit. Below is a chart indicating anchor size-internal thread length, anchor length/minimum embedmnet and required hole size.

The drop-in anchor is designed to sit flush with the concrete base material and requires that a bolt or threaded rod be inserted into it. Before inserting the anchor into the hole, clear the hole of all debris. To set the anchor, simply drop the anchor into the hole and tap lightly with a hammer until anchor is flush with base material. Insert the proper sized setting tool into the anchor and strike the tool until the lip of the anchor touches the lip of the setting tool. This will ensure that the anchor is set correctly.

Overview

These three anchors- the wedge, strike and drop in anchors- are the only mechanical type anchors that are designed for use in solid concrete only. These anchors cannot be used in brick or block based material. They provide the best, most consistent holding values among these types of fasteners. Determining which of these fasteners to use is dependent upon the application, the amount of rust resistance required, desired finished look and personal preference.