Choosing the Best Anchor to Fasten to Concrete

The process of choosing the best anchor to fasten to concrete might appear simple but the complex variables involved may quickly create complications. It is necessary to review the variables in order to simplify the process and make successful choices. The concrete anchors being considered are all mechanical type concrete fasteners. This means that the anchors derive their holding values from friction. A hole is drilled into the concrete and the concrete anchor used must be designed to go into a hole of equal size and then expand so that it becomes larger than the hole. This creates friction and this process ensures that the anchor does not come out of the hole.

What is the base material?

What is the difference between cement and concrete?

The words "concrete" and "cement" are often erroneously used as interchangable terms. Cement is an ingredient of concrete. Concrete is a mixture of water, aggregrates (i.e. small stones), sand and portland cement. Portland cement is not a brand name but rather a generic term for the type of cement used in most concrete. The cement and water harden and bind the aggregrates into a solid mass over time through a process called hydration. The hydration hardens the concrete for years, which means that concrete gets stronger as it ages. People wrongly call sidewalks "cement" and point out "cement trucks" and "cement mixers". The truth is that only concrete sidewalks and mixers exist!

How old is the concrete?

Green concrete, which is less than 28 days old, should not be drilled or anchored into. The older the concrete, the harder it gets and therefore the more difficult to drill into. In some cases, concrete also becomes more abrasive as it ages. The Tapcon® screw may not work in extremely old concrete because the lead threads deteriorate and prevent the screw from being inserted the minimum of one inch. Most other concrete fasteners will successfully work in concrete regardless of age. However, the reason there are a variety of concrete fasteners is because a variety of concrete also exists. A specific concrete anchor or fastener may not work in all applications.

What is the compressive strength of concrete?

The compressive strength of concrete is measured in psi, or pounds per square inch. The measurement is calculated by pouring concrete in a cylinder and then measuring the force needed to break the concrete after it is hardened. For example, concrete that is designated as 3,000 psi concrete is concrete that is able to carry a compressive stress of 3,000 psi after 28 days. Standard concrete is less than 7,000 psi and high strength concrete is measured between 7,000 and 14,500 psi.

How thick is the concrete?

There is a minimum embedment with all concrete anchors which is determined by the diameter of the anchor being used. As a rule of thumb, the smaller the diameter- the shorter the minimum embedment will also be. It makes sense then that the larger the diameter- the larger the minimum embedment. It is important to avoid having the working end of the concrete anchor too close to the bottom of the concrete. This will create an unsupported edge which will reduce the holding values of the concrete anchors.

Is there steel rebar in concrete?

Concrete reinforced with rebar may cause problems. The rebar may get in the way when trying to place a concrete anchor deeply or it may cause problems when drilling a hole. There are special rebar cutting bits that can be used when rebar is located in areas where the anchor must be placed. While these special rebar bits work well, they are fairly expensive and add extra work when placing anchors. Generally, the deeper the embedment of a concrete anchor, the better the pullout/holding values will be. Increased holding values will also be attained when the working end of a concrete anchor is on the far side of rebar.

How heavy is the load to be fastened?

It is important to determine the diameter of the anchor needed from the weight of the object to be fastened to the concrete. The weakest link of a concrete anchor is the concrete in which it is placed. In most cases- it is the concrete that fails, not the anchor. The best holding values is achieved when the concrete is hard and the embedment into the concrete is deep. The larger the diameter- the higher holding values the anchor will have. For example, a 3/4" diameter anchor has a minimum embedment of 3-1/4" and a 1/4" anchor has a minimum embedment of 1". The difference in these minimum embedments results in a stronger holding value for the 3/4" concrete anchor in comparison to the holding value of the 1/4" anchor.

Is it overhead on a wall or fastened to the floor into concrete?

Where the object is placed is a major consideration due to the different load values. An object placed overhead is critical for a number of reasons. First, the load is considered to be a pullout or tension load, i.e. the force of the weight is straight down. Secondly, overhead installations are critical because failure may be catastrophic, i.e. the object may fall and hurt or kill someone. An object placed on a wall puts the concrete fastener into a shear loading situation (the load is at a 90° angle to the anchor). The shear values are based on the diameter of the anchor and are not affected by embedment depth. The larger the diameter, the better the shear values will be. The values do not change with embedment depth as long as the anchor is placed at the minimum embedment or deeper. Anchoring objects to a concrete floor involves keeping the object in place so it does not move. The diameter of the anchor is usually determined by the size of the hole in the fixture. Embedment depth should be to minimum embedment or deeper.

Does the fixture vibrate?

A vibratory load is a load that is in constant motion, i.e. a fan, conveyor belt or a sign in the wind. Mechanical type concrete anchors deliver their holding values from friction. Once the friction is broken, the anchor will gradually lose its holding values. Concrete anchors in a vibratory load application may loosen or break up the concrete in the hole where it is placed. The working end of the anchor can push against the walls of the hole in the concrete. This process then deteriorates the holding values of the anchor, thereby decreasing its holding values, making the anchor loose and possibly reducing the holding values to zero.

Shock loading application?

A shock load is a load where the load values change over time. An example of a shock load would be a dock bumper. Mechanical type anchors deliver their holding values from friction. Once the friction is broken, the anchor may gradually lose its holding values. Concrete anchors in a shock loading application loosen or break up the concrete in the hole in which it is placed. The working end of the anchor pushes up against the walls of the hole. This process deteriorates the holding values of the concrete anchor- decreasing holding values, making the anchor loosen and possibly reducing its holding values to zero.

Is it a permanent fixing to concrete?

As a general rule, concrete anchors are designed to go into a hole in concrete and not come out. Some anchors are designed for the fixture to be removed at some future date such as a pallet rack or fence post. Other applications are designed so that removal is not an option. These applications include an electrical junction box or small direction sign.

Will it need to be leveled and/or shimmed?

If the fixture being placed on the concrete needs to be leveled or shimmed, then it is important to make sure that the concrete anchor protrudes high enough and with adequate threads to allow for movement up and down the anchor. In this case, the concrete wedge anchor is the only anchor that provides all of the necessary features required for leveling and shimming of a fixture.

What size hole is in the fixture to be fastened?

Many times the hole in the fixture determines which diameter of anchor needs to be used. It is important to know that the concrete anchor diameter and diameter of hole sizes do not match up. A 1/2" concrete wedge anchor will not through a 1/2" hole in a fixture. The threaded part of the anchor will fit through the 1/2" hole but not the working end. The concrete anchor diameter is matched to the hole size that needs to be drilled into the concrete to allow the anchor to protrude through the fixture. Below are anchor diameters and the corresponding fixture hole size:

Anchor Dia. Fixture Hole Dia.
1/4" 5/16"
5/16" 3/8"
3/8" 7/16"
1/2" 9/16"
5/8" 11/16"
3/4" 7/8"
7/8" 1"
1" 1-1/8"
1-1/4" 1-3/8"

 

What length of concrete fastener should be used?

If it is a male or stud anchor, the anchor length is determined by adding the thickness of the material to be fastened- plus the minimum embedment for the diameter of the anchor- plus the thickness of the nut and washer (rule of thumb is the diameter of the anchors). If a female anchor is being used, a separate bolt or screw will need to be inserted into the anchor. The bolt or screw length is determined by adding the thickness of the material to be fastened- plus any washers- plus the amount of threads in the anchor and the embedment of the anchor into the concrete.

Does it require a finished look?

Some concrete anchors are designed so that a steel rod sticks up through a nut. The length of the stud above the nut will be different heights depending on how the nut was placed on the anchor before being hammered into the concrete. The length will also depend on how many times the nut is turneed to set the anchor in the base material. This uneven look may not be what is needed for an application, therefore an anchor with a finished head is required. These types of concrete anchors would be the Tapcon®, flat or round head sleeve anchors, or any female type anchors where a specific bolt head can be chosen to be inserted into the anchor. These female type anchors would include the: drop in anchor, machine screw anchor, double expansion & single expansion anchors.

All of these considerations will play an important part in which type of concrete anchor will work best in your specific application.





 

Please remember with all fastening jobs to keep safety in mind. Always follow safety instructions on all tools, and refer to manufacturer's installation instructions when available and always remember to wear safety goggles!

Article written by: Bob Carlisle, President Concrete Fastening Systems, Inc

Back to Section