Photo: Carlos Jairo

How thick does concrete need to be to drilling anchors?

Concrete Thickness For example, if a 1/2” diameter anchor is being installed in concrete at a depth of 3”, then the slab of concrete must be at a minimum of 3-3/4” thick.

confast.com - What Fasteners to Use in Concrete.

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Purchase Anchors

Specifications

Has a particular type of fastener been specified by an engineer or the manufacturer of the item being attached? If this is the case, do not deviate from the specifications for installation.

Concrete

Concrete is measured in PSI with the standard concrete ranging in strength from about 2,000 psi on the low end and up to 6,000 psi on the high end. Generally, the harder the concrete then the better the holding strength will be for any fastener used. If the strength of the concrete is unknown, use the lowest value for any type of computation.

Concrete Thickness

The thickness of the concrete is important because the thicker concrete will provide more holding strength. The bottom end of the concrete fastener, once inserted in the hole and set, must be a minimum of 1.5 times the diameter of the anchor and placed away from the other edge or side of the concrete. For example, if a 1/2” diameter anchor is being installed in concrete at a depth of 3”, then the slab of concrete must be at a minimum of 3-3/4” thick.

Weight of Object

The heavier the item being fastened, the larger the diameter and the deeper the embedment of the anchor must be. Holding values in concrete are a factor of three considerations: the strength of the concrete, the diameter of the anchor being used and the depth that the anchor is embedded into the concrete.

Type of Load

Mechanical type fasteners should only be used in static loading type applications. A mechanical fastener is one that uses friction to derive at the holding values and should never be used in vibratory or shock loading situations. Epoxy type concrete fasteners can be used in dynamic, vibratory or static type loads. Epoxy type fasteners bind the steel anchor to the concrete along the whole length of the anchor.

Thickness of Material for Masonry Fastening

The thickness of the material being fastened is one factor in determining the overall length of the anchor. If the anchor is a male type, like the wedge anchor, then the minimum length of anchor required is equal to the thickness of the material plus the minimum embedment for the diameter of the anchor being used, plus space for the nut and washer. If the anchor is a female type, the bolt used should equal the thickness of the material being installed, plus room for the nut and washer and long enough to fully engage all the threads of the anchor.

Embedment

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Each diameter of the anchor has a specific minimum embedment depth that the anchor must be installed to reach minimum holding values. The larger the diameter of the anchor then the deeper the minimum embedment must be. Each manufacturer of concrete type fasteners will have tables available showing the various minimum embedment depths for each anchor. The requirements must be followed to ensure minimum holding values.

Hole Diameter

For male type concrete fasteners, the diameter of the hole that needs to be drilled in the concrete is equal to the diameter of the anchor being installed. For female type anchors, the diameter of the hole required in the concrete is equal to the outside diameter of the anchor being installed because the designated diameter of a female anchor is equal to the inside, or bolt diameter, used with the anchor.

Fixture Hole Size

If the anchor will be installed through the hole in the fixture with the fixture in place, the hole in the fixture must be slightly larger than the diameter of the anchor being used. For example, a 1/2” wedge anchor requires a 9/16” hole. If a female type anchor is being used, the diameter of the hole in the fixture can be the same as the diameter of the designated diameter of the anchor.

Type of Anchors

Some anchors are designed for use in solid concrete and cannot be used in brick or block. The wedge anchor and drop-in anchors should never be used in brick, block or any other type of masonry except for concrete. All anchors designed for use in brick or block can be used in concrete.

Personal Preference

Once all considerations have been evaluated, the choice of which specific anchor to use for any specific application will come down to personal preference. All anchors of the same diameter that are embedded into the same concrete at the same distance will hold about the same. If the holding values are of major consideration, move up to the next larger diameter and embed the anchor deeper. You can select from our tremendous array of fasteners and anchors by following this link.