Concrete Anchor Bolt Design: Cast-In-Place Anchors vs Post-Installed Anchors

Box of Concrete Wedge Anchors.jpg

While there are many types of anchor bolts (consisting of designs that are mostly proprietary to the manufacturing companies), there are two main types of concrete anchor bolts: post-installed anchors and cast-in-place anchors. Let’s take a look at how they differ.

Cast-In-Place Anchors Concrete Anchor Bolts

The cast-in-place anchors are non-proprietary and are the simplest type of anchor bolts. As the name implies, these anchors are cast in the wet concrete before it sets. Most designs consist of a standard bolt with a hexagonal head (hex head bolt), though there are other designs such as “hooked” J bolts and L bolts. Cast-in-pace anchors are very strong, and can be used in most anchor applications; but they are also difficult to cast. Cast-in-place anchors are recommended when the loads require large embedment lengths and high tensile strength.

Post-Installed Concrete Anchors Bolts

Once the concrete has been poured and set, you’ll have to use post-installed anchors. (Once again, the name says it all.) This type of an anchor is installed in a hole that is drilled in hardened and cured concrete. Post-installed anchors are proprietary products manufactured by several companies, and there are two main types: mechanical expansion anchors (MEA) and bonded (or adhesive) anchors.

But because there are so many types and manufacturers of these anchors,  a standard testing procedure is set forth in ACI 355. The tests determine if the anchors qualify as post installed anchors in ACI 318 Appendix D (Chapter 17).

If the anchor qualifies it can be designed with the provisions outlined in Appendix

D. (Keep in mind the ACI Code provisions are not mandatory unless adopted by the local building code.)

One big advantage to using post installed anchors over cast-in-place

anchors has to do with construction schedules. As with many fast-track projects, the concrete foundation is poured before the design is completed. When the exact anchor layout is not known at the time of the pour, it’s not possible to use cast-in-place anchors.

Here’s a brief overview of the two main types of post-installed concrete anchors:

Adhesive Anchors

Adhesive, or bonded anchor bolts come in many shapes and sizes and with different types of adhesives.These epoxy anchors are capable of reaching high bond-stress values in relatively fast cure times.

Mechanical Expansion Anchors (MEA)

When MEAs are inserted in pre-drilled holes, the anchors expand and bear against the concrete surface. They are inexpensive and easy to install, but they have relatively small tensile strength and are not recommended for use in tension zone where concrete is likely to crack. There are many types of MEAs, with wedge anchors being the most common. Other types include sleeve anchors, strike anchors, and undercut anchors.

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