Concrete Anchors – What they are and what you need to know

Concrete anchors are typically found in two distinct situations. The first occurs when the contractor/designer decides early on that post fixing the concrete anchors is the more appropriate method for the site conditions. The second, and more common, situation is in the case where alterations are required to an already cast concrete element.

Types of concrete anchors

There are two main types of concrete anchors available; chemical (aka resin) anchors and mechanical anchors.

Mechanical Anchors

Mechanical anchors are installed in post drilled holes. Here they will expand in isolated areas to apply a localized grip to the concrete. These are usually a cheaper alternative when dealing with lower forces.

The grip puts a localized pressure on the base material and it is for this reason that you should not use them in masonry. A concrete base material is better suited for mechanical anchors.

Chemical Anchors

Chemical Anchors are post drilled in the same manner, but will not expand. Instead the void will be filled with a chemical compound that will secure the bolt in place.

It is vital that the hole be thoroughly cleaned before installing the compound or the bond to the concrete will be weakened.

Chemical anchors are able to withstand much higher loading’s than mechanical anchors.

concrete anchors - chemical anchor

How will a concrete anchor fail?

In both cases there are 3 main ways in which an anchor will fail:

  1. Cone Failure: This is most common in lower strength concrete. When under tension (a pull-out force), the concrete will begin to shear along a line of approximately 45°. The shear line will extend out from the center in a cylindrical shape.
  2. Anchor/Bolt Shear: When under high loads parallel to the bolt, shear failure may occur. When reading off the shear resistance from a manufacturers loading tables you may notice the shear resistance is lower than that of a standard bolt in pure shear. This is due to the concrete crushing below the anchor before the bolt is able to shear.
  3. Resin & Mechanical failure: If the chemical anchor pulls directly out, it will almost never be due to the chemical compound. The main reason for failure is due to lack of bond between the compound and concrete. Cleaning the post drilled hole before installing the anchor cannot be stressed enough! For mechanical anchors it is a bit simpler, either the bolt mechanism has failed or the isolated forces have crushed the concrete.
  4. Anchor Failure: If the anchor has been installed in a high strength base, we can have the condition where the anchor itself will snap off. This is a function of the anchor’s yield strength that signifies the anchor was under designed rather than installed poorly.

Design considerations

From the failure methods above, we can see vital areas to consider when specifying a concrete anchor.

The depth of concrete base, embedment of the anchor and grade of concrete will all influence the cone failure. The most effective way to increase resistance against cone failure is to increase the embedment depth. Increasing the size of the bolt will help, but not as much as embedment in terms of cone failure. The position of the anchors will also have a significant effect on the strength. When placed too close together or too close to the edge of the concrete the anchor is not able to activate as much of the concrete.

Design is not strictly the correct term, specify is more suited to concrete anchors. Although you will need to consider all of the information, you will be reading off, or following calculations, provided by the manufacturer. If unsure, most of the bigger manufacturers are happy to advise on the right product for your needs. If you are a nerd like myself then you will enjoy calculating the failure methods by first principles to see how they line up with their loading tables. It’s interesting to see where their safety factors are hidden.

After all this you will still need to make sure connection can safely transfer the load to the anchor. If you’re looking for some help on steel plate design, we have an article on how to quickly size a steel plate’s thickness here.

Concrete anchor installation

The most important consideration for a chemical anchor is to ensure the hole is thoroughly cleaned before installing.

A mechanical anchor outshines its’ chemical counterpart when installing upside down, in a roof for example. When using a chemical anchor upside down, the compound will fall out onto the installers face. There are however specialist units available for these situations. These must be considered carefully, when in doubt, confirm with the manufacturer.

Drilling! We’ve talked about how to install the anchors once the hole is there, but what about when it comes to drilling these holes? In reinforced concrete you need to consider the fact that you will inevitably hit some of the existing reinforcement bars (rebar). The simplest method for reducing these effects is to specify your anchor spacings to fit within the spacing of the reinforcement. For example if the rebar is at 150mm centres, specifying your anchors at 150 or 300mm would mean you only need to one bar before adjusting the position to suit. However, anchors placed at 100mm centres would continuously hit bars along the entire length.

How to specify/order concrete anchors

When in doubt, give too much information. The information required is standard between all manufacturers around the globe and will only differ in the product names and numbers, they really do love their product numbers!

Below is a list of information that should be provided when specifying:

  • Manufacturer, type, product name and size.
  • Minimum base thickness.
  • Depth of embedment.
  • Required projection.
  • Any edge or spacing limitations.
  • Curing time.
  • Time required for base material to reach full strength (if applicable).
  • Reference to an installation guide or manufacturers details.
  • Any testing requirements.
  • Anchor type, i.e. stainless steel.
  • Any assumptions, always state your assumptions.

Many designers use the term ‘or equivalent’ when specifying chemical anchors, I personally prefer the term ‘or equivalent approved’. This means the contractor will confirm the details with the original designer before installing the anchor. This good practice will help avoid oversights on site.

Where to find high quality concrete anchors

Although not exhaustive by any means, these are the anchors I will always push for on any of my sites; Sika, Hilti and RawlPlug. One important rule in the anchor game, and life, is that you get what you pay for. Concrete anchors work hard, being cheap will cost you more down the line.

There is a comments section below for any queries, comments or experiences you may have had with concrete anchors.

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