Concrete Anchors; Types, Uses and Placement

If you are a beginner in the field of civil engineering, at some point of time, you would have wondered that how do we secure or connect any object or member with the surface of concrete and is it even possible to do so?

If this question has ever hit your mind, you are just at the right place to grab the required information. It is definitely possible to connect anything the with surface of concrete if it is to serve some purpose and concrete anchors are a possible and plausible option to be used as connectors.

Concrete anchors are mechanical fasteners that are used to connect any object or member with a concrete surface. It is important for the connection to be safe, reliable and strong so that it serves the purpose aptly and the connected object does not dismember from the concrete surface it is attached with.

To ensure the adequacy of the connection, normally, steel anchors are used to connect something with concrete. The reason steel is used is that it is strong with respect to its mechanical properties and the steel-concrete bond is a love affair we as civil engineers probably hear at birth in this field.

Types of Concrete Anchors and Their Uses

Broadly speaking, concrete anchors can be grouped into two categories; cast-in anchors and post-installed anchors. Cast-in anchors are laid out and placed prior to the pouring of concrete whereas the post-installed are ones that are laid and placed after the concrete has been poured.

Cast-in anchors generally involve a bolt or piece of rebar with some bearing-type element such as a nut, head, or hook that is embedded in the concrete to grab onto more material and provide better interlocking.  Examples include L-bolt anchor, J-bolt anchor, etc.

On the other hand, post-installed anchors can either be adhesive anchors (that are glued in to the concrete) or mechanical anchors (that are wedged, torqued, or screwed in).

Many types of concrete anchors are used for different connection works. All of these come in different sizes and have different load capacities. It is important for you to pick the right one for your project. Following are the different types of concrete anchors;

1. Wedge Anchors

Wedge anchors fall in the category of expansion anchors. The basic principle upon which these anchors work is that they expand and press against the sides of the hole in which they are inserted. This helps create a tight grip that holds the anchor in place.

These types of anchors are very popular in bigger projects because they usually come in bigger sizes and are capable of holding a great magnitude of load. They comprise a threaded rod or bolt and a wedge-shaped metal piece.

The anchor is inserted into the hole drilled in concrete and later on, the bolt is tightened as the wedge is pulled up against the metal clip. This creates a strong connection between the object and concrete.

Wedge anchors are generally nor removable; this means that you are stuck if you want to remove something attached to the concrete to reinstall it somewhere else. This also means that prior to using these anchors, you must be really sure about the location the object is to be attached and whether you expect it to get removed from the concrete surface after some time.

These types of anchors are commonly used in industrial applications and construction works to connect structural steel, machinery or any equipment to a concrete member. These anchors provide appreciable strength in bearing and can withstand high loads. However, for a specific application, their usage and installation must be assessed.

2. Sleeve Anchors

The use of sleeve anchors is very common and is seen in a lot of projects. This is because this is the only type of concrete anchor that is universal i.e., it can be used to attach something to any material such as brick, mortar, cinder block, concrete, etc. and they can also be used irrespective of the construction being old or new.

A sleeve anchor comprises a threaded metal body or rod, a cylindrical metal sleeve, a nut and washer. Just like wedge anchors, a pre-drilled hole is required in the concrete for this type of anchor to attach. The anchor is inserted into the hole and the bolt is tightened.

Sleeve anchors provide a strong and secure connection for light to medium sized objects that are to be connected with concrete. They are available in various lengths and diameters and can suit your connection needs aptly.

Just like wedge anchors, sleeve anchors are also not detachable. This means that you must not make a rash attempt of connecting something with concrete and regretting later on about the location being incorrect or the anchor falling short of design specifications.

3. Adhesive Anchors

As the name indicates, adhesive anchors use some adhesive material to connect something with concrete. The anchor is inserted into a drilled hole containing an adhesive such as some epoxy or polyester resin and the anchor is twisted and turned in the hole to make sure the adhesive gets evenly distributed.

4. Tapcon Screws

Tapcon screws feature a unique thread design that allows them to be used without pre-drilling a hole into the concrete surface. A hammer drill and a masonry bit and used to drill a hole into the concrete and the screw is then inserted into the hole and tightened to get a secured connection.

Screw anchors are removable and you can take them out if you need to move something from one place and want to install it at any other location. For this, you will simply need to unscrew the anchor and detach the fixture from the concrete surface.

5. Strike/ Impact/ Hammer Anchor

Strike anchors comprise a threaded bolt, a metal clip and a tapered cone. The anchor is inserted into a pre-drilled hole and the cone is struck or given an impact using a hammer. As a result, the metal clip expands and creates a tight grip.

These anchors are mostly used in lighter-scale projects such as mounting railings to concrete steps. However, if these anchors are used for massive or large concrete elements where it is expected for an anchor to have a greater capacity, these impact anchors may usually result in a lot of repair and replacement works because they are inherently not very strong and designed for this job.

How to Provide Concrete Anchors?

The provision of concrete anchors is made to connect something with concrete surfaces. However, several steps are involved to ensure a long-lasting and secured connection that is capable of withstanding what it was designed for. The following steps will serve as a general guideline for essentially all types of concrete anchors.

• The first step is to determine the loading requirement for which the anchors are to be designed. This is important to get an accurate picture of the type, size and length of the anchor to be used.
• Based on the design loads, the appropriate anchor type is selected. It may be a wedge anchor, sleeve anchor, adhesive anchor, etc.
• The next step is to prepare the concrete surface over which something is to be anchored. This involves cleaning the surface off of any dirt, dust or debris that might impact the bond or connection between concrete and the fixture.
• A hole is drilled into the concrete at a predetermined location using a hammer drill. The hole diameter and length must carefully be checked.
• The anchor is inserted into the hole and tightened to get a secured connection.
• Lastly, you may check the anchor by applying some load on the attached object to see if the anchor remains in place and can withstand it safely.

Factors to be Considered While Using Concrete Anchors

The choice regarding which concrete anchor should be used for your project will depend upon several factors including (but not limited to) the size and weight of the concrete member to which any object is to be attached or connected.

In addition, factors such as the required diameter of anchor and its embedment length are a direct function of the required load capacity and specific application of the anchor.

The concrete compressive strength also affects the choice of anchor to be used. The two materials that are connected must show compatibility with respect to their strengths. If the strength of concrete is high, an anchor having greater bearing capacity must be used.

In certain cases, the environmental factors such as exposure to moisture, chemicals, etc. limit the use of certain anchor types. So, these must also be considered when selecting an anchor type. Moreover, the local codes of practice can also be consulted to get a clear picture about what to go for and what not to choose.

Disadvantages of Using Concrete Anchors

The use of concrete anchors becomes inevitable in certain cases when you have got no option left to connect something with the surface of a concrete member. However, using concrete anchors also offers some disadvantages and we shall talk at length about these so that you can have your ambiguities cleared.

• Using concrete anchors is good but you cannot expect an anchor to bear too heavy a load. Therefore, there is an upper limit on the loading an anchor can safely support and you ought to know it before you use it.
• If the concrete is hard, you might face difficulties in drilling through it and anchoring the object with it, making this whole connection process challenging as well as arduous. However, if you get it perfectly secured, the outcome is worth working effortfully.
• Drilling into the concrete is not always a friendly idea as it can damage it or even lead to spalling on part of it. The surrounding concrete may also show signs of cracking due to vibrations induced by the drilling process. Therefore, if the concrete is weak or porous, when you will drill through it, there are chances of member splintering and disintegration.
• Not all anchors can be adjusted or detached to be connected somewhere else. This becomes a major concern in certain cases because if you have used an anchor type that is not detachable, the only option you have got left with you is to be break it open which will damage the concrete member.
• Almost all the anchors are made up of steel which means that these may deteriorate when exposed to moisture or other chemicals. Therefore, environmental factors ought to be given due and timely consideration.
• Concrete anchors may also disturb the aesthetics of a finished concrete surface and make it look visually unappealing.

Critical Failure Modes of Concrete Anchors

Concrete anchors can fail catastrophically and many failure mechanisms are possible. These include shear failure, tension failure, concrete failure, and steel failure. Let us discuss in detail how a particular anchor type is expected to fail when the load exceeds certain maximum value and what is the behavior of each anchor type at failure conditions.

Failure in concrete anchors can generally be categorized between steel failure and concrete failure. Steel failure may take place under tension loading or shear loading and the reason for such a failure is that the anchor bolt diameter is too small for the loads being applied. If steel channels are used, they may also fail by prying open which is a bending failure of flanges caused by localized high tensile forces.

As far as failure in concrete is concerned, under tension loads, the concrete is likely to experience a breakout failure or the anchor can pull out due to cracking at the interface of concrete and anchor. In addition, under shear loads, the concrete can pry out by exhibiting a brittle cracking failure. Failure in concrete generally occurs when the anchor embedment is not sufficient or is too low, the concrete has inadequate compressive strength, the member is subjected to dynamic loads such as vibrating effects, or the anchor type used simply does not justify the purpose and is not strong enough for the application.

The anchors used in concrete are often used as a group for stability purposes. However, this may lead to a grouped anchor failure. This means that when a number of anchors are aligned, their stress variation is likely to overlap which leads to the overlapped portion getting more stressed and the failure is expected to initiate there.

Now, the question arises that how can we avoid these potential failure modes in concrete anchors? This question is of particular importance to the design engineers but we will let you know the general design criteria of concrete anchors.

Design Criteria

The primary design criteria for concrete anchors are embedment length, diameter, edge distance, spacing and some type-specific parameters such as the wedge size of post-installed anchors or adhesion strength of epoxy, etc.

The above-mentioned design parameters are often interconnected and this creates a multi-variable engineering problem for engineers to put their minds into and solve even if it is for a simple base plate.

If you know the design loads for a particular concrete member, the product manufacturers have data catalogues for you to compare your loads to in order to see if the proposed anchors will work. In addition, there are certain manufacturers that also have a design software using which you can iterate many possible configurations to come to an optimized design. These softwares integrate engineering statics calculations, building code calculations and test data from manufacturers.

Frequently Asked Questions (FAQs)

What is meant by prying?

The term prying is used to indicate a phenomenon in which a force that acts perpendicular to the axis of anchor causes it to pull away from the joint, resulting in loss of clamping force and even failure of the connection.

The phenomenon of prying often causes concrete anchors to become overloaded. This can lead to failure in the concrete member because of cracks that sprawl across the member once the connection gets unclamped.

It is, therefore, important to design a connection to resist prying forces. The sources of these forces can be any external force such as wind load, earthquake forces, or any other force of vibration.

Prying can be prevented by using some additional reinforcement such as a washer or a large diameter bolt because doing so will increase the clamping force which will act as a resistance against the prying forces.

Which concrete anchor is the strongest among all?

Wedge anchors and sleeve anchors are the strongest among all the concrete anchors. They provide a reliable and secure connection that lasts a longer time and can bear appreciably high loads without yielding or giving in.