Setting a cast-in-place anchor in just the right location can be a daunting task. It’s so difficult, in fact, that post-installed anchors are now becoming the preferred method for attaching wood, steel, and cold-formed steel components to concrete instead of cast-in-place anchors.
Although post-installed anchors are commonly used in concrete construction, choosing the right anchor for a given application is not as simple as it might appear. There are actually several factors that affect proper anchor choice, so considering them all is essential. In this article, we’ll walk you through the process of selecting an anchor so you can be better equipped on future projects where post-installed anchors are needed or even just favored in lieu of cast-in-place anchors.
When choosing a post-installed anchor, start by considering these six main factors:1. Environmental conditions
2. Building code requirements
3. Substrates (what is the anchor being installed into)
4. Anchor configurations
5. Anchor capacities
6. Jobsite obstacles that may hamper proper anchor installation
Environmental Conditions
The environment where the anchor is to be used will often dictate your anchor choice. First, the in-service exposure conditions determine the necessary corrosion resistance of the anchor. Interior exposure only (dry service) falls into the low corrosion-resistance classification shown in Table 1, while an application in an unprotected ocean-front location falls under the severe classification. Other exposures ranging from dry exterior to wet service to elevated service will be classified as medium to high to severe, respectively. The rating on Table 1 corresponds to the type of steel or steel coating required on the anchor. Temporary anchors (those in corrosive conditions for less than six months) fall into the low corrosion-resistance classification since they typically don’t have substantial time to corrode.
Generally speaking, the potential for corrosion is heavily influenced by the galvanic activity that is likely to occur. Galvanic corrosion occurs when two electrochemically dissimilar materials contact each other in the presence of an electrolyte (in this case, water) that acts as a conductive path for metal ions to move from anodic to cathodic metals. This is called a “galvanic couple,” with the more anodic metal corroding. Table 2 shows the galvanic potential of different metals. The farther apart the metals are on the table, the greater the difference in electrochemical potential and the more rapidly corrosion will occur. Corrosion also increases with increasing conductivity of the electrolyte, such as salt water. The significant corrosion often visible in marine environments, for example, is due to the highly corrosive reaction caused by the salt in sea water.
Good detailing practices can help reduce the galvanic corrosion potential of anchors.
- Use anchors and metals with similar electrochemical potential.
- Separate electrochemically dissimilar metals with insulating materials.
- Ensure that the anchor is the anode when dissimilar metals are present. This is because the smaller/thinner items will usually corrode first. The smaller/thinner item is often the plate or fixture being attached.
- Prevent exposure to and pooling of water (which acts as the electrolyte).
Another common potentially corrosive environment occurs when anchors are used to attach preservative-treated (often called “pressure treated”) wood sill plates to concrete. This scenario often requires anchors in the medium corrosion-resistance class. Depending on the level of preservative used on the wood, zinc or mechanically galvanized anchors are most appropriate. Some manufacturers have done extensive testing with these preservative chemicals on the anchors and may have specific recommendations.
In projects such as water treatment plants, factories, or even wineries, where adhesive anchors will be exposed to chemicals, the same factors need to be considered. Is the adhesive resistant to the chemicals to which it will be exposed? What is the concentration of these chemicals? Manufacturers can choose to run ASTM testing (or similar) in order to determine the chemical resistance of each adhesive. Keep in mind that the corrosion resistance of both the adhesive and the threaded rod insert used with the adhesive needs to be taken into consideration. A manufacturer’s testing in accordance with ASTM standards may classify the adhesive as:
- Resistant (R): adhesives considered suitable for continuous exposure to the identified chemical so that samples exposed show no visible damage and have the same strength as anchors not exposed
- Non-Resistant (NR): adhesives suitable for periodic exposure to the chemical if diluted or washed off after exposure; samples would have slight damage or reduced strength after exposure
- Failed (F): adhesive destroyed by the chemical or suffering significant loss of strength
Similarly, adhesive anchor installations in damp, wet, or submerged environments can be problematic. Even when rainwater is present only during an anchor installation or the curing process, the anchor capacity can be influenced negatively. In wet or water-filled holes, choose an adhesive anchor that is approved for the identified condition:
- Dry Concrete: Cured concrete whose moisture content is in equilibrium with surrounding non-precipitate atmospheric conditions
- Water-Saturated Concrete: Cured concrete that is covered with water for a substantial period to absorb the water and then blown clear, removing standing water
- Submerged Concrete: Cured concrete that is covered with water and water saturated
- Water-Filled Hole: Drilled hole in water-saturated concrete that is clean, yet contains standing water at the time of installation
As you might expect, the temperature of the concrete or substrate can adversely affect the anchor. Typically, this is a more important factor during installation and throughout the curing process. However, in-service conditions are important, too. Generally, acrylic adhesives are great for cold weather installations but will cure very quickly at higher temperatures. Acrylics can often be installed at temperatures below freezing. Quick cure times can sometimes be an advantage, although acrylics can sometimes cure too quickly at warmer temperatures, so the installer needs to know the expected gel time and cure time. The same material that takes 24 hours to cure at 14°F might take only 20 minutes to cure at 100°F. The gel time, or “working time,” for adhesives is substantially shorter than the cure time. Note that for most adhesives installed into water-saturated concrete, the cure time must be doubled.
Epoxy adhesives are best for temperate conditions or warmer weather, since the standard cure time allows for longer installation times. Typical epoxy cure time varies from 72 hours at 50°F to 24 hours at 100°F. Installers using adhesives to install deep reinforcing bars for rebar dowelling applications will prefer an epoxy product over an acrylic due to the longer gel time (working time). Again, for water-saturated concrete the cure time should be doubled.
So let’s look at an example of how to select an anchor for a deep rebar installation in a water-filled hole in concrete where the temperature will start at 80°F in the daytime and drop to 50°F overnight. Which adhesive should be used? First, recognize that epoxy is required for rebar installations. Then from the product description, look for the acceptable jobsite conditions for installation and find an epoxy that meets all these requirements. For example, using Simpson Strong-Tie epoxies, SET-3G epoxy specifically states that it can be installed in a water-filled hole and at concrete temperatures between 40°F and 100°F. SET-3G can be used to install rebar dowels in cracked or uncracked concrete, which brings us to the next major group of selection factors, building code requirements.
Building Code Requirements
The International Building Code (IBC) refers to the American Concrete Institute (specifically ACI 318-14) for Building Code Requirements for Structural Concrete. Any anchor that is to be used in a structural application (to transmit structural loads) must meet the requirements of ACI 318-14, Chapter 17, which details “Anchoring to Concrete.” The scope of the anchor detailing requirements includes connected structural elements and even “safety-related attachments,” such as sprinkler systems, suspended pipes, or barrier rails.
In regions where earthquake loads need to be considered, the anchors must be suitable for use in “cracked concrete,” since seismic design assumes the concrete will be cracked. Cracked concrete does not mean that the concrete is cracked already. In fact, the concrete is often only 21 days old and meets all specifications. Instead, cracked concrete anchors are literally tested in a section of concrete with a crack running through the plane of the anchor when tension tested. This is to simulate what would happen in a seismic event if the concrete experienced high loading and cracked in the anchor location.
In order to gain cracked concrete approvals, these anchors must be qualified through rigorous testing in accordance with ACI 355.2 (mechanical anchors) or ACI 355.4 (adhesive anchors). Any product with a valid evaluation report stating cracked concrete approval can be used for seismic, wind, life safety, or static use and meets the intent of the building code. In areas where the anchor design is governed by wind, however, cracked concrete does not necessarily need to be assumed in the design methodology. Depending on the design requirements, anchors qualified for cracked concrete or uncracked concrete could be specified. Many designers require anchor products to have a valid evaluation report regardless of their use. In addition, many require cracked concrete–approved anchors to be used even in low seismic areas.
Another building code requirement, this time specifically listed in the International Building Code (IBC), is that structural post-installed anchors in hardened concrete must have at least “periodic special inspections” during installation. Special inspection is to be conducted by a qualified special inspector, or other authorized personnel, who has been specifically trained and authorized by the AHJ (Authority Having Jurisdiction) to perform special inspections. Special Inspection is designated, usually on the Statement of Special Inspection, as “continuous” or “periodic.” How often is periodic? As often as required by the Statement of Special Inspection, which depends on the size of the job. Although not entirely clear in the IBC, periodic inspection is typically necessary at the beginning of the anchor installations on the project and at intervals during anchor installations on the same project. The time period between inspections is related to the critical nature of the installation or the number of anchors installed on the jobsite and is often determined by the Engineer of Record.
One final building code requirement to consider is whether an anchor installation will be overhead. Mechanical anchors are typically the best choice for overhead installations; adhesive anchors can be used with reduced capacity but are typically not recommended. If an adhesive anchor is used overhead, ACI 318-14 (R17.3.1.2) requires a 0.55 strength reduction factor for sustained tension loading and that the adhesive installation be performed by an ACI certified installer unless the Engineer of Record designates otherwise. Note that these provisions apply only to adhesive anchors, not to mechanical anchors.
Substrates
As we alluded to above, the material into which the anchor is being installed is called the substrate. This could be normal-weight concrete, lightweight concrete, concrete over metal deck, concrete masonry units (CMU, either grouted or hollow), unreinforced masonry (URM), or even clay brick. Look to the anchor manufacturer’s product selection guide to discover which anchors (adhesive, screw, or expansion) are most suitable for the different substrates. Sometimes a product that is best for concrete is not the same one that is best for CMU and vice-versa.
Anchor Configurations
Some other considerations in anchor selection are how close the anchors will be placed to the edge of the concrete, the spacing between anchors, and the thickness of the substrate. For anchors close to the edge, screw anchors or adhesive anchors are well-suited since they do not impose expansion forces on the concrete. Depending on the manufacturer and the diameter, even large-diameter screw anchors can be as close as 1¾ inches from the edge and 3 inches apart (center to center). Expansion anchors, since they work by exerting an expansive force on the concrete, need to be farther from the edge and farther apart from each other. As a general rule, these distances are a function of the anchor diameter — the larger the diameter, the farther away from the edge an expansion anchor needs to be placed.
Thickness of the concrete is another important factor that is sometimes overlooked. Anchors should be installed into holes that are shallower than the depth of the concrete. Not only does drilling through the substrate create a waterproofing or installation problem, it can also influence the capacity of the anchor. Mechanical anchors are often more appropriate for thinner concrete sections, though this is not always the case.
Anchor Capacities
The capacity of each anchor is, of course, different. For most applications, an Engineer of Record should be consulted to ensure that the chosen anchor, diameter, and embedment depth are appropriate for loading that will be resisted by the anchor. Although we won’t go into depth here, often screw anchors and expansion anchors can have similar shear and tension capacities. Where concrete is thick enough to embed an anchor deeper than 12 inches, an adhesive anchor will often have better capacity, and, more importantly, thread rods and rebar are available in longer lengths to support deeper embedment depths. Always check what lengths and diameters a manufacturer has available for a specific type of anchor.
Jobsite Obstacles
The final consideration in anchor selection is something that is difficult to predict, much less control: what happens on the jobsite. While cast-in anchors in concrete are an excellent choice, what are the chances they will be installed correctly and in the proper place? Unfortunately, there are challenges involved with the installation of post-installed anchors, such as:
- Who will be performing the work?
- Are they going to drill the holes properly?
- Is the installer going to pay attention to the proper installation temperature range of the adhesives?
- Are the trades (the concrete and framing subcontractors) coordinating beforehand?
- Is a special inspector going to be on site to verify proper installation?
- Will the correct nozzle for the adhesive be used?
- Will OSHA respirable silica dust requirements be observed?
- Are the installers aware of the appropriate brush type to use for hole cleaning with the adhesive selected?
Training is the greatest preventive step for meeting or mitigating most of these challenges. Special equipment, such as dust-extraction drill bits attached to properly rated vacuums with HEPA filters to mitigate hole-cleaning problems or diminish the possibility of substantial OSHA fines, can help too.
After reading this article, you may have recognized an anchor installation that you’ve designed (or installed) that wasn’t the best anchor for the particular condition. Unfortunately, these mistakes happen, but as G.I. Joe said, “knowing is half the battle,” and perhaps we can all make better decisions on the next project. Keep in mind that although each manufacturer has specific manufacturer’s installation instructions, there are certain factors influencing anchor choice that are consistent across the industry. As you go forward in your designs and installations, I recommend starting with the worst, most restrictive factor (like a highly corrosive condition) and then narrowing in on the best choice after that has been determined. When all else fails, keep in mind that every anchor manufacturer wants a successful anchor installation and is usually happy to address installation questions and concerns.
