How Heat Treating Helps Concrete Anchoring Products Meet Tougher Load Demands

Joel Houck is a senior R&D engineer for Simpson Strong-Tie’s Infrastructure-Commercial-Industrial (ICI) group based out of the new West Chicago, IL location. He has spent the last 17 years with Simpson developing new mechanical anchors and adhesive anchor components, as well as developing a lot of the lab equipment required to test these products. This experience has given him extensive knowledge and insight into the concrete anchor industry, especially when it comes to the proper function and performance of anchors. Joel is a professionally licensed mechanical engineer in the state of Illinois.

There’s a saying in Chicago, “If you don’t like the weather, just wait fifteen minutes.” That’s especially true in the spring, when temperatures can easily vary by over 50° from one day to the next. As the temperature plunges into the blustery 30s one evening following a sunny high in the 80s, I throw my jacket on over my T-shirt, and I’m reminded that large swings in temperature tend to bring about changes in behavior as well. This isn’t true just with people, but with many materials as well, and it brings to mind a thermal process called heat treating. This is a process that is used on some concrete anchoring products in order to make them stronger and more durable. You may have heard of this process without fully understanding what it is or why it’s useful. In this post, I will try to scratch the surface of the topic with a very basic overview of how heat treating is used to improve the performance of concrete anchors.

According to the ASM Handbook: Heat Treating, heat treatment is a process of heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties.1 In practical terms, metals (usually steel in the case of most concrete anchors) are heat treated in order to improve their properties in some way over their base condition. When steel wire is formed into the complex shapes of anchors during the manufacturing process, the steel needs to be soft and formable; however, it is often beneficial to the performance of the final anchor product to be much harder and stronger than the base steel from which it’s formed. That’s where heat treating comes into play. By heating and cooling soft steel in a controlled manner, changes are made to the crystal structure of the steel in order to improve mechanical properties such as hardness, toughness, strength or wear resistance. Although the steel undergoes very complex microstructural changes during the heat treatment process, the end result is fairly straightforward – the once soft steel becomes harder and stronger as dictated by the heat treating process. As concrete anchors become more and more complex in order to meet the needs of building codes and designers, heat treating is becoming a more common and necessary component of high-strength anchors.

Figure 1. Steel microstructures: (a) soft steel example; (b) heat treated steel example.2

Depending on the desired results, there are many different types of heat treating processes that can be considered. The type of heat treatment and the parameters that are used can be customized for the steel type and the specific anchor application. There are several different types of heat treatments that are typically used for anchors. Two of the most common types are through hardening (also called neutral hardening) and surface hardening (also called case hardening).

Figure 2. Fasteners entering a heat treating furnace.3

Through hardening changes the mechanical properties (hardness, strength, ductility, etc.) of the steel without affecting its chemical composition. In order to alter the microstructure of the steel, it is heated in a furnace to a very high temperature, and then rapidly cooled, usually by submerging it in a liquid quench medium such as water or oil. This process will generally result in a very hard, but brittle material, so a secondary operation, called tempering, is employed after quenching. To temper steel, it is reheated to a lower temperature and then cooled in order to remove the stresses and brittleness created during the original quenching operation. Through hardening is useful where increased strength and toughness are required and surface wear isn’t a big concern, such as in our Crimp Drive® and split-drive anchors, setting tools for drop-in type anchors, high-strength all-thread-rod for adhesive anchors, and gas- or powder actuated fasteners. In order to effectively through harden an anchor, moderate levels of hardening elements must be present in the base steel, usually in the form of carbon. As the carbon content in the steel increases, so does the ability to harden it. The chemical composition of the steel along with the specific heat treating parameters will determine the level of hardness, strength and toughness of the final parts.

Surface hardening changes the hardness of the steel at the surface of the part by modifying the chemical composition of the steel at its surface only. This is done by altering the atmosphere in the heat treating furnace in order to get alloying elements, usually carbon, to diffuse into the surface of the steel. The increased carbon content increases the hardenability of the steel at the surface, but it can’t penetrate deeply into the steel, so a thin case forms around the surface of the steel with higher strength and hardness than the interior of the part. This creates parts that have high ductility throughout most of the interior, but that also have hard, wear-resistant surfaces. This type of heat treatment is useful in heavy-duty anchors where components of the anchors are sliding against each other during the setting process. It’s also useful in screw anchors, where the steel threads need to be very hard and wear resistant in order to cut into the concrete, but the ductility of the anchor must be maintained in order to avoid brittle failures in service. Just as with through hardening, there are many variations of surface hardening used in anchors, depending on the specific application.

Figure 3. Cross-section of surface hardened bar showing different hardness zones at the surface and in the interior.4

By using these two processes along with other heat treating processes, we are able to expand our ability to meet the higher demands placed on anchors in an industry that continues to evolve. As heat treating and steel chemistry continue to innovate, we will continue to use these developments to provide our customers with No-Equal concrete anchors that meet our high standard for performance and safety.

Mechanical Anchors

From complex infrastructure projects to do-it-yourself ventures, Simpson Strong-Tie offers a wide variety of anchoring products to meet virtually any need.


 

1 Lampman et al. (1997). ASM Handbook: Heat Treating. Materials Park, OH: ASM International.

2 “Microstructure of the AISI 4340 Steel.” Digital Image. Research Gate, n.d. Web. 14 June 2017 https://www.researchgate.net.

3 “Heat Treat Furnace.” Digital Image. ThomasNet Web Solutions, n.d. 14 June 2017 http://www.morganohare.com/heat-treating.html.

4 “Macrographs Showing Case Depth of Steels.” Digital Image. Science and Education Publishing Co. Ltd, n.d. 14 June 2017 http://pubs.sciepub.com.

Part II: Tensile Performance of Simpson Strong-Tie® SET-XP® Adhesive in Reinforced Brick – Test Results

Guest blogger Jason Oakley, field engineer

Guest blogger Jason Oakley, field engineer

This week’s blog post is written by Jason Oakley. Jason is a California registered professional engineer who graduated from UCSD in 1997 with a degree in Structural Engineering and earned his MBA from Cal State Fullerton in 2013. He is a field engineer for Simpson Strong-Tie who has specialized in anchor systems for more than 12 years. He also covers concrete repair and Fiber-Reinforced Polymer (FRP) systems. His territory includes Southern California, Hawaii and Guam.

This post is the second of a two-part series on the results of research on anchorage in reinforced brick. The research was done to shed light on what tensile values can be expected for adhesive anchors. In last week’s post, we covered the test set-up. This week, we’re taking a look at our results and findings.

To briefly recap the test set up, it was conducted in September 2014, at an office building in Burbank, Calif. Slated for demolition, this building provided an opportunity for Simpson Strong-Tie to install and test 1/2-inch diameter anchors using Simpson Strong-Tie® SET-XP® anchoring adhesive in both the face and end of the 8-1/2 inch wide reinforced brick wall. A 12-ton rated pull rig at the face and end of the wall was used to pull test the anchors to failure.

Table 1 shows the results for both face and end of wall anchors. Each data set was limited to testing three anchors of the same diameter and embedment depth. The coefficient of variation (COV) showed that the spread of the data was fairly narrow (11% maximum) for the face of wall anchors, but much higher for the end of wall anchors (24%). There are a couple of things worth noting here.

Table 1 - Tensile Results of 1/2" Diameter Threaded Rods in Reinforced Brick

Table 1 – Tensile Results of 1/2″ Diameter Threaded Rods in Reinforced Brick

Anchors 4, 5 and 6 showed that reinforced brick is capable of achieving significant capacity for anchors embedded past the grouted portion of the wall to a depth of six inches. The threaded rods were a mix of F1554 Gr. 36 (newer specification) and A307 Gr. C (older specification – likely the anchors that failed at 14,000 lbs.), which might explain the observed variation in capacity for anchors 4, 5 and 6. At what point breakout would have been achieved if higher tensile strength steel had been used is unknown but it can be estimated. What is clear is a significant reduction – probably around 60% (relative to an estimated breakout capacity of around 17,000 lbs. for an anchor embedded six inches deep far away from an edge) – can be expected for near-edge conditions, despite the presence of two #4 bars running along the edge of the wall at the window. A near-edge failure is shown in Figure 6.

Figure 6 – Anchor 13 near edge at window (anchor 14 and 15 similar)

Figure 6 – Anchor 13 near edge at window (anchor 14 and 15 similar)

At a reduced embedment depth of 4-1/2 inches, Table 1 showed that anchor location (anchors 7 through 12) had little effect on performance whether anchors were installed in the middle of the brick or in the head joint mortar. The failure modes were largely a combination of breakout and pullout as shown in Figure 7 and 8.

Figure 7 – Failures of anchors 7 through 12 (white arrows point to anchor center)

Figure 7 – Failures of anchors 7 through 12 (white arrows point to anchor center)

Figure 8 – Anchor 10 failure at face of wall (anchors 7, 8, 9, 11 and 12 similar)

Figure 8 – Anchor 10 failure at face of wall (anchors 7, 8, 9, 11 and 12 similar)

The end of wall anchor results shown in Table 1 revealed a significant reduction in adhesive tensile capacity and greater variation (COV) relative to face of wall results. Two possible contributing factors for such a high COV could be:

1) The bond strength between the grout and surrounding brick wythes is variable, and

2) The size and quantity of the voids present in the grout is probably inconsistent along the height of the wall – some areas are better than others – leading to further variation of the test results.

Figure 9 shows evidence of a slip plane failure for anchors 1, 2 and 3. Looking at the brick top and bottom surface, referred to as the bed, a scored surface can be seen running perpendicular to the length of the brick (and hence the wall surface) as shown in Figure 10. Perhaps the intent of scores is to help improve the bond strength between the brick and mortar. But this assumed benefit is limited to the bed line. The face and side of the brick are smooth. Consequently, the bond strength between the grout and brick is low enough, combined with lack of grout confinement between the two wythes, to have an appreciable effect on the anchor ultimate tensile capacity.

Figure 9 – Anchor 1, 2, and 3 failures at end of wall (1/2 inch x 6” emb.)

Figure 9 – Anchor 1, 2, and 3 failures at end of wall (1/2 inch x 6” emb.)

Figure 10 – Reinforced brick bed profile

Figure 10 – Reinforced brick bed profile

To summarize, this test program discovered that the tensile performance of 1/2-inch adhesive anchors in the face of the wall can be substantial for cases where anchors are located far enough away from a free edge. Performance is similar for anchors placed in the center of the brick or in the mortar joint, suggesting it doesn’t matter where the anchors are placed on the wall (obviously this isn’t true for anchors near a free edge). Special precautions should be taken especially for anchors located near an edge where small intermittent voids may exist in the grout. Anchor installation should ensure that sufficient quantity of adhesive has been injected into the hole. Figure 11 proves that this is possible. However, screen tubes should be considered if large voids are present, although large voids are expected to be rare in reinforced brick. End of wall anchorage applications should be designed carefully especially if significant tensile capacity is a design requirement.

Figure 11 – Anchor 2 end of wall voids filled with SET-XP® adhesive

Figure 11 – Anchor 2 end of wall voids filled with SET-XP® adhesive

Determining what the allowable load should be can be a little tricky. ICC-ES AC 58, the criteria used for adhesive anchors in masonry base material, lists several safety factors depending on whether creep and/or seismic tests have been performed. Conducting creep and seismic tests on an outdated building material like reinforced brick would be difficult because replicating 60- year-old construction accurately in the laboratory will probably be difficult. Reinforced brick has been largely replaced by grout-filled CMU as the preferred masonry building material — at least in Southern California. What safety factor should be used that would permit seismic loading of anchors in a relatively antiquated building material like reinforced brick is debatable.  Perhaps AC 60, the criteria used for assessing adhesive anchor performance in unreinforced masonry elements (URM), would serve as the best guide. It requires a minimum safety factor of five against failure and limits adhesive anchors to resisting earthquake loads only. But AC 60 also requires that the average ultimate load used not exceed an axial displacement of 1/8″ and limits the allowable load to no more than 1,200 lbs.

Despite the obvious structural dissimilarity between URM and reinforced brick and additional AC 60 requirements, Table 2 shows what the allowable loads would look like for the results of this test program if a safety factor of five was chosen. These loads are based on a wall of unknown material properties (compressive strength, tensile strength and bond, etc.) for a specific building, and may not apply to other reinforced brick buildings.

Table 2 – Allowable loads of 1/2-inch diameter threaded rods in reinforced brick using AC58

Table 2 – Allowable loads of 1/2-inch diameter threaded rods in reinforced brick using AC58

Many factors were not investigated in this test program, such as shear, creep, the simulated seismic test, just to name a few. While the evidence so far suggests that an adhesive anchor in reinforced brick performs similarly to grout-filled CMU, more testing would be necessary to substantiate this claim fully. What is very clear is the tensile tests performed on the 60-year-old Burbank office building showed that reinforced brick is a material capable of resisting appreciable anchorage forces. Of course, while a major effort is made by manufacturers to provide engineers with lab tested “code values” for design use, it can’t be ignored that the material properties of any structural element can be variable. Additional factors such as material deterioration, workmanship, etc., can all have an effect on anchorage capacity. This means that it’s never a bad idea to assess anchor performance through site-specific pull tests if gauging strength accurately is important to the anchor system design.

What have your experiences been with reinforced brick? Have you called for pull tests in this material? What were the results? Please feel free to share your experiences in the comments below.