Few things motivate change in our industry as much as a fatal failure. The Oklahoma City bombing led to new requirements for redundancy and continuity of reinforcing steel and now the Big Dig tunnel ceiling failure has led to new requirements for adhesive anchor installation.
Anchors aren’t difficult to install, but there are plenty of opportunities to make mistakes and a few things that you need to understand to do the job right. First, says Hilti’s John Silva, focus on the manufacturer’s product installation instructions (MPII). Anchors are tested by independent labs to come up with the rated capacity—that is the amount of load in tension and shear that the anchor can carry when installed according to the MPII. Each anchor, including each size of anchor, has very specific requirements for hole size and depth and a few other factors. Any deviation from this can reduce the anchor’s capacity significantly. “The MPII is specific for each anchor,” says Silva. “A change in the MPII constitutes a product change with a requirement to retest the anchor bolt.”
Types of anchors
Let’s start by understanding the various types of anchors. There are cast-in anchors and post-installed anchors. Within the post-installed category, there are mechanical anchors, adhesive anchors, and screw anchors. Drilling down even further, there are several types of mechanical anchors, including wedges, sleeves, drop-ins, and undercuts.
In general, on commercial projects, the engineer will have specified the size and type of anchor (including diameter and length) based on the capacity needed—ideally, that will include the manufacturer and specific model number. Contractors can have input at this stage based on constructability issues—many times one type of anchor can be substituted for another type that has the same capacity, if it’s more convenient for the contractor.
Cast-in anchors have the highest capacity but obviously need to be positioned precisely prior to placing the concrete. On one Indiana jobsite I visited a few years ago, Winco Construction, West Lafayette, had cast-in four anchor bolts for each of its 244 columns in a huge dairy barn. Using a 3D laser layout tool (from Trimble), they were able to get all of the bolts in the proper location.
But in many cases, post-installed anchors make more sense, since the locations may not be known so precisely prior to concrete placement. The designer then will select the anchor based on several factors, as required by the 2006 International Building Code. Is the building in a seismic zone? Could the concrete crack? How sensitive is the anchor to slight variations in installation? Only specific types of anchors can handle each situation.
The next decisions the designer needs to make concern embedment depth and edge distance. Obviously the anchor can’t be installed deeper than the concrete is thick and the anchor capacity is generally a function of the embedment depth. Edge distance is important too; an anchor installed near the edge of a concrete member will have decreased load capacity since it could blow out the side. The designer also needs to know whether the anchor is by itself or part of an anchor group—multiple anchors that will work together and that have equal embedment depth have slightly lower capacity.
All post-installed anchors require the drilling of a hole. Generally that is done with a rotary hammer. Note that a rotary hammer and a hammer drill are not the same tool. A hammer drill is a drill with a mechanism to vibrate the bit tip, which is useful for drilling holes in concrete block or soft concrete. In hard concrete, a rotary hammer is needed to drill a precise hole.
“Do not install anchors without understanding the manufacturer’s drilling instructions,” says Jeff Groom in a recent Hanley Wood University webinar. Visit www.hanleywooduniversity.com to watch the webinar, “Anchoring in Concrete: Tools and Techniques for Jobsite Safety.” In this program, he cautions that only a slightly larger hole can significantly reduce the capacity of a mechanical anchor. Groom also covers bore hole cleanliness, which is critical for all anchors. Blow the dust out of the hole with compressed air from an oil-less compressor and use the proper sized brush in the hole. Dust in the hole acts as a bond-breaker for adhesive anchors and a lubricant for mechanical anchors.
It’s important that an anchor installer understands the different requirements for a mechanical anchor and an adhesive anchor, says Bosch’s Joe Sainz in the webinar. “Mechanical anchors rely on friction to hold the anchor in the hole. Adhesive anchors require a perfectly clean hole and a hole of the proper size.” Drop-in anchors have the best holding capacity for the shortest hole depth, he says, which can be important in avoiding embedded items, such as reinforcing steel, pipes, and post-tensioning cable. But he cautions that drilling to the proper depth is critical, since the anchor uses the bottom of the hole for setting, which requires the proper setting tool to expand the anchor. A stop bit, or drop-in-anchor bit, helps achieve this correct depth (see Precision Bits for Drop-In Anchors).
Installation of adhesive anchors became an important issue following the Big Dig disaster, because the panels fell due to failure of the adhesive anchors. This prompted the National Transportation Safety Board to ask the American Concrete Institute (ACI) to take action to improve the situation. ACI teamed up with the Concrete Reinforcing Steel Institute to develop an installer certification program that has been written into the 2010 version of the ACI 318 Building Code. Specifically, 318 now requires adhesive anchors to be installed by certified installers.
The issue became even more tangled following an investigation at U.S. construction sites by three professors from the University of Stuttgart in Germany. This study revealed that some MPIIs for adhesive anchors were insufficiently detailed to ensure proper installation: The borehole diameter and depth was not always specified and the installers frequently used the wrong size or did not take care to achieve the proper depth. When adhesive anchors were later cut open, most installers had not gotten the hole completely filled with the adhesive. This study concluded, though, “that installers are generally very eager to do a good job, but ... either haven’t been properly trained, or have been given inadequate installation instructions.”1 See Things to Find in the MPII Before Installing an Anchor for the crucial information you should know before installing.
This makes ACI’s Adhesive Anchor Installer certification program even more important. This includes a training program that gives installers the opportunity to fill clear tubes with adhesive to practice getting the holes completely filled. Following a recent program in Chicago, Mike Morrison, who is running the program for ACI, says that “all thought the practice session was critical to them doing well on the practical examination. They were shocked at some of the samples passed around the room of tubes filled poorly or incorrectly. I think they thought this was easy and then realized that there was a little more to this than they had thought.”
Certification programs are being conducted across the country; contact ACI for a schedule.
1 Grosser, Fuchs, and Eligehausen, “A Field Study of Adhesive Anchor Installations,” Concrete International, January 2011.