Making it stick

by Tom Hammel

New AC 308 guidelines for cracked and uncracked concrete are driving the development of anchoring systems for post-installed anchors.

There’s a revolution going on in the anchor industry. As of January 1, 2008, ICC-ES AC58, short for “International Code Council-Evaluation Service Acceptance Criteria for Adhesive Anchors in Concrete and Masonry Elements,” was superceded by AC308.

Most anchor manufacturers are now forced into catch-up mode to reformulate, test and submit findings to the International Code Council Evaluation Service (ICC-ES), which produces certification reports that their products meet AC308 standards.

AC58 standards have evolved over time to require testing for adhesive anchoring systems under a variety of adverse conditions such as wind and seismic loads, long-term creep and static loading at higher temperatures, damp and water-filled holes, freeze-thaw conditions and critical/minimum edge distance and spacing. However, AC58 guidelines lacked something European standards have covered for years – provisions requiring mechanical and adhesive anchor designs to address cracks running through anchored areas in concrete.

Changing the rules
ICC-ES AC308 guidelines have raised the bar almost literally overnight for post-installed anchoring systems by declaring they must compensate for conditions covered by AC58 and also for cracks that may pass through anchors.

“AC308 is perhaps the biggest change ever in our industry because it forces every anchor manufacturer to invest millions of dollars to create new anchors that will perform in cracked concrete and other adverse conditions,” says Craig Pratt, product manager for Simpson Strong-Tie. The rush to earn AC308 reports for anchoring systems is complicated because of the complexity and comprehensiveness of the standards.

“Some companies are still trying to understand what they need to do to reformulate products to pass,” says Christy Johnson, product manager for ITW Red Head. “AC308 fills a book.”

The physics behind this is intriguing. Post-installed anchors create stress points in the concrete. Mechanical anchors by design exert a radial force against their concrete holes (stress point) and mechanical and chemical anchors can have stress points when under load. Chemical anchors exert almost no force unless loaded. Regardless, post-installed anchors appear to act like lightning rods for cracks, including cracks that do not originate with them.

“The theory is that a crack will tend to gravitate toward a post-installed anchor because the anchor creates a stress point,” Pratt says.

As a result, new mechanical anchors must have clips that undercut the concrete hole and expand if a crack passes through the hole after the anchor is installed.

Adhesive anchor systems can use specially designed inserts that convert the installed anchor from a bonded anchor into an expansion anchor. It wedges the hardened adhesive against the hole if a crack passes through the hole while a tension load is exerted on the anchor.

As of mid-December 2007, no U.S. manufacturer had an AC308-compliant adhesive anchor insert on the market. But by the time you read this, ITW Redhead expects to have an AC193 report for cracked concrete for at least one of its mechanical anchor systems and an AC308 report for an adhesive anchoring system. Simpson Strong-Tie’s Strong-Bolt mechanical anchor satisfies the new AC308 standard, but Simpson has yet to earn an AC193 report for an adhesive anchoring system. Powers Fasteners is conducting tests to document AC308 performance of its adhesive and mechanical anchoring systems.

Does all this mean you will have one choice in anchor systems for the foreseeable future? Not exactly.

“Construction projects will still be driven by specifications, so the question is whether a project in a jurisdiction will enforce AC308/AC193 for cracked concrete for mechanical or adhesive anchors,” Johnson says. “If they do, it will have to be reflected in the specification. But new rules take time for local governments to understand, implement and enforce.”

Will installation rules change?
This is interesting, you say, but does it change the way I have to install the danged things? Perhaps. Your jurisdiction may limit your choice of adhesive systems for a time. New adhesive formulas may also require adjusting gel and cure times or temperatures. Regardless of potential changes, installation best practices still “hold.” The experts recommend the following installation tips to anchor installation.

Ten tips for a good stick
1. Read the specs. During the AC308 changeover, double-check with your building code experts to make sure you’re meeting their requirements.

2. Drill the right hole. A pneumatic rock drill works quickly and creates a rough-walled hole that adhesives can grip better. Drill holes to close tolerances with their insert diameters to ensure best fit and less adhesive waste.

3. Lose the flour. Many anchor failures can be traced to poorly cleaned holes. Brush and blow the hole according to the manufacturers instructions, then cap it to prevent any further contamination prior to filling. Compressors work better than manual squeeze bulbs, but make sure the air is clean and free of compressor oils.

4. Use the right adhesive and check the cartridge expiration date. Everything degrades over time, including adhesives left in their cartridges, no matter how well they have been stored.

5. Use the right nozzle. Published performance figures are based on correctly mated adhesives and nozzles. Don’t mix brands.

6. Check the weather. Many epoxies can’t be dispensed below 40 F.

7. Coddle the cartridge. Some adhesives must be dispensed at room temperatures.

8. Reduction factors. Several companies publish reduction factors so contractors can calculate how much performance they can expect from an anchor installed on a 10 F day vs. a 70 F or 80 F day months after installation.

9. Use the right tools for the job. Consider pneumatic dispensing tools for large jobs.

Published in the January/February 2008 issue of Contractor Tools and Supplies magazine.

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