Volume 28, Issue 2 (March 2000)
Measurement of Anchor Strength in Non-Rigid Substrates
This study provides guidelines for measuring anchor strength in non-rigid substrates. ASTM Standard Test Method for Strength of Anchors in Concrete and Masonry Elements (E 48884) was designed to measure the “strength of anchors in concrete and masonry elements” and was never intended for use in flexible substrates. Hence, the load capacity of anchors installed in thin brittle materials like gypsum wallboard may be overestimated when measured in accordance with E 488. This is because ASTM E 488 imposes a reaction load around the anchor and delays the onset of pullout failure related to substrate bending. In contrast, the alternative test does not constrain the front surface of the substrate and can better reproduce the actual mechanisms of anchor failure in flexible materials.
The pullout strength of five anchor styles of varying rated strengths was determined following the ASTM E 488 method and by the alternative method. In both cases, the anchors were placed in a wallboard panel attached to a rigid wooden framework by suitable screws and adhesive. With no adhesive cure, the strength by the alternative method was up to 31% less than that obtained by the test following ASTM E 488 guidelines. With full adhesive cure, the strength in the alternative test was still up to 30% less than the ASTM test and up to 45% less than the rated strength. The strength differences can be linked to differences in the flexural stiffness of the test panel, and guidelines are given for determining when the substrate is flexible enough to justify the alternative test. The alternative test requires more setup time than ASTM E 488 but provides a more realistic measure of the ultimate load capacity of relatively strong anchors. In addition, the alternative test makes it possible to relate the effects of factors like screw spacing and bonding efficacy to anchor strength.