Measurement of anchor strength in non-rigid substrates

This study provides guidelines for measuring anchor strength in non-rigid s ubstrates. ASTM Standard Test Method for Strength of Anchors in Concrete an d Masonry Elements (E 488-84) was designed to measure the "strength of anch ors in concrete and masonry elements" and was never intended for use in fle xible substrates. Hence, the load capacity of anchors installed in thin bri ttle materials like gypsum wallboard may be overestimated when measured in accordance with E 488. This is because ASTM E 488 imposes a reaction load a round the anchor and delays the onset of pullout failure related to substra te 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 d etermined 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 rig id wooden framework by suitable screws and adhesive. With no adhesive cure, the strength by the alternative method was up to 31% less than that obtain ed by the test following ASTM E 488 guidelines. With full adhesive cure, th e strength in the alternative test was still up to 30% less than the ASTM t est and up to 45% less than the rated strength. The strength differences ca n 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 te st makes it possible to relate the effects of factors like screw spacing an d bonding efficacy to anchor strength.