An analysis of the effects of concrete properties on the loaded behavior of individual wedge bolt expansion anchors

The objective of this study was to evaluate the effects that certain properties of hardened concrete have on wedge bolt expansion anchors. Previous testing indicated that air content of hardened concrete could have a significant effect on these types of anchors. Thus, a program was set up which permitted an evaluation of anchor behavior with varying concrete air content, compressive strength and age. Additionally, tests to further define the loss of preload force in a wedge anchor were conducted.

The tests were performed on one brand of one-half inch wedge bolt expansion anchor. The same diameter of carbide-tipped drill bit was used for each anchor. Concrete slabs were cast with four different air contents ranging from essentially 0 to l0%. Anchors were set in groups of six and tested at predetermined compressive strengths and ages. The amount of slip each anchor underwent as well as its strength properties were analyzed. Three different methods were utilized to monitor the level of preload force remaining in an anchor prior to final loading. Statistical methods were used to evaluate the results.

The test results indicated that both during and after application of torque the amount of wedge anchor slip is affected by concrete air content. With increasing air content, above roughly 7%,the performance of an anchor begins to suffer. Variability also tends to increase for air content above this level. Depending on the air content, a wedge anchor lost roughly half of its applied preload force in the first few minutes after application. The results also indicated that further loss may occur with time.