Some experiments on the load distribution in bond tests

Abstract

Summary: Tests are briefly described in which the bond stress distribution has been obtained for tubes embedded in concrete and used as “pull-out” specimens. Electrical resistance strain gauges were inserted in straight lengthy of steel tubing to form the measuring devices which can be used for direct calibratiqn against load. These tubes were then embedded along the axis of cylinders of concrete to form “pull-out” specimens of the usual type. Readings of the strain gauges taken as the tube was being withdrawn from the concrete enabled the load distribution curves to be plotted. The variation of bond stress has been studied from the information provided by the load distribution curves, and values are given for average and maximum bond stress for various embedded lengths and types of tube surface. The results obtained from an American bar of “Hi-bond” type with a patterned surface are included. The experimental method has proved to be satisfactory in use and capable of providing sufficient data for plotting load distribution curves of reasonable accuracy. The transition of the load from the end at which it is applied to the free end of the tube has been observed, and the method enables the mechanism of bond failure to be investigated. The most important influence appears to be the character of the contact surface, and it is considered that the tests indicate that bond may be caused by (a) adhesion, (b) friction, and (c) mechanical wedging. The relative importance of these factors is apparently controlled by the type of surface, but further work on this subject is in progress.