In designing a reinforced concrete structure, the engineer takes into account the fact that cracks will eventually develop in the structure. These cracks expose the reinforcing bars to the atmosphere with consequent corrosion which can weaken the entire structure. Recent evidence uncovered at the National Bureau of Standards, however, indicates that these cracks are narrower near the surface of the reinforcing bar then at the outside concrete surface, thus exposing less of Thea bar to corrosion the was heretofore believed. The crack width measurements were made on tensile bond specimens designed to simulate a portion of the tensile zone of a reinforced concrete beam between two successive cracks. Each specimen was essentially a prism of concrete 8 inches long with a reinforcing bar embedded along the longitudinal axis. A tensile force was applied to the ends of the bar. The extension of the embedded portion of the bar, the over all extension of the concrete prism at points three-eights of an inch form the surface of the bar, and the overall change in length of the exterior surface of the prism were determined. The ratio of the width of crack at the surface of the bar to that at the exterior surface of concrete has been plotted against the applied stress, to provide the engineer with the necessary design information. This reduction of crack width in the vicinity of the surface of reinforcing bars.