PULL-OUT MECHANISM IN STEEL FIBER-REINFORCED CONCRETE

The bond between steel fibers and portland cement matrices is a critical factor in determining the strength properties of fiber-reinforced concrete structural elements. The influence of the following three major parameters on the pull-out behavior of fibers was studied: the angle of orientation of the fibers with the loading direction, the number of fibers being simultaneously pulled out from the same area, and the efficiency of random orientation. It is shown that: (1)The pull-out load of a randomly oriented fiber is not lower than that of an aligned fiber; (2)the pull-out capacity of a group of randomly oriented fibers decreases drastically when the number of fibers pulling out from the same area increases; and (3)the efficiency of fiber orientation after matrix cracking is substantially higher than efficiency factors derived from the theoretical elastic considerations. These results seem to explain why the addition to a concrete matrix of fibers with highly improved bond properties does not often lead to an equivalent improvement in the composite properties.