Abstract: A fibrous reinforcing element made from oriented polyolefin material for reinforcing a matrix material such as cement has longitudinally varying cross-sections and frayed surface portions from which fibrils extend. The element has cross-sections which vary along its length, a characteristic which combines with the fibrils of the frayed surface portions to facilitate anchoring of the element in the matrix. The anchoring and strength characteristics make the element particularly useful for constructing cement building products or any product produced from a matrix binding agent. The element is in the form of a fiber made from a polyolefin film which is stretched at least fifteen times and then cut by cutting means along lines at least partially transverse to the direction of orientation.