The tensile strength of concrete is greatly reduced as a result of internal flaws and micro-cracks. The application of fracture mechanics concepts reveals that the tensile strength is proportional to the inverse square root of flaw diameter. High tensile strengths can be realized, however, when flaws are prevented from enlarging beyond certain limits. This is accomplished by means of closely spaced wire reinforcement. For wire spacings of less than a certain predictable range, the maximum size of flaw is equal to the wire spacing. Thus, the smaller the spacing the larger the tensile strength. Theoretical results are presented in detail. The theoretically predicted relationship between tensile strength and wire spacing is substantiated by tests.