which is the law of Type 1 deterministic size effect (Fig. 2a). The purpose of the approximation made is: (a) to prevent σ σ N from becoming negative for very small D , for which the foregoing argument does not apply; and (b) to satisfy the asymptotic condition that the deterministic size effect must vanish for D / l 0 → → ∞ ∞ . Here r = positive empirical constant; the values r = 1 = or 2 have been used for concrete, while r ≈ ≈ 1.45 is optimum according to the existing test data from the literature (Fig. 2d).