An investigation on the scatter of code-type constitutive models that relate compressive strength (f(c)) with tensile strength (f(ct)) and Young's modulus (E-c) of standard concrete specimens is presented. The influence of the mix design on the accuracy of the f(c) vs. f(ct) and f(c) vs. E-c relationships is discussed, with emphasis on the lithological type and morphology of the coarse aggregates. The uncertainty of the constitutive models is analysed in probabilistic terms and random variables that model the uncertainty of the f(c) vs. f(ct) and f(c) vs. E-c relationships are proposed for reliability analyses of serviceability limit states. The suitability of the models proposed is assessed through preliminary conservative estimates of their design values.