Steel fibre reinforced concrete: a contribution to the evaluation of the limit of proportionality

Two methods for the estimate of the characteristic values of the limits of proportionality of a steel fibre reinforced concrete, f_fctk,fl are compared: a method derived directly from the classical ISO method for the determination of a statistical tolerance interval and another proposed by RILEM TC 162-TDF Committee. This is done by comparing the results obtained on 200 samples of size n = 15 and 200 samples of size n = 10, all extracted in a random manner from the same normal distribution of the values of the limit of proportionality, LOP. It is concluded that according to the ISO method, the confidence level is 95% with a 5% risk of overestimate, as generally accepted by the standards, while with the proposed RILEM method, the confidence level of the estimate (1 − α) turns out to be only 80% and hence implies a 20% risk of overestimate.