Discussion of the Models for the Crack Opening Rates

Effect of Load Ratio and Fibre Material

Figure 7 shows the effect of increasing the load ratio on the variation of crack opening rates for the three timespans considered, distinguishing between synthetic and non-synthetic fibres. The relationship between the crack opening rates and the load ratio was not significantly modified by the fibre material. Although more variability was detected for COR^0-14, this was significantly reduced afterwards and the increasing trend with respect to the load ratio was gradually moderated.

Fig. 7 Effect of load ratio on the crack opening rates

Fig. 8 Effect of toughness on the crack opening rates

Effect of Flexural Toughness

In consistency with the findings reported for the creep coefficients, the effect of fibre material on the crack opening rates was examined in conjunction with that of flexural toughness. Figure 8 shows the average trends for the crack opening rates with respect to the flexural toughness. In terms of relative magnitude, the effect of flexural toughness on crack opening rates was more moderate than that of load ratio if Figs. 7 and 8 are compared. It is also interesting to note that, if Figs. 5 and 8 are compared, the effect of increasing flexural toughness on creep coefficients was more noticeable than on the crack opening rates.

COR^{14 30} and COR³⁰ 90 were detected to slightly decrease in average when flexural toughness values increased. On the other hand, COR⁰ 14 showed a considerably higher variability, and this was one of the reasons why the first effects plot in Fig. 8 seems inconsistent with the other two.

Such inconsistency was resolved when the distinction between cases with synthetic fibres and non-synthetic fibres was considered, as shown in Fig. 9. The fibre material introduced no substantial modification to the relationship between the flexural toughness and the crack opening rates after 14 days. However, with respect to COR^0-14, it was evidenced that data corresponding to synthetic fibres followed a

different trend, thus the apparently excessive variability noticed in Fig. 8. When the fibres used are synthetic, improvements in flexural toughness clearly reduce COR^0-14. After that age, the interaction between the fibre material and the flexural toughness is not significant in terms of crack opening rates.