The MLR models summarised in Table 3 made it possible to plot response curves for the creep coefficients with respect to load ratio, concrete compressive strength and toughness. Furthermore, the differences introduced by the fibre material and their modifying effect on other variables were also scrutinised.

These aspects were examined through the contrastive effects plots for the MLR models obtained, using the open source package “visreg” in R [13]. In these plots the relative change in the creep coefficients is represented, instead of their predicted average values. This way the effect of any one variable could be analysed without assuming constant values for the other variables, which is an important advantage when interpreting complex models with many interactions.

Effect of Concrete Compressive Strength

Variations in the creep coefficients with respect to concrete compressive strength are shown in Fig. 2. Average trends as obtained from the MLR models are drawn with continuous lines, while dots correspond to individual data. Coloured bands represent the 90 %-confidence band.

It was observed that higher compressive strength values tend to reduce the creep coefficients at 30 and 90 days, following a quadratic trend. However, this relationship between concrete compressive strength and changes in the creep coefficient was not detected to be significant at 14 days.