Effect of Long Term Loads on FRC Residual Strength Properties
After unloading bending tests were performed on prisms -a to -d and also on prisms -f (that remain unloaded) with the aim of evaluating the effect of long term loadings on the FRC residual strengths. Figure 6 shows the relationship between the residual stress fR3 and the residual stress fR1 obtained during the pre-cracking process. It can be seen a good correlation between both values and that long term loads did not produce any detrimental effect on the residual strength capacity; in addition, no differences between beams of different widths were found.
Fig. 6 Relationship between the residual stress fR3 measured during rapid bending tests after unloading and the residual stress fR1 obtained during the pre-cracking process (? initial characterization tests, prisms -e; specimen not submitted to creep tests, prisms -f)
Effect of the Content of Fibres in the Cracked Section
After bending tests, the content of fibres in the fractured surfaces of each specimen was counted. In this way, it can be discussed if the beam width can affect the creep results due to variations in the fibre density or fibre orientations.
Figure 7 shows the variation in the mechanical properties measured in bending with the fibre density measured at the fracture surfaces. As it can be seen the density of fibres does not affect the first peak load but it has a significant influence on the residual stress before (fR1, Fig. 7 left) and after creep tests fR3, Fig. 7 right); in both cases the same tendency can be seen for prisms of different widths but it must be noted that the greatest variations in fibre density correspond to the thinner beams. Thus, the greater variability in the results from the 50-mm width beams is clearly justified. Analysing Table 2, it can be seen that the COV of the fibre density is only 4 and 7 % in the case of 150-mm and 100-mm beams and it increases up to 34 % in the smallest beams.
Regarding the effects of fibre density at the fracture surfaces on the results of COR, it can be seen in Fig. 8 that the highest values of crack opening rate are associated to low fibres contents; nevertheless it must be remembered that the applied sustained loads were related to the residual capacity measured during pre-cracking in each pair of companion beams (near 60 % fR1). Regardless these observations and for these loading conditions, Fig. 8 shows that the variation in crack opening rate is not so much affected by the beam width and the fibre density and then, the results of a same concrete obtained from beams of different width are comparable.
Fig. 7 Variation of mechanical properties measured in bending with the fibre density. Left and centre pre-cracking tests; right bending tests after creep loading
Fig. 8 Relationship between fibre density and crack opening rate