Crack Propagation Due to Relaxation of Cohesive Stresses
In plain concrete, the fracture process zone created by rapid loading shrinks due to relaxation of the bridging stresses. This leads to crack propagation that clearly affects load carrying capacity of the concrete (see Fig. 6).
What happens when fibres are present in such a matrix? When a strain-softening type FRC is subjected to constant load, crack propagation and widening with time
can be expected. Eventually, sudden failure may occur after tertiary creep response when the load ratio is high enough. However, when strain-hardening type FRC is subjected to constant load, it is expected that more cracks will widen with time without any failure occurring . This is coherent with the loading level effects discussed in Sect. 2 since the load on the individual fibre, which is bridging the crack, increases as the crack propagates. Figure 7a, b show some responses observed in the RILEM TC CCF Round Robin Test programme for flexural creep for PFRC and SFRC, respectively. It is seen in the plots of crack mouth opening displacement (CMOD) evolution with time, under constant load, that there is a progressive increase in crack opening with some jumps (i.e., sharp increase), each of which indicate sudden crack propagation. In a recent study, Daviau-Desnoyers et al.  also concluded that crack propagation is the mechanism that governs the crack widening in FRC due to sustained loading.
In general, when crack propagation leads to failure, the response can be expected to exhibit considerable variability (as in the case of fatigue failure) since the
Fig. 7 Responses observed for a PFRC and b SFRC
Fig. 8 The compression zone is strongly reduced due to crack propagation
microcracking characteristics, and fibre location and orientation would differ significantly from one specimen to another. However, within the secondary creep regime, the later behaviour in the same specimen may be inferred from earlier time-dependent behaviour .
When crack propagation occurs in a typical beam test that is adopted for creep characterization, the compressive stresses increase since the moment of inertia of cracked section (Icr) decreases with time. For example, analysis shows that in a SFRC beam of 150 mm depth, the crack length can be about 100 mm at CMOD = 0.5 mm. Consequently, in this situation, the compression zone would be small and the compressive strains increase as the crack propagates. Therefore, one should be careful while representing the strain increase observed as compression creep (see Fig. 8). Also, the creep coefficient obtained from flexural cracking tests will not have the same relevance as that of linear creep under compression. So formulations homologous to those used until now for compressive creep (of uncracked concrete) would not have any significance in the case of the creep of cracked FRC.