Interfacial microstructure and processes

The complexity of the processes for deformed fibres has been discussed in Section 3.4. However, even for fibres which have the ideal straight shape and circular cross section, the pull-out processes in the actual composite may be more complex than predicted by the analytical models. This section deals with special processes at the interface which are induced by (i) the special microstructure of the cement matrix in the vicinity of the fibres (ii) the special characteristics of the fibre surface itself and (iii) the geometrical and microstructural characteristics of reinforcement by strands consisting each of a large number of thin filaments. 

The speciai microstructure has two consequences (i) a porous and weak interface which wiii cause an overall reduction in bonding, (ii) a weak interface which is not at the fibre surface, but rather in the porous iayer of the iTZ, somewhat away from the fibre surface. The first characteristic directiy affects the bond and the pullout resistance, whereas the second one will have an indirect effect by influencing the mode of debonding when cracks develop in the matrix and propagate towards the fibre. 

Means taken to reduce the size and inhomogeneity of the ITZ will lead to its strengthening and the expected result is an overall increase in bond, as demonstrated by Igarashi eta/. [45] the bond of steel fibre to a paste matrix increased with mixing time 0.7, 1.0 and 1.3 MPa of average bond for 0, 20 and 40 min of agitation of the mortar matrix against the fibre (Table 3.4). 

In view of the presence of the special ITZ microstructure several models were developed, assuming a three phase material bulk, ITZ and fibre. In these models specific characteristics (i.e. modulus of elasticity) were assigned to the ITZ. Li et a/ [75] calculated a parameter related to the stiffness of the ITZ to resolve its value for different systems. Mobasher and Li [76] extended a fracture mechanics model to describe the pull-out curves in terms of adhesional and frictional bond, stiffness of the interface and interface toughness. They demonstrated that all of these parameters influence the pull-out behaviour, and their change with age could account for the variation in the pull-out behaviour, as discussed further in Chapter 5. 

As long as the interfacial microstructure inhomogeneity remains, which is frequently the case in conventional concrete reinforced with fibres, a complex mode of cracking and debonding may be induced, as outlined below. 

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