Deformed fibres

In the case of conventional fibres, with diameter of the order of 0.1 mm or bigger (called sometimes macrofibres), the adhesional and frictional bond with the cementitious matrix is not sufficient for developing adequate reinforcing efficiency. This is true for steel as well as polymeric fibres. To overcome this limitation, it is common to induce deformations in the fibre to provide it with a complex shape which will provide anchoring effects. The bonding achieved by the anchoring mechanism has been shown to be much greater than the one achieved by interfacial effects. 

This implies that it is not the length of the fibre which controls its bonding, but rather the number of crimps. Similarly, for hooked steel fibres, it was resolved that the contribution of the hook is independent of fibre length, and it is much greater than the contribution of the interfacial bond of a straight fibre of a similar length [68]. 

An alternative approach to enhance bond without changing the overall shape of the fibre is to indent or roughen the surface. This approach has been taken with steel as well as with polymer fibres [47,69] and it can result in significant influences. Singh etal. [70] demonstrated a fourfold increase in bond strength of structural polymer fibres (polypropylene with 10 GPa modulus), from about 0.5 MPa to about 2 MPa in the indented fibre. At this bond level the critical length 

Heat treatment Fibre Vickers hardness (virgin fibre) Relative peak load 

An alternative approach to develop mechanical anchoring in a fibre was recently suggested by Naaman [72]. It is based on twisting of polygonal fibres (e.g. triangular, square cross section), which allows ribs to be developed. The greater the 

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For the measurement of ventricular dimensions, intramural deformations, fibre shortening and fibre orientation, radiopaque markers were implanted at various... 

The influence of the fibre-matrix bond in the case of deformed fibres was considered later by Sujivorakul and Naaman (2003) and modelled as an important parameter in the design of high performance composites, taking into account all three components that determine fibre-matrix cooperation adhesion, friction and mechanical anchorage. 

Steel fibre volume content, % deformed fibre 0.4-1.0 0.3-0.8 0.2-0.7... 

The pull-out force is transferred from the fibre over the interfacial region 1 (deformation of the springs) to the region 2. When a temporary force equilibrium assured by friction is exceeded because of the pull-out load increment, then the motion of the deformed fibre starts in region 2. The rigid slip is observed till the moment when a new equilibrium in region 2 is reached and then further deformations of springs starts. 

The influence of the shape of the deformed fibre is usually addressed from the point of view of pull-out resistance of the aligned fibre. A shape which may lead to improved pull-out resistance of the aligned fibre may alter its sensitivity to the effect of orientation. The effect of shape on orientation was studied by Banthia and Trottier [73,74], and the compilation of their results in terms of relative peak load is presented in Figure 3.37. It clearly shows a considerable difference with... 

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. 

It was these contradictory requirements that led to the development of the deformed fibres that are currently in use, with which bonding is achieved... 

Strength also increases with increasing strength of the matrix (35-37). However, for smooth, straight fibres, beyond an aspect ratio of about 100, it is not generally possible to prepare a mix with both sufficient workability and uniform fibre distribution. Therefore, in a typical properly proportioned SFRC, failure is primarily by fibre pull-out, even with the deformed fibres shown in Figure 7.2. 

Thus, different fibre geometries and fibre-matrix interactions can affect the flexural behaviour of SFRC. This is demonstrated in Figure 7.18 [41], which shows the effects of different fibre shapes on the flexural behaviour of steel fibre shotcrete. These data suggest that the aspect ratio (//d) concept, which was developed for smooth, straight fibres, is not really useful when applied to deformed fibres,... 

Naaman and Homrich [24] studied the strengthening mechanisms in SIFCON and found that the mode of fa Mure was by fibre pull-out without fibre fracture. The tensile strength achieved with hooked and deformed fibres was roughly similar. 

Table 12.2 Tensile properties of SIFCON composites with hooked and deformed fibres in a 0.26 w/c ratio matrix (after Naaman and Homrich [24])...

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