Time-dependent behaviour mechanisms

The causes of the time-dependence of properties fall into two broad categories, regardless of the type of fibre involved. First is fibre corrosion. The nature of most hydraulic cements is alkaline. During manufacture, the alkali metal compounds present as impurities in the clays or shales used as raw materials are converted by the high clinkering temperatures into alkali oxides and/or sul- phates, which are incorporated into the structure of the cement clinker. On gauging with water during concrete mixing, these highly soluble alkalis are released into the mixing water, raising the pH significantly. As the cement hardens, the mixing water is used up and the alkalis are thus concentrated in the remaining free water, contained within the capillary porosity of the cement paste, known as the pore solution. Cement pore solution can reach a pH of up to 13.7, or 700 mmol/l (OHÿ), the OHÿ content being associated mainly with K+ ions but also with Na+ ions; the pore solution is buffered by the Ca(OH)2 (portlandite) produced by the hydrating cement, but contrary to popular belief the Ca2+ content of the pore solution is generally very low (i.e., <40 mmol/l) owing to the common ion effect (see e.g., Taylor, 1997 for more details on cement chemistry). The integrity of many fibres is compromised at these very high alkalinities and thus fibre corrosion, leading to a loss of strength of the reinforcement, is always a composite degradation mechanism that must be considered. This holds even for fibres which are supposedly immune to alkali attack, since the timescales which must be considered ± i.e. the service lifetimes of typical frc components, often decades ± are much longer than most exposure experiments. As well as reducing the strength of primary reinforcement, weakening of the fibre may also lead to a change of failure mode. As the fibre strength is reduced, lc is also reduced and thus the mode may change from fibre pull-out to fibre fracture, i.e. region IV behaviour (Fig. 9.2) may be reduced or lost. For steel fibres, the alkaline environment passivates the steel and protects it from corrosion, as in ordinary RC described elsewhere in this book.

There is also evidence that some fibres, particularly glass, may be susceptible to physicochemical attack by direct contact with calcium hydroxide in the matrix to a level over and above that caused by alkalinity alone (Proctor and Yale, 1980). In certain applications, the ingress of external agents deleterious to certain fibres, such as chlorides or acidic organics, must also be considered. Although of primary importance for steel-frc (for example, chlorides disrupt the passivation of steel fibres as with the steel in RC) this may occasionally be of concern for other frc. Frc made with natural fibres, the properties of which vary with moisture content, may be degraded by water ingress.

A variety of effects caused by the continued hydration of the cementitious matrix also have the potential to cause composite degradation. Given the continued availability of water, residual anhydrous cement in the matrix (of which there is always a small amount) will continue to hydrate, producing more hydration products and increasing the strength of the matrix. The hydrated phase assemblage itself will also tend to slowly evolve since it is essentially composed of meta-stable phases. Although this continued hydration and evolution might only involve a very small fraction of the cement paste, the resultant effects on the fibre-matrix interactions can be profound. We can see from equation 9.2 that Vfcrit is dependent on the matrix strength; as the matrix becomes stronger, a greater proportion of fibres are required to take up the load thrown onto them as the matrix begins to crack and thus transfer stress to the fibres. Thus for composites designed with a fibre content very close to Vfcrit , continued strengthening of the matrix may change the composite behaviour at first crack from ductile to brittle, as regions II and III of the stress- strain curve (Fig. 9.2) cannot be realised. If l >> lc then region IV may also be absent. Careful design of the composite should avoid this.

Continued hydration also tends to `densify’ the interface between the fibres and matrix. When frc is first made, the interface is relatively porous, with little intimate contact between the fibre and matrix. The interfacial zone is thus relatively weak, which is often beneficial for the composite for several reasons. At young ages, cracks travelling through the matrix tend to be diverted through this weaker zone. As the matrix ages, hydration products (particularly portland- ite) fill the spaces in the interfacial zone, reducing the porosity and increasing the hardness and strength; it becomes energetically, and thus statistically, more likely that a crack will travel through the fibre rather than around it. This reduces the toughness of the material. Also, fibres bridging cracks tend to become bent, owing to crack paths being tortuous and fibres generally not being aligned with the stress axis. They will have to bend through tighter radii of curvature as the matrix hardens, since the matrix immediately adjacent to the point where the fibre emerges from the crack face is less compliant. This induces greater stress in the fibres and increases the likelihood of them breaking. The bond between a fibre or strand will also increase as the interfacial zone densifies. As the bond strength increases, lc reduces. It becomes more likely that fibre failure, rather than pullout will occur, region IV behaviour (Fig. 9.2) is not developed and the toughness will be reduced (even though the tensile/bending strength may well increase appreciably, since l=lc and hence n is increased).

A related phenomenon is `bundle filling’. In young frc reinforced with strands or tows, there is normally space between the individual filaments into which the matrix has not penetrated. The reinforcement can thus act in a rope-like manner, with filaments moving over each other, adding significantly to the post-peak toughness of the composite. As the composite ages, continued hydration and migration of species within the cement matrix leads to precipitation of hydration products, especially portlandite, between the filaments and this may eventually fill all the available space. The rope-like behaviour is lost and the toughness of the composite may be compromised.

The relative importance of each of these degradation mechanisms is depen- dent on the particular fibre-matrix combination. Some combinations are particu- larly prone to specific mechanisms, while in others the dominant mechanism is still the subject of research and debate. The mechanisms are examined in more detail in Sections 9.2.4 and 9.2.5 in the context of the changing microstructure of frc and modelling of degradation.