Fibre and interfacial degradation

Although the reinforcing fibres are protected from direct exterior aggression by the embedding polymer matrix, they may experience substantial chemical and physical attack due to the ingress of moisture, alkaline or salt solutions in the FRP composite. In addition, these effects may be emphasized if additional sustained loads are applied to the composite materials. In this section, the various degradation mechanisms are briefly recalled for the main types of fibres used in construction. 

It was originally presumed that the corrosion effect was mainly related to the acidic strength of the solution (H concentration), but later studies showed that the associated anion can also play a significant role if it can form complex or insoluble species with the cations of the glass (Jones and Chandler, 1984). Such complex formation will consume leached cations and drive Equation [12.65] to the right, hence emphasizing the leaching process. This particular phenomenon may explain the severe corrosion effects observed with relatively weak acids such as oxalic acid. 

In addition, the tendency of an ion to deplete would be related to the characteristics of this ion (bond energy in the glass network, valence state, hydrated volume). More generally, the extent of corrosion depends on the nature and concentration of the acid, on the glass composition and on the manufacturing process of the glass fibres (Kumosa, 2001). 

Corrosion of glass fibres in neutral aqueous solutions 

The diffusion of chemical species is the predominant mechanism driving corrosion in neutral solutions. After water diffusion into the glass network, hydration of alkaline oxides present in all glass formulations (even the most resistant) leads to the diffusion of Na and OH ions towards the surface and the aqueous medium (Equation [12.66]). Hydroxide ions may then lead to the hydrolysis of siloxane bonds (etching) without being consumed, as shown in Equations [12.67] and [12.68] (Ishai, 1975). This is an autocatalytic process, as the rate of dissolution of the glass increases with time. 

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