Enhancements in durability through fibre and matrix modification

Choosing fibre and matrix combinations that reduce or eliminate deleterious interactions or interact in a benign manner can enhance the durability of frc. The normal approach, however, concentrates either on improving the resistance of the fibres to attack by varying their chemical composition or surface treatment, or on using modified or non-Portland cementitious matrices that are less hostile to fibres, either owing to lower pore solution alkalinity, reduced propensity to precipitate hydration products at the fibre-matrix interface, or reduced permeability to restrict the ingress of deleterious agents.

Glass-fibre development is relatively mature. It was clear from the first incarnations of glass-frc that E-glass fibre (as used by the frp industry) would be chemically unstable in the highly alkaline cement matrix. Alkali-resistant (AR) glass was developed in the 1970s by Pilkingtons, based on soda-lime-silica glass with the addition of about 16% zirconia, and marketed as Cem-FIL. Glass-frc made from AR fibres was not immune to degradation, however, and further developments were made to the soluble coating or `size’ applied to the fibre (originally applied for manufacturing purposes). Incorporation of polyhydroxy- phenols in the size modifies the hydration behaviour of the cement matrix at the interface and significantly improves durability of AR glass-frc (Majumdar and Laws, 1991, p. 13). AR-glass fibres with such coatings are known as `second- generation’ fibres and are now the industry standard. The development of these sizings is ongoing. From time to time, other glass systems are proposed, e.g. based on strontium (Karasu and Cable, 2000) or barium (Cheng et al., 2003) but these have yet to come to commercial significance. Steel fibres, although stable in the alkaline cement matrix, are susceptible to corrosion if this alkalinity is disturbed by, e.g. carbonation or chloride ions, as with RC. Thus in extreme environments such as marine applications, steel fibres may be galvanised (coated with zinc). Steel alloyed with small amounts of chromium or nickel, but not `fully stainless’, will provide significant resistance to rusting (Johnston, 2001, p. 229) but normal stainless steel (i.e., ~15% Cr) may also be used (Mangat and Gurusamy, 1988). Both galvanised and stainless fibres show significantly greater resistance to rusting during ageing compared with low carbon steel fibres (Mangat and Gurusamy, 1988). Other fibres tend to be of fixed composition (e.g., carbon, polypropylene, cellulose) and are thus not amenable to having their alkali resistance intrinsically increased.

An experimental method of increasing durability is to pre-impregnate the fibre strands with material intended either to block precipitation of hydration products between the filaments or modify the nature of hydration such that less deleterious hydration products result. Glass-fibre strands have been impregnated with microsilica or acrylic polymers (Bartos and Zhu, 1996); natural fibres have also been treated with microsilica (ToleÃdo Filho et al., 2003). In both cases, resistance to accelerated ageing and/or weathering was improved markedly. This approach has yet to be commercially adopted but would appear to be attractive.

By far the most common approach involves modifying the cement matrix, in order to improve its compatibility with fibres, either by using additives to Portland cement or using non-Portland cement matrices. A great deal of literature exists on this topic, especially concerning glass-frc; pre-1988 literature is summarised in the books by Majumdar and Laws (1991), and Bentur and Mindess (1990). Additives are generally intended to reduce the pore solution alkalinity of the matrix and/or react with the calcium hydroxide produced during hydration; thus pozzolanic materials, as used to enhance ordinary concrete, are common additives. Condensed silica fume/microsilica (csf), metakaolin, ground granulated blast furnace slag (ggbs) and pulverised fuel ash (pfa) have all been investigated for use with most fibre types. For steel-frc, the same additives are frequently used, but the intention, as with RC, is to reduce the permeability of the matrix and thus ingress of chloride ions, carbonation and water (de GutieÂrrez et al., 2005). However, few if any studies directly assess the improvements in steel-frc durability, in terms of mechanical property degradation, afforded by matrix modification.

Csf is a by-product of the ferro-silicon industry and consists of extremely fine particles (<1 um of essentially pure silica, which react rapidly with Ca(OH)2. It is used as a matrix enhancer for natural/cellulose-frc (ToleÃdo Filho et al., 2003, MacVicar et al., 1999), ceramic-frc (Ma et al., 2005), carbon-frc (Chen and Chung, 1996, Katz and Bentur, 1995, Katz 1996) (where its primary function is to aid fibre dispersion and compaction) and glass-frc (Bartos and Zhu, 1996, Marikunte et al., 1997, Zhu and Bartos, 1997). It is effective in increasing the durability of natural fibre composites exposed to weathering and cyclic wet/dry ageing, but has limited or variable effect on that of glass-frc subjected to hot- water ageing (see also Bentur and Mindess, 1990, pp. 264, 410; Majumdar and Laws, 1991, pp. 105±107). Addition levels vary between 10% and 40% replace- ment of cement, but at higher levels of replacement it has a profoundly negative effect on workability and thus requires higher water/cement (w/c) ratios for manufacture, which is undesirable. In steel-frc, 15% cement replacement by csf is reported to reduce chloride ingress and chloride diffusion coefficient by a factor of 3 (de GutieÂrrez et al., 2005); 8.5% cement replacement by csf had an uncertain effect on de-icer salt scaling resistance (Cantin and Pigeon, 1996). Yan et al. (1999) reported that 25% cement replacement by csf doubled the fatigue resistance of steel-frc.

Metakaolin is a calcined china clay which reacts readily with Ca(OH)2 in the matrix and reduces the alkalinity of the pore solution (Purnell et al., 1999). It has been extensively investigated as an additive for glass-frc to improve durability (e.g. Marikunte et al., 1997, Purnell et al., 1999, Zhu and Bartos, 1997, Sou- katchoff and Ridd, 1991, Ball, 2003, Beddows and Purnell, 2003). Normally added at 20±25% cement replacement, almost all investigators agree that it significantly improves the durability performance of glass-frc both under accelerated (i.e., hot water) and natural weathering. The degree of improvement depends on how accelerated ageing results are interpreted (see pages 349±52 and Purnell and Beddows, 2005) but it is sufficiently well established for commer- cial formulations to be used; the metakaolin is also believed to improve surface  finish and resistance to loss of appearance via surface dusting or efflorescence (e.g., Gilbert and Ridd, 2001). It does not appear to have been used to improve durability in conjunction with other fibre types except as reported by de GutieÂrrez et al. (2005) who did not monitor changes in mechanical properties with time but were concerned with chloride diffusion. Use of metakaolin as 15% cement replacement in steel-frc was reported to reduce chloride diffusion by a similar factor to csf.

Ggbs has been used with natural-frc (ToleÃdo Filho et al., 2003) and glass-frc (Majumdar and Laws, 1991, p. 102) with limited success. It was reported to have a beneficial effect on glass-frc at very high cement replacement levels (70%) but this does not seem to have spurred commercial interest, although some Japanese (Takeuchi et al., 1999) formulations use blastfurnace slag cement in conjunction with other admixtures. The DuraPact matrix (Pachow, 2001), glass-frc made with which purports to have a guaranteed 50-year lifespan, is based on blastfurnace slag cement combined with microsilica (Purnell, 1998). Ggbs does not appear to be as effective in reducing chloride diffusion in steel-frc as csf or metakaolin (de GutieÂrrez et al., 2005) even at 70% cement replacement. Pfa has been used in polypropylene-frc (Hannant, 1998) and glass-frc (Majumdar and Laws, 1991, pp. 94±102, Cyr et al., 2001). Polypropylene-frc with pfa was the material examined in the long term study by Hannant (1998) (see also Fig. 9.1). Such composites appear to retain most of their strength after 18 years of weathering but no comparison with an unmodified OPC matrix was performed. The addition of pfa to glass-frc appears to have some benefit with regard to durability (Majumdar and Laws, 1991, pp. 94±102). The durability improve- ments reported by Cyr et al. (2001) are from a hybrid system including both pfa and silica fume and thus cannot be attributed to pfa alone. For both pfa and ggbs, it should be noted that slower setting and hardening, and thus lower matrix strengths at early ages, will result from their addition to the matrix.

Another approach pioneered in glass-frc is the use of polymer modification of the matrix. Majumdar and Laws (1991, pp. 112 et seq.) dedicate a whole chapter to the topic. A wide variety of polymers have been tried but attention has most recently focused on an acrylic polymer dispersion (`Forton’). Originally designed as a curing aid for thin concrete sections to prevent water loss by evaporation leading to shrinkage cracking, it is added to glass-frc to improve workability in the fresh state and improve mouldability. It is also widely believed to improve durability and the `5/5′ formula, comprising OPC-glass-frc with the addition of 5% Forton polymer solids by volume and 5% AR-glass fibres by weight, is ubiquitous in the glass-frc literature, so much so that it is often referred to implicitly as a `base’ to which modifiers such as metakaolin are then added (e.g., Glinicki et al., 1993). The mechanism by which it enhances durability is different to the pozzolans in that it does not reduce alkalinity of Ca(OH)2 formation, but is thought to either coat the fibres with an inhibitive layer or modify hydration at the interface by occupying space that might otherwise fill with Ca(OH)2. There has been some debate as to whether polymer-modification of the matrix can actually lead to improved durability as accelerated ageing procedures often yield conflicting results (e.g., Zhu and Bartos, 1997, Qian et al., 2003). The most recent research unequivocally demonstrates that hot-water ageing is unsuitable for polymer modified glass-frc (Purnell and Beddows, 2005) and also that the addition of polymer to both plain OPC and modified matrices confers very significant durability benefits (Ball, 2003); over 19 years of natural weathering, reduction in modulus of rupture (MOR) was negligible and compared to plain OPC matrix glass-frc the degradation of strain-to-failure was reduced by 75%. Polymer modification of the matrix is uncommon with other fibre types but a summary is given by Bentur and Mindess (1990, pp. 421 et seq.). Polymer modified concrete is occasionally used as a matrix for steel-frc but the polymer is generally included to help with workability, not durability, although some authors claim it may help in that regard (Corinaldesi and Moriconi, 2004).

Other, non-Portland (nP) common cementitious systems have also been investigated for use with frc. Again, this practice is better developed for glass-frc than for other systems. The use of nP systems for glass-frc is summarised by Majumdar and Laws (1991, pp. 112 et seq.). High-alumina cement (HAC) and super-sulphated cement are both less alkaline than OPC and thus candidates for glass-frc. Worries over the integrity of the HAC matrix at moderately elevated temperatures (the `conversion’ reaction), which led to the banning of HAC for structural use in the UK, have caused glass-frc producers to be cautious over its use despite its superior durability characteristics. Super-sulphated cement also showed improved resistance to ageing but is not a common choice for glass-frc as it is susceptible to weakening on carbonation, which is enhanced for the thin sections typical of frc.

The last ten years has seen an increase in interest in nP matrices based on calcium sulpho-aluminates. These cements, with 4CaO.3Al2O3.SO4 (ye’elemite or Klein’s compound) as the major active ingredient, hydrate in the presence of a source of lime to form mainly ettringite, with little or no calcium hydroxide remaining in the hydrated phase assemblage and a lower pore solution alkalinity than OPC. Development of such cements has been led by China (Qi and Tianyou, 2003) and Japan (Takeuchi et al., 1999) but analogues are available in the UK (Gartshore et al., 1991) and the US (e.g., Molloy et al., 1995). Little data exists on the long-term properties of glass-frc made with such cements but it shows extraordinary resistance to hot-water accelerated ageing (e.g., Jiangjin et al., 2001); however, caution is required in interpreting such results (see Section 9.2.5 and Purnell and Beddows, 2005). Development of these matrices is continuing and they appear to offer the best option for completely durable glass- frc. The use of nP matrices with other fibres has received little coverage in the literature. Puertas et al. (2003) have investigated alkali-activated blast-furnace slags and pfas reinforced with polypropylene fibres, subjected to cyclic ageing. Results were unclear and did not show any significant benefit compared with OPC. Frantzis and Baggot (2000) reported a `durability’ testing procedure for the bond between steel-fibres and magnesium phosphate or calcium aluminate cements but did not suggest that such matrices conferred any durability benefits cf. Portland cement.

Combination of approaches is common. Most obvious is the combination of polymer with a pozzolanic additive, since they act via different mechanisms and thus might be expected to have a synergistic effect. The recent comprehensive paper on long-term weathering of polymer modified glass-frc by Ball (2003) reports on samples combining polymer with metakaolin and csf tested at inter- vals up to 13 years. Neither the combined or solely polymer-modified samples degraded significantly over this period thus no firm conclusion can be reached. Many of the formulations mentioned above involve combination of, e.g. sulpho- aluminate and metakaolin (Calcrete) (Gartshore et al., 1991), blast-furnace slag cement and sulpho-aluminates (Nashrin) (Takeuchi et al., 1999) or blast-furnace slag cement and microsilica (Durapact) (Pachow, 2001). Since each of the components purports to improve the durability by effectively the same mechanism, any synergy is minimal and one component tends to dominate, but since it is unlikely that combinations would reduce durability cf. unmodified matrices, the detail is only of academic interest.

The cementitious matrix in frc is susceptible to reaction with the carbon dioxide in the atmosphere, a phenomenon known as carbonation. During this reaction, portlandite in the matrix is converted to Ca(CO)3 and thus the buffering capacity of the pore solution is lost, alkalis are either converted to carbonates or absorbed in the C-S-H phase (which also carbonates, becoming decalcified) and the alkalinity of the matrix drops sharply. Its strength also increases. Thus in theory, a carbonated matrix is more compatible with most fibres (except steel). Natural carbonation can take years for frc components, but attempts have been made to apply accelerated carbonation to improve their durability. Accelerated ageing tests for natural fibre or cellulose-frc include a carbonation component since these materials tend to carbonate readily when exposed to weathering. There seems to be some confusion in the literature regarding the difference between carbonation as a treatment or an ageing process but the recent con- sensus seems to be that early curing of such composites in a CO2-rich environ- ment improves their durability (ToleÃdo Filho et al., 2003, MacVicar et al., 1999). Purnell et al. (2001b, 2003) used super-critical CO2 to effect carbonation of OPC matrix glass-frc components within hours. The treatment significantly increased the resistance of such composites to accelerated ageing, improved mechanical properties in general and improved their dimensional stability under wetting and drying (Seneviratne et al., 2002). In steel-frc, carbonation is undesirable and thus not used as a treatment.

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