ABSTRACT
Both rovings and chopped strands require careful treatment avoiding extremes of temperature and humidity. Damp and dust should be kept from contaminating the fibre, and roving boxes must be sensibly handled to prevent crushing and damaging the cheese. Ideal storage conditions are 10-25 °C at 40-65% RH.In the past, a third form of alkali resistant glassfibre has been available. This was the non-woven mesh called crenette. While very cost effective for press moulding, it has been little used and is now unobtainable. Non-alkali resistant glassfibreBefore alkali resistant glassfibre was readily available, ‘E’ glass and ‘A ’ glassfibres were used in roving and in chopped strand mat forms. The cement employed for this was aluminous cement, having the advantage of a far lower rate of attack on the fibre than ordinary Portland cement. The ‘E’ glass or borosilicate glass was more widely used and, in this case, sizes of a paraffin emulsion base have reputedly been successful in binding the filaments together. When aluminous cement is specified because sulphate attack or chemical attack is suspected, as in permanent formwork for pipes or sewers and such like, it may be cost effective to consider ‘E’ glass as the reinforcement. 5.1.2 Other fibresAs the development of fibrous cement composites continues, more economic fibres and fibre ‘blends’ have evolved. Nylon and polypropylene fibres have, for example, excellent impact properties in cement matrices but offer little tensile strength due to their low modulus and poor bonding characteristics. Blending nylon or polypropylene as a premix into the matrix and
then manufacturing by spray-up using gun chopped rovings of glassfibre can produce composites of good impact and tensile strength. The fibre addition levels may be in the order of 0.5-1% of nylon or polypropylene of 12 mm fibre length and 4-4.5% of glassfibre (both percentages by weight).One noteworthy advantage of formulating nylon or polypropylene into the composite to handle the impact requirements is that the impact value obtained remains with ageing. This contrasts with glassfibre composites which, on their own, suffer substantial reductions in impact properties after a few years weathering.Perhaps the most exciting development in blending glassfibre with other fibres is the recently developed high modulus nylon — Kevlar 49. This fibre, when introduced into the tensile zone of grc composites, considerably improves the stiffness of the product, increases the LOP value, maintains the improvement with age, and also reduces the shrinkage and creep rates. It is, however, four times the cost of alkali resistant glassfibre, and has reduced properties at elevated temperatures compared with inorganic fibres. Nevertheless, for permanent formwork units, pipes and so on it has great promise. 5.2 CEMENTSMost general applications of grc production employ the ordinary types of cement that are readily available namely: • Ordinary Portland cement • White Portland cement • Rapid hardening Portland cement • Sulphate resisting Portland cement • High alumina cement • Ultra high early strength cement. Further advice on the use of these should be sought from the manufacturers, but the following comments are offered as a guide to assist in a general consideration of their respective merits.General production should employ rapid hardening Portland cement. The extra hydration over ordinary Portland cement obtained at demoulding (usually 18 hours after casting) is sufficient to produce a marked benefit in strength, with only a marginal cost increase.If white composites are required, then naturally white ordinary Portland cement has to be used, but it may be possible to apply white cement only for the mist coat (with a polymer admixture), and back this up with rapid hardening cement mortar for the bulk of the composite. The cost differential alone makes this worthy of consideration and development.Situations that would require sulphate resisting cement may not warrant this choice with grc formulations, but it may be prudent to include pulverized-fuel ash in the matrix instead of sand.If circumstances necessitate high alumina cement in the formulation, then consideration should be given to the possibility of using ‘E’ glass as reinforcement.When rapid demoulding is required in production, regulated set cement may be considered. This cement can produce a set 5 minutes after mixing. Consequently, purpose-built mixing and dispensing equipment would be required. This rapid setting gives a strength that may allow demoulding in just 15 minutes after mixing the matrix. The rapid early strength buildup is maintained at a constant level until the normal
hydration products produce strength gains that exceed this early strength level. When considering regulated set cement, the extra cost may offset the benefits unless large production outputs are envisaged. 5.3 SAND/FILLERS
5.3.1 SandSand is introduced to reduce shrinkage of the composite. A 25% cement replacement will provide a reduction of 50% (see Figure 5.2). Shrinkage is undesirable if restraint occurs via fixings or product shape. Sufficient stress can build up resulting in cracking when the shrinkage exceeds the tensile strength of the section in question.
