The progressive substitution of asbestos in the European fibre-cement industry was initiated in the early eighties. Two main fibres successfully used for this application were wood pulp fibres and poly vinyl alcohol fibers. While the former were used as reinforcement of steam cured products for external (cladding) and internal (partitioning, ceiling) applications, a combination of both fiber types was used in air cured products mainly in roofing applications. Although many other fiber types such as polyacrylonitrile fibres (PAN), polyethylene fibres (PE), alkali resistant glass fibres (ARG), polypropylene (PP) nets and fibres, carbon fibres, (aromatic) polyamide fibres, etc. find occasional application in this industry, none of them were used to the same extent as wood pulp fibres and poly vinyl alcohol fibers. As asbestos substitution progressed globally, the need for cheap substitutes continued to increase, particularly for roofing applications.
Polypropylene fibres are synthetic fibres made from Polypropylene. The most important properties of polypropylene fibres are its low density and low softening temperature range and its hydrophobic nature. Major uses are in rugs and carpets both as face fibre and primary and secondary carpet backing and in non-woven fabrics for hygiene products where it�s very low absorbency and softness make it first choice for coverstock. It is also used in non-woven fabrics for medical uses, for geotextiles and for bulk storage bags. While ordinary PP fibers are extensively used in the field of crack prevention of concrete, they could not be used for the reinforcement of fiber-cement products as their properties were unsatisfactory.
Since PP is a relatively inexpensive polymer, worldwide available, and can be processed by classical melt spinning technologies, it remains an attractive raw material for replacement of asbestos fiber. Increasing the frictional and/or chemical interfacial bond quite effectively improves the performances of PP fiber-cement. The poor bonding of ordinary high tenacity PP fiber is due to low surface energy (hydrophobic character) and its low roughness.
The first company to use this technology for the production for fibers for cementitious composites was by Daiwabo of Japan which used fine fillers in the sheath. Improvement of interfacial bonding was due to the presence of functional groups which has ability to chemically bind with cement (coating and copolymers). An increased roughness of the surface was due to either filler or coating. Industrial production of fibre-cement roofing was initiated using some of these new fibres in different countries. The most important productions were made in Latin America (Peru, Colombia, Argentina, Chile).
External concrete gets subjected to the worst that nature has to offer and also varying degrees of harsh treatment from users. Whether it is commercial, industrial or residential hardstandings, external concrete will, over its design life, encounter natural destructive forces that lead to its early and costly deterioration. It has to withstand not only impact and abrasion resistance from use and the effects of weather in setting but it also has to resist potential damage caused by cycles of freezing and thawing.
Freeze/thaw attack after chloride-induced corrosion is the most common cause of concrete deterioration. One theory is that in freeze/thaw conditions, water molecules inside the concrete have no space to expand and contract as temperatures cool, creating tension and cracking on the surface, and allowing water and chemicals to penetrate. In turn, these molecules expand and contract, weakening the structure of the concrete and significantly reducing its lifespan. As part of a research, ADFIL Construction Fibres has devised a simple and effective method of combating freeze/thaw attack, while taking away all the variables associated with the use of AEAs. The addition of specially engineered monofilament polypropylene fibres such as Fibrin XT has, over the last two decades, allowed hundreds of contracts to successfully replace AEA. ADFIL fibres improve the early tensile strain capacity of the concrete after placing. By reducing crack frequency and size, they enable the concrete to retain more intrinsic strength, increasing its durability against the negative effects of the weather in placement, as well as freeze/thaw conditions in the hardened state. The fibre-entrained concrete will also give greater protection from joint edge deterioration and abrasion caused by the continuous impact of industrial machinery. The fibres intercept cracks during their propagation and will import a high degree of ductility into an otherwise brittle material. This will also lead to the improved impact resistance of the concrete. Scientific work by the University of Newcastle's civil engineering department supports these findings. In one of the most intensive studies of its kind, concrete cubes were subjected to 150 freeze/thaw cycles over a period of one year. Each cube was cast from concrete with a w/c ratio of 0.5 and a cement content of 350kg/m3. Polypropylene fibres of three differing lengths and types were added at a dosage of 0.91kg/m3 to the concrete cubes. One untreated cube was used as a control. A range of tests were then undertaken to study the effects that fibres had on the durability of the concrete, with the results showing 'a clear association between the use of polypropylene fibres in concrete and freeze/thaw protection, leading to subsequent enhanced durability'. Putting this into context, Kernan(5) suggested that as concrete is the most common material in modern construction, in the event that the use of monofilament fibres should become extensive, concrete would enjoy increased durability, resulting in lower environmental impact. Successful test programmes show that specialist monofilament fibres perform as well as, if not better than, AEA concrete. The CDF, RILEM test has been performed on several occasions and replicates the effects of freezing and thawing with the added problem for concrete - a salt solution is added. This test imitates de-icing methods used on some external concrete to melt ice. In practice, if ice does form on the surface of the concrete, salt should not be used retrospectively; but in reality, this is not the case. Applying salt to ice melts the ice by taking heat from the underlying concrete very rapidly. The latent heat of melting ice is massive and one or two cycles of rapid thawing can destroy a pavement surface. If vehicles enter an unsalted frozen pavement from a pre-salted highway, brine may fall from the vehicle onto the ice and damage the concrete. ADFIL fibres provide protection against frost attack by ensuring that the concrete surface remains bonded to the underlying body of concrete, so preventing the ingress of moisture and therefore the adverse effects of freezing.
Due to the growing awareness of the durability and cost benefits that monofilament polypropylene fibre can bring to concrete used in the external environment, the global demand for such concrete mixes continues to rise sharply, with significant research and testing programmes continuing to further develop the level of protection afforded to treated concrete and its uses. (By Kitchen, Andrew) Manufactured in the US by Forta Corporation, Masterfibre Econo-Net and Masterfibre Econo-Mono fibres are made from 100% homopolymer polypropylene, and are supplied in easy-to-use biodegradeable bags for fast and accurate dosing. The fibres can be added to the concrete mix at the batch plant or in the truck, with the mixing action distributing the fibres evenly throughout the concrete to produce an homogeneous mix and provide effective micro-reinforcement and crack control. Masterfibre Econo-Net incorporates a collated fibrillated fibre design that �opens up� during the mixing process to provide an effective �network� of secondary reinforcement. It can be used across a range of concrete construction applications, including slab-on-grade, overlays, toppings, kerbs, driveways, pavements and footpaths and is suitable for use in a variety of mixes, including architectural/coloured concretes, mortars and grouts. Meanwhile, Masterfibre Econo-Mono polypropylene fibres for concrete feature a monofilament design that incorporates individual fibres of a uniform length. Specifically developed to control plastic shrinkage cracking, the fibres help to reduce the formation of shrinkage cracks prior to the initial set, while also reducing settlement shrinkage.