SYNTHETIC FIBERS USED IN CIVIL ENGINEERING BASICS AND TUTORIALS

SYNTHETIC FIBERS USED IN CIVIL ENGINEERING BASIC INFORMATION
What Are Synthetic Fibers?


The fi rst synthetic fi bre was a polyamide, generally known as nylon, which started to be marketed in 1938 and found applications in wartime technical uses. Its tenacity was in the region of 0.5 N/tex and the modulus 2.5 N/tex.

Another synthetic fibre that followed was the polyester fi bre (polyethylene terephthalate), which had a similar tenacity but a higher modulus of around 10 N/tex. Varieties of these fibres developed for industrial \ applications, such as ropes and tyre cords, exhibit tenacities over 0.8 N/tex for both nylon and polyester and moduli of 9 N/ tex for nylon and 12 N/tex for polyester.

Another fi bre of increasing interest in technical applications is polypropylene. Its tenacity is in the region of 0.65 N/tex with a modulus of 7.1 N/tex and a specifi c gravity of 0.91.

Nylons, but especially polypropylene and polyester, are widely used in geosynthetics, and polyester is very much used for tensile surface structures, particularly in Europe. Other applications of polypropylene include anti-crack building products for wall surfaces.

High-performance fi bres were developed in the latter part of the twentieth century and inhibit a step change in strength and stiffness, compared to previous ones.

Advances in the 1960s led DuPont scientists in the USA to spin para-aramid fi bres from liquid-crystal solutions. These highly oriented fi bres achieved tenacities in excess of 2 N/tex and moduli reaching 80 N/tex. More-recent developments enabled some polymer fi bres to attain tenacities above 3.5 N/tex and moduli exceeding 150 N/tex.

Other interesting characteristics of para-aramids include fl ame resistance (selfextinguishing), low electrical conductivity, high chemical resistance, excellent dimensional stability, low thermal shrinkage, high toughness (work of rupture) and high cut resistance. Apart from their use in the reinforcement of composite materials, para-aramids are also of interest for ropes and cables and as a replacement for asbestos materials.

The first high-strength carbon fi bres were developed at the Royal Aircraft Establishment in the UK, and were produced by high-temperature processing of acrylic fi bres under tension. This resulted in tenacities of up to 3 N/tex and moduli of over 400 N/tex.

Carbon fibre is engineered for strength and stiffness, but variations differ in electrical conductivity, thermal, and chemical properties. The primary factors governing the physical properties are the degree of carbonization, the orientation of the layered carbon planes, and the degree of crystallization.

They have lower thermal expansion coeffi cients than both the glass and aramid fi bres, and the material has a very high fatigue and creep resistance. Much research is also now being done using carbon fi bre-reinforced plastic (CFRP) as internal reinforcement in concrete structures, such as beams and bridge decks.

The material has many advantages over conventional steel, especially in that it is much stiffer and corrosion resistant. CFRP has become of prominent importance in structural engineering, due to its cost-effectiveness in the repair of old bridges, many of which were designed to tolerate far lower service loads than they are subject to today.

Reinforcing with CFRP is a much cheaper alternative when compared with the cost of replacing the bridge. Due to the very high stiffness of CFRP, it can be used underneath spans to help prevent excessive deflections, or wrapped around beams to limit shear stresses.

Glass fi bres are used widely as reinforcements in order to increase the stiffness of composite materials. The tenacity of glass fibres may reach 1.6 N/tex and their moduli 35 N/tex, which are lower than those of aramids (on a weight basis).

The development of ceramic fi bres is primarily related to their high-temperature performance in metal and ceramic matrix composites to use in engines. However, because of their structure they exhibit high moduli, which may reach 100 N/tex, with tenacities of up to 1 N/tex, which may not be considered particularly high (on a weight basis).

Glass fi bres are divided into three classes, i.e. E-glass, S-glass and C-glass. The E-glass is designated for electrical use, and the S-glass for high strength. The C-glass is for high corrosion resistance, and it is not of common use in civil engineering applications. Of the three fibres, the E-glass is the most common reinforcement material used in civil engineering structures.

Glass fibres are also used for fi re-protective walls, fl oors and ceiling panels, as well as fi reproof curtains and partitions for indoors and outdoors. They are used for heat insulation in heating systems, power generation and incinerators.

Basalt fibres have similar applications to carbon and glass fi bres in the reinforcement of composite materials, having better physical-mechanical properties than glass and being much cheaper than carbon fibre. Basalt fibre products include: chemically resistant chopped strands for concrete reinforcement; high-strength rovings for pultruded load-bearing parts and concrete reinforcing bars; basalt woven fabrics for heat, sound insulation, and fire protection; geogrids for road and land reinforcement; and stucco nets for wall reinforcing and renovation.

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