Use of Macro Plastic Fibres in Concrete

Concrete is very strong in compression, however, it has a very low tensile strength. To improve its tensile strength, reinforcing steel is often used in the concrete. Apart from traditional steel mesh and bars, various fibres, such as steel fibre, glass fibre, natural fibre and synthetic fibre, have also been used to improve the properties of concrete.

Steel fibres can greatly improve the tensile strength and the flexural strength of concrete due to their ability to absorb energy (Beglarigale and Yazici 2015) and control cracks (Buratti et al. 2013). Their electric (Dai et al. 2013), magnetic (Al-Mattarneh 2014) and heat (Sukontasukkul et al. 2010) conductivity properties make them suitable for some special applications, such as shielding electromagnetic interference. However, corrosion of steel fibres can be detrimental and lead to rapid deterioration of concrete structures (Soylev and Ozturan 2014). Glass fibres have excellent strengthening effect (Tassew and Lubell 2014) but poor alkali resistance (Sayyar et al. 2013). Natural fibres, such as wood (Torkaman et al. 2014), sisal (Silva et al. 2010), coconut (Ali and Chouw 2013), sugarcane bagasse (Alavez-Ramirez et al. 2012), palm (Abd Aziz et al. 2014), and vegetable fibres (Pacheco-Torgal and Jalali 2011), are cheap and easily available, but they have poor durability. Synthetic fibres can be made of polyolefins (Alberti et al. 2014), acrylic (Pereira-De-Oliveira et al. 2012), aramid (Vincent and Ozbakkaloglu 2013), and carbon (Chaves and Cunha 2014). They can prevent plastic shrinkage cracks in fresh concrete (Cao et al. 2014) and improve post-cracking behaviour of concrete (Pujadas et al. 2014b).

The schematic diagram in Fig. 2.2 shows the different failure modes associated with the fibre reinforced concrete (Zollo 1997). Fibre rupture (1), pull-out (2) and debonding of fibre from matrix (4) can effectively absorb and dissipate energy to stabilise crack propagation within concrete. Fibre bridging the cracks (3) reduces stress intensity at the crack tip. In addition, the fibre bridging can decrease crack width, which prevents water and contaminants from entering the concrete matrix to corrode reinforcing steel and degrade concrete. Fibre in the matrix (5) prevents the propagation of a crack tip. Consequently, minor cracks will be distributed in other locations of the matrix (6). Although every individual fibre makes a small contribution, the overall effect of three-dimensional reinforcement is cumulative (Zollo 1997). Therefore, the fibres can effectively control and arrest crack growth, hence preventing plastic and dry shrinkage cracks (Yoo et al. 2013), retaining integrity of concrete (Yoo et al. 2014), and altering the intrinsically brittle concrete matrix into a tougher material with enhanced crack resistance and ductility (Park 2011). In

Failure mechanisms in fibre reinforced concrete. 1 Fibre rupture; 2 Fibre pull-out; 3 Fibre bridging; 4 Fibre/matrix debonding; 5 Fibre preventing crack propagation; 6 Matrix cracking (Zollo 1997)

Fig. 2.2 Failure mechanisms in fibre reinforced concrete. 1 Fibre rupture; 2 Fibre pull-out; 3 Fibre bridging; 4 Fibre/matrix debonding; 5 Fibre preventing crack propagation; 6 Matrix cracking (Zollo 1997)

order to achieve considerable reinforcement, the fibres should have high tensile strength and Young’s modulus (Yin et al. 2013).

Plastic fibres are synthetic fibres, which can be in the form of micro plastic fibres or macro plastic fibres. The micro plastic fibres refer to the plastic fibres whose diameter ranges from 5 to 100 im and length is 5-30 mm (Nili and Afroughsabet 2010). These micro fibres can effectively control plastic shrinkage cracking (Soutsos et al. 2012), which is caused by shrinkage of fresh concrete during the first 24 h after placement due to excessive evaporation of bleed water (Guneyisi et al. 2014). However, they normally do not have obvious effects on the properties of hardened concrete, as reported by Pelisser et al. (2010) and Habib et al. (2013). It is noteworthy that some micro plastic fibres, such as nylon fibres and Polyvinyl Alcohol (PVA) fibres, can provide good thermal energy storage to concrete (Ozger et al. 2013), effectively controlling shrinkage of concrete (Song et al. 2005), and also significantly improving tensile strength and toughness of concrete (Spadea et al. 2015).

The macro plastic fibres normally have a length of 30-60 mm and cross section of 0.6-1 mm2 (Yin et al. 2015b). The macro plastic fibres are not only used to control plastic shrinkage cracks (Chavooshi and Madhoushi 2013), but also mostly used for controlling drying shrinkage cracks (Pujadas et al. 2014a). Another significant benefit is the post-cracking performance provided by the macro plastic fibres (Buratti et al. 2011). The macro plastic fibres now have become increasingly popular in the construction of concrete footpaths (Alani and Beckett 2013), precast elements (Peyvandi et al. 2013) and shotcrete mine tunnels (Kaufmann et al. 2013).

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