Degradation of Plastic Fibres in Concrete
PP has a high resistance to chemical attack due to its non-polar nature (Ha and Kim 2012). For example, PP is resistant to alcohol, organic acids, esters and ketones,
Fig. 2.8 Various types of plastic fibres for pull-out tests (Oh et al. 2007)
inorganic acids and alkalis. However, it swells when exposed to aliphatic and aromatic hydrocarbons and by halogenated hydrocarbons (da Costa et al. 2007).
Brown et al. (2002) studied long-term properties of virgin PP fibres in the concrete under a reactive environment. When PP fibres were exposed to an ionic environment of sodium and chloride ions created by salt water at different temperatures of 71 °C and -7 °C for six months, the tensile properties of the PP fibres remained unchanged. Roque et al. (2009) immersed PP fibre reinforced concrete in simulated saltwater conditions for 33 months, and found that rate of stiffness reduction was only 2.34%, which was much lower than those of steel fibre (14.0%) and polyvinyl alcohol (PVA) (59.9%) reinforced concrete. It was concluded that PP has the best durability for non-structural applications in the saltwater environment. Elasto Plastic Concrete (EPC) company (EPC 2012) did advanced alkalinity testing for their product polyolefin fibre. The fibres were subjected to an alkaline solution, which simulates a concrete environment. They reported that their polyolefin fibre could last up to 100 years in an alkaline environment without any decrease of strength.
The polyolefin fibres, including PP and HDPE, show high resistance to alkaline environment, while there is no agreement about the durability of PET fibres in Portland cement matrix. The PET fibres belong to the polyester group, and polyester fibres degrade when embedded in Portland cement matrix (Alani and Beckett 2013; Won et al. 2010). The degradation tests from EPC company showed that the PET fibre only could perform well for 10 years in the concrete, after that the strength of fibre decreased significantly (EPC 2012). However, Ochi et al. (2007) and the ACI 544 (Daniel et al. 2002) reported good alkali resistance of PET fibres in mortar and concrete. Moreover, Won et al. (2010) reported recycled PET fibres and recycled PET fibre reinforced concrete are highly resistant to salt, CaCl2, and sodium sulphate, and have no significant difference in chloride permeability and repeated freeze-thaw tests compared to plain concrete.
Ochi et al. (2007) immersed recycled PET fibre into an alkaline solution, which was prepared by dissolving 10 g of sodium hydroxide in 1 dm3 of distilled water, for 120 h at 60 °C. The results showed that the tensile strength of PET fibre after immersion was 99% of that before immersion, showing minimal deterioration. Fraternali et al. (2013) did the same test on the recycled PET fibre obtained by mechanically cutting post-consumer bottles, and found that the tensile strength of the PET after alkali attack was 87% of that before attack. Therefore, the recycled PET fibre was considered to have sufficient alkali resistance in both their studies.
Silva et al. (2005) immersed recycled PET fibres in a Lawrence solution (0.48 g/l Ca(OH)2 + 3.45 g/l KOH + 0.88 g/l NaOH, pH = 12.9), which simulates a fully hydrated cement paste. Through micrographs they found that surface of the recycled PET fibres became rough after being immersed for 150 days at 50 °C. Toughness of the PET fibre reinforced concrete decreased with the age due to the degradation of PET fibres inside the concrete. Fraternali et al. (2014) submerged recycled PET fibre reinforced concrete in the Italian Salerno harbour seawater for a period of 12 months. Through the CTOD tests, the energy absorption in the heavily cracked regime (CTOD 0.6-3 mm) was found significantly decreased by 52.1%.