A holistic approach into the impact of sodium hypochlorite on polypropylene fibre reinforced concrete

The absence of available site data on the durability and performance of marine concrete, with polypropylene fibre inclusions was a fundamental issue justifying this study. At a sea defense study site on the Fylde coast in the northwest of England, dairy hypochlorite is used to eradicate biofouling, from municipal steps and walkways. This paper investigates the use of sodium hypochlorite (NaClO) and describes the effects of this destructive chemical upon the innovative use of polypropylene fibres within a marine concrete matrix. Polypropylene fibres were treated with NaClO (2.5%) and an artificial seawater (NaCl 3.5%) replicating a 6 month maintenance regime at the site. Modifications in visual, chemical and structural properties of the polymer were evaluated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX). Analysis of ATR-FTIR spectra of the untreated and treated samples revealed the appearance of new bands in the spectra of treated samples, which were attributable to the formation of carbonyl bonds (CO). This indicated the possible oxidation of the aliphatic main chain of the polypropylene fibers by exposure to NaClO. SEM observations confirmed fibre degradation, the fibre deformed, fractured and developed deep elongated pits and grooves. As a result, the visible, structural and chemical properties of the fibre changed when exposed to a minimal NaClO solution. There is an accumulative awareness of the significance of a holistic approach towards concrete durability, study and site practice. A holistic model is presented, detailing the transportation and degradation process of NaClO upon micro synthetic fibres. Microscopic investigation of study site concrete from revetment armor units was used to validate the model. This multidisciplinary study is the first to report the composition, attachment and growth of NaClO crystals upon synthetic fibres within marine concrete, laying the foundation for more research into this new phenomenon.