The biggest challenges of traditional cementitious compositesare low ductility and high fracture risk which have brought many limitations in their structural applications [1, 2]. A variety of researches have been carried outto investigate the performance and advantages of fiber reinforced compositesin the last few decades. The commonly used fibers in cementitious composites include steel, glass, polyethylene, polypropylene, polyvinyl alcohol, polyester, aramid and natural plant [3, 4, 5, 6, 7, 8, 9, 10], among which polyvinyl alcohol (PVA) fiberis one of the most popular and widely studied fibers in research to improve ductility and reduce the propagation rate of cracks [11]. The mechanism of PVA fiber is mainly to reduce stress concentrations on internal defects of cementitious composites leading to an increase in toughness and ductility [12]. Yu et al. studied the tensile behavior of PVA reinforced concrete. Their concretes had enhanced tensile stress and strain capacities because PVA fiber increased toughness [13]. Cadoni et al. found that PVA fiber addition noticeably reduced fracture energy and improved tensile strength and tensile strain capacity [14] using a dynamic tensile test. Haskett et al. concluded that after adding PVA Fibers, the crack amount in compression zone of concrete was effectively reduced [15]. Even after cracking, PVA fiber reinforced composites exhibited strain hardening behavior due to the bridging effect of fibers [16]. Ling et al. conducted an investigation on flexural bending behavior of PVA fiber reinforced cementitious composites. Their observation showed that bending strength and toughness were significantly increased by adding PVA fiber up to 1.5% volume content [17]. Many researchers have confirmed that fiber content greatly affects bending resistance of cementitious composite. Atahan et al. addressed that fiber volume content between 0.5% and 2.0% considerably improved flexural behavior of cementitious composites [18]. High amount of PVA fiber increased bending resistanceand durability of composites [19, 20].
However, a large number of studies have pointed that PVA fibers can be easily pulled out under bending due to low bond strength between matrix and fibers which weakens its bridging effect. Especially for the cementitious composites containing fly ash, low reaction rate of fly ash hurt the interfacial bond strength between cementitious matrix and PVA fibers [21]. The failure mode of fibers under bending load consists of pullout with slight abrasion, surface abrasion and a partially peeled end [22]. Similar occurrence of pullout PVA fibers was also found in fly ash geopolymer composites [23]. Such reduction in bond strength is also related to the increased porosity of composites induced by PVA fibers [23, 24]. Therefore, it is critical to find a solution for reducing porosity and improving bond strength between cementitious matrix and PVA fibers in order to enhance the bridging effect of PVA fibers in cementitious composite.
The use of nanoparticles in the concrete has been increasing in decades. Previous study results indicated that nano-SiO2 (NS) could significantly enhance strength and durability of concrete as an additive as a cementitious material due to their effects on hydration acceleration and microstructure evolution [25, 26, 27]. This improvement in concrete performance is not only attributed to its filler effect and its pozzolanic reaction, but also related to its larger surface area, which speeds up the rate of cement hydration and pozzolanic reactions [28]. Gonzalez et al. added NS to increase the compressive strength and durability of concrete [29]. A study from Zhang et al. found that 2% nanoparticles in cementitious composite noticeably promoted compressive and bending strength [30] which was associated with the filling effectin voids and between unreacted particles to decrease total porosity of the system [31, 32]. Sikora et al. used nano-Fe3O4 in cementitious composites to greatly improve the microstructure of cementitious composites was and decrease the porosity, thus increasing density of the composites [33]. Li et al. put forward that the addition of nanoparticles significantly promoted bending resistance, microstructure and ductility of cementitious composites [34]. In addition, NS has been found that it can substantially enhance the bond strength in cementitious composites due to the nanometer effects [35, 36, 37, 38]. Therefore, understanding the effects of nanoparticles on bending resistance of PVA fiber reinforced composite is important to evaluate bond strength between cementitious matrix and PVA fibers. Relative studies on the bending resistance of such composites containing NS are very limited.
Although many researches have been conducted on the bending resistance of PVA fiber reinforced cementitious composites or NS reinforced cementitious composites, the studies on the bending resistance of cementitious composites simultaneously added with PVA fiber and NS were still in lack. In present study, NS were adopted to manufacture the PVA reinforced cementitious composites. A bending test was performed on the composites containing NS with various PVA fiber contents. The effects of varied PVA fiber contents and NS addition on bending properties of cementitious composites were investigated. The recommended dosages of PVA fiber and NS particles in cementitious composite were also provided based on the experimental results.