Mechanical Properties of Polyvinyl Alcohol Fiber-Reinforced Cementitious Composites after High-Temperature Exposure

1. Introduction

Invented in the 1820s, cement has been studied for almost 200 years. Cementitious composites with cement as the primary material are extensively used in the construction of infrastructure, such as highways, bridges, dams, and houses [ 1 ]. They have become the most commonly used building material worldwide [ 2 ]. They possess the advantages of easy acquisition, high strength, convenient construction, and low cost, which make them popular in the engineering field. However, several hidden dangers are associated with the use of cementitious composites. While their compressive strength is excellent, their tensile performance is not satisfactory. The tensile strength of cementitious composite is only about 5–10% of its compressive strength. In addition, they possess low flexural strength, poor toughness, and weak deformation. Furthermore, they are heavy and prone to brittle failure without warning [ 3 ]. However, the actual failure of cementitious composite components and structures is not solely caused by external loads. Under the long-term action of natural environmental factors, the defects of cementitious composites are amplified, accelerating the aging, damage, and even failure of the components and structures, thus affecting their functionality and safety [ 4 ]. These issues considerably affect the maintenance cost and service life of infrastructure systems. Therefore, improving the properties of cementitious composites has become a focus in the engineering [ 5 6 ].

Fiber-reinforced cementitious composites (FRCCs) are composed of a hardened cement paste as the matrix mixed with non-continuous short fibers or continuous long fibers randomly distributed in the matrix as reinforcement materials. These composites overcome the deficiencies of traditional cementitious composites, namely, low tensile strength, poor toughness, high brittleness, and poor crack control. The properties of FRCCs were determined by numerous factors, such as strength of cement paste and type and content of fibers. Li [ 7 ] successfully proposed the design concept of engineered cementitious composites (ECCs) based on the principles of micromechanics in 1993. An ECC is a typical FRCC representative. Various fiber types (such as steel [ 8 ], carbon [ 9 ], polypropylene [ 10 ], glass [ 11 ], polyvinyl alcohol (PVA) [ 12 ], and basalt [ 9 ]) are used in the production of FRCCs. Compared to other fibers, PVA fibers have the advantages of good dispersibility, excellent hydrophilicity, high tensile strength, high elastic modulus, superior bonding properties with cementitious materials, and nontoxicity [ 13 ]. In addition, the good acid and alkali resistance of PVA fibers can ensure that the cement matrix is not easily eroded [ 14 ]. Therefore, the application of PVA fibers in FRCCs is promising.

Researchers from various countries have conducted numerous experiments to investigate the basic mechanical properties and durability of PVA-FRCCs [ 15 16 ]. Meng et al. [ 17 ] used PVA fibers to prepare a PVA fiber-reinforced ECC (PVA-ECC), which possessed good mechanical properties (For a typical PVA-ECC with fiber volume fraction of 2%, tensile strain capacity of 4% and ultimate strength of 4.5 MPa can be achieved) and met structural design requirements. Owing to its tight crack width and high tensile strength, the PVA-FRCC represents a new cementitious composite with great potential for effectively solving the durability problem of concrete structures [ 18 ]. In addition, the engineering performances of PVA-reinforced geopolymer composites [ 19 ] have been extensively studied. However, the applications of FRCCs are becoming more prevalent, owing to their excellent engineering properties, increasing their risk of exposure to high temperatures. For cementitious composites, high temperatures cause not only a change in appearance and mass, but also a reduction in mechanical properties [ 20 ]. The mechanical strength of cementitious composites is largely determined by the hydration products generated by the cementitious material and water. The free water in cementitious composites exposed to lower temperatures (100–200 °C) evaporates first. At 180–300 °C, a portion of the C–S–H gels is dehydrated. At 400–600 °C, calcium hydroxide undergoes dihydroxylation [ 21 ]. When the temperature is raised above 600 °C, calcium carbonate begins to decompose [ 22 ]. All of these changes decrease the mechanical strength of the cementitious composites. Moreover, melting of PVA fibers of FRCCs exposed to elevated temperatures results in the formation of pores and channels in the matrix, which further reduces the mechanical properties of FRCCs [ 12 ]. However, the presence of these pores and channels can release steam pressure and reduce the spalling of PVA-FRCCs [ 23 ].

Vejmelkova et al. [ 24 ] tested the physical performance and mechanical strength of cementitious composites containing hybrid PVA fibers at elevated temperatures. The results showed a significant increase in porosity and a reduction in tensile and flexural properties from room temperature to an elevated temperature of 600 °C. From the perspective of the microstructure, these changes were mainly caused by the decomposition of Ca(OH)and C–S–H gels at high temperatures. According to Tian et al. [ 25 ], the residual compressive and flexural strengths of cementitious composites containing 2% PVA fibers subjected to temperatures from 200 °C to 400, 600, and 800 °C decreased gradually. After exposure to 800 °C, the residual compressive strength and flexural strength of the specimens were only 37% and 17.3% of the original ones, respectively. The addition of PVA fibers accelerated the reduction rate of the flexural strength of cementitious composites at high temperatures [ 26 27 ]. However, high temperature had little effect on the mass loss rate of the specimens. When the temperature was increased from 0 °C to 800 °C, the mass loss rate of PVA-FRCC ranged from 13% to 15.8%. Furthermore, the thermal stability of mortar could be improved by the PVA fibers. The formation of a large number of pores, after the fibers were melted, reduced the probability of explosion spalling of the mortar at high temperatures [ 12 28 ]. Yu et al. [ 4 ] further investigated the impact of the cooling method on the performance of cementitious composites after exposure to elevated temperatures. They found that cooling with water was helpful for the recovery of the strength and stiffness of specimens.

Currently, most studies on PVA-FRCCs mainly focus on mechanical properties, durability, and microscopic mechanisms at room temperature [ 29 ]. However, fires have occurred frequently in recent years, and many researchers worldwide have conducted preliminary research on the high-temperature resistance of FRCCs [ 30 31 ]. There are only few relevant studies on the basic mechanical properties and damage mechanism of PVA-FRCCs exposed to high temperatures. Thus, it is necessary to further explore the high-temperature resistance of PVA-FRCCs.

This study analyzed the basic mechanical properties of PVA-FRCCs after high-temperature exposure and evaluates the influence of heating temperature, PVA fiber content, and cooling method on their mechanical properties.