# Pet Fiber Reinforced Wet-Mix Shotcrete with Walnut Shell as Replaced Aggregate
## 1 Introduction
The application of wet-mix shotcrete is prevalent in support in China because dry-mix shotcrete, producing a great deal of dust during processing, is forbidden in Chinese mine supports. Therefore, in this study, crushed walnut shells were selected as partial coarse aggregate substitute for wet-shotcrete. Tests of compression, splitting, slump, pressure drop along pipes, rebound rate and build-up thickness with different walnut shell amounts were preliminarily tested to ensure the feasibility of walnut shell as coarse aggregate replacement in shotcrete, while simultaneously determining the optimal mixture dosage. Additionally, considering that plastic shrinkage cracks may occur especially in hostile environments underground that cannot normally reach the standard requirement for curing shotcrete, which may impair the durability and serviceability of shotcrete structures, fibers were added in shotcrete to prevent such cracks. Presently, instead of traditional fibers such as steel fiber, carbon, glass and polypropylene, many studies use waste plastic fibers to reinforce concrete based on the environmental protection concept [27; 28; 29; 30; 31; 32; 33; 34]. As well known, fibers act as micro crack arrester as well as substantial energy absorption in cement composites; thus, in this study, the fibers obtained from waste polyethylene terephthalalate (PET) bottles were applied to reinforce the lightweight wet-mix shotcrete to reduce the cost of shotcrete and somewhat solve waste disposal problem. PET fiber has been applied to construction buildings such as spraying support in expressway tunnels, as reported by [PERSON] [35]; moreover, a method was described for producing PET fiber from waste PET bottles and the wetting tension of PET was measured. Mechanical properties of concrete containing PET fiber of 0.5%, 1.0% and 1.5% fraction volume were measured, including splitting tensile strength and compressive strength [36]. The results indicated that concrete mixed with proper PET fiber can enhance its performance, as reported in [37]. On the contrary, [PERSON] et al. [38] revealed that recycled PET fiber-reinforced specimens showed compressive strength decreases of 1%-9% compared to plain concrete, while the elastic modulus of concrete with PET fiber slightly decreased with the increase of fiber volume fraction. Additionally, [PERSON] et al. [39] also indicated that the addition of PET fibers decreased the compressive strength and splitting tensile of concrete specimens. [PERSON] and [PERSON] [40] investigated the rupture strength and impact resistance of precast concrete panels with different volumes PET fiber from cutting the plastic beverage bottles by hand. The results showed that the percentage of increase of rupture strength and impact resistance was 34.27% and 157.14%, respectively, for reinforced concrete compared with plain panels. The influence of PET fibers geometry on the properties of concrete was investigated, [PERSON] and [PERSON] [41; 42] showed that fiber geometry has a small effect on the workability of concrete but a significant contribution to the mechanical properties of concrete. There is a great deal of active research on the mechanical properties of concrete with PET fiber, such as splitting tensile strength, compressive strength, rupture strength and elastic modulus and so on, however, research on shotcrete mixed with PET fiber is scarce, especially on the aspect of pumpability and shootability. To author's knowledge, literature [35] only indicated PET fiber content in concrete can be freely changed in situ without fiber-ball formation and pipe clogging but no describing about shootability of PET fiber shotcrete with walnut shell. The aim of this study is to develop a type of lightweight wet-mix shotcrete with sufficient properties to meet the standards of mine supporting construction by adding PET fiber and walnut shell replacing natural coarse aggregates. Tests were divided two parts.