# Pet Fiber Reinforced Wet-Mix Shotcrete with Walnut Shell as Replaced Aggregate
## 1 Introduction  
The support technology of bolt steel mesh shotcrete is widely used in modern mine roadways [1; 2]. Therein, shotcrete, also called sprayed concrete, is a special cement-based mixture that is designed and projected pneumatically at a high velocity towards a target surface [3; 4]. Further, it is necessary that shotcrete be compacted by its own momentum, especially when sprayed on the roof [5; 6]. Otherwise, miners working under the roof sprayed with concrete would be not safe, being threaten by falling concrete bulk due to the huge self-weight of shotcrete or low adhesion stress between matrix and concrete. Therefore, in terms of the weight of concrete, lightweight shotcrete should be developedand used to reduce the debond of shotcrete from matrix and also avoid casualties resulted from dropped shotcrete. Lightweight shotcrete has been developed for many years, and made by mixing air entraining agent or lightweight aggregates [7; 8]. Because excavating aggregates from natural sources such as quarries and river bed have resulted in severe environment problems including pollution of air, soil and water and breaking of the ecosystem [9; 10], the development more optimal eco-friendly concrete was proposed; therefore, studies on waste materials and industrial byproducts, such as scoria, pumice, oil palm kernel shell, plastic particles and recycled aggregates, as a kind of coarse or fine aggregate, are being widely researched [11; 12; 13; 14]. Attempts have been made by various researchers to decrease the density of concrete without affecting the strength. For example, [PERSON] et al. [15; 16] investigated the recycling possibilities for crushed oyster shells in order to reduce coarse and fine aggregate usage, and results showed that the strength of concrete with 10% oyster shells replacement was almost equal to that of normal concrete. One recent research explored the use of lightweight fiber-cement composite mixed with rice stalk fiber (as reinforcement) and rice husk ash (as cement replacement) that are agricultural byproducts, test results indicated cement boards made with rice stalk fiber had superior properties compared to those mixed with hardwood fibers [17]. Some researchers investigated the performance of lightweight concrete with oil palm kernel shell as lightweight aggregate and found that a compressive strength between 13 and 30 MPa can be achieved [18]. According to those studies, experiments are needed to solve the problem of how to increase the substitution ratio in shotcrete mixtures to make sure the final performance of the material. Nutshell is a wood byproduct obtained during the extraction of seeds, such as walnut, which is abundantly available in food processing factories in China. According to previous research, walnut shells were mixed into a protective floor topping composition for improving the wear resistance of floors which were subjected to heavy compressive loads [19; 20; 21]. In addition, a new type of cement composition was reported in the literature [22], which mainly consisting of two materials, wherein the first material of particles included at least one of ground, crushed fruit pits or processed wood. Walnut shells were also used for processing composite particleboard to improve its requirements such as water resistance features by mixing fly ash and the optimal results indicated that maximum bending strength can reach up to 3.8 N/mm\({}^{2}\)[23]. [PERSON] et al. [24] evaluated the feasibility of almond shell as substitute to producing wood-plastic composites and indicated that almond shell as agro-waste material was a recyclable natural resource for composite industry. Due to the high hardness of walnut shell, walnut shell was treated as an abrasive that is applied to surface preparation on all types of cementitious surfaces including cast-in-place concrete floors and walls, masonry walls, and shotcrete surfaces [25; 26]. Currently, there are no published reports describing the use of walnut shell as lightweight coarse aggregate in shotcrete.