# Pet Fiber Reinforced Wet-Mix Shotcrete with Walnut Shell as Replaced Aggregate
## 3 Experimental Analysis
### Results of the Second Series of Tests  
#### 3.4.1 Results of Compressive and Splitting Tensile Strength  
The charts in Figure 12 show that concrete reinforced with different fibers gave the most satisfactory results for the splitting tensile strength of the concrete mixtures. With mixing fibers, splitting tensile strength of casting concrete (CSS) and shotcrete (SSS) achieved about 40% improvement compared with blank specimens. The range of tensile strength of mixtures with fiber was from 3.5 MPa to 4.3 MPa. This was a desirable result that concrete with more ductile behavior can be obtained by mixing PET or PP fibers.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Length** & **Cement** & **SF** & **Water** & **Sand** & **CA (kg/m3)** & **AEA** & **Fiber** \\  & **(mm)** & **(kg/m3)** & **(kg/m3)** & **(kg/m3)** & **(kg/m3)** & **Gravel** & **Walnut** & **(kg/m3)** & **(kg/m3)** \\ \hline Blank & - & 410 & 40 & 198 & 908 & 432.3 & 167 & 0.067 & \\ PP & 30 & 410 & 40 & 198 & 908 & 432.3 & 167 & 0.067 & 0.9 \\ PET & 30 & 410 & 40 & 198 & 908 & 432.3 & 167 & 0.067 & 6.5 \\ PET + PP & 30 + 15 & 410 & 4 However, the influence of fiber, PET or PP, on the tensile strength seemed to be inverse to that of the compressive strength of the samples. For example, single adding fiber reduced the compressive strength of casting concrete (CCS) and shotcrete (SCS) such as specimens of PP and PET shown in Figure 13. It might be because during concrete curing the high water absorption of walnut shell as aggregate replacement changed the water-binder ratio of concrete mixture that acted a key controlling factor in compression. Moreover, the poor bond between cement pastes with fibers may lead to the reduction in compressive strength. The other reason reported was that the hollow side of shell was difficult to fill with cement paste whereas the amount of cement paste in concrete is limited [11]. Furthermore, the compressive and splitting tensile strength of shotcrete mixture seemed to be higher than that of casting concrete, which may be caused by the compacted effect of spraying force [68]. Wherein the specimens (PP + PET) combined with two types of fibers had the interesting results of maximal value in terms of compression and splitting tensile strength in casting concrete or shotcrete.  
In the early tests, PP fiber with length of 30 mm mixed in concrete was prone to be twined together, which may cause strength loss, especially for compression. Thus, 15 mm length fiber was next mixed into mixture compounded with PET fiber (PET + PP). From the analysis, it was possible to assert that tests had confirmed the better behavior of multi-dimensions fiber than the single-mixture one and, in addition, the advantage of enhancing splitting tensile strength was much better than compressive strength after mixing fibers. The results that compressive strength became poor with mixing PP fiber concrete but splitting tensile strength improved are repeatedly reported in many studies [69; 70; 71], and multi-dimensions fiber reinforcement concrete can enhance the mechanical properties due to compounding effect of different fibers [39; 42; 69].