## 3 Results and Discussion
### Effects of Rib Spacing  
During testing, three failure modes were observed. Specimens failed in the rebar shank, failed at the grout-rebar interface (pullout) and failed due to the pipe crushing (a result of pipe properties and grouting methodology). The majority of specimens reached rebar shank failure, as was the intention of the study. Specim B-1-1 and D-1 failed by pullout. However, for specimen B-1-1 the large grout void was likely a factor in this. The failure of D-1 was also not indicative of the effects of the independent variables, as it experienced failure at the highest load of all specimens in grout annulus D (22.8 mm). This is likely due to a reduction in the area of the bolt during the rib removal process. As more ribs were removed, the reference of the original cross-section became further away and thus visually harder to match. The amount of diameter reduction to achieve the lowest value of the maximum applied load is approximately 2%; given the methodology employed, this is a reasonable explanation. Another global parameter which is often investigated, as it is important to design and use, is the stiffness. In agreement with other conducted research on smaller embedment lengths, rib spacing was found to be a negative predictor of initial stiffness [7]. Across all four selected grout annuli, it was seen that with increased rib spacing a decrease in stiffness was generally observed. An example from grout annulus A (7.7 mm) can be seen in Figure 10. This effect was strongest in annulus B (9.9 mm). However, in the largest annuli, C and D (14 mm and 22.8 mm), this was less pronounced, except in specimen C-1 which was very stiff when compared to the others. This figure also highlights the general  
Figure 9: Ribbed vs. smooth bar performance, highlighting the effects of ribs on load transfer mechanism. (**A**) Load displacement plot of B-1-i, (**B**) load displacement plot of B-6, (**C**) strain on the pipe of B-1-i, (**D**) strain on the pipe of B-6. behaviours of the specimens, with most experiencing elastic loading, a brief plastic phase, strain hardening, then rupture. The testing and loading for specimens A1-A5 were ended prematurely due to the confining pipe yielding at the grout injection hole. Specim A6 begin sliding prior to the yielding of the pipe. Specimens that began to slide generally exhibited a sinusoidal strain-softening phase which eventually reached a residual state. An example of a specimen reaching this residual state can be seen in Figure 9. It is believed that this sinusoidal pattern is a function of the deformation-based control of the MTS. Specim A7 went from elastic loading to grout failure and then sliding. Multilinear regression was conducted to afford additional insights into the qualitative observation. No outliers were found, but a slight degree of non-linearity was observed. All assumptions of independence of observation, homoscedasticity, normality and additivity were found to be met, except homoscedasticity. To address this, a robust regression was run (bootstrapping, 5000 bootstraps and 95% confidence, JASP defaults) [65]. The multiple regression model, Equation (7), statistically significantly predicts initial tangent stiffness F (2,17) = 33.002 \(p<0.001\), adj. R2 = 0.77. Coefficients can be found in Table 6. This again showed that rib spacing negatively predicts initial tangent stiffness. Both the average unconfined compress strength (UCS) of the grout and rib spacing were statistically significant. Secant stiffness at peak loading did not have a clear trend in relation to rib spacing, but appeared to be related to the maximum applied load. However, this is likely a product of the rib removal process and the effects of reduced area. \[ \begin{array}{c}Y(Predicted\ initial\ tangent\ stiffness\ (kN/mm))\\ =-456.66-0.73(rib\ spacing\ (mm))+14.12(UCS(MPa))\end{array}  \tag{7}\]  
Similar to the exploration of the effects of ribs, the strain attenuation patterns can be compared across the various grout annuli.