Interfacial Slip

Interfacial slip data were recorded at five locations along the longitudinal axis of the beams, labelled as Slip 1 through 5 and located at x = 0, 500, 1000, 2000, and 3000 mm from the west support. The data are presented for all specimens in Appendix E2 in the form of load-slip and interfacial slip profiles plots. Representative results are shown for Specimen P140 in Figures 4-5 and Figure 4-6. In both figures, the 0%, 100%, and 1000% static tests are represented by the dashed red, blue, and green lines, respectively. The sign convention adopted for the load-slip and interfacial slip plots is positive when the slab slips to the west, and negative when the slab slips to the east, relative to the steel flange.

The initial static data illustrated in Figure 4-5 displays two types of nonlinear behaviour. The first is clearly demonstrated by Slip 1, Slip 2 and Slip 5 with trends that are negative in curvature, or concave down. The second is shown in Slip 4 and Slip 5 which demonstrate a tri-linear and bi- linear trend, respectively. The discontinuities likely occur due to the chemical bond breaking between the concrete and steel at the interface, resulting in a change in stiffness. From Slip 4 and Slip 5, it is evident that the load at which the breaks was approximately 85 kN.

Once the specimens experienced fatigue loading, changes in the load-lip behaviour became pronounced. Slip 1, Slip 2, and Slip 5 all demonstrated a trend that was positive in curvature (concave up) and resulted in interfacial slip values that were larger in magnitude compared to the initial static behavior (i.e., at 0%). It is hypothesized that the change in curvature is associated with concrete damage around the weld of the stud due to fatigue loading, allowing for more slip and resulting in an initial reduction in stiffness followed by stiffening once the concrete bearing on the stud is re-engaged.

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Similar trends in the load-slip data are exhibited by the other test specimens, with Slip 1 experiencing the largest amount of slip. Since the longitudinal shear force was three times larger at the west support than the rest of the beam, Slip 1 saw the most dramatic changes in slip due to the fatigue damage. The recorded Slip 3 slip values were approximately zero which corresponds to the correct behaviour expected at the location of maximum moment. The 100% static tests results were similar to the initial results, however, once the specimens experienced significant fatigue damage (i.e., at 1000% of the predicted fatigue life), the slip increased noticeably. The direction in which Slip 3 began to slip as the number of load cycles increased was variable and not consistent throughout the specimen data. Both Slip 4 and Slip 5 appear to be good indicators of the force required to break the chemical bond between the concrete and the steel at the interface.

The interfacial slip profile for P140, plotted in Figure 4-6, represents the slip values recorded at the peak static load of 200 kN. Table 4-4 summarizes the results for Slip 1 (x = 0, west end of beam) for all of the specimens at the 0%, 100%, and 1000% tests, and also includes the slip increase compared to the 0% slip value. The interfacial slip profiles for the other specimens are found in Appendix E2, which also includes the summary tables for each slip location, similar to Table 4-4.

When considering the 0% slip values in Table 4-4, the average values for the CIP and precast specimens are 0.136 mm and 0.135 mm, respectively. A statistical Student’s t-test was performed on the CIP and precast data set to determine whether the two are likely to have come from the same two underlying populations containing the same mean Specifically, the test provides the likelihood that there is no difference between the initial CIP and precast Slip 1 behaviour. The calculation was completed assuming a two-tailed distribution and homoscedasticity, resulting in a 97% probability.

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(a) x = 0 (b) x = 500 mm

(c) x = 1000 mm (d) x = 2000 mm

(e) x = 3000 mm

Figure 4-5: Load-slip plots for (a) Slip 1 through (e) Slip 5 for Specimen P140. Note: positive

when the slab slips to the west, and negative when the slab slips to the east, relative to the steel flange.

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Figure 4-6: Interfacial slip profile for Specimen P140 at an applied load of P = 200 kN. Note:

positive when the slab slips to the west, and negative when the slab slips to the east, relative to the steel flange.

Table 4-4: West End Slip 1 Results (P = 200 kN).

Specimen 0% (mm) 100% (mm) 100% Change relative to 0% 1000% (mm) 1000% Change relative to 0%

C067 0.153 0.163 7% - - P067 0.155 0.133 -14% - - C100 0.167 0.172 3% 0.250 50% P100 0.153 0.141 -8% 0.194 26% C120 0.072 0.096 33% 0.158 119% P120 0.131 0.141 8% 0.220 68% C140 0.158 0.183 16% 0.262 66% P140 0.137 0.140 2% 0.197 44% C200 0.108 0.181 67% 0.296 173% P200 0.126 0.157 24% 0.289 129% C300 0.158 0.289 83% 0.452 186% P300 0.110 0.204 85% 0.744 575% CIP Average 0.136 Precast Average 0.135

When comparing the slip data at the 1000% fatigue life, the precast specimens P100, P120, P140, and P200 all showed a smaller increase in slip (relative to the slip at 0%) compared to their CIP equivalents. The increase in slip is associated with the decrease in shear interaction (i.e., longitudinal shear rigidity) due to fatigue cracking. This suggests that the studs in the precast specimens experienced less fatigue damage from the cyclic loading compared to the studs in the CIP specimens, resulting in longer fatigue lives. Additional slip can be present due to damage of

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the concrete around the studs, particularly around the weld collar. At the weld collar, the stud behaves most rigid and demonstrates little deformation in comparison to the rest of the shank when resisting the longitudinal shear forces. Due to this rigidity, more force is attracted to the area that may result in strain-softened concrete when cycled.

A hypothesis regarding why the precast specimens demonstrated less slip and fatigue damage compared to the CIP specimens is due to concrete shrinkage. The concrete shrinkage effectively shifts the stress histories applied to the studs into cyclic compression, or cyclic tension, depending on the direction of the resisted longitudinal shear force from the applied load and concrete shrinkage. Fatigue cracking will occur on both sides of the stud if cyclic tensile stresses are present on each side as well. This will decrease the longitudinal stiffness of a stud more rapidly compared to a stud whose fatigue cracking propagates from one side alone. A more in-depth analysis and discussion on this effect is presented in Section 5.3.3.