# Investigations on Interface Shear Fatigue of Semi-Precast Slabs with Lattice Girders
## 4 Fatigue Tests on Semi-Precast Slabs with Lattice Girders
### Test Results
#### 4.5.2 Influence of Interface Roughness  
During the test program, the interface roughness was varied by specimens in Series 0 and 1 with concrete strength of C25/30 and two rows of lattice girder. Therefore, the following interface conditions were investigated:  
* left as cast with bond breaker (very smooth, EG02)
* left as cast with a roughness depth of \(R_{\text{t}}\) = 0.24 mm determined by the sand-patch method (smooth, EG01)
* left as cast with an aspired roughness depth of \(R_{\text{t}}\approx\) 0.9 mm ('rough', EG03)
* left as cast with slightly roughening to \(R_{\text{t}}\approx\) 2.0 mm after casting (rough, EG09)  
The crack patterns and achieved number of load cycles of the four specimens are shown in Figure 16. For all investigated interface roughness, delamination of the interface could be determined. For Specimen EG02 with a very smooth surface, delamination already occurred during initial static loading. The crack formation along the interface for EG01 with a smooth surface appeared during the cyclic loading (\(N\) > 1.5 million load cycles). Both specimens failed due to shear in the interface. Since the specimens EG01 and EG02 were designed with a longer support overlay and additional stirrups along the ends of the slab, the unimpeded shear of the in situ concrete layer was additionally counteracted. For EG03 with an interface with a low roughness, the failure occurred due to delamination of the in situ concrete layer with considerable horizontal slip after a shear crack developed towards the support. Specimen EG09 with a rough interface failed in a combination of interface and shear but without complete delamination of the in situ concrete. Except for Specimen EG09 with a slightly roughened surface, all specimens exceeded \(N\) > 2.0 million load cycles. Although the applied fatigue load and the concrete properties of Specimen EG09 were similar to EG03, fatigue failure occurred after \(N\) = 9000 and \(N\) = 1,090,000 load cycles, respectively. After monotonic loading, no crack could be observed at the interface. The horizontal and vertical displacement of the interface occurred after \(N\) = 2000 load cycles and gradually increased until failure. Thus, the early failure of the rough interface can be evaluated as an aberration and needs further investigations. After exposure, the lattice girders showed fractures of the welded nodes in the area of the failure crack. By the strain measurements of the lattice girders, stress ranges of \(\Delta\sigma\) = 175 N/mm\({}^{2}\) could be determined at the welded nodes. Since the strain gauges were not placed in theimmediate area of the failure crack, the stress ranges in the fractured girder nodes might have been considerably higher. To compare the flexural behavior of the composite specimens, Figure 17a shows the load deflection curves of Specimens EG01-EG03 for the static initial loading and the first load cycle. Despite similar concrete properties, Specimen EG02 with a very smooth surface showed a lower stiffness with a deflection of 7 mm. Thus, separate load-bearing behavior of the precast slab and in situ concrete layer can be assumed. With increasing interface roughness, the inclination of the load-deflection curves increases for Specimens EG01 and EG03, which confirms an improvement of composite action. Figure 16: Influence of interface roughness: crack patterns of Specimens EG01, EG02, EG03, and EG09. Figure 17: Influence of interface roughness: load–deflection curve of Specimens EG01—EG03 (**a**), development of horizontal slip for Specimens EG02 and EG03 (**b**). A comparison of the development of horizontal interface displacement of Specimens EG02 and EG03 is shown in Figure 17b. The measured horizontal slip at the side of fatigue failure (black curve) shows interface slip after the static initial loading of 1.5 mm for EG02 and 0.7 mm for EG03. During the fatigue loading, the interface slip remained mainly constant for EG02 and increased gradually after increasing the load range.