PSI - Issue 64

Szymon Grzesiak et al. / Procedia Structural Integrity 64 (2024) 269–276 S. Grzesiak, C. de Sousa, M. Pahn / Structural Integrity Procedia 00 (2019) 000 – 000

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The segmentation of air pores inside the sample was achieved through binarization technique (assignment of 0 or 1), opting for an upper threshold. A process called “l abeling ” allows for the numbering of whole air pores and whole air structures such as cracks in the sample. Performed selection of air void structures focused only on cracks from whole labeled structures. This same process was applied for the steel reinforcement inside the concrete (this time using lower threshold option). Since the density of FRP strip is not significantly different from concrete density, and therefore difficult to segment, it was cropped out of the entire 3D data and put back in after segmentation. 3.3. Evaluation of the crack surface After the image analysis and processing, the data was visualized using a rendering tool. Three cross sections of concrete and visualization of CT Data is presented in Figure 4. In these images, some details of the crack and especially the concrete cover separation are clearly visible. Moreover, the area of the crack within the concrete volume could be estimated. The area of the projection in x and z direction is equal to 89.12 cm 2 and 50.95 cm 2 , respectively. In a simple axial loading situation, this surface area could be converted to fracture energy. Unfortunately, the geometrical configuration of the observed shear crack make it difficult to calculate this. Only the split of the fracture energy in mode I (G F ) and in mode II (G f ) allows a correct estimation. This requires further analysis and an in-situ study (Li and Zhao et al. (2023)).

Fig. 4. Cross sections of concrete with visualisation of CT Data.

3.4. Determination of the load-bearing capacity of the beam in the case of concrete cover separation During the bending tests, concrete cover separation failure occurred. At this point, the concrete cover separated at the end of the FRP strip due to the additional vertical offset between the stirrup reinforcement and the FRP strip. Subsequently, to analyze the concrete cover separation some approaches from literature like the German DAfStb guideline and fib guideline were applied (Formula 2, 3 and 4). In both guidelines, the formulas are similar, but have some differences. The distance of the FRP strip from the end support a f in DAfStb- guideline is expressed with a factor CaL, while in fib -guideline the factor CaL is equals 1.0. These formulas estimate a characteristic value. V Ed ≤V Rd,c,fe =k k ∙ (1+19.6 ∙ (100∙ρ s1 ) cρ a f CaL ) ∙ V Rd,c with ρ s1 = A s1 A c , k k =0.75 , CaL=1,0 and c ρ =0.15 (2) with value for the shear resistance for members not requiring shear reinforcement being (EN 1992-1-1): V Rd,c =( 0 γ .15 c ∙ k ∙ (100 ∙ ρ s1 ∙ f ck ) 1 3 +0.15 · σ cp ) ∙ d s1 ∙ b c with k=1+√ 2 d 00 s1 ≤2.0 and σ cp =0 (3)