PSI - Issue 64

Shamsoon Fareed et al. / Procedia Structural Integrity 64 (2024) 1057–1064 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1060

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CFRP strips used to strengthen the slabs were 1.2 mm thick, and has 4300 N/mm 2 and 238000 N/mm 2 tensile strength and Young’s modulus of respectively. Sikadur 330 epoxy with a tensile strength of 30 N/mm 2 was used with a ratio of 1:3 of resin and hardener, for external bonding of CFRP stips to the slabs. It has a bond strength of 4 N/mm 2 , and a tensile Young’s modulus of 4500 N/mm 2 . The 3D solid linear elements having an identical mesh with dimensions of 39.4 mm × 39.4 mm × 37.5 mm were used for slabs, and for columns, a mesh size of 20 by 20 by 20 mm 3 was used, whereas, steel reinforcement was modeled using a truss element. Mesh size was 61 mm. The CFRP was modeled using 3D solid linear elements. Mesh size was kept as 26.6 mm × 30.96 mm (Figure 2). The FE model, discussed above, was then used for evaluating the effect of the parameters mentioned above. Layout of placement of CFRP strips from the edge of the column is shown in Figure 3.

Table 1: Material models for FE investigation. Material

Material Model

Concrete

Concrete Damage Plasticity

Steel CFRP

Metal Plasticity

Hashin

3. Results and Discussion Results are presented and discussed for validation of the FE model, load-deflection relationships, and load-carrying capacities in punching in the subsequent subsections. 3.1. Validation of the FE model As discussed in the preceding section, FE models were validated using the results obtained from experimental investigation reported by Fareed and Khan (2022). Punching capacities of the two-way slabs, determined experimentally and numerically, are compared and presented in Table 2. It can be seen that numerically predicted punching capacities compare well with the experimentally determined capacities.

Table 2: Experimentally and numerically investigated punching capacities of slabs.

′ (MPa)

Punching Capacity - Numerical , (kN))

Punching Capacity - Experimental , (kN) 67.0

Specimen

31.0 27.2 27.4 29.8

68.7 64.5

NAC RAC NAC RAC

Without CFRP

63.0 91.5 85.0

109.4

With CFRP

96.1

3.2. Load-displacement relationships The load-displacement relationships for strengthened NAC and RAC reinforced concrete slabs with different placements of CFRP strips from the column edge are shown in Figure 4. In this part of the investigation, thickness was kept constant at 75 mm and compressive strengths were obtained from respective experimental investigation. The deflection reported herein was noted at the center of the bottom part of the slab. It is quite notablethat for the case of RC-NAC specimens, the initial response in terms of stiffness of all specimens was found to be identical, irrespective of the use of CFRP strips, as during that region the specimens remained within the elastic region and the resistance was provided mostly by the concrete alone. However, the behavior differs significantly once the punching capacity of the concrete is exceeded and the initial linear region becomes non-linear, indicating that resistance is now provided by the composite action of the reinforced concrete slabs. Similar behavior was also found for the case of the RC-RAC specimens with the reduced linear elastic region in comparison with the NAC slabs. This can be explained by considering the resposnse of RC structural elements where stresses are carried out by concrete alone before cracking, and the use of recycled aggregates exhibited less stiff response due to their inherent weakness and reduced compressive strength. When comparing the load-