# Structural Behavior of Large-Scale I-Beams with Combined Textile and CFRP Reinforcement
## 5 Discussion
### Compatibility of FRP Rebar and Grids  
Combinations of reinforcements with different material characteristics for strength, modulus of elasticity and bond are well-known from steel-reinforced concrete: Bonded prestressing strands and mild steel rebar differ substantially regarding ultimate strength, strain, and bond properties, while their modulus of elasticity is very similar. In principle, the relationships derived for steel reinforced concrete are transferable to mixed FRP reinforced sections, because FRP grids and FRP bars are of the same type (bundles of uniaxial continuous fibers with anisotropic behavior) and are subject to the same load-bearing principles. Yet, those reinforcements differ in one essential point from steel: Their lack of yield capacity and their brittle failure. In consequence, different bond and stiffness need to be considered not only in SLS (as it is usual for mixed steel reinforced sections) but also in ULS. For the beams described in this paper, this was taken into account by choosing glass grids as stirrups in the tension chords. Their lower modulus of elasticity allows higher strains in longitudinal direction parallel to the main CFRP reinforcement at rupture, and thus premature failure with spalling of the concrete was prevented. By assuming perfect bond in cross-sectional verification for bending, differences in bond strength were not considered. Pre-formed textile grids with their mechanical interlock exhibit high bond stresses, potentially better strengths than CFRP bars, depending on their surface treatment. A redistribution of tensile stresses in bending cracks towards the better bonded component can be assumed. With given bond-slip-relations for reinforcement materials, the numerical solution of the differential equation system for bond can be used to calculate the stress distribution along the embedment length. While this procedure is suitable for research, simpler detailing rules for practical applications are to be developed.  
The definition of cross-sectional area for FRP rebar (nominal, core diameter) and textile grids (effective, filament area) differs. In principle, the definition is arbitrary if the same reference value is used for stress and stiffness. This is true for cross-sectional verification of bending. However, if geometrical properties are derived from cross-sectional areas, this is disputable. For example shear verification in Eurocode 2 [64] for structural elements without shear reinforcement uses geometrical reinforcement ratio, which implicitly accounts for dowel action. It is known that resistance of dowels depends on cross-sectional stiffness of the individual reinforcement element. Also, for detailing specifications (maximum/minimum reinforcement ratios), a general definition of cross-sectional area for mixed reinforcement would be recommended.  
The large-scale tests show that compatibility of impregnated textiles and CFRP bars is given. While CFRP bars contribute large cross-sectional areas and thus high bending tensile resistance, the distributed textile shear reinforcement bridges shear cracks efficiently. Short anchorage length in compression and tension chords for the straight yarns are advantageous, while good bond and the multitude of yarns produce a dense shear crack pattern. Yet, enabling the intersection of the dense planar shear reinforcement and preformed stirrup reinforcement is challenging. With matched yarn spacing of high-quality reinforcement, which is dimensionally accurate, it becomes possible for small and medium-sized elements.