# Structural Behavior of Large-Scale I-Beams with Combined Textile and CFRP Reinforcement
## 5 Discussion
### Utilization Ratio of Carbon Bars-SLS to ULS Reserve  
CFRP reinforcement is costly, and thus a high utilization ratio of material strength is a major goal. High utilization means that reinforcement necessary for satisfying SLS deflection limits is utilized in ULS up to design strength. This discussion is known from high-strength steel in conventional reinforced concrete, which is usually utilized in flexural members only for prestressing. \begin{table}
\begin{tabular}{c c c c c|c c c|c c|c} \hline \multicolumn{1}{c|}{} & & & \multicolumn{2}{c|}{**[PERSON]/Bischoff**} & \multicolumn{2}{c|}{**M-\(\kappa\) theor.**} & \multicolumn{2}{c|}{**M-\(\kappa\) exp.**} & \multicolumn{2}{c}{**ABAQUS**} \\ \hline
**No.** & \(M_{\text{exp}}\) & \(w_{\text{exp}}\) & \(I_{\text{cr}}\) & \(M_{\text{cr}}\) & \(\gamma\) & \(I_{\text{cr}}\) & \(w_{\text{calc}}\) & \(w_{\text{calc}}\) & \(w_{\text{calc}}\) & \(w_{\text{calc}}\) & \(w_{\text{calc}}\) \\ \hline  & [**kNm**] & [**mm**] & [**cm\({}^{\star}\)\({}^{\ddagger}\)**] & [**kNm**] & [**l-1**] & [**cm\({}^{\ddagger}\)**] & [**mm\({}^{\ddagger}\)**] & [**mm**] & [**mm**] & [**mm**] \\ \hline \multirow{4}{*}{FT01} & \multirow{4}{*}{229.7} & \multirow{4}{*}{28} & & & 115.5 & 13.5 & 152,48 & 19.9 & \multirow{4}{*}{\begin{tabular}{c} \\ (-28.9\%) \({}^{\ast}\) \\ \end{tabular} } & & & & & \\  & & & & (80\%) & & (-18.9\%) & 28.2 & 25.7 & 25.6 \\  & & & & & (80\%) & & (-18.9\%) & (+0.7\%) \({}^{\ast}\) & (-8.2\%) \({}^{\ast}\) & (-8.6\%) \({}^{\ast}\) \\  & & & & & 66.6 & 1.50 & 119,666 & 25.3 & \multirow{4}{*}{\begin{tabular}{c} \\ (-9.6\%) \({}^{\ast}\) \\ \end{tabular} } & & & & & & \\ \hline \multirow{4}{*}{FT02} & \multirow{4}{*}{515.7} & \multirow{4}{*}{28} & & & 105.4 & 288,145 & 22.9 & \multirow{4}{*}{\
\begin{tabular}{c} \\ (-18.2\%) \({}^{\ast}\) \\ \end{tabular} } & & & & & & \\  & & & & & (100\%) & & (-18.2\%) \({}^{\ast}\) & & \\  & & & & 84.3 & 1.59 & 283,616 & 23.3 & 23.9 & 23.3 & 26.1 \\  & & & & & (80\%) & & (-16.8\%) \({}^{\ast}\) & (-14.6\%) \({}^{\ast}\) & (-16.7\%) \({}^{\ast}\) & (-6.6\%) \({}^{\ast}\) \\  & & & & & 60.7 & 1.63 & 279,742 & 23.6 & \multirow{4}{*}{
\begin{tabular}{c} \\ (57.6\%) \\ \end{tabular} } & & & & & & \\ \hline \end{tabular} \({}^{\ast}\) deviation to the experimental deflection of 28 mm.  
\end{table} 
Table 6: Prediction of deflection with Equation (3), with \(M\)-\(\kappa\)-relation and with FE-model. Based on strain gauge measurements in specimen FT01, the main CFRP bar reinforcement in SLS (at 28 mm short-term deflection) was utilized at 542 MPa, 30% of mean CFRP material strength. In design, the strength reserve of 70% is required for incorporation of time-dependent material strength reduction coefficients and material and load safety factors (SLS to ULS reserve). In specimen FT02 which was designed for flexural compression failure, strain measurements show that the reinforcement was subjected to at a lower stress of 450 MPa (25% of material strength). As the outermost reinforcement at ULS was strained only up to 6.74%o (1115 MPa), overall utilization ratio of reinforcement for FT02 is lower than for FT01. This ratio would further decrease if deflection in SLS increases through creep of concrete. The comparison of the reinforcement layout of both beams indicates that design for flexural compression failure while maintaining deflection limits is cost-intensive with non-prestressed reinforcement. The obvious solution for this case would be to apply low-level prestressing, staggered over the height of the beam to arrive at the same strain in ULS in all layers. Thus, strain reserves are preserved to account for necessary increase of reinforcement strain between SLS and ULS loading (see also recommendations in [81]). Meanwhile, eccentric pretensioning increases prechamber and thus reduces sag in SLS. The discussion which failure mode is more ductile, flexural tension or flexural compression failure, and thus favourable for design of CFRP reinforced structures is ongoing and not pursued in detail here.