# Delaying the Occurrence of Bar Buckling in RC Columns Confined with SRG Jacketing
## 2 Experimental Program
### Material Properties  
The same concrete mix was used for all specimens. Nine standard cylinders (150 \(\times\) 300 mm) were cast for obtaining the average compressive strength at the day of the tests. The average compressive strength was f\({}_{\rm cm}\) = 28.7 MPa (SDV = 0.74), corresponding to a characteristic compressive strength of C20/25 (f\({}_{\rm ck}\) = f\({}_{\rm cm}\) - 8 MPa) which is almost the same as B25 [1], i.e., a good quality concrete for the construction practice before the 1980s. The 12 mm diameter ribbed bars had a yield stress of f\({}_{\rm sy}\) = 540 MPa, ultimate stress f\({}_{\rm su}\) = 640 MPa, strain at the initiation of hardening \(\epsilon_{\rm h}\) = 0.005 and at fracture \(\epsilon_{\rm u}\) = 0.1, corresponding to steel quality BStIV [1] used in seismic applications in the 1970s. The 6 mm diameter smooth bar reinforcement used for the stirrups had a yield stress of f\({}_{\rm sy}\) = 360 MPa and an ultimate stress f\({}_{\rm su}\) = 467 MPa. These values correspond to BStI, which was used extensively for shear reinforcement [1]. The 3X2 unidirectional ultra-high strength steel textile was utilized for the fabrication of the SRG jackets. The term 3X2 denotes that each cord is made by twisting five wires, three straight and two wrapped with a high torque angle to enhance the interlocking with the mortar (Figure 2). The cords are thermo-welded to a fibreglass micromesh and galvanized (coated with zinc) to improve durability in a chloride, freeze-thaw and high humidity environment. Each cord has an area of 0.538 mm\({}^{2}\), a tensile strength higher than 2800 MPa, a strain to failure higher than 0.015 and an elastic modulus higher than 190 GPa. The textiles used had a density of 4 cords/in (1.57 cords/cm) and 12 cords/in (4.72 cords/cm) (Figure 2). The equivalent thickness per unit width, t\({}_{\rm s}\), for the 4 cords/in and the 12 cords/in was defined equal to 0.084 mm (=0.538 mm\({}^{2}\)/cord \(\times\) 1.57 cords/10 mm) and 0.254 mm, respectively. The axial stiffness (=190 GPa \(\times\) t\({}_{\rm s}\)\(\times\) n, n the number of plies) of the SRG jacket configurations applied (with reference to the periphery of the column) was 15,960 N/mm for the 4 cords/in single jacket (t\({}_{\rm s}\) = 0.084 mm, n = 1), 31,920 N/mm for the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen Notation** & **Group** & \multicolumn{2}{c}{**Fabric Density:**} & \\  & & **(Cords/in)** & **L = 4 or H = 12** & \\ \hline A & & - & - \\ \hline L1A & A & 4 & 1 \\ \hline L2A & tie ends bent at 90\({}^{\circ}\) & 4 & 2 \\ \hline H2A & & 12 & 2 \\ \hline B & & - & - \\ \hline L1B & B & 4 & 1 \\ \hline L2B & tie ends bent at 90\({}^{\circ}\) & 4 & 2 \\ \hline H2B & & 12 & 2 \\ \hline C & & - & - \\ \hline L1C & C & 4 & 1 \\ \hline L2C & tie ends bent at 135\({}^{\circ}\) & 4 & 2 \\ \hline H2C & & 12 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specim details. 4 cords/in two-layered jacket (t\({}_{\rm s}\) = 0.084 mm, n = 2) and 96,520 N/mm for the 12 cords/in two-layered jacket (t\({}_{\rm s}\) = 0.254 mm, n = 2). The ground used for the SRG composite system was a commercial mortar with a crystalline reaction geobinder base and a very low petrochemical polymer content and free from organic fibres. The mortar was utilised as the substrate material applied to the concrete surface of the specimens, the bonding material between the applied layers of the steel fabric and as a final cover. Its reported mechanical properties at 28 days are as follows: modulus of elasticity E\({}_{\rm m}\) = 25 GPa; flexural strength f\({}_{\rm mf}\) = 10 MPa; compressive strength f\({}_{\rm mc}\) = 55 MPa; and adhesive bond f\({}_{\rm mb}\) = 2 MPa.