While extensive experimental data is present on the axial behavior of FRP jacketed concrete members with circular cross-section, only few data is available for the members with square and rectangular cross-sections. Furthermore, the data is particularly scarce for extended rectangular cross-sections, where the cross-sectional aspect ratio is over two. In addition, most of the existing codes and/or guidelines limit the cross-sectional aspect ratio with 1.5-2.0 due to significantly reduced efficiency of FRP confinement beyond these cross-sectional aspect ratios. In this study, specimens with the cross-sectional aspect ratio of 3.0 are tested under axial compression by using a displacement-ontrolled 5000 kN-capacity Instron testing system. While keeping the thickness of external FRP jacket same in all specimens, the arrangement of FRP sheets, which are used in the form of cross-ties are changed. The concrete quality of the specimens is intentionally poor for representing existing weak structural members. Experimental results showed that the efficiency of FRP jacketing was quite high for the extended rectangular cross-sections in case of low strength concrete. In addition, the efficiency of FRP jacketing could further be increased by the use of additional FRP sheets in the form of cross-ties. In the analytical part, the increased efficiency of the FRP jacketing due to presence of FRP cross-ties is explained using the concept of efficiently confined cross-section developed before for concrete confined with steel rectilinear stirrups and cross-ties.