Axial compressive behavior of ultra-high-strength steel fiber-reinforced concrete-filled fiber reinforced polymer (FRP) tube columns

Abstract

Abstract This study presents the results of the first experimental research on the axial compressive behavior of ultra-high strength steel (UHSS) fiber reinforced concrete-filled FRP tubes (UHSSFR-CFFT). 24 circular UHSSFR-CFFT specimens were prepared and tested under axial compression to study the influence of unconfined concrete strength and fiber type. CFFTs were manufactured with glass, carbon and basalt reinforced polymer (GFRP, CFRP and BFRP) tubes with concrete compressive strengths (f’co) ranging from 35 to 105 MPa. Axial and lateral behavior were examined closely, including variation of lateral behavior along height of specimen. The results showed that the influence of concrete compressive strength on axial compressive behavior of UHSSFR-CFFTs is dependent on fiber type. For a similar normalized lateral confining pressure of FRP jacket at ultimate (flu/f’co), axial stress enhancement ratios (k1) were observed to decrease with an increase in f’co for all three fiber types. On the other hand, the obtained axial strain enhancement ratios (k2) indicated that k2 is influenced more by flu/f’co than f’co. Comparison of the recorded values of k1 and k2 for UHSSFR-CFFT with FRP-confined plain concrete showed similar trends on the influence of (f’co) on k1, but an opposite trend was observed for k2 of BFRP-confined specimens of the current study, in which k2 increased with increasing f’co. The results showed that the shape of the lateral strain-to-axial strain curves for UHSSFR CFRP- and BFRP-confined concrete specimens were similar to those of FRP-confined plain concrete, and this relationship was not influenced noticeably by f’co. Conversely, the obtained shape of the lateral strain-to-axial strain curves for UHSSFR GFRP-confined specimens was noticeably different when compared to FRP-confined plain concrete, with this behavior influenced by f’co. Finally, the results indicated that the addition of UHSS fibers in CFRP-confined specimens did not alter the influence of f’co on strain reduction factor (kɛ) where a decrease was observed by increasing f’co. However, the converse outcome was observed for GFRP- and BFRP-confined specimens where an increase in kɛ was observed when increasing f’co.