Flexural Behavior of Composite IsoTruss Reinforced Concrete Piles

Abstract

Four-point bending tests were performed in the laboratory to investigate the strength, stiffness and failure of carbon composite reinforced concrete columns for use in deep foundation applications. Two carbon/epoxy composite three- dimensional lattice structure (IsoTruss®) reinforced concrete (IRC) and two steel reinforced (SRC) concrete piles, each approximately 4.3 m (14 ft) in length and 36 cm (14 in.) in diameter, were loaded to failure while monitoring load, deflection, and strain. The steel and composite cages were designed to have equal flexural stiffness to permit a relative strength comparison. At failure, the IRC beams held nearly twice the bending moment as the SRC beams (194 kN-m vs. 101 kN-m (1,720 kip-in vs. 895 kip-in)), although the failure modes were noticeably different. As expected, SRC piles exhibit more ductile failure behavior than IRC piles, which exhibit linear load- deflection behavior to failure. At 165 kN (35 kips) - the maximum load on the SRC piles - the ductility of the SRC piles was double that of the IRC piles (0.0084 vs. 0.0042, respectively). Likewise, at failure, the SRC piles absorbed approximately twice as much total energy in flexure due to the highly ductile failure. Further investigation is required to explain the low ductility observed in the IRC piles, since higher ductility had been previously observed in similar structures in flexure. In summary, the carbon composite reinforcement in IRC piles is substantially lighter, more rigid and more corrosion-resistant than steel reinforcement, resulting in a pile that is substantially stronger, although less ductile at ultimate load, than SRC piles.