Compressive behavior of hollow concrete columns reinforced with GFRP bars

Compressive behavior of hollow concrete columns reinforced with GFRP bars

Hollow concrete columns (HCCs) reinforced with steel bars have been employed extensively for bridge piers, ground piles, and utility poles because they offer higher
structural efficiency compared to solid concrete columns with the same concrete area. Many experimental studies have been conducted to investigate the behavior of HCCs under different loading conditions and have found that the structural performance of HCCs is critically affected by the inner-to-outer diameter, reinforcement ratio, volumetric ratio, and concrete compressive strength. The improper design of the HCCs led to brittle failure behavior due to either buckling of the longitudinal bars or concrete wall crushing. Moreover, the corrosion of steel bars in HCCs is a critical issue due to their inner and outer exposed surfaces. Therefore, this research systematically investigated the fundamental behavior of HCCs reinforced with GFRP bars in compression to develop new, durable and structurally reliable construction systems.

Firstly, HCCs with different inner-to-outer diameter (i/o) ratios was investigated by testing four concrete columns 250 mm in external diameter and reinforced longitudinally with six 15.9 mm diameter GFRP bars with different inner diameters (0, 40, 65, and 90 mm). One HCC reinforced with steel bars was also prepared and tested as a control sample. Based on the experimental results, increasing the i/o ratio up to 0.36 changed the failure behavior from brittle to ductile. GFRP-reinforced HCCs exhibited higher deformation capacity and confinement efficiency compared to the GFRP-reinforced SCC and steel-reinforced HCC. The optimal (i/o) ratio was found at 0.36 as it resulted in the highest confined strength and ductility for GFRP-reinforced HCC. Similarly, reinforcing with longitudinal GFRP bars enhanced the overall behavior of HCCs.

The effect of varying the reinforcement ratio was investigated as the second study. To study this parameter, six HCCs reinforced longitudinally with GFRP bars
with different reinforcement ratios (1.78%, 1.86%, 2.67%, 2.79%, 3.72%, and 4.00%) were prepared and tested. These reinforcement ratios were achieved by changing the bar diameter (12.7 mm, 15.9 mm, and 19.1 mm) and number of bars (4, 6, 8, and 9 bars). The test results show that the increase in the bar diameter and number enhanced the strength, ductility and confinement efficiency of HCCs. For columns with equal reinforcement ratios, using a higher number and smaller diameter of GFRP bars yielded 12% higher confinement efficiency than in the columns with a lesser number and larger diameter of GFRP bars. The capacity of the GFRP-reinforced HCC can be reliably predicted by considering the contribution of the concrete and up to 3000 ue in the longitudinal reinforcement. The crushing strain of the GFRP bars embedded in the HCCs was 52.1% of the ultimate tensile strain, and was affected by the confinement provided by the lateral reinforcements and the compressive strength of concrete.

The effect of spiral spacing and concrete compressive strength was investigated as the third study. Seven large-scale HCCs with (i/o) ratio of 0.36, and reinforced with six longitudinal GFRP bars were prepared and tested. Out of these seven columns, three had spiral spacing of 50 mm, 100 mm, and 150 mm, and one had no spirals to investigate the effect of this design parameter. The fc of the other three columns were varied from 21 to 44 MPa to investigate the effect of the concrete compressive strength. Test results show that reducing the spiral spacing resulted in increasing the design load capacity, ductility, and confined strength of the HCCs due to the high lateral confinement. Increasing fc, on the other hand, increased the axial load capacity and reduced the ductility and confinement efficiency due to the brittle behavior of the high concrete compressive strength. The analytical model was then developed considering the contribution of the GFRP bars and the confined concrete core, which accurately predicted the post-loading behavior of the HCCs.

The experimental results from the three experimental studies demonstrated that the (i/o) ratio, p, pv , and fc affect the overall behavior of GFRP-reinforced HCCs. Therefore, a new design-oriented model considering the effects of these design parameters was developed in the fourth study to accurately and reliably describe the
behavior of the GFRP-reinforced HCCs. The new design-oriented model was based on the plasticity theory of concrete and considered the critical design parameters to precisely model the compressive load–strain behavior of GFRP-reinforced HCCs under monotonic and concentric loading. The results demonstrated that the proposed design-oriented model was accurate and yielding a very good representation of the axial compressive load behavior of GFRP-reinforced hollow concrete columns.

From the results of this research, a detailed understanding on how the critical design parameters affect the structural performance of GFRP-reinforced HCCs was gained. Moreover, the results from this research will provide useful information in revealing the many benefits of this new structurally efficient and non-corrosive construction system, which support the work of the technical committees engaged in the development of design provisions for GFRP-reinforced concrete columns.