Equivalent stress block parameters for fly ash-based geopolymer concrete structural elements

Research on the design of structural members made from geopolymer concrete (GPC) remains limited. This study investigates the applicability of equivalent rectangular stress block parameters, traditionally used for reinforced concrete design, in GPC structural elements. We conducted experimental tests on 20 columns (16 GPC, 4 Ordinary Portland Cement [OPC]) and 15 beams (12 GPC, 3 OPC) produced using fly ash-based GPC and standard OPC. These tests involved subjecting the specimens to various loading conditions to measure their ultimate compressive strength and strain. The findings demonstrate that these factors significantly influence the stress block parameters in GPC samples. Notably, parameters k(1) and k(3) were compatible with ACI 318 and Eurocode 2 standards, with deviations within acceptable limits, supporting GPC's potential for use in conventional reinforced concrete frameworks. The study also reveals that GPC columns and beams have higher balanced reinforcement ratios than OPC, due to GPC's increased deformation capacity and strain values. According to the results, the average balanced reinforcement ratio of GPC column specimens is 30% higher than that of OPC, while that of GPC beam specimens is 6% higher. Variations in alkaline activation and curing methods did not significantly impact the equivalent stress block parameters. The change between the average equivalent stress block parameters obtained from GPC and OPC beam samples varies between 1% and 5%, while the change in column samples is around 1%. Although the longitudinal reinforcement ratios in the tests are variable, the averages of the calculated equivalent stress block parameters are close. The experimental results align with numerical analysis, emphasizing GPC's suitability as an alternative material in structural applications. These findings provide a basis for incorporating GPC into existing design standards, with adjustments for its distinct mechanical behavior.