Why current standards fail for CFSTs with alternative binders: closing the gap

Concrete-filled steel tubes (CFSTs) with alternative binders are a promising direction for sustainable construction. However, they are hindered by the current durability testing standards. Existing international standards were developed for concrete made with ordinary Portland cement (OPC) and therefore fail to address the thermo-mechanical properties, hydration mechanisms and pore structures of innovative binders such as geopolymers, alkali-activated materials. This critical review identifies specific gaps in current testing protocols by synthesising the fragmented literature on CFST mechanics, geopolymer durability and bond behaviour. Findings reveal that rapid chloride penetration tests (RCPTs), thermal cycling procedures and bond assessment methods calibrated for OPC concrete produce misleading results for alternative binders. Moreover, the confinement of CFSTs creates durability challenges, which are not addressed in existing standards, particularly regarding the tube’s corrosion and internal reinforcement protection. Modifications to current testing frameworks are proposed, including adapted RCPT protocols for geopolymer systems, standardised thermal cycling procedures for CFSTs and enhanced push-out testing specifications. This study offers practitioners and standards bodies helpful suggestions for validating CFSTs with alternative binders by identifying critical knowledge gaps and suggesting five test domains, thereby enabling more confident adoption of sustainable composite structures.