Alkali-Activated Concrete (AAC) is considered as a promising alternative to conventional Portland Cement Concrete (PCC) due to its potential to reduce environmental impacts. However, its application in practical engineering is limited by, among others, insufficient understanding of the long-term structural behaviour of reinforced and prestressed AAC elements. To address this, a series of experiments were conducted on composite girders to investigate the long-term flexural behaviour. The composite girder is formed by a prefabricated prestressed AAC inverted-T girder with cast-in-situ ACC topping concrete. The midspan deflection of two composite girders, subjected to self-weight and additional sustained loading, were measured over a 9-month period. Subsequently, flexural tests under four-point bending configuration were performed at the age of 9 months and the reference age of 28 days. The results showed that the specimens tested at 9 months exhibited reduced initial stiffness, decreased cracking load and larger crack widths in the precast prestressed girder compared to those tested at 28 days. The reduction in stiffness likely stems from decreased elastic modulus and structural cracking. Meanwhile, the lower cracking load arises from prestress losses caused by ongoing (restrained) shrinkage and creep, consistent with AAC material test observations. Larger crack widths observed in the precast girder may result from a degradation of bond between AAC and prestressing strands over time. The distinct failure patterns of the 9-month specimens (anchorage failure for sample subjected to self-weight only and flexural failure for sample exposed to additional sustained load), highlighted the role of creep on bond behaviour between prestressing strands and AAC, particularly as a function of varying stress levels at the level of strands. Finally, analytical models were applied to evaluate the prestress loss and flexural behaviour of the specimens. The effective prestressing force and cracking loads at both testing ages were overestimated when the effects of (partially) restrained deformations between precast and cast-in-situ AAC were neglected. More accurate analytical predictions were achieved when these long-term effects and the level of restraint in the composite girder were considered.

Aimed at realizing the effective strengthening and durable repair of concrete structures, particularly in emergencies like traffic interruption triggered by broken roads and damaged bridges, nine groups of specimens were designed and tested in this paper to develop the high-early-strength Engineered Cementitious Composites (HES-ECC) featured as both high early-strength and superior long-term-deformability. The high-early-strength effect of sulphoaluminate cement, silica fume, and Portland cement on HES-ECC was compared, as well as their influence on the deformation ability of HES-ECC. Moreover, the interfacial behaviors between HES-ECC and existing concrete structure were clarified, considering the effects of interfacial agents, interfacial treatment methods, and interfacial roughness. The results indicate that HES-ECC with 6% silica fume mixed could obtain both the high early-strength and superior long-term-deformability. The flexural strength at 3 h could reach 66.67% of that at 28d. The compressive strength could reach up to 28.7 MPa at 3 h, and the ultimate tensile strain could remain 4.21% at 28d. Cement paste interfacial agent could enhance the chemical adhesive bonding between HES-ECC and existing concrete while polymer modified interfacial agent was incompatible. The increased roughness of chiseled interface was beneficial to both the bearing capacity and the deformation ability. Interfacial shear performance of the grooved interface was scarcely deteriorated even if the roughness decreased by 54.18% compared with the chiseled interface. The recommended interfacial treatment is chiseled interface combined with grooved interface, as well as a thickness of 1–2 mm cement paste interfacial agent. This study provides valuable and credible experimental data for promoting the application of HES-ECC in repairing existing concrete structures in practice.

The increasing demand for sustainable development in engineering practice has triggered researchers to explore solutions to reduce the CO2 footprint caused by ordinary Portland cement (OPC) production. Alkali-activated concrete (AAC), made of by-products using alkali activation, is of great potential as a promising alternative to conventional concrete (CC). Despite vast studies on its material properties, there is still insufficient scientific research on the structural performance of AAC, which impedes its widespread application. In this paper, an overview of the fundamental behavior of AAC beams under different loading conditions is presented. The experimental investigations on mechanical performance of AAC beams are reviewed in terms of ultimate capacity, ductility and cracking behavior. Moreover, numerical methods to predict AAC structural response as well as the applicability of existing CC design codes are summarized. It is concluded that AAC beams show comparable short-term behavior with CC counterparts. Besides, the design codes for CC turn out to be applicable but conservative for most steel-reinforced AAC beams. Though short-term flexural behavior has been widely investigated for AAC beams, the challenge remains to clarify shear behavior and long-term behavior. Furthermore, reliable guidelines are needed to be developed, providing recommendations for future structural design.