This paper presents the mechanical properties of alkali activated concrete (AAC) cured at 70°C for 24 h. The AAC mixtures were synthesized using five class C fly ashes (FAs) having different chemical and physical properties. Sodium hydroxide (SS) and sodium silicate (SH) were used as the alkali activators in this study. A conventional concrete (CC) mixture, having a compressive strength of 34.5 MPa, was synthesized using ordinary Portland cement (OPC) mixture for comparison purposes. The slump, as well as the compressive, tensile splitting, and flexural strengths were investigated at different concrete ages up to 28 days. The results revealed that with increasing the calcium content in an FA used to synthesized AAC mixture, the slump value and the mechanical properties decreased. All AAC mixtures reached approximately 92% of their 28-day compressive strength after 1 day compared with only 29% in the case of CC. Therefore, AAC can be used in applications where rapid strength gain is required, such as urgent repair, precast industry, and so forth. The measured data was also used to develop a set of equations to accurately predict the splitting tensile and flexural strengths.

This paper presents the seismic behavior of hollow-core fiber-reinforced polymer–concrete–steel (HC-FCS) columns. The typical HC-FCS column consists of a concrete wall sandwiched between an outer fiber-reinforced polymer (FRP) tube and an inner steel tube. The inner steel and outer FRP tubes provide continuous confinement for the concrete shell; hence, the concrete shell achieves significantly higher strain, strength, and ductility than unconfined concrete in conventional columns. Three large-scale HC-FCS columns were investigated in this study. Each column had an outer diameter of 610 mm (24 in.) and a height-to-diameter ratio of 4.0. The steel tube was embedded into a reinforced concrete footing with an embedded length of 1.6–1.8 times the steel tube diameter, whereas the FRP tube only confined the concrete wall thickness and truncated at the top of the footing level. In general, the columns exhibited high lateral drift, reaching to 11.6%, and failed gradually as a result of concrete crushing and local steel tube buckling. An equation to determine the steel tube development length of HC-FCS columns was introduced based on an extensive finite-element study. The finite-element analysis was validated with finite-element models. In addition, this paper introduces a quick-repair technique for HC-FCS columns. Guidelines for the preliminary design of HC-FCS columns under seismic loading are also presented to help implement this new technology.