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Mechanical and Microstructural Properties of Ultra-High Performance Concrete with Lightweight Aggregates

Although ultra-high-performance concrete (UHPC) presents superior mechanical properties and durability compared to conventional concrete; its spalling resistance to elevated temperatures is much lower compared to conventional concrete due to the high compactness and absence of capillary pores. This paper investigated the influence of lightweight aggregate (LWA) on the strength properties and microstructure of UHPC to enhance its resistance to elevated temperatures. UHPC specimens prepared with LWA as a partial replacement of silica sand were produced. The study evaluated the compressive and flexural strengths, failure mode, mass loss, and microstructure of the specimens, using SEM. The results showed that the compressive strength of the UHPC specimen was reduced with increasing the content of LWA at ambient temperature, but the compressive strength of the UHPC specimens prepared with LWA improved when exposed to elevated temperatures. The replacement of 10% of the silica sand with LWA led to an increase in the compressive strength from 100 MPa to 110 MPa after exposure to 200 °C; however, the flexural strength decreased from 23.6 MPa to 18.3 MPa. On the contrary, the flexural strength of UHPC increased with the inclusion of LWA at an ambient temperature but reduced with high-temperature exposure. The failure mode of UHPC was not significantly affected by the variation in LWA content and temperature. In addition, the SEM result confirms that LWA is an effective internal curing material for enhancing the microstructure and compressive strength of UHPC