Behaviour of ultra-high performance concretes incorporating carbon nanotubes under thermal load

•Exposure of CNT-UHPCs to elevated temperatures was investigated for the first time.•Addition of CNTs reduced the occurrence of explosive spalling.•Residual strength and mass loss of UHPCs remained unchanged on adding CNTs. Ultra High Performance Concretes (UHPC) are known for exceptional mechanical and durability properties due to optimized particle packing and a denser microstructure. However, the very tight packing of particles resulting in a dense microstructure can be a cause of problems in UHPCs when exposed to higher temperatures. The low porosity delays the movement of water vapour inside de concrete and a layer of condensed water is created, preventing water to escape the microstructure and increasing internal pressure, that may result in explosive spalling. In this research, carbon nanotubes (CNTs) were introduced in UHPCs to assess their influence on the properties after exposition to high temperatures. Specimens were subjected to slow and fast heating rates and occurrence of spalling was investigated. Mechanical tests, such as compressive strength (before heating and after heating) and splitting tensile strength were conducted to investigate the impact of CNTs on the mechanical properties. Gas permeability tests were performed to investigate alterations in porosity due to the presence of CNTs. Results show that the presence of CNTs can reduce the occurrence of spalling at additions of 0.05% and 0.10% by weight of cement. CNTs can also increase the residual strength at temperatures up to 300 °C at a slow heating rate. At the same time, the 28-day compressive and splitting tensile strengths remained unaltered and oxygen permeability coefficients were reduced when CNTs were added to the matrix.