Engineering properties and flexural performance of carbon nanofibers enhanced lightweight cementitious composite (CNF-LCC)

As a type of lightweight concrete, foam concrete is traditionally applied in building industry for its thermal and acoustic insulation properties. However, in recent years, there is a surge in interest in potential applications of foam concrete as a structural component due to its low self-weight, saving in raw materials and sustainability. The main challenge for foam concrete is to have high-performance pore walls to provide acceptable mechanical properties and other engineering properties under reduced density. In this study, a new type of structural foam concrete termed as carbon nanofibers enhanced lightweight cementitious composite (CNF-LCC) was developed based on nano-engineered ultra-high performance concrete technology. The potential application of CNF-LCC on structures was investigated by a series of comprehensive experimental programmes from the material level to the structural level and in-depth analysis of the testing results was discussed. CNF-LCC was produced by using carbon nanofibers (CNFs) enhanced ultra-high performance concrete (ceUHPC) as a base mix and then blending with homogeneous micro-foam bubbles to achieve a density of 1500 ± 50 kg/m3. The basic mechanical properties of CNF-LCC were measured in accordance with standard codes (European or American codes) and they are superior to conventional foam concrete. CNFs showed effective improvement on the mechanical properties especially flexural strength and toughness. The thermal properties of CNF-LCC under high temperature indicated its reasonably good thermal insulation properties and low thermal expansion for fire resistance. The phase transformations of CNF-LCC under high temperatures were characterised to elaborate on the experimental results and CNFs could reduce thermal shrinkage without degrading thermal insulation properties. As a critical performance for structural building material, the long-term properties of CNF-LCC were evaluated by testing the durability, shrinkage and creep. CNF-LCC presented excellent long-term performance compared to conventional normal and lightweight concrete due to the optimised UHPC base mix and modified characteristics of pore structure by CNFs. Prior to the structural tests, the bond performance between CNF-LCC and steel reinforcement was studied by pullout tests of short and long embedment length. The bond strength of CNF-LCC exceeded the traditional foam concrete and was comparable with normal concrete (NWC) and lightweight aggregate concrete (LWAC) due to the improved mechanical properties. A new analytical model was proposed to accurately predict the bond-slip performance of concrete including CNF-LCC. Finally, the flexural performance of reinforced CNF-LCC beams was investigated and the experimental behaviour surpassed conventional foam concrete and was comparable with NWC and LWAC. CNFs presented comprehensive improvement on flexural performance, especially beam ductility. Recommendations for the design and analysis of reinforced CNF-LCC beams were provided and an analytical model was proposed to predict the load-deflection relationship. In general, CNF-LCC based on nano-engineered UHPC technology presented outstanding material properties and structural performance compared to conventional foam concrete. The experimental results not only established the database in the use of CNF-LCC but also provide more test results for foam concrete. At the same time, the effect of CNFs was comprehensively investigated from the nanostructure of material to the macrostructure of elements, which was absent in previous research work. Furthermore, the related recommendations and prediction models for CNF-LCC were proposed to guide the design and analysis of reinforced CNF-LCC members in the future. CNF-LCC provided a new solution for structural lightweight concrete.Doctor of Philosoph