Constitutive Model for Plain and Fibre-Reinforced Lightweight Aggregate Concrete under direct Tension and Pull-out

In the present study, a programme of experimental investigations was carried out to ex-amine the direct uniaxial tensile (and pull-out) behaviour of plain and fibre-reinforced lightweight aggregate concrete. The lightweight aggregates were recycled from fly ash waste, also known as Pulverised Fuel Ash (PFA), which is a by-product of coal-fired elec-tricity power stations. Steel fibres were used with different aspect ratios and hooked ends with single, double and triple bends corresponding to DRAMIX steel fibres 3D, 4D and 5D types, respectively. Key parameters such as the concrete compressive strength flck, fibre volume fraction Vf, number of bends nb, embedded length LE and inclination angle ϴf where considered. The fibres were added at volume fractions Vf of 1% and 2% to cover the practical range and a direct tensile test was carried out using a purpose-built pull-out test developed as part of the present study. Thus, the tensile mechanical properties were estab-lished and a generic constitutive tensile stress-crack width  model for both plain and fibrous lightweight concrete was derived and validated against experimental data from the present studies and also previous research found in the literature (including RILEM uniaxial tests) involving different types of lightweight aggregates, concrete strengths and steel fibres. It was concluded that the higher the number of bends nb, volume fraction Vf, and concrete strength flck, the stronger the fibre-matrix interfacial bond and thus the more pronounced the enhancement provided by the fibres to the uniaxial tensile residual strength and ductility in the form of work and fracture energy. A fibre optimisation study was also carried out and design recommendations made.