Constitutive model for plain and fiber-reinforced lightweight concrete under compression

In this research work, experimental investigations of the compressive behavior of plain and fiber-reinforced lightweight-aggregate concrete have been carried out (this formed part of a wider study, which also examined tensile and flexural behaviors). Compression mechanical properties were established in the studies and a generic constitutive compressive σ–ε model for both plain and fibrous lightweight concrete was derived and validated against experimental results from the present studies and previous research in the literature involving different types of lightweight aggregates, concrete strengths, and steel fibers. The reliability of predictions of the constitutive model was also checked against existing fibrous concrete models. A fiber-reinforcing factor was also introduced taking into account the fiber volume fraction, fiber length and diameter, the number of fiber bends, and concrete compressive strength. As such, this was considered a better descriptor of the material than simply using the fiber volume fraction. The lightweight aggregates examined in the experimental study were recycled from fly ash waste and the fibers were hooked-ended with single, double, and triple bends (corresponding to DRAMIX steel fibers 3D, 4D, and 5D types, respectively). The fibers were added at volume fractions Vf of 1% and 2% and the experimental studies were carried out using standard cube and cylinder uniaxial compression test specimens. It was concluded that the higher the number of bends and fiber content, the more pronounced the enhancement provided by the fibers to the compressive strength and ductility responses. All steel fibers used in the present studies were found to significantly improve the compressive toughness, while only 4D and 5D fibers (i.e., those with double and triple end bends) enhanced the compressive strength by up to 12% and 15%, respectively. It was also found that the elastic properties of plain lightweight concrete remained unaffected by the addition of fibers.