Ductility and brittleness of lightweight aggregate concrete structures

Abstract

Lightweight aggregate concrete (LWAC) has been used in construction since ancient Greek and Roman times, for more than two thousand years. With its low density, it was mainly used to reduce the weight of the structures. LWAs were eventually manufactured and used more widely after 1917. Nevertheless, the use of LWAC in structural applications is very limited compared to normal weight concrete (NWC) even nowadays. The main concerns are its more brittle post-peak behaviour, especially in compression, its uncontrolled crack propagation, and its reduced ductility.

The main objective of the research for this thesis was to study the factors influencing LWAC’s compressive ductility, so it can provide the safe and reliable structures which are vital to making LWAC a more competitive material. Most material models and design codes treat LWAC as one material, irrespective of the type of lightweight aggregate. While some aggregates do indeed have a brittle structural response, other aggregates behave much more like normal weight concrete. The lightweight concrete used in this study, with Stalite as aggregate, is therefore compared to the design codes and material models to see if they compare it unfavourably to NWC.

The research included experimental work and a comparisons part. The experimental part included a series of stress-strain gradient tests on small concrete prisms to obtain the design parameters used later in the main experimental programme. The large-scale beam experimental programme was created to capture the confinement effect. This programme involved loading six over-reinforced beams in four-point bending. The geometry of the beams was 210x550x4500 mm (width x height x length), allowing a large compressive area of one metre between loading points. The main testing parameters we varied were the spacing of transverse reinforcement (stirrups), the amount of longitudinal compressive reinforcement, and the thickness of concrete cover. The test results were used in numerical analysis of the current material models to verify or obtain guidance for appropriate material models for structural LWAC behaviour in compression.

Compressive confinement and ductility were most influenced by the spacing of transverse reinforcement, while load capacity was determined jointly by the amount of longitudinal compressive reinforcement and the stirrup spacing. Analyses of the compressed part of the cross section with existing material models showed they can be used as a basis for obtaining a better description of LWAC in compression. However, the type of aggregate needs to be included in the models.

To test brittleness, crack propagation, and shear resistance, another large-scale beam test programme was created involving five beams. The main test parameters were the shear span ratio (a/d) and the amount of shear reinforcement. The results indicate that existing design codes underestimate the ultimate strains and shear capacity of lightweight concrete.

In summary, by using appropriate reinforcement detailing it is possible to increase the ductility of LWAC structures and achieve responses close to the response of NWC structures. Bearing in mind the major advantages of LWAC, which are reduced weight and a high strength-to-weight ratio compared to conventional concrete, structural applications of LWAC should be increased significantly.

Keywords: lightweight aggregate concrete, testing in compression, strain level, centric loading, eccentric loading, stress gradients, shear, bending, confinement, ductility, transversal reinforcement, longitudinal reinforcement, concrete cover.