Investigation on the structural behaviour of precast/prestressed hollow core concrete floor slabs used for overhangs

Abstract

This research study aims to test the structural behaviour of precast/prestressed hollow core concrete floor slabs used for overhangs. These precast slabs are commonly used in frame buildings, as they offer ease of constructability, whilst also offering greater load capacity to weight ratio than that of a cast-in situ reinforced concrete slab. These slabs have been used in the local industry over the past 50 years mainly in order to span greater distances, whilst carrying greater imposed loads from the structure above. Using an experimental research methodology, a set of slab specimens were prepared and tested in order to investigate the structural behaviour of these precast/prestressed hollow core concrete floor slabs used for overhangs in the local construction industry. The experimental programme involved preparing a testing-rig set-up within the Civil Engineering laboratory at the University of Malta. The testing programme was conducted upon a number of full-scale precast/prestressed hollow core concrete floor slabs with different configurations of concrete structural topping and steel bar reinforcement. These slabs were set-up in a way to create an overhanging end, while restraining the other end in order to prevent overturning of the slab when the cantilever end was loaded. The vertical load was applied through a load cell situated at the cantilevered end of the slab. Measuring apparatus, such as steel strain gauges, concrete strain gauges and linear variable differential transformers (LVDTs) were set up along the span of the slab. These instruments recorded the strain of the concrete at the compression side (underside), the strain in the main steel tension reinforcement within the concrete topping and the vertical displacement due to the applied load on the slab at the cantilevering end and at the midpoint between the internal supports. In total, five experimental configurations were tested in order to investigate the structural behaviour and benefits of using a reinforced concrete structural topping over the precast/prestressed hollow core concrete floor slabs in order to be able to resist the hogging moments induced within the slab by the applied loading. Quality control checks and safety parameters were used throughout the testing programme. This research study includes a descriptive and comparative analysis of the failure loads and failure modes (flexure or shear) obtained for each slab specimen. The experimental results obtained are presented graphically in order to facilitate the comparison during the loading of the slabs to failure and the corresponding post-ultimate unloading. The experimental results obtained showed that there is a significant improvement in the load-carrying capacity of the slabs with reinforced structural concrete topping compared to the control slab without topping. The experimental results were also found to compare well with analytical predictions of structural strength and failure modes This research study also includes suggestions for further research work, which are relevant to the scope of this field of research.