title: Structural performance of a novel sustainable and demountable composite floor system - recycled aggregate concrete-steel composite beam utilising demountable shear connectors
Creators: name: Wang, Mingda
Publisher: name: University of Glasgow
Year: 2025

description: This study examines sustainable construction methods, particularly the reusing of steel beams and the recycling of concrete materials. This study specifically looks at how recycled aggregate concrete (RAC) and demountable shear connectors could be used in composite floor systems, an area that has yet to be looked into enough in the past. The demand for sustainable solutions in the construction sector is rising, and this research responds to that necessity by offering an innovative flooring system that integrates RAC with demountable shear connectors, specifically for application in temporary and short-term leasing structures.

A literature study on concrete microstructure indicated that residual mortar on recycled aggregate alters the interfacial transition zone (ITZ), influencing the mechanical properties of RAC. A review was conducted on the structural uses of RAC, encompassing RAC-filled steel tubes and composite slabs. For the development of the proposed innovative floor system, two types of demountable shear connectors were evaluated, with the bolted type chosen for its practicality. Bondek II was chosen for profiled steel decking because of its benefits and prevalent application in the local market. A study gap was identified: the majority of studies on RAC employ a fixed mix design, altering the percentage of recycled aggregate instead of sustaining a consistent goal concrete strength. This complicates the assessment of whether diminished structural resistance results from the utilisation of recycled aggregates or from a reduction in design strength relative to normal aggregate concrete (NAC). A fixed design strength approach was suggested to provide target strength by augmenting recycled aggregate replacement and diminishing the water-to-cement ratio. The primary scientific challenge is ascertaining the ideal water-to-cement ratio to facilitate recycled aggregates substitution and attain the requisite concrete strength.

Two main tests are proposed. The first assessment is the push-off test, frequently employed to assess the shear behaviour of shear connections. Two categories of push-off specimens have been developed, each comprising four identical geometries, distinguished by shear connector dimensions: M20 and M24. The M20 bolts are positioned in closer proximity to establish a complete connection, whilst the larger M24 bolts are arranged at greater intervals for apartial connection. The recycled aggregate substitution ratio ranges from 0% to 30%, 70%, and 100%, 
delineating the four specimen categories. Test results demonstrate that the shear resistance of bolted connectors in RAC increases by up to 40% with a higher proportion of recycled aggregate, corresponding to a lower water-to-cement ratio. Full bending tests were performed to evaluate the overall structural performance of the proposed composite floor system. Five test specimens were produced and classified into two categories according to the size of the shear connectors. The first type employed M20 connectors at 200 mm intervals, aligning with the push-off test, and utilised three concrete mixtures: 0% recycled aggregate (baseline), 30%, and 100% replacement. The second variant employed M24 connectors at 400 mm intervals, adhering to the identical push-off test design, utilising two mixtures: 30% and 100% replacement. Test results indicate that composite beams with bolted connectors provide up to a 10% enhancement in flexural resistance in RAC relative to NAC.

Subsequent to testing, the pertinent codes were employed to compute resistances and juxtapose them with experimental outcomes. EC3 and EC4 appropriately forecasts shear resistance for bolted connectors in NAC but underestimates it in RAC by 16–51%. AISC and ACI provide more precise average projections; nonetheless, they yield unsafe estimates for NAC, underestimating by as much as 13%, and do not reliably identify the failure mode. Both EC4 and AISC accurately forecast the bending resistance of composite sections, with a mere 2% overestimation of NAC resistance. Nonetheless, they underestimate the bending resistance of RAC by as much as 9%, with AISC exhibiting better precision compared to EC4. Furthermore, the findings demonstrate that bending resistance predictions derived from push-off test data closely correspond with experimental bending test outcomes, indicating that push-off tests function as a reliable indirect approach for evaluating bending resistance with enhanced precision.