Design and Performance of Reinforced Concrete Elements with Spatial Tailoring of Concrete Properties

This thesis presents research on the design and performance of reinforced concrete elements constructed with multiple different concrete mixes which are layered in the fresh-state. This approach is called functional layering. It allows for concrete properties to be tailored to spatially varied performance requirements to make more efficient use of materials and thereby reduce the environmental impact of reinforced concrete elements. By layering in the fresh-state, hydration occurs across the interfaces between concrete layers and composite behaviour is expected. This research quantifies the embodied carbon reduction potential of layering one-way spanning simply-supported slabs, by adapting Eurocode 2 design equations to incorporate step changes in concrete properties and by assuming that composite behaviour is achieved. The embodied carbon reduction achieved through functional layering is assessed with two concurrent strategies: cement reduction of concrete mixes and weight reduction through depth minimisation. Functional layering is shown to be a promising approach for improving the efficiency of concrete use in reinforced concrete elements. A clear framework is shown to quantify the environmental performance of one-way spanning slabs without transverse reinforcement subjected to uniformly distributed loading, establish a baseline and enable the comparison of design options. Additional design constraints are imposed on layered elements, compared to conventional single-mix elements, to ensure that premature failure does not occur at concrete interfaces, or as a result of incompatibility of layered concrete properties. Two experimental programmes have been conducted for the purpose of characterising the mechanical layer interaction, and assess the influence that different mix designs, concrete properties and interface orientations relative to the applied stresses have on layered concrete behaviour. The results inform on the additional design requirements necessary for successful implementation of functional layering. The first experimental programme assesses whether layer interactions can be analytically described with single mix properties. For this purpose, the compressive strengths, the elastic moduli, the flexural and splitting tensile strengths, and the bond strengths with reinforcement of two-layer concrete samples were tested. This experimental series amounted to the production of 450 concrete specimens to conduct 756 material characterisation tests. Results show that: composite behaviour can be assumed and the ‘transformed sections’ method is suitable to predict flexural cracking; stronger layers can enhance the uncracked capacity of weaker layers through confinement from interface shear transfer; and loads can be transferred from weaker to stronger layers to delay failure. Weak planes which lead to preferential cracking and layer debonding are observed when interfaces are subjected to orthogonal tensile stress, and in these cases, the mix design pairing is shown to affect the quality of interface cohesion. Interfaces at the reinforcement level can result in different bond failure mechanisms depending on the paired properties. The second experimental programme assesses the bending behaviour of layered slab samples, to determine the accuracy of layered design assumptions used for predicting the uncracked elastic behaviour, the onset and growth of cracking, the stabilised cracking deflection and the ultimate strength and failure mechanism. This experimental series amounted to the production of 14 slab samples (9 different section designs) and 237 single mix test specimens to conduct 14 large-scale flexure tests and 426 material characterisation tests respectively. Results show that horizontal layering of a weaker concrete above a stronger concrete reduces peak reinforcement stresses in shear spans, increases the first-cracking curvature and enhances the ability to utilise uncracked regions between cracks for tension stiffening. However, a crushing capacity dictated by the weak concrete top layer requires deeper sections to ensure ductile failure. The greatest embodied carbon reduction potential is found for sections which position a concrete layer interface around the longitudinal reinforcement level, but minimisation of the strong layer thickness makes horizontally layered sections more susceptible to premature tensile cracking in the weak concrete. This internal damage is shown to lead to a reduced bond strength between the concrete and the reinforcement, allowing for bond-splitting failure to occur in horizontal planes. Vertically layered samples achieve more desirable load-deflection ductile responses than the horizontally layered arrangements, due to the higher strength concrete availability in the compression zone and due to there being no concrete interfaces in shear planes. Staged failure is observed as loads transfer to strong concrete layers.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2022

Author dc:creator

• Gimenez Fernandez, Mar

Advisor dc:contributor.advisor

• Lees, Janet M

• Concrete
• Concrete characterisation testing
• Environmental performance
• Flexure
• Functional layering
• Structural design
• Structural performance

Licence

• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.87686

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/340260