Proposed design and construction guidelines for labour-intensively built rubble masonry concrete structures with particular reference to arch bridges
Rankine, Roderick Graeme Duncan
Rubble Masonry Concrete (RMC), a particulate composite o f large uncut boulders, manually embedded into a mortar matrix, was used in Roman times for constructing robust physical infrastructure, some of which survives. A renaissance of RMC usage has resulted in recognition of its potential to provide competent infrastructure and create employment for the unskilled without costing a premium. Despite numerous successes, the potential of RMC may be threatened by an absence of knowledge of its mechanical properties and a lack of appreciation of the requirements for humans to labour effectively. This thesis explores the mechanical properties and behaviour of RMC as well as physiological factors which govern the physical work capacity of the targeted labour-force before proposing guidelines towards the rational design and manual assembly of RMC structures. In addition to factors known to govern the properties of conventional concrete, potential anisotropy (a consequence of an inherently predominant orientation of elongated inclusions),relative inclusion size and inclusion contiguity appear to characterise RMC. It is hypothesized that as a result of this contiguity, RMC may exhibit a reduced thermal contraction coefficient below its placement temperature; a contributing factor to its resistance to post-hydration cracking. Ultimate failure and deformation of RMC under uniaxial compressive load appear to be governed by mechanisms which evolve between boulder inclusions, often accompanied by rock fracturing as a result o f high inter-particle bearing stresses. Fostering of interfacial bond appears to delay the onset of these mechanisms. Physiological investigation found the cost of human energy high when quantified in terms of the equivalent energy contained in diesel fuel and bread. However, when human-labour is effectively utilised, its versatility, energy efficiency and freedom from fixed ownership costs,enhance its competitiveness, particularly where rival machinery is sub-optimally utilised.Means to improve human-labour effectiveness are discussed. Based upon these findings and experience elsewhere, RMC material specifications and design and construction guidelines for RMC arch bridges of determinable reliability are proposed.They include a proposal that a high partial material factor be applied to RMC and indicate ways to exploit RMC’s anisotropic properties to advantage in highly stressed apphcations.