Building construction with composite elements of wood-concrete compounds and timber

The proposed project "Building construction with composite elements of wood-concrete compounds and timber" aims at developing a multi-layer wall and floor system composed of cement bonded wood particles and full size timber elements in composite action. The load bearing composite elements can transmit bending (e.g. floors), compression and shear (in the case of walls). The project requires investigations on the composition of wood- concrete compounds, on the interfaces, the compatibility, and the composite action between the different components and materials of the multi-layer system. Manufacturing methods has to be analyzed, in order to optimize a composite system by using the best characteristics of each material and component. For the construction technology to be acceptable in future practical application, engineering know-how (i.e. structural behavior and appropriate design approaches and values, structural reliability) is to be amended and verified by scientifically robust background data. At the other hand the economic competitiveness and ecological aspects of the primarily wood-based multi-layer structural elements are investigated and compared to current technologies. The outcome of the project has to be communicated to the target audience in order to create attention for possibly future applications of the new construction technology. The results of the proposed research should primarily prove the practical applicability of the new structural elements, i.e. proof of readiness for first use of a new construction technology: Composition recipes for wood-concrete compounds as structural material are developed, of which the mechanical short- and long-term properties, workability characteristic and appropriate pretreatment methods for the wood particles have been preliminarily investigated. Structural composite elements, made of wood-concrete compound layers and beam or slab-type timber components, are conceptually designed for floors and walls and their structural performance is analyzed theoretically and experimentally in limited test series. Basic proposals for structural design approaches, structural modelling and structural details of the new building construction technology are developed and experimentally verified. First appraisals of the new multi-layer composite elements with regard to building physics, fire safety, as well as economic and ecological feasibility are available. The proposed technology combines concrete construction tradition with organic renewable resources as an alternative to conventional concrete or masonry, also opening opportunities to reduce carbon emissions through the increased use of wood for construction (since the carbon dioxide stocked in the wood is preserved for several decades instead of burning it). This opens new application fields for a smarter wood-based construction, with an increased volume and use of locally available wood from renewable resources. The partial substitution of energy consuming industrialized construction materials may lead to a more ecological, economic and sustainable building technology.

IntroductionThis project focuses on the development of new principles for structural elements made of wood and wood-based concrete. Besides a load-bearing function, these innovative construction elements also offer economic and ecological advantages, as they also contribute to thermal and acoustic insulation.Project descriptionThe project deals with applications of wood-based light-weight concrete in combination with timber elements on several levels: as a structural material (recipes, material laws, short-term behavior, production issues) in slab and wall elements (conceptual design, connections, short-term element behavior, structural design methods) with additional building-physical benefits (thermal insulation, heat storage, acoustic insulation), leading to economic and ecological advantages.BackgroundToday, cement-bonded wood products are mainly used for non-structural purposes, e.g. as noise or fire protection. However, wood-based concrete possibly in new mixtures could also be used in slab and wall elements and could thus contribute to load bearing. Currently, knowledge about load-bearing elements involving wood-based concrete is still too limited for practical application. In particular, data is lacking on the appropriate composition of wood-based concrete for specific uses, on the nature of joints to be used, on how completely slab and wall elements can be planned economically and on the dimensioning methods to be applied to these elements.AimDifferent mixtures of lightweight wood-based concrete are developed and assessed for their suitability as load-bearing material. Complemented by experiments on joints for the individual components, the results flow into the conceptual design of slab and wall elements. Using dimensioning methods rarely been applied before in wood construction, the bearing capacity of entire construction elements up to rupture is predicted and studied in large-scale load tests. Practice-oriented dimensioning approaches are derived from these results. Based on further pilot testing and case studies, other expected ad- vantages are assessed, e.g. thermal insulation and storage capacity and sound protection properties, as well as economic and ecological competitiveness.ResultsWood-cement compounds (or wood-based concrete), WCCs, were developed as a new structural material. They can be formulated such that they are pourable, self-compacting, lightweight, and economically viable. They contribute significantly to thermal storage capacity of construction elements, and their thermal insulation properties are equivalent to other lightweight concrete (e.g. with lightweight expanded clay aggregate LECA). Their mechanical properties (e.g. elastic modulus or compressive strength), on the contrary, are low, impeding their use as a substitution material for regular lightweight concrete. In construction elements, WCCs need to collaborate with other components in composite action. Practice-oriented proposals for modelling compressive behavior of WCC and timber were developed, derived from mechanically more sophisticated considerations. Still, WCCs can be recycled through combustion (e.g. in waste incineration), providing significant thermal energy and requiring no particular smoke treatment, nor special fire-fighting equipment. Due to important remaining from combustion, WCCs shall be combined with other combustible matter.Slab elements were conceived, tested experimentally and verified analytically, up to rupture. Practice- oriented recommendations for structural design and detailing were developed. Slabs made of timber and WCC provide structural performance apt for residential, office and school buildings. They also provide significant acoustic insulation. Wall elements made of timber and WCC were also conceived and tested experimentally, providing sufficient performance for buildings up to six stories. Practical structural design approaches were identified, in analogy to well-known methods. Such wall elements also contribute significantly to thermal insulation of buildings skins, allowing reducing (non-structural) insulation.Implications for research and practiceLoad-bearing elements containing wood-based concrete are lighter in weight than traditional timber- concrete composite elements and offer integrated sound and fire protection as well as beneficial thermal properties. Thanks to the high share of wood, these construction elements are largely based on renewable resources and they can be recycled as a combustible in heat production after dismantling. The developed dimensioning methods could make construction with wood and wood-based products more efficient and thus, contribute to the appropriate and more competitive use of Austrian forests and Austrian wood.