Lightweight-Wood-Concrete (LWC)

Transcription

1 Lightweight-Wood-Concrete (LWC) Summary Klaus Kreher Assistant Prof. Dr.-Ing. Technical University Vienna Vienna (Austria) Today s environmental awareness requires developing economically defendable products on the market. Therefore; energy-extensive products have to be replaced by more ecological ones. As a sort of regenerative material wood-chips and wood-flavour arise in the woodworking industry. These materials can be used as aggregate in the production of lightweight-concrete. Timber`s positive physical properties are applied to this lightweight-wood-concrete (LWC). The ability for sawing, screwing, nailing and drilling has positive effects on the processing. Budgeted costs are on the same level like conventional used concretes without wood. Negative influences on the hardening of concrete caused by several components of timber can be eliminated by help of suitable pre-treatment and added substances. Hence LWC is able to fulfil a big range of statically tasks. Due to the lower weight of the LWC, smaller cross sections are possible. The concrete cover can be reduced significantly with the additional use of textile fibres as reinforcement. Aim of the research was to describe the alliance between cement and fibres, to evaluate important parameters due to develop a dimensioning aid for constructions reinforced by textile fibres. The bearing capacity, bond and material constants have been pointed out with help of known analysis theories witch also have been added with new aspects. Real scale testing has to be done to show the application for LWC as pre fabricated façade elements, concrete cover for composite floor systems (stacked planks concrete), thermal or sound insulating walls. 1 INTRODUCTION In order to be able to compare measured data with analytic parameters necessary for dimensioning and design of LWC-elements, various testing has been undertaken for different applications of LWC-elements. One basic task to define was therefore parameters to describe the bondage between the textile fibres used to reinforce the LWC-elements and the matrix of the LWC, additionally to the loadbearing behaviour and the collapsing of the tested elements. Pullout-testing of textile fibres as well as pulled slabs were tested to define the bondage parameters. Four-point flexion tests and shear-strength-testing were subject of the investigations as well as the definition of the knowledge of the physical behaviour of the newly developed material. For restoration purposes used composite floor systems made of stacked planks an LWC have been developed. The favourable bearing in relation to comfort (vibrations) and sound-isolation were tested showing a big advantage compared to ordinary concrete-timber composite floor-systems. Both, the measurements as a result of the series of experiments and the modelling analysis show nearly the same bearing behaviour, either in the elastic branch or in the branch after cracking. The following are the major results of the latest research activities:

2 - Textile fibres and lightweight concrete decrease weight of structural elements - There is a pull-out phenomenon of internal fibres - Good mathematical description of alliance between cement and roving is shown by ACKtheory (proofed by finite-elements-simulation and pull-out testing, added with new aspects) - Differential equations give exact solution of displacements and stretches - Smaller sections of fibres have better alliance and less pull-out of internal fibres - Increasing degree of reinforcement (A f /A c ) provokes increasing number of cracks and better ductile load-bearing - The long-term load bearing of lightweight-wood-concrete has to be improved - Due to weight reduction and excellent stiffness timber-lwc-composite floor-systems show very good comfort according to the fundamental frequencies and sound insulation behaviour Further research of textile reinforced lightweight-wood-concrete can be recommended due to the results of the testing for construction parts which require good insulation characteristics as girder or ceiling elements. As application purposes, this material could be recommended for filigran elements such as floor covers for installation floors to replace former PVC elements or none statically used elements with increased insulation values, like building coats. Composites with stacked planks (the LWC functions as member in compression) show excellent performance in stiffness, dead load and insulation. None statically used lightweight-wood-concrete (depending on the mixture, and wood-chips content) can be also used as plastering for external surfaces, in restoration or as screed. 2 TESTING AND APPLICATIONS 2.1 Material properties In order to find mashing mixtures several ingredients had to be combined. The testing of the youngs-modulus and the compression strength defined important parameters. The creeping behaviour, weight and physical parameters have additionally been evaluated. Different cements, aggregates and adjuvants were added to find the best mixture for each application purpose. A pre-treatment of the wood aggregates was invented in order to eliminate disturbing effects of the concrete hardening caused by organic substances of the wood aggregate.

3 Fig 1: Specimen used fort he testing of material parameters; compression testing 2.2 Bondage between fibres and concrete matrix Pull out testing was done in order to get important knowledge of the bondage of fibres and the matrix of LWC. Microscopic analysis was needed to define the difference between fibre to fibre alliance and fibre to concrete contact.

4 Fig 2: Pullout testing with cubic specimen Fig 3: Microscopic view of the fibres in LWC-matrix

5 2.3 Structural elements Four point testing showed the load bearing behaviour and the collapsing of fibre reinforced LWC elements made for structural purposes as girders or plates. Fig 4: preparation of the textile fibres as reinforcement of structural LWC elements Fig 5: preparation of formwork for girder elements

6 Fig 6: different steps for the production of elements The load-bearing behaviour is almost similar to the behaviour of ordinary fibre reinforced concrete elements. An almost linear elastic load-deformation behaviour is followed by a plateau due to pull out phenomenon of the fibres. LWC and Elements made of fibre-reinforced composites show very good performance for insulation purposes, stiffness, lightweight constructing and restoration. Prefabricated façade elements were tested in combination to walls made of stacked planks for horizontal reinforcement. The so tested shear-walls showed good stiffness and ductility. Fig 7: detail of a Timber-LWC composite floor system

7 Fig 8: shear wall made of stacked planks and reinforced LWC façade element 3 CONCLUSION Due to the big variety of different mixtures of the LWC material there is a big range of different possibilities of applications. With special pre-treatment of the wood-based aggregate bad effects of organic substances can be eliminated, hence the LWC can be used for structural purposes as well as for restoration, plastering or screed. Combined with textile reinforcement very light elements for structural purposes can be produced, showing very good insulating properties and easy processing.

8 Fig 9: surface of LWC with natural finish LWC combined with stacked planks floor or wall systems (as compressed material) show excellent comfort (thermal and sound insulation, stiffness with little dead load, nice on top surface) and best processing for restoration. This offers ductile statically solutions (for earthquake safe constructions) and very good performance for reinforcement of timber-floor systems, by adding a layer of LWC. 4 REFERENCES [1] KREHER, K Textilbewehrter Holzleichtbeton; Diplomarbeit, EPFL, 1999 [2] GLINIORZ, K.-U Verbundverhalten von Glasfaserbündeln mit Leichtbeton, Présentation au Département de Génie civil de l'epfl, Thèse N 2288, IBOIS, Lausanne