Hybrid Timber Structures with Polyurethane

Transcription

1 Summary Report Zukunft Bau - research project Nr Hybrid Timber Structures with Polyurethane Research and development of extremely long-lasting, robust and weather-resistant hybrid structures composed of timber and sprayed polyurethane elastomer, which allows to extend the area of application for timber structures, timber buildings and structural elements made from timber. Authors: Prof. Dr.-Ing Volker Schmid Dipl.-Ing Özkan Yildiz Entwerfen und Konstruieren Verbundstrukturen Prof. Dr.-Ing. Frank U. Vogdt Dipl.-Ing. (FH) Falk Schaudienst M.A., Dipl.-Ing. Hendrik Kesslau Bauphysik und Baukonstruktionen submitted October 31th 2013 Hybrid timber structures with Polyurethane - Abstract 1

2 Der Forschungsbericht wurde mit Mitteln der Forschungsinitiative Zukunft Bau des Bundesinstitutes für Bau-, Stadt- und Raumforschung gefördert. (Aktenzeichen: SF / II 3 F ) Die Verantwortung für den Inhalt des Berichtes liegt bei den Autoren. Responsible: Prof. Dr.-Ing. Volker Schmid TU Berlin Fachgebiet Entwerfen und Konstruieren Verbundstrukturen Institut für Bauingenieurwesen Sekr. TIB1-B11 Gustav-Meyer-Allee Berlin Tel. Sekr.: Fax Sekr.: ek-verbundstrukturen@tu-berlin.de Univ.-Prof. Dr.-Ing. Frank U. Vogdt TU Berlin Fachgebiet Bauphysik und Baukonstruktionen Institut für Bauingenieurwesen Sekr. TIB1-B3 Gustav-Meyer-Allee Berlin Tel.: (030) Fax: (030) bauphysik@tu-berlin.de Hybrid timber structures with Polyurethane - Abstract 2

3 Summary Report: Hybrid Timber Structures with Polyurethane 1 Aim of the research project The aim of the project is the research and development of a new solution for the long-lasting protection of weathered timber structures. A 2-3 mm thick layer of 2K-Polyurethane, which is waterproof but at the same time permeable for water vapour, is sprayed on the surface of a structural timber element. This procedure is best suited for shop application but can be applied on the building site as well. The research project aims to find a solution to one of the most pressing problems in timber engineering - the long-lasting and reliable protection of timber against moisture and weathering. The findings are suited to increase the area of application for timber structures and the general use of timber, which is the most resource conserving and environmentally friendly construction material. If it can be proven that the Polyurethane thick film coating is as effective as the common constructive wood protection against weathering, an additional chemical treatment of the wood could be omitted, thereby saving costs and reducing environmental damage. The research project investigates the performance of a 2-3 mm thick PUR coating as a constructive protection against weathering for solid wood, gluelam, OSB, laminated veneer lumber and cross laminated timber. Tests are assessing the tensile strength and stiffness, the resistance to delamination, the gap-bridging capacity and the fatigue resistance of new and artificially aged PUR coatings. Full scale facade sandwich elements with PUR coating are tested in the EOTA facade testing stand, to measure the changing moisture content in the timber planks. A PUR coated roof-eaves-wall connection detail is subjected to large deformations, in order to define the efficiency and the limits of this type of hybrid system. Coated and uncoated gluelam beams are subjected to outdoor weathering conditions for more than two years, in order to investigate the changes in their moisture content. The test results indicate whether timber elements, which are protected by a 2-3 mm thick PUR-coating, can be classified in use class 2 instead of use class 3. In use class 2 the displacements due to creep would be reduced by more than 50% compared to use class 3 and the timber strength in use class 2 would be 29% higher than in use class 3. 2 Execution and results of the research project 2.1 General Since 25 years sprayed Polyurethane thick film coatings are successfully employed on concrete park decks and concrete roof slabs. The great advantage of sprayed PUR against a sealing with bituminous boards or synthetic waterproofing membranes is the easy application, especially for complicated geometries, which makes the sprayed waterproofing less prone to defective workmanship and execution mistakes. Most important for the long-lasting functioning of the thick film PUR-coating is its superior elasticity, with an elongation at break of about 300%. Together with its high resistance against delamination, the PUR coating is able to bridge large cracks, while remaining watertight. The application and performance of liquid applied waterproofing is defined in the ETAG 005: Guideline for European Technical Approval of liquid applied roof waterproofing kits. The guidelines do not specify the substrate for the sealing. Nevertheless the testing procedures described in the document assume that concrete is the preferred and common substrate. Till this day sprayed PURcoatings were applied on timber in a few pilot applications only, where the complex geometry or modern appearance of the building required new solutions to protecting the timber against Hybrid timber structures with Polyurethane - Abstract 3

4 weathering (Fig.: 1). Very important for the application of Polyurethane on timber is its permeability for vapour. A 2-3 mm thick PUR layer results in an s d -value of about 1,8 m, which allows the gradual regulation of the moisture content in the timber. Fig.: 1: Metropol Parasol, Seville: Pilot application of a 2-3 mm sprayed PUR coating on top of laminated veneer lumber V. Schmid 2.2 Costs In a preliminary design study the total capital costs are estimated for different external wall systems, which are built as a wood framed shear wall construction. The calculated costs include material, erection, maintenance and interests. The study compares external walls with four different facade systems: 1. An external facade with 2-3 mm sprayed Polyurethane on top of an OSB plate, 2. an external facade with an external thermal insulation composite system (ETICS), 3. an external facade with painted fibre cement boards and 4. an external facade with a weather boarding which is painted in any colour. In comparison to the painted weather board, which requires sanding and repainting the fading facade every three years, the maintenance of the PUR facade system is reduced to the repainting of the top coat every eight years. This top coat is responsible for the UV-protection of the Polyurethane and for the colour of the facade. Based on a live span of 80 years, which is the common presumption for Live-Cycle-Assessments, the PUR-facade system is 10% cheaper than the facade system with ETICS and 30% cheaper than the facade with painted weather boards or fibre cement boards (Fig. 2). Type of facade total capital costs for erection t=0 years total capital costs for erection t=40 years total capital costs for 80 years of maintenance capital value Weather board 75 /m² 19 /m² 126 /m² 221 /m² 2-3 mm sprayed Polyurethane 100 /m² 26 /m² 28 /m² 154 /m² Fibre cement boards 188 /m² 49 /m² 0 /m² 237 /m² ETICS 109 /m² 46 /m² 17 /m² 172 /m² Fig. 2: Comparison of the total capital costs of the four facade systems for different points in time Hybrid timber structures with Polyurethane - Abstract 4

5 stress [N/mm²] 2.3 Application Prior to the application of the Polyurethane the timber is primed with BASF Conipur 73. The Polyurethane coating consists of 2 components which are mixed in the sprayhead during the spraying process. The reaction takes less than 5 minutes, after that the coating can be walked on. The properties of the Polyurethane, including the impressive elasticity, do not change over time, because the PUR doesn t comprise any plasticizers. But it is important to maintain the top coating properly, because it is required to protect the PUR against the deteriorating effects of the UV radiation. For research purposes some of the specimens don t have a top coating and some are provided with the top coat Conipur TC-459 from BASF. In the research project the following two-component polyurethane coatings are tested: - AB PUR 550 from AB-Polymerchemie, Germany - Baytec Spray VP PU 0310/0309 from Bayer, Germany - Conipur M 803 FL from BASF. The coating company Reaku applies the different polyurethane coatings inside the great testing hall of the Technische Universität Berlin. The PUR is sprayed on different substrates: Gluelam (GL), cross laminated timber (CLT), OSB, MDF and laminated veneer lumber (LVL), all of them primed with Conipur Tensile tests with shouldered test bars of 2-3mm PUR-film At first the tensile strength and the stiffness of the 2-3 mm thick PUR-film itself is tested. For that purpose shouldered test bars are cut out of the PUR-film, which has been sprayed onto a plane and smooth surface. With each of the three PUR-coatings eight test series are performed: The test specimens are either unaged or artificially aged in water, heat or UV radiation according to ETAG 005 and either without top coating or painted with the top coat Conipur TC-459. Fig. 3 shows a typical tensile test diagram for shouldered tensile bars, using the example of Baytec Spray VP PU 0310/0309 without top coating. a) b) strain Fig. 3: PUR film in the tensile test: a) left: before test and right: before break b) tensile test diagram, using the example of Baytec Spray VP PU 0310/0309, without top coating F. Schaudienst, TU-Berlin Hybrid timber structures with Polyurethane - Abstract 5

6 The influences of the different methods of artificial ageing on the tensile strength and the strain at failure are listed in Fig. 4, using the example of AB PUR 550 from AB-Polymerchemie. As expected the modulus of elasticity increases for the test specimens with top coating, reversely their tensile strength is reduced by ca. 10%. Artificial ageing according to ETAG 005 reduces the tensile strength by another 10%. The elongation at fracture is measured in a range of 270% to 330%, depending on the method of artificial ageing and whether a top coat is applied or not. Method of artificial ageing Median value of tensile strength Without top coating Median value of elongation at fracture Median value of tensile strength With top coating Median value of elongation at fracture unaged 9,5 N/mm² 336 % 8,4 N/mm² 283 % artificial ageing in water 9,4 N/mm² 290 % 8,7 N/mm² 272 % artificial ageing with heat 8,8 N/mm² 307 % 7,9 N/mm² 271 % artificial ageing with UV-radiation 7,5 N/mm² 348 % 7,2 N/mm² 271 % Fig. 4: Tensile strength and elongation at failure, using the example of AB PUR 550 from AB Polymerchemie F. Schaudienst, TU-Berlin 2.5 Resistance to delamination for different types of wood and derived timber products The resistance to delamination of the Polyurethane coating is tested with the substrates CLT, OSB, MDF and LVL. The specimen were unaged and artificially aged according to ETAG 005. In all the test specimen the delamination occurres within the wood and not in the contact face between PUR and wood (Fig. 5). The resistance to delamination is exclusively governed by the tensile strength of the wood perpendicular to the grain. Fig. 5: Test of delamination strength of PUR on top of OSB and CLT. Failure occurs exclusively in the wood or derived wood products F. Schaudienst, Ö. Yildiz, TU-Berlin Hybrid timber structures with Polyurethane - Abstract 6

7 2.6 Test of crack bridging capacity Static tests The crack bridging capacity of the 2-3 mm thick Polyurethane coating is tested on the substrates OSB, MDF, cross laminated timber and gluelam. 75 unaged and 75 artificially aged specimens are tested in the first test series, with a crack opening from 0 mm to 1,5 mm at a temperature of -30 C (Fig. 6). The direction of the cracks is arranged either parallel or perpendicular to the grain direction, in order to assess the influence of the anisotropy of the wood. After the test the cracks are closing again. Following the crack bridging test, the watertightness is checked in the area of the crack, with all specimens passing the test. The crack opening is increased to 5 mm in the second test series with 68 specimens and again at a temperature of -30 C. All the cracks are closing after the tests and no leakage is observed. Using the example of AB PUR 550 on top of cross laminated timber, Fig. 7 shows some tensile diagrams from the test. Each curve indicates the mean value of a series of 3 to 5 tests per orientation of the crack and condition of ageing. Due to the large variance of the local timber quality and the different thickness of the tested PUR-coatings, the forces can give a rough guidance only. During the opening of the cracks, a very small localized delamination of the PUR-coating along the edge of the crack occurs and small pieces of wood below the PUR may split from the edges of the crack. This explains the discontinuity in the diagrams. Most important the result of the static tests proof that the Fig. 6: Left: Crack bridging test at -30 C. Right: Example with PUR on top of OSB V. Schmid Force [N] Force [N] Crack opening [mm] Crack perpendicular to grain Crack parallel to grain a) b) Crack perpendicular to grain, artificial ageing with heat Crack parallel to grain, artificial ageing with heat Fig. 7: Tensile test diagram for AB PUR 550: a) Series 1: crack opening 1,5 mm b) Series 2: crack opening 5,0 mm Hybrid timber structures with Polyurethane - Abstract 7

8 water content [kg/m³] relativ humidity [%] Temperature [ C] Fatigue tests 2-3 mm thick PUR coating remains water tight even for an opening of the cracks of up to 5 mm.75 unaged specimens and 75 specimens which are artificially aged in a heat treatment undergo a fatigue test in the climate chamber at a temperature of -10 C. The tested substrates are GL, OSB, CLT and LVL. The substrate is manufactured with a gap of 1 mm, which is carefully coated with the 2-3 mm Polyurethane, making sure that the gap is not filled with the PUR. During the test procedure the gap is 500 times alternately widened to 3 mm and reduced to 1mm. All tested PUR coatings pass the subsequent tests for watertightness. Fig. 8: Simultaneous fatigue test of 16 PUR coatings in the climate chamber at -10 C Ö. Yildiz, TU-Berlin 2.7 Facade tests The moisture transportation process through a sandwich wall element is tested in full scale with an external wall element of 7,34 m x 2,11 m, which is coated on the inside and outside with 2-3 mm Polyurethan. The sandwich element consists of 2-3 mm Polyurethan, 12 mm OSB, 160 mm ESP insulation, 16 mm OSB, 2-3 mm Polyurethan. Prior to the tests the moisture content in the wall element is calculated in a simulation with the software WUFI. The simulation is based on the accelerated WKI-cycles and calculates a maximum equilibrium moisture content of 80% in the cross section [mm] a) Results of simulation b) Test results relativ humidity [%] Temperature [ C] Fig. 9: a) Dynamic simulation of the moisture content in the sandwich wall b) test results for the relative humidity in the transition zone between EPS and outer OSB Hybrid timber structures with Polyurethane - Abstract 8

9 wood moisture [%] wood moisture [%] wood moisture [%] most critical location between the outer OSB plate and the outside PUR-coating and 50% between the OSB plate and the internal ESP insulation (green line in Fig. 9a). The simulation shows that there is no accumulation of moisture within the complete wall build-up. The maximum water content fluctuates between 50 and 70 kg / m³ (blue line in Fig. 9a). The light coloured lines indicate the results for the simulation of the preceding test cycles. The final test in the facade testing stand principally confirms the calculated results (Fig. 9b). Between the EPS insulation and the OSB plate a maximum equilibrium moisture content of 66% is observed, without any accumulation of moisture at this location. 2.8 Test specimens exposed to two years of direct weathering During an exposure test of two years the changing moisture contents of three types of test specimens are recorded (Fig. 10): - directly weathered timber, use class 3, without any protection. - directly weathered timber, use class 3. The timber is coated with 2-3 mm of Polyurethane. - timber below roof, use class 2, without any additional protection. For comparison only. Once per week the wood moisture is measured with a Gann Hydromette RTU 600. The measurements are taken 25 mm, 35 mm and 50 mm deep inside the wood. Due to the inaccuracy of the measurement procedure, every now and then the results give unrealistic values, for example if the outside temperature is below 0 C. Nevertheless the test results indicate reasonably well the Fig. 10: Measurement of wood moisture in gluelam. Top: directly weathered, middle: with PUR-coating, bottom: timber below roof in use class 2, for comparison only Hybrid timber structures with Polyurethane - Abstract 9

10 tendencies in the three test samples. As predicted, the moisture contend in the unprotected, directly weathered test specimens double in the first year, from 8% at the start to ca 16% and reaches its maximum value with more than 25% in the long and humid winter 2012/2013, showing significant fluctuations (Fig. 10 top). The timber below a roof in use class 2 retains a significantly lower moisture contents, with a medium value of less than 15% in wintertime (Fig. 10 bottom). Even better performs the timber which is protected by the Polyurethane coating and which is exposed to direct weathering according to use class 3. Starting from 8% the wood humidity increases to about 11% in summer and reaches only 13% in winter (Fig. 10 middle). The reduced variations of the moisture content indicate that the PUR coating has a significant damping effect. This is of great advantage for large timber structures and extended facade elements, because it reduces the shrinkage and extension in the timber. 2.9 Component test A new kind of roof-eaves-wall connection detail is developed with the aim to demonstrate the new structural possibilities which come with the 2-3 mm thick Polyurethane coating. The full scale detail is especially designed to study more complex crack geometries and failure modes. In order to simulate displacements and rotations in the tested connection, the outer end of the roof can be displaced to an extend well above the practically possible magnitude. Lowering the edge of the roof by 60 mm (Fig. 11a) results in a crack opening of 7 mm within the eaves. This induces shear and tension strains in the coating, which eventually cause the failure of the PUR-coating (Fig. 11b). However, a detailing for practical application instead for test purposes would make sure that the Polyurethane coating does not fail within the eaves. a) b) Fig. 11: Roof-eaves-wall connection detail with the forced displacement and subsequent crack inside the eaves. Ö. Yildiz, TU-Berlin 2.10 Workmanship Similar to all coating works, a high quality of workmanship is required to make sure that the sprayed Polyurethane coating protects the timber properly against weathering. The surface must be clean and dry to guaranty sufficient adhesive strength and to prevent small pores and bubbles caused by excessive humidity. Preferably the timber is coated in the shop, if applied on site, the execution work has to be attuned to the weather. As is the rule for common coating works, the edges should be rounded prior to the spraying with the Polyurethane. This avoids the sealing to be thinned out along the edge and subsequently being damaged (Fig. 12). Hybrid timber structures with Polyurethane - Abstract 10

11 Additionally the three components primer, sprayed PUR and top coat have to be applied and matched properly according to the manufacturer s instructions and approvals. a) b) Fig. 12: a) Rounded edges with constant thickness of PUR-coating b) Insufficient thickness of PUR coating along sharp edges V. Schmid, TU-Berlin 3 Conclusions The pilot projects and the research carried out at the Technische Universität Berlin suggest that a 2-3 mm thick sprayed Polyurethane coating is well suited for the constructive wood protection of timber against weathering. This technology allows architecturally and structurally new solutions for timber structures and timber buildings. Most important for the long lasting protection of the wood is the very high elasticity of the 2-3 mm thick Polyurethane coating, which allows this type of weather protection, other than the common thin film coatings, to bridge gaps and cracks and to follow the differential displacements of the substructure within a range of several millimetres without leakage. Due to the comparably long service intervals for the redoing of the top coat, the total costs for erection and maintenance are reduced against other coloured surfaces for timber facades. Timber elements which are protected by a 2-3 mm thick PUR coating experience a significantly reduced variation of wood moisture, resulting in a retarded shrinkage or expansion, which is especially beneficial for large buildings, bridges or facades. The PUR protection system is best applied as part of the prefabrication in the shop, were the quality of workmanship can be comparably easy assured. It is intended to continue the long duration tests with free exposure to weathering. Further research to look into the application in different climatic conditions and to develop efficient ways to assess the quality during application would be helpful as well as research in the different options for the non destructive testing of the finished structure. Acknowledgment This research project was funded by the Forschungsinitiative Zukunft Bau des Bundesinstitutes für Bau,- Stadt,- und Raumforschung (file reference: SF /II3 F ). We want to thank the manufacturers and companies Merk Timber, Hess Timber and IsoBouw Dämmtechnik for the donation of timber and derived timber products and AB-Polymerchemie, Bayer and BASF for providing the Polyurethane coating free of charge. Many thanks as well to the contractor Reaku who applied the primer, PUR-coating and top coats for free. The members of the advising committee Karl Moser, Ulf Cordes, Borimir Radovic and Rainer Schäpel gave their valued input, for which we thank warmly. This research project would not have been possible without the involvement and generous donations of the individuals and companies mentioned above. Hybrid timber structures with Polyurethane - Abstract 11