Motivation
When it comes to timber as sustainable and renewable construction material, it is usually equivalent to softwood, namely spruce. Hardwoods almost never appear in this context. This is despite the fact that significant amounts of hardwoods are already or will be available, e.g. in France, Germany or Austria due to the ongoing transformation of large areas of forests. It may be assumed that stronger utilization of hardwoods will result in two positive effects: better timber availability on the whole for the wood processing industry and the support of the European forestry strategy to increase the amount of hardwood as targeted in the European and national forest policies.
So far, hardwoods have seldom been used for timber structures due to versatile technical challenges, mainly specific species properties, which influence the whole building product chain. It is vital to create the required value added hardwood products to enable the economic and innovative application of hardwoods in the building sector. EU Hardwoods aims at delivering answers to recent questions for the various (production) processes which need to be considered. Appropriate processing will clearly differ from the processing of softwoods. The highest potential in the usage of hardwoods lies most commonly in glued timber products, e.g. glulam (homogenous = all lamellas are of the same species and have the same strength class as well as inhomogeneous = all lamellas are of the same species but have different strength classes) or cross laminated timber. Also hybrid products are in consideration, in which the main part consists of softwood supported by hardwood components utilizing their species dependant characteristics in an optimal way and thereby bolstering the performance of the product. Last but not least, some European hardwood species, e.g. oak, chestnut or black locust, exhibit natural resistances against weathering effects or biological attack comparable to tropical hardwoods. Thus, increased use of these wood species will substitute tropical hardwoods and protect these endangered ecosystems.
So far, hardwoods have seldom been used for timber structures due to versatile technical challenges, mainly specific species properties, which influence the whole building product chain. It is vital to create the required value added hardwood products to enable the economic and innovative application of hardwoods in the building sector. EU Hardwoods aims at delivering answers to recent questions for the various (production) processes which need to be considered. Appropriate processing will clearly differ from the processing of softwoods. The highest potential in the usage of hardwoods lies most commonly in glued timber products, e.g. glulam (homogenous = all lamellas are of the same species and have the same strength class as well as inhomogeneous = all lamellas are of the same species but have different strength classes) or cross laminated timber. Also hybrid products are in consideration, in which the main part consists of softwood supported by hardwood components utilizing their species dependant characteristics in an optimal way and thereby bolstering the performance of the product. Last but not least, some European hardwood species, e.g. oak, chestnut or black locust, exhibit natural resistances against weathering effects or biological attack comparable to tropical hardwoods. Thus, increased use of these wood species will substitute tropical hardwoods and protect these endangered ecosystems.
State of the art in hardwood application in the building sector
The approach of more extensive use of hardwoods in the building sector started only a few years ago, but the application still requires further development as innovative products are still rare. A small number of projects aiming at product development, technical approval and investigations towards standardisation have been performed so far, building the basis for the proposed project approach. These projects tackled the problems of appropriate strength classification and grading systems for the sawn hardwoods and their further manufacturing to construction components:
a) Hardwoods for construction purposes can be visually graded using national grading rules. EN 1912 allocates national (visual) grades to the European strength class system for hardwoods, i.e. so called "D" classes. Besides tropical hardwoods, beech, ash or oak from Germany as well as chestnut from Italy are allocated to strength classes as high as D40. For all other hardwood species and origins this strength classification system cannot be applied due to missing data or allocation.
b) Besides visual grading, the application of machine strength grading would be another option to allocate hardwood species to a specific strength class. However, so far no machine strength grading for hardwoods has been established, not even for glulam lamellas.
c) Only recently some technical approvals have been issued on a national basis regulating the
production and design value of some glued hardwood products:
Z-9.1-704 (DIBt) Glulam made of oak, Company Gamiz, Spain
Z-9.1-821 (DIBt) Glulam made of oak, Company Schiller, Germany
Z-9.1-679 (DIBt) Glulam made of beech or hybrid glulam combining beech and softwood, German association of producers of glued laminated timber products, Germany
d) Recently, first national approvals for adhesives have been issued on a national basis providing the evidence to be used for hardwood components, e.g. Z-9.1-807 (DIBt).
e) The former harmonized standard for glulam production EN 14080:2005 offers the possibility to produce CE-marked glulam made of hardwoods by testing of lamellas, finger-joints and whole glulam beams. However, this legal basis for hardwood products expired with EN 14080:2013 “Timber Constructions – Laminated Timber Requirements”, voted positively in EU formal vote just recently. At the moment, efforts concerning a CUAP (“Common Understanding of Assessment Procedure”) related to glued hardwoods products are made.
f) In some European countries research related to the use of hardwoods in the building sector has been performed, e.g. allocation of hardwood lamellas to European strength class system, gluing of hardwoods as well as loading capacity of these building construction elements. It has been focused on the main hardwood species, i.e. oak, beech and chestnut, accompanied by some smaller projects on ash or poplar. However, important knowledge gaps in production and characterisation of performance of hardwood glulam still exist, with the consequence that glulam made from hardwood may be manufactured and traded, but still no general admission in building regulations has been achieved such as glulam from softwood, and no discernible increase in the use of hardwoods has been reached so far. An equivalent status is reached for cross laminated timber.
a) Hardwoods for construction purposes can be visually graded using national grading rules. EN 1912 allocates national (visual) grades to the European strength class system for hardwoods, i.e. so called "D" classes. Besides tropical hardwoods, beech, ash or oak from Germany as well as chestnut from Italy are allocated to strength classes as high as D40. For all other hardwood species and origins this strength classification system cannot be applied due to missing data or allocation.
b) Besides visual grading, the application of machine strength grading would be another option to allocate hardwood species to a specific strength class. However, so far no machine strength grading for hardwoods has been established, not even for glulam lamellas.
c) Only recently some technical approvals have been issued on a national basis regulating the
production and design value of some glued hardwood products:
Z-9.1-704 (DIBt) Glulam made of oak, Company Gamiz, Spain
Z-9.1-821 (DIBt) Glulam made of oak, Company Schiller, Germany
Z-9.1-679 (DIBt) Glulam made of beech or hybrid glulam combining beech and softwood, German association of producers of glued laminated timber products, Germany
d) Recently, first national approvals for adhesives have been issued on a national basis providing the evidence to be used for hardwood components, e.g. Z-9.1-807 (DIBt).
e) The former harmonized standard for glulam production EN 14080:2005 offers the possibility to produce CE-marked glulam made of hardwoods by testing of lamellas, finger-joints and whole glulam beams. However, this legal basis for hardwood products expired with EN 14080:2013 “Timber Constructions – Laminated Timber Requirements”, voted positively in EU formal vote just recently. At the moment, efforts concerning a CUAP (“Common Understanding of Assessment Procedure”) related to glued hardwoods products are made.
f) In some European countries research related to the use of hardwoods in the building sector has been performed, e.g. allocation of hardwood lamellas to European strength class system, gluing of hardwoods as well as loading capacity of these building construction elements. It has been focused on the main hardwood species, i.e. oak, beech and chestnut, accompanied by some smaller projects on ash or poplar. However, important knowledge gaps in production and characterisation of performance of hardwood glulam still exist, with the consequence that glulam made from hardwood may be manufactured and traded, but still no general admission in building regulations has been achieved such as glulam from softwood, and no discernible increase in the use of hardwoods has been reached so far. An equivalent status is reached for cross laminated timber.
Aims of this project proposal
Previous experience indicates that possible utilisations of hardwoods are engineered products. The use of solid hardwood products is atypical mainly because production to date is too expensive. Therefore, in the future the predominantly manufactured building products made of hardwoods will be hybrid glulam or hybrid cross laminated timber (CLT) consisting of lamellas with smaller cross-sections than solid wood products. To fill the gaps of knowledge and technological developments for a broad application of hardwoods in the construction sector, the following tasks have been identified. This research proposal focuses on beech, oak, ash and Sweet chestnut with the following aspects:
i) Definition of the raw material with respect to availability and log quality. The aim is to define the potential for different roundwood qualities and grades including sawing potential / patterns.
ii) Compiling existing and/or new data to enhance the characterisation of the hardwood species of interest with respect to mechanical properties (strength, modulus of elasticity, density) to complement the database for a grading classification system (visual and machine strength grading)
as grading is the pre-requisite to widen the range of applications.
iii) Pre-normative testing and assessment aiming at certification of adhesives for the use with hardwoods.
iv) Development of solutions for the most important building products, respective components and dimensions for:
a) finger-joints in solid timber and finger-joints in lamellas,
b) homogeneous and inhomogeneous as well as hybrid glulam,
c) usage of hardwood glulam in service class 3 for weather exposed outdoor use and cladding.
v) Development of new high-tech products: cross laminated timber as a combination of hardwood and softwood lamellas. E.g. hardwood lamellas from beech can be placed in the inner zone to increase the (rolling) shear capacity in this zone which is often responsible for low strength values
of the whole plate. This fact enables the usage of a broad variety of strength grades of hardwoods in high quantity.
vi) Optimisation of the production processes of homogeneous and inhomogeneous glulam and cross laminated timber made from hardwoods on the basis of the results of i) to v) in terms of sawing and gluing.
vii) Derivation of strength models for glulam and cross laminated timber on the basis of strength properties of lamellas and finger-joints, validation through testing (see ii and iii).
The main aim is to transfer and complete the knowledge along the whole production line of hardwoods to broaden the use of hardwoods in an efficient and economic way in practice.
i) Definition of the raw material with respect to availability and log quality. The aim is to define the potential for different roundwood qualities and grades including sawing potential / patterns.
ii) Compiling existing and/or new data to enhance the characterisation of the hardwood species of interest with respect to mechanical properties (strength, modulus of elasticity, density) to complement the database for a grading classification system (visual and machine strength grading)
as grading is the pre-requisite to widen the range of applications.
iii) Pre-normative testing and assessment aiming at certification of adhesives for the use with hardwoods.
iv) Development of solutions for the most important building products, respective components and dimensions for:
a) finger-joints in solid timber and finger-joints in lamellas,
b) homogeneous and inhomogeneous as well as hybrid glulam,
c) usage of hardwood glulam in service class 3 for weather exposed outdoor use and cladding.
v) Development of new high-tech products: cross laminated timber as a combination of hardwood and softwood lamellas. E.g. hardwood lamellas from beech can be placed in the inner zone to increase the (rolling) shear capacity in this zone which is often responsible for low strength values
of the whole plate. This fact enables the usage of a broad variety of strength grades of hardwoods in high quantity.
vi) Optimisation of the production processes of homogeneous and inhomogeneous glulam and cross laminated timber made from hardwoods on the basis of the results of i) to v) in terms of sawing and gluing.
vii) Derivation of strength models for glulam and cross laminated timber on the basis of strength properties of lamellas and finger-joints, validation through testing (see ii and iii).
The main aim is to transfer and complete the knowledge along the whole production line of hardwoods to broaden the use of hardwoods in an efficient and economic way in practice.