bonding of timber

8/17/2019 Bonding of Timber

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COST Action E34

Bonding of Timber

Working Group 1: Bonding on site

Core document

Edited by

Klaus Richter and Helena Cruz

First Edition – March 2008

2008


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COST E34 - WG1: Bonding on Site

9

THE AUTHORS

(in alphabetical order)

Broughton, James Joining Technology Research Centre

School of TechnologyOxford Brookes UniversityWheatley CampusGipsy LaneOxford OX3 0BP [email protected]

Brunner, Maurice University of Applied SciencesSolothurnstrasse 102CH-2500 Biel 6 [email protected]

Cruz, Helena Laboratório Nacional de Engenharia Civil

Av. Brasil, 101P-1700-066 Lisboa [email protected]

Custódio, João Laboratório Nacional de Engenharia Civil

Av. Brasil, 101P-1700-066 Lisboa [email protected]

Lavisci, Paolo Legnopiù srlVia Borgo Valsugana, 11I-59100 Prato [email protected]

Lehmann, Martin University of Applied SciencesSolothurnstrasse 102CH-2500 Biel 6 [email protected]

Negrão João Dept. Civil Engineering U.C.Polo II - Pinhal de MarrocosP-3030-290 Coimbra [email protected]

Paula, Raquel STAPRua Marquês de Fronteira, 8, 3° Dto.P-1070-296 Lisboa [email protected]

Pizzo, Benedetto CNR-IVALSAvia Madonna del PianoI-50019 Sesto Fiorentino (FI) [email protected]

Rautenstrauch, Karl Bauhaus-University of WeimarFaculty of Civil EngineeringMarienstr. 13 AD-99423 Weimar [email protected]

Richter, Klaus EMPA, Wood LaboratoryÜberlandstasse 129CH-8600 Dübendorf [email protected]

Schober, Kay-Uwe Bauhaus-University of WeimarFaculty of Civil EngineeringMarienstr. 13 AD-99423 Weimar [email protected]


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Serrano, Erik SP Technical Research Institute of SwesenBuilding Technology and MechanicsWood TechnologySE-351 96 Växjö [email protected]

Smedley, Dave Rotafix LtdRotafix HouseUK-SA9 1UR Abercraf Swansea [email protected]

Steiger, René EMPA, Wood LaboratoryÜberlandstrasse 129CH-8600 Dübendorf [email protected]

Van Leemput, Marc CTIB-TCHN

Allée Hof ter Vleest 3B-1070 Brussels [email protected]

Acknowledgement

We express our gratitude to all authors who voluntarily and with no specific funding have contributed tothis core document within their research area.

Klaus Richter and Helena Cruz

Chairpersons of COST E34 WG1 - Bonding on site

August 2007


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TABLE OF CONTENTS

1 INTRODUCTION 13

2 TIMBER CONCRETE COMPOSITES 15 2.1 Timber-concrete composites with mechanical fasteners.......................................................... 15

2.2 Design methods........................................................................................................................ 17 2.3 Timber-concrete-composites with an adhesive bond ............................................................... 18 2.4 Research needs........................................................................................................................ 19 2.5 References ............................................................................................................................... 19

3 TIMBER WITH P ASSIVE REINFORCEMENT 22 3.1 Overview................................................................................................................................... 22 3.2 Literature survey....................................................................................................................... 22 3.3 Design methods........................................................................................................................ 24 3.4 Economic considerations.......................................................................................................... 26 3.5 Steel reinforcement................................................................................................................... 27 3.6 Research needs........................................................................................................................ 27 3.7 References ............................................................................................................................... 28

4 PRE-STRESSING OF TIMBER 29 4.1 Overview................................................................................................................................... 29 4.2 Calculation Methods................................................................................................................. 33 4.2.1 Calculation example of pre-stressed timber beam................................................................... 34 4.3 Research needs........................................................................................................................ 35 4.4 References ............................................................................................................................... 36

5 GLUED-IN RODS 37 5.1 Overview................................................................................................................................... 37 5.2 Design methods........................................................................................................................ 37 5.2.1 Basic assumptions.................................................................................................................... 37 5.2.2 Mechanics – Failure modes and design philosophy................................................................. 39 5.2.3 Design codes and code proposals ........................................................................................... 40

5.3 Typical application methods ..................................................................................................... 43 5.3.1 Manufacturing principles........................................................................................................... 43 5.3.2 Examples.................................................................................................................................. 43 5.4 Applicable standards ................................................................................................................ 44 5.5 Research needs........................................................................................................................ 45 5.6 References ............................................................................................................................... 45

6 ON SITE INTERVENTIONS ON DECAYED BEAM ENDS 46 6.1 Overview................................................................................................................................... 46 6.2 Design methods........................................................................................................................ 47 6.3 Methods of application.............................................................................................................. 50 6.3.1 Repair of decayed ends using adhesives and steel reinforcement.......................................... 50 6.3.2 Repair of decayed ends using epoxy polymer concrete and GFRP reinforcement

(bars and plates)....................................................................................................................... 51 6.4 Applicable standards ................................................................................................................ 53 6.5 Research needs........................................................................................................................ 54 6.6 References ............................................................................................................................... 55

7 REPAIR OF GLUED LAMINATED STRUCTURES 56 7.1 Description of glued laminated components............................................................................. 56 7.2 Failure types and repair options ............................................................................................... 56 7.2.1 Wood decay due to inadequate construction details................................................................ 56 7.2.2 Glue line delamination and fissures.......................................................................................... 59 7.3 Research needs........................................................................................................................ 61 7.4 References ............................................................................................................................... 61

8 F ACTORS INFLUENCING BOND PERFORMANCE 62

8.1 Introduction............................................................................................................................... 62 8.2 Environment.............................................................................................................................. 63 8.2.1 Moisture content ....................................................................................................................... 63 8.2.2 Temperature ............................................................................................................................. 64


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8.3 Materials ...................................................................................................................................66 8.3.1 Surface preparation .................................................................................................................. 66 8.3.2 Age of surface...........................................................................................................................67 8.3.3 Influence of wood species ........................................................................................................ 68 8.3.4 Treated wood............................................................................................................................ 68 8.4 Stress........................................................................................................................................ 69

8.4.1 Influence of material stiffness on stress ................................................................................... 69 8.4.2 Influence of joint geometry on stress........................................................................................ 70 8.4.3 Joint selection for the assessment of bond performance ......................................................... 74 8.5 Research Needs ....................................................................................................................... 75 8.6 References................................................................................................................................ 76

9 DURABILITY OF HISTORIC STRUCTURES REPAIRED WITH ADHESIVES 80 9.1 Overview...................................................................................................................................80 9.2 Examination of past interventions.............................................................................................80 9.2.1 ‘Structural’ type evaluations......................................................................................................81 9.2.2 Evaluations regarding the durability of the interface.................................................................81 9.3 Considerations based on the inspections................................................................................. 82 9.4 Summary and recommendations.............................................................................................. 83 9.5 Research needs........................................................................................................................ 84 9.6 References................................................................................................................................ 85

10 QUALITY CONTROL ON SITE 86 10.1 Introduction...............................................................................................................................86 10.2 Quality control of materials ....................................................................................................... 86 10.2.1 Solid timber splice..................................................................................................................... 87 10.2.2 Epoxy adhesives and grouts..................................................................................................... 87 10.2.3 Metallic rods and plates............................................................................................................ 87 10.2.4 FRP rods and plates ................................................................................................................. 87 10.3 Quality control of tools and equipment ..................................................................................... 87 10.3.1 Timber cutting and drilling slots ................................................................................................ 88 10.3.2 Surface cleaning....................................................................................................................... 88 10.3.3 Mixing and application .............................................................................................................. 88

10.3.4 Tool maintenance ..................................................................................................................... 88 10.4 Quality control on site ............................................................................................................... 88 10.4.1 Contract Preparation................................................................................................................. 88 10.4.2 Removal of decayed timber ...................................................................................................... 88 10.4.3 Drilling and slotting.................................................................................................................... 89 10.4.4 Cleaning of bonded surfaces.................................................................................................... 89 10.4.5 Mixing........................................................................................................................................ 89 10.4.6 Installation of secondary adherends......................................................................................... 90 10.4.7 Manufacture of the solid timber splice (TRS) ...........................................................................90 10.4.8 Quality control for generic repair systems ................................................................................ 90 10.4.9 Health and safety...................................................................................................................... 92 10.5 Quality plan...............................................................................................................................92 10.5.1 Responsibilities and records..................................................................................................... 92

10.5.2 Reception of materials .............................................................................................................. 92 10.5.3 Inspections and tests ................................................................................................................ 92 10.6 Certification of operatives ......................................................................................................... 94 10.6.1 Introduction...............................................................................................................................94 10.6.2 Training..................................................................................................................................... 95 10.6.3 Certification procedure.............................................................................................................. 95 10.6.4 Theoretical examination............................................................................................................ 95 10.6.5 Practical examination................................................................................................................ 95 10.6.6 Evaluation .................................................................................................................................95 10.6.7 Inspection and Testing.............................................................................................................. 95 10.7 References................................................................................................................................ 96

11 SUMMARY OF ACHIEVEMENTS 97


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Introduction 13

1 INTRODUCTION

Richter K., Cruz H., Negrão J.

Despite the development of key innovations for timber engineering and timber industries worldwide due to

the growing use of adhesives and adhesive bonding techniques, the requirements for durable high qualitybonds are demanding and remain a significant challenge; apart from the adhesive and timber quality, theapplication of the adhesives and subsequent control of the curing processes are of critical importance tobond durability.

In the glulam and general timber construction industries, the overwhelming majority of all structuralbonding processes are performed off site under factory controlled conditions. Here the control ofsignificant climatic factors, which are necessary for adequate adhesive spreading and curing, can berelatively easily maintained within acceptable limits. Similarly, surface processes like sanding, planningand pressing are almost entirely automated.

O the contrary, bonding is necessary when large structural timber parts need to be assembled at thebuilding site where process variables and the environment are difficult to control. In many cases suchassemblies are formed with steel connectors, but for special situations adhesive bonds are required to

meet the aesthetic or technical demands of the designer.Other applications of structural on site bonding are related to the rehabilitation of buildings, which is anarea of increasing economical and social importance to most European countries. A great number ofhistoric buildings are either of common timber frame construction or incorporate complex timberstructures. Both types require specific interventions, including additional reinforcement or repair due tooverloading, insect attack or decay due to fungal activity, or bond line delamination in bonded structuralelements. Similar on site bonded reinforcement and repair techniques have been applied for somedecades following procedures developed for the repair or up-grading of other structures - the adhesivesbeing used either on their own or in conjunction with steel plates, rods, fibre reinforced materials andeven concrete. Such techniques are versatile, require less time and are more cost effective thantraditional carpentry methods, and, most importantly, help to minimise any disturbance to the building andto its occupants during the intervention.

Yet, despite the fact that many historical timber structures require urgent and proper maintenance andrepair works, it is often the case that neither decision makers nor building contractors have the necessaryknowledge to apply these techniques appropriately and, as a result, damaging interventions are oftenmade.

Other concerns include a lack of suitable methods, whereby sufficient reliability of the bonded connectioncan be guaranteed. One general reason for this is that to date a long service life has not been fully provenfor synthetic adhesives; the oldest bonded joints are only approximately sixty years of age. Furthermore,reliable and representative accelerated ageing tests do not yet exist. Suitable on site quality control testmethods required for acceptance of the bonding process and conditions both of which are especiallydifficult to control on the building site, are also missing.

The variability of materials and the insufficient quality control level, typical of on site conditions, alsocontribute to reduce the confidence in glued systems. However, it should be remarked that, from all thepossible materials involved (timber, steel, GFRP’s, concrete and the adhesive itself), timber is the onefrom which most of the variability should be expected.

Of course, not all adhesives are suitable for on site bonding. It is therefore necessary to be able to selectsuitable adhesives as well as to improve existing adhesives, or, develop alternative adhesiveformulations, suitable for indoor (and even outdoor) jobs on site where, for example, the use of pressureis generally not available and clean regular bond lines are difficult to achieve.

In order to develop and assess new or improved adhesives, suitable test methods are needed. Yetexisting test methods, used to evaluate bond line performance of timber bonded joints, were originallydeveloped for phenolic or aminoplastic based products. These have been repeatedly proven to beinadequate in the assessment of the more favoured epoxy type adhesives or epoxy bonded products.Moreover, the application of these existing EN or national test and performance standards for epoxybonded products are much too penalising, since they merely impose severe conditions that are notverified on site. The need for development of European standards for the evaluation of bond durability(namely under high service temperature or humidity) as well as the long-term performance of epoxy


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14 Introduction

adhesives has already been identified by CEN, but to date no work has been undertaken. The lack ofstandards and calculation methods in this field impedes an objective evaluation of the safety level andreliability of a glued connection, causing engineers to avoid this type of solution. Thus extensive pre-normative research and thorough consideration of these problems is still required.

The brittle nature of most adhesives available for on site application needs also to be addressed. Thisissue makes difficult the accommodation of stresses caused by slight relative movements of the gluedinterfaces or withstanding intense shear stress gradients over long lengths.

In conclusion, the rehabilitation of timber structures has increasing economical, environmental and socialimportance, and timber Bonding on Site (BoS) has an important role to play in this area. However, thereare a number of issues currently hindering the wider exploitation of BoS across Europe. These havebeen identified as the following:

There is a lack of well-structured and concise knowledge on bonded reinforcement or repairtechniques for timber

BoS process and conditions are difficult to control (e.g. bondline thickness, surface properties,bondline stresses and environmental conditions)

Adhesives for BoS are not specifically developed for timber

Appropriate test methods and standards for BoS adhesives are lacking Rapid on site assessment methods (control of mixture and penetration, viscosity) are missing.

This core document therefore aims to:

Present qualitative and quantitative knowledge on structural BoS techniques

Enable the effective and safe application of reinforcement, repair and assembly of on sitebonding techniques

Disseminate knowledge to industry, research society and practitioners.


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Timber Concrete Composites 15

2 TIMBER CONCRETE COMPOSITES

Brunner M., Negrão J., Rautenstrauch K., Schober K.U.

2.1 Timber-concrete composites with mechanical fasteners

Given the low cost of concrete and its mechanical complementarity with timber, its use in timber-basedcomposites was one of the first to be considered. Mechanical fasteners were systematically used asconnecting devices. This chapter points out some relevant studies on this subject and attempts to providean overview. Though the scope of this document is focused on adhesive bonding, it is neverthelessimportant to discuss those studies concerning mechanical connection (i.e., with fasteners), because ofthe insight they provide on aspects such as ductile behaviour and partially composite behaviour.

Many aspects of timber-concrete composites behaviour have been investigated by a number of authors inthe last decade. The following list is not exhaustive and is an attempt to provide an overview of what hasbeen done and what is still missing.

Ceccotti [1] has written a very illustrative paper describing the main issues concerning the analysis anddesign of timber-concrete composite elements. Gutkowski et al [2, 3] have investigated the performanceof composite elements with the interlayer connection consisting of a notch and a shear key (Fig. 2.1).While the shear stresses were transferred by compression on the notch sloped surface, the role of theshear key was to prevent the uplift force which tended to separate timber from concrete. This system ledto a very effective composite behaviour, with reduced interlayer slippage, but no long-term or moisturecontent change effects were considered.

Figure 2.1: Notch and shear key connection

Bathon and Graf [4] have proposed the use of a two-dimensional steel mesh as a continuous shearconnector (Figure 2.2). The mesh is inserted and glued into a slot sawn in the timber. Although used as asystem component only, the adhesive plays a fundamental role in that it is the secondary stress transfermedia between timber and concrete. Therefore, from a conceptual point of view, this solution should beplaced half-way between “conventional” shear connector and an adhesive bond. More recently Bathon [5]enhanced the concept by introducing pre-stressing, prior to casting the concrete layer, achieved by anupward cambering of the timber beam. The tests were successful. The system failure was caused by the

plastic yielding of the steel mesh, whereas the adhesive bonding, timber or concrete all withstood theload.


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16 Timber Concrete Composites

Figure 2.2: Continuous mesh shear-connector

Ballerini, Crocetti and Piazza [6, 7] tested the performance of the notch plus shear key system fordifferent notch depths and angles, sizes of timber shear area and position of the screw (Figure 2.3). Intheir conclusions, they underline the need to develop a standard test set-up. Though the sentencereferred to mechanical timber-to-concrete connection, such document should desirably be suited for

testing both fastener and adhesive bonding of timber to concrete.

Hydraulic

jack

Steel Plate

Concrete

Timber

Teflon

plate

h

Lt

120 mm

Empty notch

α

LVDT

107 mm

Figure 2.3: Test set-up proposed in [6, 7]

Benitez [8] performed comparative studies between three types (Figure 2.4) of mechanical shearconnectors: smooth steel dowels at an angle of 60° to the force direction, a ring-type connector (CHS,circular hollow section) inserted into a fitted slot in the timber and fixed to it by a central screw) and an I-shaped connector made up from a UC (universal column) steel hot rolled section, fixed by screws to thetimber.

Figure 2.4: Shear connectors studied by Benitez [8]

A specifically designed shear connector was tested and FE modelled by Bou Said et al. [9]. Theconnector is “flower-shaped”, with holes in the petals to enhance the anchoring with concrete (Figure 2.5).Inoue et al. [10] used either deformed bars or special rods (Figure 2.6) as shear connectors.

Mungwa, Jullien et al. [11, 12] developed a new type of shear connector specifically designed for fastinstallation while still maintaining good stiffness, strength and ductility characteristics.

Gelfi and Giuriani [13] tested the use of stud connectors obtained from ordinary steel bars in timber-concrete connections with and without an interlayer of planks.


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Figure 2.5: Shear connector Figure 2.6: Shear connector

2.2 Design methods

A fundamental condition for the actual application of timber-concrete composite beams/floors is theavailability of design methods and recommendations.

The Pre-standard ENV 1995-2:1997 (Eurocode 5 – Part 2: Bridges) proposed a simplified method for theanalysis of composite timber-concrete beams made up with metallic shear connectors. However, most ofthese provisions were withdrawn in the newly approved version of this standard, in which still noreference is made to the interface bonded with an adhesive.

The work of van der Linden [14] is a comprehensive attempt to set design rules for the analysis ofcomposite floors made up with shear connectors. He based his proposal in the assumption of linearelastic constitutive laws for both the concrete and the timber, while on the steel connectors wereconsidered to provide a continuously distributed stiffness along the span and to follow an elastic-perfectlyplastic rule.

In Germany, the so-called "Gamma-Method" is widely used for the design of timber-concrete-compositestructures. The so-called "Gamma-factor" accounts for the fact that the shear joint between the concrete

and the timber is elastically deformable and the corresponding slippage leads to higher bending momentsin the timber and concrete components than would be the case for a stiff joint without slippage. In arecent paper, Kaliske and Schmidt [15] point out that in many practical cases ductile shear connectors areused. The use of the elastic design rules of the "Gamma-method" is quite conservative. The authorsargue for a corresponding change in the design of the ultimate limit state by taking into account thefavourable redistribution of the internal forces thanks to the plastic deformation of the connectors.

Grosse and Rautenstrauch [16] and Grosse et al. [17] proposed constitutive laws for the numericalmodelling of timber and timber-concrete composites. Such laws are based on the theory of plasticity, withflow and hardening rules conveniently adapted to the well-known parallel- and perpendicular-to-the grainbehaviour of timber. Numerical models for composite connection elements, based in these constitutivelaws, were tested, showing good agreement with the experimental data.

Frangi and Fontana [18] have formulated design equations for the shear connectors. They studied both

an elastic model for the service limit state and elastic-plastic models for the ultimate limit state. In anotherpaper [19], the same authors discussed the fire behaviour of timber-concrete slabs.

Demarzo and Tacitano [20] proposed an approximate method for the analysis of composite timber-concrete elements, but one of the basic assumptions of the method is that of the linear elastic model forthe load-slip constitutive relation of the connectors, which is severely restrictive in conditions of ultimateloading.

Dias et al [21] described an ongoing research on the development of non-linear finite element models tosimulate the behaviour of these structures.

Jorge et al [22-24] investigated the advantages of using lightweight instead of normal concrete for thetopping layer. This same type of concrete was used in the experiments of Rajčić and Rak [25] and Rajčić and Zagar [26] who tested three different possibilities of interlayer bonding, including direct bondingbetween timber and fresh lightweight concrete, with encouraging results supporting this latter solution.

Toratti et al. [27] have measured the response of timber-concrete floors in both laboratory and siteconditions and confirmed the good behaviour with regard to vibrations induced by walking. The opposite


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result was achieved by Lee et al. [28] however, who found that, in spite of the enhanced stiffness andstatic response, the lower natural frequencies due to the increased mass could result in a strongervibration response.

One important aspect with timber-concrete composites is the long-term performance, due to the differentcreep and mechano-sorptive behaviours of the component materials. Amadio et al. [29, 30] introducedand discussed a finite-element based procedure for the evaluation of these long-term effects. Long-termtesting was also conducted by Ceccotti. A brief description of the tests and main conclusions may befound in [1]. The same author and Fragiacomo [31] proposed a simplified method for accounting the long-term effects resulting from loading, shrinkage, creep and daily and yearly changes in environmentalconditions. An ongoing experimental program on the creep behaviour of composite timber-lightweightconcrete is referred to in [1]. The creep of timber-concrete bonding with metallic shear connectors hasalso been monitored for a few years by Dias [32]. Kuhlmann and Schänzlin [33] proposed the use ofeffective creep coefficients which account for the different creep rates for concrete and for timber. Theyalso propose that shrinkage could be accounted for by introducing a fictitious load. Flach and Frenette[34] discussed the application of the timber-concrete composite concept to actual bridge designs andpoint out that a deeper insight into the long-term behaviour is required in order to ensure reliability of suchsolutions.

2.3 Timber-concrete-composites with an adhesive bond

There are relatively few publications dealing with wood-based composites made up with an adhesivebond between timber and other materials. Besides the aforementioned work of Rajčić and Rak [25], inwhich adhesives were considered as one of the alternative bonding systems under study, regularresearch in this domain has only been pursued by Rautenstrauch et al. [35-39], which will be referred tolater on in this section, and Brunner et al . [40-43]. In [40] the first tests in structural size composite beamsconnected with this system are described. The theoretical failure loads were estimated with an elasticcalculation model assuming a stiff bond with no slippage between the timber and the concrete layers.There was remarkable agreement between the ultimate test loads and the expected failure loads(theoretically). A simplified evaluation of the long-term response was undertaken, in which thecontribution of timber was neglected, giving some simplifying yet restrictive assumptions. Furthermore,they state that most of the long-term deformation is apparently caused by concrete shrinkage rather thanby creep effects. In order to clarify the effect of the interaction of materials with different shrinkage andcreep behaviours, there is an ongoing research on the topic. In [41], Brunner and Gerber conclude thatbonded joints can withstand severe climatic changes - particularly with regard to moisture content -without significant strength loss, which is a point in favour of the reliability of this bonding system.

Brunner, Romer and Schnueriger [44] have published some recent work on timber-concrete-compositeslabs. They studied the wet-in-wet process, whereby the concrete is poured onto the still wet adhesive onthe timber component. They made a parameter research to find out the most favourable conditions for theadhesive bond and found out, among other things, that the best shear bond was attained when there wasa short time interval to permit the wet adhesive to stiffen before the concrete was poured. They castseveral test specimens, without and then with openings in the concrete, to simulate the many conduitswhich are often placed in concrete slabs in Switzerland. All the test specimens failed at loads whichcorresponded very well to the calculation results.

The ability of timber to be adhered to concrete is also being investigated by Negrão [45], with theexpectation of the application of such technique to composite timber-concrete beams and slabs.

Another development using timber-concrete composites for structural applications is the replacement ofcement bounded concrete by epoxy-resin bounded polymer concrete (PC). Polymer concrete is acomposite material formed by combining mineral aggregates such as sand or gravel with a monomer.Rapid-setting organic polymers are used in PC as binders. Studies on epoxy polymers have shown thatcuring method, temperature and strain rate influence, the strength and stress-strain relationships. PC isincreasingly being used as an alternative to cement concrete in many applications. Today, polymerconcrete is used for finishing work in cast-in-place applications, precast products, highway pavements,bridge decks and waste water pipes, thereby developing better PC systems and characterizing thecompressive strength in terms of constituents are essential for the efficient utilization of PC. However, thedata on epoxy PC are rather limited, and there is an increasing interest in the deformation characteristics

under working conditions in combination with other materials such as wood for composite structures.Epoxy resin-based polymer concrete can be combined with timber floors for upgrading without


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necessitating the removal of the suspended ceiling. This technique is likely to be suitable for thereinforcement of timber floors and historical structural wood components [46].

2.4 Research needs

Timber-concrete-composites with mechanical connections have been widely researched. The techniqueis frequently used in engineering practice. New research will always be necessary when new connectionsare proposed. The current calculation methods are based on elastic methods like the Gamma-Method.Some recent publications indicate that plastic design methods may lead to a more economic use of theconnectors, which are often rather expensive. This new research field needs to be intensified. Timber-Concrete-Composites with an adhesive connection are a relatively new research field which has not yetattracted the attention of many researchers. There is a need for further research activities in this field inorder to confirm and to validate the available findings. There are several open questions which need to beaddressed in an interdisciplinary approach by timber engineers and chemical engineers, e.g. with regardto an optimization of the adhesive types, the best adhesive formulations and quantities needed, as well assurface preparation. Furthermore, cyclic and durability requirements under diverse environmentalconditions need to be further analysed, this is a current research activity at Bauhaus-University of Weimartill 2008.

Apart from technical questions, research is also needed on economic issues such as:

Timber-concrete composite structures with natural bond (no mechanical fasteners)

Prefabrication: factory type of manufacturing to assure quality and to cut down costs.

Connections on site.

On site production: quality management, cost effective techniques.

2.5 References

[1] Ceccotti, A. 'Composite concrete-timber structures', Progress in Structural Engineering Materials 4(2002) 264-275

[2] Gutkowski, R., Balogh , J., Natterer , J., Brown, K., Koike, E. and Etournaud, P. 'Laboratory tests ofcomposite wood-concrete beam and floor specimens'. WCTE, Whistler, Canada, 2000, paper 8-2-1

[3] Gutkowski, R., Thompson, W., Brown, K., Etournaud, P., Shigidi, A. and Natterer, J. 'Laboratorytests of composite wood-concrete beam and deck specimens'. 1st RILEM Symposium on TimberEngineering, Stockholm, Sweden, 1999, 263-271

[4] Bathon, L. and Graf, M. 'A continuous wood-concrete-composite system'. WCTE, Whistler,Canada, 2000, paper 8-2-2

[5] Bathon, L. and Clouston, P. 'Experimental and numerical results on semi pre-stressed wood-concrete composite floor systems for long span applications'. World Conference TimberEngineering, Lahti, Finland, 2004, 339-344

[6] Ballerini, M., Crocetti, R. and Piazza, M. 'An experimental investigation on notched connections fortimber-concrete composite structures '. WCTE, Malasya, 2002, paper 4.4.4

[7] Piazza, M. and Ballerini, M. 'Experimental/numerical results on timber-concrete composite floorswith different connection systems'. WCTE, Whistler, Canada, 2000, paper P50

[8] Benitez, M.F. 'Development and testing of timber/concrete shear connectors'. WCTE, Whistler,Canada, 2000, paper 8-3-2

[9] Bou Said, E., Jullien, J. and Siemers, M. 'Non-linear analysis of local composite timber-concretebehaviour'. WCTE, Malasya, 2002, paper 2.2.5

[10] Inoue, M.e.a. 'Development of connecting system between reinforced concrete and timber'. WCTE,Malasya, 2002, paper 10.4.1

[11] Jullien, J.F., Michel, G., Mungwa, M.S. and Siemers, M. 'A new shear connector for wood-concretecomposite structures'. 1st RILEM Symposium on Timber Engineering, Stockholm, Sweden, 1999,563-570


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[12] Mungwa, M.S., Jullien, J.F., Foudjet, A. and Hentges, G. 'A new shear connector for wood-concrete composite structures'. WCTE, Montreux, Switzerland 1998, 518-525

[13] Gelfi, P. and Giuriani, E. 'Stud shear connectors in wood-concrete composite beams'. 1st RILEMSymposium on Timber Engineering, Stockholm, Sweden, 1999, 245-254

[14] Linden, M.v.d. 'Timber-Concrete Composite Floor Systems', PhD Thesis, Technical University of

Delft, The Netherlands, 1998[15] Kaliske, M. and Schmid, J. 'A new design approach for timber-concrete-composite beams'. COST

C12 Final Conference, Innsbruck, Austria, 2005,

[16] Grosse, M. and Rautenstrauch, K. 'Numerical model of timber and connection elements used intimber-concrete composite construction'. CIB W18 Meeting 37, Edinburgh, UK, 2004,

[17] Grosse, M., Lehmann, S. and Rautenstrauch, K. 'Testing connector types of laminated timber-concrete composite elements'. CIB W18 Meeting 34, Venice, Italy, 2001,

[18] Frangi, A. and Fontana, M. 'Elasto-Plastic Model for Timber-Concrete Composite Beams withDuctile Connection', Structural Engineering International-IABSE 1 (2003)

[19] Frangi, A. and Fontana, M. 'Fire behaviour of timber-concrete composite slabs'. WCTE, Montreux,Switzerland, 1998, 76-83

[20] Demarzo, M.A. and Tacitano, M. 'Alternate method to elastically coupled timber-concrete beams'.WCTE, Malasya, 2002, paper 3.2.2

[21] Dias, A., Kuilen, J.d., Cruz, H. and Lopes, S. 'Non-Linear FEM models for timber-concrete jointsmade with dowel type fasteners'. WCTE, Lahti, Finland, 2004, 371-376

[22] Jorge, L., Cruz, H. and Lopes, S. 'The Use of Lightweight Concrete in Composite Timber-ConcreteFloors (in Portuguese)'. ENCORE, Lisboa, Portugal 2003, 901-906

[23] Jorge, L., Cruz, H. and Lopes, S. 'Tests in Timber-LWAC Composite Beams with Screw-typeFasteners'. WCTE Lahti, Finland 2004, 559-564

[24] Jorge, L., Cruz, H. and Lopes, S. 'Experimental research in timber-LWAC composite structures'.Int. Symp. Adv. Timber and Timber-Composite Elements Build, Florence, Italy, 2004,

[25] Rajčić, V. and Rak, M. 'Continuous shear connecting - The best way to compose timber andlightweight (eps) concrete'. WCTE, Malasya, 2002, paper 4.4.5

[26] Rajčić, V., Zagar, Z. 'FEM models of composite timber-lightweight concrete floor systems'. WCTE,Whistler, Canada, 2000, paper P45

[27] Toratti, T., Talja, A. and Järvinen, E. 'Classification of human-induced floor vibrations in buildings: awood-concrete composite floor example'. WCTE Malasya 2002, paper 4.1.3

[28] Lee, P., Chui, Y.H. and Smith, I. 'Dynamic and static performance of wood floor with concretetopping'. WCTE, Malasya, 2002, paper 11.2.2

[29] Amadio, C., Ceccotti, A., Di Marco, R. and Fragiacomo, M. 'Numerical evaluation of long-termbehaviour of timber-concrete composite beams'. WCTE, Whistler, Canada, 2000, paper 8-2-4

[30] Amadio, C., Di Marco, R. and Fragiacomo, M. 'A linear finite-element model to study creep andshrinkage effects in a timber-concrete composite beam with deformable connections'. 1st RILEMSymposium on Timber Engineering, Stockholm, Sweden, 1999, pp. 747-756

[31] Fragiacomo, M. and Ceccotti, A. 'A simplified approach for long-term evaluation of timber-concretecomposite beams'. WCTE, Lahti, Finland, 2004, 537-542

[32] Dias, A. 'Mechanical behaviour of timber-concrete joints', PhD Thesis, Delft, The Netherlands,2005

[33] Kuhlmann, U. and Schänzlin, J. 'Time dependent behaviour of timber-concrete compositestructures'. WCTE, Lahti, Finland, 2004, 313-318

[34] Flach, M. and Frenette, C. 'Wood-Concrete Composite Technology in Bridge Construction'. WCTE,

Lahti, Finland, 2004, 289-294[35] Rautenstrauch, K., Grosse, M. and Lehmann, S. 'Forschungsvorhaben Brettstapel-Beton-Verbund,

Teil 1: Untersuchung des Tragverhaltens von Brettstapel-Beton-Verbunddeckenplatten mit


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neuartigen Verbindungsmitteln aus Flachstahlschlössern', Bauhaus-Universität Weimar.Eigenverlag, 2001

[36] Rautenstrauch, K., Grosse, M. and Lehmann, S. 'Forschungsvorhaben Brettstapel-Beton-Verbund,Teil 2: Flachstahlschlösser in Rundhölzern und Derbstangen, Betonnocken in aufgestelltenLamellen und verzinkte Lochbleche.' Bauhaus-Universität Weimar, Eigenverlag, 2002

[37] Rautenstrauch, K., Grosse, M., Lehmann, S. and Hartnack, R. 'Baupraktische Dimensionierungvon Holz-Beton-Verbunddecken'. 6. Informationstag des Instituts für Konstruktiven Ingenieurbau(IKI), Bauhaus-Universität Weimar, 2003,

[38] Rautenstrauch, K., Grosse, M., Lehmann, S. and Hartnack, R. 'Baupraktische Dimensionierungvon Holz-Beton-Verbunddecken'. 6. Informationstag des Instituts für Konstruktiven Ingenieurbau(IKI), 2003,

[39] Rautenstrauch, K., Grosse, M., Lehmann, S. and Hartnack, R. (2004) 'Modellierung undbaupraktische Bemessung von Holz-Beton-Verbunddecken mit mineralischen Deckschichten unterBerücksichtigung neuartiger Verbindungsmittel. Beitrag der Fachtagung Holz-Beton-VerbundLeipzig 2004'. In: König, G. and Holschemacher, K. (eds) Holz-Beton-Verbund. Bauwerk VerlagGmbH, Berlin.

[40] Brunner, M. and Gerber, C. 'Composite decks of concrete glued to timber'. WCTE, Malasya, 2002,

[41] Brunner, M. and Gerber, C. 'Long-term tests on a glued timber-concrete composite'. WCTEMalasya, 2002, paper 10.4.2

[42] Brunner, M. and Schnüriger, M. 'Towards a future with ductile timber beams'. WCTE, Malasya,2002, paper 11.2.3

[43] Brunner, M. and Schnüriger, M. 'Timber beams strengthened with pre-stressed fibres:Delamination'. WCTE, Lahti, Finland, 2004,

[44] Brunner, M., Romer, M. and Schnüriger, M. 'Timber-concrete-composite with an adhesiveconnector (wet on wet process)', Materials and Structures 40 (2007) 119-126

[45] Negrão, J. 'Shear testing of glued timber-concrete small-size specimens. Unpubl. Report', Dept.Civil Eng., Univ. Coimbra, Portugal, 2005

[46] Schober, K.U. and Rautenstrauch, K. 'Upgrading and repair of timber structures with polymerconcrete facing and strengthening'. WCTE, Portland OR, USA, 2006,


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22 Timber With Passive Reinforcement

3 TIMBER WITH PASSIVE REINFORCEMENT

Schober K. U., Brunner M., Negrão J., Rautenstrauch K., Lavisci P.

3.1 Overview

It is a well-known feature of timber beams that they usually fail suddenly due to the breaking of fibres onthe tensile face. This behaviour may be changed by an adequate strengthening of the tensile face, whichmay lead to a more ductile failure induced by the gentle local buckling of the fibres in the compressiveface. Such strengthening may be achieved through a number of ways:

by using a larger tensile flange

by using a much better timber grade on the critical tensile face

by bonding a passive (slack) or active (pre-stressed) reinforcement with adhesives

A number of materials may be used as reinforcement:

CFRP (carbon fibre reinforced plastics) GFRP (glass fibre reinforced plastics)

AFRP (aramid fibre reinforced plastics)

Kevlar®, steel bars and plates

When timber beams are strengthened with the above materials, the result is a composite structuralelement. In most practical cases adhesives are used for the bond line. Epoxy based adhesives have beenused in most cases for on site repair jobs, but most formulations were developed for other materials.These adhesives are generally too rigid for bonding timber and there is no chemical bonding or suitablemechanical anchorage in wood. The bond line is prone to fail because of dimensional changes in thewood induced by moisture content variations, even under Service Class 2 applications (moisture contentof timber up to 18 %). However, this group of adhesives has the potential to be the most suitable one for

on site bonding. Therefore, it is necessary to improve the existing adhesives or to develop alternativeadhesive formulations, suitable for indoor (and even outdoor) jobs on site where the use of pressure isgenerally not available and clean regular bond lines are difficult to achieve. Moreover, the existing testmethods to evaluate bond line performance were developed for other types of industrially usedadhesives, especially phenol or aminoplastic based products, and have repeatedly proved to beinadequate to assess the behaviour of epoxy type adhesives or epoxy bonded products. On siteapplication of adhesives is somewhat difficult and the consequent quality of adhesive bond is not easy toevaluate. Since properties of reinforced elements very much depend on the care put into the work, suchdifficulties have to be overcome. Procedures for applying and controlling are needed and must beestablished [1]. These procedures generally involve the use of materials compatible with and able toperform in accordance with the physical characteristics of the timber.

3.2 Literature survey

The last two decades have seen a lot of research work done on the strengthening of timber beams withfibre reinforced plastic laminates. Tingley [2] is an important pioneer. Since the 1980s, he hassystematically carried out a large number of scientific tests on glulam beams strengthened with fibrereinforced plastic laminates and also formed a company which successfully sells the technology to glulamcompanies, mostly in North America. The technology is widely used to strengthen and upgrade newbeams for construction. In the FIRP™ system a flat CFRP laminate is inserted as reinforcement betweenthe two bottom lamellas of the glulam element, in order to protect it and to enhance the contact area forshear transmission.


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Timber With Passive Reinforcement 23

Figure 3.1: FIRP™ System

European researchers started intensive research on the topic in the 1990s. One such pioneer was van deKuilen [3], who studied the strengthening of timber beams on the tensile face alone, and then on both thetensile and compressive faces. The tests were performed with larch wood reinforced with glass fibresglued with phenol-resorcinol type adhesive. Smedley has written several papers on the work he has

done, which are presented on the website of the LICONS project [4]. German researchers have also beenquite active. Blaß and Romani have written papers on strengthening of glulam beams using CFRP [5].Unlike the case in North America, there is some scepticism that the technology will be economical for newtimber structures in Europe. Many authors such as Rautenstrauch and Schober see a good marketopportunity for the technology in the strengthening of existing timber structures [6].They have alsoinvestigated two basic approaches for the use of reinforcement materials embedded in the wood, and theuse of external reinforcement resulting in a system of composite type [7]. The first results and insightshave been used successfully for renovation and reconstruction of historic roof and floor constructions asshown in the next figures.

Figure 3.2: Damaged spire of the Merseburgcathedral (D) due to high wind loads

and weathering. Cantilever joint afterstrengthening with CRPF’s

Figure 3.3: CFRP and polymer concretestrengthening of an historic ceiling

joist in waffle slab, Mansfeld castle(D)

Several experimental beam tests by Borri et al. [8], Triantafillou [9], Schober and Rautenstrauch [10]showed that the most frequent fracture mechanism is caused by the failure of the traction zone withoutthe complete plasticization of the compression region, depending on the quality of the wood. However,under particular conditions it is possible to note the other failure mechanism, which is theoreticallypreferable for several reasons. First, the section shows a more ductile behaviour, while the stresses in theFRP material with reinforcement are highly increased and therefore the composite material is moreinvolved. Initially the load defection is shown to be linear elastic up to local failures induced by thepresence of defects e.g. knots and cracks. Wood yield produced a non-linear response terminated by asudden drop of the load as a result of CFRP rupture. CFRP rupture was immediately followed by woodfracture in the tension zone, resulting in the collapse of the beams. Significantly good results, though,

have been found for the reinforcement of old floor beams where strength and stiffness needed amoderate improvement (15-20%) in order to fulfil the requirements of new codes and/or higher imposedloads. Figure 3.4 illustrates such an example, which is quite typical in Italy.


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Figure 3.4: Reinforcement with CFRP of floor beams on the tensile side. Photos: P. Lavisci.

3.3 Design methods

The service limit state of strengthened beams can generally be calculated with sufficient accuracy usingthe usual linear elastic methods. The stiffness of the composite beam can be determined with Steiner's

rule. With an adhesive bond there is no significant slippage to be taken into account. The calculation ofthe ultimate limit state is rather more difficult. The load-bearing capacity of strengthened timber beamscan be estimated by assuming linear elastic behaviour when the strengthening is relatively little. Whenthe tensile face is adequately strengthened however, the compressive face may suffer higher stressesand thus "yield" before the tensile face breaks. The failure mode of the beam will be ductile,corresponding to timber failure under compressive loading, and the load-bearing capacity can be moreaccurately estimated with plastic design methods.

There is as yet no universally recognized and accepted calculation method for strengthened timberbeams in Europe. The new Eurocodes for example do not treat the topic. The determination of theflexural strength of the composite structure can been done in the elastic range and the increasing of thebending stiffness by applying CFRP reinforcement can be defined by a fictitious modulus of elasticity,calculated from the cross-section data of the un-reinforced specimen according to EN 408:

( )( )12

12

2

1 fict

ww I 16

F F a E

−⋅⋅−⋅⋅

= λ

(1)

where F 2 -F 1 is the load increase in elastic range and w 2 -w 1 the equivalent deflection values. Thereinforcing scheme increased the capacity (E fict ) in comparison to the values measured for the un-reinforced wood beams. Different plastic calculation models have been proposed for the ultimate limitstate. Most of the models make a clear distinction between the elastic-plastic stresses on thecompressive face of the timber on the one hand, and the purely linear-elastic stresses on the tensile face.One of the older models, proposed by Tingley [2], assumes a stress distribution as shown in Fig. 3.5 with

linear strain distribution over the height of the beam

constant compressive stress over the entire compression zone

linear stress distribution on the tensile face

Some European authors such as Brunner and Blaß use a slightly modified stress distribution in thecompression zone to permit a transition from zero to the compressive stress as shown in Fig.3.6. Van deKuilen uses the more complex stress distribution shown in Fig.3.7. He assumes the hyperbolic curve (ex)proposed by Glos [11] to describe the relationship between the stresses and strains in an unreinforcedtimber beam.


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Neutral axis

f c,T

f t,T

f t,F

Legend:

f c,T: axial compressive strength

of timber

Fc,T= internal compression forcef t,T: bending strength of timber

Ft,T= internal tension force intimber

f t,F: tensile stress in FIRP

Ft,F= force in fibre

Stress distribution Internal forces:

Ft,T

Strains

Fc,T

Ft,F

e1 e2

Figure 3.5: Plastic model proposed by Tingley

Neutral axis

f c,T

f t,T

f t,F

Legend:

f c,T: axial compressive strengthof timber

Fc,T= internal compression forcef t,T: bending strength of timberFt,T= internal tension force in

timber




Ft,T

Strains

Fc,T

Ft,F

e1e2

Figure 3.6: Plastic model used by Brunner and Blaß

Neutral axis

f c,T

f t,T

f t,F

Legend:

f c,T: axial compressive strength

of timber

Fc,T= internal compression force

f t,T: bending strength of timber

Ft,T= internal tension force in

timber




Ft,T

Strains

Fc,T

Ft,F

e1e2

ex

Figure 3.7: Plastic model used by Kuilen


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Many calculation methods make the following common assumptions:

Elastic-plastic distribution of the compressive stresses in the timber. The failure stress isassumed to be equal to the axial compressive strength.

Linear-elastic distribution of the tensile stresses in the timber. Many authors assume themaximum failure stress to be equal to the bending strength of timber. Brunner argues that the

so-called bending strength of the timber beam is no true material property because it iscalculated from experiments by assuming a linear distribution of the stresses. In high gradetimbers, the compressive face will experience some slight plasticization before failure occurs.Brunner proposes that for higher grade timbers the true failure strength of the tensile face mustbe somewhat higher than the bending strength.

In cases of very great strengthening, such as with pre-stressing, the contribution of the timbertensile force is relatively small and may be neglected.

All the calculation methods are iterative and in many cases they lead to similar results. Anassumption is made for the position of the neutral axis. The failure strain of the tensile face canbe estimated from the bending strength and the Modulus of Elasticity: the characteristic strains inthe compression zone of the timber and the strain in the artificial fibre can be calculatedaccordingly. From the stress-strain diagrams, the stress distributions in the composite materials

can be calculated. The internal forces are calculated from the stress distribution in the cross-section and they must fulfil the following equilibrium equation:

No external axial force. Sum of internal forces must be zero:

Fc,T + Ft,T + Ft,F = 0 (2)

The calculation is repeated for different assumptions of the position of the neutral axis until the abovecondition is fulfilled. The bending resistance is then calculated by multiplying the internal forces on thetensile face in both timber and laminate with the corresponding distances from the compressive force inthe timber:

MR = Ft,T . e1 + Ft,F . e2 (3)

The Italian National Research Council has issued a guideline for the design of timber reinforcement with

FRPs, in order to disseminate best practices among the many professionals that already deal with thissubject [12].

3.4 Economic considerations

The use of these new reinforcement materials in a passive, not pre-stressed state raises a number ofimportant questions:

1. Does the effectiveness of the strengthened system compensate for the increase in costs resultingfrom the additional workmanship, use of complex adhesives and the still quite high (thoughdecreasing) cost of the aforementioned reinforcement materials?

2. Whilst the use of reinforcement may dramatically improve the performance of a concrete

structure, much more moderate results are to be expected with timber, because this material canwithstand both tensile and compressive stresses quite well. If, for instance, a perfectly compositebehaviour of a CFRP-reinforced timber beam is assumed and the cross-section is homogenized,one may easily conclude that a 1 mm-thick CFRP-laminate is roughly equivalent to an extra20mm timber lamella. Does it make economic sense to use reinforcement?

3. In most practical cases, the control of the serviceability limit state, in particular with regard todeflections and vibrations, is the most important factor governing the required size of the timberbeam. An analysis suggests that the use of reinforcement may not always be very helpful in thiscase. While the ratio between the design strengths of many engineered reinforcement materials,particularly CFRP and timber may be as high as 100, the ratio of their modulus of elasticity is onlyof about 20.

4. Ultimate limit state calculations of reinforced elements show that the stress in the reinforcementmaterial is well below the strength. Timber tensile failure occurs at strains of about 0.2 – 0.3%,which is well below the yield strain of many steel types or the service strain of most plastic fibres(> 0.5%), which means that we cannot make full use of the potentially high strength of thereinforcement materials.


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5. Given the high cost of the new plastic reinforcement materials, the economy of suchstrengthening of timber does not seem to justify the use in new structures. Many Europeanauthors such as Schober and Rautenstrauch suggest that the only really feasible market forstrengthening and upgrading existing structures.

3.5 Steel reinforcement

A possible alternative to the high-performance fibres is the use of the common and less expensiveconstruction rebars. In most systems steel rebars are inserted into longitudinal holes. After the rebars arein place, adhesive is injected under pressure through small holes in the top surface of the timber element.The ribbed surface of the rebars improves the bonding with the adhesive. Figure 3.8 depicts twocommercial systems i.e. Tasbeam™ and Aralam™.

One cannot help wondering whether the increase in strength compensates for the cost, because thisprocess seems to be even more complex than that of bonding fibre reinforcement strips to the tensile faceof the timber beam. Some preliminary studies indicate that the answer is no, for reasons similar to thosepointed out for the new engineered materials.

Figure 3.8: Steel reinforcement systems (left systems Tasbeam™, right: Aralam™)

3.6 Research needs

The technical aspects have been widely researched, but some difficult problems remain:

Practical solutions to premature debonding: with special adhesives, or with mechanical devicessuch as clamps, bonding in stages, etc.

Appropriate calculation methods, particularly for the ultimate state, when the timber compressiveface may plastify.

Practical application methods on site

Long-term behaviour of reinforced timber structuresThe economic aspects need to be more carefully analysed:

Possibilities of mass production

Appropriate tools and technologies to rationalise and speed up work


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3.7 References

[1] CEN 'TC, 193/SC1/WG11. Adhesives for on site assembling or restoration of timber structures. Onsite acceptance testing:Part 1: Sampling and measurement of the adhesives cure schedule. Doc. N20.

Part 2: Verification of the shear strength of an adhesive joint. Doc. N21.Part 3: Verification of the adhesive bond strength using tensile proof-loading. Doc. N22.' (2003)

[2] Tingley, D. 'FIRP Reinforcement Technology Information Packet', Science and TechnologyInstitute, Corvallis OR, USA, 1995

[3] Kuilen, J.v.d. 'Theoretical and experimental research on glass fibre reinforced laminated timberbeams'. International Timber Engineering Conference, London, England, 1991, 3.226-3.233

[4] Anonymus 'Low Intrusion Conservation Systems for Timber Structures.' 2006, Website:http://www.licons.org/

[5] Blaß, H.J. and Romani, M. 'Design model for FRP reinforced glulam beams'. International Councilfor Research and Innovation in Building and Construction. Working Commission W18 TimberStructures, Venice, Italy, 2001,

[6] Rautenstrauch, K. and Schober, K.U. 'Verstärkung von historischen Holzbauteilen mittels CFK-Lamellen'. Europäische Messe für Restaurierung, Denkmalpflege und Stadterneuerung, Leipzip,Germany, 2004,

[7] Schober, K.U. and Rautenstrauch, K. (2005) 'Strengthening of timber structures in-situ with anapplication of fiber-reinforced polymers'. In: Seracino. (eds) FRP Composites in Civil Engineering -CICE 2004. Taylor & Francis Group, London, 697-704. ISBN 90 5809 638 6.

[8] Borri, M. and et al. 'FRP reinforcement of wood elements under bending loads'. Structural Faultsand Repair, London, England, 2003,

[9] Triantafillou, T.C. 'Shear reinforcement of wood using FRP materials', Journal for Materials in CivilEngineering 9 (2) (1997) 65-69

[10] Schober, K.U. and Rautenstrauch, K. 'Experimental investigations on flexural strengthening oftimber structures with CFRP'. International Symposium on Bond Behaviour of FRP in Structures,Hong Kong, China, 2005,

[11] Glos, P. 'Zur Modellierung des Festigkeitsverhaltens von Bauholz bei Druck-, Zug- undBiegebeanspruchung. Berichte zur Zuverlässigkeitstheorie der Bauwerke 61', Universität München,1981

[12] CNR 'Studi Preliminari finalizzati alla redazione di Istruzioni per Interventi di ConsolidamentoStatico di Strutture Lignee mediante l’utilizzo di Compositi Fibrorinforzati', CNR-DT 201/2005 (2005) 60 p

http://www.licons.org/http://www.licons.org/


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4 PRE-STRESSING OF TIMBER

Negrão J., Brunner M., Lehmann M.

4.1 Overview

The idea of strengthening timber beams by reinforcing the tensile side with non-pre-stressed materials isbasically good, but in practice the economy may place a limit on its use because the high strength of theexpensive reinforcement is not fully utilised. An increasing number of European researchers believe thatthe only truly feasible market may be in the repair and strengthening of existing structures.

A small but increasing number of researchers believe that the economic efficiency of the reinforcementcould be improved by pre-stressing instead of using it as a passive reinforcement only, because its final(service) stress may be substantially increased. This research field, which is relatively new for timberstructures, will be discussed in this section.

The mechanics and calculation of pre-stressing has been clearly understood for almost a century.However, the concept has been empirically used since ancient times. The Egyptians, for example, boundwooden pieces together with heated metal rings. As the metal cooled, it firmly bound the wooden piecesto form a wine barrel. The basic concept is that of imposing onto the structure or element a stress statewhich is opposite to that expected to result from the service loading. This leads to an increase in theloading that the structure can withstand or, alternatively, to a reduction in structural material needed for aspecified loading.

The only successful practical application of pre-stressing to timber structures up to the present has beenthe case of stress-laminated decks. In this system, however, initial stresses due to pre-stressing are notan objective in its own, as in the general case. Instead, the main goal is to use these perpendicular-to-grain compressive stresses to enhance the friction between planks, thus allowing shear stresses to betransmitted through the interface, resulting in a better two-dimensional behaviour. The stress-laminateddeck has been subjected to intensive international research and there is wide agreement on the load-bearing behaviour.

Besides the stress-laminated panels, only a few references are made, in the literature, to the use of pre-stressing with timber (Fig. 4.1, 4.2). Luggin and Bergmeister [1] consider that this is a promisingtechnology. Galloway et al. [2] investigated the behaviour of Kevlar-reinforced timber beams, with andwithout the use of pre-stress.

Figure 4.1: Post-tensioned timber beams Figure 4.2: Post-tensioned timber beams

Krahemann and Fontana [3] refer to a research project in timber-concrete decks with the timber beamspre-stressed with steel bars.

δo

Figure 4.3: Pre-stressing-inducedcamber

The strengthening effect may be significantly improved if active (pre-stressed) instead of passivereinforcement is used. The ultimate limit state design can take into account the possibility of a favourableductile failure of the structural element of the compressive face instead of the brittle tensile face of the

Pre-Stressing Of Timber 29


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beam. Pre-stressing also offers a further advantage with regard to service state design. Since thedeflections are often the critical design criteria for timber beams, the upward deflection induced by theeccentrically placed pre-stressed laminate (Fig. 4.3) may prove to be of major help [4].

1 - Jacking

2 - Gluing /curing

3 - Jack release

Reinforcement

Figure 4.4: Pre-stressing bycambering

In principle, any of the previously mentioned reinforcement materials may be used for pre-stressing. Infact, fabric or laminate composite (plastic) materials are the obvious choice for strengthening orrehabilitation of existing structures, because they can be applied on the element surface, thus avoidingdrillings or any other weakening procedures. The preferred method for the actual application of pre-stressing on site is an issue still under investigation, but a possible approach could consist of bonding thereinforcement after imposing a camber to the beam and to unload after the curing of the adhesive, asschematized in Figure 4.4. This method was investigated by Lehmann [5]. In this investigation acalculation model was developed and verified using small specimens and structural sized GL24h beamswhich were reinforced using the method described in Figure 4.4. The calculations and the measurementswith strain gauges showed that the pre-stress force in the Carbon Fiber Reinforced Plastic (CFRP)

lamella is not constant over its length. It peaks in the middle of the beam where it is mostly needed and iszero towards the ends where high stress would causes delamination (Fig. 4.5). The shear stress in theglueline is constant and quite low. The pre-stress force is related to the force present in the prop used for jacking, which is itself limited to the bending strength and ability of holding the beam ends in place. Thepre-stress force introduced with this method is much lower than could be attained by stretching theCFRP-lamella before attaching it to the timber. The major benefits of this method are a significantcontribution to the service limit state (40%) and the simplicity of putting it in place as well as the reduceddanger of delamination.

Distribution of the pre-stress stress in the CFRP lamella

0

50

100

150

200

250 Stress [MPa]300

0 500 1000 1500 2000 2500 3000 3500 4000Position of the strain gauges [mm]

Figure 4.5: Pre-stress distribution in theCFRP lamella

A major problem still to be satisfactorily solved is delamination. When the usual stiff adhesives on themarket are used, the shear stresses in the interface between the timber and the plastic laminate attainhigh peaks concentrated at the beam ends. Luggin [1] reported that most of his test specimens failed

because of delamination, when the plastic laminate was suddenly peeled off the timber. The delaminationproblem has been studied by Brunner and Schnueriger [6]. They proposed the development of a “ductile”adhesive to distribute the shear stresses over a certain beam length. Unfortunately, the adhesives theyworked with proved to be incapable of withstanding the required pre-stressing force. Nevertheless,

30 Pre-Stressing Of Timber


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provided that this problem can be managed with further research work, the latter approach with ductileadhesives may prove to be the most economical and suitable for practical use.

Brunner and Schnueriger have also studied alternative methods to attach pre-stressed FRP to strengthenglulam beams [7]. They used special gradiented anchoring equipment developed by CarboLink, a spin-offcompany of the EMPA, Switzerland, to attach the fibres in stages starting at the beam centre. After eachstep the pre-stressing force was slightly reduced: thus the force was effectively anchored over aconsiderable length at both ends of the beam. They have performed two test series to date. In the firsttest series, only one pre-stressed CFRP-laminate was used per beam. There was no delamination: in allthe test specimens, the load-bearing behaviour corresponded quite well to the calculated predictions.They observed however, that the pre-stressing force was not strong enough to induce a significantplasticization of the compressive face of the timber beam. In a follow-up project, they sought to augmentthe pre-stressing force by attaching up to three pre-stressed CFRP-laminates on top of each other. Thelaminate closest to the beam had the same length as the beam and therefore extended to the supports.The outermost laminated were shorter in length and did not extend to the supports. There were no signsof delamination during the long storage period of about three months before the beams were tested.During the testing however, the outermost laminates debonded suddenly. Curiously, even with thissudden failure mode, the load-bearing capacity calculated corresponded well to the measured test values(Figures 4.6 and 4.7).

320-1 Kraft 1+2

0

10

20

30

40

50

60

70

80

90

100

-20 0 20 40 60 80 100 120 140 160

Durchbiegung wm in mm unter Pressen

L a s t F 1 + F 2

i n

k N

Figure 4.6: Typical load-deflection behaviour of the glulambeams pre-stressed with multi-CFRP [7]

Figure 4.7: Debonding of the outer-most pre-stressedCFRP in phases [7]

However, the use of these new reinforcement materials raises a number of questions still to be answered:

Does the effectiveness of the strengthening system compensate the increase in costs resultingfrom additional workmanship, adhesives and mostly the high (though decreasing) cost of theaforementioned reinforcement materials?

While the use of reinforcement may improve dramatically the performance of a concretestructure, a much more moderate result is to be expected with timber, because this material canwithstand either tensile or compressive stresses. If, for instance, a perfectly composite behaviourof a CFRP-reinforced timber beam is assumed and the cross-section is homogenized, one mayeasily conclude that a 1mm-thick laminate is roughly equivalent to an extra 20mm lamella. Is thisworthwhile?

While the ratio between design strengths of most engineered reinforcement materials(particularly CFRP) and timber may be as high as 100, that of their modulus of elasticity is onlyof about 20.

When the design of the reinforced element under the assumptions of the previous topic is made, oneconcludes that the stress in the reinforcement material is well below that leading to its effective use.Given the high cost of such materials, one may wander whether its use in timber reinforcement is worthy

or not. The effectiveness of fibres may be improved by pre-stressing instead of using them as a passivereinforcement only, because their final (service) stress may be substantially increased. The problem withthis procedure is, again, the risk of delamination, because the shear force in the interface rises in the



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Figure 4.9: Timber test specimen for withdrawal testsof glued-in pre-stressing steel wire

Figure 4.10: Epoxy specimen testing

In a feasibility study, Barroso et al [8] undertook withdrawal tests of glued-in pre-stressing steel wire(Figure 4.9). The bonding between the steel wire and timber was made up of either epoxy resin orphenol-resorcinol-formaldehyde. In all the tests, failure occurred at the interface between the steel wireand the adhesive, with the “clean” wire being pulled out. Significant values for the average withdrawalstrength (1.21 and 1.35 MPa, respectively, with COV of 19% and 16%) were found, though far below

those expected/needed. In order to clarify this issue, pull-out tests of pre-stressed steel wire glued withinepoxy specimens were made, as shown in Figure 4.10. Values in the range 5-9 MPa were obtained, withlarge COV.

Figure 4.11: Force-displacement diagram

The force-displacement diagrams were as depicted in Figure 4.11, in which the sinusoidal pattern is mostlikely due to the helical slight indentation of the wire and to the brittleness of the adhesive, which preventthe effective participation of a long glueline in the process of stress transfer between materials. This is anongoing research.

The problem of brittleness of adhesives is also pointed out by Kemmsies and Streicher [9], though in thecontext of research involving bonding of timber with steel in end connections.

4.2 Calculation Methods

Ne utr al ax is

c,T

Legend: f c,T: Axial compressive strength of timber

Fc,T: Internal compressive force timber

f m,T : Bending strength of timber

F t,T : Internal tensile force in timberf t,L : Tensile stress in FRP-laminate

F t,L : Tensile force in FRP-laminate

:


Strains

Fc,T

Ft,F

e1 e2

f t,T

f t,L

F t,T

Figure 4.12: Strain and stress distributions in a strengthened timber beam



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The calculation models described in Chapter 3 need only a slight modification in order to permit thecalculation of timber beams strengthened with pre-stressed CFRP laminates. Brunner and Schnüringer[10], for example, refer to the calculation of pre-stressed concrete and modifies the calculation modeldepicted in figure 4.12, by adding the initial pre-stressing force in the CFRP to the additional forcecorresponding to the strain level. The distribution of the stresses in the timber remains essentially thesame as depicted in fig. 4.12, though of course the neutral axis will be shifted downwards to

accommodate the larger compressive zone needed to counter-balance the greater force in the CFRPlaminate.

4.2.1 Calculation example of pre-stressed timber beam

The calculation model described above was used to predict the load-bearing capacity of a strengthenedtimber beam which was later tested in bending (figure 4.13).

The timber has the following material properties:

Dimension 140 mm width, 200mm height

GL 32h: Em=14 kN/mm2

5% fractile values according to Eurocode EN 1194: f c,k = 29 N/mm2, f m,k = 32 N/mm

2

Medium values expected in loading tests are about 1/3 higher than the 5% fractile values: – f c = 39 N/mm

2, f m= 43 N/mm2

Characteristic strains:

Tensile failure at εt = fm / E = 43 / 14000 = 3,07 ‰.

Yielding of compressive face at εc = fc / E = 39/14000 = 2,79 ‰

The FRP laminate used has the following properties:

S&P-carbon laminate type 150/2000

Cross-section 1.4x50mm

E=165 kN/mm2

Pre-stressing force: 60 kN (simplification: neglecting of losses due to creep and elasticdeformations)

It is hereby assumed that failure of the tensile face of the timber beam at a strain of 3.07 ‰ resp. a stressof 43 N/mm2 will induce collapse.

The calculation is iterative. Assuming a height z1=91mm of the tensile face of the timber leads forexample to the following results:

z2 = (39/43) 91 = 83 mm z3 = 200 – (91 + 83) = 26 mm

Maximum strain on the compressive face: εO = 2.79 ‰ (26 + 83) / 83 = 3.66 ‰

Strain in the FRP (additional to pre-stressing) = (91,7/91) 3,07 = 3,09 ‰ Additional stress in FRP = ε x E = 3,09 ‰ 165.000 = 510 N/mm2

34 Pre-Stressing Of Timber


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2 0 0 m m

Z 1

5 0 m m

1 4 0 m m

S TRA IN S

% oS TRES S ES

N /m m 2

2.79

3.07 43

39

Z 2

Z 3

51 0

IN TERN A L

F O R C E S

D 1

D 2

Z2

Z1 + Z0

εO

Figure 4.13:Calculation example of pre-stressed glulam beam

Internal forces:

Timber compressive face:

D1 = (26 x 140) x 39 / 1000 = 142.0 kN

D2 = 0,5 x (83 x 140) x 39 / 1000 = 226,5 kN

D(total) = D1 + D2 = 368,6 kN

Timber tensile face:

Z2 = 0,5 x (91 x 140) x 43 / 1000 = 273,9 kN

CFRP:

Pre-stressing force Z0 = 60 kN

Additional force Z1 = 510 x (1,4 x 50) / 1000 = 35,7 kN

Total tensile force: Z0 + Z1 + Z2 = 369,6 kN

Since the compressive forces and the tensile forces are (nearly) equal, the iterative process can beended. The distances between the forces can be found from a consideration of the geometry. Theresultant compressive force D (total) for example has the following distance from the top of the beam:

e1 = (142,0 x 13 + 226,5 x 54) / 368,6 = 38,1mm

Similarly, it can be shown that the resulting total tensile force acts at a distance of about 178 mm from thetop of the beam. The distance between the resulting compressive and tensile forces is therefore 140 mmand the expected failure moment of the pre-stressed glulam beam can be calculated as:

MU = 368,6 x 0,140 = 52 kNm

It is worth remarking here that the calculated maximum strain on the compressive face is only about 3.66

‰. Although the failure strain of structural timber under compressive loading is not listed in any normsand standards known to the authors, literature studies indicate that it may be close to the betterresearched values for small clear specimens, which many authors suggest lies at about 12 ‰. Hence thecompressive face of the timber specimen could readily accommodate larger forces. In other words, thepre-stressing force in the FRP laminate could be greatly increased before there would be any real dangerof timber compressive failure.

4.3 Research needs

Though the pre-stressing technique seems to have some potential for application to timber structures,further developments are strongly constrained by the debonding problem. The preliminary tests suggestthat, in spite of the reasonably high shear strength, the current adhesive formulations lack ductile

behaviour. As a consequence, peak shear stresses concentrate in a short length and it is not possible tomobilize the interface length required to withstand the large forces introduced by pre-stressing.



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Another relevant aspect concerns the evaluation of instantaneous and long-term pre-stress force losses.It is well documented that a second or even a third pre-stressing operation is required with stress-laminated decks. This should not be possible with pre-stressing wires or laminates glued to the timber.Besides, the creep behaviours in either the parallel or perpendicular to the grain directions are notnecessarily the same, given the orthotropy of timber.

Finally, the development of a calculation method is an essential condition for the use of the technique. Alinear elastic approach will be adequate to the range of stresses in which both the timber and the pre-stressing material are expected to work, but the real issue is the development of universally accepteddesign rules which take the ductile behaviour of the compressive face of the timber into account.

4.4 References

[1] Luggin, W. and Bergmeister, K. 'Carbon Fiber Reinforced and Prestressed Timber Beams.' 2nd Int.PhD Symposium in Civil Engineering, Budapest, Hungary, 1998,

[2] Galloway, T.L., Fogstad, C., Dolan, C.W. and Pucket, J.A. 'Initial Tests of Kevlar PrestressedTimber Beams', Nat. Conf. Wood Transport. Structures Gen. Tec. Rep. FPL-GTR-94, Madison, WI,USA, 1996

[3] Krahemann, P. and Fontana, M. 'Vorgespannte Holz-Beton-Verbundtrager. Schlussbericht zumKTI-Projekt Nr. 4617.1', IBK, ETH Zurich, Switzerland, 2002

[4] Lehmann, M., Properzi, M. and Pichelin, F. 'Prestessed FRP for the in -situ strengthening of timberstructures'. WCTE, Portland, USA, 20

bonding of timber

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