- Email: [email protected]
Adhesive systems for structural connections in timber
International Journal of Adhesion & Adhesives 21 (2001) 177}186
Adhesive systems for structural connections in timber J.G. Broughton, A.R. Hutchinson* Joining Technology Research Centre, Oxford Brookes University, Gipsy Lane, Headington Campus, Oxford OX3 0BP, UK Accepted 28 November 2000
Abstract Adhesives can be used to form load-bearing joints in timber structures, both in repair and in new-build applications. An e$cient and cost-e!ective method of making joints is to use rods or dowels, bonded into pre-drilled holes in timber elements, for transferring structural loads between such elements. This paper reviews the important issues in bonded-in rod technology, the adhesives and reinforcing materials used, the results of theoretical and experimental studies, and the steps involved in design and speci"cation. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Adhesives for wood; B. Composites; C. Joint design; Bonded-in rods
1. Introduction Repairs to timber structures may be necessary for a variety of reasons including accidental damage to or overloading of structural members, moisture ingress leading to fungal attack, insect attack, problems caused by di!erential timber or structural movement, and "re damage. Upgrading of timber members may also be necessary as a result of changes in use of the structure, for instance as consequence of increased #oor loading. The choice made in the method employed to repair a timber structure is in#uenced by a number of issues which include the location, structural requirements, access limitations, "re resistance, aesthetics, the amount of intervention into the building fabric and cost. Adhesivebonded (resin) repairs represent just one of the available methods [1,2]. Environmental issues concerning the more e$cient utilisation of timber resources have led to the development of novel manufacturing (timber conversion) processes, largely pioneered in Scandinavia and North America. These have produced e$cient structural composite lumber (SCL) materials such as laminated veneer lumber (LVL), laminated strand lumber (LSL) and parallel strand lumber (PSL), as illustrated in Fig. 1. Other established forms of composite timber, such as pre-
* Corresponding author. Tel.: #44-1865-483504; fax: #44-1865484179. E-mail address: [email protected] (A.R. Hutchinson).
fabricated glulam members, are also experiencing wider exploitation. The use of both laminated and solid underutilised timbers such as Welsh ash and oak hardwoods is currently being promoted in the UK, as is the feasibility of using green timber in order to reduce long and costly lead times presently required for seasoning [1,3,4]. The formation of e$cient joints in timber structures, particularly those involving SCLs, has been hindered by the prevalent reliance upon crude and unsophisticated connection details between the various structural members [5,6]. Concealed bonded-in connections using either steel or "bre-reinforced plastic (FRP) rods (Fig. 2) o!er one solution to the development of more e$cient joining methods [7]. Improvements include greater sti!ness and strength, reduced weight and cross-grain stress concentration, greater "re resistance and, arguably, aesthetic bene"ts (there is a less visible joint). Much of the published work has provided a greater understanding of this technology, for repair and potential &new-build' applications, but there remain concerns which include doubts over structural performance, the e!ects of moisture and long-term durability.
2. Resin-bonded methods of repair Repairs typically involve various combinations of epoxy resins, with properties speci"c to the situation of each repair, in combination with either metallic or non-metallic connection components in the form of plates or bars.
0143-7496/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 7 4 9 6 ( 0 0 ) 0 0 0 4 9 - X
178
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Fig. 1. Examples of structural composite forms (from left to right LVL, LSL, PSL and glulam).
repairs with little or no disruption to the historic fabric of a building or structure. Table 1 summarises a number of common repair circumstances [5]. Because various combinations of materials and processes can be used, the term system is often used to describe di!erent techniques, e.g. the wood}epoxy-resin (WER) system developed in Canada [6], the BETA system developed in Holland and the Resiwood system developed by Rota"x Resins in the UK [8]. 2.1. Timber replacement using resins Table 1 refers to the use of epoxy grout used to "ll the volume left by the removal of timber. Filling voids with a (self-levelling) grout generally involves enclosing the space with temporary or permanent shuttering. Since structural repairs involve the transmission of tension, compression or shear forces, reinforcing materials embedded within the grout are used to join the host timber and resin repair block together. 2.2. Structural adhesives in repair
Fig. 2. Examples of concealed bonded-in rod connections.
Resin-bonded methods of repair were developed in the 1970s and typical examples include the restoration of beam-ends and column-ends, "ssure repairs and the upgrading of beams. The resins themselves may be used either in the form of a structural adhesive or as a volume grout for replacing damaged sections of timber. Among the advantages of resin methods are minimum removal of original material and the ability to carry out in situ
Resins can be used in the form of structural adhesives to bond new pieces of timber to existing timber, such as in adding laminations, or for bonding metallic or nonmetallic plates or rods to existing timber beams. Plates can be bonded into slots cut into a beam, placed between laminations, or attached to the tension face of a beam. Rods are commonly either bonded into drilled holes or installed in multiples within the slot. Resins acting structurally are also commonly used to bond reinforcement into holes or slots in two pieces of timber which are being joined together. Very often, the resin may be acting both as a volume grout and as a load-carrying structural medium. 2.3. Regulations and concerns Resin repairs are generally subjected to Building Regulations approval, since material alteration or upgrading is often involved. Repairs must therefore follow a route
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
179
Table 1 Summary of techniques used in "ve resin repair situations Repairs
Alterations Upgrading beams
Beam -end repair
Trussed rafter and foot repair
Column repairs
Cut out decay and cast epoxy grout as a replacement material, usually with reinforcement
Cut out decay and cast epoxy grout as a replacement material, usually with reinforcement
Cut out decay and cast epoxy grout as a replacement material, reinforcing with steel bars
Cut out decay and cast epoxy grout, leaving shuttering in position
Cut out decay and cast epoxy grout, leaving shuttering in position
Bond a series of reinforcement bars into position for &&stitching'' "ssure
Bond steel plate into position (without bolts)
Bond in a replacement treated softwood splice, together with reinforcement.
Bond in a treated softwood splice to replace a trussed rafter foot or end
Fill "ssure with slow set resin and small timber pieces, then cut and shape to original.
Reinforce with steel rebars bonded into position.
that can be designed and justi"ed by calculation. Historic timber structures can be a cause of di$culty because of the concerns of conservation bodies. For example, English Heritage states that repairs should follow or satisfy the criteria of minimum intervention; that is to conserve as found, to be reversible and to use &tried and trusted' techniques. A resin repair must be detailed for good environmental durability, as for any other repair techniques. This involves eliminating the factors causing the problem, executing due care with the design and process of repair, and avoiding excessive moisture-induced movements of the timber adjacent to the repair. An industry-standard guide to good practice in timber repair is available [9], whilst European research projects, such as GIROD [10] and COLORETIM [11], have addressed design code development and the use of innovative materials as localised reinforcements in connections and repairs.
3. Materials involved in structural connections The properties of, and interactions between, at least three generic classes of materials must be considered, namely timber, resin/adhesive and connection components. Some typical short-term properties of candidate materials are collected in Table 2. Considerations related to the timber include species and grade, structural integrity, orientation of grain with respect to loading, load duration, moisture content, surface condition and service class. 3.1. Connection components Connection materials are typically in the form of plates or bars, made either from steel or FRP.
Fissure repairs
Insert steel #itch plate, bonded and bolted into position
Bars which are threaded or deformed are recommended because of the mechanical interlock conferred in addition to intrinsic adhesion. High-yield mild steel is common, with resin encapsulation in a repair providing corrosion protection. In cases where corrosion is of major concern other materials such as stainless steel, epoxy-coated rebar or zinc-galvanised rods could also be used. However, in the application of zinc-coated components it is recommended that the load transfer through the connection be attained by mechanical interlock rather than adhesion alone due to adhesion di$culties. Appropriate GFRP bars comprise a high-volume fraction of uni-directionally aligned glass "bres in a polyester, vinylester or epoxy matrix, produced by the pultrusion process. Potentially, carbon "bre-reinforced plastic (CFRP) bars could also be used to confer superior mechanical properties (see Table 2) but cost would currently rule out their use. As with steel bars, a textured or deformed FRP surface is desirable. Plates can e!ectively transmit shear forces, axial loading and bending moments between timber members, or they can be used to increase the sti!ness and strength of an existing sound member. To a certain extent, their function dictates the form and choice of material. The "bre lay-up in FRP plates is most important in order to provide the optimum properties at minimum cost (and weight). However, opportunity could be taken to mould FRP plates to particular shapes and forms to serve speci"c functions, or to use fabric preforms and laminate them with resins in situ. 3.2. Structural adhesives, resins and primers The purpose of the adhesive or resin is to provide a continuous bond between the timber and the reinforcement, to "ll voids and cavities, and to transfer and sustain loads. Thus thermosetting gap-"lling materials are
180
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Table 2 Some typical short-term properties of materials Property (at 203C)
Softwood
Polyurethane adhesive
Epoxy adhesive
GFRP rod
CFRP rod
Mild steel
Density (kg/m) Tensile modulus (GPa) Shear modulus (GPa) Tensile strength (MPa)
500 10 4 100 (para) 4 (perp) 11 (inter-laminar) *
1400 0.5 0.2 30
1500 4 1.5 30
1800 40 15 800
1600 120 45 1200
20 10 to }100
25 1 to }10
40 (inter-laminar) 2.5
60 (inter-laminar) 1.0
7800 210 80 300 (yield) * *
0.40 40
0.35 30
0.30 10
0.30 0
0.30 11
35
55
*
*
*
Shear strength (MPa) Tensile strength to fail (%) Poisson's ratio Coe$cient of thermal Expansion (10\ 3C\) Glass transition Temperature (3C)
* 4 (para) 50 (perp) *
Note that the properties of all materials (except steel) are a!ected by absorbed water.
required which exhibit good adhesion to the various materials whilst also being tolerant to variations in timber moisture content. Mays and Hutchinson [12,13] identi"ed the requirements of construction adhesives for use with steel and concrete, and these are similar for the case of structural connections in timber. Epoxies have several advantages over other resins as adhesive agents for use in timber structures and, typically, they are suitable for service temperatures in the range !30 to #603C. They may be formulated in a wide range of forms, application characteristics and mechanical properties when cured. Other generic groups include acrylics, urethanes and various combinations of phenol, resorcinol and formaldehyde resins. Epoxy resins, used for timber structural repair, may be categorised into three major groups according to the physical form of the resin when it is applied to the structure [9]. The nature of the repair normally dictates the most appropriate physical form to be used: E Grouts: A resin system with #ow characteristics allowing it to be poured, injected or pumped. Shuttering is required to contain the material in the repair area. E Adhesives: Structural systems for bonding reinforcement in place, or for injecting "ssures. E Mortars: Heavily "lled systems for non-structural applications such as small patch repairs to "ssures, notches and cavities. Where reinforcement is present, this should be coated with an epoxy binder "rst to ensure good adhesion to the mortar. Primers, coupling agents and other surface treatments can be used to enhance the bond of the epoxy resin both to timber and to reinforcement. Bond enhancement is not required to increase initial strength but may be necessary to improve long-term durability. Despite the extra cost
associated with priming, it is of particular value where connections may be subjected to repeated wetting and drying [9].
4. Adhesion and surface preparation 4.1. Timber Di!erent species of timber exhibit great variation in their bulk and surface characteristics. This can a!ect potential adhesion signi"cantly and a detailed consideration of this topic is given by Davis [14]. Oak represents an important example of timber containing acidic substances which may interfere with adhesion and which may leach out of the timber over time under moist conditions. This is also one of the most common materials used in historic buildings where this form of repair is used. Timber surfaces need to be &cut' in order to obtain good adhesion between the resin system and timber, using sharp tools. This opens up the porous cellular structure of timber and enables penetration of the resin into the microstructure of the material, promoting adhesion to cellulose and lignin components. Cutting into old timber is very important since age will have signi"cantly lowered its surface activation energy. The moisture content of timber at the time of bonding represents a most important consideration, both in terms of timber species and resin type. It can however be economically advantageous to be able to exploit timber which is relatively unseasoned and which contains moisture contents above 20% (e.g. [3]). Timber moisture contents may be related to Service Classes which re#ect di!erent service conditions. The Service Classes given in
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
181
Table 3 E!ect of timber moisture content on compressive lap shear joint performance Adhesive type
Adhesive shear strength at 203C (MPa)
Timber controls Epoxy 1
28
Species
Douglas "r Oak Oak Douglas "r Oak
Epoxy 2
25
Douglas "r Oak
Polyurethane 1
15
Douglas "r Oak
Polyurethane 2
15
Moisture content at the time of bonding (%)
Douglas "r Oak
10
18
22#
11.06 13.93 11.70 A/B 15.99 A 11.40 A 9.10 C/D 1.93 F 1.46 F 7.10 C/D 7.32 C/D
8.72 10.98 11.0 A/B 15.58 A}E 9.84 A 10.63 A 0.83 F/G 0.10 G 1.86 G 1.40 G
* * 8.60 A/B 13.57 A 8.80 C 9.40 C 0.45 G 0.01 G 1.88 G 0.01 G
Notes: Overlap area 45;45 mm.; mean shear stresses in MPa; tests carried out at 203C; A and A/B"failure deep in wood; C and C/D"failure in surface layers of timber; E"Adhesion failure; F"Thin "lm cohesive failure in adhesive; G"Cohesive failure in adhesive
Eurocode 5: Part 1.1 [15] and in BS 5268: Part 2 [16] may be interpreted as follows: E Service Class 1: Temperature and relative humidity conditions resulting in a moisture content not exceeding 12% in most softwoods, corresponding to typical indoor conditions. E Service Class 2: Temperature and relative humidity conditions resulting in a moisture content not exceeding 20% in most softwoods, corresponding to unheated but covered conditions. E Service Class 3: Temperature and relative humidity conditions resulting in a moisture content higher than Service Class 2. The e!ect of di!erent moisture contents at the time of bonding, on Douglas "r and oak, is shown in Table 3 [5]. A modi"ed compressive lap shear test was used, comprising coupons 55;45;10 mm thick with a "ne sawn "nish; the resultant bond area was 45;45 mm and the bondline thickness 1 mm. Relevant shear strength properties of the polyurethane and epoxy adhesives, and the timber species (the timber controls), are also shown. It is clear that the polyurethanes were very sensitive to moisture content, with only one of them providing signi"cant strength properties at the 10% level. The joints made with polyurethane adhesives generally failed cohesively within the adhesive layer, which had in most cases &foamed' as a result of the reaction of the adhesive with moisture in the host timber, producing CO . By contrast,
the epoxy-bonded joints fared much better than their polyurethane counterparts. The joints bonded with coupons at 10% gave shear strength values similar to those of the solid control blocks as failure occurred in the timber. At moisture contents of 18% and above there was a slight reduction in strength, generally consistent with the associated reduction in timber strength. However the oak specimens, when bonded at higher moisture contents, tended to fail nearer to the bonded interface, although still within the timber. Further tests, on imposing environmental cycling routines on joints, con"rmed the excellent durability of epoxy}timber bonds and the dubious integrity of polyurethane}timber bonds [5]. 4.2. Connection components Eurocode 5: Part 2 [15] speci"es that any metallic rods used should be threaded or deformed bars. It is therefore assumed that the adhesive will work through mechanical interlock alone at the adhesive/rod interface and will rely upon speci"c adhesion only to the timber. For optimum performance it is desirable that adhesion should also be gained to the surface of the rod. In a thick bondline joint, this is even more important because mechanical interlock is less certain. Where adhesion is relied upon, as opposed to mechanical interlock, a typical sequence of degreasing, gritblasting and dust removal should ideally be followed for both metallic and non-metallic reinforcement. Degreasing, the
182
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
removal of any millscale from steel and removal of release agents from FRP, represents the very minimum necessary surface preparation. Ideally, this should just be the "rst step prior to abrasion and further cleaning. Wire brushing of metallic reinforcement is sometimes practised but it represents a very poor method of preparation; typically the surface is merely scored and polished, and is not activated for bonding. Gritblasting of mild and stainless steels will expose a fresh clean surface, removing a variety of contaminants. Gentle blasting of FRP materials is ideal but abrasion with pads is also satisfactory, provided that dust is removed afterwards. A good standard of surface preparation for all materials is required to ensure long-term bond durability (e.g. [13,17]). The use of coupling agents or primers should also be considered where bond integrity is thought to be critical.
5. Experimental pull-out behaviour of rods in timber Beam-end repairs are fairly common, in which bars are bonded into the timber parallel to the grain. Similar joining methods may be used in many &new-build' applications, forming potential bonded-in rod connections as shown in Fig. 2. For the purposes of design and testing, this situation can be idealised by considering a small block of timber with a single bar bonded into it (Fig. 3a). 5.1. Experimental details Previous research [18] used laminated veneer lumber (LVL) for its consistent mechanical properties; it had a moisture content of around 10% by weight. The adhesives investigated included "ve types of epoxy, two types of acrylic, a polyurethane (PU) and a phenol}resorcinol}formaldehyde (PRF). All were used to bond 10 mm diameter high-yield (HY) ribbed steel dowels into 60 mm cubes of LVL, parallel to the grain. The adhesives were all two-part and curable at ambient temperature. Sharp augur bits were used to generate 14 mm diameter holes, giving a 2 mm annular bondline thickness. Some representative properties of the materials used are shown in Table 4. Further studies employed timber blocks up to 120 mm long to study the e!ects of bond length and bondline thickness for Epoxies 1 and 2. Additional work [19}21] investigated the in#uence of a double-ended test joint geometry on the pattern of results (Fig. 3b), the in#uence of counter-boring the 14 mm diameter holes to various depths (Fig. 3a) and the bene"ts of multiple rod con"gurations including rod spacing requirements (Fig. 3c), and the e!ect of moisture content on the pull-out strength of solid hardwood specimens.
Fig. 3. Schematic illustrations of test specimen con"gurations: (a) single-ended, (b) double-ended and (c) multiple-rod.
5.2. Results The pull-out data comparing the di!erent adhesive types are shown in Fig. 4. Five joints were tested for each combination. For the epoxy-bonded joints, failure took place in the timber; the average shear stresses at the adhesive/timber interfaces ranged between 5.5 and 6.6 MPa, compared with a value of around 5 MPa for the shear strength of the LVL. The results of the parametric study using Epoxies 1 and 2 are shown in Figs. 5 and 6. Fig. 5 shows that as the annular bondline thickness was increased from 1 to 6 mm, there was an almost linear increase in pull-out load for a constant bond length of 100 mm. Additional work showed that the same increase
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186 Table 4 Representative properties (at 203C) of materials used Material
Epoxy 1 Epoxy 2 Epoxy 3 Epoxy 4 Epoxy 5 Acrylic 1 Acrylic 2 Polyurethane (PU) Phenol}resorcinol} formaldehyde (PRF) LVL (parallel to grain) HY steel
Tensile modulus (GPa)
Tensile strength (MPa)
Shear strength (MPa)
7.0 5.0 3.0 2.0 1.0 0.5 0.7 0.5 0.5
30 20 35 25 20 20 15 30 15
28 25 35 25 25 35 22 15 15
10.8 210
100 500
5 250
183
since the simple and relatively cheap single-ended specimens therefore provide conservative data. Investigations involving the inclusion of an additional rod, as expected, produced twice the pull-out load of a single rod specimen. However, a minimum rod spacing of two bond diameters was necessary to obtain full strengths. Rod spacings less than this resulted in lower pull-out loads, as shown in Fig. 7. Numerical analyses were carried out on the various model joint con"gurations [18]. The data showed that large stress concentrations were associated with thinner bondlines and with very short bond lengths (less than 40 mm). Therefore, it is of note that the chosen bond length in single-end specimens (60 mm) is a reasonable minimum to avoid excessive stress concentrations at the embedment face. These excessive stress concentrations, resulting from the short embedment lengths, would have been responsible for initiating early failure within the timber, giving rise to the pattern of experimental results reported. The e!ect of moisture content, recorded at the time of bonding, on solid ash and oak hardwood specimens is illustrated in Fig. 8. The pull-out strength only decreased signi"cantly once a moisture content of 22% or above was recorded in the timber. This suggests that timber with moisture contents appropriate to Service Classes 1 and 2 can be bonded e!ectively without any concern over loss of joint integrity. 5.3. Summary E All of the epoxy adhesives investigated resulted in satisfactory performance. E The acrylic, polyurethane and phenol}resorcinol}formaldehyde adhesive types all performed poorly, producing cohesive adhesive failures or adhesion failures. The remaining observations relate to epoxy adhesives only:
Fig. 4. In#uence of adhesive type on pull-out behaviour.
in load capacity, associated with a 22 mm diameter hole, could be achieved by counter-boring the 14 mm diameter holes to 22 mm diameter (to a depth of about 30 mm). Fig. 6 shows that, for a "xed annular bondline thickness of 2 mm, there was a fairly linear increase in pull-out load with increasing bond length. Further work showed that double-ended specimens (timber blocks with steel dowels bonded into each end) produced slightly greater pull-out loads than the singleended specimens. This was unexpected, and in retrospect believed to be due to the constraint e!ects, but reassuring
E Pull-out strengths increased linearly with bondline thickness, consistent with the increase in the bonded area of timber at large hole diameters. Similar e!ects were achieved by partial counter-boring. E Pull-out strengths increased linearly with bond lengths '40 mm. E Pull-out strengths increased with the inclusion of additional rods, consistent with the increase in the bonded area, but a minimum rod spacing of 2 bond diameters was necessary to obtain maximum capacities. E The upper limits of pull-out strength will be determined by the tensile strength of the rods used if timber failure does not occur. E No signi"cant loss in pull-out strength was exhibited for solid ash and oak specimens having moisture contents, recorded at the time of bonding, up to and including Service Classes 1 and 2.
184
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Fig. 5. In#uence of bondline thickness on variation of di!erent parameters.
6. General design considerations Well designed and executed bonded structural connections in timber can be very e$cient. However, it is essential to establish a proper basis for design which involves considerations of service conditions and service class, strength, sti!ness, durability and cost. Additional factors, which may in#uence the design, include access, appearance (particularly with respect to the repair of structures with historical signi"cance) and requirements for "re resistance. The type of Service Class, or environment, to which the adhesive system is likely to be subjected to will dictate the properties of the particular resin system required. For example, formulations with high-temperature resistance may be required in certain applications or in certain countries, whilst moisture resistance may be important for repairs subjected to Service Class 3.
The limit state design approach is recommended for repairs and worked examples for beam-ends, joint repairs and beam upgrades are available [9].
7. General speci5cation requirements A detailed speci"cation should document the requirements for any contract, and this should include the following points: E design details and calculations; E details regarding access, temporary works and temporary propping; E timber speci"cations and requirements including details of new timber, decayed timber removal, maximum moisture contents prior to bonding, and methods of machining and cleaning;
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
185
Fig. 6. In#uence of embedment length on variation of di!erent parameters.
E types of grouts, adhesives or mortars, property requirements, and instructions for mixing, pumping, application and curing; E generic type, dimensions and form of reinforcement, surface texture (deformed, threaded, etc.), property requirements, details of surface preparation (and priming if required); E requirements for temporary or permanent shuttering to contain grout, including dimensions and sealing; E procedures to be followed in the event of variance outside the contract speci"cations.
8. Summary and conclusions Resin methods of repair to timber structures have a history of use of around 25 years. Surveys, recordings and long-term monitoring of repairs have been undertaken which have given information on the ways in which
resin-bonded joints behave [9]. Research undertaken by TRADA Technology Ltd and by Oxford Brookes University [5,18,20] has indicated that e$cient, highstrength joints can be made with epoxy resin adhesives. It has been shown that the e!ect of high timber moisture contents, both prior to bonding and following bonding, has minimal e!ect on the integrity of epoxy-bonded joints. Adhesive bonding can provide an e$cient and durable method for making connections in timber structures, provided that: E the joints are correctly designed using an appropriate structural approach; E suitable methods, materials and speci"cations are adopted; E the work is undertaken by experienced operatives trained in resin methods; E strict quality control is exercised both o! and on site.
186
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Acknowledgements The authors wish to acknowledge support for the work received from TRADA Technology Ltd, via DETR funding, and from EPSRC.
References
Fig. 7. In#uence of rod spacing on pull-out load.
Fig. 8. In#uence of timber moisture content at the time of bonding on pull-out load.
Experience with improved resin systems and polymer composite reinforcement is growing all the time, and the experience gained from repair is being translated into new-build for the fabrication of e$cient and innovative connections.
[1] Mettem CJ, Davis G. Resin bonded repair systems for structural timber, Part 1. Constr Repair 1996;10(2):23}8. [2] Mettem CJ, Davis G. Resin bonded repair systems for structural timber, Part 2. Constr Repair 1996;10(3):43}7. [3] Paw C, Rattray JC, Robertson AM. Building with roundwood 1. Farm Build Prog 1990;(January 99):7}11. [4] Burton R, Dickinson M, Harris R. The use of roundwood thinnings in buildings * a case study. Build Res Inf 1998;26(2):76}93. [5] Wheeler A, Hutchinson AR. Resin repairs to timber structures. Int J Adhes Adhes 1998;18(1):1}13. [6] Stumes P. Testing the e$ciency of wood}epoxy}reinforcement systems. Assoc Preservation Technol Bull Can 1975;17(3):2}35. [7] Bainbridge RJ, Mettem CJ. A review of moment-resistant structural timber connections. Proc Inst Civ Eng Struct Build 1998;128(November):323}31 [paper 11590]. [8] Anon. A guide to the use of resins in timber engineering, Ref.TE0993 (1987) and Resiwood System, Ref.RTEP/1 (1996). Rota"x Ltd, Abercraf, Swansea, UK, 1987, 1996. [9] The Institution of Structural Engineers. Guide to the restoration and repair of timber structures. London: SETO/TRADA Technology Limited, 2000. [10] GIROD Project Website 2000: http://www.sp.se/building/ wood/girod.htm. [11] COLORETIM EU contract No. FAIR-S2 9248. [12] Mays GC, Hutchinson AR. Engineering property requirements for structural adhesives. Proc Inst Civ Eng Part 2 1998; 85:485}501. [13] Mays GC, Hutchinson AR. Adhesives in civil engineering. Cambridge: Cambridge University Press, 1992. [14] Davis G. The performance of adhesive systems for structural timbers. Int J Adhes Adhes 1997;17(3):247}55. [15] DD ENV 1995-1-1:1994. Eurocode 5, Design of timber structures * Part 1.1: general rules and rules for buildings. London: BSI, 1994. [16] BS 5268:Part 2:1996. Structural use of timber * Part 2: code of practice for permissible stress design, materials and workmanship. London: BSI, 1996. [17] Hutchinson AR. Joining of "bre-reinforced polymer composite materials, Project Report 46. Construction Industry Research and Information Association, London, 1997. [18] Hutchinson AR, Broughton JG. Pull-out behaviour of reinforcement bonded into timber. Proceedings of the International Conference on Structural Adhesives in Engineering, vol. V. Bristol: Institute of Materials, 1998. p. 186}91. [19] Anon. Tensile testing of bonded-in steel rods. Internal Report, Oxford Brookes University, April 1998. [20] Broughton JG, Hutchinson AR. Pull-out behaviour of steel rods bonded in to timber. Mater Struct 2001;34(236):101}10. [21] Broughton JG, Hutchinson AR. The e!ect of timber moisture content on bonded-in rods. Constr Build Mater 2000;15(1):17}25.
Adhesive systems for structural connections in timber J.G. Broughton, A.R. Hutchinson* Joining Technology Research Centre, Oxford Brookes University, Gipsy Lane, Headington Campus, Oxford OX3 0BP, UK Accepted 28 November 2000
Abstract Adhesives can be used to form load-bearing joints in timber structures, both in repair and in new-build applications. An e$cient and cost-e!ective method of making joints is to use rods or dowels, bonded into pre-drilled holes in timber elements, for transferring structural loads between such elements. This paper reviews the important issues in bonded-in rod technology, the adhesives and reinforcing materials used, the results of theoretical and experimental studies, and the steps involved in design and speci"cation. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Adhesives for wood; B. Composites; C. Joint design; Bonded-in rods
1. Introduction Repairs to timber structures may be necessary for a variety of reasons including accidental damage to or overloading of structural members, moisture ingress leading to fungal attack, insect attack, problems caused by di!erential timber or structural movement, and "re damage. Upgrading of timber members may also be necessary as a result of changes in use of the structure, for instance as consequence of increased #oor loading. The choice made in the method employed to repair a timber structure is in#uenced by a number of issues which include the location, structural requirements, access limitations, "re resistance, aesthetics, the amount of intervention into the building fabric and cost. Adhesivebonded (resin) repairs represent just one of the available methods [1,2]. Environmental issues concerning the more e$cient utilisation of timber resources have led to the development of novel manufacturing (timber conversion) processes, largely pioneered in Scandinavia and North America. These have produced e$cient structural composite lumber (SCL) materials such as laminated veneer lumber (LVL), laminated strand lumber (LSL) and parallel strand lumber (PSL), as illustrated in Fig. 1. Other established forms of composite timber, such as pre-
* Corresponding author. Tel.: #44-1865-483504; fax: #44-1865484179. E-mail address: [email protected] (A.R. Hutchinson).
fabricated glulam members, are also experiencing wider exploitation. The use of both laminated and solid underutilised timbers such as Welsh ash and oak hardwoods is currently being promoted in the UK, as is the feasibility of using green timber in order to reduce long and costly lead times presently required for seasoning [1,3,4]. The formation of e$cient joints in timber structures, particularly those involving SCLs, has been hindered by the prevalent reliance upon crude and unsophisticated connection details between the various structural members [5,6]. Concealed bonded-in connections using either steel or "bre-reinforced plastic (FRP) rods (Fig. 2) o!er one solution to the development of more e$cient joining methods [7]. Improvements include greater sti!ness and strength, reduced weight and cross-grain stress concentration, greater "re resistance and, arguably, aesthetic bene"ts (there is a less visible joint). Much of the published work has provided a greater understanding of this technology, for repair and potential &new-build' applications, but there remain concerns which include doubts over structural performance, the e!ects of moisture and long-term durability.
2. Resin-bonded methods of repair Repairs typically involve various combinations of epoxy resins, with properties speci"c to the situation of each repair, in combination with either metallic or non-metallic connection components in the form of plates or bars.
0143-7496/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 7 4 9 6 ( 0 0 ) 0 0 0 4 9 - X
178
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Fig. 1. Examples of structural composite forms (from left to right LVL, LSL, PSL and glulam).
repairs with little or no disruption to the historic fabric of a building or structure. Table 1 summarises a number of common repair circumstances [5]. Because various combinations of materials and processes can be used, the term system is often used to describe di!erent techniques, e.g. the wood}epoxy-resin (WER) system developed in Canada [6], the BETA system developed in Holland and the Resiwood system developed by Rota"x Resins in the UK [8]. 2.1. Timber replacement using resins Table 1 refers to the use of epoxy grout used to "ll the volume left by the removal of timber. Filling voids with a (self-levelling) grout generally involves enclosing the space with temporary or permanent shuttering. Since structural repairs involve the transmission of tension, compression or shear forces, reinforcing materials embedded within the grout are used to join the host timber and resin repair block together. 2.2. Structural adhesives in repair
Fig. 2. Examples of concealed bonded-in rod connections.
Resin-bonded methods of repair were developed in the 1970s and typical examples include the restoration of beam-ends and column-ends, "ssure repairs and the upgrading of beams. The resins themselves may be used either in the form of a structural adhesive or as a volume grout for replacing damaged sections of timber. Among the advantages of resin methods are minimum removal of original material and the ability to carry out in situ
Resins can be used in the form of structural adhesives to bond new pieces of timber to existing timber, such as in adding laminations, or for bonding metallic or nonmetallic plates or rods to existing timber beams. Plates can be bonded into slots cut into a beam, placed between laminations, or attached to the tension face of a beam. Rods are commonly either bonded into drilled holes or installed in multiples within the slot. Resins acting structurally are also commonly used to bond reinforcement into holes or slots in two pieces of timber which are being joined together. Very often, the resin may be acting both as a volume grout and as a load-carrying structural medium. 2.3. Regulations and concerns Resin repairs are generally subjected to Building Regulations approval, since material alteration or upgrading is often involved. Repairs must therefore follow a route
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
179
Table 1 Summary of techniques used in "ve resin repair situations Repairs
Alterations Upgrading beams
Beam -end repair
Trussed rafter and foot repair
Column repairs
Cut out decay and cast epoxy grout as a replacement material, usually with reinforcement
Cut out decay and cast epoxy grout as a replacement material, usually with reinforcement
Cut out decay and cast epoxy grout as a replacement material, reinforcing with steel bars
Cut out decay and cast epoxy grout, leaving shuttering in position
Cut out decay and cast epoxy grout, leaving shuttering in position
Bond a series of reinforcement bars into position for &&stitching'' "ssure
Bond steel plate into position (without bolts)
Bond in a replacement treated softwood splice, together with reinforcement.
Bond in a treated softwood splice to replace a trussed rafter foot or end
Fill "ssure with slow set resin and small timber pieces, then cut and shape to original.
Reinforce with steel rebars bonded into position.
that can be designed and justi"ed by calculation. Historic timber structures can be a cause of di$culty because of the concerns of conservation bodies. For example, English Heritage states that repairs should follow or satisfy the criteria of minimum intervention; that is to conserve as found, to be reversible and to use &tried and trusted' techniques. A resin repair must be detailed for good environmental durability, as for any other repair techniques. This involves eliminating the factors causing the problem, executing due care with the design and process of repair, and avoiding excessive moisture-induced movements of the timber adjacent to the repair. An industry-standard guide to good practice in timber repair is available [9], whilst European research projects, such as GIROD [10] and COLORETIM [11], have addressed design code development and the use of innovative materials as localised reinforcements in connections and repairs.
3. Materials involved in structural connections The properties of, and interactions between, at least three generic classes of materials must be considered, namely timber, resin/adhesive and connection components. Some typical short-term properties of candidate materials are collected in Table 2. Considerations related to the timber include species and grade, structural integrity, orientation of grain with respect to loading, load duration, moisture content, surface condition and service class. 3.1. Connection components Connection materials are typically in the form of plates or bars, made either from steel or FRP.
Fissure repairs
Insert steel #itch plate, bonded and bolted into position
Bars which are threaded or deformed are recommended because of the mechanical interlock conferred in addition to intrinsic adhesion. High-yield mild steel is common, with resin encapsulation in a repair providing corrosion protection. In cases where corrosion is of major concern other materials such as stainless steel, epoxy-coated rebar or zinc-galvanised rods could also be used. However, in the application of zinc-coated components it is recommended that the load transfer through the connection be attained by mechanical interlock rather than adhesion alone due to adhesion di$culties. Appropriate GFRP bars comprise a high-volume fraction of uni-directionally aligned glass "bres in a polyester, vinylester or epoxy matrix, produced by the pultrusion process. Potentially, carbon "bre-reinforced plastic (CFRP) bars could also be used to confer superior mechanical properties (see Table 2) but cost would currently rule out their use. As with steel bars, a textured or deformed FRP surface is desirable. Plates can e!ectively transmit shear forces, axial loading and bending moments between timber members, or they can be used to increase the sti!ness and strength of an existing sound member. To a certain extent, their function dictates the form and choice of material. The "bre lay-up in FRP plates is most important in order to provide the optimum properties at minimum cost (and weight). However, opportunity could be taken to mould FRP plates to particular shapes and forms to serve speci"c functions, or to use fabric preforms and laminate them with resins in situ. 3.2. Structural adhesives, resins and primers The purpose of the adhesive or resin is to provide a continuous bond between the timber and the reinforcement, to "ll voids and cavities, and to transfer and sustain loads. Thus thermosetting gap-"lling materials are
180
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Table 2 Some typical short-term properties of materials Property (at 203C)
Softwood
Polyurethane adhesive
Epoxy adhesive
GFRP rod
CFRP rod
Mild steel
Density (kg/m) Tensile modulus (GPa) Shear modulus (GPa) Tensile strength (MPa)
500 10 4 100 (para) 4 (perp) 11 (inter-laminar) *
1400 0.5 0.2 30
1500 4 1.5 30
1800 40 15 800
1600 120 45 1200
20 10 to }100
25 1 to }10
40 (inter-laminar) 2.5
60 (inter-laminar) 1.0
7800 210 80 300 (yield) * *
0.40 40
0.35 30
0.30 10
0.30 0
0.30 11
35
55
*
*
*
Shear strength (MPa) Tensile strength to fail (%) Poisson's ratio Coe$cient of thermal Expansion (10\ 3C\) Glass transition Temperature (3C)
* 4 (para) 50 (perp) *
Note that the properties of all materials (except steel) are a!ected by absorbed water.
required which exhibit good adhesion to the various materials whilst also being tolerant to variations in timber moisture content. Mays and Hutchinson [12,13] identi"ed the requirements of construction adhesives for use with steel and concrete, and these are similar for the case of structural connections in timber. Epoxies have several advantages over other resins as adhesive agents for use in timber structures and, typically, they are suitable for service temperatures in the range !30 to #603C. They may be formulated in a wide range of forms, application characteristics and mechanical properties when cured. Other generic groups include acrylics, urethanes and various combinations of phenol, resorcinol and formaldehyde resins. Epoxy resins, used for timber structural repair, may be categorised into three major groups according to the physical form of the resin when it is applied to the structure [9]. The nature of the repair normally dictates the most appropriate physical form to be used: E Grouts: A resin system with #ow characteristics allowing it to be poured, injected or pumped. Shuttering is required to contain the material in the repair area. E Adhesives: Structural systems for bonding reinforcement in place, or for injecting "ssures. E Mortars: Heavily "lled systems for non-structural applications such as small patch repairs to "ssures, notches and cavities. Where reinforcement is present, this should be coated with an epoxy binder "rst to ensure good adhesion to the mortar. Primers, coupling agents and other surface treatments can be used to enhance the bond of the epoxy resin both to timber and to reinforcement. Bond enhancement is not required to increase initial strength but may be necessary to improve long-term durability. Despite the extra cost
associated with priming, it is of particular value where connections may be subjected to repeated wetting and drying [9].
4. Adhesion and surface preparation 4.1. Timber Di!erent species of timber exhibit great variation in their bulk and surface characteristics. This can a!ect potential adhesion signi"cantly and a detailed consideration of this topic is given by Davis [14]. Oak represents an important example of timber containing acidic substances which may interfere with adhesion and which may leach out of the timber over time under moist conditions. This is also one of the most common materials used in historic buildings where this form of repair is used. Timber surfaces need to be &cut' in order to obtain good adhesion between the resin system and timber, using sharp tools. This opens up the porous cellular structure of timber and enables penetration of the resin into the microstructure of the material, promoting adhesion to cellulose and lignin components. Cutting into old timber is very important since age will have signi"cantly lowered its surface activation energy. The moisture content of timber at the time of bonding represents a most important consideration, both in terms of timber species and resin type. It can however be economically advantageous to be able to exploit timber which is relatively unseasoned and which contains moisture contents above 20% (e.g. [3]). Timber moisture contents may be related to Service Classes which re#ect di!erent service conditions. The Service Classes given in
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
181
Table 3 E!ect of timber moisture content on compressive lap shear joint performance Adhesive type
Adhesive shear strength at 203C (MPa)
Timber controls Epoxy 1
28
Species
Douglas "r Oak Oak Douglas "r Oak
Epoxy 2
25
Douglas "r Oak
Polyurethane 1
15
Douglas "r Oak
Polyurethane 2
15
Moisture content at the time of bonding (%)
Douglas "r Oak
10
18
22#
11.06 13.93 11.70 A/B 15.99 A 11.40 A 9.10 C/D 1.93 F 1.46 F 7.10 C/D 7.32 C/D
8.72 10.98 11.0 A/B 15.58 A}E 9.84 A 10.63 A 0.83 F/G 0.10 G 1.86 G 1.40 G
* * 8.60 A/B 13.57 A 8.80 C 9.40 C 0.45 G 0.01 G 1.88 G 0.01 G
Notes: Overlap area 45;45 mm.; mean shear stresses in MPa; tests carried out at 203C; A and A/B"failure deep in wood; C and C/D"failure in surface layers of timber; E"Adhesion failure; F"Thin "lm cohesive failure in adhesive; G"Cohesive failure in adhesive
Eurocode 5: Part 1.1 [15] and in BS 5268: Part 2 [16] may be interpreted as follows: E Service Class 1: Temperature and relative humidity conditions resulting in a moisture content not exceeding 12% in most softwoods, corresponding to typical indoor conditions. E Service Class 2: Temperature and relative humidity conditions resulting in a moisture content not exceeding 20% in most softwoods, corresponding to unheated but covered conditions. E Service Class 3: Temperature and relative humidity conditions resulting in a moisture content higher than Service Class 2. The e!ect of di!erent moisture contents at the time of bonding, on Douglas "r and oak, is shown in Table 3 [5]. A modi"ed compressive lap shear test was used, comprising coupons 55;45;10 mm thick with a "ne sawn "nish; the resultant bond area was 45;45 mm and the bondline thickness 1 mm. Relevant shear strength properties of the polyurethane and epoxy adhesives, and the timber species (the timber controls), are also shown. It is clear that the polyurethanes were very sensitive to moisture content, with only one of them providing signi"cant strength properties at the 10% level. The joints made with polyurethane adhesives generally failed cohesively within the adhesive layer, which had in most cases &foamed' as a result of the reaction of the adhesive with moisture in the host timber, producing CO . By contrast,
the epoxy-bonded joints fared much better than their polyurethane counterparts. The joints bonded with coupons at 10% gave shear strength values similar to those of the solid control blocks as failure occurred in the timber. At moisture contents of 18% and above there was a slight reduction in strength, generally consistent with the associated reduction in timber strength. However the oak specimens, when bonded at higher moisture contents, tended to fail nearer to the bonded interface, although still within the timber. Further tests, on imposing environmental cycling routines on joints, con"rmed the excellent durability of epoxy}timber bonds and the dubious integrity of polyurethane}timber bonds [5]. 4.2. Connection components Eurocode 5: Part 2 [15] speci"es that any metallic rods used should be threaded or deformed bars. It is therefore assumed that the adhesive will work through mechanical interlock alone at the adhesive/rod interface and will rely upon speci"c adhesion only to the timber. For optimum performance it is desirable that adhesion should also be gained to the surface of the rod. In a thick bondline joint, this is even more important because mechanical interlock is less certain. Where adhesion is relied upon, as opposed to mechanical interlock, a typical sequence of degreasing, gritblasting and dust removal should ideally be followed for both metallic and non-metallic reinforcement. Degreasing, the
182
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
removal of any millscale from steel and removal of release agents from FRP, represents the very minimum necessary surface preparation. Ideally, this should just be the "rst step prior to abrasion and further cleaning. Wire brushing of metallic reinforcement is sometimes practised but it represents a very poor method of preparation; typically the surface is merely scored and polished, and is not activated for bonding. Gritblasting of mild and stainless steels will expose a fresh clean surface, removing a variety of contaminants. Gentle blasting of FRP materials is ideal but abrasion with pads is also satisfactory, provided that dust is removed afterwards. A good standard of surface preparation for all materials is required to ensure long-term bond durability (e.g. [13,17]). The use of coupling agents or primers should also be considered where bond integrity is thought to be critical.
5. Experimental pull-out behaviour of rods in timber Beam-end repairs are fairly common, in which bars are bonded into the timber parallel to the grain. Similar joining methods may be used in many &new-build' applications, forming potential bonded-in rod connections as shown in Fig. 2. For the purposes of design and testing, this situation can be idealised by considering a small block of timber with a single bar bonded into it (Fig. 3a). 5.1. Experimental details Previous research [18] used laminated veneer lumber (LVL) for its consistent mechanical properties; it had a moisture content of around 10% by weight. The adhesives investigated included "ve types of epoxy, two types of acrylic, a polyurethane (PU) and a phenol}resorcinol}formaldehyde (PRF). All were used to bond 10 mm diameter high-yield (HY) ribbed steel dowels into 60 mm cubes of LVL, parallel to the grain. The adhesives were all two-part and curable at ambient temperature. Sharp augur bits were used to generate 14 mm diameter holes, giving a 2 mm annular bondline thickness. Some representative properties of the materials used are shown in Table 4. Further studies employed timber blocks up to 120 mm long to study the e!ects of bond length and bondline thickness for Epoxies 1 and 2. Additional work [19}21] investigated the in#uence of a double-ended test joint geometry on the pattern of results (Fig. 3b), the in#uence of counter-boring the 14 mm diameter holes to various depths (Fig. 3a) and the bene"ts of multiple rod con"gurations including rod spacing requirements (Fig. 3c), and the e!ect of moisture content on the pull-out strength of solid hardwood specimens.
Fig. 3. Schematic illustrations of test specimen con"gurations: (a) single-ended, (b) double-ended and (c) multiple-rod.
5.2. Results The pull-out data comparing the di!erent adhesive types are shown in Fig. 4. Five joints were tested for each combination. For the epoxy-bonded joints, failure took place in the timber; the average shear stresses at the adhesive/timber interfaces ranged between 5.5 and 6.6 MPa, compared with a value of around 5 MPa for the shear strength of the LVL. The results of the parametric study using Epoxies 1 and 2 are shown in Figs. 5 and 6. Fig. 5 shows that as the annular bondline thickness was increased from 1 to 6 mm, there was an almost linear increase in pull-out load for a constant bond length of 100 mm. Additional work showed that the same increase
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186 Table 4 Representative properties (at 203C) of materials used Material
Epoxy 1 Epoxy 2 Epoxy 3 Epoxy 4 Epoxy 5 Acrylic 1 Acrylic 2 Polyurethane (PU) Phenol}resorcinol} formaldehyde (PRF) LVL (parallel to grain) HY steel
Tensile modulus (GPa)
Tensile strength (MPa)
Shear strength (MPa)
7.0 5.0 3.0 2.0 1.0 0.5 0.7 0.5 0.5
30 20 35 25 20 20 15 30 15
28 25 35 25 25 35 22 15 15
10.8 210
100 500
5 250
183
since the simple and relatively cheap single-ended specimens therefore provide conservative data. Investigations involving the inclusion of an additional rod, as expected, produced twice the pull-out load of a single rod specimen. However, a minimum rod spacing of two bond diameters was necessary to obtain full strengths. Rod spacings less than this resulted in lower pull-out loads, as shown in Fig. 7. Numerical analyses were carried out on the various model joint con"gurations [18]. The data showed that large stress concentrations were associated with thinner bondlines and with very short bond lengths (less than 40 mm). Therefore, it is of note that the chosen bond length in single-end specimens (60 mm) is a reasonable minimum to avoid excessive stress concentrations at the embedment face. These excessive stress concentrations, resulting from the short embedment lengths, would have been responsible for initiating early failure within the timber, giving rise to the pattern of experimental results reported. The e!ect of moisture content, recorded at the time of bonding, on solid ash and oak hardwood specimens is illustrated in Fig. 8. The pull-out strength only decreased signi"cantly once a moisture content of 22% or above was recorded in the timber. This suggests that timber with moisture contents appropriate to Service Classes 1 and 2 can be bonded e!ectively without any concern over loss of joint integrity. 5.3. Summary E All of the epoxy adhesives investigated resulted in satisfactory performance. E The acrylic, polyurethane and phenol}resorcinol}formaldehyde adhesive types all performed poorly, producing cohesive adhesive failures or adhesion failures. The remaining observations relate to epoxy adhesives only:
Fig. 4. In#uence of adhesive type on pull-out behaviour.
in load capacity, associated with a 22 mm diameter hole, could be achieved by counter-boring the 14 mm diameter holes to 22 mm diameter (to a depth of about 30 mm). Fig. 6 shows that, for a "xed annular bondline thickness of 2 mm, there was a fairly linear increase in pull-out load with increasing bond length. Further work showed that double-ended specimens (timber blocks with steel dowels bonded into each end) produced slightly greater pull-out loads than the singleended specimens. This was unexpected, and in retrospect believed to be due to the constraint e!ects, but reassuring
E Pull-out strengths increased linearly with bondline thickness, consistent with the increase in the bonded area of timber at large hole diameters. Similar e!ects were achieved by partial counter-boring. E Pull-out strengths increased linearly with bond lengths '40 mm. E Pull-out strengths increased with the inclusion of additional rods, consistent with the increase in the bonded area, but a minimum rod spacing of 2 bond diameters was necessary to obtain maximum capacities. E The upper limits of pull-out strength will be determined by the tensile strength of the rods used if timber failure does not occur. E No signi"cant loss in pull-out strength was exhibited for solid ash and oak specimens having moisture contents, recorded at the time of bonding, up to and including Service Classes 1 and 2.
184
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Fig. 5. In#uence of bondline thickness on variation of di!erent parameters.
6. General design considerations Well designed and executed bonded structural connections in timber can be very e$cient. However, it is essential to establish a proper basis for design which involves considerations of service conditions and service class, strength, sti!ness, durability and cost. Additional factors, which may in#uence the design, include access, appearance (particularly with respect to the repair of structures with historical signi"cance) and requirements for "re resistance. The type of Service Class, or environment, to which the adhesive system is likely to be subjected to will dictate the properties of the particular resin system required. For example, formulations with high-temperature resistance may be required in certain applications or in certain countries, whilst moisture resistance may be important for repairs subjected to Service Class 3.
The limit state design approach is recommended for repairs and worked examples for beam-ends, joint repairs and beam upgrades are available [9].
7. General speci5cation requirements A detailed speci"cation should document the requirements for any contract, and this should include the following points: E design details and calculations; E details regarding access, temporary works and temporary propping; E timber speci"cations and requirements including details of new timber, decayed timber removal, maximum moisture contents prior to bonding, and methods of machining and cleaning;
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
185
Fig. 6. In#uence of embedment length on variation of di!erent parameters.
E types of grouts, adhesives or mortars, property requirements, and instructions for mixing, pumping, application and curing; E generic type, dimensions and form of reinforcement, surface texture (deformed, threaded, etc.), property requirements, details of surface preparation (and priming if required); E requirements for temporary or permanent shuttering to contain grout, including dimensions and sealing; E procedures to be followed in the event of variance outside the contract speci"cations.
8. Summary and conclusions Resin methods of repair to timber structures have a history of use of around 25 years. Surveys, recordings and long-term monitoring of repairs have been undertaken which have given information on the ways in which
resin-bonded joints behave [9]. Research undertaken by TRADA Technology Ltd and by Oxford Brookes University [5,18,20] has indicated that e$cient, highstrength joints can be made with epoxy resin adhesives. It has been shown that the e!ect of high timber moisture contents, both prior to bonding and following bonding, has minimal e!ect on the integrity of epoxy-bonded joints. Adhesive bonding can provide an e$cient and durable method for making connections in timber structures, provided that: E the joints are correctly designed using an appropriate structural approach; E suitable methods, materials and speci"cations are adopted; E the work is undertaken by experienced operatives trained in resin methods; E strict quality control is exercised both o! and on site.
186
J.G. Broughton, A.R. Hutchinson / International Journal of Adhesion & Adhesives 21 (2001) 177}186
Acknowledgements The authors wish to acknowledge support for the work received from TRADA Technology Ltd, via DETR funding, and from EPSRC.
References
Fig. 7. In#uence of rod spacing on pull-out load.
Fig. 8. In#uence of timber moisture content at the time of bonding on pull-out load.
Experience with improved resin systems and polymer composite reinforcement is growing all the time, and the experience gained from repair is being translated into new-build for the fabrication of e$cient and innovative connections.
[1] Mettem CJ, Davis G. Resin bonded repair systems for structural timber, Part 1. Constr Repair 1996;10(2):23}8. [2] Mettem CJ, Davis G. Resin bonded repair systems for structural timber, Part 2. Constr Repair 1996;10(3):43}7. [3] Paw C, Rattray JC, Robertson AM. Building with roundwood 1. Farm Build Prog 1990;(January 99):7}11. [4] Burton R, Dickinson M, Harris R. The use of roundwood thinnings in buildings * a case study. Build Res Inf 1998;26(2):76}93. [5] Wheeler A, Hutchinson AR. Resin repairs to timber structures. Int J Adhes Adhes 1998;18(1):1}13. [6] Stumes P. Testing the e$ciency of wood}epoxy}reinforcement systems. Assoc Preservation Technol Bull Can 1975;17(3):2}35. [7] Bainbridge RJ, Mettem CJ. A review of moment-resistant structural timber connections. Proc Inst Civ Eng Struct Build 1998;128(November):323}31 [paper 11590]. [8] Anon. A guide to the use of resins in timber engineering, Ref.TE0993 (1987) and Resiwood System, Ref.RTEP/1 (1996). Rota"x Ltd, Abercraf, Swansea, UK, 1987, 1996. [9] The Institution of Structural Engineers. Guide to the restoration and repair of timber structures. London: SETO/TRADA Technology Limited, 2000. [10] GIROD Project Website 2000: http://www.sp.se/building/ wood/girod.htm. [11] COLORETIM EU contract No. FAIR-S2 9248. [12] Mays GC, Hutchinson AR. Engineering property requirements for structural adhesives. Proc Inst Civ Eng Part 2 1998; 85:485}501. [13] Mays GC, Hutchinson AR. Adhesives in civil engineering. Cambridge: Cambridge University Press, 1992. [14] Davis G. The performance of adhesive systems for structural timbers. Int J Adhes Adhes 1997;17(3):247}55. [15] DD ENV 1995-1-1:1994. Eurocode 5, Design of timber structures * Part 1.1: general rules and rules for buildings. London: BSI, 1994. [16] BS 5268:Part 2:1996. Structural use of timber * Part 2: code of practice for permissible stress design, materials and workmanship. London: BSI, 1996. [17] Hutchinson AR. Joining of "bre-reinforced polymer composite materials, Project Report 46. Construction Industry Research and Information Association, London, 1997. [18] Hutchinson AR, Broughton JG. Pull-out behaviour of reinforcement bonded into timber. Proceedings of the International Conference on Structural Adhesives in Engineering, vol. V. Bristol: Institute of Materials, 1998. p. 186}91. [19] Anon. Tensile testing of bonded-in steel rods. Internal Report, Oxford Brookes University, April 1998. [20] Broughton JG, Hutchinson AR. Pull-out behaviour of steel rods bonded in to timber. Mater Struct 2001;34(236):101}10. [21] Broughton JG, Hutchinson AR. The e!ect of timber moisture content on bonded-in rods. Constr Build Mater 2000;15(1):17}25.