Resin-bonded repair systems for structural timber
Wood Information Sheet Section 4 Sheet 22
Subject: Structural uses
Revised July 2001
Epoxy resin-bonded systems for structural timber have been in use in the UK for over 20 years. They are used to reinforce existing timbers to accommodate an increase in load or to repair or replace parts of structural members which have deteriorated through decay, insect attack, mechanical damage or structural failure.
Resin repairs should be undertaken by specialist contractors. However, this Sheet provides an overview of the ways in which epoxy resins are currently used in repair work and gives surveyors, designers and owners or keepers of structures some guidance as to the suitability of resin repair techniques.
TRADA Technology has undertaken research in this area and guidance is published in a more comprehensive document Resin repairs to timber structures: Guidance and selection. For engineers, calculated and annotated design examples for a tension member, a beam end repair and an eaves joint repair are published in Resin repairs to timber structures: Design examples to Eurocode 5.
Although this sheet concentrates on epoxy resin repair systems, there are alternative solutions. These include replacement of damaged sections with new timber, and reinforcement or strengthening by mechanical means. The survey work necessary to establish the need for and the choice of the repair or strengthening system, is considered in Resin repairs to timber structures: Guidance and selection and in a WI Sheet Assessment and repair of structural timber .
Uses of epoxy resin systems
Epoxy resin systems are used for a number of purposes:
Repair of damaged members to restore their strength and / or appearance
The damage may have been caused by fungal decay, insect infestation, mechanical damage, fire damage, building movement or inferior design or detailing. The cause of the damage must be remedied before the repair is undertaken, eg by improving detailing to prevent the accumulation of moisture.
The choice of repair system will be influenced by the structural requirements, accessibility, fire resistance requirements, and the appearance of the finished repair.
Fissures usually occur during drying. They may also be caused by moisture movement in the members of a joint assembled in green timber, which subsequently dries and contracts. Small fissures are a normal feature of structural timber and should not necessarily be considered a defect. In solid softwood or glulam sections, fissures should not occur provided that good practices have been followed (see WI Sheets Moisture in timber and Glued laminated timber - an introduction).In green oak construction, fissures are part of the normal in-situ drying out process, which occurs over many years, and should be recognised, rather than causing concern. Filling fissures may be necessary as they tend to retain moisture which may allow the timber to decay. They may also decrease the timber’s resistance to shear stresses and may reduce the fire resistance of the member. Fissures occurring at joints, eg along bolt lines will decrease the efficiency of the joint.
This may be necessary to increase the strength or stiffness of a timber member when a change of use of a building increases the design load. The considerations are similar to those for repairing damaged sections.
Epoxy resin systems may be used in varying exposure conditions. Four categories of exposure are usually recognised for structural timber. These are shown in Table 1 with an indication of the properties required of an epoxy resin adhesive in such a situation.
TABLE 1 Epoxy resin adhesive systems - conditions of use
Exposure category | Conditions | Examples | Epoxy resin requirements |
Exterior high hazard Equivalent to: EC5 Service
Class 3 | Full exposure to weather | Exterior components or assemblies where the
resin/timber joint is exposed to the elements. Partial exposure to
weather or where moisture trapping is possible eg timber built into
wall or where timber has some protection from bridge deck or eaves
overhang. | Adhesive should exhibit long-term water resistance.
Rate of cure should suit the ambient temperature: a catalyst may be
added to assist reaction at low temperatures;
<5o. For high temperatures a resin with a
high glass transition temperature should be used. |
Exterior low hazard. | Exposed to the weather but protected from sun and
rain. | Inside roof of open sheds and porches. Underneath
some types of bridge deck. |
Interior high hazard. | In closed buildings with warm damp
conditions. | Unventilated roof spaces, swimming
pools. |
Chemically polluted atmospheres. | Chemical works, such as salt stores,
tanneries. | Resins with good resistance to chemicals concerned.
Curing agents with good chemical resistance As with all categories,
good packaging for reliable batching; good open assembly and work
handling characteristics. |
Interior low hazard. Equivalent to: EC5 Service
Class 1. | Heated and ventilated buildings where the moisture
content of the wood will not exceed 20%. | Inside heated buildings. |
The resin performs a variety of functions in a resin-bonded repair or upgrading system:
Replacement of damaged section
Various formulations of epoxy resin grout adhesives are used. These are filled epoxies with a relatively low viscosity, making them self-levelling. The space to be filled must be enclosed by permanent or temporary shuttering and sealed to prevent leakage. When the area to be repaired is inaccessible or vertically orientated, thixotropic resins are usually specified. These have low viscosity when stirred or agitated but thicken when left undisturbed.
Use as an adhesive to bond new pieces of timber or reinforcing rods or plates to the timber
When resins are used to bond two pieces of timber together, the movement that may occur due to moisture and temperature fluctuations should be considered. In common with all types of adhesives, high differential movement between the two timbers may affect the bond at the timber/ resin interface. Similar considerations apply when bonding steel plates or FRP pultrusions to timber. Reliance should not be placed on bonding the end-grain of timbers.
Use as an adhesive to bond reinforcing rods or plates into existing timbers. Two types of reinforcement are commonly used:
Dowels and rods
Mild steel, stainless steel or FRP pultrusions (usually fibreglass strands in a polymeric heat-cured matrix) are most commonly used, bonded to the timber using thixotropic resin. The surface of the metal has an important influence upon the performance of the bond. Smooth stainless steel dowels should not be used. Figures 1, 3 and 5 show schematic principles of such repairs.
Steel plates
Steel plates are used where damaged material has been removed, to join a replacement section to the remaining sound section. They may also be used to increase the stiffness and strength of a sound member to upgrade its performance.
