Wood is a living material that must be in optimum environmental conditions or at least it should have a preventive treatment according to the place of exposure, to avoid damage caused by biological or abiological pathologies, which can affect the structural safety of the building. In most cases the middle of the beam is still in excellent condition and can carry the load as before. However, the ends are loaded by shearing forces and a real danger of severe damage of the structure exists. In the case of the timber elements that are very damaged, it must be replaced by other treated wood with an insecticide-fungicide product or the lost geometry of the element must be recovered by using a timber prosthesis with epoxy resin. It is a rarely employed technique but it has a minimal impact on the structural parts, because it can reduce the repaired area and it may conserve as much of the original timber element, with a very low visual impact. This technique have been employed for repairing damaged timber elements in the building case study called “Cal Trepat” in Tàrrega (Spain)
The factory studied was built in the 1930s. Currently, it integrally retains all its architectural structure, formed by a set of nineteen industrial warehouses aligned one alongside another. The scenic value of the factory is remarkable as a representative element of Tàrrega city. The five industrial warehouses inspected had a gabled timber roof formed by 8 trusses and 117 straps every roof.
In some trusses was detected excessive twist in supports, which was considered dangerous to mechanical effects. Several bottom chords had cubic rot at the supports with the pillars. In other cases, it was observed that some of the supports of the trusses didn’t reach the 25 cm long on the pillar, too short to ensure the stability of the structure. Some bottom chords had a strong presence of Hylotrupes bajulus deteriorating several centimeters of the section. We used four different types of prosthesis timber-timber with epoxy resins during the intervention. The wood provided was conifer with the same characteristics that the old wood, in terms of quality, humidity and scantling, giving it a finished old surface and the timber moisture content in the moment of gluing was approximately 13% with a difference of moisture content of the timber prosthesis less than 5%.
A double tooth dog prostheis was used in the bottom chords degraded by xylophages insects. The repair consists of removing the damaged wood (Fig. 1a) and adding wood until completing the bottom chord. The junction between repaired timber and provided timber is performed by means of a double tooth dog (Fig. b). Six pairs of galvanized screws of 400 mm of length were screwed diagonally along of double tooth dog joint and also in the prosthesis of the ending part of the bottom chord, in order to improve the shear stress of the joint between the new and old timber (Fig. 1b). In the bottom chord sixteen pairs of holes were made in the longitudinal plane allowing the injection of the resin (Fig. 1c). Rothoblaas XEPOX resin was used for the double tooth dog joint and the joint of the timber prosthesis with the ending part of the bottom chord because was more fluid and it permitted a best distribution of resin into the joint (Fig. 1c).
Fig. 1. a, b) Provided timber and screws used c) Injection of resin and gluing of the timber (Photos: Labèrnia, 2011)
Rotten supports of the bottom chords were repaired replacing the affected area with a timber prosthesis. In the repair the damaged timber was removed. Then, in the repaired timber two holes of 20 mm of diameter separated one third of the thickness of the bottom chord were done. In these holes were placed the steel threaded rods of 16 mm of diameter and one meter of length with Hilti HIT-RE 500 resin, which helped to link the new timber piece and to strengthen the joint (Fig. 2a). The Hilti resin was less fluid and its employment was better for joining the timber with the rods. The rods were introduced 700 mm in the bottom chord and 300 mm in the timber prosthesis. Seven pairs of holes in the longitudinal plane of gluing were performed in the bottom chord, allowing the injection of the Rothoblaas XEPOX resin in order to fill the gluing plane between two timbers and the threaded bars (Fig. 2b and 2c).
Fig. 2. a) Removal of damaged timber and performing of holes for insert the rods b) Provided timber c) Injection of resin and gluing of timber (Photos: Labèrnia, 2011)
The bottom chords with short support on the pillar were suspect in terms of stability. To solve this problem the supports were elongated using a contribution of timber of 250 mm of length to ensure a sufficient contact surface with the pillar. In the repair six holes of 20 mm of diameter were made, separated approximately one third of the thickness and height of the bottom chord. In these holes were placed the steel threaded rods of 16 mm of diameter and one meter of length with Hilti HIT-RE 500 epoxy resin, that besides of reinforcing the union allowed coupling the new timber piece (Fig. 3a and 3b). The rods were introduced 850 mm in the bottom chord and 250 mm in the timber prosthesis. Between the repaired and provided timber a contact gasket was left, which was filled with epoxy formulation of Rothoblaas XEPOX (Fig. 3c).
Fig. 3. a) Holes for insert the rods and provided timber b) Steel threaded rods glued in the timber c) Gluing of the provide timber with the epoxy resin (Photos: Labèrnia, 2011)
Sometimes the supports of the bottom chords on the pillar besides being too short also had fungi, showing a double structural deficiency. The reparation is the same that the two before cases, but only using an unique prosthesis, which replaced the rotten wood and provided 250 mm of length improving the contact surface of the bottom chord with the pillar.
Fig. 4. a) Removal of damaged timber, insert the rods and provide timber b) Coupling of the wood graft c) Injection of the resin and the gluing of the timber (Photos: Labèrnia, 2011)
Repairs done in “Cal Trepat” industrial building comply with the current trend, permitting the recovery of the load capacity of the repaired part with a very low visual impact. These type of solutions performed in the work have been tested in the laboratory by different researchers overcoming the bending and shear strength tests, aside from using the resins in outdoor environments. At the end of the 1980s, some tests showed that the humidity is a limiting factor, between timber and epoxy resins, as it does not reach the cohesion strength of the timber. This moisture must be optimal for a correct gluing between the wood and the steel bars, because got the greatest tensile strength (14,96 to 20,98 kN) with a timber moisture content between 10% and 22%, unfortunately with a humidity higher of 25% the tensile strength is lower (7,44 kN). Other authors were observed that the steel threaded rods were the best option to ordinary ribbed low-carbon bars. The thread improves the adherence to the epoxy mortar and the nature of the material reduces concerns and provisions against rust. Besides, the tensile strength is much higher than that of the ordinary steel. Glass fiber reinforced polymer bars, in spite of its high tensile strength, have a lower modulus of elasticity, resulting in larger deformations than those observed for steel reinforcement. Different studies showed that the glued length of the rods is important, so the steel rods of 16 mm of diameter and glued of 100 to 350 mm of length have a shear stress of 40 to 150 kN and when the separation between the glued steel rods is near of the 40 mm is obtained the better tensile strength (47kN).
Choosing epoxy resins repairs to timber structures can be difficult for architects and engineers, especially when confronted by strong opinions of the conservation bodies and little information about the design or the performance of some methods. Nowadays the tests carried out demonstrate that the particular epoxy adhesives used are suitable for bonding timber. The greatest advantage of resin methods of repair is the ability to carry out in situ repairs with little or no disruption to the historic fabric of the building. Furthermore is a strengthening technique which can offer an elegant solution for the renovation of deteriorated beam ends and the use of the epoxy resins is easy, because there are dispensers with mixing nozzles that permit employing the quantity of mix wished with the exact dosing.
Extract of paper “Timber structure repair of an emblematic catalan industrial building with wood grafts and epoxy resins” presented by Vilches M., Rodríguez V. & Labèrnia C., at The 2nd International Conference on Structural Health Assessment of Timber Structures – SHATIS2013. Italy: Trento, September 4-6.