Exposed Glulam Delamination - Should Exposed Glulam be Sealed

It can be assumed that stresses will develop along glue-lines in exposed glulam, simply because the grain orientation, ring orientation, wood density, response to moisture etc. thus will differ between adjacent laminations. European specifications for exposed glulam state that the growth rings in the laminations must all be oriented the same way, and they show a diagram of flat-sawn laminations all with the pith-side downwards.This is hardly practical in New Zealand with radiata as the ring orientation is likely to change across a finger-joint, and many laminations will be quarter-sawn, flat sawn and everything in between.Therefore delamination is to be expected but generally should not penetrate more than 20 mm in properly cured glulam, made with resorcinol adhesive. Exposed treated solid timber is likely to develop similar checking. The reason for this is that the moisture fluctuations that give rise to the stresses do not penetrate far, i.e. they are damped out by the resistance of the wood to diffusion of moisture. Forest Research has tested glulam from a coolstore that showed obvious delamination. There appeared to be little effect on strength unless the delamination goes right through. There have been experiments to determine how much delamination can be tolerated before an effect on shear is noticed. Where the glue-line was artificially narrowed by placing adhesive tape along the laminations prior to gluing, there was no effect down to 25% of the width remaining. When the glued area was reduced by placing adhesive strips across the wood at intervals, an immediate effect was noticed. It all has to do with the stress-raisers generated by the delamination. Some type of sealing is certainly helpful, and painting is excellent but it must be maintained. An oil-based preservative such as creosote is effective because the oiliness acts as a water repellant. I have seen a thick tacky substance applied to glulam bridge stringers, same as can be applied to steel as a rust preventative. The bridge in question is at the entrance to the Whakarewarewa village in Rotorua and is still giving good service after 40 years. There are several proprietary formulations of water repellent sealants on the market, and some manufacturers apply “Ensele” as a matter of course.