Strength and durability of hot-press glued-laminated timber beams

Abstract

Hot-press glued-laminated timber was manufactured exclusively by one Canadian laminator between 1967 and 1970. Members made by this process have been found to be capable of seriously delaminating in service. The purpose of this study was to determine the strength and durability of lightly-delaminated hot-press beams. Eight such beams, plus two conventionally manufactured Glulam beams, provided material which was tested in either flexural-shear, flexural-tension-perpendicular-to-glueline after accelerated ageing, or vacuum-pressure cycling. For the accelerated ageing tests, material was subjected to either zero, one, two, four, six, or eight cycles of alternating high (85%) and low (35%) relative humidity at 50 °C, corresponding to equilibrium moisture contents of 16 and 6%, respectively. Flexural-shear strength of the hot-press material was somewhat lower than that expected of conventional material. Vacuum-pressure cycling of hot-press specimens resulted in delamination values exceeding the permissible amount. The average flexural-tensile strength of conventional material was substantially higher and less variable than that of hot-press material, both before and after accelerated ageing. Moreover, the flexural-tension-perpendicular-to-glueline tests after accelerated ageing cycles resulted in increased amounts of gluebond failure in the hot-press material. The results of the accelerated ageing and vacuum-pressure cycling tests indicate that even lightly-delaminated hot-press material has an inherent ability to delaminate and may perform poorly in situations of cyclic moisture environments.