Investigation on bonding between timber and ultra-high performance concrete (UHPC)

Bonded composite constructions from timber and ultra-high performance concrete (UHPC) are investigated as a highly innovative structural element. In a first step, mechanical models for the description of the bond between timber and UHPC are documented and analyzed. The theory of the compliant bond, which was already introduced by Volkersen (1953) [28], and the linear elastic fracture mechanics (LEFM) are possible approaches for this purpose. Extensive experimental investigations on the bond behaviour are accomplished. From this it is found that the failure of the bond occurs in timber close to the bond-line in most cases. The kind of the surface treatment of UHPC does not affect the bond behaviour. In general, the ultimate loads can be estimated by Volkersens theory, but in the case of short bond lengths, tension stresses in timber, orthotropic to the bond-line affect bending strength. With numerical studies, this effect could be examined and it was found that the mentioned tension stresses do not affect the strength for bond lengths of 300 mm and more. On the basis of the bond investigations, recommendations for the design of bending members are given, and validated by two bending tests.

Research highlights
► Theoretical models describe the behaviour of the bond between timber and concrete.
► Tests depict, that bonding failure equals timber failure in most cases.
► Nonlinear effects occur with increasing bond length.
► Parameters for the bond theory are established. The maximum bond length equals 400 mm.
► Theoretical results could be verified by two bending tests.