Effects of various admixtures and shear keys in wood–concrete composite beams

Wood–concrete composite beams are a layered system, which essentially utilize a concrete layer in compression and a wood layer in tension. This layered system offers a way to construct or rehabilitate wood floors in historic timber structures while increasing the floors’ stiffness and load carrying capacity. This research paper investigates past problems with poor consolidation of the concrete, transverse shrinkage cracks in the concrete, swelling of the wood, moisture loss from the concrete, and the resulting reduced composite efficiency. The research presented herein describes how these problems can be mitigated and thereby increase the composite efficiency of the wood–concrete composite system. By painting the specimens with a water proofing paint, the swelling of the wood can be reduced, which helps to maintain a tight interface between the wood and concrete. To improve consolidation, a self-leveling concrete was designed with a 28-day compressive strength of 34.5 MPa and a slump of 279.4 mm. Nylon fibers and Type I steel fibers were used as admixtures to the fresh concrete to determine their effect on the composite efficiency and the reduction of shrinkage cracks. Twelve full size specimens were constructed and tested to failure. It was found that the most common mode of failure was combined bending and tension at mid span in the wood. An average composite efficiency of 83.4% was reached in the full size test specimens when placed in four point bending.