Wood-concrete composites are floor and deck systems, which comprise of a concrete slab integrally connected to wooden beams (or a slab of laminated wood) beneath by means of a shear connector. The use of this construction method can significantly improve a deck’s strength and stiffness (up to approximately 2 times and 4 times, respectively) when compared to that of a non-composite structure – resulting in a highly efficient use of materials. Sound and vibration performance as well as fire resistance are also improved when compared to timber floors.
Although wood-concrete slabs have been historically used in North America, recent applications are limited. In Europe, both research and construction of wood-concrete slabs (and other structural components) has advanced as of late. New interest in the environmentally friendly material wood combined with strong code requirements concerning sound and fire protection make the WCC system a viable option. A need to upgrade existing wooden structures also drives demand for this method. In bridge applications, the wood reduces the overall bridge weight while the concrete provides a wear surface and weather protection that surpasses wooden decking.
While the short-term structural performance of wood-concrete composites can be evaluated using a mechanics-based approach, long-term system performance is not, as yet, well understood – particularly for fluctuating outdoor climates. It is known that wood and concrete exhibit creep under prolonged loading. For both materials, the amount of creep-related deflection is a function of load duration as well as applied stress level. For wood, however, its creep behavior is also influenced by moisture and temperature changes due to its hygroscopic nature. Since wood and concrete are adjoined in a WCC, different creep rates (concrete creeps faster than wood) may also have a detrimental effect. In addition to magnified deflections, the differential creep also tends to migrate internal forces from the concrete to the wood leading to higher stresses in the wood.
To understand the long-term behavior and to test a wood-concrete system in the New England climate, we constructed a 14 ft. slab (the “bridge”), loaded it and have been measuring deflections and strain in the structure since November 2005.
- HBV Systeme – The manufacturer of the shear connector