A composite, structural concrete topping is commonly used in floor construction with hollowcore slabs. The composite action is desirable to add stiffness and strength for gravity loads and may also be required for load transfer within a diaphragm. When a composite topping is used, consideration must be given to its strength, detailing and quality assurance.
The required compressive strength of the topping may be determined from the hollow core slab design requirements. Load tables provided by local producers will normally indicate that either a 3000 psi or 4000 psi concrete is required. Diaphragm requirements may necessitate a higher strength topping concrete.
Since the composite topping and hollow core slabs interact to create the final structural element, it is imperative that the topping bond well with the slabs. While the building designer may only be interested in the final product, the process of achieving a well bonded, composite topping is very important. The hollow core producer is dependent on a properly bonded topping, yet is not involved in specifying, designing or installing the topping. The hollow core producer is responsible for supplying a slab that is capable of bonding with a topping. The installer of the topping is responsible for surface preparation, topping concrete mix design and curing to assure proper bond.
Design of hollow core slabs for composite action is usually limited to a horizontal shear strength of 80 psi according to section 22.214.171.124 of ACI 318-95. Through limited published and unpublished testing, the machine finished surface has been found to meet the requirements of that section. The horizontal shear check should be based on the shear diagram rather than using an average horizontal shear over the distance from zero moment to maximum moment when checking compliance with the 80 psi limit.