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S ince the 1950s, prestressed concrete hollow-core slabs have been used in a variety of structural applications, including residential and commercial buildings, parking structures, and short-span bridges. The slabs contain voids that run continuously along their length, which helps reduce dead weight and material cost. Figure 1 shows hollow-core slabs with two different top surface conditions. Hollow-core slabs are economical, have good fire resistance and sound insulation properties, and are capable of spanning long distances with relatively shallow depths. Common depths of prestressed hollow-core slabs range from 6 to 10 in. (150 to 250 mm) for spans of approximately 30 ft (9 m). More recently, hollow-core slabs with depths as great as 16 in. (410 mm) have become available and are capable of spanning more than 50 ft (15 m).In practice, a concrete topping layer is often cast in place onto the top surface of hollow-core slabs to create a continuous level finished surface. The topping layer is typically 2 in. (50 mm) deep. The topping may increase the flexural strength, shear strength, and bending stiffness of the slab if composite action is developed with the hollow-core units.Composite action between the cast-in-place concrete topping and the hollow-core unit is developed primarily through bond at the interface. Steel reinforcement to promote composite action across concretes cast at differ-■ This paper reports tests on two types of hollow-core units (dry mix and wet mix) to determine the interfacial shear strength between them and their cast-in-place concrete toppings.■ Tests conducted using push-off specimens designed to generate shear stresses at the interface showed that interfacial shear strength correlates with surface roughness, sandblasting to remove laitance (from wet-mix specimens), and grouting.