Fire Resistance of Composite Concrete Decks

The ACI-318 building code defines composite concrete flexural slabs as elements constructed from separate layers of pre-cast and poured in place concrete, but so interconnected that all of them respond to a load as a single unit. These separate concrete layers must be attached to each other to keep them intact so that the combined unit can be loaded.

Consequently, the code provides a guide for construction and design of the contact faces with the nominal horizontal shear strength values to be used for design. At this point, the code only addresses the strength of composite decks needed to sustain the imposed live and dead loads. It assumes an environment where all components of the concrete decks share the same internal temperature and respond in unison to temperature changes without adding any additional horizontal shear stress to their interconnecting surfaces.

However, when the environment changes (as happens during a building fire) and the internal temperature of the concrete in the adjacent components begin to vary, the design criteria given by the code may no longer be adequate or viable. The new phenomenon affecting the strength and safety of composite concrete decks is layer’s exposure to various levels of heat.

The heat variation of concrete deck surfaces during a fire may reach a level of 1500° F (See UL Test of the Filigree deck). Under such conditions, the actual shear stress at the contact faces of layers becomes unpredictable, and may well exceed the values permitted by the code. This may consequentially cause separation of the layers and de-lamination the composite deck.

Since credible data that can reliably predict the behavior of a composite concrete deck during a building fire is not available, the only option is an actual and complete fire test to determine the structure’s sustainability and safety.

The standards for such fire tests are given by the building code as “Standard Fire Tests of Building Construction and Materials UL263”, ASTM E119, and NFPA, No. 251.

The code requires that only fire resistant components may be used for construction of publicly used structures. All pre-cast concrete elements used for floor construction carry a United Testing Laboratory Certified Fire Resistant Rating.

The International Building Codes (IBC), past and current, mandate that the same requirements for fire resistance rating applies to all structures using composite concrete decks of any type or denomination.