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	Design Criteria Installation Guide Quality Assurance UL/FM Classifications

Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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Structural Concrete Roof Decks
Under the classification of structural concrete roof decks, there are numerous types of concretes mixed from an almost unlimited quantity of design mixes that yield concretes of various physical and performance properties. The two general types of structural concrete roof decks are described in the following section. These decks differ by the manner in which they are reinforced; by whether they are poured, or cast on or off the project site; and by design mix.

The Two General Types of Structural Concrete

Normal Structural Concrete: Aggregate, usually stone or crushed gravel, mixed with sand, portland cement, water, and in some design mixes, various chemical additives produces normal structural concrete. Steel bars or wire mesh are added to reinforce the concrete. The density of reinforced normal structural concrete is generally about 150 pounds per cubic foot. Do not confuse reinforced normal structural concrete with either lightweight structural concrete with a dry density between 85 to 120 pounds per cubic foot, or lightweight insulating concrete with a dry density between 20 to 40 pounds per cubic foot. For normal structural concrete roof decks, a smooth surface shall be achieved by screeding and trowel finishing the concrete. The surface shall be finished to a texture that is equivalent to either a light power metal trowel finish or a firm wood trowel finish, without depressions or irregularities.

Lightweight Structural Concrete: Lightweight structural concrete roof decks are similar to normal structural concrete roof decks, although lightweight structural concrete roof decks are approximately two thirds the weight of normal structural concrete decks. Lightweight aggregates are used in lightweight structural concrete mixes instead of the sand and crushed stone mixed into normal structural concretes. These lightweight aggregates are made from natural occurring substances such as shale, clay and slate. The most commonly used lightweight aggregate is expanded shale. The shale is crushed and heated to a high temperature until it becomes like plastic. The small amounts of water that are naturally present turn to steam and expand, popping the shale particles like popcorn. Adding water in the blast furnace expands the shale slag while it is in a molten state. Lightweight structural concrete roof decks made with these types of aggregate and portland cement have a density between 85 to 120 pounds per cubic foot, which is about 80% of the density of normal structural concrete. Although lighter in weight, lightweight structural concretes retain most of the characteristics of normal cast in place structural concrete.

Three General Types of Normal & Lightweight Structural Concrete Roof Decks

1.	Cast In Place Structural concretes that are poured and formed at the project site are referred to as cast in place concretes. Common configurations of cast in place concrete roof decks include: Joist and slab or beam and slab; waffle slab; flat plate; flat slab; and slab band. See Illustration 5.

Illustration 5: Cast in place concrete roof decks

Post Tensioned Cast in Place Post tensioned structural concrete contains high strength steel strands called tendons that are embedded in the concrete when it is poured in place at the project site. These tendons are sheathed or encased in steel or plastic tubes so as to prevent the concrete from bonding to the tendon. After the concrete has dried, the tendons are tightened, or tensioned, with a hydraulic jack. Post tensioned concrete can carry a much heavier load than could be carried if the concrete were just reinforced with steel rods or wire mesh in the conventional manner. Post tensioning is done on the project site. See Illustration 6.

Illustration 6: Post tensioned cast in place concrete roof decks

In posttensioning, the concrete is not allowed to bond to the steel strands during drying.

After the concrete has dried, the strands are tensioned with a hydraulic jack and anchored to the ends of the beam. if the strands are draped, as shown here, higher structural efficiency is possible than with straight strands.

Precast Panels: Precast structural concrete panels are poured into forms, allowed to dry, then transported to the project site for installation of the roof deck. Most precast concrete units are prestressed. Prestressing is accomplished by pouring concrete around pretensioned steel strands stretched across a pouring form; then allowing the concrete to sufficiently dry and bond to the strands. The strands are cut from their anchors outside either end of the dried panel. The release of the strands puts the precast concrete panel into compression. Prestressing generally results in camber, that is upward deflection, raising the center of the span above the elevation of the supports. See Illustration 7.

Illustration 7: Prestressed precast concrete panels

The first step in presenting is to stretch the steel prestressing strands tightly across the casting bed.

Concrete is cast around the stretched strands and dried. The concrete bonds to the strands.

When the strands are cut the concrete goes into compression and the beam takes on a camber enabling the concrete panel to accommodate greater loading.

The precast panel is prestressed prior to being installed on the project site. Prestressing the panel by putting it into a state of compression allows the panel to carry greater loads during service. Prestressed concrete panels are manufactured in the following shapes: Solid slabs, hollow core slabs, single tees, and double tees.

Illustration 8: Prestressed precast concrete panels shapes

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