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What are the different types of Concrete roof decks?

What are the different types of Concrete roof decks?

Structural Concrete Deck – A cast-in-place concrete deck serves as an integral part of the structure and designed to carry heavy loads.  Normal weight concrete is typically used for cast-in-place construction. Beneath the roof cover, structural concrete roof decks are commonly topside insulated using adhered insulation panels or cast-in-place lightweight insulating concrete.

Structural Concrete Composite Deck – A supported steel panel deck system filled with normal weight or structural lightweight concrete. The steel deck panels are embossed in a manner which when filled with concrete, allows the elements of the construction to respond to load as a single unit. Structural concrete composite decks are somewhat uncommon at the roof level. They are often however, part of a building design when future above roof level floors are contemplated. Like structural concrete roof decks, composite decks are commonly topside insulated using adhered insulation panels or cast-in-place lightweight insulating concrete.

Precast Concrete Decks – Systems comprised of load bearing precast concrete members placed upon a structural support system. Commonly, the precast members are comprised of reinforced, pre-stressed or post tensioned normal weight concrete. In many cases, the precast assembly is monolithically covered with a topside placement of lightweight or structural lightweight concrete which serves as a leveling layer. Thereafter, precast systems are commonly topside insulated, below the roof cover, using adhered insulation panels or cast-in-place lightweight insulating concrete.

Lightweight Insulating Concrete Decks – Non-structural, low-density, lightweight insulating concrete (LWIC) can be placed upon a variety of structural deck systems that have been designed to carry loads. LWIC can be placed atop any of the above mentioned concrete deck types or directly upon a properly supported corrugated steel deck system.  LWIC roof decks often contain encapsulated expanded polystyrene insulation and are placed in a manner to provide roof drainage.

What are the different materials in concrete?

Normal Weight Concrete – Concrete having a typical cast unit weight of 145 lbs/ft³ resulting in a 28 day compressive strength of 2500 psi or greater.  Concrete having structural quality containing normal weight aggregates.

Structural Concrete – Concrete designed to carry structural load while forming an integral part of a structure.

Structural Lightweight Concrete – Structural concrete having a cast unit weight not more than 115 lbs/ft³.  Concrete comprised using lightweight aggregates and resulting in a 28 day compressive strength of 2500 psi or greater.  Concrete having a structural quality.

Lightweight Concrete – Concrete having a cast unit weight of not more than 115 lbs/ft³.  Concrete comprised using lightweight aggregates and resulting in a 28 day compressive strength of not less than 1200, but not more than 2500 psi. Considered non-structural. Lightweight concrete is often used for fill or leveling.

Lightweight Insulating Concrete – Low-density, non-structural concrete comprised using ultra-lightweight expanded mineral aggregates or preformed foam.  Lightweight insulating concrete (LWIC) is defined as having oven-dry unit weights of 50 lbs/ft³ or less.  Compressive strengths from 125 -350 psi are common for LWIC used in roof deck construction. LWIC placements serve as insulating systems and are often constructed containing encapsulated expanded polystyrene insulation. They are commonly placed in a manner which provides drainage slope and serve to receive the attachment of the roof membrane. Types of lightweight insulating concrete:

1. Aggregate: LWIC which attains its low-density characteristic using ultra lightweight exfoliated or expanded minerals such as vermiculite or perlite.

2. Cellular: LWIC is neat cement paste concrete made very light and cellular by adding a pre-generated aerosol foam (Also known as foamed concrete).

How do I determine what type of concrete deck I have?

Underside Examination – Cast-in-place structural concrete decks will appear monolithic from the underside. Pre-cast structures will be segmental. Composite decks will have a relatively wide spaced I-beam support system. The steel deck is commonly a wide rib type and will exhibit apparent embossment in the corrugations. LWIC placed upon steel decks are generally supported by light framing systems. The most common steel deck types used in conjunction with LWIC are inverted profile form deck, wide rib “B” deck and deep profile “N” deck. Often, the decking will have slots or perforations broken into the bottom of the corrugations for venting. All steel deck used in conjunction with LWIC are galvanized.

Topside Examination – A useful tool for the examination of concrete decks is a wood handled masons hammer. Striking a structural material with a modest blow will result in a ringing of the hammer head. Striking non-structural materials result in a dull hollow sound and an impact divot. If determined as being a non-structural material, lightweight concrete and the various types of LWIC can often be visually identified. By way of the visual examination of a deck fragment, the presents of sand aggregate would indicate lightweight concrete. Concrete with a tan coloration having aggregate particles with a brownish silver mica appearance would be a vermiculite or Zonolite LWIC. Concrete containing aggregate particles appearing as white specks would be perlite LWIC. Concrete containing no apparent aggregate and numerous macro air voids would be cellular LWIC.

For more information, visit American Institute of Concrete (ACI).

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