Steel in Contact with Concrete

According to the Steel Structures Painting Manual, Vol. I, Good Painting Practice (Steel Structures Painting Council, 40 24th Street, Suite 600, Pittsburgh, PA 15213):

1. Steel that is embedded in concrete for reinforcing should not be painted.
Design considerations require strong bond between the reinforcing and the concrete so that the stress is distributed. Painting of such steel does not supply sufficient bond. If the concrete is properly made and of sufficient thickness over the metal, the steel will not corrode.
2. Steel that is encased in exposed lightweight concrete that is porous should be painted with at least one coat of good-quality rust-inhibitive primer. When conditions are severe, or humidity is high, two or more coats of paint should be applied, since the concrete may accelerate corrosion.
3. When steel is enclosed in concrete of high density or low porosity, and when the concrete is at least 2 to 3 in thick, painting is not necessary, since the concrete will protect the steel.
4. Steel in partial contact with concrete is generally not painted. This creates an undesirable condition, for water may seep into the crack between the steel and the concrete, causing corrosion. A sufficient volume of rust may be built up, spalling the concrete. The only remedy is to chip or leave a groove in the concrete at the edge next to the steel and seal the crack with an alkali-resistant calking compound (such as bituminous cement).
5. Steel should not be encased in concrete that contains cinders, since the acidic condition will cause corrosion of the steel.
Structural steel is a noncombustible material. It is therefore satisfactory for use without protective coverage in many types of buildings where combustibility loading is low, from the viewpoint of either building ordinances or owners preference.
When structural steel is used in this fashion, it is described as exposed or unprotected. Unprotected steel may be selected wherever building codes permit combustible construction.
Exposed or unprotected structural steel is commonly used for industrial-type buildings, hangars, auditoriums, stadiums, warehouses, parking garages, billboards, towers, and low stores, schools, and hospitals. In most cases, these structures contain little combustible material. In others, where the contents are highly combustible, sprinkler systems may be incorporated to protect the steelwork.
Steel building frames and floor systems should be covered with fire-resistant materials in certain buildings to reduce the chance of fire damage. These structures may be tall buildings, such as offices, apartments, and hotels, or low-height buildings,  such as warehouses, where there is a large amount of combustible content.
The buildings may be located in congested areas, where the spread of fire is a strong possibility. So for public safety, as well as to prevent property loss, building codes regulate the amount of fire resistance that must be provided.
The following are some of the factors that enter into the determination of minimum fire resistance for a specific structure: height, floor area, type of occupancy (a measure of combustible contents), fire-fighting apparatus, sprinkler systems, and location in a community (fire zone), which is a measure of hazard to adjoining properties.

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