The value of gypsum and cement plaster as fire resistive materials has been known for centuries. So it should come as no surprise that the plastering industry was called on to supply the materials for fireproofing when heavy gauged steel buildings opened up a new concept in construction. Large mid-rise and high-rise buildings rose quickly over the skylines of many cities.
The problem that was encountered however was the fact that steel begins to lose strength as temperatures begin to rise. The obvious need then was to provide protection to the steel structure to prevent the collapse of a building in the event of a fire. In its infancy, steel construction was often enclosed in masonry, clay tile or in some cases forms were made around the steel beams and columns and concrete was actually poured around them. These methods seemed a bit self-defeating given the speed of erection of the steel. Moreover the sizing of the steel had to be increased to accommodate the extra weight of the masonry or concrete used as fireproofing.
It was these circumstances that really ushered in modern lathing and plastering techniques that not only offered protection for the steel, but also made it possible to provide a finished surface for heavy steel beams and columns. Gypsum plaster is endothermic (absorbs heat) in nature, in that it has trapped water contained in its molecules, that it gives back as steam in a fire event. In its hard state gypsum plaster is called “hemihydrate” which essentially means that it has returned to the rock like state of crystallization it had before it was mined from the earth. As it consumes heat in a fire event (calcines) it disassociates from the water molecules it needed in its installation. The temperature at which this transpires is over 212° F. This knowledge has made it possible to design assemblies that dependent upon the thickness of the plaster can hold up to fire for up to four hours.
Although lath and plaster is still used in fire rated assemblies, the introduction of spraying machines have greatly reduced cost and speeded up the way plasterers install fire resistive materials in a direct-to-steel operation. As a result, many products have been developed over the years that use gypsum base or portland cement in conjunction with cellulose fibers to form a lightweight easily applied fire resistive material. Newer types of fireproofing materials include intumescent paints which expand when subject to fire.
- High degree of resistance against the passage of heat and flame.
- Helps maintain the integrity of the building structure in a fire.
- Allows occupants time to get out of a dangerous situation by inhibiting smoke development and restricting toxic gasses.
- Bides firefighters time to see and rescue victims that may be trapped by a blaze.
- Slows the growth of flames which allows firefighters to gain a foothold of control over the building before there is any threat of collapse.
- Helps create safe havens that shelter firefighters while they assemble and fight the fire.
- Provides a redundant passive fire suppression system in support and balance of more active fire suppression measures such as sprinklers.
SPRAY GYPSUM CELLULOSE APPLICATION
The fireproofing product is brought to the job site in large sacks and put into a hopper of the spraying machine. Rotating blades in the hopper break the treated cellulose up so that air pressure can move it through a supply hose that runs to the application wand controlled by the plasterer. The wand has four nozzle openings spaced in the collar that is attached to the hose. Water is supplied through a separate hose and atomized through the nozzles in the collar. As the cellulose and gypsum material is pushed through the supply hose, it mixes with a water mist created by the nozzles to compact the material and make it sticky enough to apply to the steel.
The mixing and spraying operations are slightly different. These types of spray cellulose materials are mixed with water to make a slurry in a plaster type tumbler mixer, and then pumped directly through hoses to a wand controlled by the plasterer. Compressed air is also supplied to the wand by a small hose which forces the material out onto the steel. The supply of the material is controlled by changing the air pressure and changing the orifice size of the nozzle at the end of the wand.
The plasterer needs to be skilled in determining that the steel surface is being covered in a uniform application, because the thickness of the spray fireproofing determines the fire rating that must be met. In this respect, the industry has developed measuring instruments which are invaluable in determining whether the proper thickness has been applied.
MORE INFORMATION/OTHER USEFUL LINKS
“Fireproofing, Sprinklers and Mrs. O’Leary’s Cow.” The Minnesota Lath and Plaster Bureau newsletter “Hot Spot,” October 2001
AWCI the Association of the Wall and Ceiling Industry has many excellent publications on Fireproofing at http://store.awci.org/cgi-bin/awci
- AWCI Sprayed Fire Resistive Material (SFRM) Industry Standards
- Technical Manual 12-A, 4th Edition; Standard Practice for the Testing and Inspection of Field Applied Sprayed Fire-Resistive Materials; an Annotated Guide
- Technical Manual 12-B, 3rd Edition; Standard Practice for the Testing and Inspection of Field Applied Thin Film Itumescent Fire-Resistive Materials; an Annotated Guide
The Alliance for Fire and Smoke Containment and Control was established in 1999 by building enforcement, construction, design, and manufacturing professionals in response to the need for a well-coordinated, educational effort to promote the value of a balanced fire protection design in the built environment. See http://afscc.org/ for more information.