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Pre-cast concrete foundation systems are a new construction feature in homebuilding. Unlike a poured concrete foundation, which requires a separate pour for concrete footings and can mean weeks before a dwelling can be erected on it, a pre-cast concrete foundation can be assembled into place in days and the structure - whether a modular or stick-built house - can be assembled right away.
What are the benefits? The cost saving can be 30% or more, and there are no concrete footings or setting up and removing forms for the concrete foundation walls - the pre-cast system is assembled at the construction site and bolted together.
The process begins when the site selected for the foundation is excavated below the frost line for that particular geographic location. The next step is to level and compress a base of pea gravel (very small gravel, about three-eights of an inch in diameter) with a vibrating power tamp. The compressed graveled surface is all that is required for the pre-cast walls to be assembled on. No concrete footings are required.
A reinforced footer beam is incorporated in each wall section of the pre-cast foundation. The foundation wall sections are transported to the job site via a flatbed tractor-trailer. With the assistance of a crane, the pre-cast wall sections are lifted into place and bolted together, forming a full foundation perimeter. A cement floor slab will be poured inside the foundation walls.
Once the walls are assembled in place, the perimeter around the outside of the foundation wall is backfilled by an excavator. This is a delicate operation; a rock or stone cascading down and striking the foundation wall could crack and or penetrate the thin exterior wall surface. The actual wall thickness face shell of the pre-cast foundation wall is 1Â¾ to two inches.
A one-inch extruded polystyrene foam insulation board is incorporated into the inside of each wall section. The manufacturer highly recommends that the inside wall be protected with sheetrock - but who enforces that recommendation? The interior wall studs are pre-cast concrete two-by-sixes with a wooden treated wall nailer attached so sheetrock can be installed. Each stud has a pre-drilled hole that can be used for installing electric cable.
This is a new foundation system for the construction market. The question for the fire service is how will this hold up to a heavy-content, free-burning hot fire? At a recent fire, severe spalling occurred on the interior wall. One of the photos on page 103 shows that the blue polystyrene foam board was obliterated from the wall. The brown discoloration is the spalling.
What is spalling? Spalling is the loss of surface material that occurs when concrete is heated to a temperature of about 800 to 900 degrees Fahrenheit; the heating causes the expansion of moisture in the concrete. Spalling occurs violently, throwing chunks of concrete-like projectiles.
I witnessed the effects of spalling at a fire in a department store. A floor area about 300 by 300 feet was fully involved in fire. The contents were totally consumed. Large craters in the concrete floor above were gouged out. The craters were from two to four inches deep and covered an area of two feet by three feet throughout sections of the concrete ceiling above. Steel reinforcement bars were exposed in some locations.
What effect does severe spalling have on the integrity of a pre-cast wall? Can it hold up as well as a poured-foundation wall when exposed to a free-burning fire? Remember the manufacturer highly recommends a fire rating. Does the builder or homeowner always comply? This should be mandatory and the building inspector must inspect and insist on compliance with any recommendations by manufacturers of any type of new systems and lightweight engineered products. This is the minimum protection for the safety of firefighters and civilians.