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Many homes and other structures that are severely damaged in storms like Katrina, Rita and Ike have been built on concrete slabs.
The question that is on many homeowners’ minds is “Can I reuse the slab?” The answer will require homeowners to ask other questions, such as:
Is the slab in good structural condition?
In Louisiana, the slab and the foundation for the structure are built in one piece. We call this type of slab a “monolithic slab.” Before the concrete was poured, trenches were dug into the soil and steel reinforcement rods were placed in the trenches and became the foundation for the building. Forms were constructed above these trenches and the slab was poured. With this understanding of the condition of the slab, one must consider the soil below the foundation, the foundation built into the slab and the surface concrete of the slab.
The condition is rather easy to see as we look from the top. Small stress cracks are often seen in all slabs. These cracks will not cause problems. Larger cracks with the surface of one side lower than the other or cracks that are ¼ inch wide will likely mean more serious problems. Either of these situations likely means the foundation has shifted possibly because the soil beneath foundation has for some reason given way.
Many soils in Louisiana are clay-based and expand greatly as the moisture content goes up. These soils shrink as they dry out. After a flood and, then a dry season, we have the potential for foundation problems.
Other soils in the area are high in organics and can actually rot if exposed to oxygen. When they rot there is nothing to support the slab foundation and, again, shifting and cracking can occur.
Other damages can occur to slabs by heavy equipment used in the cleanup of the structure following the hurricanes. Most damage can be seen from the surface and will show up as cracks and settling.
How old was the building?
This question is not about the slab itself but what is in the slab, such as the plumbing. Again, in Louisiana, when a structure is built on a slab the piping for both the fresh water and the drain system are located in the slab. Over time, piping deteriorates either from external shifting in the slab or from internal damage due to acid water or minerals in the water or from the pipe rusting. Therefore, you must consider the condition of the plumbing. Its condition is normally related to age. You will need a plumber to help you with this question.
Will the new building fit on the old slab?
You may not want to limit yourself to designing a new structure to fit the old slab. If the plumbing was good, the new baths and kitchen must be located to connect to the plumbing in the slab. Unless you add to the slab, the size and shape of the new house must fit the old slab. Ask yourself if this is okay.
What elevation is now required for the first floor?
These last two questions may be the first ones to get answered. If the structure was damaged to the point that the only thing left is the slab, regulations and financial institutions are going to require the new structure built at the 100-year base flood elevation on existing Flood Insurance Rate Maps or the Advisory Base Flood Elevation maps. These elevations are usually determined by the U.S. Army Corps of Engineers, based on historic observations and calculations made from new knowledge gathered from computer models. The elevation data are provided to FEMA for use in making flood insurance and floodplain regulation maps. If your old slab is not at the required elevation, you cannot use it as the first floor of the new structure. You can consider elevating over the old slab and using the old slab surface for non-inhabited space. Foundation design considerations limit this concept and would require engineering assistance, as well as approval of the local building officials.
Do new building codes require a different foundation design?
Louisiana has adopted new building codes that will directly affect whether the existing slab can be reused. These codes require soil strength tests run and foundations be designed based on the results of the soil loads allowable. It is likely that the old foundations in the existing slab will not meet the new code requirements. Also in the new codes are increased requirements regarding attachments connecting structures to slabs.
