Is It Possible to Build Over or Add on to an Existing Concrete Slab Foundation?
When considering the expansion or alteration of a building that has an existing concrete slab foundation, the question arises: is it possible to build over or add onto this structure? The answer is yes, but the approach depends on the specific conditions and objectives of the project. Below, we explore three primary methods and a real-world example of adding to an existing concrete slab foundation.
1. Integrating the Existing Slab into the New Design
One of the most common methods to build over an existing concrete slab foundation is to treat the current slab as a crucial element in the overall design of the new structure. This involves considering the existing slab and surrounding soil conditions (such as rocks, sand, clay, and landfill) as integral components of the new foundation. The existing slab can be a plus, providing a stable base, or a minus, requiring additional reinforcement or integration into the new design.
2. Ignoring the Existing Foundation
Another approach is to ignore the existing system when building additional stories or levels. In this case, new columns are erected, and these columns are supported by new pad footings or isolated footings that are either cut into the existing work or sit apart from it. This method allows for a clean start, ensuring that the new structure is not influenced by the conditions of the old foundation.
3. Extending the Existing Foundation
For some projects, it is possible to extend and incorporate the existing foundation into the new structure. This can be achieved by connecting the new elements with dowels or other interlocking methods. Essentially, the new construction matches the existing foundation, either connecting to it or installing new elements to float alongside the old but adhere to the same structural integrity.
The choice of method depends on the specific requirements of the project, the stability of the existing slab, and the design objectives. Each method has its own advantages and considerations, making them suitable for different scenarios and conditions.
Options for Addressing Uneven or Damaged Slabs
If the concrete slab is not in excellent condition but is still usable, various methods can be employed to improve its condition and usability. For a slab that is merely uneven, different materials can be applied to topdress it, such as drypacking, using gypcrete, or self-leveling compounds to flatten or resurface the surface.
However, if the slab is sunken, shifted, or cracked and uneven, a more thorough approach is necessary. In such cases, the problematic sections of the slab may need to be completely removed and replaced with competent concrete. This process can be extensive and often requires careful planning and execution to ensure the structural integrity of the new slab.
Real-World Example: A Case Study in NYC
One notable example of building over an existing concrete slab foundation can be found in New York City (NYC) at a location near the United Nations building. The site originally housed a Pepsi bottling plant, which was initially constructed as a kerosene bottling plant and later became a sugar refinery before being transformed into a soda bottling facility. Over time, various floors had to be added due to the deteriorating effects of sugar on the concrete surfaces. Plastic sheets and other measures were used to separate the different concrete floors, but they proved ineffective.
Finally, it was decided to replace the older concrete floors with a new, more competent concrete floor. As the old floors were being removed, the air began to smell like a brewery, a result of the varnished sugar residues that had accumulated over the years. This real-world case illustrates the complexities and challenges that can arise when working with a deteriorating or problematic existing foundation, emphasizing the importance of considering thorough solutions and planning.