Concrete slab foundations, commonly called slabs, are the most common foundation system found in commercial buildings. Concrete is a mixture of Portland cement, sand, gravel, and water. When concrete is allowed to cure or harden, it can support the load of a building.
There are two basic types of slabs found in commercial properties: monolithic and stem wall foundations. A monolithic slab is one whose entire slab is poured as one element and is typically the same thickness across its whole depth. However, depending on the region, the exterior might be made thicker to prevent frost heaving and provide better support for the exterior walls. A stem wall slab has frost footings or walls that are placed first, and the slab is poured on top or inside of these footings. This helps support the exterior load and prevents frost heave.
Regardless of the slab type, it is placed on undisturbed soil or a compacted base to specifications determined by the building engineer. Undisturbed soil refers to soil that hasn’t been excavated. In either case, a layer of sand or gravel is added first and compacted before the slab is poured on top of it to help drainage and prevent future movement.
Compression vs. Tension
Concrete has excellent compressive strength. When concrete reaches its full strength and hardness, its strength is determined by the force placed on it. This means that pressure placed on the concrete can be evenly distributed across the mass with little damage or effect on the concrete. Conversely, concrete has very poor tensile strength. Tensile strength is the stretching and bending of the concrete. Concrete can be easily cracked as it is bent or when forces other than pressure are placed on it. This type of force might include impact.
Steel is added to concrete to improve the concrete’s tensile properties or tensile strength. It is known as reinforced concrete when it is poured around steel bars or welded steel wire mesh. The steel bars are called rebar, short for reinforcement bar, and the mesh is typically called WWR for welded wire reinforcement. The amount of rebar or wire mesh used will affect the tensile strength of the slab. Engineers typically design this, and reinforcement is rarely visible after the slab cures. A failure might occur when this reinforcement is visible during an inspection.
During the beginning stages of development, an engineer or architect typically visits a construction site to survey the property and determine the location of the finished slab. Either party will also determine the height of the slab, which establishes the height of the floor. The finished height of the floor aids in the calculation of all upward and downward measurements of the building, and the floor is always assumed to be level.
The slab must be placed on undisturbed soil or a compacted base. Most of the time, there will need to be some excavation to either level the construction site or place the slab in the appropriate location. To determine the depth needed to excavate, the finished height of the slab is subtracted by the depth of the slab plus the amount of gravel needed.
For example, if the slab is 6 inches thick, 6 inches of gravel will need to be placed under the slab, so the excavation will need to be 12 inches deep.
Using this example, under the slab, the undisturbed soil would be 12 inches. If the excavation needs to go deeper than 12 inches, then the disturbed soil needs to be brought to compaction equal to that of the undisturbed soil, or additional gravel will need to be used to balance this area of over-excavation.
Once the entire footprint of the slab is created, excavation forms are placed to create the outer boundaries or shape of the slab. This is the area where liquid concrete will be poured and allowed to cure and dry. Also located in this formed area will be reinforcement. This method will create a monolithic slab foundation.
There will be a few additional steps needed if the slab is a stem wall. The first extra step would be the excavation of the footings. Footings are placed around the perimeter of the footprint at a depth equal to or greater than the frost depth in the building’s region. Depending on the region, these depths could be 24 to 50 inches. Next, forms are placed to create the locations for the footings. The footings are placed on undisturbed soil or a compacted base. Finally, the footings are reinforced, and concrete is poured into the footing forms to cure and dry.
Once the footings are cured, another set of foundation forms is placed on the footings to provide a stable base for the slab and perimeter walls. Reinforcement is added within the foundation forms. Then, concrete is poured, cured and dried, and the stem wall is ready to have the slab poured on top of it or inside the footing.
These stem walls will provide a stable foundation to transfer the load of the exterior walls to the soil. The monolithic slab also transfers the load, but could be subject to movement or failure during frost conditions.
The basic principles of construction hold with slabs. A slab should be level, and a stem wall should be plumb. While inspecting the slab, the first thing a commercial property inspector should identify is the levelness of the slab. Areas not level may show signs of water ponding, dips and ridges, or humps or rises.
The heat produced by concrete during the curing process is called an exothermic reaction. It is during this heating process that the majority of cracking occurs in the slab. Therefore, during construction, a series of control joints or seams are added throughout the slab to help protect it from errant cracking. The goal of these joints is to allow the concrete to ideally crack only at the joints. Inspection includes reviewing these joints for any areas that might provide trip hazards or indications that movement in the slab might have occurred.
Going Beyond a Visual Inspection
An inspector will not be able to determine the depth of the slab. The slab could be 4 inches, 6 inches, or maybe thicker, depending on the need of the building. For example, facilities with printing presses or other heavy loads might require thicker slabs. Determining the exact thickness of the slab would require coring the slab and measuring the thickness of the core. This service is beyond the ComSOP but may be added to your inspection agreement if it’s of interest to the client.
An inspector also cannot see the condition of the base below the slab. A potential failure under the slab may be caused by hollow spots where the slab is not fully supported. Placing loads on the slab over inadequately supported areas may cause the slab to crack and potentially settle or sag. Continuous pressures on these areas could cause structural problems, or at least concerns. Most of these will not be visible to the inspector and are challenging to identify.
Beyond a visual assessment, the inspector could perform an auditory assessment using a prop like an iron bar, wooden dowel, or broom handle. When the prop is dropped onto the adequately supported concrete, a solid sound will ring out, and the bar or handle will sharply bounce back upward. However, if the prop is dropped onto an inadequately supported spot or hollow base area, it will not spring back upward, and the sound will be hollow.
While not conclusive, this technique can at least give some evidence of the potential that there are issues with the support of the slab in that particular area. This could suggest that a core sample be taken in that area, or a recommendation for further structural review by an engineer. In most cases, the reinforcement within the slab will allow this hollow area to be bridged, but extreme compressive loads could cause issues.
Concrete slabs are the most common foundation type in commercial buildings. A commercial property inspector needs to understand the construction methods and the basic types of failures that might occur. Remembering to review the slab for levelness and trip hazards, and identifying areas of movement should be part of every inspection, and any findings noted in the client’s report.
Article Written By: Rob, Claus, CMI®
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