Our aim is to help you understand the basic construction of the most common concrete slab foundation utilized within central Texas communities.
The most common concrete slab foundation used in residential construction these days are what is known as “slab-on-grade” foundations. In short, the foundation is designed to keep the cost of production down while maintaining the necessary support. The construction of the foundation consists of three components of monolithic nature, all of which utilize shallow-subsurface and surface soils for their support.
Prior to forming the concrete slab foundation for production, the lot is graded and/or filled, then compacted to meet the requirements for a particular design and location. For slab on grade foundations, after the lot has been configured, channels are excavated and formed for final assembly.
Take a moment and imagine a square moat with a tic-toe pattern excavated inside of the square. The excavated intersecting lines inside of the square moat are referred as interior grade beams. The square moat itself is referred as the exterior or perimeter grade beams. What’s poured above the interior and exterior grade beams will be the slab. The excavated channels coupled with the forms allow for all the three components; the interior, exterior grade beams and the slab to be of a monolithic pour. Grade beams will vary in depth and size but, typically measure twenty to thirty-six inches in depth for a residential slab on grade foundation.
Once the grade beams are excavated and formed its time to add a moisture barrier and some strength before concrete can be poured. The moisture barrier will be placed on top of the surface soils within the areas of the foundation to protect the home from subsurface moisture. Concrete inherently by itself is weak; we need to add interlaced steel rebar, steel cables (post-tension), wire mesh or fiber to reinforce the concrete. At this point if you were to look down into the unfinished foundation, you’ll find the excavated channels, the moisture barrier and the steel reinforcement ready for concrete.
The final step is pouring the concrete. When the concrete is poured it will fill the channels of the interior and exterior grade beams then it will form the slab. Slabs are typically poured three to four inches thick. While pouring the concrete, the contractors typically vibrate and screed the concrete to make sure it has been tightly compacted and smoothed out. Presto…we now have a slab on grade foundation.
In conclusion concrete slab foundations come in many shapes, sizes and functions for a given geographical zone. We’ve just discussed the basic construction of a residential slab on grade foundation found in many central Texas communities like San Antonio.