Monolithic Slab Foundation
A monolithic slab or also commonly called a slab on grade is a continuous concrete slab poured as one unit. Other foundations might be constructed by pouring exterior and interior beams then pour the slab separately. A pier and beam foundation consists of a concrete beam resting on piers and in most cases wooden floor joist.
Warmer climates
In warmer climates a monolithic foundation is more popular as there is surface freezing only for short periods. Also the cost is much less expensive and if properly constructed has the same longevity. If you live in a colder climate, your foundation will probably be a “Pier and Beam” or a “Basement” foundation due to the depth of the frost.
Construction of slab
When constructing a monolithic slab there are many details to be addressed. The most important factor is preparing the substrate also called the pad. You have heard the saying that a good house depends on a good foundation; yes, and the substrate is the foundation’s foundation.
Determine the perimeter of the house and bulldoze away a six to eight inch layer of grass and unstable top soil down to what is termed 98% stable soil. If you suspect that there has been unknown fill brought in over the years, you may want to remove up to twelve inches. If there has been a history of slab failure in your area either from a fault or poor soil, you would be wise to drill bell bottom piers down to a stable clay stratus which could be 6 to 10 feet.
Next you want to establish the height of your finished floor. If you have a fairly level lot, customarily the finished floor level will be eighteen inches higher than the crown of the street in front of your house. If you are building on a hillside, make sure that during heavy rains the water will hastily drain from your slab.
Sandy loam
Your fill dirt should be what is termed, ‘Sandy Loam” or in construction terminology, “Select Fill”. This is a very stable soil that doesn’t expand and contract from a wet to a dry season. It will compact and support a slab foundation with very little heaving. Be sure the entire fill is consistent as you do not want a differential of movement from one end of your slab to the other.
The sandy loam (select fill) soil will compact very easily and the best manner to accomplish this is to have a bulldozer spread the soil as it is being dumped and let the pneumatic tires of the dual-axle trucks run over it as each lift is applied, however you must use a compactor afterward to assure thorough compaction. If the soil is extremely dry, as you are spreading the dirt apply a small amount of water to aid in the compaction. It is imperative that there be no loose soil as it will settle over time leaving the concrete over that area with no earth under it.
The normal thickness of a residential slab is four inches; therefore you want to bring the select fill pad within six inches from the final grade, and then bring in a two inch layer of sand for a cushion between the pad and the concrete slab.
Grade beams
Set your wood forms to the desired grade and then dig your grade beams. These beams are normally eighteen to twenty four inches deep and twelve to fourteen inches wide. If your foundation plans have been properly engineered; the size of the beams, how many and their placement will be specified. A vapor barrier using 6m poly-film or an equal should be placed on top of the prepared pad. The reinforcement steel will also be specified by the engineer and should be strictly adhered to.
In the case of heavy rains, be sure to remove any water from the grade beams and let dry for a couple of days. A light rain is of no concern and will not damage your pad.
Final stage
The final stage is pouring the concrete in the beams and forms. Do not let the concrete finishers water down the concrete. Strength improves with lower water to cement ratios. A recommended psi (pounds per square inch) is 4000 or more. You want what is called a .50 water cement ratio which is calculated by dividing the weight of the water in one cubic yard of the mix by the weight of the cement in the mix. Example, if one cubic yard of the mix has 235 pounds of water and 470 pounds of cement- the mix is a .50 water to cement ratio. This ratio will give you the strength you need yet be wet enough to allow enough time to achieve proper finishing. On a hot summer day, once the surface has hardened enough, set up a water sprinkler to cool down the hot concrete and slow down the curing. If the surface cures much faster than the rest of the slab, you will have some surface cracking.
If you should have a longer more rectangular slab with offsets, you may want to consider saw cutting several control joints to a 1/4″ depth in suspect areas. Even in the very best conditions with all phases done properly, you may get some surface cracking. A control joint will dictate where the crack will occur. This is especially important if your finish floor will be tile, marble, or the like.
Substrate phase most critical
Each phase of the construction of a monolithic aka slab on grade foundation is of great importance to achieve a superior foundation for your home. If you asked me which phase is the most important, I would hastily tell you that the substrate is the most critical due to ground movement and moisture.
Last but not least, you want to make sure you keep equal moisture around your foundation; this can be achieved by proper drainage and an irrigation system for uniformity of moisture.
Category: Foundation
