A concrete slab foundation, often called a slab-on-grade, is a monolithic structural base that rests directly on the ground. This foundation is a thick, single layer of concrete that forms both the structural support and the ground floor of the residence. It is a popular choice in residential construction, especially where ground freezing is not a major concern, due to its straightforward construction. This design offers a durable base for the building and is often cost-effective compared to foundations requiring deep basements or extensive crawl spaces.
Preparing the Site and Setting Forms
Site preparation begins with removing all vegetation, topsoil, and debris from the construction area. Establishing a clean, stable base is necessary to prevent future settling or movement of the slab. Rough grading is then performed to ensure the site slopes away from the structure, promoting proper surface drainage.
Excavation is carried out to the required depth, determined by the local frost line and the foundation design specifications. The subgrade soil must be compacted to a specified density using heavy mechanical equipment. Inadequate compaction can lead to the foundation settling unevenly. During this phase, ensure all required permits have been obtained and the site is ready for inspection by the local building authority.
Next, the perimeter formwork is constructed to contain the fluid concrete and define the slab’s final dimensions. This mold is typically built using 2x lumber set on edge and held in place by wooden stakes. The forms must be securely braced from the outside to resist the pressure exerted by the wet concrete.
Accuracy in setting the forms is important, requiring diagonal measurements to ensure the perimeter is square. The top edges must be leveled precisely to the final height of the slab. The forms should be set to the specified thickness, commonly four to six inches for the main floor area. Outer edges are often thickened to 12 to 24 inches to provide extra support for exterior load-bearing walls.
Installing Sub-Base, Vapor Barrier, and Reinforcement
Once the forms are in place, a granular sub-base material is laid inside the formwork to provide a level surface and facilitate drainage. This material is typically a two to three-inch layer of crushed stone, gravel, or sand, which prevents moisture from wicking upward into the concrete. The sub-base must be uniformly spread and mechanically compacted to achieve maximum density and stability.
A vapor barrier, consisting of heavy-duty polyethylene sheeting, is installed over the compacted sub-base to block water vapor migration into the house. This barrier protects finished flooring and interior space from moisture intrusion. The plastic sheets must overlap by several inches at the seams and be sealed with tape to create a continuous, impermeable membrane.
Steel reinforcement, either wire mesh or rebar, is positioned within the formwork to increase the concrete’s tensile strength and control cracking. Since concrete is strong in compression but weak in tension, the steel handles stresses caused by temperature changes and settling. For maximum effectiveness, the reinforcement should be suspended to sit in the upper half of the slab’s depth, using small concrete blocks or specialized wire chairs.
The reinforcement is commonly laid out in a grid pattern, with rebar spacing ranging from 18 to 24 inches on center, depending on structural requirements. This network must be securely tied together at the intersections to maintain placement during the pour. Maintaining the correct distance from the bottom and edges, known as concrete cover, protects the steel from corrosion and ensures its structural function.
Placing and Initial Leveling of Concrete
Before ordering, precisely calculate the volume of concrete needed based on the formwork dimensions to ensure a continuous pour. For residential slabs, the mix design typically specifies a compressive strength between 3,500 and 4,000 pounds per square inch (PSI) after 28 days of curing. This strength provides the necessary load-bearing capacity and resistance to wear. The specific PSI is often dictated by local building codes and climate conditions.
Delivery logistics must be coordinated so the concrete arrives in a steady flow, allowing for even distribution across the entire formwork without significant delays. As the ready-mix concrete is discharged, it is spread using shovels and rakes. Ensure the material is evenly distributed and consolidated around the reinforcement and into the thickened edge areas. Avoid excessive handling or spreading, which can lead to aggregate segregation and compromise final strength.
Vibrators are used immediately after placement to eliminate trapped air pockets and ensure the concrete flows densely around all reinforcing steel. This consolidation process removes voids, improving the overall density, strength, and durability of the finished slab. Once consolidated, the initial leveling process, known as screeding, begins. Screeding involves pulling a long, straight edge across the formwork to remove excess material and bring the surface to the required elevation. Following screeding, the surface is treated with a bull float. This long-handled tool pushes down aggregate particles and brings a layer of smooth cement paste, or ‘cream,’ to the surface, eliminating high or low spots and preparing the slab for final finishing.
Finishing and Curing the Slab
After bull floating, a waiting period allows the bleed water—excess water that rises to the surface—to evaporate before final finishing begins. The timing is determined by the concrete’s setting time, ambient temperature, and humidity. Starting the finishing too soon, while bleed water is present, results in a weak, soft surface layer susceptible to dusting and scaling.
Once the surface moisture has disappeared and the concrete can support a finisher’s weight, the final steps of hard troweling and edging begin. Troweling uses a smooth tool to create a dense, hard-wearing surface resistant to abrasion. Edging involves rounding the perimeter of the slab with a specialized tool to prevent chipping and improve durability.
To manage the material’s natural tendency to shrink and crack, control joints (contraction joints) are introduced. These joints are tooled or saw-cut shortly after finishing to create planes of weakness where cracking can occur predictably. Control joints should be cut to a depth of at least one-quarter of the slab’s total thickness. They must be placed within the first six to eighteen hours after the pour, before random shrinkage cracks develop.
Curing immediately follows finishing and is important for ensuring the concrete reaches its specified compressive strength. Curing involves maintaining adequate moisture and temperature for a minimum of seven days, supporting the hydration reaction that makes the concrete harden. Methods include covering the slab with plastic sheeting, continuously misting the surface, or applying specialized liquid curing compounds. The concrete achieves approximately 70% of its ultimate strength within the first week, with full design strength typically reached after a 28-day curing period.