What is a Concrete Slab?
A concrete slab is a flat, horizontal element made of cast concrete, commonly used as a floor or foundation for buildings, patios, driveways, and walkways. Slabs on grade are poured directly on the ground over a compacted subbase, while elevated slabs are supported by beams and columns. The most common residential application is the slab-on-grade foundation, which serves as both the structural base and the finished floor of a building.
Proper concrete slab construction requires careful preparation including soil compaction, gravel subbase placement, vapor barrier installation, reinforcement placement, and accurate formwork. The thickness and reinforcement design depend on the intended loads and soil conditions at the site.
Slab Volume Formula
Since thickness is usually specified in inches, convert to feet by dividing by 12 before calculating volume.
Thickness Guide
| Application | Min. Thickness | Recommended | Strength (psi) |
|---|---|---|---|
| Sidewalk / Patio | 3.5" | 4" | 3,000 |
| Residential Driveway | 4" | 4-5" | 3,500 |
| Garage Floor | 4" | 5-6" | 3,500 |
| Shed / Workshop | 4" | 4" | 3,000 |
| Commercial Parking | 5" | 6" | 4,000 |
Slab Reinforcement
Most concrete slabs require reinforcement to control cracking and add structural strength. The two most common methods are welded wire mesh (WWM) and rebar. Wire mesh (6x6 W1.4/W1.4) is standard for patios and sidewalks, while #4 rebar on 18-inch centers is used for driveways and structural slabs. Fiber reinforcement added to the concrete mix can supplement but not replace steel reinforcement in structural applications.
Worked Example
A 20 ft x 20 ft patio slab that is 4 inches thick with 10% waste:
With waste = 133.33 × 1.10 = 146.67 ft³
Cubic Yards = 146.67 ÷ 27 = 5.43 yd³
You would need approximately 5.5 cubic yards of ready-mix concrete, or about 245 bags of 80-lb pre-mixed concrete.
Frequently Asked Questions
How many bags of concrete do I need for a slab?
One 80-lb bag yields approximately 0.6 cubic feet. For a 10x10 ft slab at 4 inches thick, you need about 33.3 cubic feet, which equals approximately 56 bags of 80-lb mix. For larger slabs, ready-mix delivery is more economical and ensures consistent quality.
Do I need gravel under a concrete slab?
Yes, a 4-6 inch compacted gravel base is recommended under all concrete slabs. The gravel provides drainage, prevents soil movement from undermining the slab, and creates a uniform bearing surface. In areas with expansive clay soils, a thicker base may be required.
How long does a concrete slab take to cure?
Concrete reaches about 70% of its design strength in 7 days and full strength at 28 days. Keep the slab moist for at least 7 days by covering with plastic sheeting or applying a curing compound. Avoid placing heavy loads on the slab for at least 28 days.
When should I order ready-mix vs. use bags?
For projects requiring more than 1 cubic yard (about 45 bags of 80-lb mix), ready-mix delivery is usually more economical and produces better results. Ready-mix ensures consistent quality, saves significant labor, and allows the entire slab to be poured in one operation, avoiding cold joints.