A concrete slab is a flat, horizontal, structural element made of cast concrete, often reinforced with steel (rebar), used as a sturdy foundation, floor, roof, or pavement in buildings and infrastructure. They provide a durable, stable surface for supporting loads and come in various types, like slab-on-ground, suspended, waffle, or precast, with thicknesses ranging from a few inches (for patios) to over a foot (for heavy-duty use).
A concrete slab is a structural feature, usually of constant thickness, that can be used as a floor or a roof. A slab-on-ground is supported on the subsoil and is usually reinforced with reinforcing bars or welded wire mesh.
Residential concrete slabs in Australia must adhere to Australian Standards AS 2870 and AS 3600. Typically, the thickness of these slabs ranges from 85mm to 100mm.
In the context of construction, a slab refers to a flat, horizontal, and typically reinforced concrete element used to form floors, ceilings, and roofs.
There are several types of slabs including one-way slabs which carry load in one direction, two-way slabs which carry load in two directions, joist slabs which have concrete ribs for support, and precast slabs which are constructed off-site and transported.
A Site Classification is carried out in accordance with the Australian Standard AS2870: “Residential Slabs and Footings”. Classes include S (slightly reactive), M (moderately reactive), H1-H2 (highly reactive), E (extremely reactive), or P (problem site).
Ground-bearing slabs, also known as "on-ground", "slab-on-grade" or "concrete bed", are commonly used for ground floors on domestic and some commercial applications.
In Australian slang, a "slab" refers to a carton of beer, typically containing 24 cans or bottles, often purchased from a bottle shop (liquor store) and named for its flat, "slab-like" shape compared to older carton styles. It's a common term for buying a large quantity of beer for a gathering, like saying, "Let's grab a slab from the bottle-o".
A 20x20 foot concrete slab generally costs between $1,600 and $4,800 installed, averaging around $2,400-$3,200, depending heavily on thickness (4" vs. 6"), site prep, rebar, and labor rates, with 4-inch slabs on the lower end and 6-inch reinforced slabs higher; expect about $4 to $8 per square foot for basic material and installation, but costs rise with complexity.
Concrete can last up to 100 years, depending on several external factors that have a marked effect on its durability. In addition to its extended lifespan, concrete is one of the most utilized substances in the world for several reasons. It repels fire and water and is affordable for large-scale construction.
“Slab” issues that most often arise in disputes between builders and owners relate to the size of cracks that may appear, and the thickness of the concrete slab installed. In Western Australia the most common slab thickness is between 85mm and 100mm.
The 10-20-30-40 concrete rule is a rule of thumb for mix proportions by volume: roughly 10% cement, 20% water & air, 30% sand, and 40% gravel/stone, aiming to use as much cost-effective aggregate as possible while the cement paste binds it. This ratio creates a strong, well-graded mix where smaller particles fill gaps in larger ones, reducing void space and cement needs, making it economical and durable for general construction.
One-Way Slab
This type of slab is commonly used in rectangular areas where the length is more than twice the width. One-way slabs are ideal for residential buildings and office spaces, where the structural design requires straightforward load distribution. They are efficient and economical for small to medium spans.
The best way to determine which type you need is to review the breadth and width of a particular project. If one direction is longer than the other by a ratio of two to one, then you should consider a one-way slab.
Poured concrete can be a slab, or it can be formed walls.
This material is typically produced at a utility sluice pond site by dumping raw ash into the pond and allowing it to hydrate and harden into a working platform. Additional raw ash is placed on top of the platform in thin lifts, watered, compacted, and allowed to hydrate and harden.
It no secret concrete is generally cheaper, when considering the price per square foot and when it comes to installation.
Typically a 4" slab on grade would get wire mesh , like in a sidewalk or driveway for a home. A 6" slab, like for a garage floor designed for heavier vehicles, might call for heavier reinforcement, like #4 bar . Really depends on what the slab is designed to do.
10 Common Concrete Construction Mistakes to Avoid
The AS 2870 Australian standard is widely used for concrete slabs and footings.
Pavers are small, interlocking units made of concrete, stone, or brick, designed to move slightly without cracking. Slabs, on the other hand, are large, continuous sections of poured concrete that form a solid base.
If you want the slab to be level with the surrounding earth, you'll have to dig into the ground to compact the soil and lay down a base before putting it down. The right depth depends on the height of your slab, local climate, the condition of your soil, and what you plan to use the slab for.
A 20x20 foot concrete slab generally costs between $1,600 and $4,800 installed, averaging around $2,400-$3,200, depending heavily on thickness (4" vs. 6"), site prep, rebar, and labor rates, with 4-inch slabs on the lower end and 6-inch reinforced slabs higher; expect about $4 to $8 per square foot for basic material and installation, but costs rise with complexity.
Types of Concrete
FAQ – Concrete Slabs vs Pads
Slabs are structural and cover large areas. Pads are smaller, isolated foundations used for specific items like decks or sheds.