Suspended concrete slabs typically use 32 MPa or higher, with the exact mix, slump and aggregate size specified by the project engineer rather than chosen on site. That is the honest answer and it is not a dodge. A suspended slab carries its own weight across a span with nothing underneath it, so the concrete is doing structural work that a slab on ground never has to do, and the consequences of getting it wrong are serious rather than cosmetic. Your structural drawings will state the grade, and usually the slump, aggregate size and reinforcement as well. What this guide does is explain what those numbers mean and why the engineer chose them, so you can read your own specification, order correctly and spot a problem before the truck arrives.
| Specification item | Typical suspended slab value | What it controls |
|---|---|---|
| Strength grade | 32 MPa, often 40 MPa | Structural capacity and durability |
| Exposure classification | A1 internal, B1 or higher exposed | Minimum grade and cover |
| Slump | 100 to 120 mm | Workability through congested steel |
| Maximum aggregate size | 10 mm or 14 mm | Flow around bars, cover compliance |
| Placement method | Pumped, almost always | Access to an elevated pour |
| Reinforcement | Bar, mesh or post-tensioned | Tension capacity, deflection control |
The Engineer Specifies It, Not the Supplier
This is the part worth being blunt about, because it separates a suspended slab from every other pour on a residential site. Nobody should be choosing this mix by rule of thumb.
A slab on ground is supported continuously by the earth beneath it, so if the concrete is slightly under specification the ground still carries the load. A suspended slab has air underneath. It spans between beams, walls or columns, it carries its own dead weight plus everything you put on it, and it is designed to AS 3600 with calculated reinforcement, deflection limits and cover requirements. Change the mix and you change the assumptions the design was built on. We will supply exactly what the drawings call for and we will happily talk it through with you, but we will not nominate a grade for a structural slab, and no supplier should. If you do not have engineered drawings for a suspended slab, that is the thing to sort out before anything else.
Why 32 MPa Is the Common Starting Point
Most suspended slabs you will encounter on residential and light commercial work specify 32 MPa, and it is worth understanding why that figure recurs. It is rarely about raw load capacity.
Thirty-two MPa is generally the minimum that satisfies the durability requirements for a suspended slab in anything other than a fully protected internal environment, and it produces a denser, less permeable concrete that protects the reinforcement inside it. That matters more than it sounds, because the most common long-term failure in suspended slabs is corrosion of the steel rather than crushing of the concrete. Rust occupies more volume than the steel it replaces, so corroding bars crack and spall the slab from the inside. A denser mix slows the ingress of water and chlorides that starts the process. Forty MPa appears where exposure is harsher or spans and loads are greater. If you want the underlying explanation of what the number means, our driveway MPa guide covers compressive strength in plain terms.
Exposure Classification Drives the Grade
This is the piece most people miss, and it explains why two slabs carrying identical loads can be specified at different grades. Location changes the requirement, not just load.
AS 3600 assigns an exposure classification based on where the concrete sits and what it is exposed to, and each classification carries a minimum characteristic strength. A protected internal floor sits at the mild end. A balcony, an external walkway, a slab over a carport or anything exposed to weather sits higher, and coastal or marine environments higher again. As a rough sense of the progression, a sheltered internal slab may only require around 20 to 25 MPa on durability grounds, an exposed external slab commonly lands at 32 MPa, more aggressive exposure at 40 MPa, and marine or aggressive soil conditions at 50 MPa. Concrete cover over the reinforcement increases alongside it. So a suspended balcony slab will often be specified stronger than an internal floor spanning the same distance, and that is durability rather than structure talking. Your engineer applies the classification. Do not assume it from the load alone.
Slump and Workability Matter Because It Is Pumped
Grade gets all the attention, but on a suspended slab the slump is arguably the more practical number. Get it wrong and a correctly specified mix still gives you a bad slab.
Suspended slabs are almost always placed by concrete pump, since the pour is elevated and a truck cannot discharge directly into it. The concrete has to travel through a line, then flow around dense reinforcement, formwork edges, penetrations and services without segregating or leaving voids. A slump around 100 to 120 millimetres is typical for this, giving enough workability to compact properly around the steel. The critical rule is that workability is adjusted at the plant with admixtures, not with a hose on site. Adding water on site to make a stiff load easier to place reduces strength, increases shrinkage and voids the mix design the engineer specified. If a load arrives too stiff to place, that is a conversation with the supplier, not a bucket of water. Because pump access shapes the whole day, plan it early, and our note on site access for concrete trucks covers arranging a line pump where the truck cannot reach.
Aggregate Size and Reinforcement Congestion
Maximum aggregate size is the specification item most often overlooked on an order, and on a suspended slab it has a direct physical consequence. It is not a detail.
Suspended slabs are thinner and far more heavily reinforced than slabs on ground, so the concrete has to pass between closely spaced bars and still achieve full cover underneath them. Twenty millimetre aggregate, standard for a driveway, can bridge across tight bar spacing and leave voids beneath, which compromises both the bond to the steel and the cover protecting it. This is why 10 millimetre or 14 millimetre aggregate is commonly specified for suspended work. Smaller aggregate also pumps more reliably through a line. If your drawings nominate an aggregate size, order it explicitly, because it is not what a supplier will assume by default.
Post-Tensioned Slabs Have an Extra Requirement
If your slab is post-tensioned, the specification carries a timing element that conventionally reinforced slabs do not. Early strength becomes a scheduling constraint.
In a post-tensioned slab, steel tendons running through the slab are tensioned after the concrete has been poured and has gained enough strength to take the force. That means the mix has to reach a nominated strength within a set number of days so stressing can happen on programme, and the specification will state both the 28 day characteristic strength and the strength required at stressing. Cylinders are usually tested to confirm it rather than working from the calendar. Tell your supplier at the point of order that the slab is post-tensioned and when you intend to stress, because the mix may need to be designed for early strength gain rather than simply meeting the 28 day figure.
How to Read Your Structural Drawings
Everything you need is on the drawings, usually in one place, and knowing where to look saves a phone call. It is worth five minutes before you order.
Look for the concrete notes or specification panel, generally on the first structural sheet or in the general notes. You are looking for the grade written as a number followed by MPa or as an N-grade such as N32, the exposure classification, the slump, the maximum aggregate size and the required cover to reinforcement. The reinforcement schedule sits separately and covers bar sizes, spacing and laps. If any of those are missing, ambiguous or contradict another sheet, ring the engineer rather than making an assumption, because a supplier cannot resolve a drawing conflict for you. Photograph the notes panel, since it is the fastest way to give a supplier exactly what they need.
What to Send Us When You Order
Ordering for a suspended slab is straightforward once you have the specification in front of you. We would rather see the drawings than take a grade over the phone.
Send us the concrete notes from your structural drawings, along with the volume, the date and time you need it and the placement method. If you are pumping, tell us so we can supply a mix suited to it, and if the slab is post-tensioned, tell us that too. Our concrete calculator will get your volume close, though on a suspended slab it pays to confirm against the drawings and allow for deflection in the formwork, which can absorb noticeably more concrete than the nominal thickness suggests. We supply structural grade pre-mix concrete across Broadford, Kilmore, Wallan, Seymour and the surrounding region. Send your details through our contact page or call +61 3 5784 1500 and we will confirm the mix against your specification before anything is batched.
Frequently Asked Questions
What MPa concrete is used for a suspended slab?
Thirty-two MPa is the most common specification for residential and light commercial suspended slabs, with 40 MPa where exposure or loading is more demanding. The figure comes from your structural drawings, since it is set by the engineer against both structural and durability requirements.
Can I use the same mix as my house slab for a suspended slab?
No. A slab on ground is supported continuously by the earth beneath it, while a suspended slab spans an opening and carries load in bending. The grade, slump, aggregate size and reinforcement are all specified differently, and substituting one for the other is not safe.
Do suspended slabs need a concrete pump?
Almost always, because the pour is elevated and a truck cannot discharge into it directly. A line pump or boom pump is standard, and the mix is designed with that in mind. Arrange the pump when you book the concrete rather than on the day.
Can water be added on site to a suspended slab mix?
No. Adding water reduces strength, increases shrinkage cracking and invalidates the specified mix design. If a load is too stiff to place, contact the supplier so it can be corrected properly at the plant rather than adjusted with a hose.
