Concrete Slump Test Explained: How to Check Workability
The slump test measures how workable fresh concrete is. For most residential pours in Sydney, you want 80–120 mm slump. Pump mixes need 120–160 mm. A slump that's too low means the concrete is hard to place; too high may mean excess water that weakens the final slab.
What Is Slump and Why Does It Matter?
Slump is the standard field measurement for concrete workability: how easily the mix flows, fills formwork, and compacts around reinforcement. It's measured in millimetres using the Abrams slump cone test (AS 1012.3.1), and it's the single most common quality check performed on ready-mix concrete in Australia.
When your concrete truck arrives on-site, slump is the first thing to verify. A mix with the wrong slump can be almost impossible to place correctly. Too stiff and you'll get honeycombing and voids; too wet and you lose strength, increase shrinkage, and risk surface defects during curing.
According to CCAA guidelines, the acceptable tolerance on slump is typically ±25 mm of the ordered value. Concrete outside this range should be assessed before placing.
Slump Ranges by Application
Different pours need different workability. Here are the typical slump ranges used in Sydney residential and light commercial work:
| Application | Typical slump | Notes |
|---|---|---|
| Footings & strip foundations | 40–80 mm | Stiff mix; placed directly into trenches, minimal reinforcement |
| Driveways & slabs on ground | 80–120 mm | Standard workability for screeding and finishing |
| Pump mixes | 120–160 mm | Higher slump needed to flow through the pump line; usually achieved with plasticiser, not extra water |
| Columns & walls | 100–140 mm | Must flow around steel; vibration helps compact lower-slump mixes |
| Exposed aggregate | 60–100 mm | Slightly stiffer to hold aggregates in position during finishing |
| Self-compacting concrete (SCC) | 650–800 mm* | *Measured by slump flow test (different method), not standard cone |
Ordering tip: When booking through MixHub, specify your application type and the plant will select the appropriate slump. If you're pumping, tell us and we'll match you with a pump-friendly mix.
How to Perform a Slump Test
The test takes 2–3 minutes and should be done within 5 minutes of the truck arriving, before any concrete is discharged into formwork. You need a slump cone (Abrams cone, 300 mm tall, 200 mm base, 100 mm top), a tamping rod, a flat base plate, and a tape measure.
Dampen the cone and base plate
Wet the inside of the slump cone and the base plate to prevent the concrete sticking. Place the cone on a flat, level surface.
Fill in three layers
Fill the cone in three roughly equal layers. Tamp each layer 25 times with the rod, distributing strokes evenly across the cross-section.
Strike off the top
After tamping the final layer, strike off the excess concrete level with the top of the cone using the tamping rod in a rolling motion.
Lift the cone vertically
Lift the cone straight up in 3–5 seconds with a steady pull. Do not twist or tilt. Place the cone upside-down next to the concrete.
Measure the slump
Lay the tamping rod across the top of the inverted cone. Measure the distance from the rod down to the highest point of the slumped concrete. This distance in millimetres is the slump value.
Reading the Result
There are three types of slump to watch for:
- True slump: the concrete subsides evenly all around. This is the normal, expected result.
- Shear slump: one side shears off and slides. This indicates a harsh mix and the test should be repeated.
- Collapse: the concrete collapses completely into a flat puddle. This means the mix is too wet or has excess water.
What Affects Slump on the Day
Slump isn't fixed; it changes between the batching plant and your site. Understanding why helps you anticipate problems:
- Travel time: slump drops 10–25 mm per 30 minutes of travel in Sydney conditions. Longer hauls from distant plants arrive stiffer.
- Temperature: hot days accelerate cement hydration. A mix batched at 100 mm slump in Penrith at 38°C can arrive at 60 mm. The plant can add retarder to compensate.
- Waiting time on-site: if the truck sits on-site for 20+ minutes before discharge, expect slump loss. Drum rotation slows but doesn't stop the loss.
- Admixtures: plasticisers (water reducers) increase slump without adding water. They're the proper way to adjust workability. Superplasticisers can add 100+ mm of slump.
- Water content: every 10 litres of water added to a 6 m³ load increases slump by roughly 20–30 mm but reduces 28-day strength by 2–3 MPa. This is why site-added water is discouraged.
When to Reject a Concrete Load
You're paying for concrete that meets specification. Rejecting a non-conforming load before it's placed is far cheaper than dealing with a failed slab. Reject if:
- Slump is more than 25 mm below your specified value and the driver cannot adjust with admixture, meaning the concrete will be too stiff to finish properly
- Slump is more than 25 mm above your specified value, likely from excess water, which means lower final strength
- The concrete shows visible segregation (stones separated from the paste, or bleed water pooling on the surface immediately)
- The batch ticket doesn't match what you ordered (wrong grade, wrong slump target, wrong admixtures)
What to do when rejecting
Photograph the slump test result and the batch docket. Note the time and the truck number. Call the plant immediately; they can often dispatch a replacement load within 30–60 minutes. Through MixHub, rejected loads are logged and the replacement is prioritised automatically.
Frequently Asked Questions
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