Slump (concrete workability test)
Slump is the workability measure of fresh concrete: filled into a cone, cone lifted, vertical drop measured. Residential slab pours typically 80-100 mm slump.
Ask Chalkline about this →Slump is the measure of fresh concrete’s workability via the slump test (also called the Abrams cone test), conducted under AS 1012.3.1:2014. The test fills freshly batched concrete into a standardised metal cone (300 mm tall, 100 mm top diameter, 200 mm base), lifts the cone vertically, and measures the vertical drop of the concrete mass in millimetres. The drop is the slump. Higher slump = more workable (flowable), but higher slump means higher water content, which reduces strength if not balanced with cement.
The test procedure (AS 1012.3.1):
- Wet the inside of the cone (cone sits on a wet baseplate).
- Fill the cone in three equal layers, each layer rodded 25 times with a 16 mm tamping rod.
- Strike off the top with the tamping rod, level with the cone rim.
- Lift the cone vertically in 5-10 seconds, no twisting.
- Measure the vertical drop from the original cone height (300 mm) to the highest point of the slumped concrete.
- Record in millimetres.
The test takes about 5 minutes and is typically done at the concrete pour at every truck arrival (or at least the first truck) on a residential slab pour.
Typical residential specifications:
| Element | Specified slump (mm) |
|---|---|
| Slab on ground (residential, waffle/raft) | 80-100 mm |
| Footing pad | 80-100 mm |
| Bored pier (handworked) | 100-150 mm |
| Pumped slab (concrete pumped from truck) | 100-120 mm |
| Suspended slab / structural | 80-100 mm |
| Decorative exposed-aggregate slab | 80-100 mm |
| Self-compacting concrete (SCC) | Not measured by slump (uses slump flow test) |
Slump and water content (the danger):
Higher slump = more water added at the batch plant OR water added on site. Water added on site is typically prohibited by AS 1379:
- Strength reduces ~5-10% per 25 mm of additional water-induced slump increase.
- Shrinkage increases.
- Cracking risk increases.
If the concretor wants higher slump, the BATCH PLANT must add water AND adjust cement to maintain strength. Adding water on site without batch plant approval voids the strength guarantee.
Slump check at delivery (the standard practice):
- Concretor performs slump test on the truck arrival.
- If slump is below spec: truck batch was too dry; either accept (if concretor judges OK) OR call for water-cement adjustment from the batch plant. Adding water on site without approval is forbidden.
- If slump is above spec: truck batch was too wet OR water was added en route. Reject the truck if significantly above; document for the batch plant.
- Record slump on the delivery docket: documentary evidence at certifier inspection.
Common builder issues:
- No slump test on residential pours: cheap practice but loses QA on the concrete.
- Slump test on first truck only: subsequent trucks may differ; ideally test 1 in 4 trucks or any visually different load.
- Water added on site by concretor: strength compromised; if discovered, batch may need replacement.
- Wrong cone or no rodding: test invalid; results unreliable.
- Slump test on hot day, late in the pour: concrete starts setting; slump reads lower than at batch.
The bigger picture: workability vs strength:
A “workable” concrete (high slump) is easier for the concretor to spread, finish, and pump. A “low slump” concrete is stiffer, more cement-strong, but harder to work. The balance depends on the application:
- Hand-placed footings: high slump (100-150 mm) for workability.
- Pumped slabs: medium-high slump (100-120 mm) to flow through the pump.
- Structural high-strength: low slump (60-90 mm) to maximise strength.
A skilled concretor specs slump appropriate to the pour and verifies on delivery.
For builders:
- Spec slump on the concrete order with the batch plant: e.g. “N25 slump 90 mm at delivery”.
- Have a slump test cone on site for any residential pour: $80-$150 to buy, reusable.
- Watch the slump test at first truck arrival: confirm it matches spec.
- Document the slump on the delivery docket: written record protects you on strength disputes.
- Don’t accept water-added-on-site unless the batch plant authorises and the adjusted mix is documented.
Also known as: slump test, concrete slump, Abrams cone test.
Category: Concrete / quality control / AS 1012.
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Last updated: 2026-05-16. Verified: 2026-05-16.