Concrete Slump Requirements
Reviewed by a working tradesman with 17+ years in the field.
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Slump tells you how stiff or fluid the concrete is. It is set when you order, checked when the truck arrives, and never changed with a hose.
Tap your level and the boxes below open to the advice for you. Remembered on this device.
By the end you can
- Say what slump measures and what it does not.
- Watch a slump test and read the result in mm and inches.
- Work out the accepted range from the ordered slump and tolerance.
- Fix a stiff load the right way, through the supplier.
Do not add water, sand or cement to a load to change its slump. Extra water makes concrete easier to place and permanently weaker, and it raises the water-to-cement ratio that the mix was designed around. If the slump is wrong, hold the load and call the supplier.
Wet concrete can burn skin, so wear gloves, long sleeves, tall waterproof boots and eye protection when you handle test samples too.
The big idea
Slump tells you how the concrete will flow, pump and finish. It does not tell you its strength. The slump is decided when the concrete is ordered, from the placing method and the work. It is not something the crew chooses at the truck.
Concrete in Canada is ordered and tested to CSA A23.1 and A23.2, and the slump test is CSA A23.2-5C. A slump test is a quick check that the load matches the order.
The slump test
The test follows CSA A23.2-5C. A Canadian testing lab describes the steps this way:
- Set up. Put the Abrams cone (300 mm / 12 in tall, 200 mm / 8 in across the base, 100 mm / 4 in across the top) on a flat, rigid, non-absorbent plate.
- Fill in three layers. Rod each layer 25 times with a 16 mm (5/8 in) rod. On the second and third layers the rod goes about 25 mm (1 in) into the layer below. Overfill the last layer.
- Strike off. Level the top flush with the cone.
- Lift straight up. No twisting or sideways motion. The lab guide gives 5 to 12 seconds for the lift and about 2 1/2 minutes for the whole test. The standard sets the exact times.
- Measure. Lay the rod across the cone and measure down to the top of the slumped concrete. Record it to the nearest 5 mm (1/4 in).
The concrete should slump and keep roughly its shape. If one side shears off or the whole mound collapses, take a fresh sample and test again.
The step values above come from a Canadian testing-lab guide that describes CSA A23.2-5C. I could not read the standard itself. A general source gives the same cone and rodding steps. The standard governs, so a trained tester should run the test.
Reading slump
- Low slump is a stiff mix that holds its shape and needs vibration.
- High slump is a fluid mix that flows easily but is more likely to segregate and bleed.
Slump is read in mm. Conversions: 25 mm is about 1 in, 50 mm about 2 in, 75 mm about 3 in, 100 mm about 4 in, 150 mm about 6 in.
| Description | Slump | Source |
|---|---|---|
| Stiff | under 75 mm (3 in) | Canadian testing lab guide |
| Workable | 75 to 150 mm (3 to 6 in) | Canadian testing lab guide |
| Fluid | over 150 mm (6 in) | Canadian testing lab guide |
| General construction: slabs, beams, footings | about 50 to 100 mm (2 to 4 in) | Canadian testing lab guide |
These are common bands, not rules. Your drawings, your supplier and the placing method decide the number you order. Self-consolidating concrete is measured by spread, not slump, and is ordered specially.
A slump test is not suited to concrete that is too fluid. Aggregate larger than 38 mm (1 1/2 in) also falls outside the test.
Tolerance: what reading is acceptable
A load is never exactly on the ordered number, so CSA A23.1 allows a tolerance. A Canadian testing lab summarizes it this way:
| Ordered slump | Tolerance |
|---|---|
| under 80 mm (3 1/8 in) | ± 20 mm (3/4 in) |
| 80 to 180 mm (3 1/8 to 7 1/8 in) | ± 30 mm (1 3/16 in) |
| over 180 mm (7 1/8 in) | ± 40 mm (1 9/16 in) |
Examples. Order 70 mm (2 3/4 in): acceptable is 50 to 90 mm (2 to 3 9/16 in). Order 80 mm (3 1/8 in): acceptable is 50 to 110 mm (2 to 4 5/16 in).
I could not read CSA A23.1 itself, so these tolerances come from a secondary source. Write the slump and the tolerance on the order so the crew and the testing agency read the same numbers, and confirm them with your supplier.
Adjusting slump the right way
- Ask the supplier. Adjustments are made by or with the supplier, recorded on the ticket, and the load is retested. A municipal specification I read says slump and air must be measured before and after a water-reducing admixture is added.
- Water at the truck is where jobs go wrong. Some specifications forbid it, some allow it only with the supplier's written permission. A US industry guide (NRMCA CIP 26) says it may only be done within the specified slump and water ratio, with the amount recorded on the ticket. Treat it as the supplier's call, never the crew's.
- How much water is a lot? The same US guide gives a rule of thumb of about 5 L per m³ for each 25 mm (1 in) of slump. Example: 8 m³ × 5 L = 40 L (about 10 US gal) for one inch. Water raises slump but lowers strength and raises cracking risk.
- If a load is too wet, reject it or ask the supplier for direction. Do not stiffen it with sand or cement.
- A reading higher than ordered often means water was added somewhere. Look into it before you accept the load.
How to choose a slump
- Formed work: enough flow to fill the form and get around the steel, with vibration, but not so wet it bleeds or pushes hard on the form.
- Flatwork: enough to place and screed easily, not so wet it bleeds for hours.
- Pumping: enough to move through the line, which may mean an admixture from the supplier, not extra water.
Tell the supplier how you will place the concrete and let them recommend the number. The engineer's number wins on engineered work.
Common mistakes
- Adding water to make it "easier"
- Ordering a higher slump than the work needs because it is easier
- Not vibrating low-slump mixes
- Over-vibrating wet mixes so they segregate
- Pumping stiff concrete through a long line
- Finishing concrete that is too wet
If you cannot work the concrete as ordered, the fix is a better placing plan or an admixture from the supplier, not a hose. Every litre of site water takes strength and durability out of the pour.
Hot weather makes slump drop quickly after batching, which tempts crews to add water. Plan for it with a retarder or a closer delivery schedule. Cold slows the set, so the concrete may stay workable longer than you expect.
The ticket, the slump reading against the ordered value and tolerance, and any admixture recorded on the ticket. On a municipal job I read, a slump test was required at least once per 50 m³ placed and at least once each day of placement. Your project may differ.
You ordered 80 mm (3 1/8 in) slump. The tolerance for 80 to 180 mm is ±30 mm. What readings are acceptable?
You ordered 70 mm (2 3/4 in) slump. What is the acceptable range?
What does slump measure, and what does it not?
A load tests too stiff. What is the right fix?
A tester says a rule of thumb is about 5 L of water per m³ per 25 mm of slump. How much water would lift an 8 m³ load by 25 mm (1 in)?
Why does hot weather tempt crews to add water?
- Nice to haveTesting: a slump cone with foot pieces, a 16 mm (5/8 in) tamping rod about 610 mm (24 in) long, a flat rigid base plate, and a ruler or tape in mm (the lab or supplier usually runs the test)
- Must-haveRecording:
- Rent itPlacing by machine: a pump or chute and a hose
- Must-havePlacing by hand: rakes and shovels set up for the ordered slump
- Rent itConsolidating: an internal vibrator, because stiff mixes need it
- Must-havePPE:
Home DIY
Laborer or helper
1st-year apprentice
Apprentice, 2nd to 4th year
Journeyperson
Lead hand or foreman
Contractor or business owner
Starting a new business
The technique is the same across Canada. Concrete is ordered and tested to CSA A23.1 and A23.2 everywhere. What changes is the code edition and who tests.
- BC: BC Building Code 2024 (based on NBC 2020).
- AB: National Building Code 2023 Alberta Edition, in force 1 May 2024, based on NBC 2020.
- ON: 2024 Ontario Building Code (O. Reg. 163/24), NBC 2020 plus the Ontario amendments.
- QC: Quebec Construction Code, Chapter I Building, with NBC 2020 (amended), in force 17 April 2025.
- Other provinces and territories: confirm the code edition and local testing rules with the local authority.
The code editions above are carried over from earlier research and were not re-checked this pass. See Canada by Province for the full comparison.
- Hoskin Scientific, Concrete slump testing in Canada (a secondary source: CSA A23.2-5C steps and CSA A23.1 tolerances as summarized)
- Wikipedia, Concrete slump test (cone, rod and slump types; secondary)
- NRMCA CIP 26, Jobsite addition of water (US; rule of thumb for scale only)
- City of Winnipeg concrete specification (a municipal spec, not the standard: water-reducer and test frequency items)
- CSA A23.1 and A23.2 are paid standards and were not read directly.
- Tolerance ranges, conversions and the water example are calculated and re-checked.
Last reviewed October 2026. General information, not engineering or legal advice.