The concrete slump test — formally ASTM C143 — is the most common field test for measuring the workability and consistency of fresh concrete. It tells you how fluid the mix is at the point of delivery, which directly affects how easily your crew can place and finish it.
Slump is reported in inches. A higher slump means more fluid concrete; a lower slump means stiffer concrete. The test is fast, requires minimal equipment, and gives you an immediate read on whether the load matches your specification.
The test uses a 12-inch tall steel cone (the Abrams cone), open at both ends, and a tamping rod. Place the cone on a flat, non-absorbent surface. Fill it in three equal layers, rodding each layer 25 times with the tamping rod. Strike off the top, then carefully lift the cone straight up. Measure the distance from the top of the cone to the top of the settled concrete. That distance — in inches — is the slump.
The entire test takes under two minutes. Run it on the first truck and at regular intervals throughout the pour. If slump drops significantly between loads, it signals a change in mix consistency, water content, or conditions.
Standard flatwork — sidewalks, driveways, warehouse floors. Enough workability to screed and finish without excessive bleed water.
Lower slump for mass concrete that does not need a fine finish. Stiffer mix reduces bleed and segregation in deeper placements.
Higher slump required for pumpability. The mix needs to flow through the pump line without segregating. Superplasticizers are often used to achieve high slump without adding water.
Needs to flow around reinforcement and consolidate with vibration. Too low and you get honeycombing; too high and you risk segregation.
Most specifications allow ±1 inch from the target slump. If a load arrives outside that tolerance, reject it or work with your ready-mix supplier to adjust before placement.
Temperature has a direct impact on slump. Concrete batched at 5 inches of slump at the plant can arrive at the job site at 3.5 inches on a hot day, especially with a long haul. Higher temperatures accelerate hydration and evaporate mix water during transit, stiffening the concrete before it reaches your forms.
This is why hot weather pours often require a retarder in the mix or a higher target slump at the plant to account for slump loss during delivery. It is also why haul time matters more in summer — every additional minute in the drum on a 95°F day costs you workability.
Here is the point that many crews miss: slump tells you about the concrete in the truck. Evaporation rate tells you about the conditions on the slab. You can have perfect slump — 5 inches, right on spec — and still get plastic shrinkage cracks if the evaporation rate is above 0.15 lb/ft²/hr.
Slump and evaporation rate measure different things. Slump measures workability at the point of delivery. Evaporation rate measures how fast the environment is pulling moisture off your slab after placement. Both need to be within acceptable ranges for a successful pour. Checking one without the other leaves you exposed.
Every slump test result should be recorded in your pour log — along with the truck number, time of arrival, and ambient conditions. You can check conditions ahead of time with our free evaporation rate calculator. PourDay's pour logging feature lets you capture this data alongside real-time evaporation rate and weather conditions, building a complete record of every pour. When a question comes up about a slab six months later, your log shows exactly what was delivered, what conditions were present, and what decisions were made.
Slump tells you about the truck. PourDay tells you about the slab. Free on iOS and Android.
The number slump cannot tell you — how fast moisture is leaving your slab after placement.
Read articleHow to keep slump where you need it — when a retarder or accelerator makes the difference.
Read articleWhat happens when slump is fine but evaporation rate is not — the gap that catches crews off guard.
Read articleReal-time evaporation rate. 16-day forecast. Pour logging. Free for contractors.