Concrete slump test field guide

What the slump test measures, and what it doesn't

Slump is a measure of how fresh concrete flows and consolidates, what the trade calls consistency and workability. It is not a strength test. A 6 in slump and a 3 in slump can come off the same mix design and reach the same 28-day strength, because slump tells you how wet and mobile the concrete is on the day you place it, not how strong it cures.

The distinction matters because people reach for slump as a quality stamp and it is not one. What controls strength is the water to cement ratio, and slump only hints at water content when everything else in the mix is held constant. Add water to a stiff load to bring the slump up and you raise the w/c ratio and lose strength. Add a water reducer instead and you raise the slump with no strength penalty, because you changed the flow without changing the water. Same slump number, opposite outcome for the concrete.

What slump does tell you, reliably, is whether the load batched consistent with the design and whether it will place and finish the way the crew expects. A slump well off the ticket is a flag that something changed: water, aggregate moisture, admixture dose, or time on the drum. That is the real job of the test. It is a fast, cheap consistency check at the point of placement, and a trigger to look harder before the concrete goes in the forms.

How do you run an ASTM C143 slump test?

Run the slump test to ASTM C143. Dampen the cone and set it narrow end up on a rigid, non-absorbent, level base, then stand on the foot pieces to hold it down. Fill in three layers, each about one third of the cone by volume, not by height. Because the cone is wider at the bottom, the first layer comes up only to roughly 2-5/8 in and the second to about 6-1/8 in.

Rod each layer 25 times with the 5/8 in diameter, 24 in long tamping rod that has the rounded tip. Spread the strokes evenly across the cross section. Rod the bottom layer through its full depth. For the second and third layers, drive the rod just into the layer below, about 1 in. After rodding the top layer, strike it off flush with a screeding and rolling motion of the rod.

Now lift the cone straight up, no twist, in 5 plus or minus 2 seconds, and do not bump the concrete on the way. Invert the empty cone next to the slumped concrete, lay the rod across the top, and measure down to the displaced center of the original top surface, to the nearest 1/4 in. That is the slump.

The whole test, from the start of filling to the measurement, runs without interruption inside 2-1/2 minutes. Drag it out and the concrete stiffens in the cone and reads low. Work clean and work fast.

The cone, the rod, and the base

Three pieces of the setup decide whether your number means anything: the cone, the rod, and the base.

The cone is sheet metal, 8 in across the bottom, 4 in across the top, 12 in tall. Dampen it before every test. A dry cone pulls water out of the concrete skin and drags the lift, which reads as a lower slump than the load actually has. The base has the same trap. It has to be rigid, level, non-absorbent, and damp. A piece of plywood is the classic rookie base and it is wrong twice: it absorbs water and it flexes when you stand on it, and both corrupt the lift. Use a steel plate or a clean, damp, smooth slab.

The rod is not a piece of rebar. It is 5/8 in in diameter, 24 in long, with a hemispherical tip the same diameter as the rod. The rounded tip matters because a flat or cut end punches and tears the concrete instead of consolidating it. Twenty-five strokes is the count, every layer, no rounding to about 25.

Keep the gear clean between trucks. Set concrete on the cone wall changes the inside dimension and grabs the lift. Rinse it, wipe it, dampen it, and you remove a whole category of argument about why two techs got two numbers off the same truck.

True slump, shear slump, or collapse?

A slump test gives you one of three shapes, and only one of them is a valid measurement.

True slump is the concrete settling evenly, the mass keeping its form as it sinks. You measure to the displaced center and record it. That is the number you want.

Shear slump is when the top half slides off to one side along a diagonal plane, leaving a lopsided cone. It usually means a harsh, lean, or non-cohesive mix, and sometimes just a sloppy lift. ASTM C143 says if you get a shear slump, take another test on a fresh portion of the sample. If the second test also shears, the concrete lacks the plasticity and cohesion the test needs, and the slump result is not valid for that load. Note it. A persistent shear is telling you something about the mix, not the technician.

Collapse is the whole cone falling apart and spreading out, which happens on very wet or self-consolidating mixes. A collapsed or very high slump is past the useful range of the test. For those mixes the slump cone is the wrong tool and you move to slump flow under ASTM C1611. The practical range where the slump test behaves is roughly 1/2 in to about 9 in. Outside that, the number stops meaning what you think it means.

Field example: a 5 in target on a hot afternoon

Take a real load. The ticket calls a 5 in nominal slump on a 4000 psi mix, the truck shows up 40 minutes after batch, ambient is 88°F. You sample at the chute per ASTM C172, run the cone, and read 3-1/2 in.

Before anyone touches a hose, work the context. A 5 in nominal target with the C94 tolerance of plus or minus 1 in gives an acceptable band of 4 to 6 in, so at 3-1/2 in the load is below the band. But it is also 40 minutes out on a hot day, so some of that loss is slump loss, not a bad batch. Check the ticket for water held back at the plant. If the design allows it, a one-time water addition to bring the slump up is legal, as long as it does not push the w/c ratio over the mix maximum and you are inside the discharge time and revolution limits. Add the water, mix 30 drum revolutions at mixing speed, retest.

Say the retest comes back 5 in. Now you place. You record the original slump, the water added, the retest slump, the temperature, the elapsed time, and who approved it. The number that protects you later is not the 5 in. It is the documented path from 3-1/2 to 5.

What slump is acceptable?

Acceptable slump is whatever the project specification and mix design say it is, within the ASTM C94 tolerances. There is no universal good slump. A pump mix might be specified at 5 in, a bridge deck at 3 in, a column at 7 in with a superplasticizer. The target lives in the contract documents and the approved mix design.

ASTM C94 handles tolerance two different ways, and which one applies changes the math. When the spec gives a single nominal target, the tolerance is plus or minus and depends on the target. When the spec gives a maximum or not-to-exceed slump, the tolerance is one-sided, because there is no credit for being wetter than the cap. Know which one your spec wrote before you accept or reject.

Read the table the way the spec is written. If the documents say 4 in maximum, a 5 in load is out, full stop, even though 5 in would pass as a nominal 4 in target. The project spec controls, and a sharp inspector reads the slump line on the spec before the first truck, not after a dispute.

Does slump predict strength?

Slump does not predict strength, and treating it like it does is the most expensive misunderstanding in fresh concrete. Strength is governed by the water to cement ratio. The slump test never measures w/c ratio. It measures consistency, which only tracks water content when everything else in the mix is fixed.

Here is where it goes wrong on site. A load comes in stiff, someone hoses water into the drum to make it place easier, the slump jumps from 3 to 6 in, and the concrete goes in looking great. That extra water raised the w/c ratio, and the cylinders break low a month later, and now the structural engineer is in the conversation. The slump looked better and the concrete got worse.

The reverse is also true and worth carrying. A high-range water reducer raises slump sharply with no added water, so a 9 in superplasticized mix can be stronger than a 4 in conventional one. Same family of materials, opposite relationship between slump and strength. So when slump is high, the right question is not is this weak, it is did this get wet from water or from admixture. One hurts the concrete. The other was designed in. The cylinders, cast under ASTM C31 and broken under C39, are what actually answer the strength question. Slump just tells you to ask it.

Where and when to take the sample

Where and when you grab the sample changes the number, so ASTM C172 pins it down. For acceptance, sample at the point of placement when you can, because that is the concrete that goes in the forms. The chute at the truck is convenient and common, but on a hot day or a long pump line the concrete placement sees has lost slump the chute sample never showed.

C172 wants a composite sample for most tests. Take it from at least two portions of the discharge, from the middle of the load, after about the first 10 percent is out and before the last 10 percent. Skim the very first and the very last of the load. Catch the whole stream by passing the receptacle through it, do not scoop one side. Combine the portions, remix with a shovel, and keep the time between the first and last portion of the composite inside 15 minutes.

Then move. Start the slump, temperature, and air tests within 5 minutes of getting the final portion, and have the strength cylinders molded inside 15 minutes. Keep the sample out of the sun and wind while you work, because a sample drying on the plate reads stiff. The point of C172 is that two techs sampling the same truck the same way get the same concrete to test.

How often is a slump or strength test required?

How often you test for acceptance is set by the project specification, usually pointing at ACI 318 and ACI 301. The common floor under ACI 318 is one strength test, a set of cylinders, for each class of concrete placed each day, and not less than once for each 150 cubic yards placed, and not less than once for each 5000 square feet of slab or wall surface area, whichever gives the most tests. A strength test there is the average of at least two cylinders from the same sample.

Slump runs on a different and usually tighter schedule. Slump, temperature, and air are field acceptance tests, and many specs call for them every time strength cylinders are cast, plus any time the concrete looks off, plus on a per-truck basis when the inspector or spec requires it. On a tight structural pour it is common to slump every truck. On a long flatwork pour it might be every few loads once the mix proves consistent.

These are minimums and the spec can be stricter, so read it. The trouble crews get into is rarely testing too little overall, it is testing the wrong loads. Test the first truck of the day, the first after a mix change, anything that looks dry or wet, and anything that ran long getting to the site. Those are the loads where a number actually changes a decision.

Slump loss and the clock

Slump drops between the plant and the forms, every time, and the rate depends mostly on heat and time. The cement starts hydrating the moment it meets water, the aggregate keeps absorbing, and the mix stiffens on the drum. A load that batched at 6 in can show 4 in by the time it has driven across town and waited in line.

Heat speeds all of it up. On a 90°F afternoon you can lose 1 to 2 in of slump in 30 to 45 minutes, where the same mix in cool weather barely moves in that time. That is why the chute reading and the point-of-placement reading disagree on hot pours, and why sampling at the chute on a hot day flatters the concrete the crew actually has to place. On a hot pour, check slump at the back of the pump line or the point of placement, not at the truck.

The clock is the part people forget. ASTM C94 sets an outside limit of 90 minutes or 300 drum revolutions from the time water hits the cement to complete discharge, whichever comes first, unless the spec or conditions allow otherwise. That window exists because past it the slump loss and the strength picture both go uncertain. Watch the batch time on the ticket, not the time the truck showed up. A truck that sat 50 minutes at the plant before it rolled is already half spent when it arrives.

Adding water on site without wrecking the mix

Adding water on site is legal under ASTM C94, but only inside hard limits, and only if you write it down. The rule: water may be added one time, to bring the slump within the specified range, provided the addition does not push the water to cement ratio over the maximum in the mix design and does not exceed the maximum specified slump. After the water goes in, mix at least 30 revolutions of the drum at mixing speed before you discharge or retest.

The w/c limit is the one that matters and the one that gets ignored. The mix was designed to a maximum w/c ratio to hit a strength. The plant usually holds back some of the design water so the field can trim slump without breaking that ratio, and that withheld water is the only water you have to spend. Once you have added it, you are done. Hosing in more to make a stiff load place easy is not retempering, it is wrecking the mix, and it shows up in low cylinders.

No water gets added after more than about 1/4 cubic yard has discharged, and nothing happens past the 90 minute or 300 revolution window. Every addition gets measured and recorded on the ticket: how much, when, by whom. Water added and not recorded is the single most common reason a load with a strength problem can never be explained later. The undocumented gallon is the one that hurts you.

Should you reject the truck?

Not on a slump number alone. A reject decision uses the mix design, the spec tolerance, the elapsed time, the water-add history, and the companion tests together. A single number out of band is the start of the investigation, not the verdict.

Walk it in order. First, what does the spec say the slump is, nominal or maximum, and what is the C94 tolerance around it. Second, where and when was the sample taken, chute or point of placement. Third, how long has the load been out and what was the temperature, because some low slump is just loss you can legally correct. Fourth, has water already been added, and is there design water left to add. Fifth, if it is a shear slump, did you retest. Only after all of that do you decide.

When you do reject, reject for cause and write the cause down. Slump 7 in against a 4 in maximum, no water added, sampled at point of placement, 35 minutes out, load returned, is a defensible record. Looked wet, sent it back, is not. And know the cost of what you are deciding. A wrongly rejected load is real money and a hole in the pour schedule. A wrongly accepted load can be a structural defect you cannot see. Be right, and be able to show why.

Why is my slump too low or too high?

When slump comes back off target, the cause is usually on a short list. Run it before you touch the load.

Low slump, most often: the load sat too long or got hot and lost slump on the drum. Less water was batched than the design called for, or the aggregate came in drier than assumed and drank more. The admixture was under-dosed. A stiff load that is just time and heat can be brought back with the design water you have left. A load that batched dry is a plant problem.

High slump, most often: water was added at the plant or on site beyond the design, the aggregate came in wet and carried free water nobody accounted for, or the admixture was over-dosed. High slump from a water reducer is fine. High slump from extra water is a strength problem hiding behind a good-looking number. The test that separates them is unit weight under ASTM C138. A load watered down reads lighter than the design unit weight, because water is lighter than the cement and aggregate it displaced.

The mistake is reacting to the number without finding the why. Two loads can read 7 in for opposite reasons, one harmless and one a defect. The ticket, the batch time, the temperature, and the unit weight are how you tell them apart before the concrete is in the forms and the decision is gone.

The companion fresh tests and when each triggers

Slump never travels alone. It is one of a set of fresh tests, and each answers a question slump cannot. Run the ones the spec calls for, and run the extra ones when slump or the load looks wrong.

Air content is the big one on anything exposed to freeze-thaw or deicers. Entrained air protects the paste from freezing damage, and you verify it with the pressure method, ASTM C231, on normal-weight concrete, or the volumetric method, ASTM C173, on lightweight or porous aggregate where the pressure method misreads. Concrete temperature, ASTM C1064, is quick and tells you whether you are inside the spec hot or cold weather limits, which is also context for slump loss. Unit weight and yield, ASTM C138, catches a load that was watered down or has the wrong air, and confirms you are getting the cubic yards you paid for. And the strength cylinders, cast and cured under ASTM C31, are the only test on the list that answers the strength question slump cannot.

The order on a typical acceptance set: temperature and slump first because they change fastest, then air, then cast the cylinders, all inside the C172 timing. A tech who only runs slump is running a quarter of the test.

Slump or slump flow? Pump mixes, slabs, and SCC

Slump is the wrong test once the concrete is too fluid to hold a cone. Self-consolidating concrete, SCC, flows and levels under its own weight with no vibration, and dropped in a slump cone it just collapses and spreads. The number from a collapse is meaningless. For SCC you measure slump flow under ASTM C1611. You lift the same cone and measure the diameter of the spread instead of the drop in height, usually two diameters at right angles, averaged. Typical SCC spreads land roughly in the 18 to 30 in range, with the target set by the mix and the placement.

Conventional pump mixes are a different case. A pumpable mix usually sits in the 4 to 6 in slump range and the standard slump test is right for it. Higher pumped slumps come from water reducers, not water, and the slump test still works as long as the concrete holds a cone shape and does not collapse.

Slab on grade is conventional too, usually a moderate slump that finishes well, and the slump test fits. The line to remember: if the mix is designed to flow and self-level, it is an SCC and you measure spread with C1611. If it holds a cone, it is a slump test under C143. Running a slump cone on SCC and reporting collapse as a failure is a tourist move. The mix is doing what it was designed to do, and you brought the wrong test.

Common mistakes

• Wetting nothing or using an absorbent base. A dry cone or a plywood base pulls water from the skin and reads the slump low.
• Rodding the wrong count, or filling by height instead of filling each layer to one third the volume.
• Dumping the concrete into the cone in one go instead of three rodded layers.
• Twisting or jerking the cone on the lift, or taking longer than 5 plus or minus 2 seconds.
• Letting the test drag past 2-1/2 minutes so the concrete stiffens in the cone and reads low.
• Measuring to the high point of the slumped concrete instead of to the displaced center.
• Accepting a shear slump as a real number instead of retesting on a fresh portion.
• Adding water on site without recording how much, when, and by whom.
• Sampling at the chute on a hot pour and calling it the point-of-placement slump.

Field checklist

What to document

The slump reading is only worth what you wrote down when you ran it. When a pour comes into question, the record is what separates a load that was handled right from one that has to be assumed bad. Write it at the truck, not from memory after the trucks are gone.

Capture the ticket and mix ID, the batch time and the sample time, where you sampled, the measured slump and whether it was a true slump or a retest off a shear, the concrete temperature, the air if tested, the unit weight if tested, any water added with the amount and time, the retest slump, the spec target and tolerance you judged against, the accept or reject decision, who approved it, and photos if the load is disputed.

Standards and references

ASTM C143 is the test method for slump itself: the cone, the three layers, 25 strokes, the lift, and the measurement. ASTM C172 governs how you sample the fresh concrete so the test sees representative material. ASTM C94 is the ready-mixed concrete specification, and it carries the slump tolerances, the jobsite water-addition rules, and the 90 minute or 300 revolution discharge limit. Those three cover most of what a slump decision rests on.

The companion fresh tests each have their own method: ASTM C231 and C173 for air content by the pressure and volumetric methods, ASTM C1064 for temperature, ASTM C138 for unit weight and yield, ASTM C31 for casting and curing field strength specimens, and ASTM C39 for breaking them. ASTM C1611 is the slump flow method for self-consolidating concrete, the test that replaces the slump cone once the mix is designed to flow.

Acceptance and frequency live on the design side. ACI 318, the structural concrete code, and ACI 301, the specifications for structural concrete, set how often you test and how strength results are judged for acceptance. ASTM and ACI provisions change between editions, so confirm the method year and the code edition the project actually adopted, and let the project specification control where it is stricter than the floor these documents set.

Units, terms, and conversions

Slump is read in inches in the US and millimeters most other places, and the two show up side by side on international jobs and imported equipment. Slump is recorded to the nearest 1/4 in, which is about 5 mm. The cone is 12 in, or 305 mm, tall.

Concrete volume is ordered and recorded in cubic yards on US jobs and cubic meters elsewhere, where 1 cubic yard is about 0.76 cubic meters. The water to cement ratio, the number that actually drives strength, is a dimensionless mass ratio and reads the same in any unit system. Keep the units straight between the ticket, the spec, and the mix design, because a target written in millimeters and judged in inches is a dispute waiting to happen.

Slump

The drop in height of fresh concrete after the cone is lifted, a measure of consistency and workability

Consistency / workability

How readily fresh concrete flows, places, and consolidates without segregating

Water to cement ratio (w/c)

Mass of water divided by mass of cementitious material, the main driver of strength

Retempering

A one-time, recorded jobsite water addition allowed under ASTM C94 within the design w/c limit

Shear slump

A lopsided slump where the top slides off to one side; retest on a fresh portion

Slump flow

The spread diameter of self-consolidating concrete, measured per ASTM C1611 instead of slump

Nominal max aggregate

Largest standard sieve size; coarse aggregate over 1-1/2 in is wet-sieved out before a slump test

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• How much concrete should I order?
• What is a concrete admixture?
• What is the difference between a mechanical and an adhesive anchor?
• What is the difference between integral color and stain?
• Why does concrete crack?