Concrete Field Testing and Slump Control

A truck backs in and the load looks stiff. The finisher wants water, the driver wants to get unloaded, and the technician has the cone out. What gets decided in the next ten minutes can follow that concrete for 50 years, and it can also start a dispute that costs far more than the load did.

You already know what slump measures and why extra water hurts. This guide is about the calls around the truck: what each test really tells you, how much water a load can take, and how to tell bad concrete from a bad test.

Acceptance tests must be run by a trained, certified person. Most specifications call for ACI Concrete Field Testing Technician Grade I or an equal program. Reading this does not qualify you to run them. It helps you protect the tests, read the results, and speak up when something is off. The job specification controls every limit below, and ASTM standards are revised over time, so check the edition your job cites.

What You Will Be Able to Do

• Explain what each field test tells you and what it cannot tell you.
• Spot a poor sample or a mishandled cylinder before it becomes a false failure.
• Work out how much water a load can take and record it properly.
• Read a strength report against the acceptance rules.
• Trace a low break to its most likely causes.

The Tests at the Truck

When cylinders are made for acceptance, the technician also tests temperature, slump, density and air on the same sample. Those numbers go on the report, and they are the first place you look if a break comes back low.

The pressure meter in ASTM C231 is used on normal stone. The rolling meter in ASTM C173 is used on lightweight aggregate, which holds air inside the stone and would fool a pressure meter.

Getting a True Sample

A sample has to stand for the whole load. If it does not, every number after it is wrong.

1. Sample from the middle of the load. The first and last 10 percent of the discharge are not representative.
2. Catch the whole stream. Swing the full chute stream into a wheelbarrow rather than scooping off the edge.
3. Take at least two portions. No more than 15 minutes can pass between the first and the last portion.
4. Get enough. The minimum is 1 cubic foot.
5. Protect it. Cover the sample against sun, wind and contamination, and move it to the test spot.
6. Remix it, then start. Slump, air and density tests begin within 5 minutes. Cylinders start within 15 minutes.

A quick slump from the first 1/4 cubic yard is allowed as a check on consistency, for example when you are deciding on water. It is not an acceptance test, and nobody should reject a load on it alone.

Warning: Wet concrete burns skin and eyes, and the burn may not hurt until hours later. Wear waterproof gloves, boots and eye protection when you handle samples, and wash off contact right away with clean water.

Reading Slump in the Field

When a job gives a target slump, ASTM C94 has allowed these tolerances. Some specifications set a maximum slump instead, so check which one you have.

A 4-inch target therefore accepts 3 to 5 inches. A 5-inch target accepts 3-1/2 to 6-1/2 inches.

Three things are worth knowing:

• High slump is not always water. A mix with a water reducer can show 7 inches with less water than a plain mix at 4 inches. Read the ticket before you judge.
• A shear is not a slump. If one side of the pile falls away or shears off, throw that test out and run another on a new part of the sample.
• Slump drops with time and heat. A load can leave the plant on target and arrive low after a long haul on a hot day. Find out how old it is before you decide what is wrong.

How Much Water Can This Load Take?

Water at the truck is allowed under ASTM C94 only inside limits. It is a controlled step, not a favor to the crew.

1. Know the limits before the pour. You need the maximum slump, the maximum water-cement ratio, and the name of the one person allowed to approve water. Settle this at the pre-pour meeting.
2. Check the ticket. It shows batch time and any water held back at the plant. That held-back water is part of the design.
3. Estimate the slump from the first part of the discharge.
4. Figure the water. A working rule is about 1 gallon per cubic yard for each 1 inch of slump you want to gain.
5. Add it to the whole load, so the volume is known. That means before more than a small part has come out. CIP 26 from the National Ready Mixed Concrete Association (NRMCA) puts the limit at 1/4 cubic yard.
6. Mix 30 revolutions at mixing speed.
7. Measure and record the gallons, the time and who approved it. Unrecorded water is the first thing an engineer will suspect.

Stop adding water once you reach the maximum slump or the maximum water-cement ratio. Water beyond the design needs the purchaser's approval, and the purchaser then takes responsibility for the result. Avoid adding water to concrete more than 1-1/2 hours old.

Worked example. An 8-yard load has 564 pounds of cement per yard and 265 pounds of water per yard as batched. The specification caps the water-cement ratio at 0.50. Slump is 2-1/2 inches and the crew wants 5.

1. Ratio as batched: 265 / 564 = 0.47.
2. Most water allowed: 0.50 x 564 = 282 pounds per yard.
3. Room left: 282 - 265 = 17 pounds per yard. Water weighs about 8.34 pounds per gallon, so that is about 2 gallons per yard.
4. For the whole truck: 2 x 8 = 16 gallons.
5. Slump gain from 2 gallons per yard: about 2 inches, so about 4-1/2 inches.

The crew wanted 2-1/2 inches of gain, which needs about 2-1/2 gallons per yard. That is 21 pounds and a ratio of 286 / 564 = 0.51. It breaks the limit. The answer is 4-1/2 inches, or a water reducer dosed as the plant directs. Admixtures can raise slump without changing the water-cement ratio, which is why they are the better fix.

Air and Temperature

Air. ASTM C94 has allowed a tolerance of plus or minus 1.5 percentage points on air-entrained concrete, so a 6 percent target accepts 4.5 to 7.5 percent. Low air means poor freeze-thaw protection on a driveway. High air costs strength. Heat can knock air down, so a load that tested fine in the morning can test low in the afternoon.

Air also matters indoors, in the other direction. NRMCA recommends no more than 3 percent air on a slab that gets a hard troweled finish, because entrained air raises the risk of delamination.

Temperature. Many specifications, following ACI 301 and ACI 305.1, cap concrete at 95°F at discharge. That limit can be changed or waived by the purchaser if the slump is fine without excess water. Cold weather minimums depend on the thickness of the section. Temperature is the quickest test to run, and it explains a lot of what the load is doing.

Cylinders: Where Good Concrete Gets a Bad Report

Most low breaks trace back to how the cylinders were made and handled. Almost every mistake lowers the result. Untrained workers must not make or handle them. Your job is to protect them.

The most damaging errors are poorly finished tops, initial curing over 80°F, frozen cylinders, extra days in the field, and damage in the truck. A cylinder left on the tailgate in July is not a test of your concrete. On a hot job, the curing box may need ice or refrigeration. On a cold one, it needs heat. Keep a minimum and maximum thermometer in the box.

Standard-cured and field-cured cylinders are different things. Standard-cured cylinders stay in controlled conditions and decide acceptance. Field-cured cylinders sit next to the structure and show what the concrete in place is doing, so they are used to decide when to strip forms or load a member. Do not mix them up.

A set is usually made at least once per 150 cubic yards, but your specification may ask for more.

Reading a Strength Report

One strength test is the average of two 6 x 12 cylinders or three 4 x 8 cylinders, broken at the same age, usually 28 days. Under ACI 318 and ACI 301, the concrete passes when both of these are true:

• The average of any 3 consecutive tests equals or beats the specified strength.
• No single test falls more than 500 psi below it. Over 5,000 psi specified, no single test falls below 90 percent of it.

A 7-day break is only an early look. It is not a reason to reject concrete that has a 28-day requirement.

Worked example. The specification calls for 4,000 psi.

Test 2 is below 4,000 psi, but only by 70 psi, and the first three average 4,137. That is acceptable. A single test below the specified strength is not a failure by itself. Test 4 fails both ways: it is 530 psi low, and the running average has dropped to 3,910.

Tracing a Low Break

Collect every report before you react, not only the failing one. Then work down this list.

If the concrete itself is in question, the engineer decides what happens next. Cores are a common step. Under ACI 318, the average of three cores must reach 85 percent of the specified strength, with no core under 75 percent. Do not drill, patch or load anything until you are told to.

Common Mistakes

Check Your Understanding

1. The first part of the discharge shows a 3-inch slump. The target is 5 inches. Is the load out of tolerance?

Not necessarily. That is a quick check, not an acceptance test. A 5-inch target has a tolerance of 1-1/2 inches, so the low limit is 3-1/2 inches. Check the ticket, and then decide on water within the limits.

2. A 10-yard load has room for 15 pounds of water per yard under the water-cement limit. How many gallons can go in?

15 / 8.34 = about 1.8 gallons per yard, or about 18 gallons for the truck. Record it, and mix 30 revolutions.

3. Half the truck is in the forms and the placing crew asks for water. What do you say?

No. Water goes into a full, known volume, before more than a small part has come out. Ask about an admixture instead.

4. On a 90°F day, a new laborer offers to set the cylinders on the tailgate out of the way. Why stop them?

They would sit in the sun above 80°F, which lowers the measured strength. They belong in a curing box that holds 60 to 80°F, on a level surface where nothing will bump them.

5. Spec is 3,500 psi. A single test comes back at 3,100 psi, and the last three average 3,620. Does it pass?

Yes. It is 400 psi low, which is inside the 500 psi limit, and the running average beats 3,500. Keep watching the next results.

Key Takeaways

• A bad sample ruins every test after it, so sample the middle of the load.
• Slump is about consistency. Check the ticket before you judge a high or low number.
• Water at the truck has limits: maximum slump, maximum ratio, one person approving, and every gallon recorded.
• Protect cylinders at 60 to 80°F. Most low breaks trace back to handling.
• Acceptance uses the average of 3 tests and a 500 psi floor on any single test.
• When strength is truly in doubt, the engineer decides what comes next.