Before a single cylinder is broken, the field technician has already gathered most of the evidence anyone will use to explain the result. Slump, air content, temperature and unit weight describe the concrete that arrived on that truck at that time. When a break comes in low weeks later, the first thing a reviewer reaches for is the field sheet from the day it was cast. This guide walks through the standard tests on fresh concrete, what each one tells you, and the habits that make the record useful.
Why does every fresh test start with the sample?
Every fresh concrete test depends on a representative sample. ASTM C172 describes how to sample from a truck mixer or other sources, how to combine portions into a composite sample, and how to protect it before testing. A sample taken carelessly, such as the first concrete out of the chute, can make every later test unrepresentative.
Record the truck number, ticket number, batch time, sample time and any water added on site. Water added after batching is one of the most useful facts for explaining both slump and strength.
What is the slump test (ASTM C143)?
The slump test measures consistency. The cone is 12 in tall, 8 in across the bottom and 4 in across the top. It is filled in 3 layers, each rodded 25 times, struck off, and lifted straight up. The slump is how far the concrete settles, measured to the nearest 1/4 in.
Slump is sensitive to timing and technique. Lift the cone steadily, without twisting, and measure promptly. If the concrete shears off to one side, it is not a valid slump; follow the standard on what to do next. Compare the result with the range the project specifies.
How is air content measured (ASTM C231 or C173)?
Entrained air protects concrete exposed to freezing and thawing, and too much air costs strength. The pressure method, ASTM C231, is common for normal-weight concrete. The volumetric method, ASTM C173, is used where the pressure method is not suitable, such as with lightweight or porous aggregates. Record which method you used, because the result is only meaningful alongside it.
Why is concrete temperature recorded (ASTM C1064)?
Concrete temperature affects setting time, workability and early strength gain. ASTM C1064 covers measuring it with a suitable thermometer embedded in the fresh concrete. Record ambient temperature and weather too. Together with the cylinder curing record, they help explain early-age results.
What does unit weight tell you (ASTM C138)?
ASTM C138 covers the density (unit weight) of fresh concrete, measured by filling a container of known volume and weighing it. The standard also uses it to calculate yield and air content by gravimetric means. A unit weight that is out of line with the mix design is a useful signal that something about the batch differs from what was ordered.
How do the fresh test results fit together?
Each fresh test is more useful alongside the others than on its own. A slump at the high end of the range plus a note that water was added on site suggests a higher water-cement ratio than the mix design intended, which may show up later as lower strength. A unit weight lower than expected with an air content near the top of the range points the same way, since entrained air reduces both density and strength. A high concrete temperature on a hot afternoon can explain a stiffer mix and faster slump loss.
None of these is a verdict on its own. They are context. A lab that records all of them, consistently and with times, gives its reviewer and the engineer of record a far better chance of explaining an unusual break than a lab that records only the cylinders.
It also helps when a contractor asks about rejected concrete. A clear record of what was measured, when, and against which specified range is much easier to discuss than a recollection from the field.
In what order are fresh concrete tests run on site?
Take the composite sample. Start the tests promptly, as the standards describe, and mold cylinders under ASTM C31 from the same sample. Record every result with time, location and placement, and compare each with the project specification while the truck is still there, so out-of-range concrete can be raised with the contractor at the time rather than discovered at the break.
What formulas are used?
- 01Slump cone: 12 in tall, 8 in bottom, 4 in top; 3 layers, 25 rods each; read to the nearest 1/4 in
- 02Strength = max load ÷ (π × D² ÷ 4), D = average of two diameters, nearest 10 psi
What does a worked example look like?
Inputs
A 4 × 8 in cylinder molded from the sampled truck and broken at the acceptance age: D1 4.00 in, D2 4.00 in, length 8.00 in, maximum load 50,000 lbf.
Result
Area 12.57 in², L/D 2.00 (no correction), strength 3,980 psi. If this were low against the specification, the field sheet for the same sample (slump, air, temperature, water added, curing temperatures) is where the explanation would start.
Calculated by the same tested functions as the ProofGrit free calculators.
What should happen when a fresh test is out of range?
The tester's job is to measure accurately and report promptly. Record the value as measured, tell the person the project names for acceptance decisions while the truck is still there, and note who was told and when. Acceptance or rejection is their decision.
If cylinders are molded from that load anyway, tie them to the out-of-range result on the field sheet. Weeks later, when the breaks come in, that link is what lets a reviewer connect a strength result to what happened at the truck.
What are the most common fresh concrete testing mistakes?
Testing the first concrete out of the chute.
Not recording water added on site.
An air result with no method noted.
Slump measured long after sampling.
Field sheets that do not tie to the cylinder identifiers, so a low break cannot be traced back to its truck.
Which standards cover fresh concrete testing?
ASTM C172 (sampling), C143 (slump), C231 and C173 (air content), C1064 (temperature), C138 (density, yield and gravimetric air) and C31 (cylinders). Self-consolidating concrete uses ASTM C1611 for slump flow. Each has its own page on the ASTM site.
See the standards index for every standard covered on this site.
| Test | Standard | What it tells you |
|---|---|---|
| Sampling | ASTM C172 | A representative sample for every other test |
| Slump | ASTM C143 | Consistency and workability |
| Air content, pressure | ASTM C231 | Air in normal-weight concrete |
| Air content, volumetric | ASTM C173 | Air with lightweight or porous aggregate |
| Temperature | ASTM C1064 | Concrete temperature at placement |
| Density, yield | ASTM C138 | Unit weight, yield and gravimetric air |
Frequently asked questions
Which fresh concrete tests are required?
That depends on the project specification. Slump, air content, temperature and cylinders are common; unit weight is often required too.
Can water be added on site?
That is a project and supplier decision, not a testing one. The tester's job is to record it accurately, with the amount and time.
Pressure or volumetric air?
The pressure method (C231) is common for normal-weight concrete; the volumetric method (C173) suits lightweight or porous aggregates. Record the method with the result.
Why record ambient temperature as well as concrete temperature?
Ambient conditions affect setting and early curing, and they help explain early-age strength results.
What if a fresh test result is outside the specified range?
Record it as measured and tell the person the project names for that decision while the truck is on site. Note who was told and when. Acceptance is their call; the record is yours.
How ProofGrit helps
The ProofGrit free tools focus on the calculations that follow the fresh tests: the concrete compressive strength calculator for the breaks and the ACI 318 acceptance calculator for the acceptance series. They run in the browser and save nothing, so keep your field sheets as your record.
Related guides and tools
- ASTM C31: making and curing concrete cylinders in the fieldA walk-through of making and curing field cylinders, the initial curing temperature ranges, and how handling errors show up in acceptance results.
- ASTM C39: compressive strength of concrete cylindersHow a cylinder break works, how strength is calculated from two diameters and the maximum load, when the L/D correction applies, and what to check.
- CMT lab accreditation: AASHTO R18, ISO/IEC 17025 and recordsWhat AASHTO re:source accreditation, AASHTO R18 and ISO/IEC 17025 mean for a small CMT lab, and the records your lab and its software need to keep.
- Construction materials testing: a practical lab guideA practical overview of construction materials testing: how field observations, laboratory methods, calculations and reviewed reports fit together.
- Concrete strength calculatorCylinder strength from two diameters and the maximum load, with L/D correction and the set average.
- ACI 318 acceptance calculatorCheck strength tests against both ACI 318 acceptance criteria: running average of three and the single-test limit.
- Standards indexEvery ASTM, AASHTO and ACI standard explained on this site.
- All concrete testing guidesBrowse the full guide library by topic.
Sources and further reading
- ASTM C172/C172M, sampling freshly mixed concreteASTM International
- ASTM C143/C143M, slump of hydraulic-cement concreteASTM International
- ASTM C231/C231M, air content by the pressure methodASTM International
- ASTM C173/C173M, air content by the volumetric methodASTM International
- ASTM C1064/C1064M, temperature of freshly mixed concreteASTM International
- ASTM C138/C138M, density (unit weight), yield and gravimetric air contentASTM International
Standards are summarized in our own words. Read the current edition from the publisher before you test.
This guide is a plain-English summary. Always follow the current standard and your project specifications.