22 Sampling Fresh Concrete (ASTM C172) Practice Questions & Answers

Every Sampling Fresh Concrete (ASTM C172) practice question from the ACI Concrete Field Testing Technician Grade I Practice Test, with the correct answer and a short explanation.

Start practice test →

1. A project engineer asks the field technician to use ASTM C172/C172M to find out whether the concrete at the start of a truck's discharge differs from the concrete near the end of the same load. How should the technician respond?
- A.Use C172 but test each portion separately before remixing, since remixing them into a composite would hide any differences
- B.Explain that within-batch uniformity checks fall under ASTM C94, since C172 builds one composite for the whole batch✓ Answer
- C.Use the C172 paving-mixer pile method on the truck load, since sampling several parts of the pile reveals variation in a batch
- D.Use C172 with one portion from the very first and one from the very last concrete, since the composite then captures both ends
ASTM C172/C172M is a practice for obtaining a representative composite sample of the concrete as delivered; it expressly does not cover sampling to judge uniformity within a batch. Mixer-uniformity checks compare separate samples taken at defined points of discharge under the uniformity provisions of ASTM C94/C94M, so the request belongs to that procedure.
Ref QB-M3RY75Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (scope: excludes within-batch uniformity sampling); ASTM C94/C94M, Standard Specification for Ready-Mixed Concrete (mixer uniformity)Report a problem with this question
2. To save time, a technician fills one bucket with the first concrete out of the chute and uses it for slump and for molding cylinders. The cylinders later break low. What is the most defensible conclusion?
- A.The low breaks are valid for the first part of the load, so only that part of the placement needs to be removed
- B.The low breaks are not reliable evidence about the batch, because the sample did not represent the delivered concrete✓ Answer
- C.The low breaks prove the delivered concrete is deficient, because the cylinders were molded and cured by the standard
- D.The low breaks reflect only a curing problem, because the sampling location has no effect on measured strength
A test can be no more representative than the sample it is made on. C172/C172M requires portions from the middle of the discharge, combined and remixed into a composite, precisely because the very first concrete out of a mixer can differ from the rest; a single first-out bucket violates the practice, so neither acceptance nor rejection of the batch can rest on it.
Ref QB-4KJTM4Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (significance and use: representative composite sample)Report a problem with this question
3. Assume the practice allows no more than 15 min between taking the first and the final portions of a composite sample. A technician takes the first portion at 10:00, the truck then stops to reposition, and the final portion is taken at 10:19. What should the technician do?
- A.Run every test on the combined portions and write the 19-minute sampling interval on the report
- B.Use the portions for slump and air, then take one fresh portion from the truck just for cylinders
- C.Discard these portions and take a new composite sample from the batch within the allowed window✓ Answer
- D.Add one more portion from late in the discharge so the composite is balanced across the delay
The interval between the first and final portions is part of what makes the composite representative of one moment in the batch's life. Once that window is exceeded the composite no longer meets the practice, and recording the deviation or adding portions does not cure it; the technician should discard it and resample from the batch within the limit.
Ref QB-K3DN24Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (time limit for obtaining the composite sample)Report a problem with this question
4. Assume the practice requires slump, temperature and air tests to start within 5 min after the final portion is obtained, and molding of strength specimens to start within 15 min after the composite is made. The final portion is taken at 2:03 and the composite is remixed by 2:05. What are the latest permitted start times?
- A.Slump, temperature and air by 2:20; molding cylinders by 2:20
- B.Slump, temperature and air by 2:10; molding cylinders by 2:20
- C.Slump, temperature and air by 2:08; molding cylinders by 2:20✓ Answer
- D.Slump, temperature and air by 2:18; molding cylinders by 2:08
Each limit runs from its own reference point. The stated 5 min for slump, temperature and air counts from the final portion (2:03 + 5 = 2:08), while the stated 15 min for molding counts from completion of the composite (2:05 + 15 = 2:20). Swapping the windows, or measuring both from the same moment, gives the wrong deadlines.
Ref QB-7KMT58Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (time limits for starting slump, temperature, air content and molding)Report a problem with this question
5. Why does ASTM C172/C172M set deadlines for starting the slump, temperature and air content tests and for starting to mold strength specimens?
- A.Sampled concrete must rest briefly so entrapped air escapes, so the deadlines mark when testing may finally begin
- B.Sampled concrete keeps stiffening, losing air and changing temperature, so late tests misstate the delivered concrete✓ Answer
- C.Sampled concrete bleeds after sampling, so the deadlines make sure enough bleed water has risen before testing
- D.Sampled concrete must be tested before the truck leaves, so the deadlines exist mainly to settle delivery disputes
Fresh concrete changes from the moment it is sampled: hydration and evaporation reduce slump, handling and time reduce air content, and the sample warms or cools toward ambient. The start deadlines keep the tests and specimens reflecting the concrete as delivered rather than an aged sample; they are maximum delays, not waiting periods.
Ref QB-P3FNJ7Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (time limits for starting tests and molding specimens)Report a problem with this question
6. Which sequence correctly describes how a composite sample is formed from a revolving-drum truck mixer?
- A.Take portions from the middle of the discharge, carry them to the test site, then combine and remix them with a shovel✓ Answer
- B.Take portions from the middle of the discharge, run slump on each one, then combine only the portions that pass the test
- C.Take portions from the middle of the discharge, combine them at the truck, then remix them until the tests are started
- D.Take portions from the middle of the discharge, remix each one in its own receptacle, then test each portion separately
C172/C172M has the individual portions transported to the place of testing or molding, where they are combined and remixed with a shovel just enough to make the composite uniform. Remixing longer than needed changes the concrete, and testing or screening portions separately defeats the purpose of a single representative composite.
Ref QB-6K9GY6Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (transporting, combining and remixing the composite sample)Report a problem with this question
7. A ready-mix driver plans to add water at the site after the superintendent eyes the first concrete in the chute. When should the technician sample this load for acceptance testing?
- A.Before any jobsite water is added, so the tests reflect the concrete exactly as it was batched at the plant
- B.From the first concrete in the chute, so the tests match what the superintendent judged before adding water
- C.Right after the water goes into the drum, so the effect of the added water is captured before it disperses
- D.After all water and admixtures are added and the drum has mixed the load until it is uniform again✓ Answer
The acceptance sample must represent the concrete actually placed. C172/C172M therefore requires that all water, including any jobsite addition, and any admixtures be in the load and mixed to uniformity before portions are taken; sampling earlier, from the first concrete, or before the addition is mixed in represents concrete that is not the concrete being placed.
Ref QB-N34BD6Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (sampling from revolving-drum truck mixers: after all water and admixtures are added)Report a problem with this question
8. Concrete is pouring from a truck chute so fast that the technician cannot pass the wheelbarrow through the stream without spilling. How should the discharge rate be controlled for sampling?
- A.Have the driver partly close the discharge gate so that only a narrow stream of concrete reaches the chute
- B.Have the driver stop discharging so that portions can be scooped from the concrete resting in the chute
- C.Have the driver slow the drum's rotation so that the stream eases while the gate opening stays unchanged✓ Answer
- D.Have the driver lower the chute toward the forms so that the stream drops a shorter distance to the receptacle
For truck mixers and agitators, C172/C172M regulates the rate of discharge by the rate of drum revolution, not by the size of the gate opening. Choking the gate or sampling concrete that has stopped in the chute promotes segregation, so the sample would not represent the batch.
Ref QB-ZJGT4ZSource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (truck mixers: discharge rate regulated by drum revolution, not gate opening)Report a problem with this question
9. While sampling from a truck mixer, how should the technician use the sampling receptacle in the discharge stream?
- A.Hold it at the edge of the stream, where the flow is slower and the concrete is less likely to spill
- B.Wait for the stream to stop, then hold it under the chute end, where the last concrete settles evenly
- C.Place it at the center of the stream, where the concrete is assumed to be the most thoroughly mixed
- D.Pass it through the entire stream, or divert the whole stream into it, so every part is collected✓ Answer
A discharge stream is not uniform across its width: mortar and coarse aggregate separate toward different parts of it. C172/C172M therefore requires passing the receptacle through the entire stream or diverting the whole stream into the container, so the portion contains the same proportions as the concrete being delivered.
Ref QB-G3BNHYSource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (passing the receptacle through the entire discharge stream)Report a problem with this question
10. At what point in the discharge of a truck mixer should the portions for a composite sample be taken?
- A.From the very first and the very last concrete, so the composite averages both ends
- B.From the middle part of the batch's discharge, at regularly spaced intervals✓ Answer
- C.From the very first concrete out of the chute, since it shows the mix before any change
- D.From the very last concrete in the drum, since by then the whole batch is fully mixed
The very beginning and very end of a discharge are the parts most likely to differ from the batch as a whole. C172/C172M has the portions taken at regularly spaced intervals during discharge of the middle part of the batch so that the composite represents the bulk of the concrete delivered.
Ref QB-Y9S8XDSource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (truck mixers: portions from the middle portion of the batch)Report a problem with this question
11. A technician must sample concrete from a stationary central-plant mixer (not a paving mixer) as it discharges into a hopper. Which procedure follows C172/C172M?
- A.Take one portion as the discharge begins, since the first concrete reflects the mixer's freshest batch
- B.Narrow the mixer's discharge to a trickle so that the receptacle fills slowly and evenly without spilling
- C.Stop the mixer once it is loaded and scoop portions directly from the concrete still inside the drum
- D.Take portions at regular intervals during the middle of the discharge, catching the full stream each time✓ Answer
For stationary mixers other than paving mixers, C172/C172M calls for two or more portions at regularly spaced intervals during discharge of the middle part of the batch, with the receptacle passed through the whole stream or the stream diverted into it. It also warns against restricting the flow from the mixer in a way that causes segregation.
Ref QB-GSXDHESource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (sampling from stationary mixers, except paving mixers)Report a problem with this question
12. Concrete for a small job is produced by a continuous (volumetric) mixer mounted on a truck. How should the composite sample be obtained?
- A.As for a stationary mixer: take portions at regular intervals from the middle of the discharge, not its ends✓ Answer
- B.As for a calibration run: take the first concrete produced, since it shows how the mixer is proportioning
- C.As for a paving mixer: discharge the whole amount needed into a pile, then sample several parts of that pile
- D.As for a final check: take the last concrete produced, since the mixer has reached steady output by then
C172/C172M treats continuous mixers like stationary mixers: portions are taken at regularly spaced intervals during discharge of the middle part of the production, avoiding the very beginning and end. The pile method belongs to paving mixers, and first-out or last-out concrete is exactly what the practice tells the sampler to avoid.
Ref QB-X9EWRCSource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (sampling from continuous mixers)Report a problem with this question
13. On a highway job, concrete comes from a paving mixer. How does C172/C172M have the technician obtain the composite sample?
- A.Discharge the whole batch, then take one large portion from the top center of the resulting pile
- B.Discharge the whole batch, then take portions from several different parts of the resulting pile✓ Answer
- C.Take portions at regular intervals from the stream while the paving mixer discharges onto the grade
- D.Take portions from the first concrete discharged, before it spreads out and touches the subgrade
A paving mixer dumps its batch rather than delivering a controllable stream, so C172/C172M has the entire batch discharged and portions taken from several different parts of the pile (the practice specifies a minimum number of locations). Taking from one spot or from the first concrete would not represent the whole batch.
Ref QB-FSK3CDSource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (sampling from paving mixers)Report a problem with this question
14. A paving mixer will discharge onto a dry, absorptive granular subgrade. What should the technician do to protect the sample?
- A.Set shallow containers on the subgrade and discharge across them, so the sample never touches the grade✓ Answer
- B.Scoop each portion from the bottom of the pile, so the sample includes the concrete in contact with the grade
- C.Let the pile sit on the grade until bleeding stops, so excess water drains out before portions are taken
- D.Take each portion with some subgrade fines, so the sample reflects the conditions of the in-place concrete
C172/C172M requires that a paving sample not be contaminated with subgrade material and not stay in prolonged contact with an absorptive subgrade, which would draw water from the concrete. Placing shallow containers on the subgrade and discharging the batch across them is the method the practice describes for meeting both requirements.
Ref QB-W93KKBSource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (paving mixers: avoiding subgrade contamination and absorption)Report a problem with this question
15. It is hot, sunny and windy, and the testing station is about 100 ft (30 m) from the truck. How should the composite sample be handled between sampling and testing?
- A.Hold it in a dry plywood box so the wood soaks up bleed water and the surface stays firm until testing
- B.Leave it uncovered in the open so it gives off heat and comes to ambient temperature before the tests start
- C.Keep it covered and shaded from sun and wind, and away from dust and debris, until the tests are started✓ Answer
- D.Sprinkle a little water on it as it dries so it keeps the same workability it had when it left the chute
C172/C172M requires protecting the sample from sun, wind and other sources of rapid evaporation, and from contamination, between sampling and testing. Evaporation, added water, absorptive containers or deliberate cooling would all change the concrete, so the tests would no longer describe the delivered batch.
Ref QB-ES8R6CSource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (protection of the sample from evaporation and contamination)Report a problem with this question
16. Why should the wheelbarrow and shovel used for sampling be nonabsorptive and dampened before use?
- A.So that a thin film of water mixes into the sample and makes it easier to remix with a shovel
- B.So that the tools take no water from the sample and its water content and slump stay unchanged✓ Answer
- C.So that the damp metal cools the sample enough to keep it inside the allowed temperature range
- D.So that the extra moisture replaces water the concrete will lose by evaporation during the walk
A dry or absorptive surface pulls water out of the fresh concrete it touches, lowering slump and altering the sample. Dampened, nonabsorptive equipment is neutral: it neither removes water nor is intended to add any, which keeps the sample faithful to the delivered concrete.
Ref QB-V9Q8EASource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (sampling equipment: nonabsorptive, dampened receptacles)Report a problem with this question
17. Assume the practice sets a minimum composite size of 1 ft³ (28 L) when strength specimens will be molded. For one truck, the technician will run only slump and temperature and no cylinders. What governs the size of that sample?
- A.Just enough to fill the slump mold once, since temperature can be read in the leftover concrete
- B.Exactly half of the strength minimum, since routine tests always use a fixed fraction of that size
- C.The same 1 ft³ (28 L) minimum, since every composite must be strength-sized regardless of the tests
- D.The needs of the planned test methods and the aggregate size, since a smaller sample is permitted✓ Answer
The larger minimum applies to samples from which strength specimens are made, because several specimens must come from one uniform composite. For routine slump, temperature and air tests C172/C172M allows smaller samples, sized by the requirements of the test methods and the maximum aggregate size rather than by a fixed fraction or a single-fill amount.
Ref QB-DSVDZASource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (size of sample: strength tests versus routine slump, temperature and air tests)Report a problem with this question
18. Assume the practice requires a composite of at least 1 ft³ (28 L) for strength tests. After remixing, a technician realizes the composite is only about 0.6 ft³ (17 L), yet slump and a set of cylinders are both required from this truck. What is the correct action?
- A.Obtain a new composite of adequate size from the batch, following the full sampling procedure✓ Answer
- B.Mold fewer cylinders from the small composite and note the reduced set count on the report
- C.Mold the cylinders from the small composite and skip the slump test to save enough concrete
- D.Top up the small composite with concrete from the next truck so that the total reaches 1 ft³
A composite below the stated minimum for strength tests does not satisfy C172/C172M, and dropping required tests or specimens leaves the batch incompletely evaluated. Mixing in concrete from another truck destroys the link between the sample and one batch, so the only compliant course is a new, properly sized composite taken from the same batch within the time limits.
Ref QB-U9CW89Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (size of sample for strength tests)Report a problem with this question
19. A mass-concrete mix contains coarse aggregate larger than the air content test method permits, so the oversize particles must be removed by wet sieving. At what point in the sampling procedure is the wet sieving done?
- A.After the portions are carried to the test site and before they are remixed into the composite✓ Answer
- B.After the air test is run on the unsieved concrete, so that the result can be corrected afterward
- C.After the composite is remixed and the slump is run, sieving only what is left for the air test
- D.At the truck, sieving each portion as it is caught, before the portions are carried to the test site
C172/C172M places wet sieving after the sample is transported and before it is remixed, so the sieved material is then remixed into one uniform composite for testing. Sieving at the truck lengthens the sampling time, and sieving only a leftover part, or after testing, means the test is not run on the material the method requires.
Ref QB-CSH3U9Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (removal of large maximum-size aggregate by wet sieving)Report a problem with this question
20. During wet sieving to remove oversize coarse aggregate, mortar is clinging both to the retained stones and to the sieve frame and screen. How is each handled?
- A.Discard the stones with their mortar, and return the mortar on the sieve and equipment to the sample✓ Answer
- B.Wipe the mortar off the stones back into the sample, and discard the mortar on the sieve with the stones
- C.Discard the stones with their mortar, and discard the mortar on the sieve and equipment along with them
- D.Wipe the mortar off the stones back into the sample, and scrape the mortar on the sieve back in as well
C172/C172M has the oversize particles discarded as they come off the sieve, without wiping off the mortar that coats them, because scraping that mortar back would return material to the sample in an uncontrolled amount. Mortar left sticking to the sieving equipment, however, belongs to the material that passed the sieve, so it is returned to the sample before remixing.
Ref QB-V7S3EASource: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (wet sieving: handling of oversize particles and adhering mortar)Report a problem with this question
21. How should a technician physically carry out wet sieving of a sample containing oversize coarse aggregate?
- A.Rinse the stones on the sieve with a hose, so the mortar washes through and the sample regains its paste
- B.Load the whole sample onto the sieve at once and shake it briefly, so the sieving stays within the time limits
- C.Sieve small amounts at a time, shaking until no undersize material remains on the designated sieve✓ Answer
- D.Pick the large stones out of the composite by hand, so no mortar is lost to the sieve or the equipment
Overloading the sieve traps undersize material with the oversize stones, while hand-picking and hosing change the sample in ways the practice does not allow; added water in particular alters the concrete. C172/C172M has the concrete passed over the designated sieve in increments small enough that shaking or vibrating clears all undersize material through it.
Ref QB-CQKWT9Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (wet sieving procedure)Report a problem with this question
22. A specification requires air content on the full concrete, but the test was run on a wet-sieved sample from which oversize aggregate was removed. How does the air content of the sieved fraction compare with that of the full concrete?
- A.It is lower, because sieving strips out most of the entrained air bubbles along with the large stones
- B.It is the same, because the air bubbles are spread evenly through every part of the concrete mixture
- C.It is higher, because the extra handling during sieving whips additional air into the remaining mortar
- D.It is higher, because the removed stones contained no air, so the remaining fraction is richer in paste✓ Answer
Air is held in the paste, not in the coarse aggregate particles. Removing the large stones raises the proportion of paste, so the sieved fraction shows a higher air content than the full concrete, even though the added handling tends to lose some air rather than add it; the full-concrete value must be calculated, not read directly.
Ref QB-U7ER89Source: ASTM C172/C172M, Standard Practice for Sampling Freshly Mixed Concrete (wet sieving: effect on air content of the sieved fraction)Report a problem with this question

1. A project engineer asks the field technician to use ASTM C172/C172M to find out whether the concrete at the start of a truck's discharge differs from the concrete near the end of the same load. How should the technician respond?

2. To save time, a technician fills one bucket with the first concrete out of the chute and uses it for slump and for molding cylinders. The cylinders later break low. What is the most defensible conclusion?

3. Assume the practice allows no more than 15 min between taking the first and the final portions of a composite sample. A technician takes the first portion at 10:00, the truck then stops to reposition, and the final portion is taken at 10:19. What should the technician do?

5. Why does ASTM C172/C172M set deadlines for starting the slump, temperature and air content tests and for starting to mold strength specimens?

6. Which sequence correctly describes how a composite sample is formed from a revolving-drum truck mixer?

7. A ready-mix driver plans to add water at the site after the superintendent eyes the first concrete in the chute. When should the technician sample this load for acceptance testing?

8. Concrete is pouring from a truck chute so fast that the technician cannot pass the wheelbarrow through the stream without spilling. How should the discharge rate be controlled for sampling?

9. While sampling from a truck mixer, how should the technician use the sampling receptacle in the discharge stream?

10. At what point in the discharge of a truck mixer should the portions for a composite sample be taken?

11. A technician must sample concrete from a stationary central-plant mixer (not a paving mixer) as it discharges into a hopper. Which procedure follows C172/C172M?

12. Concrete for a small job is produced by a continuous (volumetric) mixer mounted on a truck. How should the composite sample be obtained?

13. On a highway job, concrete comes from a paving mixer. How does C172/C172M have the technician obtain the composite sample?

14. A paving mixer will discharge onto a dry, absorptive granular subgrade. What should the technician do to protect the sample?

15. It is hot, sunny and windy, and the testing station is about 100 ft (30 m) from the truck. How should the composite sample be handled between sampling and testing?

16. Why should the wheelbarrow and shovel used for sampling be nonabsorptive and dampened before use?

17. Assume the practice sets a minimum composite size of 1 ft³ (28 L) when strength specimens will be molded. For one truck, the technician will run only slump and temperature and no cylinders. What governs the size of that sample?

18. Assume the practice requires a composite of at least 1 ft³ (28 L) for strength tests. After remixing, a technician realizes the composite is only about 0.6 ft³ (17 L), yet slump and a set of cylinders are both required from this truck. What is the correct action?

19. A mass-concrete mix contains coarse aggregate larger than the air content test method permits, so the oversize particles must be removed by wet sieving. At what point in the sampling procedure is the wet sieving done?

20. During wet sieving to remove oversize coarse aggregate, mortar is clinging both to the retained stones and to the sieve frame and screen. How is each handled?

21. How should a technician physically carry out wet sieving of a sample containing oversize coarse aggregate?

22. A specification requires air content on the full concrete, but the test was run on a wet-sieved sample from which oversize aggregate was removed. How does the air content of the sieved fraction compare with that of the full concrete?

Other topics in this exam