24 Making & Curing Specimens (ASTM C31) Practice Questions & Answers

Every Making & Curing Specimens (ASTM C31) practice question from the ACI Concrete Field Testing Technician Grade I Practice Test, with the correct answer and a short explanation.

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1. A project specification calls for compressive-strength cylinders to decide whether the delivered concrete meets the specified strength (acceptance testing). How must those cylinders be cured?
- A.By standard curing: initial curing on site, then moist or lime-water curing in the laboratory✓ Answer
- B.By field curing on site until the forms are stripped, then dry storage in the laboratory
- C.By either method, as long as every cylinder from the same truck gets the same treatment
- D.By field curing: kept on the structure beside the slab so they share its actual exposure
Acceptance testing compares the mixture's potential strength against the specification, so every set must be cured under the same controlled conditions; standard curing provides that, while field curing reflects the varying conditions of the structure and must never be used for acceptance.
Ref QB-WJZ525Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Significance and Use: standard-cured specimens for acceptance testingReport a problem with this question
2. A contractor wants to know when the shoring under an elevated slab can be removed. Which specimens provide the information needed for that decision?
- A.Standard-cured cylinders broken early, since early-age results already predict the slab's strength
- B.Lab-cured cylinders made from a trial batch, because they reflect the mixture design exactly
- C.Standard-cured cylinders, because controlled curing shows the strength the mix can reach
- D.Field-cured cylinders kept on the slab with the same protection as the concrete they represent✓ Answer
Form and shoring removal depends on the strength the concrete in the structure has actually developed, and only specimens exposed to the same temperature, moisture and protection as that concrete estimate it; standard-cured specimens show the mixture's potential, not the in-place condition.
Ref QB-C3GN93Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Significance and Use: field-cured specimens for form and shoring removalReport a problem with this question
3. Field-cured cylinders are being made for a bridge deck. Which handling is consistent with field curing under ASTM C31/C31M?
- A.Keep them on the deck for one day, then soak them in lime water until the scheduled test date
- B.Store them on the deck, strip the molds about when the forms come off, and test them as they are✓ Answer
- C.Move them to the lab moist room the next day so their curing matches the acceptance cylinders
- D.Keep them in a temperature-controlled curing box on site until the deck forms are stripped
Field-cured cylinders must experience what the represented concrete experiences: they stay in or on the structure with the same protection, come out of their molds about when the forms are removed, and are tested in the moisture condition that treatment produces; a controlled box or lab storage would make them standard-cured in effect.
Ref QB-VJNTU3Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Field curing of cylindersReport a problem with this question
4. Why does ASTM C31/C31M call for cylinders to be molded on a level, rigid surface free of vibration, as close as practicable to where they will be stored?
- A.So the concrete temperature can be read in the molds rather than in a separate container
- B.So the technician can skip covering the cylinders, since they will not be moved anywhere later
- C.So the molds stay plumb and the fresh concrete is not disturbed or segregated by moving it✓ Answer
- D.So the slump of the batch can be judged from how the concrete settles in each of the molds
Fresh concrete in a mold is easily damaged: tilting, jarring or vibration can cause segregation, bleeding channels or a sloped top, so molding where the specimens will stay and on a stable surface avoids moving them while they are still plastic.
Ref QB-B35B42Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Place of moldingReport a problem with this question
5. The composite sample for cylinders was obtained per C172/C172M. Assume molding must begin within 15 minutes of obtaining that sample. What should the technician do first?
- A.Fill the molds straight from the truck chute so the concrete in them is as fresh as possible
- B.Wait until the slump and air tests are done, then take a new sample from the truck to mold
- C.Remix the sample in the receptacle, then start filling the molds within the allowed time✓ Answer
- D.Let the sample rest so bleed water rises, then pour it off before filling any of the molds
Specimens must come from the same representative sample used for the fresh tests, and remixing it restores uniformity after any settling or segregation in the receptacle; filling from the chute or taking a second sample breaks the link between the specimens and the tested sample.
Ref QB-UJAGP2Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Sampling concrete for specimens; remixing before moldingReport a problem with this question
6. The coarse aggregate has a nominal maximum size of 1-1/2 in (37.5 mm). Assume the cylinder diameter must be at least three times the nominal maximum aggregate size. Only 4 x 8 in and 6 x 12 in molds are on site. Which choice is correct?
- A.Use either mold size, because the three-times rule applies only to flexural beam specimens
- B.Use the 6 x 12 in molds, because 6 in meets the three-times rule and 4 in does not✓ Answer
- C.Use the 4 x 8 in molds after picking the largest stones out of the sample by hand
- D.Use the 4 x 8 in molds, because a 4 in diameter is more than twice the aggregate size
Three times 1-1/2 in is 4-1/2 in, so a 4 in mold is too small and the 6 in mold is required; the minimum-diameter rule exists so large particles do not dominate the cross-section, and removing stones by hand changes the concrete being tested.
Ref QB-A3SYWYSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Cylinder size relative to nominal maximum aggregate sizeReport a problem with this question
7. A mass-concrete mix contains aggregate larger than the 6 x 12 in molds on site can accept (assume the limit for those molds is 2 in). How should the strength specimens be made?
- A.Dry-sieve the coarse aggregate at the plant and have a special batch made for the molds
- B.Wet-sieve the sample over a 2 in (50 mm) sieve per C172/C172M, then mold what passes✓ Answer
- C.Mold the full mix as delivered and note the oversize aggregate on the specimen report
- D.Pick the large stones out by hand while filling each layer, then rod the layer as usual
When the aggregate exceeds what the mold size allows, C31/C31M directs that the oversize particles be removed by wet sieving per C172/C172M so the specimen still represents the mortar and smaller aggregate of the delivered concrete; hand-picking is not a controlled method, and a special batch no longer represents the delivered load.
Ref QB-TJX5HZSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Specimen size and wet sieving of oversize aggregateReport a problem with this question
8. When a technician molds strength cylinders from a sample, which fresh-concrete tests does ASTM C31/C31M expect to be made on that sample and recorded?
- A.Slump and density only, because air content is measured later on the hardened cylinders
- B.Air content and temperature only, because slump has no effect on cylinder strength results
- C.Slump, air content and temperature, all determined on the sample the cylinders came from✓ Answer
- D.Slump, air content, temperature and density, since C31 requires density for every set
Strength results are interpreted in light of the concrete's condition when the specimens were made, so slump, air content and temperature are determined on the same sample and reported with the specimens. Air content cannot be measured on hardened cylinders, and C31 does not make a density (unit weight) test part of every set. Density may still be required by ASTM C94 or the project specification, but not by C31.
Ref QB-H39B59Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Fresh concrete tests accompanying strength specimensReport a problem with this question
9. Assume concrete with a slump of 1 in (25 mm) or less must be consolidated by vibration. A paving mix measures 3/4 in (19 mm) slump. How should its cylinders be consolidated?
- A.By vibration, because a slump this low is in the range where rodding is not allowed✓ Answer
- B.With extra rod strokes on every layer to make up for the very stiff consistency
- C.By rodding, because rodding is the reference method for concrete of any slump
- D.By rodding or vibration at the technician's choice, as both methods apply at this slump
Stiff, low-slump concrete cannot be reliably consolidated by a hand rod, so voids would remain no matter how many strokes are added; 3/4 in falls within the stated vibration-only range, and the choice between rodding and vibration exists only above it.
Ref QB-2KEGQ9Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Selection of consolidation method based on slumpReport a problem with this question
10. When rodding the upper layers of a cylinder, why should the rod pass about 1 in (25 mm) into the layer below?
- A.To check the depth of each layer and confirm that all of the layers are equal
- B.To drive entrapped air down into the bottom layer, where it does no harm
- C.To knit the layers together so no weak plane or voids remain between them✓ Answer
- D.To push excess paste downward so the top surface is easier to finish later
Each layer is placed on concrete that has already been consolidated, and rodding slightly into it blends the two so the cylinder behaves as one mass; a seam or void between layers would become a weak plane and lower the measured strength.
Ref QB-EW6ADESource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Rodding procedure for cylindersReport a problem with this question
11. A technician rods all layers of a 6 x 12 in cylinder and taps the outside of the mold only once, after the final layer. What is wrong with this practice?
- A.Tapping should happen before rodding each layer so that the rod can pass through more easily
- B.Tapping should be skipped entirely, because it drives bleed water up to the top surface
- C.Tapping must follow each layer, to close rod holes and release trapped air bubbles✓ Answer
- D.Extra rod strokes should replace the tapping, since both serve the same purpose
Rod holes and entrapped air form in every layer, and once the next layer is placed on top they can no longer be closed from outside; tapping right after rodding each layer closes them while the concrete is still accessible.
Ref QB-WDZ4TDSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Rodding procedure: tapping after each layerReport a problem with this question
12. Single-use cylinder molds made of light-gauge sheet metal could be dented by a mallet. How should their sides be tapped after each layer is rodded?
- A.With an open hand, which closes the rod holes without deforming the thin mold✓ Answer
- B.With no tapping at all, since a thin mold vibrates enough while being rodded
- C.With the tamping rod, which strikes harder than a mallet and saves a little time
- D.With a steel hammer, which is heavier and closes the rod holes in fewer blows
Tapping is still needed to close rod holes, but a deformed mold produces an out-of-round or out-of-square specimen; an open hand gives enough energy for a light mold without denting it, while a rod or steel hammer would damage it.
Ref QB-FWHMJFSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Rodding procedure: tapping single-use light-gauge moldsReport a problem with this question
13. When consolidating cylinders with an internal vibrator, which practice is correct?
- A.Pull the vibrator out quickly so the concrete does not segregate around the element
- B.Let the vibrator rest on the bottom of the mold so the lowest concrete is fully consolidated
- C.Keep vibrating each layer until paste stops rising and a layer of water covers the top
- D.Insert it without touching the mold, then withdraw it slowly so no voids are left behind✓ Answer
Touching the mold can damage it and transmits vibration unevenly, and a fast withdrawal leaves a hole the concrete cannot close; vibration stops once the surface is relatively smooth and large air bubbles cease, because continuing past that point causes segregation and bleeding.
Ref QB-XDCFYESource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Vibration procedure for cylindersReport a problem with this question
14. Which item belongs on the report for standard-cured cylinders made under ASTM C31/C31M?
- A.The relative humidity in the lab on test day and the name of whoever broke the cylinders
- B.The supplier's mix design approval number and the batch plant's scale calibration date
- C.The cement type and mill certificate number, plus the admixture dosage on the ticket
- D.The initial curing method with its maximum and minimum temperatures, and the final method✓ Answer
The C31/C31M report documents how the specimens were made and cured so the strength results can be trusted: identification, location represented, who molded them and when, the fresh test results, the curing method and, for standard curing, the initial curing method with its recorded temperature extremes and the final curing method; plant, material and breaking details belong to other records.
Ref QB-CWFM2CSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — ReportReport a problem with this question
15. After the last layer of a cylinder has been consolidated, how should the top be finished?
- A.Struck off flush with the rim using the rod, float or trowel, leaving it flat✓ Answer
- B.Left rough and unfinished so the cap applied at the lab will bond to it better
- C.Troweled repeatedly until bleed water shows, which proves the surface is dense
- D.Left slightly mounded above the rim so that capping later removes any low spots
A flat top level with the rim keeps the specimen at its full length with an end that can be capped or ground uniformly; a mound, a rough surface or an overworked top creates an uneven or weakened end that concentrates load during testing.
Ref QB-UDAFFBSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Finishing of cylindersReport a problem with this question
16. Where should the identification be marked on a freshly molded cylinder?
- A.On the outside of the mold, since lids are easily removed and swapped between molds✓ Answer
- B.On the delivery ticket only, since the lab can match cylinders by order of arrival
- C.On the fresh concrete surface, by scratching the number into the top with a nail
- D.On the plastic lid, since that is the first surface the laboratory technician will see
Identification has to stay with the specimen without damaging it; lids come off and get mixed up, and scratching the fresh top damages the end that will bear the load, so the outside of the mold is the reliable place.
Ref QB-DWTX7DSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Identification of specimensReport a problem with this question
17. Right after finishing, why must cylinders be covered with a nonabsorbent, nonreactive cap or plastic sheet?
- A.To keep rain off, so in dry weather the cylinders can safely be left uncovered
- B.To keep water from evaporating, which would weaken the top and lower the strength✓ Answer
- C.To hold the identification label in place until the molds are stripped at the lab
- D.To keep the concrete warm, since covering raises its temperature and speeds strength gain
Cement needs its mixing water to hydrate, and moisture lost from the exposed top in the first hours leaves a weak, crazed end that lowers the measured strength; a nonabsorbent, nonreactive cover holds the water in without adding or reacting with anything.
Ref QB-VDNSMCSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Protection of specimens to prevent evaporationReport a problem with this question
18. Cylinders for a 6000 psi mix are in an insulated curing box on site. Assume the required initial curing range for this strength is 68 to 78 F (20 to 26 C). The max-min thermometer shows a high of 82 F (28 C). What should the technician do?
- A.Treat it as compliant, since the wider range for ordinary mixes is the one that governs
- B.Discard the cylinders and mold replacements from a later truck of the same mixture
- C.Extend the lab curing period by one day so the cylinders end up equivalent
- D.Treat it as out of range, correct the box, and report the readings as a deviation✓ Answer
A high of 82 F exceeds the stated 68 to 78 F band, so the environment must be corrected and the recorded maximum and minimum reported as a deviation; replacement cylinders would represent different concrete, and extra curing time cannot undo early overheating.
Ref QB-JWKM3KSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Standard curing: initial curing temperature for higher-strength concrete; ReportReport a problem with this question
19. Why must cylinders in initial curing be shielded from direct sunlight and radiant heaters?
- A.Sunlight fades the identification marks so the lab may be unable to read them
- B.Heat makes plastic molds swell, so the cylinders come out larger than their nominal size
- C.High early temperatures speed early gain but tend to lower later-age strength✓ Answer
- D.Warm cylinders bond to their molds, so the molds must be cut off at the lab
Excess heat in the first hours accelerates hydration but produces a coarser structure, so the later-age strength measured is lower than the mixture's potential; sunlight and radiant heat also dry the specimens, which is why the initial environment must be controlled and recorded.
Ref QB-2EEFGJSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Standard curing: initial curing environmentReport a problem with this question
20. In a laboratory moist room, a leaking pipe lets water run continuously over one rack of cylinders during final curing. How does this relate to ASTM C31/C31M?
- A.It meets the standard, because free water on every surface is all that is required
- B.It is the preferred setup, because running water keeps cylinders cooler than still water
- C.It meets the standard as long as the room temperature stays inside the required range
- D.It violates the standard, because specimens must not be exposed to dripping or running water✓ Answer
Final curing requires free water on all surfaces, but running or dripping water can leach lime and erode the surface of the specimens, so the standard excludes it; the leak must be fixed and the exposed cylinders noted.
Ref QB-KN5SJ9Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Standard curing: final curingReport a problem with this question
21. Assume final curing must begin within 30 minutes after the molds are removed. Cylinders are demolded at 9:00 a.m. and sit on a bench until 10:15 a.m. while paperwork is completed. What is the proper response?
- A.Place them in final curing now with no note, since demolding time is not controlled
- B.Place them in final curing now and record the late start as a deviation on the report✓ Answer
- C.Restart their age count at 10:15 a.m. so that the scheduled test date is unaffected
- D.Let them air-dry for a few more hours so cold curing water does not shock their surfaces
75 minutes exceeds the stated 30-minute limit, so the cylinders lost surface moisture; they should go into final curing immediately and the deviation be reported so anyone reading the results knows; age always counts from molding, and further drying only makes the error worse.
Ref QB-46BX5ASource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Standard curing: final curing after mold removal; ReportReport a problem with this question
22. Assume specimens may not be moved until at least 8 h after final set and transport may not exceed 4 h. Which transport plan meets ASTM C31/C31M?
- A.Leave 12 h after final set, cushioned and wrapped in plastic, arriving 5 h later
- B.Leave 10 h after final set, cushioned and wrapped in plastic, arriving 3 h later✓ Answer
- C.Leave 10 h after final set, loose in the truck bed and uncovered, arriving 2 h later
- D.Leave 6 h after final set, cushioned and wrapped in plastic, arriving 3 h later
Only the plan that waits past the stated minimum age, stays within the stated transport time and protects the specimens satisfies all conditions; moving them too early or loose risks cracking young concrete, and an overlong trip or no wrap lets them dry, freeze or be jarred.
Ref QB-MNG4RASource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Transportation of specimens to the laboratoryReport a problem with this question
23. Why must flexural beam specimens not be allowed to dry between their removal from lime-water storage and testing?
- A.Wet surfaces let loading blocks seat evenly, while dry beams slip on the supports
- B.Drying makes beams stronger, so the result would overstate the true strength of the concrete
- C.Dry beams weigh less, and the testing machine is calibrated for saturated specimens
- D.Surface drying sets up shrinkage stresses in the tension face that lower the result✓ Answer
Flexural strength is governed by the extreme tension face, and even brief surface drying makes that face shrink against the moist core, adding tensile stress that cracks the beam at a lower load; that is why beams are kept saturated right up to testing.
Ref QB-56P9BBSource: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Standard curing of flexural strength specimensReport a problem with this question
24. Assume beams are rodded once for each 2 sq in (14 cm²) of top surface area in every layer. How does this differ from rodding cylinders?
- A.Beam strokes depend on the specimen's top area, while cylinders use a set count by size✓ Answer
- B.Beam strokes are not used at all, since long specimens must always be vibrated
- C.Beam strokes match the count for a 6 in cylinder, regardless of how long the beam is
- D.Beam strokes are set by slump, while cylinder strokes depend on the mold's top area
Beams come in different lengths and widths, so tying the stroke count to top surface area gives every part of the beam the same consolidating effort, whereas cylinders of a given size always have the same area and so use a fixed count; rodding is still permitted for beams when the slump allows it.
Ref QB-HNE487Source: ASTM C31/C31M, Standard Practice for Making and Curing Concrete Test Specimens in the Field — Molding beam specimens: roddingReport a problem with this question

1. A project specification calls for compressive-strength cylinders to decide whether the delivered concrete meets the specified strength (acceptance testing). How must those cylinders be cured?

2. A contractor wants to know when the shoring under an elevated slab can be removed. Which specimens provide the information needed for that decision?

3. Field-cured cylinders are being made for a bridge deck. Which handling is consistent with field curing under ASTM C31/C31M?

4. Why does ASTM C31/C31M call for cylinders to be molded on a level, rigid surface free of vibration, as close as practicable to where they will be stored?

5. The composite sample for cylinders was obtained per C172/C172M. Assume molding must begin within 15 minutes of obtaining that sample. What should the technician do first?

6. The coarse aggregate has a nominal maximum size of 1-1/2 in (37.5 mm). Assume the cylinder diameter must be at least three times the nominal maximum aggregate size. Only 4 x 8 in and 6 x 12 in molds are on site. Which choice is correct?

7. A mass-concrete mix contains aggregate larger than the 6 x 12 in molds on site can accept (assume the limit for those molds is 2 in). How should the strength specimens be made?

8. When a technician molds strength cylinders from a sample, which fresh-concrete tests does ASTM C31/C31M expect to be made on that sample and recorded?

9. Assume concrete with a slump of 1 in (25 mm) or less must be consolidated by vibration. A paving mix measures 3/4 in (19 mm) slump. How should its cylinders be consolidated?

10. When rodding the upper layers of a cylinder, why should the rod pass about 1 in (25 mm) into the layer below?

11. A technician rods all layers of a 6 x 12 in cylinder and taps the outside of the mold only once, after the final layer. What is wrong with this practice?

12. Single-use cylinder molds made of light-gauge sheet metal could be dented by a mallet. How should their sides be tapped after each layer is rodded?

13. When consolidating cylinders with an internal vibrator, which practice is correct?

14. Which item belongs on the report for standard-cured cylinders made under ASTM C31/C31M?

15. After the last layer of a cylinder has been consolidated, how should the top be finished?

16. Where should the identification be marked on a freshly molded cylinder?

17. Right after finishing, why must cylinders be covered with a nonabsorbent, nonreactive cap or plastic sheet?

18. Cylinders for a 6000 psi mix are in an insulated curing box on site. Assume the required initial curing range for this strength is 68 to 78 F (20 to 26 C). The max-min thermometer shows a high of 82 F (28 C). What should the technician do?

19. Why must cylinders in initial curing be shielded from direct sunlight and radiant heaters?

20. In a laboratory moist room, a leaking pipe lets water run continuously over one rack of cylinders during final curing. How does this relate to ASTM C31/C31M?

21. Assume final curing must begin within 30 minutes after the molds are removed. Cylinders are demolded at 9:00 a.m. and sit on a bench until 10:15 a.m. while paperwork is completed. What is the proper response?

22. Assume specimens may not be moved until at least 8 h after final set and transport may not exceed 4 h. Which transport plan meets ASTM C31/C31M?

23. Why must flexural beam specimens not be allowed to dry between their removal from lime-water storage and testing?

24. Assume beams are rodded once for each 2 sq in (14 cm²) of top surface area in every layer. How does this differ from rodding cylinders?

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