ACI Concrete Field Testing

Exam blueprint

Sourced from ACI Concrete Field Testing Technician — Grade I Certification Policies and Examination Requirements

• ASTM C172 — Sampling fresh concrete12%
• ASTM C143 — Slump test18%
• ASTM C231 — Air content, pressure method18%
• ASTM C173 — Air content, volumetric method10%
• ASTM C1064 — Temperature7%
• ASTM C138 — Unit weight + yield15%
• ASTM C31 — Making + curing test cylinders15%
• Job-site safety + general practice5%

1. 01Sampling + Temperature — the gateway tests

   ~60min

   Every other test depends on a properly drawn sample at a representative temperature. ASTM C172 and C1064 set the procedural foundation. Get sampling wrong and every downstream result is invalid.

   • ASTM C172 — Sampling truck-mixed concrete

     Standard procedure for truck-mixed concrete: sample is the COMPOSITE of two or more portions taken from the MIDDLE PORTION of the batch — NOT from the very first or very last discharge. Specifically, the truck must be discharged AT THE NORMAL OPERATING SPEED of the chute; the sample is taken either by (a) passing a receptacle through the discharge stream (preferred) or (b) diverting the stream into a sample container at appropriate intervals. MINIMUM SAMPLE SIZE for routine tests (slump + air + cylinders): 1 cubic foot. The sample MUST be remixed in the wheelbarrow with a shovel or scoop before any test is performed (segregation occurs immediately on transfer). All field tests must be STARTED within 5 minutes of obtaining the final sample portion; cylinder casting must be COMPLETED within 15 minutes of obtaining the sample. Time-stamp the sample on the test report.

     Reference: ASTM C172 Sampling Freshly Mixed Concrete

   • ASTM C1064 — Concrete temperature

     Measure with a thermometer accurate to ±1°F (or ±0.5°C), embedded so the SENSOR IS COVERED BY AT LEAST 3 INCHES OF CONCRETE in all directions. Wait for the reading to STABILIZE — at least 2 minutes, no more than 5 minutes. Record to the nearest 1°F. Common spec limits: minimum 50°F, maximum 90°F (varies by jurisdiction and project; cold-weather concrete per ACI 306 + hot-weather per ACI 305 have additional restrictions). Always log the AMBIENT temperature alongside the concrete temperature — they are different data points and both go on the test report. The temperature test is FAST and CHEAP — it should always be the FIRST test performed after sample acquisition because slump and air are temperature-sensitive (cold concrete reads stiffer slump; warm concrete reads more slump for the same mix).

     Reference: ASTM C1064 Temperature of Freshly Mixed Hydraulic-Cement Concrete

   Practice questions (2)

   1. 1. A ready-mix truck arrives. The driver is in a hurry and asks the field tech to take the sample from the very first discharge. The tech should:

      • A.Comply — the first discharge is most representative
      • B.Refuse and require sampling from the MIDDLE PORTION of the batch per ASTM C172✓ correct
      • C.Take samples from both the first and last portions for a comparison
      • D.Wait until the truck is empty, then sample what remains

      ASTM C172 prohibits sampling from the very first or very last portion of a batch — both are non-representative (first is often segregated from washout; last contains accumulated coarse aggregate). The composite sample is drawn from at LEAST TWO REGULARLY SPACED PORTIONS of the middle of the batch. (A) is the textbook violation. (C) wastes time and still includes a non-representative portion. (D) is the opposite error — sampling the very last portion.

   2. 2. A field tech inserts the temperature probe to a depth of 1.5 inches into the wheelbarrow sample. After 30 seconds the reading is 72°F. Issue?

      • A.No issue; reading is acceptable
      • B.Probe is not deep enough — must be covered by at least 3 inches of concrete on all sides✓ correct
      • C.Reading time is insufficient — wait 30 minutes
      • D.ASTM C1064 prohibits sampling concrete in a wheelbarrow

      ASTM C1064 requires the probe to be covered by AT LEAST 3 INCHES of concrete in all directions to give a representative core temperature; surface readings are biased by ambient air. The 30-second reading time is also short — minimum 2 minutes after the temperature stabilizes. (A) ignores both the depth and time issues. (C) overstates — 30 minutes is unnecessary. (D) is wrong; wheelbarrow sampling is standard.

2. 02ASTM C143 — Slump test

   ~60min

   The slump test is the single most performed concrete field test. It measures workability — the apparent fluidity of the mix. The procedure is exact and the candidate must perform it flawlessly on the performance exam.

   • Equipment + setup

     Equipment: SLUMP CONE (frustum, 8 inches base diameter × 4 inches top diameter × 12 inches tall), TAMPING ROD (5/8 inch diameter × 24 inches long, hemispherical tip), MOISTENED NON-ABSORBENT BASE (a flat steel or hardboard plate, dampened with water, NOT pooling). The cone must be DAMPENED with water on the inside before each test — NOT oiled (oil affects slump). Place the cone on the moist base, large opening down. The tester stands ON the foot pieces of the cone to hold it in place during filling.

     Reference: ASTM C143 Slump of Hydraulic-Cement Concrete

   • Procedure — three layers, 25 rods each, lift in 3-7 sec

     Fill the cone in THREE LAYERS, each approximately 1/3 of the volume. After each layer, ROD 25 TIMES with the hemispherical end of the tamping rod. (1) FIRST LAYER: rod 25 times, distributing strokes evenly across the cross-section, just penetrating the layer below — NOT touching the base. (2) SECOND LAYER: rod 25 times, penetrating just into the first layer. (3) THIRD LAYER: heap concrete above the cone before rodding; rod 25 times, penetrating just into the second layer. Strike off the top with the rod or trowel. Remove any spilled concrete from the base. LIFT THE CONE STRAIGHT UP in 3-7 SECONDS without lateral or twisting motion — the cone must clear the concrete with no pause. Place the cone next to the slumped concrete (inverted), lay the rod across the inverted cone, and measure the DISTANCE FROM THE BOTTOM OF THE ROD TO THE DISPLACED ORIGINAL CENTER OF THE TOP SURFACE of the slumped concrete. Record to nearest 1/4 inch. Total elapsed time start-of-fill to slump measurement: 2.5 MINUTES MAXIMUM.

   • Slump types + when to discard the test

     Three slump shapes: TRUE SLUMP (uniform symmetrical settling) — VALID measurement. SHEAR SLUMP (one half slides off relative to the other) — DISCARD and re-test from a fresh sample portion. COLLAPSE (concrete completely falls apart) — typical of very high-slump (>9 inches) or improperly proportioned mixes; record but understand the result is unreliable for QC purposes. Common procedural errors that invalidate the test: cone lifted in less than 3 sec or more than 7 sec; lateral movement during cone lift; incorrect number of rods per layer (must be exactly 25); base not properly moistened. The performance examiner will deliberately test whether the candidate detects shear slump and knows to retest.

   Practice questions (2)

   1. 1. A field tech fills the cone in three layers, rods 25 times per layer, and lifts the cone in 4 seconds. The result shows one side of the concrete sliding off while the other side remains nearly upright. The tech should:

      • A.Record the slump measurement at the lower side
      • B.Average the slump from both sides
      • C.Discard the test as a SHEAR SLUMP and retest with a fresh portion of the same sample✓ correct
      • D.Record it as 'collapse' and proceed

      A shear slump indicates the concrete is not behaving cohesively and the slump value is unreliable. ASTM C143 requires DISCARDING a shear slump and retesting from a FRESH PORTION of the same sample (the sample, not a new truck). If the second test ALSO shears, the concrete is reported as 'shear slump' and is generally not acceptable as representative. (A) and (B) treat the unreliable result as if valid. (D) confuses shear with collapse — collapse is total failure to hold any shape, not a one-sided slide.

   2. 2. ASTM C143 requires the cone to be lifted in:

      • A.1-3 seconds with a slight twist
      • B.3-7 seconds with no twist or lateral motion✓ correct
      • C.10-15 seconds slowly
      • D.As fast as possible to prevent re-settling

      ASTM C143 specifies lifting in 3-7 seconds, vertically, with NO twisting or lateral motion. Lifting too fast (under 3 sec) creates a 'whipping' motion that distorts the slump; too slow (over 7 sec) allows water to migrate and biases the measurement low. Twist or lateral movement disturbs the concrete and invalidates the test. (A) introduces twist — explicitly prohibited. (C) is too slow. (D) ignores the lower bound and the no-motion requirement.

3. 03Air content — C231 (pressure) + C173 (volumetric)

   ~75min

   Air entrainment provides freeze-thaw durability. The field tech measures total air content with a meter — pressure type (Type B) for normal-weight concrete; volumetric (rollameter) for lightweight concrete where pressure method gives biased readings.

   • ASTM C231 — Pressure method (Type B air meter)

     Used for NORMAL WEIGHT (>140 lb/cu ft) concrete. The Type B meter consists of a 0.25 cu ft (or larger) measuring bowl with a clamp-on top assembly that has a calibrated pressure gauge and an internal air chamber. Procedure: fill the bowl in 3 LAYERS, RODDING 25 TIMES PER LAYER (same as slump cone), then TAP THE SIDES 10-15 TIMES with a rubber mallet AFTER EACH LAYER to release entrapped air without consolidating away entrained air. Strike off level with the rim. Clean the rim and seat the top assembly. With both petcocks open, fill water through one petcock until water exits the other (this purges air). Close petcocks. PUMP UP the internal chamber to the initial pressure mark on the gauge. Press the main valve — pressure equalizes between chamber and bowl, gauge needle drops to the AIR CONTENT reading. Record to the nearest 0.1%. CALIBRATE the meter against known volumes (the calibration vessel) before the day's testing — meters drift.

     Reference: ASTM C231 Air Content of Freshly Mixed Concrete by the Pressure Method

   • ASTM C173 — Volumetric method (rollameter)

     Used for LIGHTWEIGHT or POROUS-AGGREGATE concrete where the pressure method gives FALSELY HIGH readings (the porous aggregate compresses under pressure, mimicking air). Procedure: fill the bowl portion of the rollameter in 3 layers, rodding 25 times per layer + tapping. Strike off. Clamp on the upper neck. Fill water + isopropyl alcohol (alcohol prevents foaming) into the neck to the zero line. Invert the apparatus and ROLL it through 1/4 turn back-and-forth for at least 45 seconds (the agitation extracts air bubbles into the water column). Stand upright; let stand for 1 min; check the water level — the drop from the zero line indicates the percentage of AIR in the original sample. Aggregate-correction factor may be required for some lightweight aggregates. The rollameter test is more time-consuming than C231 and is reserved for lightweight applications.

   • Air content target ranges + spec verification

     Typical specified air content for freeze-thaw exposure is 5-7% with tolerance ±1.5%. ACI 318 / project specs reference exposure class (F0/F1/F2/F3) — F2/F3 (severe freeze-thaw) requires the higher air content. NON-AIR-ENTRAINED concrete (interior structural, no exposure) targets 1-3% (entrapped air only, no entrainment admixture). The field tech reports the measured air against the SPEC tolerance, not against a generic 6% — read the project spec carefully on the test report. Air content drops over time as the concrete sits in the truck (normal hydration consumes some); always run the air test within the C172 5-minute window from the final sample portion.

   Practice questions (2)

   1. 1. A spec calls for 6% ± 1.5% air content. The field tech runs the C231 pressure-method test on a normal-weight 4,000 psi mix and reads 4.2%. Action?

      • A.Acceptable — within spec tolerance
      • B.Below spec lower limit (4.5%) — REJECT or require admixture adjustment per truck✓ correct
      • C.Above spec — reject
      • D.Switch to C173 volumetric method

      Spec tolerance: 6% − 1.5% = 4.5% lower limit; 6% + 1.5% = 7.5% upper limit. Measured 4.2% is BELOW the lower limit — concrete is non-conforming. Field action: notify the contractor and the ready-mix supplier; the load is typically rejected OR the supplier may add air-entraining admixture and re-test (only if allowed by spec + within elapsed-time limits). (A) misreads the tolerance band. (C) inverts the direction. (D) is wrong — C173 is for lightweight aggregate, not for getting a 'better' reading on normal-weight.

   2. 2. Why is ASTM C173 (volumetric) preferred over C231 (pressure) for LIGHTWEIGHT concrete?

      • A.C173 is faster
      • B.C173 uses a smaller sample
      • C.Lightweight (porous) aggregate compresses under pressure, biasing C231 readings high✓ correct
      • D.C231 cannot be calibrated for lightweight

      Porous aggregate has internal voids that compress when pressurized in a Type B meter, registering as 'air' on the gauge — the apparent air content is HIGHER than the actual entrained air. C173 uses agitation in water (no pressure) so aggregate porosity does not bias the result. (A) is wrong — C173 is SLOWER. (B) sample sizes are similar. (D) is incorrect; C231 calibration is independent of aggregate type, but the systematic bias remains.

4. 04ASTM C138 — Unit weight + yield + gravimetric air

   ~60min

   Unit weight is the density of fresh concrete; yield is the volume of concrete actually produced per batch (compared to design volume — tells the contractor whether they got what they paid for); gravimetric air content is calculated from unit weight as a cross-check on C231/C173.

   • C138 procedure — fill, rod, tap, strike, weigh

     Use the same measuring bowl as C231 if certified for both. (1) Determine the EMPTY WEIGHT of the bowl (Wmt) and its calibrated VOLUME (Vm) — record once per bowl. (2) Fill the bowl in 3 LAYERS, RODDING 25 TIMES PER LAYER + tapping the sides 10-15 times after each layer. (3) Strike off level with a strike-off plate or trowel — slightly heap, then strike. (4) Clean the rim. (5) Weigh the filled bowl (Wm). UNIT WEIGHT = (Wm − Wmt) / Vm, recorded to the nearest 0.1 lb/cu ft. (6) YIELD = (total batch weight from delivery ticket) / unit weight; compare to the BATCH DESIGN VOLUME — typical acceptance ±2%. Less than batch design = short load (overage to be applied to subsequent loads). (7) GRAVIMETRIC AIR CONTENT = (theoretical density − measured density) / theoretical density × 100. The theoretical density is computed from the mix design absolute volumes — should match the C231 reading within ~1%.

     Reference: ASTM C138 Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete

   • Yield — what the contractor actually cares about

     Yield discrepancies hit the GC's pocketbook directly: a 28-yard ordered slab pour that yields 27.5 yards short means the GC has to re-order or the slab is short of design. Typical sources of low yield: (1) ready-mix supplier under-batched (rare; most suppliers over-batch slightly); (2) excessive AIR ENTRAINMENT pushing the volume up artificially per yard at the plant but normalizing in the field; (3) high-slump concrete losing volume during placement; (4) consolidation differences. The C138 yield calculation is the contractor's audit tool. CCM examination heavily features yield calculations: given a mix design and a measured unit weight, calculate yield in cubic yards from a batch ticket weight. Practice the math: 1 cu yd = 27 cu ft.

   Practice questions (1)

   1. 1. A batch ticket shows 8,100 lb total batch weight (cement + water + aggregate + admixture). The field tech measures unit weight at 145 lb/cu ft. What is the yield in cubic yards?

      • A.2.07 cu yd✓ correct
      • B.2.07 cu ft (a unit-confusion error)
      • C.0.55 cu yd
      • D.4.0 cu yd

      Volume produced = batch weight / unit weight = 8,100 / 145 = 55.86 cu ft. Convert to cu yd: 55.86 / 27 = 2.07 cu yd. (B) is the same number with wrong units — this is a common test error where the candidate fails to convert cu ft to cu yd. (C) inverts the division (would imply higher density than weight allows). (D) implies a unit weight of about 75 lb/cu ft, which is below normal-weight concrete by a wide margin.

5. 05ASTM C31 — Making + curing test cylinders

   ~75min

   Compressive-strength cylinders are the contract acceptance test for hardened concrete. The field tech CASTS the cylinders properly; a lab tech later breaks them at 7 and 28 days. A miscast cylinder produces a low strength result that triggers core sampling, project delay, and dispute.

   • Mold selection + filling

     Standard mold size: 6 in. diameter × 12 in. tall (4 in × 8 in. is acceptable per ACI 318 for compressive strengths up to 8,000 psi when noted in the spec). Molds are SINGLE-USE plastic with a tight-fitting lid; reusable steel molds are also acceptable. Place the mold on a level surface NEAR THE CASTING LOCATION and away from vibration. Fill in TWO LAYERS for a 6×12 mold (THREE layers for some mold sizes — check the standard for the chosen size). For each layer, ROD 25 TIMES with the 5/8-inch hemispherical-tip rod. After rodding each layer, TAP the SIDES OF THE MOLD 10-15 TIMES with a rubber mallet to close any rod holes. After the final layer, strike off the top level with the rod or trowel and place the lid (sealed) on the cylinder.

     Reference: ASTM C31 Making and Curing Concrete Test Specimens in the Field

   • Initial curing — first 24-48 hours

     Initial curing is the FIRST 24-48 HOURS after casting (specifically 24 to 48 hours per ASTM C31 unless the project specifies different). The cylinder must be: (1) PROTECTED from FREEZING; (2) maintained at a TEMPERATURE OF 60-80°F (or 68-78°F for high-strength concrete > 6,000 psi); (3) covered to prevent moisture loss. Acceptable methods: insulated curing box with thermostatic control, water-bath cabinet, or a buried/insulated storage location with appropriate temperature data logging. PILING the cylinders in a hot truck cab or in direct sun is a contract violation. After initial curing, the cylinders are TRANSPORTED to a certified lab within ~48 hours; further curing is in a LAB MOIST ROOM at 73 ± 3°F until break date.

   • Cylinder identification + records

     Each cylinder must be labeled with: project ID, location of pour, date + time cast, mix design ID, ticket number, slump and air results from the same sample, and the field tech's initials. Most projects use TYVEK TAGS WIRED to the mold (water-resistant). Unmarked or mis-labeled cylinders break the chain of evidence; a low strength reading on an unidentified cylinder cannot be associated with a specific pour and is functionally useless for spec compliance. The 28-day break is the contract acceptance test (some specs add 56-day break for high-strength or supplementary cementitious materials). Always cast AT LEAST ONE EXTRA cylinder per set as a backup — the standard practice is 4 cylinders per sample (one tested at 7 days, two averaged at 28 days, one held).

   Practice questions (2)

   1. 1. A field tech casts six 6×12 cylinders for a slab pour and stores them on top of the contractor's job-site truck-bed in 95°F direct sun until the lab pickup at end of shift (8 hours). 28-day breaks come back at 3,400 psi (spec was 4,000 psi). Most likely cause?

      • A.Concrete supplier delivered substandard mix
      • B.Cylinders were not subjected to proper INITIAL CURING per ASTM C31; high temperature + sun damaged the specimens✓ correct
      • C.Mold size was wrong
      • D.Slab itself is below strength

      Initial curing at 60-80°F (or 68-78°F for high-strength) is REQUIRED in the first 24-48 hours. Unprotected exposure to 95°F sun for 8 hours causes premature drying, internal stress, and depressed strength readings. The cylinders' low result REFLECTS THE CYLINDER, NOT the slab — coring the slab will likely show in-place strength meeting spec. (A) is presumptuous without re-testing. (C) is unrelated to curing. (D) jumps to the most expensive remediation without ruling out the curing fault first. The proper response is to investigate the chain of custody and curing first, BEFORE accepting the cylinder result as proof of slab failure.

   2. 2. How many times is each layer rodded when filling a 6×12 cylinder mold per ASTM C31?

      • A.10
      • B.15
      • C.25✓ correct
      • D.50

      ASTM C31 specifies 25 RODS PER LAYER with the 5/8-in. hemispherical-tip rod for a 6×12 mold (and for 6×6 cubes, slump cone, C231 air bowl, C138 unit-weight bowl). 4×8 cylinders use a smaller rod and fewer rods per layer — 25 with the small rod for the smaller mold. Memorize 25 as the standard for 6-inch-equipment. (A) and (B) are too few; (D) is too many and over-consolidates.

External resources

• Official
  ACI Concrete Field Testing Technician — Grade I Certification Policies ↗

  ACI's official certification page with the candidate handbook, exam application, study workbook order, and renewal policy. The Grade I Workbook (CP-1) is the canonical study guide and includes all seven ASTM standards in summarized form.

• Official
  ASTM Volume 04.02 (Concrete and Aggregates) ↗

  The ASTM volume containing C172, C143, C231, C173, C1064, C138, and C31 in their authoritative form. Many testing agencies subscribe; individual standards can also be purchased. The performance examiner expects candidate familiarity with the actual standard text, not just summarized flashcards.

• Third-party
  ACI 318 — Building Code Requirements for Structural Concrete ↗

  ACI 318 references the field test standards as the basis for spec compliance and acceptance. Reading Chapter 26 (Construction Documents) gives the field tech context for WHY the test is required and how the result feeds back into the strength acceptance decision.