ACI Strength Testing Exam Flashcards

Q: What does the ACI Concrete Strength Testing Technician certification cover?

Four ASTM practices for laboratory strength testing of hardened concrete: C39/C39M (compressive strength of cylinders), C78/C78M (flexural strength, third-point loading), C617/C617M (sulfur mortar capping), and C1231/C1231M (unbonded neoprene caps). It does not cover field sampling of fresh concrete — that is the separate ACI Concrete Field Testing Technician Grade I credential (ASTM C31).

Q: What is the ACI Strength Testing exam format?

Two parts. The written exam is closed-book, one hour, about 40 multiple-choice questions, with at least 8 questions on each of the four ASTM practices. The performance exam requires the candidate to physically demonstrate C39, C78, C617, and C1231 procedures on real equipment for an examiner.

Q: What is the passing score for the ACI Strength Testing exam?

At least 70% overall on the written exam AND at least 60% on each of the four individual ASTM sections. A candidate scoring 85% overall but only 55% on the C617 section still fails. The performance exam is pass/fail on completing every required step correctly.

Q: How long is ACI Concrete Strength Testing Technician certification valid?

Five years from the date both the written and performance exams are successfully completed. There is no continuing-education renewal path — technicians must retake both exams to recertify after 5 years.

Q: What is the most heavily tested fact on the ASTM C39 portion of the exam?

The compression loading rate: 35 ± 7 psi/s (0.25 ± 0.05 MPa/s), meaning 28 to 42 psi/s, applied during the second half of the anticipated loading phase. A faster rate can artificially inflate the measured strength.

Q: Are there prerequisites for the ACI Strength Testing Technician exam?

No formal prerequisites — anyone may register through a local ACI sponsoring chapter. Hands-on familiarity with compression machines, flexural beam fixtures, sulfur capping equipment, and neoprene pad retainers is strongly recommended before attempting the performance exam.

Question 1

ASTM C39: what is the required compression loading rate?

Accepted Answer

35 ± 7 psi/s (0.25 ± 0.05 MPa/s), i.e. 28 to 42 psi/s, applied continuously and without shock during the second half of the anticipated loading phase. A faster-than-required rate can artificially inflate measured strength.

Question 2

Cross-sectional areas of the two standard cylinder sizes used in ASTM C39

Accepted Answer

6 in. × 12 in. cylinder: area = π(3)² ≈ 28.27 in². 4 in. × 8 in. cylinder: area = π(2)² ≈ 12.57 in². Compressive strength f'c = maximum load ÷ cross-sectional area.

Question 3

ASTM C39: to what precision is compressive strength reported?

Accepted Answer

To the nearest 10 psi (0.1 MPa). Strength is calculated from the maximum (peak) load reached during the test, not the first-crack load or a post-peak average.

Question 4

ASTM C39 length-to-diameter (L/D) correction factor table

Accepted Answer

L/D = 1.75 → 0.98; L/D = 1.50 → 0.96; L/D = 1.25 → 0.93; L/D = 1.00 → 0.87. The calculated strength is multiplied by this factor; interpolate for intermediate ratios.

Question 5

ASTM C39: minimum allowable L/D ratio, and when do correction factors apply?

Accepted Answer

Minimum allowable L/D is 1.00 — below that, results are not usable. Correction factors from the C39 table apply when L/D (after end preparation) is between 1.00 and 1.75; at L/D > 1.75 no correction is needed.

Question 6

ASTM C39: specimen end planeness tolerance

Accepted Answer

Ends must be plane within 0.002 in. (0.050 mm). Ends exceeding this tolerance must be sawed, ground, or capped (per C617 or C1231) before testing.

Question 7

ASTM C39: specimen end perpendicularity tolerance

Accepted Answer

The ends must not depart from perpendicularity to the cylinder axis by more than 0.5° — approximately 1/8 in. in 12 in. (3 mm in 300 mm).

Question 8

ASTM C39 fracture Type 1

Accepted Answer

Reasonably well-formed cones on both ends, with less than 1 in. (25 mm) of cracking through the caps — the classic "double cone" failure, typical of a well-prepared specimen.

Question 9

ASTM C39 fracture Type 2

Accepted Answer

A well-formed cone on one end plus vertical cracks running through the caps, with no well-defined cone on the other end.

Question 10

ASTM C39 fracture Type 3

Accepted Answer

Columnar vertical cracking through both ends, with no well-formed cone on either end — often points to a capping or end-preparation problem when it recurs.

Question 11

ASTM C39 fracture Type 4

Accepted Answer

A diagonal fracture with no cracking through the ends of the specimen.

Question 12

ASTM C39 fracture Types 5 and 6

Accepted Answer

Type 5: side fractures at the top or bottom of the cylinder. Type 6: similar to Type 5 but more pointed. Both occur more often with unbonded caps and can signal damaged pads, incorrect Shore A hardness, or eccentric/misaligned loading.

Question 13

ASTM C39: how is cylinder diameter measured?

Accepted Answer

To the nearest 0.01 in. (0.25 mm), by averaging two diameters measured at right angles to each other at about mid-height of the specimen; this average is used to compute the cross-sectional area.

Question 14

ASTM C39: minimum specimen diameter relative to aggregate size

Accepted Answer

The specimen diameter must be at least 3 times the nominal maximum size of the coarse aggregate, so no single aggregate particle disproportionately influences the result.

Question 15

ASTM C39 (via ASTM E4): testing machine verification requirements

Accepted Answer

The machine must be verified at least annually, within one year of the last verification, and after any repair, adjustment, or relocation. Accuracy must be within ± 1.0% of the indicated load over the verified loading range.

Question 16

ASTM C39: what moisture condition must a specimen be tested in?

Accepted Answer

The moist condition — kept surface-wet with a wet burlap or blanket from the moment of removal from curing until tested. Allowing the specimen to dry before testing significantly lowers the measured strength.

Question 17

ACI 318 acceptance criteria for compressive strength test results

Accepted Answer

Investigation is triggered if ANY individual strength test is more than 500 psi below f'c (or more than 0.10 × f'c when f'c > 5,000 psi), OR if the average of any 3 consecutive tests is below the specified f'c. Both conditions must be satisfied for concrete to be considered acceptable.

Question 18

Per ACI 318, what constitutes one "strength test"?

Accepted Answer

The average compressive strength of at least two 6 in. × 12 in. cylinders (or at least three 4 in. × 8 in. cylinders) broken at the same test age. A single cylinder result is not considered a valid "strength test."

Question 19

ASTM C78: modulus of rupture formula when fracture is within the middle third of the span

Accepted Answer

R = PL / (b × d²), where P = maximum applied load, L = span length, b = average width, d = average depth at the fracture — the standard third-point loading formula.

Question 20

ASTM C78: modulus of rupture formula when fracture is outside the middle third (but within 5% of the span)

Accepted Answer

R = 3Pa / (b × d²), where a = the average distance from the fracture line to the nearest support on the tension face. If fracture occurs more than 5% of the span outside the middle third, the result is discarded.

Question 21

ASTM C78: required span-to-depth relationship for the test beam

Accepted Answer

The span length must equal 3 times the beam depth, within a 2% tolerance. For a standard 6 × 6 × 21 in. beam, that gives an 18 in. span (3 × 6 in.).

Question 22

ASTM C78: required stress loading rate

Accepted Answer

During the second half of the anticipated loading, load is applied at a rate that constantly increases the extreme fiber stress between 125 and 175 psi/min (0.9 to 1.2 MPa/min).

Question 23

ASTM C78: to what precision is modulus of rupture reported?

Accepted Answer

To the nearest 5 psi (0.05 MPa) — finer than the 10 psi reporting precision used for ASTM C39 compressive strength.

Question 24

ASTM C78: spacing of the third-point loading points

Accepted Answer

The two load-applying blocks sit at the 1/3 points of the span, so they are 1/3 of the span length apart and each is 1/3 of the span from the nearest support (6 in. apart on an 18 in. span).

Question 25

ASTM C78: required moisture condition for beam testing

Accepted Answer

Beams are tested in the moist condition immediately after removal from curing; the tension surface must be kept wet (e.g., wet burlap) until tested. Drying the tension face sharply reduces the measured modulus of rupture.

Question 26

ASTM C78: when is a beam flexural result discarded outright?

Accepted Answer

When the fracture initiates on the tension surface more than 5% of the span length outside the middle third. Fractures within the middle third use PL/(bd²); fractures within 5% outside it use 3Pa/(bd²).

Question 27

ASTM C78: how are beam width (b) and depth (d) measured for the calculation?

Accepted Answer

Across the fractured cross-section after testing — one measurement at each edge and one at the center, each to the nearest 0.05 in. (1 mm) — then averaged. Nominal mold dimensions may not be substituted.

Question 28

How does modulus of rupture typically compare to compressive strength for normal-weight concrete?

Accepted Answer

Modulus of rupture is roughly 10-15% of compressive strength, approximated as MR ≈ 7.5√f'c (psi). For f'c = 4,000 psi, MR ≈ 474 psi.

Question 29

ASTM C617: planeness tolerance of a finished sulfur mortar cap

Accepted Answer

The finished cap must be plane within 0.002 in. (0.05 mm) in 6 in. — the same tolerance ASTM C39 requires for uncapped specimen ends. Caps failing this must be removed and remade.

Question 30

ASTM C617: sulfur mortar cap thickness limits

Accepted Answer

Average cap thickness should be about 1/4 in. (6 mm), with no point exceeding 5/16 in. (8 mm). A cap thicker than 5/16 in. at any point must be remade — over-thick caps deform under load and depress measured strength.

Question 31

ASTM C617: minimum sulfur mortar cap hardening times before testing

Accepted Answer

At least 2 hours for cylinders expected below 5,000 psi (35 MPa); at least 16 hours for cylinders expected at 5,000 psi (35 MPa) or greater. Testing too soon on high-strength concrete gives falsely low results.

Question 32

ASTM C617: target temperature of molten sulfur mortar for capping

Accepted Answer

Approximately 265°F (130°C), per the manufacturer's guidance — never exceed the manufacturer's stated maximum. Overheating causes the sulfur to polymerize and weakens the cap.

Question 33

ASTM C617: why must the cylinder end be dry before sulfur capping?

Accepted Answer

Free water on the end surface causes the molten sulfur to foam and steam, producing a weak, defective cap. The end must be dry (or dried) before capping — never saturated or left damp.

Question 34

ASTM C617: capping plate planeness tolerance and material

Accepted Answer

The plate's working (pouring) surface must be plane within 0.002 in. (0.05 mm) in 6 in., checked with a straightedge and feeler gauge. Plates are made of steel, machined glass, or similarly hard, rigid, non-reactive material.

Question 35

ASTM C617: minimum capping plate diameter relative to the cylinder

Accepted Answer

The capping plate must be at least 1 in. (25 mm) greater in diameter than the specimen being capped, so molten sulfur fills the cavity without the cylinder overhanging the plate.

Question 36

ASTM C617: perpendicularity tolerance of the capping apparatus alignment bar

Accepted Answer

The vertical alignment bar must be perpendicular to the base plate within 0.5° — about 1/8 in. in 12 in. — matching the perpendicularity required of the finished cylinder end.

Question 37

ASTM C617: reuse limits for sulfur mortar capping compound

Accepted Answer

For cylinders below 5,000 psi, the pot may be recharged so the oldest material is not used more than 5 times. For cylinders at or above 5,000 psi (35 MPa), reuse of recovered/old cap material is NOT permitted — fresh compound is required.

Question 38

ASTM C617: why does sulfur capping require ventilation and PPE?

Accepted Answer

Molten sulfur mortar contaminated with organic material (paraffin, oil) can emit hazardous hydrogen sulfide (H₂S) gas, whose rotten-egg odor fatigues quickly and can't be relied on as a warning. Capping requires a ventilated hood/exhaust plus heat-resistant gloves and eye protection.

Question 39

ASTM C1231: compressive strength range where unbonded caps are permitted

Accepted Answer

Unbonded caps may be used for acceptance testing between 1,500 psi (10 MPa) and 12,000 psi (80 MPa). Outside that range, unbonded caps are not permitted without additional qualification.

Question 40

ASTM C1231: neoprene pad Shore A durometer vs. concrete strength matching

Accepted Answer

50 durometer: 1,500–6,000 psi (10–40 MPa). 60 durometer: 2,500–7,000 psi (17–50 MPa). 70 durometer pads are used for higher-strength concrete above these ranges. Using too-hard a pad on low-strength concrete can cause unrepresentative side/point fractures (Types 5/6).

Question 41

ASTM C1231: tolerance on neoprene pad Shore A durometer hardness

Accepted Answer

± 5 points. A pad labeled 60 durometer may measure anywhere from 55 to 65 and still meet the standard.

Question 42

ASTM C1231: nominal neoprene pad thickness

Accepted Answer

1/2 in. (13 mm), ± 1/16 in. (about ±2 mm). Pads outside this range don't function properly — too thin can't compensate for end irregularities; too thick loses lateral confinement.

Question 43

ASTM C1231: retainer ring dimensional requirements

Accepted Answer

Inside diameter must be between 102% and 107% of the specimen diameter (to allow the pad to bulge laterally under load), and the cavity depth must be at least twice the pad thickness. The ring must be steel or another hard, rigid, non-deforming material.

Question 44

ASTM C1231: reuse and discard rules for neoprene pads

Accepted Answer

Unqualified pads may be reused up to about 100 times within their hardness/strength range. Inspect every pad before each use, and discard/replace it if it shows cracks or splits exceeding 3/8 in. (10 mm) in length, regardless of depth.

Question 45

ASTM C1231: where must unbonded caps be applied on a specimen?

Accepted Answer

On both ends of the cylinder for a valid test, unless one end is already bonded-capped per ASTM C617 (mixing one sulfur cap with one unbonded cap is permitted).

Question 46

ACI Concrete Strength Testing Technician: exam structure

Accepted Answer

Two required parts: a closed-book written exam (about 40 multiple-choice questions, 1 hour, with at least 8 questions on each of the four ASTM practices) and a hands-on performance exam demonstrating C39, C78, C617, and C1231 procedures on real equipment.

Question 47

ACI Concrete Strength Testing Technician: passing requirements

Accepted Answer

Written exam: at least 70% overall AND at least 60% on each individual ASTM section. Performance exam: pass/fail, requiring every step of each procedure to be completed correctly. Both the written and performance exams must be passed to certify.

Question 48

How long does ACI Concrete Strength Testing Technician certification remain valid?

Accepted Answer

5 years from the date both the written and performance exams are successfully completed. There is no continuing-education renewal — technicians must retake both exams to recertify.

Question 49

Which standard governs lab accreditation and personnel qualification for concrete strength testing?

Accepted Answer

ASTM C1077 (Standard Practice for Laboratories Testing Concrete and Concrete Aggregates) sets minimum lab requirements. It requires personnel performing strength testing to hold ACI Concrete Strength Testing Technician certification (or an equivalent recognized qualification).

Question 50

What does the ACI Concrete Strength Testing Technician certification NOT cover?

Accepted Answer

Field sampling and testing of fresh concrete (slump, air content, temperature, making and curing field specimens per ASTM C31) is outside this credential's scope. That is covered by the separate ACI Concrete Field Testing Technician Grade I certification.

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