Free ACI Strength Testing Exam Flashcards

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.

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.

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.

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.

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.

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.

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).

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.

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

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.

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

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.

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.

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.

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.

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.

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.

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."

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.

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.

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.).

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).

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

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).

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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

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.

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.

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.

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).

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.

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.

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.

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).

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.