2.6 Load Indicators, Density Determination & Report Contents

2.6 Load Indicators, Density Determination & Report Contents

The accuracy of a compressive strength result is only as good as the device that measures the load and the completeness of the report that records it. ASTM C39 devotes an entire subsection (Section 6.4, Load Indication) to the requirements for dial and digital load indicators, and the Job-Task Analysis explicitly lists requirements for load-indicating dials, requirements for digital load indicators, determining density of test specimens (when requested), and which information is to be included on reports as separate examinable items. This section teaches each of those requirements in the exact terms the written exam uses.

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Why Load Indication Is Tested Separately

The testing machine can apply a perfectly controlled load, but if the indicator cannot be read precisely or does not hold the peak value, the recorded compressive strength will be wrong. Because concrete is load-rate sensitive and fails suddenly, the technician rarely watches the exact instant of peak load — the machine must capture and hold the maximum for later reading. ASTM C39 therefore sets minimum resolution and accuracy rules for both traditional analog dials and modern digital indicators.

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Dial (Analog) Load Indicators

When the machine registers load on a dial, ASTM C39 Section 6.4.3 requires:

The 100× rule is a favorite exam trap. If the smallest readable increment on the scale is 100 lbf, the verified range does not include any load below 10,000 lbf — a low reading in that region is not trustworthy and must not be reported.

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Digital Load Indicators

Digital indicators are held to an equivalent standard (Section 6.4.4):

• The numerical increment displayed must be equal to or less than 0.10% of the full-scale load of the loading range in use.
• As with dials, the verified loading range must never include loads less than the minimum numerical increment multiplied by 100.
• The indicated load must be accurate to within 1.0% for any value displayed within the verified loading range.
• A true-zero adjustment at zero load must be provided.

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The Maximum-Load (Peak-Hold) Requirement

Whether the machine uses a dial or a digital display, ASTM C39 requires a device that records or indicates, until reset, the maximum load applied to the specimen to within 1.0% system accuracy (Section 6.4.2). Concrete cylinders fail abruptly and the indicated load drops instantly after fracture, so a peak-hold feature is the only reliable way to capture the true $P_{max}$ used in the $f'_c = P/A$ calculation.

[!WARNING] If the peak-hold device is missing or malfunctioning, the technician cannot reliably capture the maximum load, and the machine does not conform to ASTM C39. The examinee is not penalized for faulty equipment on the performance exam, but on the written exam you must recognize the peak-hold requirement as mandatory.

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Determining Density of the Test Specimen (When Requested)

ASTM C39 permits the density (unit weight) of the hardened specimen to be determined and reported when it is requested by the specifier. Key rules:

• Density is determined before the specimen is capped, because capping material adds mass and changes the volume.
• The technician measures the mass of the specimen and computes the volume from the measured average diameter and length (volume of a right circular cylinder, $V = A \times L$).
• Density is then $\rho = m / V$, reported in lb/ft³ [kg/m³].
• Determining density is optional — it is only performed and reported when the project or specification requests it.

This links back to the scope of C39, which applies only to concrete with density greater than 50 lb/ft³ [800 kg/m³]; the requested-density measurement confirms the specimen falls within the standard's applicable range.

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Information Required on the C39 Report

The JTA item know which information is to be included on reports maps to ASTM C39 Section 10 (Report). A compliant compressive strength report includes:

1. Specimen identification number.
2. Diameter (and the two right-angle measurements averaged) and length, in inches or mm.
3. Cross-sectional area.
4. Maximum load carried by the specimen, to the nearest whole number in lbf or N.
5. Compressive strength, computed to the nearest 10 psi [0.1 MPa], with any L/D correction factor applied and noted.
6. Type of fracture (one of the six standard patterns) if other than the usual cone.
7. Age of the specimen at test.
8. Defects in the specimen or caps, and the capping method (sulfur mortar, unbonded caps, or ground ends) when relevant.
9. Density, when it was requested and determined.

Omitting a required field — most commonly the fracture type, the applied L/D correction, or the capping method — is a reporting error even when the arithmetic is correct.

Worked Example: Applying the 100× Verified-Range Rule

A laboratory's dial machine has a 300,000 lbf full-scale range with a smallest readable graduation of 200 lbf.

• Verified-range floor = $100 \times 200\text{ lbf} = 20{,}000\text{ lbf}$.
• A 6×12 in. cylinder (area ≈ 28.27 in²) that fails at a load producing a reading of 15,000 lbf falls below the 20,000 lbf floor.
• Therefore the reading is outside the verified range and must not be reported as a valid strength; the technician should use a lower loading range or a machine whose verified range covers the expected load.

This is exactly why laboratories keep multiple loading ranges: a range appropriate for high-strength 12,000 psi cylinders is too coarse for low-strength 2,000 psi specimens.

Density Calculation Example

A specifier requests density on a 6×12 in. cylinder. Before capping, the technician records:

• Mass = 28.5 lb
• Average diameter = 6.00 in. → area = $\pi(3.00)^2 = 28.27\text{ in}^2$
• Length = 12.00 in. → volume = $28.27 \times 12.00 = 339.3\text{ in}^3 = 0.1964\text{ ft}^3$

$\rho = \frac{28.5\text{ lb}}{0.1964\text{ ft}^3} \approx 145\text{ lb/ft}^3$

This result (~145 lb/ft³) is a normal-weight concrete density, comfortably above the 50 lb/ft³ scope floor.