5.1 Purpose, Equipment, and the Calibrated Measure

ACI Concrete Field Testing Technician Grade I Study Guide Exam

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1 Orientation: Certification Requirements, Written Exam, Performance Exam, and Study Sources

1.1 Credential Purpose and Official Definition 1.2 Eligibility and the Two-Part Certification Path 1.3 Written Exam Format and Dual Passing Rule 1.4 Performance Exam Format and Grading 1.5 Current Study Sources and CP-1 41st Edition 1.6 Validity, Recertification, and Myth Control

2 JTA, Performance Checklists, Common Fail Points, and Field Mindset

2.1 JTA as the Exam Blueprint 2.2 Written Knowledge Versus Performance Skill 2.3 Performance Checklists and the Six Demonstrations 2.4 Timing, Sequence, and Verbal Description Discipline 2.5 Common Fail Points and Result Recording 2.6 Field Mindset, Exam Habits, and Second Trials

3 ASTM C172: Sampling Fresh Concrete and Field Sample Control

3.1 Representative Sampling Purpose and Exam Role 3.2 Sampling Locations and Middle-Discharge Control 3.3 Composite Sample Timing, Remixing, and Protection 3.4 Sample Size, Large Aggregate, and Wet Sieving 3.5 Sequence With Slump, Temperature, Air, Density, and Specimens 3.6 Performance Exam Verbal Description and Field Habits

4 ASTM C1064 Temperature and ASTM C143 Slump

4.1 ASTM C1064 Purpose, Equipment, and Timing 4.2 Sensor Placement, Stabilization, and Reporting 4.3 Temperature Field Errors and Performance Habits 4.4 ASTM C143 Scope, Equipment, and Setup 4.5 Filling, Rodding, Lifting, and Measuring Slump 4.6 Invalid Slump Results, Reporting, and Integrated Habits

5 ASTM C138: Density, Unit Weight, Yield, and Gravimetric Air

5.1 Purpose, Equipment, and the Calibrated Measure 5.2 Sampling, Filling, and Consolidation Choice 5.3 Strike-Off, Cleaning, Mass, and Density 5.4 Yield, Relative Yield, and Batch Comparison 5.5 Gravimetric Air and Theoretical Density 5.6 Reporting, Validity, and Calculation Traps

6 ASTM C231: Pressure Air Method, Type B Meter, and Aggregate Correction

6.1 Scope, Principle, and When Pressure Air Fits 6.2 Type B Meter Equipment, Calibration, and Setup 6.3 Sampling, Bowl Filling, Consolidation, and Strike-Off 6.4 Type B Sequence: Water, Petcocks, Pressure, and Reading 6.5 Aggregate Correction Factor and Final Air Content 6.6 Reporting, Validity, and Invalid-Result Traps

7 ASTM C173: Volumetric Air Method, Rollameter, Alcohol Correction, and Validity

7.1 Scope, Equipment, and When the Volumetric Method Fits 7.2 Sample, Bowl Filling, Consolidation, and Strike-Off 7.3 Water, Alcohol, Initial Liquid Level, and Sealing 7.4 Inversion, Shaking, Rolling, and Air Release 7.5 Reading Agreement, Out-of-Range Steps, and Invalid Results 7.6 Alcohol Correction, Calculation, Reporting, and Performance Habits

8 ASTM C31: Making and Curing Cylinders and Beams, Initial Curing, and Transport

8.1 Purpose, Sample, Equipment, and Specimen Plan 8.2 Cylinder Molding, Consolidation, Finishing, and Identification 8.3 Beam Molding, Flexural Specimens, and Finishing 8.4 Initial Curing, Protection, Temperature, and Moisture 8.5 Standard Curing, Field Curing, Final Curing, and Interpretation 8.6 Transport, Lab Handoff, Reporting, and Common Failures

9 Integrated Field Workflow: Test Order, Documentation, and Quality Control

9.1 Field Sample to Report Workflow 9.2 Test Order and Parallel Timing 9.3 Documentation and Report Accuracy 9.4 Invalid Results and Retest Decisions 9.5 QA/QC Communication and Acceptance Context 9.6 Contamination, Delays, and Handoff Control

10 Written Exam Strategy: Numerical Values, Calculations, and Category Scoring

10.1 One-Hour Written Exam Pacing 10.2 Category Scoring and the No-Compensation Rule 10.3 High-Yield Numerical Values and Units 10.4 Density, Yield, and Gravimetric Air Calculations 10.5 Air Content Corrections and Validity Checks 10.6 Method-by-Method Practice Plan

11 Performance Exam Strategy: Equipment Setup, Examiner Communication, and Remediation

11.1 Performance Exam Map and Mindset 11.2 Equipment Setup, Station Control, and Safety 11.3 Examiner Communication and Speaking the Work 11.4 One-Step Errors, Timing, and Method Control 11.5 C172 Verbal Description Under Pressure 11.6 Remediation, Retest Planning, and Field Confidence

12 Final Review: Recertification, Career Paths, and Next ACI Credentials

12.1 Two-to-Six Week Final Review Plan 12.2 Final Week and Test-Day Checklist 12.3 CP-1, JTA, and Source Control 12.4 Recertification, Five-Year Cycle, and Re-Exam Requirement 12.5 Career Paths After Field Testing Grade I 12.6 Next ACI Credentials and Growth Map

5.1 Purpose, Equipment, and the Calibrated Measure

Key Takeaways Explain This Topic AI

ASTM C138 determines fresh concrete density (unit weight) and from it computes yield, relative yield, cement content, and gravimetric air content.

Density equals the net mass of concrete divided by the volume of the measure: D = (Mc − Mm) / Vm.

The measure is calibrated by the water-fill method, sealing a glass plate over water and dividing water mass by the density of water (62.4 lb/ft³ or ~997–998 kg/m³).

Measures of 0.2 ft³ or larger are recommended; the measure volume must be re-verified at least annually and after any damage.

The mallet weighs 1.25 ± 0.50 lb for measures ≤0.5 ft³ and 2.25 ± 0.50 lb for larger measures.

Last updated: June 2026

What ASTM C138 Does and Why It Matters

ASTM C138/C138M , "Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete," is one of the seven performance tests an ACI Concrete Field Testing Technician Grade I must master. A single field measurement — the density (also called unit weight ) of freshly mixed concrete — feeds four separate calculations:

Yield — the volume of concrete actually produced from a known batch of materials.

Relative yield — whether the batch delivered exactly the volume the mix was designed for.

Cement content — the actual mass of cementitious material per unit volume of concrete produced.

Gravimetric (theoretical) air content — the air percentage derived by comparing measured density to a theoretical air-free density.

Because every one of these answers flows from density, an error of even a few pounds per cubic foot ripples through all four results. That is why C138 procedure precision — filling, consolidating, striking off, and weighing — is tested so heavily on the exam.

The Density Equation

The core relationship the exam expects you to recite is:

D = (Mc − Mm) / Vm

where D is density (lb/ft³ or kg/m³), Mc is the mass of the measure filled with concrete, Mm is the mass of the empty measure, and Vm is the calibrated volume of the measure. The difference (Mc − Mm) is the net mass of concrete alone — the empty measure mass is always subtracted out. Forgetting to subtract Mm is one of the most common conceptual traps.

Worked density example

An empty 0.25 ft³ measure weighs 9.50 lb . Filled and struck off, it weighs 45.50 lb . Net concrete mass = 45.50 − 9.50 = 36.00 lb . Density = 36.00 / 0.25 = 144.0 lb/ft³ . A normal-weight concrete density of 140–150 lb/ft³ is a sanity-check range; a result far outside it signals a procedural or weighing error before you ever report it.

Required Equipment

C138 prescribes specific apparatus, and the exam asks you to match tools to the right measure size:

Equipment Specification
Measure Watertight cylindrical metal container; 0.2 ft³ or larger recommended; volume calibrated and known
Tamping rod 5/8 in. dia × 24 in. (≤0.5 ft³ work) or 3/4 in. × 24 in.; hemispherical rounded tip
Mallet 1.25 ± 0.50 lb for measures ≤0.5 ft³; 2.25 ± 0.50 lb for larger measures

Strike-off plate Flat metal plate ≥1/4 in. thick (or ≥1/2 in. glass/acrylic), at least 2 in. larger than the measure

Balance/scale Accurate to 0.3% of the test load
Internal vibrator ≥9,000 vibrations per minute, used only for low-slump mixes

The measure should be the same one used for the air-pressure (C231) test in many field setups, but its volume must still be independently calibrated for C138.

Calibrating the Measure (Water-Fill Method)

The volume Vm is not assumed — it is calibrated by water . You fill the measure with water at a known temperature, slide a glass plate across the top to seal it with no air bubbles trapped underneath, dry the exterior, and weigh. The net water mass divided by the density of water at that temperature gives the volume:

Vm = (mass of water) / (density of water)

The density of water is approximately 62.4 lb/ft³ (about 997–998 kg/m³ at typical 16–29 °C field temperatures; 1000 kg/m³ only near 4 °C). Using 62.4 lb/ft³ is the standard inch-pound shortcut.

Calibration example

A measure holds 15.60 lb of water (net). Vm = 15.60 / 62.4 = 0.250 ft³ . Calibration is repeated at least annually and whenever the measure is dented, because a deformed measure changes Vm and silently corrupts every density and yield result that follows.

Note the temperature sensitivity: water is densest near 4 °C (about 1000 kg/m³, 62.43 lb/ft³) and slightly less dense at field temperatures. At 60–80 °F (16–27 °C) the density is roughly 997–998 kg/m³ , which is why the metric value is given as a range rather than a flat 1000. For inch-pound calibration, the 62.4 lb/ft³ value is the accepted working figure unless an unusually hot or cold day demands the temperature-corrected number.

Why Density Drives Everything Else

It is worth pausing on how central a single number is to this method. Once D is in hand, yield , relative yield , cement content , and gravimetric air content all follow by arithmetic — no further field measurement is required. This makes C138 efficient but unforgiving: a 2% error in density (about 3 lb/ft³ on a normal mix) becomes a roughly 2% error in yield and shifts the computed air content by close to a full percentage point.

That sensitivity is why the standard is so prescriptive about consolidation, strike-off, cleaning, and calibration. Every one of those steps exists to make (Mc − Mm)/Vm represent the true concrete as placed. A technician who treats the procedure casually — heaping the measure, leaving slurry on the outside, or trusting an uncalibrated stamped volume — is not just risking the density number; they are corrupting four downstream quality-control values that the producer and owner rely on. Keep that chain in mind: clean apparatus and disciplined procedure are the whole point of the method.