3.2 Sampling Locations and Middle-Discharge Control

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

3.2 Sampling Locations and Middle-Discharge Control

Key Takeaways Explain This Topic AI

For stationary and revolving-drum truck mixers, collect two or more portions at regularly spaced intervals from the middle of the batch.

Do not sample the first or last portions of the discharge, which are not representative of the batch average.

For paving concrete, obtain the sample from at least five different portions of the discharged pile, then composite.

The container must intercept the entire discharge stream or a fully diverted stream so mortar and coarse aggregate stay in proportion.

Last updated: June 2026

Matching the Procedure to the Concrete Source

Most ACI candidates picture C172 sampling at a ready-mix truck, and that is the most common field case, but C172 gives a procedure for each delivery type. The unifying concept is source control : whether concrete comes from a stationary mixer, a revolving-drum truck mixer, an open-top or agitating truck, or a paving operation, the technician must obtain concrete that represents the batch and is not contaminated or segregated by the act of sampling.

For stationary mixers and revolving-drum (truck) mixers , the rule is the same: sample the concrete by collecting two or more portions at regularly spaced intervals during discharge of the middle portion of the batch . The first concrete out can be affected by startup flow, chute wetting, or material that has not fully mixed. The last concrete can be affected by tailing, segregation, and cleanup. By concentrating on the middle of the batch and avoiding the very first and very last material, the composite represents the batch average. As a working rule of thumb, do not sample from the first or last 10 percent of the load.

For paving concrete discharged onto the grade or into a hopper, the technician collects the sample from at least five different portions of the pile after the mixer is fully discharged, then combines them into one composite. Five well-spread portions are needed because a paving discharge spreads and segregates more than a chute stream.

Concrete source C172 sampling rule Bias it avoids
Stationary mixer Two or more portions, middle of batch First/last unrepresentative material
Revolving-drum truck mixer Two or more portions, middle of discharge Startup and tailing variation
Paving concrete At least five portions of the discharged pile Spread and segregation of the pile
Any source Capture full stream / full diverted flow Loss of mortar or coarse aggregate
Capturing the Stream Without Bias

The container matters as much as the timing. The receptacle should intercept the entire discharge stream , or receive a fully diverted stream, so mortar and coarse aggregate stay in their true proportions. Scooping from one edge of the chute can bias the sample because the stream is not uniform across its width. Letting aggregate bounce out of the pan or paste wash away changes the material before any test begins.

In the verbal performance exam, do not rush past the sampling location. Many weak answers say only that the technician gets concrete from the truck and starts testing. A stronger answer states where in the discharge the portions come from, that two or more portions are taken (or five for paving), and why the first and last material are avoided. That shows the examiner you are describing the ASTM practice, not a habit from one jobsite.

Sampling must also be coordinated with the placing crew. The technician needs room and time to collect portions without interfering with placement. The field habit is to choose the sampling point before concrete is flowing; waiting until the chute is already discharging leads to rushed or unrepresentative sampling.

Performance-exam checklist:

Identify the concrete source and sampling point before discharge starts.

For mixers, take two or more portions from the middle of the batch; for paving, at least five portions of the pile.

Avoid the first and last material of the discharge.

Capture the full stream into a clean, damp, nonabsorbent container.

Combine, remix, protect, and move immediately into the timed tests.

When the Truck Will Not Cooperate

Real discharges are not textbook-clean, and the performance examiner may probe how you adapt. If the drum is turning too slowly to discharge, the technician asks the driver to bring the truck to mixing speed for the number of revolutions the delivery ticket or specification requires before sampling, because concrete that has not been remixed in the drum is not representative. If discharge is interrupted, the technician must still keep the first-to-final portion interval within 15 minutes ; if the gap will blow that window, the right move is to discard and start a fresh composite rather than report a stale blend.

A common trap is sampling to adjust the mix rather than for acceptance . C172 is for obtaining a representative sample for the standard tests. It is not a tool for fishing out the wettest concrete to make a slump pass. If the technician suspects the load is wrong, the answer is to sample correctly, report the true result, and let the responsible party decide.

Equipment That Keeps the Sample Honest

The sampling equipment itself can introduce error if it is wrong for the job. Containers and tools must be clean, damp, and nonabsorbent . A dry wooden or absorbent container pulls water out of the surface paste and can shift slump and air. A dirty pan adds contamination. The receptacle must be large enough to catch the entire stream for a truck or to hold the combined portions for paving without overflowing.

Tool Correct condition Error if wrong
Sampling receptacle Clean, damp, nonabsorbent, large enough Absorbs water, contaminates, overflows
Wheelbarrow / pan Smooth, mortar-tight Mortar leaks, aggregate bounces out
Shovel Clean, used only to remix to uniformity Over-mixing or contamination
Cover/shield Available before sampling Sun and wind change the sample

Thinking about equipment in advance is part of source control. A technician who shows up with a damp, clean, properly sized receptacle has already removed several ways the sample could go wrong before the truck arrives.