3.3 Composite Sample Timing, Remixing, and Protection

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

3.3 Composite Sample Timing, Remixing, and Protection

Key Takeaways Explain This Topic AI

The elapsed time between obtaining the first and final portions of the composite sample must not exceed 15 minutes.

Slump, temperature, and air-content tests must be started within 5 minutes after obtaining the final portion of the composite sample.

After transport, the sample is combined and remixed with a shovel only enough to make it uniform, never retempered with water.

The sample must be protected from sun, wind, rapid evaporation, and contamination throughout sampling and testing.

Last updated: June 2026

The 15-Minute Composite Window

C172 timing begins the instant the first portion of the composite sample is obtained. The elapsed time between the first and final portions must not exceed 15 minutes . This limit prevents the composite from becoming a blend of concrete taken at materially different ages. Fresh concrete keeps hydrating, loses or gains moisture, changes temperature, and stiffens as time passes, so portions taken 25 minutes apart would not represent the same concrete.

In practice, the 15-minute window rarely feels tight when sampling a single truck, because a few portions are collected during one discharge. It becomes important when discharge is interrupted, when the technician is also setting up equipment, or when sampling spans a slow pour. The rule is a hard ceiling: if more than 15 minutes pass between the first and last portion, the composite is no longer valid and the technician should start over.

The 5-Minute Test-Start Window

Once the final portion is obtained, the clock tightens for the fresh-property tests. Slump, temperature, and air-content testing must be started within 5 minutes after obtaining the final portion of the composite sample. Strength-specimen molding has its own start requirement tied to fabricating the composite (covered in the next section), but the habit is the same: do not let the sample wait. Waiting to wash tools, hunt for a thermometer, or assemble an air meter can consume the 5-minute window and let the sample age before the test even begins.

Action C172 control Field meaning
First to final portion Maximum 15 minutes Collect all portions promptly
Final portion to slump start Within 5 minutes Have the slump cone damp and ready
Final portion to air start Within 5 minutes Pre-assemble and clean the air meter
Final portion to temperature Within 5 minutes Have the thermometer verified and at hand
Composite handling Remix to uniformity Remove portion-to-portion bias
Transport, Remix, and Protect

After the portions are carried to the testing location, the technician combines and remixes the sample with a shovel only enough to make it uniform. The goal is uniformity, not retempering. Adding water, overworking the concrete, letting it segregate, or shuttling it between containers all change the very properties the downstream tests are meant to measure. Remixing corrects the portion-to-portion variation that exists right after collection; it is not permission to adjust the mix.

Sample protection is mandatory, not optional. C172 requires that the sample be protected from sun, wind, rapid evaporation, and contamination . Direct sun raises temperature and dries the surface; wind strips moisture from a shallow pan quickly; rain adds water; and dust, soil, form oil, or wash water contaminate the concrete. Use clean equipment, shield the sample when conditions are harsh, and keep the container close to the test area so transport time stays short.

A practical exam habit is to stage equipment before sampling : the slump cone damp and ready, the thermometer verified, the air meter assembled and clean, and the cylinder molds positioned where they can be filled on time. C172 does not stand alone; it launches a timed sequence that must support every later method.

Verbal sequence under exam pressure:

Keep the first-to-final portion interval within 15 minutes.

Combine the portions in a clean receptacle and remix with a shovel to uniformity.

Protect the sample from sun, wind, evaporation, and contamination.

Start slump, temperature, and air-content tests within 5 minutes of the final portion.

Why the Clocks Are So Tight

The 15-minute and 5-minute limits are not arbitrary; they exist because fresh concrete changes measurably on that timescale. Slump loss begins almost immediately as cement hydrates and aggregate absorbs water, so a slump measured 15 minutes late can read an inch or more lower than the true value at the point of sampling. Air content can change as the concrete is handled, jostled, and allowed to sit, and entrained-air bubbles can coalesce or escape. Temperature drifts toward ambient. Each delay therefore moves the measured value away from the property the concrete actually had when it left the truck.

Because the tests evaluate the concrete as delivered, the standard pins the measurement to a narrow window after sampling. The 5-minute rule protects the as-sampled condition. A technician who starts slump at minute 9 has reported a property the concrete no longer has.

Two Clocks, Not One

A frequent point of confusion is treating C172 as having a single time limit. There are two distinct clocks that start at different events:

Clock 1 (15 minutes): runs from the first portion to the final portion of the composite. It governs collection.

Clock 2 (5 minutes): runs from the final portion to the start of slump, temperature, and air tests. It governs the fresh-property tests.

A third limit (15 minutes from fabricating the composite to the start of molding strength specimens) is covered with sample size in the next section.

Clock Starts at Ends at Limit
Collection First portion obtained Final portion obtained 15 min max
Fresh-test start Final portion obtained Slump/temp/air begins 5 min max
Specimen molding Composite fabricated Molding begins 15 min max

Keeping the three straight is a classic written-exam discriminator. Identify which event just happened (last portion obtained? composite fabricated?) and apply the matching clock. The 5-minute window is only for slump, temperature, and air; do not apply it to cylinder molding.