Study guide
9.4 Invalid Results and Retest Decisions
ACI Concrete Field Testing Technician Grade I Study Guide Exam
Track your progress as you read
Finished sections get a check mark.
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
9.4 Invalid Results and Retest Decisions
Key Takeaways Explain This Topic AI
Invalid results trace to sample problems, equipment problems, procedural error, blown C172 time windows, or genuine out-of-spec concrete.
Under ASTM C94 a single retest is permitted: if a slump or air test fails, retest a new portion, and only the average of two failing tests confirms nonconformance.
C94 allows tempering water only up to the design water-cementitious ratio, mixed at least 30 revolutions, and all water additions completed within the discharge window.
A questionable result is handled through the applicable ASTM method and the project communication chain, never by editing the number until it looks acceptable.
Last updated: June 2026
Treat Invalid Results as Quality Events
A field test can be performed carefully and still produce an invalid result. Distinguish two situations the exam tests hard:
Invalid test — the procedure or sample was compromised (blown 5-minute window, leaking air meter, dropped slump cone, segregated sample). The number does not represent the concrete and must be discarded and the test repeated when possible.
Out-of-spec result — a valid test shows the concrete is outside the specification limit (slump too high, air too low). The number is real and must be reported and escalated , not erased.
Common causes of invalid results include: a non-representative sample (drawn from the first/last 10 percent, or not remixed); equipment faults (uncalibrated air meter, damaged slump cone, leaking pressure gauge); procedural error (wrong rod count, wrong layer count, improper strike-off); and timing failures where the test began outside the C172 window.
The ASTM C94 Single-Retest Rule
When a fresh-property test fails, the technician does not condemn the load on one reading. ASTM C94 permits a single retest: if the result of a slump or air-content test falls outside the specified tolerance, a second test is run on a new portion of the same sample. The concrete is considered to fail only if the second test also fails — and many specifications evaluate the failure based on the second test, not an average that masks a bad batch. The key exam point is that one failing fresh test is not automatic rejection; a confirming retest is required.
- Property C94 tolerance (typical) On a fail
- Slump (target) ±0.5 to ±1.5 in. by target value Retest one new portion
- Air content ±1.0 percentage point of specified Retest one new portion
- Discharge time Stated on ticket (no default 90 min) Reject if exceeded
If the retest also fails, the concrete is reported as nonconforming and the decision passes up the QA/QC chain.
Water Additions and Never "Fixing" the Number
A tempting but wrong response to low slump is unauthorized water. ASTM C94 limits jobsite water: water may be added only up to the maximum design water-cementitious ratio , the addition must be mixed at least 30 revolutions at mixing speed, all water additions must be completed within the discharge time limit , and the fresh tests must be repeated after the addition. Every gallon must appear on the ticket. Adding water beyond the design w/cm to chase a slump number is a quality failure that weakens the concrete.
The cardinal rule is that a questionable reading is handled through the applicable ASTM method and the project communication chain , never by editing the number. If the air meter leaks, the result is invalid and the meter is repaired or replaced before retesting — the technician does not nudge the gauge to a plausible value. Documenting why a result was invalid (and what was done about it) is itself an act of quality control, because it protects everyone who relies on the record.
A Decision Tree for a Questionable Result
When a number looks wrong, the technician works through a disciplined sequence rather than reacting emotionally:
Was the sample representative? Drawn from the middle of the batch, remixed, not from the first/last 10 percent? If not, the result is invalid — obtain a new portion or sample.
Was the equipment sound and calibrated? A leaking air meter, dented cone, or out-of-tolerance scale invalidates the result. Repair or replace, then retest.
Was the procedure correct? Right number of layers and rod strokes, proper strike-off, correct lift time on the slump cone, test begun inside the C172 window? A procedural slip means repeat.
If sample, equipment, and procedure were all sound, the result is VALID — and if it is outside spec, it is an out-of-spec finding to report , applying the C94 single-retest rule before any nonconformance is declared.
Out-of-Spec Versus Invalid: A Critical Distinction
Mixing up these two leads to bad outcomes. " Treating an invalid result as out-of-spec unfairly condemns good concrete and triggers needless investigation. The exam tests whether the candidate can tell them apart from the cause: a blown time window, contamination, or broken equipment makes a test invalid ; a clean test that exceeds the slump or air limit is out-of-spec but real . In both cases the data and the response are documented, but the path differs — discard and repeat for invalid, report and escalate (after one retest) for out-of-spec.
Never resolve either by quietly editing the number; that converts a recoverable quality event into falsified records.
The single-retest provision exists precisely so a technician does not have to choose between two bad options — accepting a marginal first reading or fabricating a better one. When slump or air fails, the disciplined move is simply to draw a fresh portion and run the test again , then let the two valid results speak. This protects the technician as much as the project: a documented retest sequence is defensible, whereas an unexplained adjusted number is not, and it keeps a single anomalous reading from either condemning sound concrete or excusing a genuinely defective batch.