Study guide
4.5 Filling, Rodding, Lifting, and Measuring Slump
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
4.5 Filling, Rodding, Lifting, and Measuring Slump
Key Takeaways Explain This Topic AI
The cone is filled in three layers of approximately 1/3 the mold volume: the first layer to about 2-5/8 in., the second to about 6-1/8 in., the third overfilled.
Each layer is rodded exactly 25 times, distributed uniformly, with upper layers penetrating about 1 in. into the layer below.
The mold is raised vertically 12 in. in 5 plus or minus 2 seconds with no lateral or twisting motion.
Slump is measured immediately as the difference between the 12 in. mold height and the displaced original center, to the nearest 1/4 in. (5 mm); the test is completed within 2.5 minutes.
Last updated: June 2026
Filling by Volume, Not by Height
Once the slump test begins it must proceed without interruption. The mold is filled in three layers, each approximately one-third of the mold volume . Because the cone is tapered, one-third by volume is not one-third by height. The widely taught fill heights are: first layer to about 2-5/8 in. (67 mm) , second layer to about 6-1/8 in. (155 mm) , and the third layer heaped above the 12 in. top before rodding and strike-off. Scooping concrete to a flat one-third-height mark would under-fill the bottom and over-fill the top, biasing consolidation.
Move the scoop around the perimeter as each layer is added so the concrete is distributed evenly rather than dumped on one side.
The reason for filling by volume is geometric. The cone is wide at the bottom (8 in.) and narrow at the top (4 in.), so equal volumes correspond to shrinking heights as you move up. If a technician instead split the 12 in. height into three equal 4 in. layers, the bottom layer would hold far less than one-third of the volume and the top far more, so the 25 strokes per layer would over-consolidate the bottom and under-consolidate the top. Hitting the ~2-5/8 in. and ~6-1/8 in. marks keeps the energy per unit volume roughly constant.
Rodding Each Layer 25 Times
Each layer is rodded exactly 25 strokes with the 5/8 in. hemispherical-tip rod, distributed uniformly over the cross section of the layer. Rod the bottom layer throughout its depth , holding the rod slightly inclined and making about half the strokes near the perimeter in a spiral toward the center, without forcing the rod against the base. Rod the middle and top layers throughout their depth so the rod just penetrates about 1 in. into the layer below , knitting the layers together.
During the top layer, keep a surplus of concrete above the mold by adding more before rodding is complete if the level drops below the rim, so concrete remains over the top throughout the 25 strokes.
- Layer Fill level before rodding Rod strokes Penetration
- 1 (bottom) About 2-5/8 in. (1/3 volume) 25 Through depth, not into base
- 2 (middle) About 6-1/8 in. (2/3 volume) 25 ~1 in. into layer 1
- 3 (top) Heaped above 12 in. rim 25 ~1 in. into layer 2
All strokes use the rounded end. Stabbing the center repeatedly, hammering the base, or rodding only the wall produces uneven consolidation and a distorted slump.
A useful mental model for distributing strokes: imagine the cross section divided into rings. On each layer, place several strokes near the perimeter, then spiral inward toward the center, so all 25 strokes are spread over the area rather than concentrated. For the bottom layer the rod is inclined slightly near the wall to follow the taper; for the upper two layers the rod is driven straight through the current layer and about 1 in. into the layer beneath so the boundary between layers is consolidated and not left as a weak plane.
Maintaining surplus concrete on the top layer is its own checkpoint. As the rod consolidates the heaped third layer, the surface drops. If it falls below the rim before the 25th stroke, the technician adds concrete to keep a surplus above the mold so the layer stays full while rodding finishes. Only after all 25 strokes are complete is the surface struck off flush. Striking off before the layer is fully rodded, or rodding an under-filled top layer, both bias the slump.
Strike-Off, Lift, and Measure
After the top layer is rodded, strike off the surface flush with the top of the mold using a screeding and rolling motion of the rod, then clean spilled concrete from the base around the mold. Immediately remove the mold by raising it vertically 12 in. (300 mm) in 5 plus or minus 2 seconds , in one smooth motion with no lateral or torsional movement. The whole filling-through-removal operation is performed without interruption and completed within 2.5 minutes .
Measure slump immediately after removal. The slump is the vertical difference between the top of the mold (12 in.) and the displaced original center of the top surface of the specimen . A common technique inverts the empty mold beside the slumped concrete and lays the rod across the top so the ruler can read down to the displaced original center. Record to the nearest 1/4 in. (5 mm).
Worked example: the rod laid across the inverted mold sits at the 12 in. datum; the ruler reads 4-3/16 in. down to the original-center high point of the concrete. Rounded to the nearest 1/4 in., the reported slump is 4-1/4 in.
Sequence memory list:
Fill three layers by volume (~2-5/8 in., ~6-1/8 in., heaped).
Rod each layer exactly 25 strokes; upper layers ~1 in. into the layer below.
Strike off flush and clean the base area.
Lift 12 in. in 5 plus or minus 2 seconds, straight up, no twist.
Measure to the displaced original center, nearest 1/4 in., within 2.5 minutes total.