ASTM C173/C173M Overview
ASTM C173/C173M, "Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method," represents a critical component of Domain 6 in the ACI Concrete Field Testing Technician Grade I certification. This test method provides an alternative approach to measuring air content when the pressure method (ASTM C231) cannot be used due to the presence of highly porous aggregates or other materials that would compromise pressure-based measurements.
While ACI doesn't publish exact percentages, Domain 6 typically accounts for 5-10 questions on the 55-question written exam, making it worth approximately 9-18% of your total score. Combined with the performance demonstration requirement, mastering C173 is essential for certification success.
The volumetric method differs fundamentally from the pressure method covered in Domain 5 by directly measuring air volume through water displacement rather than relying on pressure relationships. This makes it particularly valuable when testing concrete containing lightweight aggregates, slag, or other porous materials that would absorb air under pressure.
When to Use the Volumetric Method
Understanding when to apply ASTM C173 versus other air content measurement methods is crucial for both field practice and success on the challenging ACI exam. The volumetric method becomes the preferred or only viable option in specific circumstances that field technicians must recognize.
Primary Applications
The volumetric method is specifically required when concrete contains aggregates or admixtures that would interfere with pressure-based testing. Lightweight aggregates, such as expanded clay, shale, or slag, contain internal voids that absorb air under pressure, leading to falsely low air content readings with ASTM C231. Similarly, certain chemical admixtures may react with pressurized air or contain components that affect pressure measurements.
Using the wrong air content test method can result in significant measurement errors and potentially unsafe concrete. On the ACI exam, expect questions about method selection based on mix design components and project specifications.
Aggregate Considerations
Normal weight aggregates with absorption rates below 2% typically work well with either method, but as absorption increases, the volumetric method becomes more reliable. Aggregates with absorption rates exceeding 3% almost always require volumetric testing to obtain accurate air content measurements.
| Aggregate Type | Typical Absorption % | Recommended Method | Reason |
|---|---|---|---|
| Dense Limestone | 0.5-1.5% | C231 or C173 | Either method acceptable |
| Sandstone | 2-4% | C173 preferred | Moderate porosity affects pressure |
| Expanded Clay | 10-25% | C173 required | High porosity invalidates pressure method |
| Blast Furnace Slag | 1-8% | C173 recommended | Variable porosity creates uncertainty |
Required Equipment and Apparatus
Proper equipment setup and calibration form the foundation of accurate volumetric air content testing. The ACI performance exam evaluates not only your testing technique but also your equipment knowledge and pre-test verification procedures.
Primary Apparatus
The volumetric air meter consists of a bowl with a capacity of at least 0.075 ft³ (approximately 2.1 L), fitted with a transparent top section graduated to measure air content directly. The apparatus must be equipped with a funnel for water addition, a method for agitating the sample, and provisions for isolating the air content measurement.
Bowl construction typically uses aluminum or stainless steel with smooth interior surfaces to prevent air entrapment. The graduated neck section requires clear markings at 1% intervals with the capability to read to the nearest 0.5%. Rubber gaskets and clamps must create an airtight seal between the bowl and top assembly.
Before each test, verify that all gasket surfaces are clean and undamaged. A poor seal will allow air to escape during testing, resulting in erroneously low air content readings. This inspection step is often evaluated during the ACI performance exam.
Auxiliary Equipment
Additional required equipment includes a tamping rod meeting ASTM specifications (3/8 inch diameter, 12 inches long with rounded ends), a rubber mallet for consolidation, measuring tools for water addition, and cleaning supplies. Many technicians also carry backup equipment to handle field contingencies.
Isopropyl alcohol serves as a critical component for breaking surface tension and ensuring accurate air measurement. The alcohol concentration should be approximately 70% for optimal effectiveness in eliminating foam and surface effects that could compromise measurement accuracy.
Step-by-Step Test Procedure
The ASTM C173 procedure requires careful attention to detail and proper sequencing to achieve reliable results. This section breaks down each critical step that you'll need to demonstrate during the ACI performance examination across all seven domains.
Sample Preparation
Begin with a representative sample obtained according to ASTM C172 sampling procedures from Domain 2. The sample size must be sufficient to fill the volumetric air meter bowl with some excess material remaining. Typically, this requires approximately 0.1 ft³ of concrete, depending on the specific apparatus size.
Remove particles larger than 1.5 inches through wet sieving, collecting the retained material for later calculations. This step ensures that oversized aggregates don't interfere with the consolidation process or create voids that would be mistaken for entrained air.
Complete the air content test within 15 minutes of obtaining the sample to prevent air loss or changes in concrete properties. This timing requirement frequently appears on ACI exam questions and is strictly enforced during performance evaluations.
Filling and Consolidation
Fill the volumetric air meter bowl in two approximately equal layers, consolidating each layer with 25 strokes of the tamping rod. Distribute the tamping uniformly across the surface, ensuring the rod penetrates the previous layer by approximately 1 inch. This consolidation removes large air voids while preserving the intentionally entrained air.
After consolidation, strike off excess concrete level with the rim using a sawing motion with a straightedge. Tap the sides of the bowl lightly to close surface voids created during striking off, then clean the rim thoroughly to ensure proper sealing with the top assembly.
Assembly and Initial Water Addition
Attach the top section of the volumetric air meter, ensuring all gaskets seat properly and clamps achieve uniform pressure. Add water through the funnel until it appears in the graduated neck, recording the initial water level. This baseline reading accounts for the volume of concrete and any initial air present.
Agitation and Air Removal
The critical phase involves vigorous agitation to dislodge entrapped air while avoiding creation of additional air through excessive agitation. Roll, invert, and shake the apparatus for approximately 45 seconds to 2 minutes until no additional air bubbles rise to the surface.
Add isopropyl alcohol through the funnel to break surface tension and eliminate foam. The alcohol addition helps distinguish between entrained air (which remains in the concrete) and entrapped air or foam (which rises to the surface).
Under-agitation fails to remove entrapped air, while over-agitation can break down intentionally entrained air bubbles. Finding the right balance requires practice and forms a key evaluation criterion on the ACI performance exam.
Calculations and Reporting
Accurate calculations distinguish successful ACI candidates from those who struggle with the quantitative aspects of concrete testing. The volumetric method calculations involve several steps that must account for aggregate corrections and proper unit conversions.
Basic Air Content Calculation
The fundamental calculation determines air content as a percentage of the concrete volume. The formula accounts for the difference between the theoretical volume of concrete materials and the actual volume measured in the apparatus:
Air Content (%) = (V₁ - V₂) / V₁ × 100
Where:
- V₁ = Total volume of concrete sample
- V₂ = Volume of concrete materials (cement, aggregates, water)
The apparatus graduation typically provides direct reading capability, but understanding the underlying calculation helps troubleshoot unusual results and demonstrates competency during the ACI examination process.
Aggregate Correction Factor
When coarse aggregate particles larger than 1.5 inches are removed by wet sieving, apply a correction factor to account for the volume reduction. This correction assumes the air content in the removed portion equals that measured in the tested portion:
Corrected Air Content = Measured Air Content × (1 - F)
Where F represents the volume fraction of removed aggregate relative to the original sample volume. This correction ensures results represent the actual concrete mixture rather than the modified test sample.
Common Testing Errors to Avoid
Understanding typical mistakes helps prevent errors during both field testing and the ACI certification exam. These errors often stem from procedural shortcuts or equipment issues that compromise result accuracy.
Procedural Errors
Insufficient consolidation represents the most common procedural error, leaving large air voids that inflate air content readings. Conversely, over-consolidation can drive out entrained air, producing artificially low results. Proper consolidation technique requires consistent tamping pressure and uniform coverage across each layer.
Timing violations frequently occur when technicians attempt to test samples that have begun setting or have been exposed to temperature extremes. These conditions alter the air void system and invalidate test results, regardless of procedural compliance.
Extreme temperatures affect both concrete workability and air void stability. Test concrete at temperatures between 40°F and 80°F when possible, and never test concrete that has begun to set or shows signs of segregation.
Equipment-Related Errors
Leaky gaskets or improper assembly allow air to escape during testing, consistently producing low air content readings. Regular equipment maintenance and pre-test inspection prevent these issues, which can be difficult to detect during testing but become obvious when results consistently fall below expected ranges.
Calibration errors in the graduated neck section affect all subsequent readings. Periodic verification using known volumes ensures measurement accuracy and helps identify equipment problems before they compromise test results.
Volumetric vs Pressure Method Comparison
The relationship between ASTM C173 and ASTM C231 pressure method testing frequently appears on ACI examinations, requiring thorough understanding of each method's advantages, limitations, and appropriate applications.
| Aspect | Volumetric Method (C173) | Pressure Method (C231) |
|---|---|---|
| Test Duration | 15-20 minutes | 5-10 minutes |
| Aggregate Limitations | None - works with all aggregates | Limited by aggregate porosity |
| Precision | ±0.2% typical | ±0.1% typical |
| Equipment Complexity | Moderate | Higher (pressure gauge required) |
| Operator Skill Required | High | Moderate |
| Field Portability | Excellent | Good |
Method Selection Criteria
Aggregate properties primarily drive method selection, but other factors including project specifications, time constraints, and equipment availability also influence the decision. Some specifications require specific test methods regardless of aggregate characteristics, making method familiarity essential for field technicians.
When both methods are acceptable, many technicians prefer the pressure method due to its speed and simplicity. However, the volumetric method provides greater versatility and eliminates uncertainty about aggregate porosity effects on test results.
ACI Exam Preparation Strategy
Success on the ACI Domain 6 examination requires both theoretical knowledge and practical skills demonstration. The written exam tests understanding of procedures, calculations, and troubleshooting, while the performance exam evaluates technique and result interpretation.
Focus equal attention on procedural details and calculation methods. The comprehensive ACI study guide recommends spending 15-20% of your preparation time on Domain 6, given its moderate weight on the overall exam.
Written Exam Focus Areas
Written questions typically cover equipment requirements, procedural sequences, calculation methods, and result interpretation. Pay particular attention to timing requirements, consolidation procedures, and aggregate correction factors, as these topics frequently appear in multiple-choice format.
Mathematical problems may require air content calculations with aggregate corrections or comparisons between different test methods. Practice these calculations until you can complete them quickly and accurately using a non-programmable calculator.
Performance Exam Preparation
The hands-on demonstration evaluates your ability to properly execute each procedural step while explaining the rationale behind critical decisions. Practice the complete procedure multiple times, focusing on smooth transitions between steps and consistent timing.
Expect questions about equipment inspection, troubleshooting unusual results, and method selection criteria. The examiner may present scenarios requiring adaptation of standard procedures to field conditions or specification requirements.
Key Practice Areas
Targeted practice in specific areas strengthens weak points and builds confidence for both examination components. Understanding current ACI pass rates and performance trends helps prioritize study efforts for maximum effectiveness.
Calculation Practice
Work through problems involving various aggregate correction factors and unit conversions. Practice scenarios might include:
- Calculating air content when 15% of coarse aggregate volume is removed
- Converting between different measurement units (ft³, liters, etc.)
- Determining appropriate sample sizes for different apparatus configurations
- Interpreting results that fall outside expected ranges
Procedure Memorization
Develop muscle memory for the complete testing sequence, including timing requirements and quality checkpoints. Create mental checklists covering equipment inspection, sample preparation, testing execution, and result calculation phases.
Practice explaining procedures aloud, as the performance exam requires verbal description of key steps and decision points. This preparation also helps during the written exam when questions ask about procedural rationales or troubleshooting approaches.
Domain 6 connects closely with sampling procedures from Domain 2 and density testing from Domain 4. Review these connections as part of your comprehensive practice testing strategy to understand how different test methods work together in field applications.
Equipment Troubleshooting
Learn to identify and resolve common equipment problems that could compromise test results. Practice scenarios include detecting air leaks, calibrating graduated scales, and adapting procedures when backup equipment differs from primary apparatus specifications.
Understanding equipment limitations helps explain unusual results and demonstrates competency during practical examinations. This knowledge also proves valuable in field situations where perfect conditions rarely exist.
For additional practice opportunities and to test your knowledge across all domains, utilize comprehensive practice tests that simulate actual exam conditions and provide detailed explanations for both correct and incorrect answers.
ASTM C173 uses the volumetric method that directly measures air volume through water displacement, while ASTM C231 uses pressure relationships. C173 works with all aggregate types including porous materials, while C231 is limited to dense aggregates with low absorption rates.
The volumetric method is required when concrete contains highly porous aggregates like expanded clay, slag, or lightweight materials with absorption rates typically exceeding 3%. It's also used when specifications specifically require this method or when pressure equipment is unavailable.
The volumetric method typically achieves precision of ±0.2% for single-operator repeatability and ±0.3% for multi-laboratory reproducibility. Results should be reported to the nearest 0.5% as specified in the standard.
The complete ASTM C173 test typically takes 15-20 minutes from sample preparation through final reading. This includes consolidation, assembly, agitation, and measurement phases. The test must be completed within 15 minutes of sampling to ensure accurate results.
Particles larger than 1.5 inches must be removed by wet sieving before testing. An aggregate correction factor is then applied to account for the volume reduction, assuming the air content in the removed portion equals the measured air content.
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