Simplified Guide for Testing Aggregates for Concrete (IS 2386 Part IV)
This guide explains the mechanical property tests for aggregates as per IS 2386 (Part IV) - 1963, focusing on aggregate crushing value, aggregate impact value, abrasion value, and crushing strength. These tests assess the strength and durability of aggregates, critical for ensuring concrete quality in construction projects like buildings, roads, and bridges.
Why Test Mechanical Properties of Aggregates?
Aggregates constitute 60-75% of concrete’s volume, and their mechanical properties directly influence the concrete’s strength, durability, and resistance to wear. Testing ensures aggregates can withstand the stresses of mixing, placement, and service conditions.
1. Aggregate Crushing Value (ACV) Test
What It Measures
The aggregate crushing value indicates the resistance of an aggregate to crushing under a gradually applied compressive load. A lower ACV indicates a stronger aggregate.
Why It Matters
- Aggregates with high crushing strength are essential for high-strength concrete used in structures like bridges or high-rise buildings.
- Weak aggregates can lead to concrete failure under load.
How to Test
- Apparatus:
- A 15-cm diameter open-ended steel cylinder with a plunger and base plate (or a 75-mm cylinder for smaller aggregates), as shown in Fig. 1 of the standard.
- A tamping rod (16 mm diameter, 45-60 cm long, rounded at one end).
- A compression testing machine capable of applying 40 tons in 10 minutes.
- IS sieves (12.5 mm, 10 mm, and 2.36 mm).
- A balance accurate to 1 gram.
- Procedure:
- Use aggregates passing the 12.5-mm sieve and retained on the 10-mm sieve, in a surface-dry condition. If dried, heat at 100-110°C for no more than 4 hours and cool to room temperature.
- Fill the cylinder in three equal layers, tamping each layer 25 times with the tamping rod to a depth of 10 cm.
- Level the surface, weigh the sample, and place the cylinder in the compression testing machine.
- Apply a load of 40 tons over 10 minutes.
- Sieve the crushed material through a 2.36-mm sieve and weigh the material passing (fines).
- Calculate ACV = [(Weight of fines passing 2.36-mm sieve) / (Total weight of sample)] × 100%, reported to the nearest whole number.
- Notes:
- Larger aggregates typically have higher ACV, indicating lower resistance to crushing.
- Conduct a duplicate test using the same weight of material for accuracy.
Real-Life Example
For a highway pavement project, you test coarse aggregates and find an ACV of 20%. This indicates that 20% of the material is crushed into fines under the test load. If the project specification requires an ACV below 25% for road aggregates, your material is suitable, ensuring the pavement can withstand heavy traffic loads.
2. Aggregate Impact Value (AIV) Test
What It Measures
The aggregate impact value measures the resistance of aggregates to sudden impact or shock, simulating dynamic loads like those from traffic or blasting.
Why It Matters
- Aggregates with low AIV are tougher and better suited for applications like road bases or railway ballast, where sudden loads are common.
- High AIV indicates brittle aggregates, which may fail under impact.
How to Test
- Apparatus:
- An impact testing machine with a total weight of 60 kg, including a metal base (22-30 kg) and a cylindrical cup (as shown in Fig. 2).
- A tamping rod (16 mm diameter, 50-60 cm long, rounded at one end).
- IS sieves (12.5 mm, 10 mm, and 2.36 mm).
- A balance accurate to 1 gram.
- Procedure:
- Use aggregates passing the 12.5-mm sieve and retained on the 10-mm sieve, in a surface-dry condition.
- Fill the cup in three layers, tamping each layer 25 times. Strike off surplus aggregate.
- Fix the cup firmly on the machine’s base. Raise the hammer (weighing 13.5-14 kg) to 380 mm and allow it to fall freely 15 times.
- Sieve the material through a 2.36-mm sieve and weigh the fines.
- Calculate AIV = [(Weight of fines passing 2.36-mm sieve) / (Total weight of sample)] × 100%.
Real-Life Example
For a railway ballast project, you test aggregates and find an AIV of 15%. The specification requires an AIV below 20% for ballast to resist train vibrations. Your aggregates pass, ensuring durability under dynamic loads.
3. Abrasion Value Test (Using Deval Machine or Los Angeles Machine)
What It Measures
The abrasion value assesses the resistance of aggregates to wear and tear, critical for surfaces exposed to friction, like roads or pavements.
Why It Matters
- Aggregates with low abrasion values are more durable, reducing maintenance costs for roads or runways.
- High abrasion loss indicates aggregates may wear out quickly, affecting surface integrity.
How to Test (Deval Machine)
- Apparatus:
- Deval abrasion testing machine with hollow cast iron cylinders (20 cm diameter, 34 cm deep), rotated at 30° to the shaft.
- IS sieve (1.70 mm).
- Abrasive charge: 6 cast iron or steel spheres (48 mm diameter, 390-415 g each, total 2500 ± 10 g).
- A balance accurate to 1 gram.
- Procedure:
- Prepare a test sample based on specific gravity:
- Over 2.8: 5500 g
- 2.4-2.8: 5000 g
- 2.2-2.4: 4500 g
- Less than 2.2: 4000 g
- Sieve aggregates to the required grading (as per Part III of the standard), wash, and dry.
- Place the sample and abrasive charge in the cylinder and rotate for 10,000 revolutions at 30-33 rpm.
- Sieve the material through a 1.70-mm sieve and weigh the fines.
- Calculate Abrasion Value = [(Weight of fines passing 1.70-mm sieve) / (Total weight of sample)] × 100%.
- Prepare a test sample based on specific gravity:
How to Test (Los Angeles Machine)
- Apparatus:
- Los Angeles abrasion testing machine with a hollow steel cylinder (as shown in Fig. 3).
- IS sieve (1.70 mm).
- Abrasive charge: Steel spheres (48 mm diameter, 390-445 g each), with the number and weight varying by grading (e.g., 12 spheres for Grading A, 5000 ± 25 g).
- Procedure:
- Prepare a test sample (e.g., 5000 g for Gradings A-D, 10,000 g for Gradings E-G) based on grading specified in the standard.
- Place the sample and abrasive charge in the cylinder and rotate for 500 revolutions (Gradings A-D) or 1000 revolutions (Gradings E-G) at 20-33 rpm.
- Sieve the material through a 1.70-mm sieve and weigh the fines.
- Calculate Abrasion Value = [(Weight of fines passing 1.70-mm sieve) / (Total weight of sample)] × 100%.
Real-Life Example
For a runway project, you test aggregates using the Los Angeles machine and find an abrasion value of 25%. The specification requires a value below 30% for pavement aggregates. Your aggregates are suitable, ensuring the runway surface resists wear from aircraft landings.
4. Crushing Strength Test
What It Measures
The crushing strength measures the compressive strength of individual aggregate particles, indicating their ability to resist direct compressive forces.
Why It Matters
- This test is critical for aggregates used in high-strength concrete, such as in dams or heavy-duty structures.
- It helps identify weak aggregates that could fail under load.
How to Test
- Apparatus:
- A compression testing machine with a spherical seating platen (radius ≤ 2 cm).
- A well-ventilated, thermostatically controlled oven (100-110°C).
- Procedure:
- Select individual aggregate particles (e.g., passing 12.5-mm sieve, retained on 10-mm sieve).
- Dry the particles in the oven for 4 hours and cool to room temperature.
- Test each particle under compression, recording the load at failure.
- Calculate the crushing stress for each particle and report the average to the nearest 5 kg/cm².
- Note any defects (e.g., seams, fissures) that may affect results.
Real-Life Example
For a dam project, you test granite aggregates and find an average crushing strength of 1000 kg/cm². The design requires aggregates with a minimum strength of 800 kg/cm² to withstand the dam’s compressive loads. Your aggregates meet the requirement, ensuring structural integrity.
Key Tips for Students and Engineers
- Select Appropriate Tests: Choose tests based on project needs (e.g., ACV for structural concrete, AIV for roads, abrasion for pavements).
- Ensure Proper Calibration: Use calibrated equipment (e.g., sieves, compression machines) for accurate results.
- Consider Aggregate Type: Crushed aggregates (e.g., granite) typically have lower ACV and AIV than uncrushed gravel, affecting test outcomes.
- Follow Standards Strictly: Adhere to IS 2386 (Part IV) for precise procedures and calculations.
Common Mistakes to Avoid
- Improper Sample Preparation: Failing to dry aggregates to a surface-dry condition can skew results.
- Inconsistent Tamping: Uneven tamping in ACV or AIV tests can lead to inaccurate measurements.
- Ignoring Grading: Using incorrect aggregate sizes or gradings can invalidate abrasion test results.
- Overlooking Defects: Not noting flaws like fissures in crushing strength tests can misrepresent aggregate quality.
Practical Application
Imagine you’re designing concrete for a high-rise building. You test the aggregates:
- ACV: 18% (suitable for structural concrete, <30%).
- AIV: 15% (suitable for dynamic loads, <20%).
- Los Angeles Abrasion: 22% (suitable for pavements, <30%).
- Crushing Strength: 900 kg/cm² (meets high-strength requirements). These results confirm the aggregates are robust for the building’s foundation, ensuring durability and safety.
Notes on Document Issues
- The document contains typographical errors (e.g., “AGREAGATES,” “MECHANAL,” “CONCRT”) and inconsistent numbering (e.g., “IS : 2366” vs. “IS : 2386”). Always refer to the official IS 2386 (Part IV) - 1963 for accurate details.
- Some sections are heavily truncated, limiting procedural details. Consult the full standard for complete test methods.
- Amendments (e.g., No. 2, September 1981; No. 3, October 1983) update apparatus specifications, such as referencing IS 9376-1979 for the crushing value apparatus.
For detailed procedures and calculations, refer to the full IS 2386 (Part IV) - 1963 document.
