Aggregate Impact Value and Its Significance in Construction

The construction industry, a cornerstone of infrastructure development in the United States, relies heavily on the quality of materials used in its various projects. Among these materials, aggregates, which are a fundamental component in road construction, play a pivotal role in ensuring the longevity, stability, and safety of our roads. As someone deeply involved in the construction industry, you understand the significance of selecting the right aggregates for each project. In this comprehensive blog post, we will explore the Aggregate Impact Value (AIV) test, a crucial assessment tool that allows construction professionals like you to determine the suitability of aggregates for road construction.

Roads serve as the arteries of our transportation network, facilitating the movement of people and goods across the nation. To uphold the integrity and reliability of these vital arteries, it is imperative to choose aggregates that can withstand the rigors of constant traffic and environmental factors. The AIV test, as per IS: 2386 (Part IV) – 1963, serves as a robust method for assessing the impact resistance of aggregates. By delving into the details of this test, its apparatus, theory, procedure, and the significance of its results, you can gain a deeper understanding of how to select the right aggregates, ensuring the strength and durability of our road infrastructure. Let’s embark on this informative journey to explore the world of Aggregate Impact Value and its pivotal role in the construction industry.

Aggregate Impact Value Test Apparatus

The AIV test requires specific apparatus to ensure accurate results. The apparatus, as per IS: 2386 (Part IV) – 1963, consists of:

1. Testing Machine

  • Weighing 45 to 60 kg.
  • Metal base with a painted lower surface of not less than 30 cm in diameter.
  • Supported on a level and plane concrete floor of minimum 45 cm thickness.
  • Provisions for fixing its base.

2. Cylindrical Steel Cup

  • Internal diameter 102 mm.
  • Depth 50 mm.
  • Minimum thickness 6.3 mm.

3. Metal Hammer (Tup)

  • Weighing 13.5 to 14.0 kg.
  • Cylindrical in shape, 50 mm long, 100.0 mm in diameter, with a 2 mm chamfer at the lower edge.
  • Case hardened.
  • Free fall of hammer should be within 380±5 mm.

4. Cylindrical Metal Measure

  • Internal diameter 75 mm.
  • Depth 50 mm for measuring aggregates.

5. Tamping Rod

  • 10 mm in diameter.
  • 230 mm long.
  • Rounded at one end.

6. Balance

  • Capacity not less than 500g.
  • Readable and accurate up to 0.1 g.

Theory of Aggregate Impact Test

The AIV test measures the property of a material known as toughness. Toughness is crucial because, as vehicles traverse roads, aggregates are subjected to impact, which can lead to their breakage into smaller pieces. Therefore, aggregates must possess sufficient toughness to withstand this impact. The AIV is a measure of resistance to sudden impact or shock, which may differ from resistance to gradually applied compressive load.

Procedure of Aggregate Impact Test

The procedure for conducting the AIV test is as follows:

  1. Sample Preparation
  • Use aggregates sized between 10.0 mm and 12.5 mm.
  • Dry the aggregates by heating at 100-110°C for 4 hours and then cool them.
  • Sieve the material through 12.5 mm and 10.0 mm IS sieves. The aggregates passing through the 12.5 mm sieve and retained on the 10.0 mm sieve comprise the test material.
  1. Filling the Cylinder
  • Pour the aggregates to fill about one-third of the depth of the measuring cylinder.
  1. Compaction
  • Compact the material by giving it 25 gentle blows with the rounded end of the tamping rod.
  1. Layering
  • Add two more layers in a similar manner until the cylinder is full.
  1. Surplus Removal
  • Strike off the surplus aggregates.
  1. Weighing
  • Determine the net weight of the aggregates to the nearest gram (W).
  1. Testing Machine Setup
  • Ensure the impact machine is rigid and its hammer guide columns are vertical.
  • Fix the cup firmly in position on the machine’s base.
  1. Testing
  • Place the entire test sample in the cup and compact it by giving 25 gentle strokes with the tamping rod.
  • Raise the hammer until its lower face is 380 mm above the surface of the aggregate sample in the cup and allow it to fall freely on the sample. Repeat this process for 15 blows at intervals of not less than one second between successive falls.
  1. Sieve Analysis
  • Remove the crushed aggregate from the cup and sieve it through a 2.36 mm IS sieve until no further significant amount passes in one minute.
  1. Calculating Aggregate Impact Value
    • Weigh the fraction passing the sieve to an accuracy of 1 gram (W2).
    • Also, weigh the fraction retained in the sieve.
    • Compute the Aggregate Impact Value using the formula:
      Aggregate Impact Value (percent) = (W2 / W1) × 100
    • Take the mean of two observations, rounded to the nearest whole number, as the Aggregate Impact Value.

Observations and Results

The observations in the Impact Test are recorded, including the total weight of the dry sample (W1 gm), weight of portion passing the 2.36 mm sieve (W2 gm), and the calculated Aggregate Impact Value in percent.

Significance of Aggregate Impact Value

The Aggregate Impact Value is used to classify aggregates based on their suitability for different types of road construction. The classification is as follows:

  • <20%: Exceptionally Strong
  • 10 – 20%: Strong
  • 20-30%: Satisfactory for road surfacing
  • >35%: Weak for road surfacing

The Indian Roads Congress provides specified limits of the percent Aggregate Impact Value for different types of road construction. These values ensure that the aggregates used are appropriate for the intended purpose, thereby contributing to the longevity and safety of road infrastructure.

Recommended Aggregate Impact Test Values by Indian Roads Congress

  1. Wearing Course
  • Bituminous surface dressing: ≤ 30
  • Penetration macadam: ≤ 30
  • Bituminous carpet concrete: ≤ 30
  • Cement concrete: ≤ 30
  1. Bitumen Bound Macadam Base Course: ≤ 35
  2. WBM Base Course with Bitumen Surfacing: ≤ 40
  3. Cement Concrete Base Course: ≤ 45

Conclusion

In the construction industry, the Aggregate Impact Value test is a critical tool for assessing the quality and suitability of aggregates for road construction. It helps ensure that the materials used in building our roads possess the necessary toughness to withstand the impact of vehicular traffic, ultimately contributing to the safety and longevity of our roadways. By adhering to the recommended AIV values provided by organizations like the Indian Roads Congress, construction professionals can make informed decisions and build robust road infrastructure.

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