The shape and surface texture of coarse aggregate particles play a significant role in determining the workability, strength, and durability of concrete. One key parameter that quantifies coarse aggregate shape is the angularity number. This value measures the angularity of particles relative to a theoretical perfectly rounded shape. The standard procedure is described in IS 2386 Part 1, which covers test methods for aggregates relating to particle size and shape. Understanding this property helps engineers select aggregates for different concrete applications. For related guidance on aggregate quality testing, refer to the Aggregate Impact Value Testing method as per IS 2386 Part IV, which evaluates the toughness of aggregate particles under impact loading.
What Is Angularity Number and Why It Matters in Concrete Technology
The angularity number of coarse aggregate is a numerical index that indicates how angular or rounded the aggregate particles are. It is defined as the percentage of voids in the aggregate after compaction in excess of 33 percent, which is the void content of a perfectly rounded aggregate of the same grading. The higher the angularity number, the more angular the particles are. Angular aggregates have a higher surface area to volume ratio compared to rounded aggregates, which affects the paste requirement in concrete mix design.
The practical significance of angularity number in concrete technology can be summarised across several aspects:
- Workability: Angular aggregates require more cement paste to coat their surfaces and fill the additional voids, reducing workability for the same water-cement ratio compared to rounded aggregates.
- Bond strength: Angular particles develop a stronger mechanical interlock with the cement paste matrix, leading to higher bond strength at the interfacial transition zone.
- Compressive strength: The interlocking nature of angular aggregates contributes to higher compressive strength in concrete, particularly in high-strength mixes.
- Mix design adjustments: Aggregates with a high angularity number require adjustments to the fine aggregate content and water demand in the mix design process.
Before conducting the angularity number test, it is important to determine the specific gravity and water absorption of the aggregate sample, as these values are used in the final calculation. The Specific Gravity and Water Absorption Test of Coarse Aggregate as per IS 2386 Part 3 provides the standard method for obtaining these essential parameters.
Equipment Required and Sample Preparation Procedure
The angularity number test requires specific laboratory equipment and carefully prepared aggregate samples to ensure consistent and repeatable results. The apparatus used for this test is designed to produce a standard degree of compaction so that the void content measurement is comparable across different aggregate samples.
List of Equipment and Apparatus
- Balance with a capacity of 0 to 10 kg and accuracy of 1 gram
- IS sieves of sizes 20 mm, 16 mm, 12.5 mm, 10 mm, 6.3 mm, and 4.75 mm
- Standard metal cylinder of known volume
- Tamping rod conforming to IS specifications
- Scoop for transferring aggregate
- Ventilated oven capable of maintaining 100 to 110 degrees Celsius
In recent research, the angularity characteristics of aggregates have been studied in the context of sustainable construction materials. One interesting area involves the Experimental Investigation on Concrete with Replacement of Coarse Aggregate by Demolished Building Waste with Crushed Concrete, where the angularity of recycled aggregates significantly influences the concrete properties.
Sample Preparation and Sieve Requirements
The test sample consists of coarse aggregate retained between specific pairs of IS sieves. The aggregate must be tested in the size fractions listed in the table below. For each size fraction, a separate determination is made, and the results are averaged or reported individually as required.
| Passing Through Sieve | Retained on Sieve |
|---|---|
| 20 mm | 16 mm |
| 16 mm | 12.5 mm |
| 12.5 mm | 10 mm |
| 10 mm | 6.3 mm |
| 6.3 mm | 4.75 mm |
The prepared aggregate sample must be dried in a well-ventilated oven maintained at a temperature of 100 to 110 degrees Celsius for at least 24 hours before testing. This ensures that all moisture is removed, and the weight measurements reflect only the dry aggregate mass. The dry aggregate is then allowed to cool to room temperature in a desiccator before the test begins.
Step-by-Step Test Procedure for Angularity Number Determination
The test procedure for determining the angularity number involves compacting the aggregate into a standard metal cylinder under controlled conditions. The compaction process is designed to achieve a reproducible packing density that forms the basis for calculating the void content.
Follow these steps in sequence to perform the test correctly:
- The metal cylinder is placed on a firm horizontal surface and filled with the prepared dry aggregate in three layers of approximately equal depth.
- Each layer is compacted by giving 100 blows using the standard tamping rod. The rod is lifted 5 cm above the surface of the aggregate and allowed to fall freely at a rate of 2 blows per second.
- The blows must be distributed uniformly over the surface of the aggregate to ensure even compaction across the entire cross-section of the cylinder.
- After compacting the third layer, additional aggregate is added to fill the cylinder to overflowing. The excess material is struck off using the tamping rod as a straight edge, ensuring a level surface flush with the top of the cylinder.
- The cylinder with the compacted aggregate is weighed. The weight is recorded to the nearest gram.
- Three separate determinations are made for the same aggregate sample, and the mean weight of the aggregate in the cylinder is calculated from these three readings.
This compaction approach is similar to the standard used in other aggregate tests. The Aggregate Crushing Value Test Procedure as per IS 2386 Part 4 also employs a comparable compaction protocol as part of its testing method, though with different loading conditions and evaluation criteria.
Calculation of Angularity Number with Formula and Worked Example
The angularity number is calculated from three measured parameters: the mean weight of aggregate filling the cylinder, the volume of the cylinder, and the specific gravity of the aggregate. The underlying principle is that the angularity number represents the percentage of voids in the compacted aggregate in excess of 33 percent (the void content of perfectly rounded particles).
The Formula
The angularity number is calculated using the following formula:
Angularity Number = [ (C – W / Gs) / C ] x 100 – 33
Where:
- W = mean weight of the aggregate filling the cylinder (in grams)
- C = weight of water required to completely fill the cylinder (in grams), which is numerically equal to the volume of the cylinder in cubic centimetres
- Gs = specific gravity of the aggregate (determined as per IS 2386 Part 3)
The term (C – W/Gs) represents the volume of voids in the compacted aggregate. Dividing by C gives the void proportion, multiplying by 100 converts it to a percentage, and subtracting 33 removes the baseline void content of rounded particles to isolate the angularity contribution.
Worked Example
Consider a cylinder with a volume (C) of 3000 cubic centimetres. The mean weight of aggregate (W) from three determinations is 4350 grams. The specific gravity (Gs) of the aggregate is 2.65.
Step 1: Calculate the absolute volume of aggregate solids:
Volume of solids = W / Gs = 4350 / 2.65 = 1641.5 cm3
Step 2: Calculate the volume of voids:
Volume of voids = C – Volume of solids = 3000 – 1641.5 = 1358.5 cm3
Step 3: Calculate the percentage of voids:
Percentage of voids = (1358.5 / 3000) x 100 = 45.28 percent
Step 4: Calculate the angularity number:
Angularity Number = 45.28 – 33 = 12.28
The reported value, expressed to the nearest whole number, is 12. This indicates a relatively angular aggregate, which would require careful consideration in mix design. For further reading on how shape characteristics influence concrete performance, see the article on Angularity Number of Aggregate Testing Method and Significance in Concrete.
Interpreting Angularity Number Results and Practical Applications
The angularity number typically ranges from 0 for perfectly rounded aggregates such as river gravel to about 15 for highly angular crushed rocks. In practice, most crushed stone aggregates used in construction fall between 8 and 12. Understanding where a particular aggregate falls on this scale helps engineers make informed decisions about mix proportions, water demand, and admixture requirements.
| Angularity Number Range | Aggregate Shape Description | Typical Source |
|---|---|---|
| 0 to 3 | Rounded to nearly rounded | River gravel, seashore deposits |
| 4 to 7 | Sub-angular to irregular | Pit-run gravel, partially crushed |
| 8 to 11 | Angular | Crushed stone (standard crushing) |
| 12 to 15 | Highly angular | Crushed stone (special crushing) |
When designing concrete mixes with angular aggregates, several adjustments are necessary. The higher void content means more fine aggregate and cement paste are needed to achieve the same workability. This affects the overall cost and performance of the concrete. Understanding the properties of Coarse Aggregate Concrete Construction helps in selecting the appropriate aggregate type for specific structural requirements.
The angularity number also has implications for pavement design and road construction. In bituminous mixes, angular aggregates provide better interlock and resistance to rutting under traffic loads. In concrete pavements, the angularity affects the finishing characteristics and the ultimate surface texture of the pavement.
Precautions, Reporting, and Quality Control Considerations
Safety Precautions
- Use safety shoes, dust mask, and protective aprons while performing the test.
- Thoroughly clean and dry the metal cylinder and all apparatus before beginning the test.
- Ensure that no external air currents affect the balance readings during weighing.
- Clean all sieves with a soft brush after the test to prevent particle entrapment between uses.
- Handle hot aggregates and oven equipment with appropriate heat-resistant gloves.
Reporting of Results
The angularity number is reported as the mean value of three separate determinations, expressed to the nearest whole number. If the individual determinations vary by more than a specified margin, additional tests may be required to confirm the result. The report should include the size fraction tested, the specific gravity used in the calculation, and any observations about the aggregate condition such as surface moisture or coating.
Quality control in aggregate testing is essential for producing consistent concrete. The angularity number test is one of several tests that provide a complete picture of aggregate quality. To understand how aggregate characteristics influence the final concrete product, read about How Coarse Aggregate Affects Mix Design of Concrete, which explains the relationships between aggregate properties and concrete performance in practical construction scenarios.
The angularity number test according to IS 2386 Part 1 remains a fundamental quality control tool in concrete technology. When combined with other tests such as crushing value, impact value, and specific gravity, it provides engineers with a comprehensive understanding of aggregate suitability for specific construction applications. Proper interpretation of angularity number results enables more economical and durable concrete mixes, ultimately contributing to safer and longer-lasting structures.