The shape of aggregate particles plays a critical role in determining the workability, strength, and durability of concrete. Among the various shape indices used in civil engineering, the angularity number stands out as a practical measure for quantifying how angular or rounded an aggregate particle is. This concept, introduced by Shergold, provides engineers with a reliable method to assess aggregate shape through a straightforward void content test. Understanding the angularity number helps in selecting appropriate aggregates for different construction applications, from concrete production to pavement construction. For those involved in quality control of construction materials, knowing the angularity characteristics of aggregates is as essential as understanding other mechanical properties such as the aggregate crushing value test to determine aggregate crushing strength, which evaluates the resistance of aggregates to crushing under gradually applied compressive loads.
What Is Angularity Number and How It Defines Aggregate Shape
The angularity number is a numerical index that represents the shape characteristics of aggregate particles. It was developed by Shergold as a qualitative measure to classify aggregates based on their angularity. The fundamental principle behind this concept is that angular particles have higher void content when compacted compared to rounded particles. This is because angular particles interlock with each other, leaving more empty spaces between them, whereas rounded particles tend to pack more tightly together.
The angularity number scale ranges from 0 to 11 for aggregates suitable for concrete production. An angularity number of 0 represents the most practicable rounded aggregate, meaning it has the lowest void content and therefore the least angularity. At the other end of the scale, an angularity number of 11 indicates the most angular aggregate that can still be used for making concrete. Aggregates with angularity numbers above 11 are generally considered too angular for practical concrete production as they would require excessive cement paste to fill the voids and achieve adequate workability.
The concept draws an important parallel to other dimensionless numbers used in civil engineering for characterizing material behavior. Just as the Froude number or Reynolds number for scale model similarity in hydraulic engineering helps engineers predict fluid behavior in scaled models, the angularity number provides a standardized reference for comparing aggregate shapes across different sources and types.
Test Procedure for Determining Angularity Number
The angularity number test is a straightforward laboratory procedure that determines aggregate angularity based on void content after compaction under standardized conditions. The test procedure follows these steps:
- A sample of single-sized aggregate is prepared by sieving the aggregate through standard sieves to obtain particles of uniform size range.
- A metal cylinder with a capacity of exactly 3 liters is cleaned and dried thoroughly.
- The single-sized aggregate sample is filled into the metal cylinder in three layers, with each layer being compacted using a standard tamping rod.
- After compaction, the surface is leveled and the net weight of the aggregate in the cylinder is recorded.
- The solid volume of the aggregate is calculated by dividing the weight of the aggregate by its specific gravity.
- The percentage of voids is then calculated using the formula: Void content (%) = 100 x (Cylinder Volume – Solid Volume) / Cylinder Volume.
- The angularity number is derived from the void content using the standard relationship.
The calculation of angularity number is based on the relationship between void content and a baseline value. When the void content of a compacted aggregate sample is 33 percent, the angularity number is taken as 0. When the void content reaches 44 percent, the angularity number becomes 11. The formula used is:
Angularity Number = (Void Content in Percent – 33) / 1
This linear relationship means that every one percent increase in void content above 33 percent corresponds to an increase of one unit in the angularity number. Some references also express this as Angularity Number = 67 minus Percent Solid Volume, which is mathematically equivalent.
| Void Content (%) | Angularity Number | Aggregate Description |
|---|---|---|
| 33 | 0 | Most practicable rounded aggregate |
| 35 | 2 | Well-rounded aggregate |
| 37 | 4 | Rounded to sub-rounded aggregate |
| 39 | 6 | Sub-angular aggregate |
| 41 | 8 | Angular aggregate |
| 44 | 11 | Most angular aggregate suitable for concrete |
Interpreting Angularity Number Values for Quality Assessment
Understanding what each angularity number value means is essential for making informed decisions about aggregate selection. The interpretation of test results requires knowledge of how angularity affects concrete properties. Aggregates with low angularity numbers (0 to 4) are well-rounded and pack more densely, requiring less cement paste to fill the voids. This makes them economical for concrete production as they reduce the cement content needed per cubic meter.
However, rounded aggregates also provide lower inter-particle friction and reduced mechanical interlock. This can affect the bond between the aggregate and the cement paste. On the other hand, aggregates with higher angularity numbers (7 to 11) provide better interlocking and stronger mechanical bonds within the concrete matrix. The increased surface area of angular particles also provides more area for the cement paste to adhere to, potentially improving the overall strength of the concrete. The trade-off is that angular aggregates require more water and cement to achieve the same workability as rounded aggregates of the same size.
When designing concrete structures, the choice of aggregate size and shape must be carefully considered alongside other factors. As noted by experts, in designing concrete structures normally maximum aggregate sizes are adopted with ranges from 10mm to 20mm and an increase of maximum aggregate size can benefit the structures in certain applications by reducing the cement requirement and minimizing thermal cracking in mass concrete. The angularity of these aggregate particles further modifies these benefits depending on whether the particles are rounded or angular.
Impact of Angularity Number on Concrete Properties
The angularity number directly influences several key properties of both fresh and hardened concrete. In the fresh state, aggregate shape affects workability, pumpability, and compactability. Rounded aggregates with low angularity numbers produce more workable concrete mixes for a given water-cement ratio, making them preferred for heavily reinforced sections where good flow is required. Angular aggregates, while more difficult to work with in the fresh state, contribute to higher strength in the hardened state due to better mechanical interlock.
The relationship between angularity and concrete strength can be understood through the following key points:
- Angular aggregates develop stronger mechanical bonds with the cement paste matrix, improving compressive strength.
- Higher angularity numbers correlate with increased flexural strength due to better interlocking between particles.
- The void content of angular aggregates requires careful adjustment of the mix design to maintain workability without compromising strength.
- Aggregates with angularity numbers between 4 and 8 offer the best balance of workability and strength for most general construction applications.
- For high-performance concrete requiring maximum strength, angular aggregates in the upper range of 7 to 11 are often preferred despite higher cement demands.
The angularity number is closely related to other aggregate quality indices such as the aggregate impact value testing complete guide to IS 2386 part IV method for coarse aggregate quality assessment, which measures the resistance of aggregates to sudden impact loads. Both tests provide complementary information about aggregate suitability for different construction applications, with angularity focusing on shape characteristics and impact value focusing on toughness.
Practical Applications and Selection Criteria for Construction
In practical construction applications, the angularity number guides engineers in selecting appropriate aggregates for specific uses. For road pavements and base courses, angular aggregates are generally preferred because they provide better interlocking and load distribution characteristics. The higher friction between angular particles resists displacement under traffic loads, contributing to pavement stability and longevity. For asphalt concrete, angular aggregates improve the resistance to rutting and permanent deformation.
For concrete production, the choice between angular and rounded aggregates depends on the specific requirements of the project. Mass concrete works such as dams and large foundations benefit from rounded aggregates with lower angularity numbers because they require less cement and generate less heat of hydration. Structural concrete in columns, beams, and slabs may benefit from moderately angular aggregates that provide good strength without excessive cement demand. The selection process must also account for the fact that angularity affects the coarse aggregate in concrete construction, as the coarse aggregate fraction typically has the most significant influence on the overall concrete properties due to its large volume proportion.
The angularity number test is just one of several tests used to characterize aggregates for construction. A comprehensive approach to aggregate properties testing includes evaluating gradation, specific gravity, water absorption, crushing strength, impact resistance, and shape characteristics. Each of these properties contributes to the overall assessment of aggregate quality and suitability for specific applications. The angularity number fills an important niche by quantifying shape in a standardized, reproducible manner that complements other test results.
Conclusion: The Role of Angularity Number in Quality Concrete Production
The angularity number remains a valuable tool in the civil engineer’s repertoire for assessing aggregate shape characteristics. Its simplicity and reproducibility make it suitable for routine quality control testing in both laboratory and field settings. The concept, introduced by Shergold, provides a clear numerical framework for classifying aggregates from rounded (angularity number 0) to highly angular (angularity number 11), enabling engineers to make informed decisions about material selection.
Understanding the angularity number of aggregates used in a project helps optimize concrete mix designs, balance workability and strength requirements, and ensure long-term durability of concrete structures. When combined with other test methods such as the aggregate impact value test and crushing value assessment, the angularity number contributes to a complete picture of aggregate quality and performance potential. For construction professionals seeking to produce high-quality concrete that meets specification requirements and performs reliably over its design life, paying attention to the angularity number of coarse aggregates is a worthwhile investment in quality assurance.