Aggregates are one of the most important components in construction materials like concrete, mortar, and asphalt. Their physical and mechanical properties significantly influence the strength, durability, and workability of the final product. This article explores essential properties of aggregates, including bulk density, specific gravity, void content, water absorption, and permissible limits of impurities, along with their typical values and significance in construction.
1. Bulk Density, Specific Gravity, and Voids of Aggregates
Bulk Density
Bulk density is defined as the mass of aggregate per unit volume, including the voids between particles. It affects the mix design and the overall density of concrete. Typical bulk density values for common aggregates are:
- River sand (Fine): 1.44 g/cm³
- River sand (Medium): 1.52 g/cm³
- River sand (Coarse): 1.60 g/cm³
- Beach or river shingle: 1.60 g/cm³
- Broken stone: 1.60 g/cm³
- Stone screenings: 1.44 g/cm³
- Broken granite: 1.68 g/cm³
Specific Gravity
Specific gravity is the ratio of the density of the aggregate to the density of water. It is crucial for calculating mix proportions and estimating the void content. Values for some typical aggregates are:
- Trap rock: 2.9
- Granite: 2.8
- Gravel: 2.66
- Sand: 2.65
Voids
Voids represent the percentage of empty space between particles in the aggregate mass. The void content affects the amount of cement paste required to fill these spaces. Average void percentages for different aggregates include:
- Fine river sand: 43%
- Coarse river sand: 35%
- Mixed and moist sand: 38%
- Mixed and dry sand: 30%
- Broken stone (maximum size 25 mm): 46%
- Stone screenings: 48%
Note: These values are indicative and can vary based on source and processing.
2. Limiting Values of Mechanical Properties
Mechanical properties such as crushing value, impact value, abrasion resistance, and soundness determine the aggregate’s ability to withstand stresses during handling and service life.
| Property | Wearing Surfaces (Max %) | Other Surfaces (Max %) |
|---|---|---|
| Crushing value | 30 | 45 |
| Impact value | 30 | 45 |
| Abrasion (Los Angeles) | 30 | 50 |
| Soundness (Na2SO4) | Fine: 10, Coarse: 12 | Fine: 15, Coarse: 18 |
| Soundness (MgSO4) | Fine: 10, Coarse: 12 | Fine: 15, Coarse: 18 |
Source: IS: 383 – 1970
3. Water Absorption Characteristics
Water absorption indicates the ability of aggregates to absorb water, which affects the water-cement ratio and workability of concrete.
- Average sand: ~1.0%
- Pebbles and crushed limestone: ~1.0%
- Trap rock and granite: ~0.5%
- Porous sandstone: ~0.5%
- Very light and porous aggregates may absorb up to 25% by weight
Generally, coarser aggregates absorb less free water on their surface, which is important for accurate mix water calculations.
4. Limits of Deleterious Materials in Aggregates
Deleterious materials such as coal, clay lumps, shale, and excessive fine particles can weaken concrete and impair durability. Standards specify maximum permissible limits as follows:
| Material | Fine Aggregate (%) | Coarse Aggregate (%) |
|---|---|---|
| Uncrushed | Crushed | |
| Coal and lignite | 1.0 | 1.0 |
| Clay lumps | 1.0 | 1.0 |
| Materials finer than 75 micron sieve | 2.0 | 15.0 |
| Shale | 1.0 | – |
| Total deleterious materials | 5.0 | 2.0 |
Note: Mica is excluded from the deleterious materials category.
5. Bulking of Sands Relative to Moisture Content
Bulking is the phenomenon where the volume of sand increases due to moisture causing particle separation. This affects the volume measurements in construction and must be accounted for.
| Moisture (%) | Fine Sand (%) | Medium Sand (%) | Coarse Sand (%) |
|---|---|---|---|
| 1 | 16 | 8 | 6 |
| 3 | 32 | 22 | 15 |
| 5 | 38 | 29 | 18 |
| 10 | 32 | 22 | 12 |
| 20 | 9 | 0 | 0 |
Bulking reduces with increasing moisture beyond a certain point as the sand becomes saturated.
6. Surface Water Content in Aggregates
Surface water retained by aggregates affects the effective water content in the concrete mix. Approximate surface water contents include:
| Aggregate Type | % by Mass | Liters per Cubic Meter |
|---|---|---|
| Very wet sand | 7.5 | 120 |
| Moderately wet sand | 5.0 | 80 |
| Moist sand | 2.5 | 40 |
| Moist gravel/crushed rock | 1.25–2.5 | 20–40 |
Coarser aggregates carry less surface water compared to finer sands.
Conclusion
Understanding the properties of aggregates—such as bulk density, specific gravity, voids, mechanical strength, water absorption, and permissible impurities—is critical for producing high-quality concrete and other construction materials. Adhering to standard limits ensures durability, strength, and long-term performance of structures. Proper selection and testing of aggregates, guided by standards like IS: 383 and IS: 456, remain foundational in civil engineering practices.