The properties and performance of concrete depend significantly on the characteristics and properties of aggregates used. Aggregates must meet several essential criteria to ensure the quality of concrete. They should be clean, hard, strong, properly shaped, and well graded. Furthermore, aggregates must exhibit chemical stability and resistance to abrasion, freezing, and thawing. The presence of harmful substances that might cause physical or chemical reactions—such as cracking, swelling, softening, or leaching—should be avoided.
The key properties of aggregates can be categorized as follows:
1. Mechanical Properties of Aggregates
i. Strength of Aggregate
The strength of the aggregate directly influences the strength of the concrete. For typical concrete, as long as the aggregate strength is higher than the concrete matrix, it suffices. However, in high-strength concrete subjected to localized stress, aggregate strength becomes critical.
To evaluate aggregate strength, the following tests are commonly used:
- Aggregate Crushing Value (ACV): Indicates the ability of the aggregate to resist crushing under a compressive load. A maximum crushing value of 45% is recommended for general concrete (IS: 383–1970), with stricter limits (30%) for wearing surfaces such as roads and pavements.
- Aggregate Impact Value (AIV): Assesses toughness, or the ability to resist sudden impacts. The value should not exceed 45% for general concrete and 30% for wearing surfaces (IS: 2386 Part IV–1963).
- 10% Fines Value: Determines the load required to produce 10% fine particles. For wearing surfaces, British Standard BS: 882–1965 specifies a minimum value of 10 tonnes, and 5 tonnes for other uses.
ii. Toughness and Hardness
- Toughness: Resistance to failure under impact is a crucial property for aggregates subjected to wear. It can be measured using the AIV test.
- Hardness: Defined by the aggregate’s resistance to wear, hardness is evaluated using the Los Angeles Abrasion test (IS: 2386 Part IV–1963). Acceptable abrasion values are 30% for wearing surfaces and 50% for non-wearing surfaces.
iii. Resistance to Freezing and Thawing
Aggregates exposed to harsh weather must resist damage from freezing and thawing cycles. This property is influenced by factors like porosity, water absorption, and pore structure. Fully saturated aggregates may fail due to the expansion of frozen water within their pores.
iv. Modulus of Elasticity
Aggregates with a higher modulus of elasticity typically produce concrete with a correspondingly higher modulus. This property also impacts creep and shrinkage behavior. While highly compressible aggregates reduce concrete stress during volume changes, overly rigid aggregates may lead to cracks in the surrounding cement paste. Therefore, aggregates with moderate strength and elasticity are often preferred for enhancing concrete durability.
2. Particle Shape and Texture
The physical attributes of aggregates, such as shape, texture, and surface roughness, play a critical role in influencing the workability of fresh concrete and the bond strength between the aggregate and the cement matrix. Aggregates are broadly classified into four categories based on shape: rounded, irregular, angular, and flaky.
- Rounded Aggregates: These are smooth, water-worn particles, often found in riverbeds or along seashores. Their rounded shape reduces water and cement paste requirements for achieving a desired workability. Using rounded aggregates can lower water content by 5–10% and sand content by 3–5%.
- Irregular Aggregates: Partially rounded particles, typically sourced from pitsands or gravels, are partially shaped by natural attrition and have slightly rounded edges.
- Angular Aggregates: These aggregates have sharp, well-defined edges formed at the intersection of flat surfaces. They are obtained by crushing rocks. Angular aggregates improve the strength of concrete due to better interlocking and bonding with the cement paste, potentially increasing compressive strength by 10–20%. For concrete with a low water–cement ratio (below 0.4), this strength increase can reach up to 38%.
- Flaky Aggregates: Derived from laminated rocks, these aggregates have a small thickness compared to their width or length. They tend to reduce workability because of their high surface area-to-volume ratio. Their orientation during mixing can trap air and water, further decreasing efficiency. The flakiness index of coarse aggregates is typically restricted to 25%.
Surface Texture of Aggregates
The surface texture—whether glassy, smooth, granular, rough, crystalline, porous, or honeycombed—is critical in determining the bond strength between aggregate and cement paste.
- Aggregates with a rough and porous texture are preferred over smooth ones as they enhance the aggregate-cement bond by up to 75%. This stronger bond can improve the compressive and flexural strength of concrete by up to 20%.
- The surface pores of aggregates allow suction of cement paste into them, creating a stronger mechanical bond. Interestingly, some smooth-looking aggregates may still form strong bonds due to microscopic surface porosity.
The shape and surface texture of fine aggregates significantly affect their void content, which in turn influences the water demand of the concrete mix. Crushed or manufactured sands with well-defined shapes, textures, and gradings enable the production of concrete with minimal void content and improved workability.
3. Specific Gravity of Aggregates
The specific gravity of an aggregate is the ratio of the mass of the solid material in a sample to the mass of an equal volume of water at the same temperature. Since aggregates contain voids, different types of specific gravities are defined to account for varying conditions.
Types of Specific Gravity
i. Absolute Specific Gravity:
This considers only the volume of solid material, excluding all voids. It is the ratio of the mass of the solid aggregate to the weight of an equal void-free volume of water at a specified temperature.
ii. Apparent/Bulk Specific Gravity:
When the volume includes voids between aggregate particles, the specific gravity is termed bulk specific gravity. Apparent specific gravity, on the other hand, accounts for the volume including impermeable voids only and is defined as:
iii. Saturated Surface Dry (SSD) Specific Gravity:
This condition considers the aggregate with its surface saturated but without excess moisture. Water absorbed into the aggregate’s pores does not participate in the cement’s chemical reactions and is thus treated as part of the aggregate. This specific gravity is most commonly used in concrete mix design for calculating the yield of concrete or determining the quantity of aggregates needed.
Importance of Specific Gravity
- A higher specific gravity generally indicates a harder and stronger aggregate.
- Deviations from the expected specific gravity range for a particular type of aggregate can signify changes in the aggregate’s shape, texture, or grading.
The specific gravity of natural aggregates typically ranges between 2.5 and 2.8.
Determination of Specific Gravity
The method for determining specific gravity is outlined in IS: 2386 (Part III)–1963. The specific gravity is calculated as:
Where:
- a = Mass of saturated surface-dry aggregate in air
- = Mass of saturated surface-dry aggregate in water
- c = Mass of oven-dry aggregate in air
This property provides critical insights into the aggregate’s quality and suitability for concrete production.
4. Bulk Density of Aggregates
Bulk density refers to the mass of aggregate material per unit volume, typically expressed in kilograms per liter (kg/L). It is influenced by the degree of compaction achieved during measurement. Key factors that impact bulk density include particle shape, size, grading, and moisture content. Among these, the shape of the particles plays a significant role in determining how tightly the aggregate can be packed.
For a coarse aggregate with a specific gravity, a higher bulk density indicates reduced void spaces, which means less sand and cement are required to fill the gaps. Bulk density is a useful parameter for assessing the quality of aggregate by comparing it against the standard density values for that specific type. It also helps determine the suitability of the aggregate for different types of concrete applications.
Moreover, bulk density is essential when converting mix proportions from weight-based to volume-based measurements. The procedure for determining bulk density is outlined in the Indian Standard IS: 2386 (Part III) – 1963.
5. Voids
The spaces between aggregate particles that remain unoccupied are referred to as voids. These voids represent the difference between the total (gross) volume of the aggregate and the actual volume occupied solely by the particles themselves.
The void ratio of an aggregate can be determined using the specific gravity and bulk density of the aggregate. The formula for calculating the void ratio is as follows:
6. Porosity and Absorption of Aggregates
During the formation of rocks, air bubbles may become trapped, or certain minerals may decompose due to atmospheric effects. This leads to the development of small holes or cavities within the rock, commonly referred to as pores. The size of these pores varies widely, ranging from microscopic dimensions to those visible to the naked eye. These pores can either be entirely enclosed within the aggregate or open to its surface.
The porosity of aggregates typically ranges from 0% to 20% for commonly used rocks. Since aggregates make up approximately 75% of the total volume of concrete, their porosity significantly influences the overall porosity of the concrete. Porosity affects critical properties such as permeability, absorption, bond strength between the aggregate and cement paste, resistance to freezing and thawing, chemical durability, abrasion resistance, and the aggregate’s specific gravity.
Pores within aggregates can act as reservoirs for moisture. The absorption of aggregates refers to the percentage of water an aggregate can absorb when immersed in water. An aggregate that is fully saturated but has no surface moisture is termed saturated surface-dry (SSD) aggregate.
The process for determining water absorption is detailed in IS: 2386 (Part III) – 1963. Two absorption types are commonly defined:
- Oven-Dry Basis: The percentage of water absorbed by an aggregate that has been oven-dried at 105°C to a constant weight before immersion in water for 24 hours.
- Air-Dry Basis: The percentage of water absorbed by an air-dried aggregate after 24 hours of water immersion.
Understanding the absorption characteristics of aggregates is vital for accurate concrete mix design calculations, ensuring optimal performance and durability of the concrete.
7. Moisture Content of Aggregate
The moisture content of an aggregate is defined as the surface moisture expressed as a percentage of the weight of the saturated surface-dry (SSD) aggregate. Since the SSD condition already accounts for the water absorbed within the aggregate, the moisture content represents the excess water present on the surface. Thus, the total water content of a moist aggregate is the sum of its absorption and moisture content. The procedure for determining the moisture content of aggregates is outlined in IS: 2386 (Part III) – 1963.
Determining the moisture content is crucial for calculating the accurate water–cement ratio in a concrete mix. Excess moisture in aggregates can significantly increase the effective water–cement ratio, potentially leading to weaker concrete unless suitable adjustments are made.
IS: 2386 (Part III) – 1963 specifies two methods for determining moisture content:
- Displacement Method: This method calculates moisture content as a percentage of the SSD sample’s mass. The accuracy of this method relies heavily on the precise determination of the specific gravity of the material in its SSD condition.
- Drying Method: In this method, moisture content is expressed as a percentage of the mass of the dried sample. This measurement typically reflects the total moisture content, which includes both free and absorbed water.
These two methods may yield differing results due to the way they measure moisture. The drying method generally provides the total moisture content, whereas the displacement method is more dependent on the specific gravity data for accuracy. Understanding and accounting for moisture content is essential to ensure the consistency, quality, and strength of the final concrete mix.
8. Bulking of Fine Aggregate
The phenomenon where the volume of fine aggregate increases due to the presence of moisture is referred to as bulking. This occurs because thin films of water surrounding the particles create a separation effect, pushing them apart. The extent of bulking depends on the moisture content and the fineness of the sand.
Bulking generally increases with rising moisture content up to a certain level, after which it starts to decrease as additional water causes the water films to merge. When the sand becomes fully saturated (inundated), the bulking effect diminishes completely. For most ordinary sands, bulking typically ranges between 15% and 30%.
Finer sands exhibit greater bulking, with maximum bulking occurring at higher moisture content levels compared to coarser sands. In extremely fine sand, bulking can reach up to 40% at a 10% moisture content. However, such fine sand is unsuitable for concrete. In contrast, coarse aggregate experiences negligible volume increase due to free water, as the thickness of the moisture film is insignificant compared to the size of the particles.
The percentage of bulking can be determined using methods prescribed in IS: 2386 (Part III) – 1963.
Impact of Bulking on Concrete Mix
When sand is measured by volume without accounting for bulking, the concrete mix may become richer than intended. This happens because moist sand occupies a larger volume than dry sand of the same mass. If no adjustments are made, this can lead to:
- A sand-deficient mix, increasing the chances of segregation and honeycombing in concrete.
- Reduced yield of concrete.
To ensure the correct proportion of sand in a nominal mix, the measured volume of moist sand should be increased by the percentage of bulking. For instance, if no adjustment is made for 15% bulking, a nominal mix of 1:2:4 would effectively become 1:1.74:4. Adjustments are critical as bulking variations can significantly affect the concrete’s properties. An increase in bulking from 15% to 30% may enhance concrete strength by as much as 14%, while failing to account for bulking may lead to strength variations of up to 25%.
