Coarse aggregate is a crucial component of concrete and takes up the most space in the mix, making it a significant factor in concrete mix design. The properties of coarse aggregate, including strength, maximum size, shape, and water absorption, have an impact on the water demand, cement and fine aggregate quantities in the concrete mixture.
A larger maximum size of coarse aggregate can result in lower water demand in the mixture since the aggregate has a smaller surface area compared to smaller coarse aggregate sizes. The shape of the coarse aggregate also plays a role, with a rounded shape being more economical for normal strength concrete, while an angular shape is preferable for high strength concrete.
The risk of segregation can be minimized by properly grading coarser aggregate, highlighting the importance of good grading in concrete mix design. Moreover, higher aggregate strength can lead to higher concrete strength, provided that other controlling factors are adequately addressed.
1. Maximum Aggregate Size
The water demand and quantity of fine aggregate required for a cohesive mix are influenced by the maximum size of coarse aggregate in use. A larger maximum size results in a lower surface area of coarse aggregates, while a smaller maximum size increases the surface area of coarse aggregate. As surface area increases, so does the water demand to coat the particles and create the necessary workability.
Using a smaller maximum size of coarse aggregate would necessitate a greater amount of fine aggregate to coat the particles and maintain mix cohesiveness. Hence, for achieving the same workability, 40mm down aggregate would have a lower water/cement ratio, leading to higher strength than 20mm down aggregate. By reducing water demand, a higher maximum size of coarse aggregate can help to lower cement consumption.
The clear cover and minimum distance between reinforcement bars usually limit the maximum size of aggregate. To produce dense and homogeneous concrete, a maximum size of coarse aggregate less than the clear cover or minimum distance between reinforcement bars is preferred. This allows aggregates to pass through the reinforcement in congested areas.
2. Grading of Coarse Aggregate
The determination of the particle-size distribution of aggregate, known as grading, has a significant impact on the properties of concrete. It affects the amount of cement and water needed, the workability and pumpability of the mixture, and the durability of the final product. In particular, proper grading of coarse aggregate is crucial to achieve cohesive and dense concrete. This is because smaller coarse aggregate particles can fill the voids left by larger ones, resulting in a more uniform mixture.
By following the coarse aggregate grading limits outlined in ASTM C33/C33M and IS 383 – 1970 – table 2, Clause 4.1 and 4.2, segregation of the mixture can be minimized, especially when higher workability is required. Furthermore, properly graded coarse aggregate improves the compatibility of concrete, leading to a higher quality end product. These limits apply to both single size aggregate and graded aggregate, ensuring that the desired particle-size distribution is achieved.
3. Shape of Coarse Aggregate
Coarse aggregates can be categorized based on their shape, such as round, angular, or irregular. Among these shapes, rounded aggregates are known to have the lowest water demand due to their lower surface area. They also require less mortar paste, making them the most economical choice for concrete grades up to M35. However, for higher concrete grades such as M40 and above, angular aggregates with more surface area are preferred to avoid bond failure.
Flaky and elongated coarse aggregates have negative impacts on the properties of concrete. They increase the water demand and segregation tendency, while reducing the flexural strength of concrete. To ensure the quality of concrete, the Ministry of Surface Transport has established specifications that limit the combined weight of flaky and elongated particles to 30% of the weight of coarse aggregates.
4. Strength of Coarse Aggregate
The strength of coarse aggregate material can be determined through various tests such as the crushing strength of rock, aggregate crushing value, aggregate impact value, and aggregate abrasion value. In order to meet the standards set by the Indian Standards (IS), the aggregate crushing value, aggregate impact value, and aggregate abrasion value must fall within the prescribed limits. These limits have been established by the IS in order to ensure that the coarse aggregate material possesses the necessary strength characteristics to perform optimally in various construction applications. Therefore, it is crucial for engineers and construction professionals to conduct these tests and ensure that the coarse aggregate material being used meets the required standards for strength.
5. Aggregate Absorption
The application of a correction factor for aggregates in dry condition and the determination of water demand for concrete in saturated surface dry condition involve the use of aggregate absorption. This process accounts for the fact that aggregates, in their bone dry state, can absorb water up to 2% of their weight. However, it is important to note that some aggregates may exhibit higher absorption rates, reaching up to 5% in certain cases.
By taking aggregate absorption into consideration, concrete mix designs can be more accurately formulated. This is because the amount of water needed for a concrete mix is influenced by the moisture content of the aggregates. Inaccurate estimation of aggregate absorption can result in concrete mixes with insufficient or excess water, which can compromise their strength, workability, and durability.
Therefore, it is crucial to have a thorough understanding of the absorption characteristics of the aggregates used in concrete production. This information can be obtained through testing and analysis, and it should be taken into account when determining the water demand for concrete mix designs. By doing so, concrete producers can ensure that their products meet the required standards of performance and quality.