3.7.3. Maximum Size of Aggregates

• Larger size → Reduced water and cement requirements (for same workability)

Using larger aggregates in concrete mixes can lead to a reduction in the amount of water and cement needed to achieve the same workability level. This is because larger aggregates have a lower surface area relative to their volume, which means they absorb less water and require less cement paste to coat their surfaces for effective bonding.

Imagine a sponge versus a rock. A sponge can soak up a lot of water because it has many tiny holes (a larger surface area). A smooth rock, in comparison, doesn’t soak up much water because its surface is solid and less porous. Similarly, larger aggregates in concrete act like rocks, requiring less water and cement because they don't absorb as much.

• Smaller size → Better surface area for bond, increased strength

Smaller aggregates provide a greater surface area for bonding with the cement paste, which can enhance the overall strength of the concrete. When the aggregate particles are smaller, they fit better together and create a denser mix, leading to improved mechanical interlocking and bonding with the surrounding cement matrix.

Think of how puzzle pieces fit together. Smaller pieces can create a more intricate design compared to larger ones, which might leave gaps. In concrete, using smaller aggregate sizes allows the particles to fit together more closely, much like how smaller puzzle pieces complete a picture more effectively.

• Limitation: Should not exceed 1/4 of the minimum member thickness • Should not be more than 20 mm for thin sections or pumpable concrete

There are limitations on the maximum size of aggregates used in concrete to ensure the structural integrity and workability of the mix. Specifically, the size of aggregates should not exceed one-fourth of the thickness of the concrete member to prevent issues such as difficulty in mixing or placing concrete and to avoid creating weak points in the structure. Additionally, for thin sections or areas that will be pumped, the maximum size is capped at 20 mm to ensure proper flow and compaction without blockages.

Imagine trying to pour a large bag of marbles through a very small opening. If the marbles (large aggregates) are too big, they won't fit through. Similarly, in concrete, if the aggregates are too large for the thickness of the structural element, they can cause problems in mixing and placement, just like the marbles would get stuck.

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