1.3. Causes

• Poorly graded aggregates.

Aggregates are the materials that make up the majority of concrete volume, and they come in various sizes. When they're poorly graded, it means they are not well-mixed in terms of size distribution. Ideally, the mix should have a range of aggregate sizes that fit together tightly to create a solid structure. If the size distribution is off, it can lead to voids or gaps which reduce the strength and durability of the concrete.

Think of poorly graded aggregates like jigsaw puzzle pieces that don't fit well together. If you have only large pieces or too many small pieces, the gaps left over might weaken the puzzle – just like the gaps in concrete can weaken a structure.

• Excessive vibration or compaction.

Vibration is often used in concrete work to help settle and eliminate air bubbles in the mix. However, too much vibration can cause the heavier aggregates to settle too quickly, which leads to segregation. It's about finding the right balance; proper compaction should help the mix settle without separating its components.

Imagine mixing a salad. If you're too rough while tossing it, the heavier ingredients like cucumbers may sink to the bottom, leaving the lighter ingredients like lettuce floating on top. Just like the salad, concrete needs gentle handling to keep everything mixed evenly.

• High water-cement ratio (overly wet mixes).

The water-cement ratio is crucial in concrete mixes. A higher ratio means more water in relation to cement, which might seem beneficial for workability, but it significantly weakens the finished concrete. More water creates a more fluid mix, but when it evaporates after setting, it can leave voids that result in low strength and durability.

Consider making a cake. If you add too much water to the batter, the cake becomes too moist and may collapse while baking. Similarly, too much water in concrete can weaken its integrity, making it crumble under pressure.

• Dropping concrete from excessive heights.

When concrete is poured from a high place, it impacts the ground forcefully, causing the heavier aggregates to separate from the lighter paste. This separation can result in a non-homogeneous mixture that compromises the structure’s strength.

Think of dropping a bag of marbles from a height. When they hit the ground, the heavier marbles may roll away from the lighter ones, making a mess. The same happens in concrete when it's dropped from a great height – the components lose their intended mix and balance.

• Insufficient cohesive strength of the mix (low fines or improper mix design).

Cohesive strength refers to how well the different parts of the concrete mix stick together. A mix with too few fine particles (like sand) or an improper design can lead to a weak bond between aggregates and the cement paste, resulting in segregation.

Imagine trying to hold together a cluster of grapes with toothpicks. If you use too few toothpicks or the wrong type, the grapes will fall apart. In the same way, concrete needs the right mix of fine particles to hold everything together.

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