2.3. Causes

• Poorly graded aggregates.
• Excessive vibration or compaction.
• High water-cement ratio (overly wet mixes).
• Dropping concrete from excessive heights.
• Insufficient cohesive strength of the mix (low fines or improper mix design).

The causes of segregation in fresh concrete can be attributed to several factors. Firstly, poorly graded aggregates mean that the mix has a wide variation in particle size, leading to separation during movement or pouring. Secondly, excessive vibration or compaction can disturb the balance of materials, causing them to separate. A high water-cement ratio results in a very fluid mix that can exacerbate segregation by allowing heavier aggregates to settle. Additionally, if concrete is dropped from excessive heights, the impact can lead to separation of the components. Lastly, if the mix lacks sufficient cohesive strength, it can lead to disintegration of the mixture.

Imagine making a salad with different ingredients: if you use only large chunks of vegetables without any smaller ingredients, when you toss the salad, the large pieces will clump together at the bottom. Similarly, in concrete, if the ingredients are not well balanced (well-graded), they won't hold together effectively during mixing and pouring.

• High water-cement ratio.
• Over-sanded or under-sanded mix.
• Poor cement quality or insufficient fines.
• Use of poorly graded aggregates.

Bleeding occurs when excess water rises to the surface of freshly placed concrete. This can happen for several reasons. A high water-cement ratio means there is more water relative to cement, making it easier for the water to escape to the surface. If the mix is poorly sanded, either having too much or too little sand, it can affect how the solids settle, leading to bleeding. Additionally, low-quality cement or insufficient fine aggregates can also contribute, as they do not bind the water effectively. Finally, poor grading of aggregates can lead to uneven distribution within the mix.

Think of a sponge soaked in water. If you press the sponge, excess water oozes out. In concrete, when there is too much water mixed in, pressing or settling leads to water oozing out to the surface as well—just like the excess water from the sponge.

• High ambient temperature and wind.
• Rapid hydration of cement.
• Evaporation of water due to high temperature or low humidity.
• Delays in transportation, placing, or compaction.
• High cement content leading to faster stiffening.
• Inadequate use of water-retaining or workability-enhancing admixtures.

Slump loss refers to the decrease in workability of fresh concrete over time. High ambient temperatures and windy conditions accelerate the evaporation of water from the concrete mix, leading to quicker stiffening. Rapid hydration of cement can also cause the concrete to harden faster than desired. If there are delays in transporting or placing the mix, this can also lead to slump loss as the concrete begins to set before it is properly placed. High quantities of cement contribute to quicker setting times, and if there are insufficient admixtures to enhance workability, it can further worsen the slump loss.

Consider how ice cubes melt on a hot day—if you leave them out too long, they lose shape and begin to stick together. Similarly, if fresh concrete is left exposed to high temperatures without proper handling, it loses its workability and becomes difficult to manage.

• Weakening of concrete due to disrupted hydration.
• Reduced compressive and tensile strength.
• Increased permeability and shrinkage.
• Inconsistent setting times leading to cracks.
• Reduced bond strength between aggregate and paste.

Re-tempering involves adding water to concrete that has begun to set, which can lead to a host of problems. The addition of water disrupts the hydration process that is crucial for developing the strength of concrete, thereby weakening it. Over time, this can reduce both compressive and tensile strength, making the concrete less stable. It can also increase permeability, making the concrete more prone to cracking and shrinkage. Additionally, inconsistent setting times can result in poor bonding between the aggregates and the cement paste, jeopardizing the overall integrity of the structure.

Think of baking a cake: if you add extra ingredients halfway through without considering how they mix, the final product might not rise properly or could be dense and uneven. In the same way, adding water to concrete that’s already starting to set can cause weakness and inconsistencies in the final product.

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