3.3. Causes

In this section, we will explore the various causes that lead to issues in fresh concrete such as segregation, bleeding, slump loss, and re-tempering.

This chunk serves as an introduction to understanding why fresh concrete may fail to perform optimally. Each of the identified issues—segregation, bleeding, slump loss, and re-tempering—has specific causes that can be linked to methods of mixing, material choices, and environmental factors.

Think of concrete like a cake batter. If you don't mix all the ingredients properly, or if some ingredients are too heavy and sink to the bottom, your cake will not rise evenly, just like concrete may not set well if it's not mixed properly.

• 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).

Segregation occurs when the components of the concrete mix separate, leading to weaknesses. Poorly graded aggregates mean that the sizes of particles are not well balanced, which can lead to larger stones sinking. Excessive vibration can disturb the mix too much, separating water and aggregates. A high water-cement ratio makes mixtures too fluid, encouraging separation. Dropping concrete from heights can also lead to the materials spreading out unevenly, while inadequate cohesion can result from the mix itself not being designed properly.

Imagine pouring a fruit salad where heavier ingredients like grapes sink to the bottom while lighter items like oranges float to the top. The same effect happens in concrete if aggregates are not properly mixed or placed.

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

Bleeding occurs when water rises to the surface of freshly poured concrete due to the settling of solid particles. When the water-cement ratio is too high, more water is available to escape, causing bleeding. An improper sand mix can also either retain too much water or not enough, contributing to this issue. Poor quality cement can have varying effects on hydration, leading to uneven outcomes, and poorly graded aggregates can cause inadequate packing, resulting in excess water pooling.

Think of making a smoothie. If you add too much liquid, eventually, the liquid will rise and separate from the fruit, just like how water can separate out in concrete.

• 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 reduction in the concrete's slump over time, affecting workability. High temperatures or windy conditions can speed up hydration, causing the mix to thicken faster than desired. When water evaporates quickly, the remaining mix can dry out, making it hard to work with. If transportation or placement is delayed, the concrete may begin to set prematurely. High cement content leads to quicker setting times, making it less workable. Finally, not using appropriate additives to retain moisture can aggravate the slump loss.

Imagine leaving a scoop of ice cream out in a hot room; it melts quickly, and if you wait too long, you can't scoop it anymore. Similarly, fresh concrete can lose its workability if exposed to heat for too long.

Re-tempering is the addition of water or mixing of concrete after initial setting has started to regain workability.

Re-tempering refers to a common practice where extra water is added to concrete that has already begun to set in order to regain its workability. This is done when delays occur on the site, but it can lead to significant issues such as reduced strength and increased porosity, compromising the overall integrity of the concrete structure. Over time, this disrupted hydration can create inconsistencies in how it sets and cures, potentially leading to cracks or other failures.

Think of a sponge. If you add more water to a sponge that’s already soaked, it won't just absorb it all properly again; it may become overly soggy and fall apart, similar to how re-tempered concrete becomes weaker.

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