7.7. Field Problems and Troubleshooting in Fresh Concrete

• Cause: Over-vibration, excessive slump, improper mix grading. • Solution: Use viscosity modifying admixtures (VMAs), proper mix proportioning.

Segregation occurs when different components of the concrete mix separate from each other, which can lead to poor quality in the finished product. Over-vibration can cause too much movement in the mix, and if the slump is too high, the concrete becomes too fluid and unstable. Improperly graded aggregates can also cause segregation by not providing a good balance of sizes for the mix. To combat this, viscosity modifying admixtures can be added to enhance the stability of the mix, so it stays homogeneous even when pumped.

Think of mixing a salad. If you toss it too vigorously, the lettuce might end up on the bottom and the tomatoes on top, leading to an uneven distribution of flavors. Similarly, too much vibration can cause materials in concrete to separate.

• Cause: High ambient temperature, prolonged transport, high cement content. • Solution: Add retarders, limit mix travel time, use chilled water.

Rapid slump loss happens when the concrete mixture becomes stiffer over time, making it harder to work with. High temperatures can accelerate this process as the heat causes evaporation of water from the mix. If the concrete is transported for too long, it can dry out and lose workability. A high cement content can also lead to quick setting. To address this, retarders can be added to the mix to delay the setting time, the transport duration should be minimized, and using chilled water for mixing can help cool the mix and maintain its consistency.

Consider ice cream on a hot day. If you keep it in the sun too long, it'll melt and become hard to scoop. Similarly, if concrete is exposed to high temperatures for too long, it becomes difficult to work with.

• Cause: Excessive water content, poorly graded aggregates. • Solution: Reduce water-cement ratio, use supplementary cementitious materials (SCMs).

Bleeding occurs when water rises to the surface of freshly poured concrete, causing a layer of water to pool at the top. This typically happens when there is too much water in the mix or when the aggregates are not well graded. To fix this problem, the water-cement ratio should be decreased to limit the amount of water in the mix. Additionally, using supplementary cementitious materials like fly ash or slag can improve the density of the mix, helping to prevent bleeding.

Think about pouring a drink with ice cubes; if there’s too much ice and you pour too quickly, the liquid may bubble up and spill over. Similarly, too much water in concrete can lead to surface issues like bleeding.

• Cause: Coarse grading, insufficient fines. • Solution: Re-balance fine/coarse aggregate ratio, use fly ash or fine sand.

A harsh mix is difficult to work with and doesn't flow well, making tasks like finishing challenging. One reason for this can be the use of a high ratio of coarse aggregates compared to fine aggregates, or an insufficient amount of fine materials (like sand). To resolve this issue, the mix's proportion of fine to coarse aggregates should be balanced to ensure the mix is workable. Incorporating materials like fly ash or fine sand can also help achieve a better texture and workability.

Imagine trying to spread a thick peanut butter on bread; if it's too thick, it’s hard to work with. Adding a bit of smooth jelly (fine material) can help it spread more easily.

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