Pouring concrete in columns and walls is a critical process that requires great caution and attention to detail. Proper techniques are essential to ensure the concrete achieves its designated strength and durability, which directly impacts the safety and longevity of the structure. This article outlines best practices for pouring concrete in columns and walls to avoid common problems such as segregation, honeycombing, cracks, and corrosion of reinforcement.
The Risks of Improper Concreting
Incorrect concreting procedures can severely compromise the integrity of concrete columns and walls. Problems such as segregation — where the coarse and fine aggregates separate — can weaken the concrete mass. Honeycombing, which leaves voids and gaps, and the development of cracks also degrade structural performance. Moreover, if the concrete fails to properly encase steel reinforcement bars, corrosion may occur, leading to early deterioration and eventual failure of the element.
Recommended Procedures for Pouring Concrete
General Guidelines
To achieve a consolidated and durable concrete mass, the concrete should be placed as close to its final position as possible. Pouring should be done in layers rather than all at once. Each layer must be compacted thoroughly using vibration to ensure there are no voids or gaps, but care must be taken to avoid over-vibration, which can cause the aggregates to separate and reduce the overall quality of the concrete.
Specific Guidelines for Columns and Walls
- Short Columns and Walls: For forms of relatively short height, pour concrete directly from the top.
- Tall Columns: Use layers approximately 30 to 50 cm thick to ensure proper compaction.
- Concrete Walls: Limit each layer to a maximum thickness of 60 cm per pass to prevent segregation.
Timing Between Layers
It is crucial to pour subsequent layers within a certain timeframe to avoid cold joints—weak planes between layers of concrete. Under normal conditions, this interval should not exceed 30 minutes. In hot weather, reduce this to 20 minutes to prevent premature setting.
Pouring Deep Walls
For walls deeper than 1.2 meters, use vertical trunks or chutes placed approximately 2.4 meters apart to control concrete flow. The concrete should freefall from the chute end over a distance of 0.9 to 1.5 meters (sometimes limited to 0.6 meters) to avoid segregation caused by excessive drop. The freefall must be continuous and should not strike reinforcement or embedded objects, which can lead to concrete separation.
Where possible, discharge concrete directly into formworks without using chutes or hoppers, reducing segregation risk.
Vibration and Compaction
Each concrete layer must be vibrated immediately after placement to remove air pockets and consolidate the mass. The vibrator should be lifted only after fully compacting the layer, and its tip should extend about 10 to 15 cm into the previously placed layer to create a good bond. Avoid moving the vibrator laterally, which can cause segregation and voids.
Managing Interruptions During Pouring
If there is an interruption in the concrete supply, cold joints can form between batches, weakening the structure. To mitigate this, set retarders like sugar can be applied to the concrete surface to delay setting time by up to four hours, allowing better bonding with the next batch. When pouring resumes, the new layer should be thinner than the previous one, and vibration must extend into the earlier layers.
Special Considerations for RC Columns and Narrow Walls
Pouring should start with a 5 to 10 cm grout layer to prevent loose coarse aggregates from collecting at the bottom, which could cause honeycombing. The grout should have the same slump and equal or lower water-cement ratio as the main concrete mix. Alternatively, use a concrete mix with half the normal amount of coarse aggregate at the bottom layer with a thickness between 15 and 30 cm.
If concrete pouring must be paused for a day or more, roughen the surface after the initial set using a brush to improve bonding with the next concrete layer.
Signs of Improper Concrete Pouring
Sloping layer lines, water leakage, and honeycombs in walls are clear indications of improper pouring techniques. These defects often result from uneven placement, inadequate vibration, or excessive freefall distances.
Summary and Best Practices
To avoid common problems in pouring concrete for columns and walls:
- Avoid placing the full depth of concrete at a single point.
- Use lower slump concrete for upper layers to reduce segregation.
- Maintain vertical movement of the vibrator and avoid lateral motion.
- Thoroughly vibrate each layer to consolidate the concrete properly.
By adhering to these guidelines, the structural integrity and durability of concrete columns and walls can be effectively maintained, reducing the risk of defects and premature failure.