When placing concrete from heights, several precautions are essential to ensure the quality and integrity of the final structure, primarily focusing on preventing segregation and achieving proper placement.
Key Precautions for High-Level Concrete Placement
The primary goal when placing concrete from a height is to avoid segregation, which occurs when the aggregates separate from the cement paste. This leads to inconsistent strength and potential defects in the hardened concrete.
Based on standard practices and the provided reference, key precautions include:
- Limit the Drop Height: Concrete should not be dropped from heights greater than 2 metres. This strict limit is crucial specifically to avoid segregation.
- Minimize Discharge Fall: The overall distance the concrete falls from the point of discharge into the formwork should be kept to a minimum.
- Utilize Drop Chutes: If minimizing the drop height directly is not feasible due to the structure's design, drop chutes (also known as tremie pipes or elephant trunks) can be used. These enclosed systems guide the concrete directly to the placement location, significantly reducing the free-fall distance and preventing segregation.
- Avoid Using Vibrators for Flow: Vibrators are intended for consolidation, removing air voids within the concrete once it is in place. They should not be used to move concrete horizontally or make it flow from the point of discharge. This can also cause segregation.
- Employ Acceptable Placement Means: The method of placing the concrete should be suitable for the height and volume being placed. This might involve pumps, conveyors, or cranes with proper hoppers and drop chutes, ensuring controlled and careful placement.
These measures collectively help maintain the homogeneity of the concrete mix during placement, leading to durable and strong concrete elements.
Why is Limiting Drop Height Important?
Dropping concrete from excessive heights allows heavier aggregates to separate from the lighter cement paste and water. This results in:
- Areas with too much aggregate (honeycombing)
- Areas with excess paste (laitance)
Both conditions compromise the concrete's strength, durability, and appearance. Limiting the drop height, specifically to not greater than 2 metres to avoid segregation, is the most direct way to combat this issue.
Summary of Precautions
Here is a quick overview of the essential steps:
- Ensure placement is done by acceptable means.
- Do not drop concrete from heights greater than 2 metres to avoid segregation.
- Keep the discharge fall to a minimum.
- Use drop chutes if necessary to control the fall.
- Do not use vibrators to make concrete flow.
Applying these precautions meticulously ensures that the concrete retains its designed mix proportions as it settles into the formwork, leading to a high-quality final product.
| Precaution | Purpose | Method/Why |
|---|---|---|
| Limit Drop Height | Prevent Segregation | Avoid dropping concrete > 2 metres. |
| Minimize Discharge Fall | Control Placement | Keep the distance from discharge point to final location as short as possible. |
| Use Drop Chutes | Guide Concrete Flow | Recommended when significant height difference exists. |
| Proper Placement Means | Ensure Controlled Delivery | Use suitable equipment (pumps, conveyors, hoppers). |
| Correct Vibrator Use | Consolidate, Not Flow | Vibrators are for removing air, not moving concrete horizontally. |
