Top down construction is increasingly popular due to its productivity benefits. This concrete construction method allows for above ground construction work to be carried out at the same time as excavation of the basement, resulting in significant time savings. Commonly used in urban areas, top down construction is often used for high rise buildings with deep basements and for underground structures such as car parks and transit systems.
The construction typically begins with:
- Execution of diaphragm walls
- Construction of piles with plunge columns
- Construction of concrete floor slabs
Concrete challenges in top down construction
There are typically space constraints in top down construction, particularly when pouring foundations underground. In these confined spaces, it’s next to impossible for workers or large machines to easily access concreting areas to vibrate concrete or move it around safely. The concrete needs to flow easily.
In some instances, long slump retention is also needed, sometimes for as long as 12 hours. The poured concrete in foundation piles needs to retain slump so that steel king posts can be inserted after the pour is completed.
Control flow concrete
Control flow concrete is ideal for this construction method. Unlike conventional concrete, which requires a great deal of labor to move and place, control flow concrete flows more readily, yet is still segregation-resistant. It has high passing ability through congested formwork. There’s also minimal need for vibration during placement due to its high flow and high responsiveness to external energy.
Top down construction projects can utilize control flow concrete to gain numerous advantages, including:
- Reduced labor costs. Control flow concrete flows easily, making it much less labor intensive to place and finish. This means the workers can be much more efficient.
- Improved safety. With control flow concrete, workers can move concrete with less strain. And with less reliance on vibration machines, there’s less chance of injury.
- Increased profitability. CONCERA® concrete admixtures allow for the production of control flow concrete, without the costs of specialty mixes or the additional quality assurance costs that are typically needed with self-compacting concrete. It also extends the slump life of concrete, which helps simplify the work process. The long slump time saves money, because there is no need to pay for king post hauling machines to be on-site for long periods of time.
- More consistency. Control flow concrete has excellent tolerance to moisture variation. It also offers a predictable slump flow and air control properties, enabling batch plant adjustments and consistent properties at the job site.