Placing concrete from bottom up
Fresh concrete cast into a vertical element such as a wall or column is normally carried out from the top down, typically with a crane/skip with tremie or a pump. In certain instances, this is not possible for example:
• Heavily congested rebar or other inclusions above or within the element.
• Casting under an existing floor, pile cap or transfer beam.
• Underground, basements or tunnels linings.
• Strengthening or within existing concrete.
One method to overcome the above is to cast top down via a letterbox at set intervals, but this can be fraught with compaction issues. Another method is to pour from the bottom of the element upwards by pumping using a valve opening in the formwork. The concrete needs to be self- compacting as it must remain mobile to fill the form. Attempting to compact a less fluid concrete as it is pumped would probably cause a blockage.
Other benefits to consider is the reduction in access staging and ease of handling. It takes less air into the concrete and allows faster casting rates than pumping or skipping from the top. However, the formwork pressure will be higher than the static head above the valve and needs to be accounted for in the formwork design. The pressure in the forms will increase as the pump delivers concrete and more so on restarting the pump after a stop has occurred.
After pumping from the bottom is complete, the valve is closed and locked. This will leave a obtrusion or impression of the valve on the concrete face once struck. The protruding concrete can be removed after striking of the formwork, however if the visual appearance of the element is of importance, then this would need to be detailed out.