Q. I work for a concrete contractor in the San Francisco Bay area. We recently came up against a tricky forming condition that I felt lent itself to the use of self-consolidating concrete (SCC). The project was a water feature that required forming a trough along the entire top of the wall that did not allow any room for concrete placement. Based on information I gathered from reading your magazine, I decided to give SCC a try, although none of us here had used it or even seen it used. We pumped from the bottom up, at the higher end of the sloped wall to allow the SCC to flow. I've attached photos so you can see how it came out. Everyone was quite pleased with the results.
I do have a question though. In an article in your October 2004 issue by Joe Nasvik, there is a photograph of a pump connection to the (Doka) formwork. How do you disconnect the pump when placement is complete? Since there is considerable pressure built up in the system in order to force the SCC through the formwork, disconnecting the hose would relieve that pressure and allow the SCC to come back out of the forms. Our water feature wall was small and uniquely shaped so we built the forms out of wood. We used a PVC pipe inserted at the bottom of the form with an eighth bend then a vertical section of pipe, which we inserted the pump hose into. We found that when the forms were about ¾ full, the back pressure caused SCC to bleed out the end of the PVC. We had to wrap the pump/PVC connection with rags and duct tape (universal problem solver), which sealed the connection and allowed us to complete placement. After the concrete had set, we simply broke off the PVC and ground the end flush. The surrounding finish grade will cover it. This worked well for our little wall but would be impractical for anything much bigger. How have other contractors dealt with this problem?
A. Thank heavens for duct tape. Where would we be without it? But there is a much better way to solve your problem. You're right; the fluidity of SCC would cause problems if there wasn't a good way to seal the pipe opening into the form.
The best way to solve your problem is to mount a shut-off valve (also referred to as a guillotine valve) where the pipe enters the form. You can slide the guillotine closed when you are done placing concrete, allowing you to disconnect the pump hose. There are two ways to do that: either a shut-off valve with an attached framework that makes it possible to bolt the unit directly to the formwork or a shut-off value that attaches to a flange that is pre-installed on the formwork. Each type of valve has a standard concrete hose fitting for an easy connection to the concrete hose. You normally use a hammer to shut the valve.
There are good reasons to consider placing concrete through the bottom of forms, especially SCC. Consolidation around reinforcing steel is good, fewer bug holes result on the form surfaces (especially because the fresh concrete isn't mixing with air as it falls around the reinforcement), less labor is needed during the placement and for repairs afterwards, and vibration of the concrete isn't necessary for SCC. You need to be sure, though, that the SCC retains enough fluidity to complete the process of filling the forms.
One source for purchasing shut-off valves is Construction Forms, Inc., located in the Milwaukee area. It is an after-market supplier serving the concrete pumping industry and can be reached at 800-223-3676 or online at www.conforms.com.