Q.: I sometimes run into problems with water and mud while pouring footers and would like some guidelines. Can you pour concrete directly into water? Does the concrete just displace the water? How do contractors who build bridge piers set their forms? At the time the concrete is poured is there water in the forms? What effect does the water have on the concrete?
A.: The methods used for placing bridge piers require more equipment and preparation than may be justified on most foundation jobs where the amount of water involved is considerably smaller. On big projects where concrete must be placed under water the concrete is placed through a pipe of large diameter (often 10 or 12 inches) which extends below the water to the point of placement. The process is described in "Tremie Concrete," Concrete Construction, September 1962, page 261. Essentially the concrete does simply displace the water, but only if the work is done properly. The precautions necessary to keep the concrete from mixing with the water to any significant extent are given in Chapter 8 of ACI 304, "Recommended Practice for Measuring, Mixing, Transporting, and Placing Concrete."
There are several methods of setting forms in water for bridge piers. One is to drive piles to bedrock with their tops just below the river bottom. A coffer is built that reaches from above the water line to a point well below the tops of the piles. Within this coffer concrete is placed under water by tremie methods to produce a footing of adequate thickness that seals the bottom. The coffer is then pumped out and the pier constructed on the footing. This and other methods are described in M. K. Hurd's Formwork for Concrete, Fourth Edition, American Concrete Institute Publication SP-4, pages 9-54 to 9-56.
The purpose of tremie methods is to keep the concrete from becoming mixed with any significant amount of the water under which it is being placed, since any additional water would increase the water-cement ratio and weaken the concrete. If concrete were mixed with water that is muddy, the contamination by mud would weaken the concrete further.
American Concrete Institute Committee 332, Residential Concrete, is about to publish a recommendation about water in footings: "Pools of rainwater that have collected in footing forms must be pumped out and all water that has collected in forms or on grade should be removed before placing concrete. It is not always possible to get the surface completely dry, particularly where the water table is high. If so, the concrete should be placed in a manner that displaces the water without mixing it into the concrete." ("Guide to Residential Cast-in-Place Concrete Construction," scheduled to be published soon in Concrete International)