A major concern in underwater concreting is that the water in which concrete is placed will wash away cement and sand or mix with the concrete and increase the water-cement ratio. However, if the lower end of the tremie is kept immersed in concrete during a placement, most people believe that new concrete flows under and is protected by previously placed concrete. Experiments have shown that concrete exiting a tremie pipe may exhibit different flow patterns, exposing more concrete to water than was originally expected.
FLOW PATTERNS OBSERVED IN UNDERWATER CONCRETE
A tremie placement box 20 feet long, 4 feet wide and 4 feet high was constructed and filled with water to a height of 3 1/2 feet. A 10-inch-diameter pipe was used to place six cubic yards of colored concrete, each two cubic yards being a different color. After the concrete was placed and allowed to harden, the water was drained and cores were removed from the concrete section to establish the fresh concrete flow pattern. The flow patterns didn't verify the typical explanation of how fresh concrete flows from a tremie pipe and is protected from mixing with the water. Instead two different flow patterns were observed: layered and bulging flow. In the layered flow pattern, the new concrete flowed upward around the tremie pipe and then outward over the slope, much like the flow of lava from a volcano. Slopes were very steep as the fresh concrete flowed away from the tremie. The bulging flow pattern, on the other hand, produced a general displacement of the concrete, resulting in much flatter slopes. It provided better quality concrete at the far end of the placement and produced much less laitance, making it the preferred flow pattern for actual performance. This type of flow pattern is apparently made possible by a reduction in the shear resistance of the fresh concrete.