Q: We have been lining manure pits with concrete on slopes of 2H:1V for some years and have always specified (per ACI recommendations) the use of long chutes or pumps to place the concrete as near to its final position as possible. Some of our contractors would like to place the concrete by depositing it near the top of the slope and letting it slowly move down the slope like a lava flow. The slump would be 2 to 3 inches and not plasticized. Very little raking for consolidation would be done, and a roller with etching would be used for a final roughened surface finish.

We are concerned about aggregate separation, contamination of the concrete with subbase material, and voids down the slope of the steel reinforcement. Do you have any experience with this type of placement method? Can it be successful? Should it be allowed at all? Is it an industry standard to place concrete very near its final position?

A: Yes, industry standard is absolutely to place concrete as near its final position as possible. ACI 325.9 ("Concrete Pavements and Bases"), Section 9.1, says to deposit the concrete directly from the mixer "near its final position." ACI 309.2 notes that horizontal movement of concrete causes segregation of the concrete aggregate and results in honeycombs. The ACI Concrete Craftsman Series book Slabs on Grade states, "If a slab is on a slope, start at the low end and work uphill" We would insist on some method to get the concrete closer to the point of placement.

With such low slump concrete, we don't feel that the concrete would even flow down a 2:1 slope and we would expect it to roll down the slope rather than slide, picking up subbase material as it goes. Finally, you imply that there is steel reinforcement in the concrete. Without some kind of vibration, we don't think you will get good consolidation around steel with such a low slump mix.