Q: In June 2001, a Problem Clinic question was posed about the advisability of depositing concrete near the top of a 2:1 slope for a manure pit lining and letting it flow to the bottom. The questioner indicated that the slump was 2 to 3 inches. We responded that industry standard is to place concrete as near as possible to its final position that letting it slide or roll to the bottom of the slope could cause segregation and poor consolidation, especially around reinforcing steel. A contractor who actually does this work protested, stating that using conveyors or pumps to get the concrete to the bottom of the slope is cost-prohibitive and that they actually use 4.5-inch slump concrete that flows without noticeable segregation. We asked two industry experts for their thoughts on this situation.
A: Dick Miller, longtime concrete expert in California, responded, "I have looked up the original question and your answer, and I agree wholeheartedly. I feel it is risky to place concrete in this manner, especially without any type of vibration. The only type of successful placement I have observed similar to this is in concrete ditch linings, but that was done with a vibrating, traveling form. Since concrete exposed to manure is subjected to sulfate attack, as well as other chemicals, I would expect a short life. Durability would be my first concern. Because vibration is lacking, and therefore consolidation, plus the potential contamination from the subbase, I do not see this as a good method. The original question does not mention water-cementitious ratio, but I think ACI 201.2R should be followed." (ACI 201.2, "Guide to Durable Concrete," recommends for concrete subjected to moderate sulfate attack that the w/c ratio be kept at 0.50 and that Type II cement be used.)
On the other hand, Bruce Suprenant with Baker Concrete takes a more performance-based approach by recommending that the contractor try to prove his point by coring through a bar. "If the core density is uniform, with few or no voids, and the concrete is consolidated around the bar, then his placing method is not creating a segregation or bond issue. He can show that to the inspector, or ask the inspector where to core. Placing the concrete as close as possible to the final position may eliminate segregation and bond issues or it may not. This slope is about 22 degrees with rebar. It's difficult to say how much sliding there will be without watching it. And, even if there's sliding, it does not necessarily mean segregation. Think of a wall pour concrete is always sliding somewhere. Core ... core ... core ... core .... someone will then win the argument either the contractor or the inspector. It will cost about $500 for the core."