Constructing the spillway crest for Willow Creek Dam could have been a dangerous and expensive job if conventional concrete placement techniques had been used. Instead, wet mix process shotcreting methods saved money and made construction indisputably safer. The job also produced some test results which suggest that running strength and air content tests on concrete shot directly into the cylinder mold or air meter bowl may produce more realistic results.


Savings of millions of dollars on this dam in Heppner, Oregon had already been made possible by using roller compacted concrete (RCC) methods. However, RCC construction procedures did not allow exact enough shaping of the spillway. An accurate parabolic curve was needed at the top of the spillway for hydraulic reasons and the spillway crest had to be level within plus or minus 1/2 inch of the design elevation along its entire 380-foot length. The idea of using shotcrete in lieu of more conventional placing techniques was proposed and accepted under the value engineering clause of Corps of Engineers construction contracts.


During each day of production the Corps of Engineers cast a series of companion compressive test cylinders from a sample of wet mix from the truck. This was rodded into the mold in three layers. Strengths of these cylinders were compared with the strengths of shotcrete samples made by shooting directly into the 6x12-inch cylinder molds. Significantly and consistently higher strengths resulted when the mix was shot directly into the mold: 35 percent higher at 7 days and 15 percent higher at 28 days. It is also of importance that there was less variability of the strength specimens when the mix was shot directly into the molds.