Effect of Pore Water Pressure on Friction between Concrete and Slipform Panel During Slipforming

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Title: Effect of Pore Water Pressure on Friction between Concrete and Slipform Panel During Slipforming

Author(s): Kjell T. Fossa and Magne Maage

Publication: Materials Journal

Volume: 101

Issue: 2

Appears on pages(s): 117-123

Keywords: concrete; friction; slipform; stress.

Date: 3/1/2004

Abstract:
Slipforming is a construction technique that is often used for vertical concrete structures such as bridge columns, towers, and offshore platforms. Friction will occur in the interfacial zone between the slipform panel and the concrete as a consequence of the lifting of the slipform. One of the main parameters controlling the friction (net lifting stress) is the pore water pressure in the concrete. When the concrete is newly placed, the pore water pressure is positive. During the early setting, the pore water pressure will decrease due to the chemical shrinkage accompanying cement hydration. This under-pressure in the pore water will result in a higher effective pressure against the slipform panel. Tests carried out show that higher effective pressure produces a higher stress during lifting. Effective and pore water pressure in concrete has not been considered in connection with slipforming before. The minimum pore water pressure is defined as the pore water pressure at the time of maximum lifting stress. Minimum pore water pressure occurs in these tests before initial set and depends on the concrete mixture composition as well as the lifting technique. Lower air content and a finer pore system will lead to a lower minimum pore water pressure. Also lower lifting frequency and lower lifting height of the slipform panel will lead to a lower minimum pore water pressure. Lower minimum pore water pressure will result in a higher maximum lifting stress. It is assumed that higher lifting stress will increase the risk for any surface damages during slipforming.