Dear Editor, In your September 2000 issue I was interested by the article on concrete faced rockfill dams (CFRDs) in China, especially in their use of slip-formed face concrete.
I am a student currently completing my final year project at Curtain University in Western Australia. I am investigating various construction methods of reinforced concrete placed on steep, sloping reservoir walls.
The main problem with placing concrete on a slope, when used on sites where water will be stored, is the amount of vertical joints required between each cast panel. The joints are potential leakage zones which result in lost revenue. Limiting the amount of vertical joints can directly result in reduced leakage losses. The slope that I am working on is a 1m vertical to 1.2m horizontal, with a vertical height of 10m (15.6m along the slope). These are the typical slope parameters for reservoirs constructed locally. This is obviously a lot smaller in scale than panel sizes used on a dam. The slip-form methods described in your article on CFRDs in China incorporate the same basic construction methods used on local reservoirs.
Local reservoirs have traditionally been constructed in 9m wide reinforced concrete panels. The panels are slip-formed vertically (up the slope) and cast in a checkerboard pattern of alternate bays, which allows for curing and shrinkage before the alternate bays are filled in. This proven method provides a surface of high quality but results in extensive construction joints (at every 9m). To pour each panel can take an average of 6 hours.
I am currently investigating the feasibility of pouring larger continuous sections of up to 50m in length between construction joints. The proposed system would follow the same slip-forming method but would require each vertical slip-formed panel to be poured quickly enough to cast the adjacent panel and form a green joint. Theoretically if this were possible, one could pour large sections continuously without joints.
Each 9m wide slip-formed panel would need to be cast in 30-40 minutes. The side form/guide rail would then be removed, and the next panel would use the green concrete on one side as a support. The system would then continue in this manner until a construction joint is formed as designed.
This proposed method faces many potential problems, such as: concrete slumping due to very quick slip-forming rates (rapid travel times); shrinkage and honeycombing at the green joint; shrinkage at green joints; impervious concrete due to honeycombing; uneven cover to reinforcement, due to potential slumping of the previously laid panels; and slumping caused in previous green panel by secondary vibrations.
Other problems include removing the side form quickly as soon as the panel is poured, and can green concrete act as a rail to support the slip-form during the adjacent pour? Alternatively, can the rails be in two parts and the lower half removed for a second trailing slip-form to commence before the entire previous panel is finished? It is my task to research the feasibility of this proposed method of construction. I am required to report back on the feasibility (or otherwise) of this proposed method, or on any other methods that could significantly reduce the amount of joints required.
The problems faced here are similar to those experienced on slip-formed CFRDs. I would like to hear from your readers in the engineering community who deal with such construction methods and would be grateful for any information on the topic.