A successful and fruitful symposium10 June 1999
Shen Chonggang reports from the International Symposium on RCC Dams, held in Dujiangyan, China, on 21-25 April
The Advisory Committee of the Symposium on RCC Dams had proposed four questions to the symposium delegates. They were considered on the afternoon of 23 April, and the resulting discussions are summarised by Shen Chonggang as follows.
•Question 1. Which type of structure is best for the upstream face of a high RCC dam? Is any additional protection necessary to guarantee its impermeability? Most speakers considered that good RCC will have high impermeability, if the surface of the layers is well treated. In general, the coefficient of filtration will be less than 10-6-10-8m/sec, and this has been confirmed in the laboratory, from cores taken on site, and tested on site on borehole samples. The quality of RCC should in fact be better than CVC.
The question had been raised by the owners and designers: they did not have so much confidence in RCC, so they recommended different kinds of anti-seepage structures or other protection. Many agreed, however, that if the RCC technology was used correctly, additional protection should not be necessary. Some considered that if additional protective structures were used contractors might not pay enough attention to the treatment of the surfaces of layers and the quality of the RCC. In such cases the risk of leakage would indeed be increased.
In China the so called grout-enriched vibratable RCC (GEVR) is now used. In this technique a mortar using cement and pozzolan is added to the RCC on the upstream surface against the forms, then vertical vibrators are used to compact it. Such technology improves the smoothness of the upstream surface and also decreases the coefficient of filtration by about 0.1m/sec, as shown by tests on cores obtained from Jiangya dam.
•Question 2. The technology of slope–layer placement of concrete.
The contractors at Jiangya dam developed a measure to decrease the area of placement of concrete with a 1:10 to 1:12 slope-layer. The advantages are: the area of concrete placement is reduced and the time required to cover it with a new layer of concrete is shorter than with a horizontal layer, so it is easier to treat the surface layer; when the temperature of concrete being placed is lower than the air temperature, the loss of heat in the precooling concrete will be less than horizontal layers; water drains from the surface more easily than with a horizontal layer; if the slope is inclined to the upstream, the shear strength will be higher than with horizontal layers. It was warned that the technology should be applied under strict conditions to avoid any segregation: if the slope direction is from one abutment to another, the end of the slope must be compacted carefully without any access from upstream to the downstream.
This technology was used at Jiangya and when cores were drilled they showed good results when compared with horizontal layering. However, leakage on the downstream face should be carefully checked after the water is impounded in the reservoir. Some experts observed that from the point of view of dam safety, the technique should be used in a small dam first, before being used in large dams.
•Question 3. Additives for RCC mixtures.
Additives were used successfully in all RCC dams in China They were effective because they reduced water, improved workability and promoted a longer setting time for the concrete, producing a higher quality of RCC. This is a significant benefit in the construction of RCC dams, but worldwide around 40% of RCC projects do not use additives due to their high price.
Air content is a very important factor in the durability of RCC. Several Chinese high RCC dams have used an RCC mixture with composed additives intended to contribute to both water reduction and air entraining. Good results have been obtained.
•Question 4. General discussion on the benefits of RCC dams.
The unit price of RCC compared with CVC is about 15-20% cheaper, and in China the reduction can be as much as 30% cheaper. In other countries different prices apply, and the competitiveness of the RCC dams varies. In some areas they are highly competitive, for example there is a good chance of RCC being chosen in Brazil.
The construction time depends on many factors. Generally it will be shorter than CVC dams, sometimes shorter than CFRD. However, unless material preparation is good and investment arrives on time, the construction time will always be longer than expected.