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Roller-Compacted Concrete Dams

The first step to Knowledge is to know that we are ignorant) – Cecil

OMER A. BILBAS omerbilbas@yahoo.com

Introduction Dams are among the most important and beautiful creation of mankind. They usually surpass other constructions in volume and cost, as well as in social importance and risk. Gravity masonry dams which were predominantly developed by Egyptians, Greeks and Romans lasted thousands of years. Dams require the most developed methods of construction and design, and organization of their building requires the most important skills and the widest experience in civil engineering. Concrete dams have some intrinsic advantages over the fill type structure, they are resistant to

erosion and maintenance is usually less than required for a fill type structure. On account at construction joints, concrete dams have the advantage of preventing temperature cracks, but the complicated arrangement for pipe cooling and joint grouting make them less economical than the conventional embankment dams. Thus, a new construction approach seems necessary for future concrete dams. The new method must not only be better from economic considerations but must be simple quick and with less technical problems inherent in mass concrete construction.

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What is Roller Compacted Concrete Roller compacted concrete (RCC) is a dry (concrete) material that has been consolidated by external vibration utilizing vibratory rollers. It differs from conventional concrete principally in its required consistency. For effective consolidation, RCC must be dry enough to support the weight of the vibratory equipment but wet enough to permit adequate distribution of paste binder throughout the mass during the mixing and vibration process. The consistency requirements have a direct effect on the mixed proportion requirements. The term roller- compacted concrete describes concrete used in the construction process, which combines the economical, and rapid placing techniques used for fill dams with the strength and durability of concrete. RCC is concrete with a no-slump consistency in its un-hardened state that is transported, placed, and compacted using fill – dam construction equipment. The properties of hardened RCC are similar to those of traditionally-placed concrete. Placement of RCC is in horizontal layers similar to the techniques of earth and rock-fill dams. The dry, nonflowable nature of RCC makes the use of a wide range of equipment for construction and continuous placement possible. End and bottom dump trucks and /or conveyors can be used for transporting concrete from the mixer to the dam. Mechanical spreaders, such as caterpillars and graders, place the material in layers or lifts .Self-propelled, vibratory, steel-wheeled, or pneumatic rollers along with the dozers perform the compaction (figure1).

The thickness of the placement layers, ranging from (20 to 60 cm) is established by the compaction capabilities. With the flexibility of using the above equipment and continuous placement, RCC dams can be constructed at significantly higher rates than those achievable with conventional mass concrete. Using this construction procedure RCC dams can be raised at a rate close to (10m) per month or more .The number of joints between the relatively thin layers and the related quality control can have a large influence on the overall stability of the dam in term of uplift water pressure, tensile and shear (cohesion) strength at the joints between the layers. The majority of RCCs contain mineral admixtures, commonly fly ash, as an active constitute of the concrete and one of the major advance in concrete technology brought about by the development of the RCC dam has been the greater understanding of the performance of mineral admixtures in concrete. The objective of RCC dams is to place, as much concretes possible without construction joints to maximize production and to reduce the costs associated with the treatment of joints. A typical cross section of an RCC dam is shown in (figure2) Brief History of RCC Dams The first dams to be built of RCC were gravity dams and this type remains by far the most common. Nevertheless by the end of 1996 there were two examples arch-gravity RCC dams in South Africa, one arch RCC dam in China .The properties that can be achieved with RCC are more than adequate for the slightly

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increased loading with these sections when compared to the traditional gravity section. RCC has also been used for a considerable number of coffer dams where the advantage of speed of construction can be fully utilized, some RCC cofferdams have been constructed at vertical rates of over 30m/month.In addition some have been overtopped at an early age without damage. The interest in RCC dams is driven by economic considerations and also where speed of construction is an important element. Given an adequate foundation, RCC dams commonly have a lower cost than the equivalent fill dams when the saving in cost of diversions and spillways are taken into account. Roller-compacted concrete has been in regular use since the last 1920s mostly as a base for highways and airfield pavements .In this application it is commonly known as lean–mix or dry-lean concrete and similar terms. The first suggestion that RCC could be used in dam construction was in 1941 in the cofferdam (Chinese Taipei-Taiwan) .The concrete was used as an impervious core and was placed using earth-fill methods and was roller compacted. The first use of RCC in large volumes was at Tarbela Dam (Pakistan) in 1975 and the first major RCC dam was the 52-m high in (USA) completed in 1982.The RCC in this dam had a low cementitious content in the dam body (47 kg/m3) of cement and (19 kg/m3) of mineral admixtures .Pre-cast concrete panels formed the up stream face. The next RCC dam was the 40m high (Australia) completed in 1984 .This dam contained an RCC with 80Kg/m3 of cement in the central zone and 80Kg/m3 of cement and 30Kgm3 flyash in the outer zones of the dam. In 1985 and 1986 two dams on opposite sides of the world (Spain and China) introduced the concept of using higher cementitious content in RCC with a high proportion of mineral admixtures .These two dams were the first dams by two of the countries now leading the development of RCC for dam construction. Soon after these dams, RCC dams were also initiated in South Africa, Brazil and Mexico. With greater experience and gain in confidence in the material, the next step forward for RCC was the extension of its use to

arch-gravity and eventually arches dams. Essentially RCC has effectively replaced traditional concrete for the construction of dams. ADVANTAGES OF RCC DAM CONSTRUCTION The advantages of the RCC dam method of construction are many. Some of the advantages are primarily applicable with certain types of mixtures , structural designs, production methods, weather or other conditions .Each RCC project must therefore be thoroughly evaluated based on technical merit and cost and compared with alternative methods of construction. The major advantages of RCC in dam construction as compared with traditional-concrete dams include: 1-More rapid construction (2.5 to 3m vertical progress per week can be achieved in larger rates has been achieved in smaller dams). -Effective use of conventional equipment (trucks, dozers, vibratory, rollers, etc)٢ -A reduced cost of construction as a sequence of the above.٣ 4-Thinner layers which lead to increased safety during construction by reducing the differences in levels between placements. -Safety is also enhanced by the reduced dependence on formwork. ٥ 6 -Shorter river diversions (both in terms of length and time) during construction. 7 -The dam is capable of passing floods during construction by over-topping without damage. At the end of 1986 there were only 15 compacted RCC dams in the world: six in the USA, two in each of South Africa, Australia, Japan, and one in each of Brazil, Spain and China .It is of interest that these seven countries are still leading proponents of RCC dam construction .At the end at 1996, there were at least 157 completed RCC dams and a further 32 were under construction. At the end of 1996 there were 157 completed RCC dams in some 20 countries. The countries that had completed RCC dams at the end of 1996 are shown in the tableNo. (1): Nowadays close to 343 RCC dams completed or under construction as shown in the table No. (2); Besides its use in dams, RCC can find economical use in the construction of roads, airport runways, seaport pavements and berths, mass concrete floors, check dams large size water storage tanks, large size stilling basins, …etc. References 1-Roller-Compacted Concrete Dams, Bulletin 126-2003, International Commission on Large Dams (ICOLD) 2-Roller Compacted Concrete Dam, by V.K Mehrotra, First Edition; 2004.

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No. North America No. Asia

39 USA 102 China 7 Mexico 46 Japan 2 Canada 1 Kyrgyzstan 48 Total 3 Thailand

South America 2 Iran 37 Brazil 3 India 2 Honduras 3 Jordan 1 Argentina 2 Philippines 1 French Guyana 162 Total 2 Chile Europe 43 Total 22 Spain

Oceania 6 France 11 Australia 2 Romania 11 Total 5 Greece

4 Turkey 343 Gant Total

39 Total Africa

14 South Africa 15 Morocco ١ Angola 2 Algeria 2 Tunisia 2 UAE

6 Vietnam

42 Total

No. North AmericaNo. Asia

28 USA 23 China 5 Mexico 28 Japan 2 Canada 1 Kyrgyzstan

35 Total 1 Thailand South America 53 Total

9 Brazil Europe

2 Honduras 19 Spain

1 Argentina 6 France

1 French Guyana 2 Romania

1 Chile 1 Greece

14 Total 28 Total

Oceania Africa 8 Australia 11 South Africa 8 Total 7 Morocco

157 Grant Total 1 Angola

19 Total

Table No. (2)

Table No. (1)

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