Roller-compacted concrete dams have steadily taken their place in the menu of possible dam types over the past four decades, with over 650 of them completed or under construction, according to HydroWorld magazine. China is far and away the leading RCC dam builder, with 165 as of 2012; of those, 40 are higher than 100 meters. Japan, the United States, Brazil and Spain are the other most prolific RCC dambuilders.

Roller-compacted concrete dams evolved in the 1970s to satisfy a particular need by dambuilders and owners. The percentage of dams built with concrete dropped steadily between 1950 and the late 1970s, from 38 percent to 16.5 percent. This decline occurred “in wide-valley sites, where concrete-gravity dams were being replaced by less costly earth-and-rock embankments,” according to the 1991 book “Roller-Compacted Concrete Dams” by Kenneth D. Hansen and William G. Reinhardt.

The increased popularity of embankment dams led Engineering News-Record to editorialize in our March 6, 1969, issue, saying, "The technology of mass concrete construction simply has not kept pace with the art and science of earthmoving. It is time for a study into ways of reducing the cost of concrete dams. … Dams must be conservatively designed and carefully built. But it does seem that in all the years since Hoover Dam, there should have been more change in the bucket-by-bucket method of moving mass concrete into place. What’s needed is a lot more systems analysis and a bit less ‘grandpa-ism.’ ”

The dambuilding community in the United States organized two conferences: A 1970 conference was titled “Rapid Construction in Concrete Dams,” and the second, in 1972, was called “Economical Construction of Concrete Dams.” Since embankment dams were more prone to failure, the conference attendees were looking for a new type of dam that could combine the safety advantages of concrete and the efficiencies of embankment-dam construction.

The first major step toward the development of RCC dams, a hybrid structure combining elements of both concrete and embankment dams, was Italy's 172-m-high Alpe Gera Dam, completed in 1964. It maintained the traditional cross section of a concrete gravity dam while reducing the cost of placing the concrete. Among other factors, the cost savings was achieved by reducing the cement content in the concrete mix used for the interior of the dam, where stresses are low and durability requirements are minimal.

The book “Roller-Compacted Concrete Dams” pinpointed the method used for that project, noting, “Placing this lean concrete mixture using earthfill construction methods was the greatest step forward.... Instead of building the concrete dam in vertical lifts to form cantilever blocks, horizontal placement was introduced. Dump trucks delivered the interior concrete to the dam, rather than buckets moved by crane or cableway. Side forms for blocks were eliminated, as were cooling coils. The consolidation of the lean concrete by internal immersion vibration, rather than external roller compaction, was about all that kept Alpe Gera from being the first RCC dam.”

Another critical step in the development of RCC dams was the design and construction of the Barney M. Davis cooling-water reservoir dike, completed in 1973, in Corpus Christi, Texas. The 6.7-m-tall, 10-kilometer-long ring dike was more economical than a conventional sandfill embankment and had soil-cement protection on both sides. “Besides being the only large dam constructed entirely of soil-cement, the Barney M. Davis reservoir embankment marked the first recorded use of vibratory rollers to compact soil-cement," note Hansen and Reinhardt.

RCC dam design evolved in three different directions during the 1970s. The U.S. Army Corps of Engineers conducted field tests that confirmed the basic construction method and yielded information on material properties and the strength of the bond between successive layers of RCC. After several less-than-perfect attempts, the Corps’ Willow Creek Dam, completed in 1982, in Oregon was the world’s first major dam to be built entirely of RCC.

Roller-compacted concrete shares the same ingredients as conventional concrete but in different ratios and with partial substitution of fly ash for portland cement. Containing less water, the produced mix is drier and essentially has no slump. By minimizing cement and/or pozzolan contents, costs are reduced.

And if no pozzolans or admixtures are used, storage, mixing and materials delivery are simplified. Lean mixes also eliminate the need for vertical joints, further reducing costs and complexity.

Other differences between RCC dams and conventional concrete dams involve construction methods. Because the RCC mix is too dry to be combined in ready-mix trucks, it is usually mixed at a temporary plant on-site, transported by conveyor belt and then compacted with a vibratory roller in 1-ft-thick layers, or lifts. Continuous placement of RCC is usually specified to minimize cold joints between the horizontal concrete layers that could inhibit bonding.

Several gigantic RCC dams are under construction in Ethiopia. The Gibe III Dam, the largest of a series of four dams on the Omo River, has been under construction since 2008 and is expected to be completed this year. The 243-m-tall, 630-m-long dam will contain 6.1 million cu m of RCC and generate 1,870 MW of electricity when the last of the 10 turbines is installed in 2016. Impregilo is the EPC contractor of the $2-billion project. It is expected to have a transformative impact on East Africa, double Ethiopia’s electric output and, once a 1,068-km-long transmission line is completed in 2018, supply lower-cost electricity to neighboring Kenya.

Casting an even longer shadow both physically and politically, the Grand Renaissance Dam, under construction on the Blue Nile in Ethiopia, is a $4.7-billion job. It includes a 170-m-tall, 1,800-m-long main dam that contains 10 million cu m of concrete; a 50-m-tall, 5,000-m-long rockfill saddle dam; and two powerhouses that generate a total of 6,000 MW of electricity. Construction started in 2010 and is expected to be completed in 2017. Impregilo is the contractor, while China is providing about $1.8 billion in turbines and equipment financing.

For several years, Egyptian officials protested that the Grand Renaissance Dam would disrupt the vital supply of the Nile, Egypt’s principal water source, in violation of the 1929 treaty governing water allotments from the Nile between East African nations. Ethiopian officials maintained that, when completed, the dam’s reservoir would be filled slowly and not threaten Egypt’s water supply. Last week, defusing tensions, Egyptian President Abdel Fattah al-Sisi addressed the Ethiopian Parliament and announced a three-way accord among Egypt, Ethiopia and Sudan concerning the water allotments from the river.

Elsewhere in Africa, the Lauca Dam, under construction on the Kwanza River in Angola, has been under construction since 2012. It will be 132 m tall, 1,075 m long and will contain 2.6 million cu m of RCC. It is expected to be completed in 2018. Its installed capacity of 2,070 MW will more than double Angola’s current hydroelectric generating capacity. It was designed by COBA, an Algerian engineering firm, in partnership with Lahmeyer International; the EPC contractor is Odebrecht of Brazil.