Taking advantage of RCC

Roller Compacted Concrete (RCC) has been rapidly developing over the past 40 years and is now commonly used for mass concreting operations typically in a Gravity Dam application. With a widely varied methodology of design theories and project specific considerations, RCC is not only extremely practical from a constructability standpoint; it is also, along with other advantages, very cost effective.

Although there have been many applications of a dry mixed concrete in the history of concrete construction and differing opinions of its inception, one of the first known substantial applications of RCC was in the mid 1970’s at the Tarbela Dam project on the River Indus in Pakistan (then referred to as “Rollcrete”). Several other very successful applications of Rollcrete were used at Tarbela for various reasons until 1980 (E Scrhader).

The application at Tarbela Dam was for an emergency backfill operation after a malfunctioning gate failure in the powerhouse headrace tunnel released a high volume of water downstream creating a massive cavity in the plunge pool area and immediately downstream of the Earthfill dam foundation. With concerns about the integrity of the newly built Tarbela Dam comprised of some 127Mm3 of Earth and Rock fill material, the Engineers on the project collectively proposed an emergency backfilling operation with a “0 slump dry mix concrete” During this application of the Rollcrete backfill operation some 18,000m3 was placed in a single 24 hr period.

The Application of RCC at Tarbela was extremely successful and became quite significant over the next 30 years and a new age of concrete dam construction was born. Specifically, the development of RCC.

In the early 1980’s the first RCC dams were being proposed and successfully built in the US, Australia and Asia. By the end of 1985 there had been only seven large (greater than 15m; icold) RCC dams completed. By the end of 1990 this number had risen to 59 RCC dams completed (Dunstan 1992) and as of 2010 over 450 RCC dams have been built worldwide (Hydropower & Dams 2010) with another 50 planned or under construction worldwide, with some now reaching heights approaching 300m.

Advantages

There are many things to take into consideration when designing a large dam > 15m tall (ICOLD). This paper will not go into the design aspects in detail but when considering a RCC for a project there must be the proper resources available to the site to make it more economical. These site specific conditions include the following:

• Adequate foundation conditions;

• Adequate aggregate sources in close proximity to the dam site;

• Supply of cement;

• Supply of a natural Pozzolan (flyash).

Particular and critical resources are the access of Flyash or natural Pozzolanic material as most RCC dams use considerably more flyash or pozzolan than conventional concrete (CVC) and in some cases mixes use double the flyash than cement. This is a tremendous advantage over CVC dams as the use high amounts of flyash (pozzolantic materials) are in most cases cheaper, generate considerably less heat of hydration, and promote better workability, as well a high past that enables an effective bond between layers of RCC placed during rapid construction without the use of a bedding material that has potential to slow production of RCC construction.

Further advantages from a design standpoint is the strength of the RCC mass. The amount of cementitious material for a RCC mixture can vary from lower than 100kg/m3 to as high as 250kg/m3 depending on the design parameters with corresponding compressive strengths of approximately 10 mpa to as high as 45 mpa. Test results for high cementitious mix designs for the recently completed Yeywa RCC dam and the ongoing Nam Gnouang dams in direct tension across the lift joints are as high as 3.0 and 2.21 Mpa respectively, which is considered excellent. Generally most RCC dams are now designed using the high past (HCRCC) philosophy that is > 150kg/m3 of cementitious material. It should be further noted that several RCC dams have been built with lower amounts of cementitious material with great success including several in North America and as recent as the Taum Sauk reservoir and Saluda dam in South Carolina (discussed further below). Most dams over 100m tall including the recently completed 134m tall Yeywa RCC dam (Myanmar), the Kodiat Acerdoune RCC dam (Algeria) and the ongoing 131m tall Dong Nai 4 (Vietnam) , as well as aseveral others RCC dams in Southeast Asia and worldwide are adopting the “High Cementitious”approach (HCRCC). The choice of a mix design is highly dependent on the region in which the dam is being built, the height of the dam as well as the resources available.

Another significant advantage is the rate of placement achieved during the construction of RCC dams as compared with CVC dams. At Longtan dam (China) a Maximum daily rate of placement was as high as 18,000m3 in a single 24hr period with a corresponding average monthly rate of 144,691m3 while at Yeywa (Myanmar) the maximum daily rate was 7700m3 with a corresponding average monthly rate of 72,188m3. Maximum monthly rates for the some of the fastest constructed RCC dams range from approximately 125,010m3 Yeywa (Myanmar) and 400,755m3 Longtan (China). It should be further noted that there have been several RCC dams with monthly rates of placement of between 20,000m3 and 40,000 m3 as the wrong equipment was procured for these projects in an attempt to save initial costs but substantial financial losses in time and particularly power generation were realised in the final delivery of such projects. These loses could have been mitigated with the implementation of proper expertise in the early development of the project leading to proper equipment procurement and set up as well as effective planning from initial quarry development, aggregate production, to RCC batching, delivery and placement equipment. Table 1 is accurate illustration of the rates of placement achieved at 12 of the fastest constructed large RCC dams worldwide from 1987 thru 2008 and containing volume of approximately 1 million m3 to 4.6 million m3 as well as the respective contractors that achieved these rates of placement1. It should be noted that for some projects where extreme weather was encountered the RCC placement operation was stopped for more than a month due. These non-productive months were not considered in the average monthly rates over the total calendar period of RCC placement.

Comparison with CFRD and Earthfill dams

There have been many dams that were originally designed as concrete faced earthfill dams (CFRDs) and then changed to RCC when the RCC option proved more economical. One such structure was the Al Wehdah RCC dam in Jordan (See IWP&DC August 2009). Al Wehdah still remains in the top twelve fastest constructed RCC dams with 1.4Mm3 RCC placed in 19 months

Another similar case was the recently completed Taum Sauk Upper Reservoir Rebuild Project for the pumped storage facility near Lesterville, Missouri. The original dam was a CFRD and suffered a 215m long breach after being overtopped. The structure was replaced by a low cementitious content (LCRCC) RCC (118kg/m3) and had a RCC construction period of approximately 21 months with an average monthly rate of placement of 117,198m3 with a corresponding maximum daily rate of placement of approximately 13,281m3

It should be noted that the original CFRD was completed in 1963 and took four years to construct. The rebuild project was constructed by Ozark Constructers with design being done by Paul C. Rizzo Associates. The project was a huge success and is currently a state-of-the-art pumped storage facility as the RCC dam was built in an emergency rebuild operation using nearly all of the existing CFRD material, which was processed into RCC for the complete dam structure which includes an emergency spillway.

Another example where a RCC dam was built for an emergency ballast fill behind an earth fill dam was the Saluda project in South Carolina, US. Here an existing zoned earth fill dam was deemed unsafe following an intensive investigation within the downstream and existing foundation of the dam, which showed that the dam could be subject to liquefaction and catastrophic failure as a result of an earthquake event. Due to the serious status of the aging structure it was concluded that an RCC dam be built immediately downstream of the original dam for safety concerns. This project was a huge success containing some 1Mm3 RCC and achieved a rate of placement of 13,000 m3 in a single 24hr period.

Another significant advantage is the environmental concerns of the respective types of dam construction. RCC dams have a smaller footprint and require substantially less mass than earthfill dams and CFRD’s. The amount of natural environmental disturbance ie quarries, clearing and large foot print excavation as well as substantially larger borrow areas and environmental runoff can be as much as half that of CFRD and earthfill dams.

Another advantage that must be realized with the development of RCC dam design and construction is the fact that the diversion schemes can be significantly reduced in size with further cost savings, and the overall construction schedule can be minimised as the RCC dams can be overtopped during flood stages without any significant damage to the main structure.

Such overtopping events have happened to several RCC dams during construction. As part of the design, cofferdams can be built with a lean RCC mixture to prevent failure during a flood event. Many designers, constructors and owners alike are realizing this potential for flood events and mitigate the risks by planning for such events using an integrated (insitu) cofferdam within the structure to prevent downstream scouring of the RCC foundation similar to the massive erosion mentioned at Tarbela. This method was used and properly implemented at the 155m Ralco RCC dam in southern Chile (See IWP&DC August 2009). Here, the construction of the 60m tall earth and rock fill cofferdam was nearly complete when it was partially destroyed just before the start of RCC placement. The partially destroyed cofferdam was then rebuilt with RCC to a lower elevation and withstood three more overtopping events with no damage to the RCC main dam. Only a few days were lost due to these flood events because the proper precautions were taken to withstand three successive flood seasons during its RCC construction period. Ralco still remains in the top 12 fastest constructed RCC dams at average rate of placement of 76,000 m3 / month.

The same design concept was adopted at the Yeywa RCC dam (Myanmar) as an integrated spillway and cofferdam was implemented into the works to protect the powerhouse. This project was only overtopped once during the first flood season in 2006 during the early stages of RCC construction.

Another more recent example of an RCC dam with a planned overtopping and integrated spillway is the 70m tall Nam Gnouang currently under construction in Lao PDR. The design of the dam was for the 50-year return period. All the cofferdams to protect the works were earthen and Rockfill dams. The first flood event came in late August 2010 and the 25m upstream cofferdam completely failed as the water in the river rose at a rate of 1m/hr after a Tropical Storm engulfed the catchment area. A huge volume of water was sent over the 140,000m3 RCC already placed with an integrated spillway similar to the Yeywa project. The partially built RCC dam withstood a surge of water passing over the dam at a rate of 200 m3 per second while the diversion was passing some 50m3 per second. The entire dam construction works was inundated and the downstream cofferdam was totally destroyed. It should be noted that was no damage to the partially built RCC dam and the RCC operation continued immediately after the flood waters receded (see photos).

A second flood occurred on 15 September where the rebuilt cofferdam failed again and the RCC was overtopped with a volume of 2500m3 per second.

A third event caused another overtopping which in turn caused further erosion downstream. The RCC dam and integrated spillway passed some 3000m3 per second for nearly 10 days. There was still no damage to the RCC dam other than minor erosion as the RCC was only 10 hrs old when the flood occurred. Total erosion of RCC was negligible at some 10m3 (see photo). Again the RCC started immediately after the overtopping event reseeded. Had this dam been an earthfill or a CFRD as originally considered, the author believes that major losses of time and damage to the dam would have been inevitable.

During the third overtopping event it was possible to continue RCC production at the left abutment as the dam was designed to overtop, and the integrated spillway within the RCC dam allowed the contractor to continue work at the left abutment while the river was flowing over the spillway.

Conclusions

From an owner’s standpoint as well as that of the designer and constructor, the application of RCC dams is developing as a more economical, safe and practical solution for water storage projects. The applications of RCC has significantly developed from the early days at Willow Creek where the first RCC dam was designed and constructed. Since then many other applications of RCC have been used as a ballast for larger dams, spillways in existing earthen structures as well as pavements.

The applications in dam design and construction has been extremely significant as there are nearly 500 of these types of dams in the world today ranging from 15m tall to 272m tall planned or currently under construction by the end of 210. The tallest RCC dam constructed to date is the Longtan RCC dam in China at 192m tall (to be raised to 216.5m). Some of the tallest under construction are the Diamer Basher dam (Pakistan) at 272m tall, and the ongoing 243m tall Gibe III project in Ethiopia.

The author believed the application of RCC in gravity dams is a clear choice for safe dams worldwide. Although not discussed in detail in this paper many arched dams have also been designed and constructed using RCC as compared with Conventional Vibrated Concrete (CVC) Dams.

Authored by Ted Warren. Editing and additional authoring completed by Alex Denoyer and Aygun Ulas. Photographs by Ted Warren, Aygun Ulas and Alex Denoyer

Information on Saluda provided by Paul Rizzo associates

Mr Warren is a construction civil engineer specializing in the development of RCC dams and hydropower facilities and has worked on several large RCC dams and water resource projects in over 10 countries and on six continents worldwide during his 30 year career. He has worked with engineers from around the world, primarily in developing countries on projects from US$1M to as large as US$2B including dams from 15m tall to 243m tall and containing some 10Mm3 of RCC