Alternative concrete for dams

February 2012 » Features » PROGRESSIVE ENGINEERING
Grout Enriched roller compacted concrete is gaining acceptance in the United States to produce an economical, low-permeability upstream facing for dams.
Thomas Fitzgerald, P.E.
GERCC was used to produce an inexpensive, aesthetically pleasing finish for the non-overflow portions of the downstream face of the 188-foot-high Hickory Log Creek Dam in Canton, Ga.

Roller compacted concrete (RCC) is frequently used for the construction of mid-sized to large mass concrete applications, including dams, because of its faster placement rate, lower heat of hydration, and reduced cost compared with conventional concrete. RCC is a no-slump concrete that is typically spread in lifts as thick as 12 inches with earthmoving equipment and compacted with a smooth vibratory roller. Early experience on RCC dam applications in the 1980s showed a tendency for seepage to develop along the 1-foot-thick lift lines due to the tendency at the time to use a lean RCC mix. Therefore, many RCC dam designers started including an upstream facing system as a watertight barrier in combination with careful attention to lift joint preparation and treatment. The upstream barrier is typically constructed of a zone of conventional concrete, an exposed liner system, or precast concrete panels with or without a liner syste

An alternative facing material that has been used extensively overseas and is starting to gain more widespread acceptance in the United States is Grout Enriched RCC (GERCC). This innovative process includes addition of a neat cement grout to the uncompacted RCC at each lift along the upstream face. After the grout has soaked into the RCC, immersion vibrators are used to mix and to consolidate the grout and RCC to produce a seamless zone similar to conventional concrete. The surface of the RCC lift is then compacted with a vibratory roller.

This process is typically used with higher-paste RCC mixes with cementitious contents (cement plus Class F fly ash) of 300 pounds or more per cubic yard and has a Vebe time less than 20 seconds. The grout simply consists of a mixture of portland cement and water with a 1:1 ratio by weight. The water-cement ratio of the grout may be reduced with the use of a water-reducing admixture to approximately 0.7:1.0 to improve the strength and durability of the GERCC facing. The GERCC facing is typically applied to produce a facing with thickness between 12 and 24 inches.

Laboratory tests have shown that the grout may not evenly disperse throughout the RCC by using immersion vibrators. However, the basic intent for using GERCC is to ensure that enough paste is made available to fill any voids in the RCC that is placed against formwork and foundation contacts. Unless the parent RCC mix has an excessively low Vebe time (less than 10 seconds), compacting the RCC against formwork or rock surfaces without the additional grout results in honeycombing and therefore poor aesthetics of the finished surfaces and poor bonding to foundation surfaces. The proper application of GERCC has proven to provide conventional concrete-like surfaces and good bonding capacity.

Workers conduct a final cleaning of the GERCC downstream steps in the spillway of the Deep Creek Watershed Dam 5D.

The first reported use of GERCC for an upstream facing system was on the cofferdam for Puding Dam in China in 1994. Subsequently, GERCC was used on the 430-foot-high Jiangya Dam in Hunan Province, China, for the upstream face and to create a bond along the rock abutment interface. Placement of the 1,439,000 cubic yards of RCC was successfully completed in 1999. According to a listing of RCC Dams worldwide published in Hydropower and Dams World Atlas 2010, more than 75 dams have been completed or are currently under construction using GERCC in at least 22 countries, with 35 constructed in China alone.

Since its inception, the GERCC process has been used for the exposed upstream and downstream face of the dam for improved aesthetics, to bond RCC against rock abutments, for the construction of drainage galleries, and around conduits and other penetrations through dams. Numerous minor variations in the process have been developed over the years for various dams. For example, Grout Enriched Vibrated RCC (GEVRCC) involves placing the grout along the upstream face of the previously compacted lift of RCC, then spreading a loose lift of RCC over the grout and using immersion vibrators to draw the grout up into the RCC. This process typically has been limited to very high-paste RCC mixes. The most recent development involves placing RCC and grout into a transit mixer on the lift surface and producing a mixture referred to as GERCC-M to be deposited in place, consolidated, and compacted.

Workers apply neat cement grout to the uncompacted RCC at the downstream formwork for the Deep Creek Watershed Dam 5D.

GERCC has been proven to produce a consistent, economical, low-permeability upstream face for dams. One of the primary reasons for its popularity worldwide is that it uses a simplified procedure that can be performed simultaneously with the RCC placement and does not interfere with overall RCC production rates. Other facing systems such as precast panels and conventional concrete require careful coordination and timing so they do not slow down RCC production. Another advantage is that the grout used in GERCC is mixed directly with the parent RCC and there is no potential for delamination at the critical location between the facing material and the RCC mass.

In the United States, the use of GERCC technology has been fairly limited. This can be attributed to the infrequent construction of new large dams in the United States, the early general trend to use dryer/leaner RCC mixes that are less conducive to successful GERCC, and concern over freeze-thaw durability. Integration of air entraining admixtures into GERCC has not been successful to date. In addition, the majority of new dams using GERCC have been constructed in developing countries with inexpensive labor. Since the GERCC installation process is labor intensive and rates are typically higher in the United States, this process can be less economical than in other parts of the world.

The first domestic use of GERCC occurred in 2002 on the construction of the 309-foot-high Olivenhain Dam for the San Diego County Water Authority. For this case, GEVRCC was used to produce a relatively smooth upstream face, suitable for the installation of an exposed membrane liner after RCC construction was completed, and at the foundation contacts. In 2007, GERCC was used to produce an inexpensive, aesthetically pleasing finish for the non-overflow portions of the downstream face of the 188-foot-high Hickory Log Creek Dam in Canton, Ga.

Deep Creek Dam
The recently completed Deep Creek Watershed Dam 5D represents the first use of GERCC as the sole upstream barrier for a dam in the United States. This multi-purpose flood control and water supply dam in Yadkinville, N.C., was designed by Schnabel Engineering. A composite dam design was selected to make efficient use of the complex existing foundation conditions. Shallow competent bedrock on the right side of the valley was used to support a RCC gravity section with a maximum height of 74 feet and a length of approximately 750 feet. On the left side of the valley, rock was deeper and more variably weathered. A zoned earthen embankment was designed to wrap around the end of the RCC dam and extend roughly 800 feet to the left abutment.

During design, Schnabel primarily focused on three types of RCC facing including formed RCC, conventional concrete, and GERCC. Several factors were considered in the selection including constructability, permeability, durability, cost, and appearance. GERCC was selected based on its proved performance worldwide, lower anticipated cost, and expected production advantage compared with other facing alternatives. GERCC also offered flexibility along the dam/abutment and foundation interface where surfaces were expected to be irregular.

A vertical core taken of a GERCC facing through an upstream PVC waterstop shows excellent bond and no honeycombing.

Design of the proposed dam was completed in 2004; however, due to lack of funding, the project was not bid until October 2008. The project was awarded to the low bidder, Haymes Brothers Construction Inc. from Chatham, Va. ASI Constructors Inc. of Colorado was responsible for RCC and conventional concrete as a subcontractor to Haymes. The project was completed in August 2010 and includes a 16-inch-thick GERCC upstream face and a 12-inch-thick GERCC downstream face. Since initial filling, the face has performed successfully with very minimal seepage.

In the United States and its territories, since the completion of Deep Creek, GERCC has been used to produce an aesthetically pleasing downstream face for the exposed RCC overtopping protection project at Fox Creek #4 Dam in Flemingsburg, Ky., and is currently being used for the upstream and downstream facing for Portuguese Dam in Puerto Rico, designed by the U.S. Army Corps of Engineers, and at the San Vicente Dam near San Diego where the existing 220-foot-high concrete dam is being raised 117 feet with RCC.

Thomas Fitzgerald, P.E., is senior associate, and Randall Bass, P.E., is principal with Schnabel Engineering. They can be contacted at tfizgerald@schnabel-eng.com and rbass@schnabel-eng.com, respectively.

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