By Ernest K. Schrader
Facings and contraction joints are important aspects to consider during the design of a roller-compacted-concrete dam. The author – who has been involved in the design and construction of more than 100 RCC dams in 35 countries – shares his experience and recommendations for these critical elements.
Facings on roller-compacted-concrete (RCC) dams offer durability against freezing and thawing, provide a means to construct a face steeper than the natural angle of repose of the RCC, result in an aesthetically pleasing surface, and can be designed to control seepage of water through lift joints. Seepage also may be controlled by other methods. Both the upstream and downstream faces of an RCC dam can be designed using any of a number of options. This article provides details about these options.
With regard to contraction joints, proper design to control cracking is necessary to safeguard the long-term integrity of the dam. This article provides specifics on proper design of contraction joints.
Options for designing upstream facings
If the upstream face of the dam is sloped, the unformed face may be left exposed as long as lift-joint seepage is either tolerable or controlled with bedding or higher cementitious content RCC and the look is aesthetically acceptable. If total watertightness is needed and the dam is not being built using special mixes and rigorous lift joint inspection, a flexible geomembrane can be placed over the sloping face. (See sketch on left in Figure 1 on page 40.) If necessary, the membrane could be protected from removal or damage by vandals by hanging chain link fencing or another form of a metal “mattress” over it. However, a recent review of more than 50 projects using geomembranes reveals that there has not been a documented case of vandalism during project operation.
Below the level of backfill against the face of the dam, the membrane is protected from damage by a layer of sand. As the sketch on the left in Figure 1 on page 40 shows, shotcrete could be used for protection on a sloping surface where anchors are not necessary. Shotcrete could be used on a vertical surface if anchors are installed, but this method has not yet been employed.
A reinforced conventional concrete wall or slab placed over the face of the dam after RCC is placed uses the same concept as an upstream face on a rockfill dam. If wall is thick enough, it can be built before the RCC is placed using traditional slip-formed or jump-formed construction. This allows RCC to be placed directly against the conventional concrete wall. There is no forming to delay RCC operation because the wall is the form. The wall also acts as thermal shock protection for the RCC. Properly designed and constructed, such a wall can provide an attractive and watertight facing.
This method requires slabs with waterstops in the vertical and horizontal joints. A “plinth” or watertight tie-in to the foundation and abutments is required. Two-way reinforcing distributes shrinkage cracks throughout the slab so that cracks are small and closely spaced. By limiting crack width to about 0.15 millimeter, the cracks will be essentially watertight and typically undergo autogenous healing or calcification with time, even under hydraulic head. A drain system is needed between the slabs and the RCC. Anchors are needed to hold the slabs to the dam. These should be designed for the force due to horizontal acceleration in an earthquake and be corrosion-proofed. It is possible (but difficult) to position the anchors in the RCC when it is placed; drilling and grouting them afterward is the alternative. This concrete-facing method is often considered but, primarily because of cost, is seldom designed or used. Its application seems to be more plausible and useful for very high and large dams where the facing is most critical and the cost of it is relatively low compared to the overall volume of RCC.
|There are many methods available for designing the upstream face of roller-compacted-concrete dams.|
An extension of the above concrete facing option includes a second facing of porous concrete that acts as a total drain between the RCC and the conventional concrete wall that forms the upstream face. This option also isolates the RCC from shrinkage and potential cracking or joint requirements in the facing, and the porous concrete acts as thermal insulation to reduce gradients near the face. This method was included as an option for the RCC design at Kapachira Dam in Malawi, but a different type of dam was constructed. However, the concept of a more pervious region immediately behind an impervious face and before the start of impervious RCC has been used on many projects. This occurs inherently when RCC is placed against the face without special attention to achieve an impervious contact to the facing.
Another upstream facing option is to place the RCC directly against conventional forms (see center sketch in Figure 1). Threaded anchors to the forms can be compacted into the RCC. After the RCC has been placed high enough that the next form can be positioned and anchored, the lower form can be slid out along the anchor, away from the RCC mass. The void between the RCC and form can then be filled with conventional concrete that bonds to the young RCC and is mechanically held by the anchor. Instrumentation using strain gages on the anchor bars has shown that by controlling the rate of placement and set time of the concrete using this type of procedure, form pressures can be developed that will stress the anchors and “prestress” the concrete face in place.
An additional upstream facing option, precast panels, makes an attractive, economical, and crack-free facing (see Option 1 on the right in Figure 1). However, the panel joints are not watertight. Anchors needed to hold the panels in place are minimal, usually about 2.76 square meters of panel per square centimeter of steel anchor area. Watertightness can be provided using a flexible polyvinyl chloride (PVC) membrane (about 2 millimeters thick with welded field seams) attached to the back of the precast panel. A nut and washer tightened against the membrane with epoxy provides a watertight seal for the anchor.
|These three options are methods for using conventional concrete on the upstream face of a roller-compacted-concrete dam.|
This procedure has been very successful in construction and operation and in tests to a head of 183 meters. A small amount of bedding is recommended between the membrane and RCC. This acts as a “cushion” to minimize the possibility of sharp coarse aggregate particles puncturing the membrane (if crushed aggregate is used in the RCC). It also seals the RCC at the lift joints so that any seepage that might get past the membrane flows along the interface between the membrane or facing and the impervious RCC, to a system of drains. The drains are there to relieve pressure, not necessarily to carry any large flow. Experience has shown that, as with a simple sheet of plastic against sheet pile, the hydraulic head differential keeps the membrane tight to the supporting surface so that the only water that can get past it is at the pin hole of a small puncture. This never results in high flow but can result in pressure if it is not relieved.
Drains that can be effectively monitored and maintained should be provided behind the membrane to collect seepage if it occurs. In some instances, where design and construction of the drainage system has been rigorous and to a high standard and the drains can be monitored and maintained, the amount of calculated uplift can be reduced. If uplift pressure gets into a lift joint, it reduces the downward force of the concrete above it, thereby decreasing stability that comes from the effective gravity load. If uplift exceeds the downward gravity load, undesirable tensile stress can develop.
|The downstream face of a roller-compacted-concrete dam, or any other sloping face, can be designed using any of the options shown.|
Watertightness can be established using an exposed PVC membrane placed directly against the RCC1,2,3 (see Option 2 on the right in Figure 1 on page 40). The membrane requires an anchored but unbonded procedure specifically developed for concrete dam facings. A variety of synthetic membranes (such as high-density and low-density polyethylene and lower-quality PVC) have been used in earlier projects, either with or without drains. These systems worked satisfactorily, but not totally. A specially formulated PVC membrane produced and installed by Carpi was subsequently adapted to RCC, using technology and performance from its application to provide watertightness at older conventional concrete dams with seepage and permeability problems. This has routinely provided total watertightness at the upstream face. The installations include a face drain system. Drains between the membrane and RCC can improve stability through additional uplift reduction.
Simply extending the bedding mix downstream along the lift joint for a distance equal to at least 8 percent of the hydraulic height can provide watertightness, if it is done correctly 100 percent of the time (see Option 3 on the right in Figure 1 on page 40). However, in practice, this is not possible. Normal construction with good inspection results in about a 95 percent reduction in seepage. This may be technically adequate but is not aesthetically acceptable.
A number of RCC dams, and the facings of rooms and walls at other RCC mass applications, have been built using various procedures (see Figure 2 on page 41). This results in an attractive conventional concrete face. Usually, the facing has no anchors to the RCC and no reinforcing bars (see Option 1 in Figure 2 on page 41). If a low-water/ low-cement/low-shrinkage conventional concrete mix containing a high-range water reducer is carefully used and controlled, a virtually crack-free facing can result – even without vertical joints. The mix should not be thicker (horizontal dimension) than about 0.3 meter, or thermal and shrinkage cracks will probably result. Excellent curing must be provided. Without these precautions, tight cracks at spacings of about 1.2 to 3 meters can be expected. Normal construction with a reasonable mix will be crack-free if joints are provided in the facing about 7.6 meters apart. The problem with joints in a facing is that it is very difficult to install waterstops. At projects where this procedure has been used, the result has been less-than-watertight joints. The facing does not make the horizontal lifts watertight. If placing proceeds quickly (about four to six lifts per day), the fact that the successive layers are placed before the previous layer has fully set will improve watertightness of these joints.
A modified version of the procedure described above uses a temporary “blockout” near the upstream face at every other RCC lift (see Option 2 in Figure 2 on page 41). The blockout is removed before placing conventional facing and the next RCC lift. Each facing placement covers two RCC lifts. Added watertightness can be achieved by using a simple “swelling strip” waterstop that is impregnated with chemical grout and laid along the facing mix lift surface. If seepage penetrates the lift joint, moisture causes the strip to swell and create a watertight pressure against the adjacent lift surfaces. The swelling strip should be suitable for the hydraulic head to which it will be subjected. Experience has shown that strips impregnated with chemical grout work well at relatively high heads. Swelling materials that use a clay such as bentonite are effective for very low heads but are not suitable for dams. Under typical dam pressures, the bentonite extrudes and washes away.
A third version of this procedure involves using precast and slip-formed interlocking upstream-facing elements (see Option 3 in Figure 2 on page 41). The upstream area covered by each precast piece has been only about 1 square meter of exposed surface area, so production and placing becomes labor-intensive and slow. The small area is a result of the weight of the thick and overlapping shape. The joints are not watertight, and there is concern about stability of the facing if it is not anchored to the RCC. Horizontally slip-formed facings can slow production of RCC on dams with short axes, but the procedure and equipment is better suited to long dams with a large volume of RCC per lift. Careful control of the mix and its delivery are critical, and the facing will develop small shrinkage cracks. RCC can be placed against the facing the same day it is slipped, but usually not more than two lifts per day. Consideration should be given to the bond between the unanchored facing and RCC. This may require a high paste RCC mix against the slip-formed facing. Sandblasting may be necessary to achieve a bond if the facing is old before the RCC is placed against it. The possibility and consequences of saturation and freezing at the bond line should be evaluated.
Dams in steep canyons, and some large dams, can benefit from an upstream concrete wall placed across the valley. Such a wall acts as an upstream form for the RCC or as a starting wall for concrete facing and membrane systems. It also protects the foundation by containing water and debris. And it allows fill to be placed against the upstream side of the wall, thereby making a practical work area that extends to the face of the dam.
At some projects, companies have saved time and money by placing backfill lift by lift with the RCC to create a vertical upstream face without forms. This is fast and effective and provides a long level surface upon which subsequent forms can be set when the maximum practical height of backfill is reached.
Regardless of the procedure used for watertightness, a tight contact is essential at the interface between the upstream face and foundation. Details for this contact and how it varies from one upstream facing and/or water barrier method to another are beyond the scope of this article.
Options for designing downstream facings
The downstream face of the dam, or any other sloping face, can be designed using any of the options shown in Figure 3 on page 42. A common, economical, and practical method uses steps with a small amount of bedding placed against reusable-form panels (see left side of Figure 3 on page 42, Option 1). The panels are one to three lifts high, moveable without equipment, and held by simple methods such as pins hammered into the RCC after compaction. By changing the width of the steps, any average or changing downstream slope can be achieved.
If a conventional concrete appearance or protection from weather is desired, conventional concrete can be used for the facing. Larger steps, for example if needed in a spillway, can be built using variations of this method. Reinforcing steel and anchors are not required with a monolithic construction procedure. However, it is essential that the conventional concrete be placed first with a mix that will have lost its slump but not reached initial “set” by the time the RCC is spread against it and compacted down into it. If the RCC is placed first, the result will look good at the surface, but there will be no reliable contact or interface between the two mixes. Anchorage is then necessary and the RCC should first be compacted at the edge (see left side of Figure 3 on page 42, Option 2).
Smooth spillways and downstream sloped surfaces have been designed and constructed using a variety of methods, including:
- Slip-formed concrete with anchors and two-way reinforcement placed after completion of the RCC and after substantial chipping or preparation of the RCC (see right side of Figure 3 on page 42, Option 3);
- Unreinforced and unanchored conventional concrete facings placed monolithically with the RCC and having about a 0.3-meter minimum width (see right side of Figure 3 on page 42, Option 4); and
- No treatment except for hand trimming (see right side of Figure 3, Option 5).
Costs of facings
The cost of the facing methods depends on many factors, including: the contractor’s experience with the technique, equipment that must be purchased, the size and scope of the project, and the site-specific cost of materials and labor. In some cases, the cost of precast panels can be less than the cost to form huge areas of an upstream face, then remove and reset the forms. Other costs to be considered include the effect of each facing method on the placement and speed of construction of the RCC itself. The facing method chosen should not slow RCC production.
The principal function of vertical contraction joints is to control cracking due to foundation restraint, foundation geometry, and thermal volume change. Contraction joints also have been used as formed construction joints that divide the dam into independent work areas or monoliths. Depending on the mixture, climate, and approach to design, some RCC projects have included many contraction joints, while others have had no contraction joints.
Concerns over cracking
Before describing the design of contraction joints, it is important to understand why cracking and its prevention are necessary. The main concerns for cracking in massive gravity sections are structural stability, appearance, durability, and leakage control. Seepage through transverse upstream to downstream cracks and joints will result in uncontrolled leakage and an undesirable loss of water. It also can create operational or maintenance problems and be visually undesirable. This situation normally does not create a direct stress and stability concern, but if leakage enters a crack or monolith joint and then traverses out onto lift joints, it can cause uplift conditions that were not part of the design. This can result in compromised factors of safety.
Seepage into longitudinal cracks parallel to the axis of the dam is likely to be more serious than the situation mentioned above. If water under the reservoir or abutment groundwater pressure fills a longitudinal crack, it can dramatically affect sliding and overturning stability. The consequences can be catastrophic. These types of cracks have occurred in both conventional concrete and RCC structures, and their potential consequences have been demonstrated.
Solving the problem
Remedial measures typically include draining hydrostatic pressure from within the crack with a comprehensive series of drilled drain holes. Even with drainage, the mass upstream of the crack may not be effective, and the effect on stability can be serious if not catastrophic. Some longitudinal cracking has been corrected by grouting and post tensioning across the crack, but this is not very practical unless the reservoir can be drained or lowered (such as in a navigation lock).
When thermal or other analysis indicates that an unacceptable longitudinal crack is possible, a groutable joint can be constructed at that location. The joint should be grouted after thermal contraction has caused it to open significantly, but grouting may be needed earlier due to the schedule for filling the reservoir. In this case, a re-groutable joint should be designed if possible, or the joint and grout should be designed so that the grout will continually expand over time as the joint tends to open more.
The location and spacing of joints depends on: foundation restraint, temperature change and the time period over which it occurs, the tensile strain capacity of the concrete at the time in question, creep relaxation, the coefficient of thermal expansion of the concrete, and applied loads. For many projects, joints are carefully formed to go through the entire dam. Other designs use partial joints to provide a weakened plane along which cracks will propagate. Waterstops and drains are usually an integral part of a complete joint design.
Seepage control methods of transverse contraction joints vary widely. Seepage control methods for RCC dams include:
- A surface control joint with waterstops;
- A membrane placed over the upstream surface (either a membrane with precast concrete panels or an exposed membrane); and
- Conventional concrete face of jointed slabs placed after the RCC.
Transverse contraction joints with surface control and waterstops have been used in numerous RCC dams. Typical details consist of a formed crack inducer in the upstream face with a waterstop in the facing concrete, followed by crack inducement in the RCC lift by one of the methods described above.
Arch dams and gravity arch dams require contact across transverse contraction joints in order for the structure to function in the three-dimensional manner in which it has been designed. This normally requires grouting with effective “grout stops” at both the upstream and downstream faces of the dam, similar to conventional concrete arch dams. Unlike conventional concrete, RCC typically is not post-cooled to assure full contraction before grouting, so the concerns discussed above for longitudinal joints also apply to these joints. However, if the RCC has a high Poisson ratio and low modulus of elasticity during the time the dam is constructed, and if thermal contraction is kept to a minimum, joints in the lower portion of the dam may remain in close contact even without grouting. The same applies to straight axis dams in tight “V” shaped canyons, which also can benefit from three-dimensional effects similar to arch dams. s
Dr. Schrader may be reached at Schrader Consulting, 1474 Blue Creek Road, Walla Walla, WA 99362 USA; (1) 509-529-1210; E-mail: [email protected] columbiaenergyllc.com.
- Nawy, E.G., Concrete Construction Engineering Handbook, Chapter 20, CRC Press, Boca Raton, Fla., 2007.
- Schrader, Ernest K. and A. Rashed, “Benefits of Non-Linear Stress-Strain Properties & Membranes for RCC Dam Stresses,” Roller Compacted Concrete (RCC) Dam Construction in the Middle East 2002, Jordan University of Science and Technology (JUST) & Technische Universitaet Muenchen (TUM), 2002.
- Scuero, A, and G. Vaschetti, “Synthetic Geomembranes in RCC Dams: Since 1984, A Reliable Cost Effective Way to Stop Leakage,” Roller Compacted Concrete Dams, International RCC Dam Symposium, Swets & Zeitlinger, Lisse, Netherlands, Lisse, November 2003, pages 519-530.
Ernie Schrader, PhD, P.E., is a consultant with more than 30 years of experience in roller-compacted concrete (RCC). He has been involved in more than 30 RCC dams that are complete and operational, several under construction, and many undergoing design and feasibility studies. The projects range from the world’s highest and largest to the smallest RCC dams.
This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.