Brian Forbes* has been closely associated with RCC dams since 1983. He explains the development of grout-enriched RCC and its proposed use in Malaysia’s Kinta dam
Numerous methods of constructing the upstream and downstream faces of RCC dams have been adopted since the first RCC dam was completed in 1982. The most popular to date has been an internally vibrated conventional concrete facing, placed simultaneously with the RCC. Other methods have included: precast concrete panels; reinforced concrete placed either ahead of or following RCC construction; PVC membranes, either backing onto precast panels or placed on the dam face in direct contact with the reservoir; and RCC itself.
The main objectives of the facing are to provide a durable surface and to enhance the overall impermeability of the structure. Because of the relatively low workability of formed RCC faces, they are generally highly voided, of low strength and poor durability. The more workable RCC mixes, if heavily compacted in sub-lifts with smaller vibrating rollers or plates, can provide very acceptable off-form surfaces, as in some of the RCC dams in Spain.
One of the major concerns in using conventional vibrated concrete placed simultaneously with RCC, is the difficulty in achieving full compaction of the RCC along its interface with the conventional concrete facing. The result can be a porous, weak zone of RCC material and, as has occurred in some cases, the development of a longitudinal crack along the interface between the two types of concrete.
By adjusting the RCC mix proportions with the addition of a cement-water grout, the RCC can be brought to a more workable consistency such that it can be effectively consolidated using internal vibration methods, as is traditionally used for conventional slump concrete. In 1987, in a trial at the Yantan coffer dam in China, a cement water grout was poured over the spread RCC surface. When it had soaked into the RCC, the mixture could be successfully compacted with poker vibrators along the upstream vertical formwork so as to form a homogeneous impervious facing; the Chinese engineers at that time termed the material ‘anomaly concrete’.
The technique was used for the second time in 1989, at the 53m high Rongdi RCC dam in China, where the entire upstream face of the dam was constructed in ‘anomaly concrete’. The third use was in 1992-3 at the 75m high Puding RCC arch dam, also in China; here it was used on both the upstream and downstream faces.
There is no seepage of any significance through the face at Puding dam. Investigation core drilling through the face concrete and water pressure testing showed the material to be of excellent quality, so a plan for additional mortar rendering of the face to enhance impermeability, using a specially developed synthetic rubber latex sand cement mixture, was omitted.
Since the success of Puding, nearly all Chinese RCC dams have been constructed using ‘anomaly concrete’ as the facing material. At China’s 88m high Fenhe II dam, for example, completed in 1998, and the 131m high Jiangya dam, completed in 1999, the overall quality of the upstream face is excellent.
The method has been used with similar success at the recently completed Cadiangullong dam in Australia and Horseshoe Bend dam in New Zealand, and it is currently in use at the Beni Haroun dam in Algeria.
The term now generally used for grout enrichment of RCC to achieve an internally vibratable concrete is GE-RCC or GEVR.
Benefits and limitations
The main benefits of grout enriched roller compacted concrete are:
•High quality off-form finish.
•Homogeneous and monolithic with the adjacent RCC.
•Simple to construct.
•Only one mixer and transport system is required, ie that for the RCC.
•Grout can be mixed by hand or by grout plant.
•Reinforcing steel, waterstops etc can be incorporated into the RCC.
•It can be used between abutment rock and the RCC body to achieve good bond and filling of all rock cavities, irregularities etc.
The main requirements and limitations of grout enriched RCC are:
•The parent RCC needs to be medium to high paste and reasonably workable (VeBe around 20sec).
•Quality control relies on good inspection, and an understanding of the requirements by those applying the grout and those carrying out compaction. Any defective zones evident when the forms are stripped can be easily repaired.
•Lift joint treatment is necessary, as with any conventional concrete lift surface.
•Achieving a trowelled, level surface, say for exposed step surfaces, is not as easily achieved as with conventional concrete, since it is less workable.
The quantity of grout required can be determined by laboratory or field trials. Use of a large (0.5m sided) container with a perspex wall will enable a visual assessment to be made in the laboratory of the quantity of grout required to achieve complete distribution. Typically about 8 l/m/0.4m width for a 0.3m-thick RCC lift, where the parent RCC contains about 160-180kg/m3 cementitious material, has been adequate. By maintaining a grout water:cement ratio equal to that of the RCC similar compressive strengths will be achieved for the two materials. Admixtures such as water reducers, set retarders, air entrainers and plasticisers can be added to the grout if necessary. Grout can be applied to the surface of the lower GE-RCC lift or the top of the spread new RCC lift, or both, or in the body of the RCC lift as it is being spread.
The natural tendency during internal vibration is for the paste and mortar fractions to rise to the surface; if the grout is applied to the spread RCC surface, then the viscosity of the grout should be such that it will flow into the loose RCC, not sit on the surface. During compaction the surface of the GE-REC will be mobile underfoot — indicating that compaction is taking place effectively. If this is not evident to the operator then a higher dose rate is necessary.
On removal of the vibrator, any holes left by the poker should be ‘tramped’ to close them up, or the grout dose rate increased.
The size of the poker vibrator depends on the maximum aggregate size, workability of the original RCC, quantity of grout etc. At Jiangya (131m high, China, 1996-9) a set of four 150mm diameter units mounted on a transom attached to a mobile rig were used. This was originally provided for large-mass conventional concrete pours; it was more than was necessary for the 300mm thick GE-RCC lifts. Elsewhere, such as at Cadiangullong (43m high, Australia, 1997-8) vibrators as small as 50mm diameter were used successfully.
Mixing of the grout can be carried out by hand. This was done initially at Jiangya, and also at Cadiangullong, where 40kg pockets of cement were mixed with the required volume of water in a loader bucket or small wheelbarrow respectively. Later at Jiangya, as at Beni Haroun (123m high, Algeria, 1999+) and the Sharpei (132m high arch) and Dachaoshan (118m high) dams, both presently under construction in China, the grout is mixed in a grout mixing plant and piped to the face. In this case mixing is very thorough and control of proportions is good, but application rate is difficult to judge. At Cadiagullong it was poured over the lift on a ‘bucket-length-width’ basis as described earlier, the mixing, pouring and compaction requiring only three labourers to keep up with RCC placing.
On completion of a length of GE-RCC, roller compaction of the RCC with the large vibrating rollers should take place up to the treated zone, and beyond if the formwork will permit, so that the contact between the two is fully compacted. If the GE-RCC has been overdosed with grout it will be displaced upwards by the roller. This is not of concern, as long as it is no more than 10-20mm; otherwise dose rates should be reduced and a larger internal vibrator used for the GE-RCC.
Lift joint treatment will depend on the age of the lift surface and degree of laitance produced on the surface. Generally the laitance is not ‘bleed water’ as in conventional concrete, but paste with a similar water–cement ratio as the RCC. Having a formwork system which has regularly spaced removable panels or ‘doors’, to provide openings, enables the lift surface to be high-pressure washed and the washings easily sluiced off the dam through the opening. If the next lift is placed within the specified time, no special treatment is necessary.
At Cadiangullong the full RCC lift surface was treated with a 6mm thick layer of 150mm slump bedding mortar. This tended to thicken to >20mm at the upstream and downstream faces during the spreading process and, with the grout dosed over the top of the spread RCC layer, a very good bond was achieved along the GE-RCC lift surface. At the recently completed Horseshoe Bend dam (14m high, New Zealand, 1999), in order to achieve a freeze-thaw resistant facing, the grout was heavily dosed with an air entraining admixture to achieve 3-4% residual air in the GE-RCC facing.
Initial trials on site were unsuccessful, as the highly ‘foamed’ grout would not soak down through the spread RCC layer. Applying the air-entrained grout at the bottom of the lift, so that it could flow up through the RCC during compaction may possibly have been more successful.
Sampling of the GE-RCC after compaction for slump testing (typically 5-10mm) and manufacturing of test samples for compressive strength etc. should form part of the quality assurance programme. Horizontal coring through the face, either through the body of the lift or along the lift joint, will confirm homogeneity, density, GE-RCC and lift joint strengths. Vertical coring can also be carried out.
Other uses for GE-RCC
At the Dachaoshan dam in China the downstream stepped portion of the spillway chute has been constructed in 0.9m high steps using GE-RCC. To ensure good quality concrete results the whole area has been covered in straw matting and is being continuously water cured. Velocities down the spillway chute are high and finished surface quality and alignment was an important issue to the designer.
In the past it has been usual practice to encase the transverse contraction joint waterstops in conventional concrete. Unless carried out correctly there is the potential for a line of weakness or porosity between the RCC and the conventional concrete, such that seepage will bypass the waterstop. In the more recent Chinese dams and at Cadiangullong and Horseshoe Bend the waterstop detail has been encased in GE-RCC. At these projects, GE-RCC, instead of conventional concrete, has also been used for the interface between the abutment rock and the RCC. Where reinforcing steel has been used, such as around gallery openings, and where GE-RCC has replaced conventional concrete, the steel appears to be as well encased in the GE-RCC as would be achieved using conventional concrete. This indicates the feasibility of incorporating reinforcing into RCC in these locations, as well as in other areas, such as along the upstream faces of the higher dams to distribute any cracking potential evenly between transverse joints.
GE-RCC at Kinta dam
Kinta will be Malaysia’s first RCC dam. Prequalification of contractors was called in April 1999 and it is expected that a contract for construction will be let in November 1999.
The dam will be 85m high and will contain almost one million cubic metres of roller compacted concrete. It is located near the city of Ipoh in peninsular Malaysia, north-west of Kuala Lumpur, and is being developed to augment Ipoh’s present water supply.
The RCC will use a fully crushed granite aggregate with a maximum size of 62mm and with 6-8% finer than 75 micron, all sourced from a quarry to be developed close to the dam. It is expected that the mix will contain a total of about 170kg of a portland cement, ground blast furnace slag and fly ash blend.
The RCC is scheduled to be placed over a 12 month period commencing in May 2001.
The typical cross section of the dam, as shown in the figure below, will use GE-RCC for the full upstream and downstream faces, including the 100m wide stepped spillway portion, the abutment contact concrete and the transverse joint waterstop encasement.
The spillway is an uncontrolled ogee crest capable of discharging the PMF of 2250m3/s. Energy dissipation is provided by the stepped downstream face of the dam, with residual energy absorbed through a roller bucket at the heel of the dam. Energy absorption over the steps varies from approximately 90% at lower flows to 60% at the PMF. The ogee crest, spillway side walls, roller bucket, diversion conduit/outlet works and foundation levelling/dental concreting works are expected to be the only conventional concrete used in the dam.
To enhance RCC lift bond the contractor will be required to use a 6mm thick vigorously-spread layer of highly workable bedding mortar on all RCC lift surfaces which exceed the initial set. Use of set retarders in the RCC will be permitted, as will the recently developed ‘sloped layer method’ of RCC lift construction. Using this method it is possible to place RCC continuously across the dam from one abutment to the other in 300mm thick lifts, each being placed within an unretarded initial set time of around 2 hours.
Laboratory trial mix designs are due to commence in May and will investigate the material proportions for the RCC and enrichment grout and the required grout dose rates. Full scale trials by the contractor, prior to commencing RCC construction, will fine-tune the mix proportions and trial equipment and techniques for RCC and GE-RCC construction. Design of Kinta dam and project engineering services are being provided by GHD Pty Ltd, Australia and Ankasa-GHD, Malaysia.
The developer is the Metropolitan Utilities Corporation who, in 1989, obtained the 20+15 year supplemental agreement for the build-own-transfer contract to provide Ipoh’s water supply.