7. PERFORMANCE
7.5. Performance of dam facing systems
A variety of dam facing systems have been used in RCC construction over the past three decades. Initially the facing systems included pre-cast panels (with or without a membrane on the upstream side), slip-formed concrete kerb systems and conventional CVC placed concurrently with the RCC. However, all these systems have lost popularity in favour of the all-RCC facing systems GERCC, GEVR and IVRCC.
In the subsequent text, the relative advantages and disadvantages of the various facing systems are discussed.
7.5.1. Formed CVC facing
Many RCC dams constructed through the mid 1990’s utilized CVC concrete placed simultaneously with the RCC placement; either placed before the RCC is spread, or placed in the void between the spread RCC and the formwork. The overall performance of such a dam facing is satisfactory, as long as the RCC and CVC are compacted simultaneously. This, however, is not always easily achieved and performance issues have been observed, at the CVC-RCC interface. For CVC placed before RCC, localized lack of compaction can occur due to the instability of equipment compacting the RCC above the fluid CVC. Alternately, if RCC is placed first, difficulty in consolidation can occurred due to the inability of poker vibrators to consolidate low-workability RCC. Frequently, the interface between the CVC and RCC is consequently low density and porous.
Often CVC facing will suffer shrinkage cracking at closer spacings than applicable for the RCC mass, although these cracks do not generally penetrate deeply below the dam surface. Induced cracks, with or without induced contraction joints have been successful in limiting crack widths in the CVC facing to tolerable levels. Sealants placed in “V” notch crack inducers have been effective with proper installation, as follows; (1) clean the concrete in the crack, (2) placing a “foam “backer rod” in the notch and (3) using the proper sealant in the notch over the backer rod.
Most dams in Brazil have utilized a 0.5 m wide CVC facing with embedded wasterstop and drains with satisfactory performance, see Figure 7.20. Reasons for this include well-proportioned facing mixtures, good overall quality control, low cementitious RCC mixtures with added fines and low thermal cracking potential in a favourable climate. However, some Brazilian dams constructed in very remotes areas using less experienced construction staff have shown some leakage due mainly to inadequate compaction/vibration. See Figures 7.21 and 7.22.
Fig. 7.21
Leakage on the downstream face of RCC dam due to inadequate consolidation of facing concrete (Photo: Andriolo, 2015)
Fig. 7.22
Upstream impermeable membrane repair at Saco de Nova Olinda Dam to reduce leakage, approximately 25 years after construction (Photo: Andriolo, 2015)
7.5.2. Grout-Enriched RCC (GERCC) and Grout-Enriched Vibrated RCC (GEVR) facing Both Grout-Enriched RCC (GERCC) and Grout-Enriched, Vibrated RCC (GEVR) facings have become the preferred method of constructing RCC facing in the past 15 years and both have demonstrated very satisfactory performance. Forbes documented the development of and successful performance of GERCC (Forbes, 2003). Some of the key aspects of GERCC performance include the following:
1. “Unless a superplasticizer is used in the added grout, the compressive strength of GERCC may be slightly lower, (perhaps approximately a 4 to 5% reduction in strength) (Dolen, 2003).
2. For properly placed GERCC with the sloping layer placement method, there will be no layer joint and shear and tensile strength will be that of the parent material. To improve the strengths on more mature layer joints, it is necessary to green cut the surface to remove any excess set grout (laitance) and expose the surfaces of the aggregate. Use of bedding mortar will be necessary, as with the adjoining parent RCC surface (McDonald, 2002).
3. Limited success has been achieved with air-entrained GERCC for freeze-thaw durability.
Tests performed by the U.S. Army Corps of Engineers to entrain air at the North Fork Hughes River Test Section were generally unsuccessful. One section with “air-entrained RCC” achieved satisfactory freeze-thaw performance. Note: both GERCC and GEVR were used with air-entrained HCRCC at the FST for Muskrat Falls Dam, Lower Churchill Project in Canada. However, the dam facing itself remained CVC due to the severe freeze-thaw environment.
4. The durability of GERCC against erosion from flow over a stepped spillway will essentially depend on the compressive strength of the GERCC and the quality of the aggregates. After some 6 months of spillway flow to depths of nearly 200 mm (8 in) over the ogee crest, the surface of the 2-foot-high steps (of Kinta Dam) was still in a good condition.
5. In essence, (the appearance of) GERCC is a low slump vibratable conventional concrete.
Being low slump, it is subject to surface defects such as voids or honeycombing from insufficient consolidation as well as loss of paste from gaps in the formwork with consequent surface voids/honeycombing, just as is the case with a CVC facing. An important aspect of the construction process is that operators of the immersion vibrators are trained to know when sufficient compaction has been achieved and not to move on to the adjacent zone too soon.
6. Some projects have used GERCC with generally good results, such as at Tannur Dam. In this case, a PVC wasterstop was placed between the RCC dam and the later constructed downstream concrete stepped spillway section. Both vertically and horizontally placed sections needed to accommodate the (1.2 m) four foot high stepped spillway section. The wasterstop was incorporated into the GERCC facing of the spillway and on inspection after stripping the formwork, it was clear that the wasterstop had been successfully embedded without any sign of voiding on the underside of the horizontal sections, or under the bends, etc (Forbes, Hansen & Fitzgerald, 2008).
The performance of GEVR tends to be better than GERCC, due to a superior basic principle and reliability, with the lightest material (paste) rising to the surface and demonstrating full consolidation.
Some contractors’ methodology and labour force experience are better suited to GEVR over GERCC or vice versa, while some RCC mixes will not work well with GERCC without a significantly dosage of superplasticiser. Both GERCC and GEVR were utilized at Gibe 3 Dam. Early on, the contractor experienced difficulties with consolidation of GERCC that resulted in honeycombing on the surface and around wasterstops, as well as de-bonding of RCC layers. The contractor subsequently changed to GEVR with satisfactory results. Both GERCC and GEVR were specified for Portugues Dam, Puerto Rico (U.S. Army Corps of Engineers, 2008). GEVR was again finally selected over GERCC for this project, due to superior performance.
One aspect critical to the satisfactory performance of both GERCC and GEVR is the size of the internal vibrators. Large size vibrators (75 to 150 mm diameter), powered by tracked equipment will provide more satisfactory consolidation and are likely be more effective with higher Loaded VeBe time mixtures. Smaller sized (40 to 75 mm diameter), hand held vibrators are effective for mixtures with lower Loaded VeBe times. Additionally, the surface appearance of GERCC and GEVR may be affected by the formwork construction and tightness. Gaps in forms or inadequate support may lead to excessive grout leakage and poor performance.
GEVR appears to be a more suitable method for placement of RCC at foundations and abutments simply because the grout is spread directly onto the rock surface, like slush grout, resulting in better filling of voids and rock discontinuities.
It can be concluded that both GERCC and GEVR can be fully successful for dam facings, but that GEVR tends to be more reliable in a wider range of circumstances.
7.5.3. Performance of impermeable PVC membranes
Impermeable, PVC membranes have been constructed on RCC dams using either the standard post-construction methodology (exposed PVC membrane plus a geo-textile drainage layer attached with
“profiles”), by incorporating the membrane with pre-cast concrete panel systems, “in-the dry” localized repairs and a few underwater applications. The performance of a 3 mm and 2.5 mm thick, post construction PVC membrane at the 176 m Miel I dam in Colombia was reported (Vaschetti, Jimenez &
Cowland, 2015). A maximum total leakage of approximately 3.9 l/s was recorded, with an average of approximately 2 l/s for a 31,500 m2 surface area. Approximately 25-30 l/s of seepage was measured through the abutments. Some small tears have been observed and repaired as regular dam maintenance over a time of 11 years of dam operation. Comparison testing of a sample of exposed membrane from the dam and virgin material concluded “there was no reason or evidence that they can be relevant to a significant alteration of the properties of the geo-composite” after 13 years.
The performance of two geo-membrane installations on RCC dams in France, Riou Dam and Rizzanese Dam, has been reported (Delorme, 2015). The 26 m Riou dam utilized an exposed 2 mm thick geomembrane/geo-composite drainage system. Performance reported after 6 and approximately 25 years indicated overall leakage declining from approximately 700 l/min to 210 l/min after eight months and declining further to approximately 45 l/min under normal operations. Seepage through the geomembrane was documented at approximately 35 l/min, the primary source of seepage being the absence of a positive connection between the anchoring upstream “Plinth” and the grouting curtain constructed from the gallery.
At Rizzanese Dam, a geocomposite system comprising a PVC geomembrane and a geotextile was attached to vertical, precast concrete panels. Drainage was ensured by a second geotextile placed on the inside of the precast panels, with half-round pipes (300 mm in diameter) for drainage collection installed in CVC behind the geomembrane system. The lack of support of the geomembrane within the voids of the collector drains caused localized tearing of the geomembrane and some failures of the welded joints between panels, resulting in a total leakage through the system of 1,650 l/min. Initial repairs involving filling the drainage collector voids mainly with granular materials decreased leakage to 1,200 l/min, while subsequent epoxy resin grouting reduced leakage further to 200 l/min. Dry repairs one year later decreased the seepage flow to 50 l/min. It should be noted that the drainage collector design associated with the geomembrane system applied was non-standard.
It can be concluded that a properly installed impermeable membrane system, with rigid support of the membrane, will have very satisfactory performance. Successful performance requires rigid support under the membrane to prevent localized tearing from breaching the membrane.
7.5.4. Performance of spray-on membranes
The performance of post construction, spray-on membranes has been reported for several RCC dams. Galesville Dam utilized a spray-on coal-tar based elastomeric membrane shortly after completion of the RCC dam. The membrane adhered to the surface of the CVC facing, but did not effectively bridge post-construction thermal cracks occurring within 6 to 12 months of completion of the dam and failed locally at the cracks. An elastomeric membrane was sprayed onto the face of Upper Stillwater Dam approximately 17 years after completion and over a distance of approximately 6 m on either side of 14 thermal cracks. Again, the membrane adhered well to the prepared surface, but did not span over the cracks during regular thermal contraction. The cause of both membrane failures was insufficient un- bonded surface area on either side of the cracks. The Upper Stillwater Dam membrane failures were repaired, allowing for sufficient un-bonded surface adjacent to the cracks, and the membranes have since performed satisfactorily. It is noted there were no induced contraction joints on either of the two dams.
A sprayed two-component polyurea membrane was applied on the upstream surface the lowest part of Gibe 3 Dam, where the hydrostatic head exceeds 200 m. The sprayed polyurea adheres to the substrate and overlaps with an external free-to-deform thermo plastic elastomer previously installed in
correspondence to the dam contraction joints (already protected by two internal neoprene waterstops and a drain pipe in between) and thus generating a continuous 3 mm thick impervious surface.
An extensive laboratory testing programme was carried out to define the application procedures and to improve the bonding between sprayed membrane and substrate. The key issue for the successful implementation of this system is the preparation of the RCC upstream surface, which included the careful removal, by means of high pressure washing or sanding, of all traces of dirt and loose material and the application of an epoxy-cementitious primer on the RCC surface.
The reservoir level reached about 90% of the maximum head at the end of the 2016 rainy season. Performance of the dam, in terms of seepage through the dam upstream face, is, until now, fully satisfactory (less than 10 l/s) (Pietrangeli, 2017).
It can be concluded that a properly applied spray-on membrane system can adhere to the facing of RCC dams. Spray-on membranes should not be relied on as a substitute for contraction joints due to the inability to bridge cracks. Successful performance requires proper surface preparation by sand blasting or other high-pressure methods.