9. RCC ARCH DAMS
9.6. THERMAL CONTROL
Methods and systems for the temperature control of RCC are discussed in Chapters 2 and 5 and only those aspects that are of particular relevance to RCC arch and arch-gravity dams are discussed in this Chapter.
The reduced section thickness and total concrete volume associated with an arch, or arch- gravity configuration can offer schedule benefits, but when the actual dam structure construction is not on the critical path, these characteristics can sometimes allow artificial cooling to be avoided, or reduced, through RCC placement only during the cooler periods of the year. Such an approach has been successfully applied at a number of smaller-volume RCC arch-gravity dams.
9.6.2. Pre-cooling
In RCC gravity dams, pre-cooling is applied in order to ensure that surface-gradient and mass- gradient effects do not develop any deleterious structural impacts. For an RCC arch, or arch-gravity dam, pre-cooling fulfils the same purpose, as well as representing one of the possible tools in the overall dam design, as mentioned under Section 9.5. Pre-cooling is used for all RCC arch dams in China.
9.6.3. Post-cooling
Concrete post-cooling is obviously beneficial for an arch dam and essential for a thin and/or double-curvature arch in which no shrinkage compensation additives are used, particularly in the case of a more extreme climate. Post-cooling of RCC arch dams, however, is more difficult and impacts construction efficiency significantly more than is the case for CVC dams. Post-cooling has generally only been used in Chinese-built RCC arch dams and very large gravity dams.
Practical experience in the application of post-cooling systems in RCC arch dams has demonstrated the following important design and construction requirements (Du, 2010):
• Cooling pipe systems and installation must be designed to minimise impact on RCC placement rates;
• Pipe systems must be designed to be resistant to damage from RCC placed on top, as well as RCC spreading and compaction plant and equipment;
• No plant, or equipment must be allowed on, or in direct contact with the cooling pipes;
• High-density polyethylene (HDPE), particularly composite polyethylene (PE), should be used, with reel lengths of 200 to 250 m, allowing minimal, or no jointing within an RCC layer;
• Appropriate construction procedures must be developed;
• Pipe connections must be thoroughly sealed and jointed in a manner to eliminate the possibility of pull-out during RCC spreading and compaction;
• Before and after RCC spreading, the cooling pipes should be tested for leakage;
• Cooling pipes should be placed on freshly-compacted RCC prior to initial set, with loops anchored in place using small diameter inverted U-shaped reinforcing bars;
• Cooling pipes should be covered with a minimum RCC layer depth of 25 cm;
• Dumping, spreading and compaction of RCC must be initiated at one side of the cooling pipe network, subsequently always working away from this point and accordingly avoiding the need for traffic moving over the pipes, etc.
Although PE material has a lower thermal conductivity than traditional steel cooling pipes, research and construction practice has demonstrated that the consequential cooling is not significantly affected (Zhu, 1999), partly due to the thin-walled PE pipes used.
In Chinese RCC arch dam practice, post-cooling is performed in two or three phases. The initial phase is performed immediately after the compaction of the RCC layer above the post-cooling pipes and is maintained for a period of 14 days to reduce the peak hydration temperature. The final phase of post-cooling is initiated at least one month prior to grouting of the transverse contraction joints, to reduce the concrete temperatures to the target closure temperature. In climatically more extreme areas, an intermediate phase of post-cooling is applied to reduce temperature gradients across the RCC section.
Where conventional transverse contraction joints (fully de-bonded joints) exclusively are applied, full post-cooling to the closure temperature will be completed to allow joint grouting before reservoir impounding. For a mix of conventional and induced (partially de-bonded) transverse joints, partial, or full post-cooling can be applied and while all joints are subsequently grouted prior to impoundment, a second grouting during operation will also be foreseen. When only induced joints are used, post-cooling will generally not be applied, except occasionally to limit temperature gradients.
9.6.4. Joint forming systems
Transverse contraction joints are formed in RCC arch dams by inserting de-bonding systems over either part, or all of the cross-section area of the defined joint, in a similar manner to RCC gravity dams. In the case of arch dams, however, the systems applied must generally include grouting facilities to re-establish structural continuity after post-hydration temperature drop shrinkage.
In Chinese arch dams, a system that creates de-bonding over the full joint contact area is termed a “conventional transverse contraction joint”. A system that de-bonds between 1/6 and 1/3 of the joint area by including a de-bonding system only on certain layers is termed an “induced joint”, whereby a plane of tension weakness is created for the purpose of initiating a crack during post-hydration heat dissipation.
Elsewhere in the world, a conventional contraction joint is considered to apply to a wholly formed, rather than induced joint, whereby RCC is placed initially on one side of the joint against formwork and subsequently on the other side of the joint once the formwork has been removed, in the same manner applicable for a CVC dam. A conventional transverse contraction joint has been used in various RCC arch-gravity dams for diverse reasons, usually related to construction logistics and/or planning. Such joints have also been used where specific shear resistance capacity is required on the joint under dynamic loading (Shaw, 2015). In such instances, all joints not involving placing RCC against formwork would be termed an induced joint.
9.6.5. Groutable transverse contraction joint
1. System used for RCC arches in China
Groutable transverse contraction joints have been included in all larger (> 70 m) RCC arch dams in China and the system generally used comprises pre-cast concrete blocks (Zhu, 2003). The joints are formed using two types (A and B) of precast concrete block of 1 m length and 0.3 m height (equal to the layer thickness), with a bottom width of 0.3 m. The sloped side of the block in contact with RCC is formed with “teeth” to promote bonding. In the type A blocks, the holes for installation of the grout feed and vent pipes are included and type A blocks are installed in every fifth or sixth layer, with type B blocks installed in between, as illustrated in Fig. 9.4. The concrete blocks are aligned on the line of the contraction joint and anchored into the RCC with steel bars.
Fig. 9.4
Transverse contraction joint (a) and precast concrete blocks used to form the joint (b) (Photo: Du, 2015)
(a) Type A (b)
Type B A
A
B2 B1 B1
0.3$m
Inlet*pipe (a)
The blocks are installed on a bedding mortar to improve bonding and impermeability with the receiving RCC layer surface and RCC is placed and compacted on either side. The joints are grouted prior to the initial reservoir impounding.
2. Post-compaction system
A joint inducing and grouting system comprising perforated HDPE pipes installed inside a folded HDPE sheet was developed for Wolwedans Dam in South Africa (Geringer, 1995) & (Shaw, 2003) and these were installed in the process of RCC placement. These systems were termed “groutable crack directors”. Problems were experienced in the sideways movement of RCC under roller compaction, which resulted in pipe joints being pulled apart and subsequently prompted the development of an improved system for Changuinola 1 Dam in Panama.
By creating a trench in the compacted RCC on the alignment of the contraction joint using a specially manufactured wide vibrating blade attached to a backactor, it was possible to insert a perforated HDPE pipe inside a folded HDPE sheet, with inlets and outlets on the downstream face and connector pipes in the upstream GEVR to form loops (see Error! Reference source not found.). On t he alignment of the joint, groutable crack directors were installed in every fifth layer, with folded HDPE sheets without pipes installed in the third layer, resulting in a 50% de-bonding of the cross-section on the joint. During the installation of these systems, it is essential to clear the pipes with compressed air for the first few days after installation, due to the accumulation of laitance and water, which enters the system despite the pipe perforations being sealed with rubber sleeves.
Fig. 9.5
Typical tranverse joint inducing and grouting system, as used at Changuinola 1 Dam, Kotanli Dam & Koröğlu Dam (Photo:
Shaw, 2010)
9.6.6. Joint grouting
1. Joint grouting systems
As for a CVC arch dam, joint systems in RCC arch dams are provided to accommodate contraction associated with post-hydration temperature drop and are consequently generally grouted to re-establish the monolith arch structure and the associated structural continuity. Over the history of RCC arch dams, a number of strategies and technologies have been developed to grout induced contraction
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• A once only grouting system with post-cooling of the RCC mass;
• A system that is grouted in two, or three stages; and
• A re-injectable grouting system.
When RCC is post-cooled to remove hydration heat and to artificially achieve the closure temperature, a single grouting exercise will be sufficient, similar to CVC arch dams. When an arch is allowed to cool naturally, depending on its structural configuration, it may require grouting of the contraction joints in stages. Typically, a double-grouting system will allow two grouting stages, while a re-injectable grouting system will theoretically allow repeated re-grouting, as and whenever necessary.
As implied, a double grouting facility includes two independent grouting systems within each grouting compartment.
In most of the Chinese RCC arch dams, re-injectable grouting systems are installed in all joint types, while at the 96.5-m high Linhekou double curvature RCC arch dam, a triple grouting system was installed in the five induced joints and three conventional joints.
2. Re-injectable grouting system
A re-injectable grouting system was developed in China specifically for grouting transverse contraction joints in RCC arch dams (Chen, Ji & Huang, 2003).
Fig. 9.6
Detail of re-injectable grouting outlets
As illustrated in Figure 9.6, the key component of the re-injectable grouting system is a “tube- à-manchette” arrangement comprising a perforated steel pipe inside a rubber sleeve. The highly elastic rubber sleeve fits tightly around the steel pipe and functions as a one-way valve. Only when the internal pressure in the grouting pipe exceeds 60 to 150 kPa (0.6 to 1.5 bar) will the rubber sleeve open to allow grout to be released into the contraction joint. On completion of grouting, the grouting pipe system is washed out with low pressure water, allowing subsequent re-use when further contraction joint opening develops.
Once instrumentation indicates that a joint has opened sufficiently, grouting is typically undertaken, while the provision to re-grout is provided in case sufficient further joint opening develops during dam operation. Two conventional joints and two induced joints were constructed in the Shapai RCC arch dam, each including a re-injectable grouting system. The conventional joints were grouted prior to the initial reservoir impounding, while the 1st-stage grouting of the induced joints ensued over the following 9 months (April to December 2001) and the 2nd-stage grouting was completed in April 2003, two years after completion of the dam construction.
Experience to date has demonstrated that most of the induced joints in an RCC arch dam will not be open again after initial grouting. However, there have been exceptions where cracks have subsequently developed between joints, while the induced joints themselves remained closed. At Puding Dam, for example, the two induced joints remained closed several years after operation commenced, while cracks occurred in the concrete on the left and right abutments. The probable cause of these cracks is considered to be either higher placement temperatures in conjunction with
Rubber sleeve
Perforated steel pipe
Pipe coupling Grout outlets
inappropriate vertical construction joints (Chen & Xu, 2000) in these areas, or simply an inadequate number of induced joints on the extreme flanks.
A similar concept of a re-injectable grouting system has also been used outside China, with 40 mm HDPE pipes (instead of steel) perforated with 10 mm holes at 100 mm centres, which are similarly covered with a rubber sleeve in a “tube-à-manchette” system, to allow grouting, washing and re-use.
3. Important lessons learned from RCC joint grouting
The problems that can be encountered during the grouting process on non-formwork, or induced joints are blockage and leakage. Important lessons were learnt during the induced-joint grouting at Wolwedans RCC arch-gravity dam approximately 4 years after completion in 1993 (Hattingh, Heinz &
Oosthuizen, 2003) and the system was substantially improved for application at Changuinola 1 Dam.
While blockage and pull-out problems at connection points were substantially eliminated through installation after, rather than before RCC compaction, the most significant problem experienced at Wolwedans Dam during grouting was a consequence of the presence of a highly permeable zone between the RCC and the CVC facing (as discussed in Chapter 2), at both the up- and downstream faces. While this situation and leakage of grout around waterstops prevented the development of the intended pressure in the induced joints during grouting, where the contraction joints were open, successful grout filling was achieved. At Changuinola 1 Dam, only those joints that could not be accessed after impoundment were grouted and in this case, target grouting pressures were achieved without difficulty.
9.6.7. Special materials
As discussed elsewhere in this Chapter, shrinkage compensating cement additives and low stress-relaxation creep RCC mixes can be used to reduce the impacts of mass-gradient thermal effects, while high stress-relaxation creep RCC mixes can be used to reduce the impacts of surface-gradient thermal effects.