2. DESIGN OF RCC DAMS
2.5. Thermal considerations
stress-relaxation creep, resulting in differential shrinkage of the core in relation to the outer surface, with surface compression and core tension subsequently developing as the hydration heat is dissipated.
Surface gradient thermal effects are short-term and result in surface cracking, while mass gradient thermal effects are longer-term and result in internal cracking.
The impact of both surface and mass gradient effects is determined by the magnitude of the total hydration temperature rise, the temperature at placement, the stress-relaxation creep experienced during hydration-related thermal expansion and the extremes of the applicable external ambient temperatures. Surface gradient effects are typically more intense during winter, while mass gradient effects are more intense in more extreme climates.
The most important mass gradient thermal effect relates to tensions caused by physical restraint under the thermal shrinkage that occurs as the concrete of the dam structure dissipates its hydration heat. With the consequential shrinkage in RCC dams almost always accommodated in induced transverse contraction joints, the extent of the total temperature drop to be accommodated is a function of the “zero stress” (or closure) temperature compared to the final winter-season equilibrium temperature. The “zero stress” temperature is a function of the placement temperature, the total hydration temperature rise and the stress-relaxation creep.
Higher levels of stress-relaxation creep during the hydration process result in a lower susceptibility to surface gradient effects and a higher impact of mass gradient effects. The reverse is true for lower levels of stress-relaxation creep.
An RCC with higher stress-relaxation creep is more susceptible to the development and propagation of long-term cracking both perpendicular and parallel to the dam axis, but less sensitive to short-term surface cracking due to thermal gradients. An RCC with low stress-relaxation creep will be less susceptible to mass gradient thermal effects and is less likely to develop cracking parallel to the dam axis, or cracking perpendicular to the dam axis between the induced joints. By contracts, this type of RCC is more susceptible to surface gradient effects and is consequently particularly sensitive to the development of surface cracks in more extreme climatic conditions, such as horizontal cracks that can subsequently compromise vertical tensile strength and/or result in leakage that can reach the galleries.
Reduction of the peak temperature experienced during hydration reduces both surface and mass gradient effects.
Surface gradient thermal tensions are superficial and can lead to accelerated weathering and/or leakage, while compromising structural function particularly under dynamic loading. Cracking consequential to mass gradient tensions can compromise structural function and can give rise to leakage. Some consequential cracking can often be tolerated as part of the dam design, while cracking in the placement surface during periods of extended exposure can sometimes be demonstrated to close due to consequential re-heating from the hydrating RCC in the layers placed above. In all cases, however, comprehensive analysis over the full hydration heating and cooling cycle is necessary to ensure that any such cracks will not propagate upwards, or downwards.
Stress-relaxation creep is dependent on cementitious materials types and contents, the amount of heat evolved during hydration, aggregate structure and the applicable level of structural restraint. An RCC with a 70%/30% flyash/OPC blend and well-shaped, well-graded aggregates with a low void content might indicate stress-relaxation creep between 0 and 75 micro strain, dependent on structural restraint. By contrast, an RCC with a low cement content, without pozzolan, and with a high Loaded VeBe time could be expected typically to indicate stress-relaxation creep above 150 micro strain.
2.5.3. Thermal analysis and design
The key materials parameters necessary to understand, model and predict thermal behaviour
• coefficient of thermal expansion,
• thermal conductivity,
• stress-relaxation creep, and
• the age development of tensile strain capacity.
To predict the impact of both surface and mass gradient thermal effects during the period from placement until the hydration heat is fully dissipated requires detailed thermal modelling, whereby a thermal analysis is considered to include both the temporal evolution and dissipation of temperatures and the consequential stress/strain (thermo-mechanical) behaviour. Such modelling should reflect the construction programme and take into account all external conditions and inputs, such as placement temperature, climatic temperature variations, solar radiation, the presence of water at the upstream face of the dam, the period of surface exposures, etc. To determine the consequential impacts of these thermal effects, simplified generic rules can be used in the case of smaller dams, although particular caution must be applied to take cognisance of the different stress-relaxation characteristics of different RCC materials when applying generic rules of thumb developed for CVC. For large RCC dams and smaller dams under critical thermal conditions, however, a relatively detailed Finite Element (FE) thermal analysis is essential to develop an adequate understanding of the consequential structural performance of the applied design.
Typically, a thermal FE analysis should model the full temperature development and dissipation cycle, predicting the consequential stress evolution and evaluating stresses at various time increments against the stress capacity of the RCC at the corresponding age. The thermal analysis should identify the maximum placement temperature at which consequential tensions (and possibly cracking) do not exceed tolerable levels. Modelling the construction with appropriate temperature controls, as necessary, the thermal analysis should be extended to identify the most appropriate spacing of transverse induced joints to avoid uncontrolled intermediate cracking.
It should be noted that a thermo-mechanical analysis based on the temporal development of concrete modulus of elasticity does not fully, or correctly recognise the actual RCC stress-relaxation creep behaviour.
A realistic understanding of the actual stress-relaxation creep to be experienced in a particular RCC during the hydration cycle is essential to allow a meaningful level of accuracy for a thermal study of an RCC dam. A conservative assumption is not possible, as surface gradient effects will be underestimated for a high assumption and mass gradient effects will be underestimated for a low assumption. Particular attention is necessary in the RCC close to the foundation interface, where high tensions can develop due to structural restraint, without relief from longitudinal contraction joints.
Thermal analysis is of particular importance in the case of an RCC arch dam, as a result of the 3-dimensional structural action being compromised by the opening of contraction joints as the concrete cools and the fact that the associated stress condition will continue to change until the hydration heat is fully dissipated. The thermal analysis is similarly important to establish the consequential requirement and timing for post-cooling and effective joint grouting.
2.5.4. Temperature control measures
With a maximum placement temperature, or temperatures for different parts of the structure, identified by means of the thermal analysis, the related requirements must be compared with the climatic and related conditions anticipated on the dam site during construction. Depending on the magnitude of the difference between the uncontrolled maximum temperature and the maximum allowable placement temperature, different placement temperature control measures will be required.
In CVC dams, traditionally both surface and mass gradient thermal effects have been minimised by reducing the concrete temperature at the time of placement, selecting cementitious materials blends with lower heats of hydration and/or suppressing the maximum hydration temperature experienced through post-cooling. In more extreme weather conditions, the surface of the concrete may also be insulated as a means to reduce surface gradient thermal effects. The same measures are applied for
RCC dams, although post-cooling is not preferred due to its greater influence on construction efficiency, while surface insulation becomes less practical at the typically high rates of placement and for the large placement areas consequentially associated with RCC. The application of extended concrete design strength ages, however, allows higher levels of pozzolan replacement in the cementitious materials, while a lower water requirement can allow reduced total cementitious contents, both implying a lower total hydration temperature rise in RCCs. In an RCC dam, mass gradient tensions parallel to the dam axis are released through induced transverse contraction joints, while tensions perpendicular to the dam axis are usually limited to a tolerable level to avoid cracking, without recourse to longitudinal contraction joints. Post-cooling is generally avoided wherever possible.
The measures commonly applied to reduce the placement temperature of RCC are addressed in detail in Chapter 5: Construction and these can be summarised as follows (Edwards & Petersen, 1995):
• A reduction in the hydration heat - through the use of low heat cements and high percentages of SCM.
• Pre-cooling of the RCC materials through:
o Cooling the coarse aggregates - In its simplest form, this might involve shading, or spraying the aggregate stockpiles to develop evaporative cooling. The most efficient method for a continuous cooling process has been found to be the use of a “wet belt” conveyor.
o Additional coarse aggregate cooling using chilled air.
o Fine aggregate cooling with chilled air.
o Replacement of mixing water with chilled water, or flaked ice.
o Adding liquid nitrogen at the mixer.
• Placement scheduling – To reduce cooling costs in warmer climates, it can be advantageous to schedule RCC placement in thermally critical parts of the dam structure during the cooler periods of the year and by avoiding mixing and placing RCC during the warmer times of the day.
• Evaporative cooling – Heat gains due to solar radiation can be reduced using evaporative water cooling, particularly in conditions of low relative humidity. Fogging over the placement area can also be used to similar effect.
• Post-cooling – Post-cooling with pipe loops placed on the compacted RCC surface has been successfully implemented on a number of RCC gravity and arch dams (Du, 2010) to reduce the time required to dissipate the hydration heat and to reduce the maximum hydration temperature experienced.
The temperature of RCC as discharged from the mixer is of less importance than the temperature of the same compacted RCC when covered by the subsequent placement layer.
Consideration must accordingly always be given to measures that reduce temperature gain after mixing, such as exposure to solar radiation during transportation and after spreading and compaction. In this regard, rapid construction develops benefits in reduced exposure times, etc.
2.5.5. Contraction joints
In early RCC development, it was postulated that the reduced mass gradient thermal effects associated with lower cement content could be accommodated without contraction joints. Prototype experience, however, quickly demonstrated this not to be the case, with some severe unplanned contraction cracking experienced in a number of cases. It is consequently now common practice to include contraction joints in all RCC dams, although the reduced hydration temperature rise and lower stress-relaxation creep typical of RCC generally allows a wider spacing than is the case in CVC dams (typically exceeding 20 m).
Whereas the construction of a CVC dam as a series of separate vertical monoliths allows post-
There are four forms of transverse contraction joints that are used in RCC dams (discussed in greater detail in Chapter 5):
• Post-formed joints that are formed by vibrating a de-bonding system into each RCC layer after compaction (sometimes after spreading and before compaction) to effectively divide the dam into a series of fully de-bonded blocks.
• Induced joints in which only part of the surface area of the joint is de-bonded. An induced joint is formed in the same manner as a post-formed joint, except that the crack inducers are only vibrated into every second, third, or fourth layer, effectively creating a tensile weakness and ensuring that subsequent contraction forces preferentially develop a crack on the alignment, or cross section of the joint.
• Formed contraction joints against formwork in a similar manner to traditional mass concrete.
• Partial (induced) joints in which only part of the joint is formed, acting as an initiator and thereby allowing thermal contraction forces to develop the remainder of the joint across the full cross-section. Partial joints should only be initiated from the upstream face.
The most common forms of transverse contraction joints in modern RCC dams are post-formed and induced joints. In both cases, de-bonding is usually achieved through the insertion of a folded plastic, or metal sheet, while in the latter case, the de-bonded area on the alignment of the joint has been as little as 16%, but is generally greater than 25%.
While the practice has largely become obsolete due to the ease with which joints can be vibrated into compacted RCC, in the earlier years of RCC dam construction the de-bonding system was often installed on the surface of the receiving layer immediately prior to spreading of the new RCC layer.
With joint spacing design optimised in conjunction with the maximum allowable placement temperature through the thermal analysis, the location of contraction joints should also take into account specific unavoidable conditions and features likely to cause stress concentrations. Particular aspects to be accommodated include foundation discontinuities, abrupt changes in the abutment gradient, rock mass zones of significantly variable stiffness, adjacent blocks of significantly different height and inserts and appurtenant works in the RCC, such as access galleries, discharge conduits, outlets, etc.
Gravity dams are designed for stability in 2-dimensons and consequently, the opening of transverse joints to accommodate thermal contraction is of no structural consequence. In the case of very high (and broad-section) gravity dams, however, it can prove necessary to include a (longitudinal) contraction joint parallel to the dam axis for a certain height above the restraint of the foundation. To maintain structural integrity in such instances, it is necessary either to include a facility to grout (when the concrete has cooled to equilibrium temperatures), or to drain such a joint, when left open. In addition, measures must be included to prevent upward crack propagation.
On the alignment of a contraction joint, an inducing and sealing system is commonly provided in the upstream facing (CVC/GERCC/GEVR/IVRCC). A typical arrangement comprises one or two PVC centrebulb (or similar) waterstops (between 250 and 500 mm in width) located at a distance from the upstream face of 300 to 500 mm and separated from each other by 400 to 1500 mm. A drain hole is usually provided midway between the waterstops and sometimes a second drain is provided downstreasm of the last waterstop. In higher RCC dams, more than two waterstops are often included in the upstream facing. In RCD dams, the first waterstop is usually located between 500 and 1000 mm from the upstream face, with a second a further 500 to 1000 mm downstream and a drain hole another 500 to 1000 mm downstream.
Contraction joints in arch dams typically require grouting to re-establish 3-dimensional structural continuity. Through placement temperature control and the use of very low stress-relaxation creep RCC at a number of RCC arch/gravity dams, however, it has proved possible in mild climatic conditions to limit the total effective temperature drop sufficiently to avoid the need for grouting (see Chapter 9). The structural requirement for grouting of contraction joints should accordingly be evaluated very carefully on a case-by-case basis.