6TH INTERNATIONAL SYMPOSIUM ON ROLLER COMPACTED CONCRETE (RCC) DAMS Zaragoza, 23 – 25 October 2012
PROPOSING A STANDARDIZED APPROACH TO STRESS-STRAIN INSTRUMENTATION FOR RCC DAMS Dr.-Ing. Marco CONRAD AF-Consult Switzerland Ltd, Baden, Switzerland Dr. Quentin SHAW ARQ Consulting Engineers (Pty) Ltd., Pretoria, South Africa Dr Malcolm R.H. DUNSTAN MD&A, Buckfastleigh, United Kingdom
It has recently been demonstrated that different types of roller-compacted concrete (RCC) in large dams can display quite different early stress-strain behaviour, which must be accurately modelled for meaningful thermal analysis and dam design, particularly in the case of high dams. A need has consequently been identified to motivate the RCC dams industry to investigate and to develop data on this important characteristic across a broad range of RCC and Hardfill (CSG) types, with a view to establishing ‘rules of thumb’ for design and appropriate methods for verification in the mix development process and during early construction. The installation of a standardized instrumentation arrangement in a large number of RCC dams is perceived as a best first approach to encourage a broad investigation and consequently to swiftly advance the understanding of the early stress-strain behaviour for a wide range of RCC types. On the basis of their combined experience, the authors of this paper correspondingly propose a standardized stress-strain instrumentation scheme for RCC dams, designed to evaluate the response for the different material combinations and RCC types in respect of stress-strain behaviour, in-situ elastic modulus development, relaxation/creep in tension and compression and other non-thermal volume changes. An arrangement of long-base strain gauges and so-called effective concrete stressmeters is proposed, due to the well-proven performance of these instruments under the harsh conditions inherent to RCC placement for dams.
Despite a history in dam construction of some 30 years, our understanding of the early-age behaviour of different types of RCC in large dams continues to develop and with these advances new opportunities and new problems that must be considered in thermal analysis and dam design become apparent. With recent research demonstrating that different types of RCC and different cementitious materials can give rise to quite different levels of early stress-relaxation creep, it is particularly important for dam design that we can measure and eventually predict these characteristics. To continue to advance our understanding of the associated issues and to develop our ability to take full advantage of the associated behaviour in the design of large RCC dams requires data from more dams and the broad involvement of the community of RCC dam professionals in consequential investigation and analysis. Until very recently, we have assumed that all types of RCC behave in a similar manner during the hydration cycle and that the typical stress-strain behaviour witnessed in conventionally-vibrated concrete (CVC) is similarly evident in all RCCs. While we construct CVC dams in separate vertical monoliths to account for the related behaviour, we assume that the induced joints in RCC dams open to allow the dam to behave as a similar series of monoliths. But if certain RCCs experience negligible creep and consequential shrinkage during the hydration cycle, at best, the considerations that we must apply for thermal analyses must be appropriately adjusted and at worst, we might be incorrect in treating our gravity dams essentially as 2-dimensional structures.
With evidence demonstrating that RCC can now be designed for a range of stress-relaxation and conventional creep characteristics, it is important to note that the consequential behaviour of the RCC can be manifested in effects and performance that have not previously been acknowledged in general RCC dam design. In large RCC dams, particularly in narrower valleys, these consequential effects can possibly be deleterious if not understood and appropriately managed. The early stress-strain behaviour of each particular RCC mix is a very important parameter that will influence the appropriate approaches to dam design, thermal stress analysis, placement temperature control requirements and induced joint systems and arrangements. Conversely, the requirements of a particular dam site might well determine the requirements of the RCC in respect of early stress-strain and creep behaviour.
Through a definition of the currently available instrumentation that is typically most suitable for the measurement of early stress-strain behaviour in RCC and the proposal of a standard layout of instruments, it is intended to encourage fellow RCC dam practitioners to investigate the related behaviour of as many different types of RCC and Hardfill (CSG) as possible. With more data and more technical analysis, using the same reference framework, it will be possible to build a more definitive understanding of the specific characteristics that determine the extent of creep that occurs during the hydration cycle and the range of stress-relaxation creep possible across the various different RCCs. In the current situation, we can develop only indicative information through laboratory testing, while some better information can only really be produced by the time of the construction of the Full-Scale Trial, which comes rather late to be of more than of corroborative value for dam design. With a good data-base produced for different RCC types using a standard configuration of instruments, it will eventually be possible to develop the necessary knowledge and understanding on which basis to establish ‘rules of thumb’ for different RCC types and cementitious materials blends to be applied during the dam design process.
DETERMINATION OF EARLY-AGE BEHAVIOUR
The recent research into the early-age behaviour of RCC demonstrated that it is the processes that occur during placement and over the first few days that are the key to the stress-relaxation creep and consequential shrinkage behaviour of an RCC. With the method and mechanisms of compaction and the extent of restraint undoubtedly playing significant roles, the real conditions that develop over the first few days in bulk RCC cannot realistically be recreated and measured in a laboratory, nor fully replicated in a Full-Scale Trial. While it is this very problem that has compromised the development of our understanding of this early behaviour to date, it required long-term comparative instrumentation measurement on RCC dams, recreated in analytical simulation, to prove the different stress-relaxation creep and volume-change characteristics of different RCC types. A key aspect with a strong influence on the stress-relaxation characteristics of a specific RCC is the autogenous volume change of the constituent cementitious materials during hydration and this is probably the only related characteristic that can realistically be measured in a laboratory. Consequently, and in the absence of any ‘rules of thumb’, it is necessary to rely on instrumentation to establish the associated anticipated level of stress-relaxation creep.
RCC DAM INSTRUMENTATION
Instrumentation for RCC dams has been developed over the years on the same general principles as applied for traditional mass concrete dams, to measure foundation uplift pressures, foundation and dam body seepage, concrete and water temperatures, structural deformations and displacements, foundation bearing loads, seismic response, etc. In addition, specific strain measurement instruments have been developed to measure the openings on induced joints, while RCC dams will also often use a substantial number of thermocouples, or thermistor temperature sensors, in order to accurately establish the distributions of temperature at various intervals during the hydration cycle. More recently, effective stress measurement gauges, originally designed for use in CVC, have been successfully applied in RCC.
In this paper, the authors focus specifically on instrumentation suitable for the measurement of stress-strain behaviour in RCC. While strain instruments can be used for measuring joint opening and structural deformation over the full operational life of the dam, it is the behaviour of the RCC over the duration of hydration temperature rise and heat dissipation, or the hydration cycle, that is of greatest interest to the dam designer. To this end, it has been found that the location and orientation of instruments is of particular importance to avoid the development of distorted stress-strain data, which will often be the case, for example, when strain gauges are located across induced joints. In addition, measured strains and restraint stresses can only realistically be interpreted for stress-relaxation creep and non-thermal volume changes taking into account the full picture of the concurrent transient temperature state. An enhanced monitoring of thermal gradients therefore needs to accompany all stress-strain measurement.
Due to the exaggerated influences of thermal effects and restraint caused by the proximity of the foundation, the development of useful information in relation to the early stress-strain behaviour of RCC during the Full-Scale Trial (FST) construction has not generally been successful. While the standardized arrangement proposed in this paper has been developed for installation in the body of the dam itself, it is perhaps worth making some comment on arrangements likely to be more suitable for an FST, as early information in respect of this behaviour characteristic is obviously of particular value.
For the FST, or FSTs, a comparison of the early stress-strain behaviour under restrained and un-restrained conditions is of specific value and it is suggested that part of the FST be constructed on competent rock, while another part be constructed on well-compacted dry sand. Blinding and a heavy HDPE de-bonding sheet should also be used to minimise foundation restraint beneath the section of the FST constructed on sand and dry sand should also be used for thermal insulation over the full top surface of the completed trial embankment, in order to replicate as closely as possible the maximum hydration temperature to be experienced within the core of the dam body. Alternatively, to avoid the insulation, a larger number of ‘training’ layers could be placed in the bottom part of the FST (which would be beneficial in any event), where the installed stress-strain instrumentation would be insulated by the overlying RCC.
STADARDIZED APPROACH TO STRESS-STRAIN INSTRUMENTATION FOR RCC DAMS
As discussed in the paper “The Influence of Low Stress-Relaxation Creep on Large RCC Arch & Gravity Dam Design”, the early stress-strain behaviour of different types of RCC can be seen to substantially alter the requirements for dam design and analysis, placement temperature controls, allowable thermal gradients and exposure times between the layers. Furthermore, an accurate thermal analysis is simply not possible without knowledge of the actual stress-relaxation characteristics of the particular RCC to be used. Investigations have further demonstrated that deleterious 3-dimensional effects can become particularly problematic for high gravity dams in narrow valleys and it is quite possible that a different approach would be appropriate for the induced joints in cases where foundations indicate significant variations in deformability. It is consequently apparent that an additional parameter exists for RCC that requires detail consideration in the design and mix development process. On the basis of the importance of the early stress-strain behaviour of the various different types of RCC and the difficulties inherent to the prediction and measurement of this characteristic, the most appropriate means by which to improve the related understanding within the dam engineering industry is considered to be the production, gathering and analysis of as much related data as possible, ideally produced on a comparable basis. To realise this objective, a standardized arrangement of specific instrumentation is advocated, that can be simply and relatively cheaply installed in as many RCC dams as possible over the coming years.
From the early days of RCC construction, instrumentation designed specifically to monitor the behaviour of the material itself has generally included thermocouples and thermistors and different types of strain gauges. In respect of temperature, instrumentation has been installed to establish the temperature profile and consequently the thermal gradients at various stages during construction and early operation, while allowing the monitoring of the hydration temperature development and dissipation cycle. Strain gauges have been installed with the primary function of monitoring the opening of induced joints, as the body of the dam cools. With some early RCC dams only including crack inducers in the surface zone, and not in the body of the RCC, Long-Base-Strain-Gauge-Temperature Meters (LBSGTMs) were developed with a view to intercepting an induced joint opening that could occur over an area, as opposed to on a precise, pre-determined alignment. With the advent of crack directors placed in the RCC at intervals varying between every layer and every fourth layer, it has been possible to reduce the length of the LBSGTMs from 1 m to 600 or 700 mm. These instruments, which use vibrating wire technology, have proved very successful and reliable in operation and are well suited to the conditions of any type of RCC dam construction. Paradoxically, shorter (250 mm) and apparently more robust stainless steel strain gauges have proved more unreliable and more susceptible to indicating variable strain results. More recently, effective concrete stressmeters (Munich type) and fibre optic instruments have been successfully installed in RCC dams. Fibre optics offer the benefit of being able to recover densely distributed data over an extended length and the ability to measure temperature, strain and deformation. However, the disadvantage of fibre optics is the cost of the readout units. These units are expensive, when looking at the specifications usually desired for the application in an RCC dam, and not commonly available. Consequently, it is not realistically possible to maintain standby equipment, unless an Owner is constructing several large RCC dams in a short period of time, and accordingly any malfunction could lead to important data being permanently lost. While effective concrete stressmeters have been installed in RCC, it is not considered that accurate measurement using these instruments can be universally assured. Consequently, it is considered sensible to support a stressmeter with simultaneous, adjacent strain measurement, which will allow better validation and calibration of the indicated stress levels, taking into account in particular the variable restraint conditions. It is also important that all stress and strain measurements are accompanied by temperature measurements and the use of vibrating wire systems will allow coincident strain and temperature measurements within the same gauges. Enhanced thermal gradient monitoring, with a higher density of thermocouples or thermistors in the vicinity of the
stress-strain instrumentation, will allow a more accurate analysis of stress-strain behaviour. Restraint, both internal and external, will impact stress and strain development and, while it is important to understand the associated influence on the early stress-relaxation creep of a particular RCC, this is only of real value if compared with the equivalent situation without any significant influence of restraint. Consequently, to develop a meaningful understanding of the early stress-strain behaviour of a particular RCC, it is necessary to make strain measurement during the hydration temperature rise and dissipation under unrestrained conditions and to take stress and strain measurement, both parallel and perpendicular to the dam axis, in the core and external zones of the dam body. To achieve this objective, it is necessary to include appropriate instrumentation in the FST (see Fig. 1), as well as the body of the dam itself. It is also recommended that part of the FST be constructed to simulate unrestrained conditions (see sub-Section 3.3).
Fig. 1: Designed Instrumentation in the FST for Dak Mi 4, Vietnam While the FST serves a number of purposes, it is considered likely that only an indicative evaluation of the early stress-strain behaviour of the constituent RCC can be made on the basis of the FST and it is the body of the dam that is likely to yield the most definitive information. Consequently, it is proposed that the main set of standardized instrumentation be installed in the body of the main dam. Instrumentation under minimised impacts of restraint and hydration temperature peak suppression resulting from the proximity of the foundation rockmass or exposed surfaces, where steep thermal gradients will develop, should be installed at least 10 m above the foundation level and ideally in the monolith centre. However, it is also considered beneficial to install instrumentation in areas of maximum restraint, i.e. close to the foundation and close to facings. Long-term placement breaks also represent an additional area of interest. On the basis of successful experience to date, a system developed using effective concrete stressmeters (Munich type) and LBSGTMs is advocated and the arrangement considered most practical and appropriate is indicated in Fig. 2 and Fig. 3.
Fig. 2: Schematic Section of Proposed Standardized Instrumentation for Early Stress-Strain Behaviour Measurement (excluding Temperature Monitoring)
Fig. 3: Schematic Layout of Proposed Standardized Instrumentation for Early Stress-Strain Behaviour Measurement (considering instrument redundancy) 5.
In proposing and presenting a standardized layout of instrumentation for the measurement of the early stress-strain response of RCC in large dams, the authors are attempting to encourage the development of a better understanding of this previously inadequately addressed behaviour characteristic across the full range of RCC types. In tandem with laboratory testing to determine the autogenous shrinkage characteristics of the applicable cementitious materials blend, the proposed instrumentation is seen as a necessary and appropriate inclusion in all large dams to investigate and verify the applicable early stress-relaxation creep parameters to be applied in thermal analysis and structural design. The findings from the standardized stress-strain instrumentation proposed are expected to be of significant structural and economic importance, with benefits including reducing adverse dam structure behaviour, eliminating the need to grout induced joints on RCC arch dams and allowing higher maximum admissible RCC placement temperatures. A better understanding of the in-situ stress-relaxation creep behaviour will also enable a more accurate determination of the potential for surface and mass gradient cracking in RCC dams both during construction and operation.
Shaw, Q.H.W., and Dunstan, M.R.H., “The Low Stress-Relaxation Characteristics of Flyash-rich RCC”. Proceedings. 6th International Symposium on RCC Dams. Zaragoza, Spain. October 2012. Shaw, Q.H.W., “The Influence of Low Stress Relaxation Creep on Large RCC Arch & Gravity Dam Design”. Proceedings. 6th International Symposium on RCC Dams. Zaragoza, Spain. October 2012. Conrad, M, Aufleger, M.G. and Husein Malkawi, Al., “Investigations on the Modulus of Elasticity of Young RCC Dams.” Proceedings. 4th International Symposium on RCC Dams. Madrid, Spain. November 2003. Conrad, M, Hoepffner, R and Aufleger, M.G., “Innovative Monitoring Devices for an Integral Observation of Thermal Stress Behaviour of Large RCC Dams.” Proceedings. 5th International Symposium on RCC Dams. Guiyang, China. November 2007.
Dams: Dak Mi 4 Keywords: Keywords Trial Mix Programme, Full-Scale Trial, instrumentation, standardized layout, long-base strain gauges, effective concrete stressmeters, early age, stress