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Marco CONRAD AF-Consult Switzerland Ltd,

Switzerland Dr

Quentin SHAW ARQ Consulting Engineers (Pty) Ltd


South Africa Dr Malcolm R



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[1] 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,

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,

placement temperature control requirements and induced joint systems and arrangements


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


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


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



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,

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,

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,

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,

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


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



As discussed in the paper “The Influence of Low Stress-Relaxation Creep on Large RCC Arch & Gravity Dam Design”[2],

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


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,

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,

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


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)[3] and fibre optic[4] 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,


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,


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


it is considered sensible to support a stressmeter with simultaneous,

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


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


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

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

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


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


it is also considered beneficial to install instrumentation in areas of maximum restraint,

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


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


“The Low Stress-Relaxation Characteristics of Flyash-rich RCC”



October 2012

“The Influence of Low Stress Relaxation Creep on Large RCC Arch & Gravity Dam Design”



October 2012



“Investigations on the Modulus of Elasticity of Young RCC Dams


November 2003



R and Aufleger,

“Innovative Monitoring Devices for an Integral Observation of Thermal Stress Behaviour of Large RCC Dams


November 2007

Dams: Dak Mi 4 Keywords: Keywords Trial Mix Programme,

Full-Scale Trial,

effective concrete stressmeters,