The raising and/or stabilising of existing concrete gravity dams by continuous concrete buttressing is a viable solution and, in some cases, it is the only solution available. There are few medium-large dams in Australia currently under consideration for raising with continuous buttressing. Two of the major issues to be surmounted are: (a) the existing dam should not be subjected to cracking (particularly on the upstream face) due to heat-hydration effects, and (b) the requirement for the two dam bodies to resist the hydrostatic and other loadings as a monolith (unified dam). However, there is great need for understanding the mechanisms involved in selecting an appropriate heat-of-hydration model and in calculating thermal stresses rationally. Due to such lack of understanding, expensive precautions, mostly with compounding conservatisms, would be adopted in concept and detailed designs eg. shear-keys on the interface, artificial cooling, post-grouted interface, anchor bars at the interface, concrete with high cement contents. On the other hand, unsafe designs could be the result. The paper discusses these issues highlighting that a rational approach can be adopted to economise the design and construction processes. An example is also presented to demonstrate how the potential for temperature-induced cracking in new and old dam bodies can be evaluated with reduced uncertainty by considering all the mechanisms involved in a holistic way.

1. Hydration-induced Stresses in
Concrete Buttressing of Existing
Concrete Dams
By
Nihal Vitharana (Arup, Sydney)
Nuno Ferreira (Arup, London)

2. 22
What is Concrete Buttressing ?
• It involves placing concrete behind an existing concrete
dam to either raise or strengthen it.
• This is a viable solution and sometimes the only
solution available.
• This presentation doesn’t cover the background behind
buttressing or other design details (refer: N Anderson
and N Vitharana paper at NZSOLD 2013).
• This paper concentrates only on the heat-of-hydration
and its effects on the existing dam and the buttress.

3. 3
Concrete Buttressing: Typical Arrangement
Foundation Drains
Foundation drainage access
platform
Horizontal outlet
drains
Interface Drains
Drain cap
Vertical foundation
drains
NZSOLD-2003: Anderson &
Vitharana

4. 44
Two Major Challenges in Concrete Buttressing
1. Existing dam should not be subjected to cracking
(particularly on the upstream face) due to heat-of-
hydration effects, and
2. Two dam bodies to resist the hydrostatic and other
loadings as a monolith (unified dam).
(Second point is discussed by Anderson & Vitharana
NZSOLD-2003 with extensive mathematics involved).

5. 55
Why heat-of-hydration is serious in buttressing ?
• Traditionally covered by: USBR-1965 and ACI committee 207
manuals dealing with mass concrete.
• However, the existing dam should not suffer damage (cracking
resulting in degradation and instability)
• In recent times eg
 Different cement types (fly ash, slag, silica fume)
 Different characteristics (too fine cement particles)
 Faster construction times
 Cost implications (mantra is to keep the cost down !)
 Different delivery modes with accountabilities assigned to various
parties (eg D&C contracts)

6. 66
Heat-of-Hydration calculations: what does this involve ?
1. Concrete mix design (Materials engineering/
constructability)
2. Heat generation (Physics)
3. Heat transfer (Chemistry)
4. Stress calculation (Engineering)
5. Asked questions if cracked (Potential litigation)
6. Fixing cracks (Repair technologists)
(Although appears simple, it is complex with intertwined
disciplines => big picture)

7. 77
Earlier work on this subject: Challenge the tradition
ANCOLD paper in 2002 (Adelaide) by Vitharana &
Wark:
Thermal Crack Occurrence in Large Concrete
Placements: Theory and Applications
Canning dam anchoring project => challenged the
tradition => faster construction without risk of concrete
cracking
However, more to do on fundamentals and with
buttressing……

8. 88
Why thermal loading is different from applied loadings
1. It is a strain-induced loading (ie, caused by a strain) =>
similar to others eg shrinkage & swelling, AAR
2. They are self-equilibriating (eg, vertical direction in a
concrete gravity dam) => no direct influence on stability
3. Being strain-induced stresses, they depend on stiffness:
=> thermal stress = Young’s modulus x thermal strain
This means => with “cracking”, stresses will be relaxed or
disappear

9. 99
Incorrect procedure => leads to conservatism
• With increasing load levels, thermal stresses relax
• If not well understood, it will lead to over-
conservatism in the estimation of thermal stresses
Vitharana and Priestley (ACI Structural Journal, 1998)

10. 1010
It is a serviceability issue, then why bother in dams ?
In dams, there is a secondary effect:
If cracking is caused by combined thermal and applied
stresses, it will modify the uplift pressures
unfavourably => affecting its stability
In a water-environment, cracking will also accelerate
deterioration mechanism with rapid ingress of water, eg, Alkali-
Aggregate-Reaction (AAR)

11. 1111
Modification of uplift pressures due to thermal cracking
Eg, a dam “without” drains:
Casagrande (1961) showed the benefit of drains for all adverse
conditions => install drains wherever possible

12. 1212
Fundamentals of Heat-of-Hydration
1. It is exothermic (ie, produces heat)
2. It is thermally-activated (rate of reaction depends
on the temperature regime of the hydrating
environment) as shown by Rastrup 1954 (Chemist
from Denmark)
=> it reacts slowly at low temperatures

13. 1313
Heat-Generation Characteristics
• Adiabatic is a special case (all boundaries are
thermally insulated) eg ACI 207 data
• Use this with extreme caution if accurate results are
required.
Temperature at time t:
Rate of heat generation J/kg/m3 of concrete:

14. 1414
Heat-Generation Characteristics => ACI 207
• Popular document => ACI 207 “Mass Concrete for
Dams and Other Massive Structure”
• Adiabatic curves for “four” cement types

15. 1515
Heat-Generation Characteristics => ACI 207
• Dependence on placing-temperature T0
• Implicit admission of thermal activation => higher
the temperature => higher the hydration rate

16. 1616
What is wrong if using Adiabatic Curve ?
• Hydration is a thermally-activated process.
• Two concurrent processes takes place:
- 1. Cement produces heat (hydration)
- 2. Heat is lost to the ambient (heat-transfer)
• We need to couple heat-generation and heat-transfer
< Use of traditional adiabatic model is fundamentally
wrong because it decouples the two processes >

17. 1717
Heat-Generation Characteristics: Rastrup (1954)
• Rastrup (1954) introduces a model for varying-
temperature environment => (Vitharana & Sakai
1995)
• Where te couples hydration and heat-transfer with an
equivalent time
• Suitable for any varying-temperature regime

18. 1818
Heat-Generation Testing
• Input to Rastrup model is via “Heat of Solution” tests
• In HoS tests, small cement samples are tested under
constant reference temperature Tr.
• It can be used for any cement composition by
separately testing the binders.
• Now becoming popular and recognised in many
international standards eg, ISO, Norwegian, ASTM

19. 1919
Difference in Temperature : Adiabatic vs Rastrup
• Eg. Low-heat cement with cement content of S=350
kg/m3
• Adiabatic with T0=20 0C and Rastrup at Tr=20 0C
A significant difference with adiabatic over-estimating
the rate

20. 2020
Difference in Thermal stresses: Adiabatic vs Rastrup
• 200 and 600mm thick walls with S=400 kg/m3 ; OPC
(Vitharana & Sakai 1995)
Adiabatic over-estimates thermal stress by 40%

21. 2121
Heat Transfer in a dam
• Generalised heat-transfer can be described by:
• Q0 is rate of heat generation
• Ambient interaction => allow for heat transfer due to
convection, radiation through dam's surfaces

22. 2222
Type and Removal Time of Formwork
• Eg. 750mm thick wall with steel and wood
formworks (Vitharana 1998)
=> Thermal shock occurs on removal of wood formwork

23. 2323
Typical Cost for Lowering Placing Temperature
• T0 has a significant effect on rate of hydration and
tensile thermal stress

24. 2424
Tensile Strength Development
• Hydration is also thermally-activated
• Standard cylinder test is not simulating actual
hydration state within the dam.
• Maturity function of CEB-FIP model code, modified
te by (Vitharana & Sakai 1995)
Ft is tensile strength at time t

25. 2525
Mechanism of Hydration-Induced Cracking
• Cracking occurs when
Thermal stress > Tensile strength at a given time t
Temperature is not causing cracking, it is the ’Thermal
Stress”

26. 2626
Schematic Representation of the Mechanism
• At early-age, change in concrete’s modulus E is the
main reason for thermal stresses; ref; Vitharana &
Wark 2002, Vitharana & Sakai 1995

27. 2727
Creep Relaxation ?
• Creep relaxation can reduce the thermal stresses by
40-50% at early-age
• Eg 600mm thick wall: stress distribution with and
without creep (Vitharana & Sakai 1995)

28. 2828
Simple Thermal Stress Calculation
Once the net thermal and creep strain is known, stress
increment at any time is given by:
Derivation is given in (Vitharana 1998)

29. 2929
Finite Element modelling; The Need ?
• In a buttress dam, the mechanical interaction between
existing dam and buttress is complex => Need finite
element modelling
• Reliable thermal and material input parameters are
more important than just operating a FE Model !
• Most commercial packages have severe limitations in
modelling early-age thermal behaviour => check
carefully what it gives you !

30. 3030
Example Buttressing
• 35m high dam raised by 4 m and stabilised with a
1v:0.75h d/s slope
• Low-heat cement with heat = 273 kJ/kg of cement
• Unit cement content of 350 kg/m3
• Placing-temperature of 20 0C
• Time gap between 1.2m thick lifts: 5-7 days
• Heat of Solution and Hot-Box trials were undertaken
• total temperature rise would be about 36 0C or total of 56 0C
(adiabatic) with To=20 0C

31. 3131
Example Buttressing: Dimensions

32. 3232
Temperature distributions
• Maximum at any time was 47 0C (compared with
traditional adiabatic = 56 0C)
Any time End of Construction

33. 3333
Vertical Stress
• Maximum on upstream face at any time was 0.8MPa
< tensile strength
Any time End of construction

34. 3434
Thermal Stress vs Tensile Strength with time
• At 4m above the foundation level
Any time End of construction

35. 3535
Deterministic vs Probabilistic
• Japanese Concrete Institute (1986) provides a
probabilistic approach.
• Thermal stress/tensile strength ratio is related to
probability of cracking (based on extensive site data)

36. 3636
Concluding remarks
1. Early-age thermal stress calculation is complex.
2. Important to evaluate your input parameters with accuracy
before embarking on complex analyses.
3. Hydration characteristics play a major role.
4. Creep relaxes thermal stresses by 40-50%.
5. With proper testing and sophisticated structural modelling,
economy in construction can be achieved with a good
understanding of the risks involved.
6. Example dam analyse proved (5).
Thank you !