Hydration-induced Stresses in Concrete Buttressing of Existing

Concrete Dams

By

Nihal Vitharana (Arup, Sydney)Nuno Ferreira (Arup, London)

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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.

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Concrete Buttressing: Typical Arrangement

Foundation Drains

Foundation drainage access platform

Horizontal outlet drains

Interface Drains

Drain cap

Vertical foundation drains

NZSOLD-2003: Anderson &Vitharana

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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).

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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)

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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)

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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 ApplicationsCanning dam anchoring project => challenged the tradition => faster construction without risk of concrete cracking

However, more to do on fundamentals and with buttressing……

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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, AAR2. They are self-equilibriating (eg, vertical direction in a concrete gravity dam) => no direct influence on stability3. Being strain-induced stresses, they depend on stiffness:

=> thermal stress = Young’s modulus x thermal strainThis means => with “cracking”, stresses will be relaxed or disappear

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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)

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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)

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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

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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

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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:

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Heat-Generation Characteristics => ACI 207

• Popular document => ACI 207 “Mass Concrete for Dams and Other Massive Structure”

• Adiabatic curves for “four” cement types

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Heat-Generation Characteristics => ACI 207

• Dependence on placing-temperature T0

• Implicit admission of thermal activation => higher the temperature => higher the hydration rate

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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 >

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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

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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

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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

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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%

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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

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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

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Typical Cost for Lowering Placing Temperature

• T0 has a significant effect on rate of hydration and tensile thermal stress

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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

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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”

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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

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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)

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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)

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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 !

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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

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Example Buttressing: Dimensions

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Temperature distributions

• Maximum at any time was 47 0C (compared with traditional adiabatic = 56 0C)

Any time End of Construction

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Vertical Stress

• Maximum on upstream face at any time was 0.8MPa < tensile strength

Any time End of construction

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Thermal Stress vs Tensile Strength with time

• At 4m above the foundation level

Any time End of construction

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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)

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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 !