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First National Dam Safety Conference
CWC, TNWRD and IITM
A Brief Study of Arch Dams Behaviour
Dr. B. R. K. Pillai
Zika Smiljkovic
Dr. A. K. Dhawan
Project Director, DRIP
Central Water Commission, New
Delhi
Email: dir‐drip‐[email protected]
Dam Design Engineer
EGIS Eau
Montpellier, France
Dam Instrumentation Specialist, EGIS India
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Causes for Potential Unusual (Irrecoverable)
Behavior of Arch Dams
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Alkali Silica Slow Reaction (ASSR)
Flow chart of stages of alkali silica formation: Mechanism: Alkali solution in cement paste pores attacks
reactive minerals of concrete aggregate producing thus
alkali silica gel which in presence of water swells. The gel
usually builds up within the aggregate fragments which
then swells causing expansion of concrete and cracking of
surrounding cement paste. Affects initially the aggregate.
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ASSR Displays
• Long lasting irreversible chemical process
causing: irreversible upstream deflection of
dams crest, upheaval of dam crest and mostly
continuous cracking of upper galleries.
• Expansive reaction from a few to approx. 30
years. Initiation, development and, slow down
phase following exhaustion of alkalis.
• Impairing the concrete strength and integrity,
and state of equilibrium of a dam.
5 • Cases reported: Chambon dam, upheaval of 3.6mm/yr; Arch dams in Alpine region, U/S drift up to 1mm/yr. • Cracking of inspection galleries reported: Pian
Telesio Dam(Italy), Portodemouros dam (Spain).
• Karun Dam, Iran, 205m height and 14 yr old, exposed to ASSR.
• ASSR limited with service compressive field > 6MPa, as reported. Tensile stresses are less constrainable to ASSR development.
• Upstream drift mechanism: lower RWL with higher concrete temperature →upstream deflection of upper arches → tensile stress on U/S dam face → pronounced U/S dam face swelling→ irreversible upstream dri . • Galleries crack mechanism: lower RWL with lower concrete temperature → tensile stress in the zone of the galleries → pronounced swelling in the tensile zones → increase of service tensile stresses → cracking of the galleries. 6
Delayed Ettringites Formation (DEF)
Potential Flow Chart of (DEF) process (as reported): Mechanism: Expansion of cement paste forming gaps around the non‐expansive aggregate fragments in which ettringite crystals generate. The crystals my grow under the process described above followed by expansion and spalling of concrete . Affects initially the
cement paste.
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DEF Displays
• DEF is expected to be limited in arch dams areas with pronounced service compressive field. The service tensile stresses are less efficient to DEF development.
• Upstream drift mechanism: lower RWL with higher concrete temperature →upstream deflection of upper arches → tensile stress on U/S dam face → pronounced U/S dam face swelling due to DEF→ irreversible upstream drift. • Galleries crack mechanism: lower RWL with lower concrete temperature → tensile stress in the zone of the galleries → pronounced swelling due to DEF in the tensile zones → increase of service tensile stresses → cracking of the galleries. 8
Concrete creeping effects to behaviour of arch dams
• Creeping under the action of compressive stresses.
• Pronounced usually at D/S face whilst max WL and min concrete temperature, when dam expected to deflect D/S. • Creeping Criterion: σcreep = (0.35‐0.40)fc’
• Since max compressive stresses expected to D/S face, downstream deflection due to creep expected. • Creeping is mainly aligned with creeping of cement paste.
• The temperature of concrete increases the creeping process in concrete. • Creeping of concrete can be enhanced by ASSR process. There, creeping can mask ASSR process going on. • Produces irreversible creep strains with age of an arch dam. 9
Empirical vs computed models for creeping of arch dams (ICOLD)
Total Strain per Unit Stress J(T, t) 0.079
0.078
0.077
0.076
0.075
0.074
0.073
0.072
0.071
0.07
0
15
20
25
30
35
Years of Creeping t
n
T
m
︶
 
10
1
1E
,
T
J
︵
5
            


Empirical model of elastic + creeping strain of Ross Dam USA, applied on an 35 old arch dam of similar properties
[A] Years, [B]&[C] Normalized & Irrecoverable strains
Calculated normalized strain due to seasonal variation of loading, including creep effects, at Zervreila arch dam
Empirical model(Ross Dam) and Calculated Strain for chosen function of Central Cantilever deformation (Zervreila Dam) indicated possibility for leveling off of creeping strains after a time. 10 Influence of Block Joints Carbonation to Behavior of Arch Dams
Distorted Scale Mechanism: Ca (OH)2 + CO2 = Ca CO3 + H2O
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Joints Carbonation Displays
• Precipitation of lime to calcium carbonate while long‐term presence of stagnant water in the reservoir.
• Blocks joints separation, occurring seasonally whilst lower RL and higher concrete temperatures when arches tend to deflect upstream.
• Carbonate infill stiffens during a season, making thus a firm peg in the joint. Result: Obstruction to joints to close again i.e. capturing upstream deflection of dam. • May enhance ASSR development In presence of water in joints. 12
Thermal Effects to Arch Dams Deformation
• Irreversibility of upstream movements of arch dams are also attributed to global irreversible heating which are often linked with global decrease of reservoir level. • Solar radiation effects may amplify the concrete temperature to more than 45 degrees centigrade.
• Consequences: for approximately 50% increase of upstream dam deflection. • Accompanying effects: Enhancing of ASSR where already developed. 13
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Conclusions
ASSR develops within aggregate of concrete. A Swelling process which drifting the upper stretches of arch dams towards upstream and simultaneously their upheaving. Monitoring of a number of arch dams indicated their max upstream drift of 1mm/Yr. Some of them exhibited upheaval of max 3.6mm/Yr.
Usually, levelling off process on long term basis. DEF(Ettringites) develops within concrete paste. A swelling process inducing upstream dams deflection. Creeping of concrete can be distinctive to arch dams affected by considerable aging effects, as a result of which the mechanical properties of concrete became inferior. Some events reported to be levelling – off process.
Block joints carbonation while their seasonal upstream separations my prevent joints subsequent closing. This, in turn may contribute to differential upstream arch dams drifting. 14
References
Particular attention to conference Attendees is recommended to be paid to following references whish were used while drafting of this paper: References No. 2, 3, 4, 6, and 10. Thank you for your Attention.
Merci de Votre Attention, Madames et Monsieurs.