Workshop. Granada - Spain. CESI RICERCA s numerical modeling of dams affected by ASR: two case-studies. A. Frigerio, G. Mazzà

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1 Workshop Granada - Spain CESI RICERCA s numerical modeling of dams affected by ASR: two case-studies A. Frigerio, G. Mazzà 10/2007

2 Forward Topics of the presentation: problems related to ageing and deterioration of materials: the presence and the effects of alkalisilica reaction (ASR) the key role of mathematical models in the evaluation of structural behaviour of dams the safety reassessment and rehabilitation of existing dams

3 CESI RICERCA s research on ASR Definition of a screening criteria to classify concrete dams according to their potential to be affected by ASR Map of Italian concrete dams which might be affected by ASR; evaluation based on the characteristics of the construction materials (aggregates and cement): high potential (red), medium potential (yellow), low potential (green)

4 CESI RICERCA s research on ASR Parameters identified making reference to lab testing In our in-house computer software, CANT-SD, the mathematical model proposed by Charlwood et al., in 1994, was implemented This model considers the effective concrete expansion to be stress-dependent, i.e. compressive stresses are assumed to reduce the concrete free expansion 4

5 Piantelessio dam: general data Concrete arch-gravity dam located in Piemonte, North-West of Italy Construction completed in 1956 Crest level 1919 m a.s.l. Max height 80 m Max storage level 1917 m a.s.l. Crest length 515 m Storage reservoir 23.5 Mm 3 5

6 Piantelessio dam: ASR 6 Downstream [mm] Upstream [mm] Vertical section CN

7 Piantelessio dam: ASR The presence of ASR was ascertained by laboratory tests (i.e. expansion tests at 38 C and 80 C in NaOH) Mechanical tests on specimens extracted from different locations in the dam body were carried out to characterize the material 7

8 Piantelessio dam: Benchmark Theme proposed at the 6 th International Benchmark Workshop on Numerical Analysis of Dams October 17-19, 2001, Salzburg Austria Aim of the analyses proposed as a Benchmark: Structural identification of the present stress-strain state of a concrete arch dam, making reference to the measurements collected with the monitoring system Forecast of the future stress-strain trend in the next 20 years 8

9 Piantelessio dam: Finite Element model Dam foundation system Peripheral joint (joint elements) nodes e 2044 linear finite elements 4 elements in the dam thickness 9

10 Piantelessio dam: numerical analyses Loads applied to the structure: Dead weight Hydrostatic pressure in the reservoir Volumetric expansion due to the ASR Numerical models to simulate the joints behaviour: Elastic shear joint (ESJ) Frictional Coulomb joint (FJ) Numerical models to simulate the expansion process: Uniform volumetric deformation, applied as a thermal load (TE) Volumetric deformation dependant from the local stress state (CH) 10

11 Horizontal downstream-upstream crest displacement in 1999 assumed as the reference value for the identification process Case A Case B ESJ ASR TE ESJ ASR CH Δ Δ Δ Case C FJ ASR CH Δ Δ Δ 11

12 Piantelessio dam: results in 1999 Max principal stresses in 1999 Min principal stresses in 1999 Case A - ESJ ASR TE Case A - ESJ ASR TE Case B - ESJ ASR CH Case B - ESJ ASR CH Case C - FJ ASR CH Case C - FJ ASR CH 12

13 Forecast of the future behavior: horizontal crest displacement in the period Case B - ESJ ASR CH

14 Maximum and minimum principal stresses on the downstream dam face in 1999 and 2020 Max principal stresses Case B - ESJ ASR CH (MPa) Dam: max=1.95 Pulvino: max=3.51 Min principal stresses Dam: max=3.87 Pulvino: max=5.65 (MPa) Dam: min=-7.81 Pulvino: min= Dam: min= Pulvino: min=

15 Comments on results Max principal stresses in 1999 Case B - ESJ ASR CH Case C - FJ ASR CH Modeling of joints and interfaces is fundamental for the interpretation of the structural dam behavior Different limit states relevant to the overcome of concrete strength require suitable models capable to keep into account cracks formation and propagation 15

16 Poglia dam: general data Concrete hollow gravity dam, located in Lombardy, North of Italy Construction completed in 1950 Crest level 632,4 m a.s.l. Max height 50 m Max storage level 632,0 m a.s.l. Crest length 137,1 m Storage reservoir 0.5 Mm 3 Upstream view from the right abutment during construction Downstream view 16

17 Poglia dam: general data Longitudinal section Dam layout 17

18 Poglia dam: general data Horizontal and vertical section of the main buttress 18

19 Poglia dam: ASR A drift in the displacements was observed both in the vertical direction (1mm per year) and in the upstreamdownstream direction (0.2mm per year) in the main block, both detected by plumb lines, collimation and levelling networks) Chemical-physical tests carried out on concrete have clearly stated the presence of alkali content, mainly in the aggregates, and of an amorphous gel which can be ascribed to ASR Downstream view of the main block 19

20 Poglia dam: Finite Element model Numerical analyses have been carried out with CANT-SD, a program for linear and non-linear static and dynamic analysis Discontinuity such as structural joints and rock-concrete interfaces have been included in the model. Upstream view Downstream view 20

21 Poglia dam: model calibration Comparison between collimation and levelling measurements on the dam crest and values computed by means of the numerical model 21

22 Poglia dam: rehabilitation Thanks to the calibrated model, it has been possible: to evaluate the future trend of displacements and stresses on the basis of the expected residual concrete expansion to forecast the effects of the joints cutting which is the structural rehabilitation methodology chosen to reduce the stress-strain state in the dam body 22

23 Poglia dam: rehabilitation Construction site preparation for the cutting of the contraction joints Facility for the diamond-wire cutting of the contraction joints Facility at work for the cut of a joint shield and the copper strip End of the cut work 23

24 Poglia dam: rehabilitation Hydro demolition of the concrete joint shields Facility at work for the demolition of the concrete joint shields 24

25 Poglia dam: Benchmark Water heights to be considered Theme proposed at the 8 th International Benchmark Workshop on Numerical Analysis of Dams October 23-30, 2005, Wuhan China Aim of the analyses proposed as a Benchmark: Ultimate Water Height: to be found Structural identification of the present stress-strain state of the main block of a large concrete hollow Operational gravity Water dam, with reference Height: to the ultimate strength against the 630 hydrostatic = 47.9 m load The opening and sliding state of the dam-foundation interface Comparison of the load-displacement curves obtained with or Elevation: without ASR expansion m a.s.l. For both cases the ultimate water level has to be found 25

26 Poglia dam: numerical analyses Loads applied to the structure: Dead weight + ASR + hydrostatic load Dead weight + hydrostatic load Numerical models to simulate the joints behaviour: Friction-cohesion Coulomb, no tension (CANT-SD) Friction Coulomb, no tension (ABAQUS) Numerical models to simulate the concrete behaviour: Elasto-plastic (Lade, 1988, with CANT-SD) Concrete Damaged Plasticity (Fenves, 1998 with ABAQUS) 26

27 Poglia dam: Finite Element model CANT-SD ABAQUS 4594 nodes parabolic brick elements 407 dam 405 rock foundation 52 joint elements 3464 nodes linear tetrahedron elements 8181 dam 3398 rock foundation 2 contact surfaces 27

28 Poglia dam: results CANT - ASR CANT - NoASR ABAQUS - ASR - Friction 37 ABAQUS - NoASR - Friction 37 All curves are cleaned by the ASR displacement 60 Water Height [m] CANT-SD NoASR CANT-SD upstream downstream Displacements (w.h.=30m) CANT-SD Opening upstream downstream ABAQUS NoASR 0 Sliding Top Downstream Displacement [mm] 28

29 Poglia dam: results Water Height [m] Top Downstream Displacement [mm] noaar AAR notens_noaar notens_aar 29

30 Poglia dam: results Dead weight ASR Ultimate Water level - Max Principal Stresses Internal view Dead weight ASR Ultimate Water level - Min Principal Stresses Internal view External view External view 30

31 Comments on results Self-balanced actions such as ASR does not influence sliding limit equilibrium condition Modeling of joints and interfaces is fundamental for the interpretation of the structural dam behavior The presence of lift joints in the structure deserves the improvement of numerical constitutive models After the rehabilitation works, it is suitable: To check the reliability of the numerical models adopted to support the choice of interventions To re-calibrate the model, if necessary, to improve the future forecasting 31

32 Main lessons learned Dams in general present structural discontinuities: their modeling is fundamental for the correct assessment of the real dam behavior Different limit states relevant to the overcome of concrete strength require the use of reliable models capable to keep into account cracks formation and propagation as well as smeared damage which can cause stiffness reduction of the materials The complexity level of the models used to carry out numerical analyses has to be adequate to data completeness and quality 32