SCconcretemodelexcha nge6062006

Crack modeling of the SUPERCONTAINER main body Overview and status on 6/6/6Johan Bel ONDRAF/NIRASAlain Van Cotthem TRACTEBEL/SUEZ: 

Crack modeling of the SUPERCONTAINER main body Overview and status on 6/6/6 Johan Bel ONDRAF/NIRAS Alain Van Cotthem TRACTEBEL/SUEZ Crack!!

PLAN: 

PLAN General context Supercontainer buffer : concrete choice - First step 2D modeling of the main body Concrete choice confirmation: the need for large scale tests Conclusion

General context: 

General context Starting point for the choice of a concrete buffer for the Supercontainer: ONDRAF/NIRAS with help of experts laid out basic requirements categorized in Absolute must Recommended « nice to have » The need exists to demonstrate feasibility for such a large concrete container in terms of: Workability (lab tests) Staging of the fabrication process (next study stages) Control of cracking : 2D/3D calculation Intermediate large scale tests

General context: 

General context Basically an iterative process between: Ondraf/Niras Consultancy (Magnel, TBL) Manufacturer (Socea, Ronveaux,…) Corrosion and concrete experts panel Supplier of basic components (Carmeuse, Degussa, …) GTA and SFC coordination team

Slide5: 

WP1:concrete lab testing (characterization) and feasibility study WP2 : test of fabrication methods (classic, spun concrete, self-compacting,..) WP3: small scale testing 2nd and 3rd phase Galson study WP4: crack behavior modeling WP5: supercontainer mock-up (scale 1/1) WP6: supercontainer heater test We are here Design and manufacturing specification General program :

Concrete choice : a typical mass concrete problem: 

Concrete choice : a typical mass concrete problem hydration residual stresses workability (homogeneity, segregation,…)

Concrete choice : a phasing problem (on going study): 

Concrete choice : a phasing problem (on going study)

Slide8: 

Labo tests on small samples were carried out by two independent companies (Socea and Ronveaux) with large industrial experience Initially, the composition from the “Galson” report was used : Concrete choice, First step : Is the initial composition adequate ?

Slide9: 

Workability of mortar without super plasticizer is poor => very dry concrete (compaction problems, risk for segregation,…) Concrete choice, First step : Is the initial composition adequate ?

Slide10: 

Different possibilities to tackle this problem: Higher Water/Cement ratio: up to 0.55 - 0.6 Use of superplasticizer (polycarboxylate based) Self Compacting Concrete =SCC (filler + super plasticizer) “Spun” concrete :

Slide11: 

Crack modeling (Preliminary 2D approach) Modeling has run in parallel with experimental adequacy lab tests First 2D-modeling study carried out by Belgatom in collaboration with university of Ghent (prof De Schutter) 2 D Modeling deemed necessary on order to: verify overall crack risks depth due to hydration and waste heat in the middle section Help set up experimental program for unknown or important parameters advice on some design related aspects (use of steel bars, use of anchors, closing details, shape adjustment,…) => 3D modeling needed for this purpose

Slide12: 

Crack modeling : basic assumptions CEM I 32.5 C30/37 350 kg/m³ 2 Dmodel Simplifications (I) : no iterative process has been used to take into account the reduced section in tension no material strength properties reduction due to T (15 % max) creeping effect is taken into account by using a reduced young’s modulus curve. based exclusively on bibliographical data’s it is only valid at the mid section (end effects are not taken into account)

Slide13: 

Crack modeling : basic assumptions 2 D model Simplifications (II) : Systus® software was used based on integral solving  but material history has to be considered => incremental solving (↔ non linearity) was replaced by : Constants : Thermal expansion coefficient Thermal conductivity Specific Heat capacity

Slide14: 

Crack modeling : basic assumptions the behaviour is closer to the adiabatic state then isothermal Two curing conditions: 5 W/m²°C applied on the inner wall and 16 W/m²°C on the outer wall (ventilated hall) 5 W/m²°C on both boundaries (adiabatic hall) The annular gap filling with cementitious grout or dry powder/pellets (lime or portlandite) (other E ) is not modelled 2 D model Simplifications (III) :

Slide15: 

Crack modeling : basic assumptions 2 D model Simplifications (III) : adiabatic curves from literature /standards

Slide16: 

Crack modeling : basic assumptions basic formula’s (I) Compressive strength, tensile strength, modulus based on EUROCODE and variable with equivalent time Compressive strength tensile strength Young modulus

Slide17: 

Crack modeling : basic assumptions basic formula’s (II) Hydration heat

Slide18: 

Crack modeling : Model Cross-section

What is exactly calculated ?: 

What is exactly calculated ? Step 1 : Temperature distribution due to hydration heat (solve heat equation) Step 2 : Corresponding displacement and stress distribution (longitudinal, radial, circumferential) in every point of the Supercontainer as a function of time from the beginning of the hardening (~8 hours) until equilibrium Thermo-elastic calculation (T,M) => T,H,M not necessary ? Only the first stage in the fabrication proces is modelled (2D) => further modelling will consider also other fabrication and handling stages and will be more detailed (3D) Both heat sources (cement hydration + HLW) are simulated Calculated tensile tresses are compared with tensile strength to evaluate risk for cracking Crack widths are NOT calculated

Slide20: 

Crack modeling : Results prefab(I) Temperature distribution (from red to blue ) Max after 36 hours Equilibrium after 16 days

Slide21: 

Crack modeling : Results prefab(II) Radial stress (+ : tensile; - compressive ) Ventilated room

Slide22: 

Crack modeling : Results prefab(III) Circumf and long stresses (+ : tensile; - compressive ) : Stress inversion with time Residual stress after cooling down due to T gradient history non linearities creep Ventilated room

Slide23: 

Crack modeling : Results prefab (IV) Tensile stresses remain below tensile strength during fabrication : admissible computed

Slide24: 

Only 5 to 10 cm of (non reinforced) the outside border of the Supercontainer may present micro-cracking : Figure shows final stress state at equilibrium (extrados) Crack modeling : Results prefab + waste (V)

Slide25: 

Crack modeling : Results (VI) Stresses due to hydration are dependant of the curing and drying conditions “Steam” curing may be envisaged and will lead to compressive residual main stresses at equilibrium and potentially better contact between envelope and concrete Stresses in stainless steel envelope are very low No separation between stainless steel envelope and concrete buffer is expected in the central section but verification on end plugs is necessary “Adiabatic” curing conditions provide better contact between concrete and steel

Slide26: 

Concrete choice confirmation tests Unusual concrete (angulous aggregate, CEM I,..) hence no reliable bibliographical data’s Missing information at young age on creep and hydration Confirmation of strength parameters with time Confirmation of Gas basic calculation values : gas entry pressure Real scale observation of homogeneity, segregation and workability Acquire reliable values for a complete 3 D model that will also predict the behaviour at the end piece (lid) and at the annular gap interface. WHY (I) ?

Slide27: 

Concrete choice confirmation tests No more simplifications : incremental, tensile zone, creep, shrinkage, use of hydration rate α(t), based on adapted parameters values from tests WHY (II) ?

Slide28: 

Concrete choice confirmation tests HOW ? On representative 6 m high samples with different concrete composition

Slide29: 

Concrete choice confirmation tests HOW ? A A’

Slide30: 

Concrete choice confirmation tests HOW ?

Slide31: 

Concrete choice : Latest decisions Isothermal test shows CEM I 42.5 could be used availibility in Belgium Guarantee of supply on the long term SCC concrete composition is considered as the most promising one but « normal » concrete with superplasticizer will also be considered ( vibration !!) Spun concrete on hold (but only solution without superplasticizer) Tests set-up under discussion with Magnel/Cstc/TBL Interim report for 2d-modeling (Feb 2006) will be updated before testing

Slide32: 

Conclusions We are close to concrete choice, narrow down to few proposals Choice will be strongly justified through a step by step approach Cracking will likely be controlled Pending : end plug (lid) and annular gap filling necessity and behaviour of the steel enveloppe (3 D modeling needed) Behavior under dynamic loading (normal and accidental) => modeling+tests

Requirements for the supercontainer buffer material : 

Requirements for the supercontainer buffer material CATEGORY 1 : ABSOLUTE MUSTS (no compromise is to be accepted ) BUF_R1.1 use of pure PORTLAND cement CEM I BUF_R1.2 Cement HSR:  to better resist to sulphur species present in Boom Clay pore water BUF_R1.3 Aggregates only based on calcareous (CaCO3) materials:  sand and filler (if applicable) included ! No siliceous materials allowed BUF_R1.4 No other organic additives but the superplasticizer

Slide34: 

CATEGORY 2 : RECOMMENDED (a certain margin or flexibility of the requirement can be accepted ) BUF_R2.1 Cement with limited  hydration heat production to avoid or limit cracking  : preferably CEM I 32.5 but if availabity is a problem 42.5 LH (low heat) & N (normal hardening) can be acceptable (hydration heat test Q=f(t)  to be performed to compare 42.5 with 32.5 ) BUF_R2.2 Tensile strength 2 MPa (caracteristic value)  This value of 2 MPa may be reviewed if another cement (e.g. 42.5) is chosen BUF_R2.3 Materials should be available in sufficient quantitities during long periods of time (use on an industrial scale only planned within several decennia) BUF_R2.4 Superplasticizer preferably based on polycarboxylate (unifunctional) BUF_R2.5 Good workability  - preferably pompable so S4 or even S5 during at least 60 to 90 minutes

Slide35: 

CATEGORY 2 : RECOMMENDED (a certain margin or flexibility of the requirement can be accepted ) BUF_R2.6 Compressive strength sufficient to resist to mechanical normal and accidental (fall,..;)  loads  : no exact values can be given but a C30/37 or better concrete is a good starting point BUF_R2.7 Micro-cracks are allowed (and cannot be avoided) but radial, through-going cracks that might jeopardize the radiological shielding capacity should be avoided BUF_R2.8 Good quality, homogeneous and dense concrete (no quantitative values imposed)

Slide36: 

CATEGORY 3 : NICE TO HAVE (if not fulfilled, this requirement will not jeopardize the concept ) BUF_R3.1 Water/cement factor : 0.4 à 0.45 no exact value imposed : can be derived from other requirements BUF_R3.2 Compressive strength after 1 day, 7 days,... : no precise requirements ; fabrication  (e.g. time to keep casting form around concrete)  will be adapted to results BUF_R3.3 cement content min 300 kg/m3 BUF_R3.4 avoid use of rebars or steel fibres back