DEF coupled to thermal history: Recent advances

Transcription

1 DEF coupled to thermal history: Recent advances François Toutlemonde, Badreddine Kchakech*, Othman Omikrine-Metalssi, Renaud-Pierre Martin IFSTTAR / Paris-Est University * Ph.D. co-sponsored by IFSTTAR and EDF, presently at Marrakech private University

2 Delayed Ettringite Formation (DEF) as coupled to T Autogenous expansive reaction (no necessary supply of external sulphates) leading to cracking and serious serviceability and structural capacity concerns. DEF affects structures submitted to early-age ettringite destabilization (T >~ 65 C) e.g. in case of bad controlled thermal curing of precast elements or massive structures DEF risk is proven on cement paste, mortar (Barbarulo 2002, Famy 1999) and only isolated results on concrete, when heating occurs at a mature stage (T > ~ 65 C) DEF is known as qualitatively affected by maximum temperature reached and heating duration. Non fully satisfactory models (Baghdadi 2008, Martin 2012) Quantification is required for : Precise reassessment (coupling law) Adjusted prevention guidelines Confirmation of a threshold value? Confirmation of pessimum effect? Clarification of mechanism(s) 2

3 Experimental program Jointly sponsored Additional data to Baghdadi s, Brunetaud s, Martin s and Pavoine s PhDs Prism specimens 11 x 11 x 22 cm w/ automated axial swelling monitoring Various thermal treatments at early age and 100 days (mature stage) 3 specimens for each condition kept in separate water baths to promote swelling Trapezoidal-shaped temperature profiles applied 2 hour after casting or at 100 days: (T max 61 to 86 C, 1 to 28 days duration) T plateau controlled w/ deviation < 0,15 C Heating process controlled w/ deviation < 0,5 % Satisfactory control of the treatment process! + Realistic temperature profiles for validation Companion specimens for physical / mechanical characterization: Tensile and compressive strengths Poisson s ratio, Young s modulus Porosity, control of alkali leaching 3

4 Experimental program Jointly sponsored Both mixes: W/C = 0.46 Porosity 15.5 % Siliceous, non ASR-reactive aggregate Concrete SO 3 (% C) C 3 A (% C) Na 2 O eq (% C) R1 f c28 = 50.5 MPa f c99 = 52.0 MPa R0 f c28 = 52.4 MPa f c99 = 63.9 MPa Thermal treatments T ( C) Duration of the plateau at maximal T (days) 61 C (early) (R1 +R0) - 66 C (early) (R1) - 71 C (early) 2 (R1) 7 (R1) 12 (R1) 14 (R1) 28 (R1) 81 C (early) 1 (R1+R0) 3 (R1+R0) 5 (R1) 7 (R1+R0) 14 (R1 + R0) 81 C (late) 1 (R1+R0) 3 (R1+R0) - 7 (R1+R0) 14 (R1) 86 C (late) (R1) - - 4

5 Raw results: Influence of maximum temperature Highly non-linear response: for this concrete, threshold between 61 and 66 C Very low scatter Latency times lower for higher maximum temperature Pessimum effect confirmed on the amplitude of final expansion 5 5

6 Raw results: Influence of heating duration (71 C) Latency and characteristic times lower for longer heating durations Pessimum effect of heating duration confirmed on amplitude of final expansion 6 6

7 Raw results: Influence of heating duration (81 C) Latency and characteristic times lower for longer heating durations (except 14 days) Pessimum effect of heating duration confirmed on amplitude of final expansion 7 7

8 Confirmation of high combined sensitivity to Tmax and duration Expansion (%) R1_J_V_74 R1_J_V_ Time (Days) R1_J_V_74 (Thermal treatment Tmax=74 C immersed in a water bath in an oven) vs. R1_J_V_79 (Thermal treatment Tmax=79 C in heated pool) 8 8

9 Raw results: R1, late heat treatment 2.0% 1.8% 1.6% Expansion (%) 1.4% 1.2% 1.0% 0.8% 0.6% 0.4% 0.2% 0.0% Time (days) R1_T_81_1 R1_T_81_3 R1_T_81_7 R1_T_81_14 R1_T_86_5 R1_T_V_79 Confirmed risk of significant expansion for R1 after heating when mature Pessimum effect with respect to final expansion (decreases after 7 days) Similar kinetics whatever the thermal treatment duration 9 9

10 Raw results: R0, late heat treatment Expansion (%) 0.20% 0.18% 0.16% 0.14% 0.12% 0.10% 0.08% 0.06% 0.04% 0.02% 0.00% Time (days) R0_T_81_1 R0_T_81_3 R0_T_81_7 Confirmed risk of significant expansion for R0 after heating when mature Pessimum effect with respect to final expansion (3 days > 7 days) Immediate swelling initiation whatever the thermal treatment duration 10

11 Processing: Quantitative description of the results 1.0% 0.9% 0.8% 0.7% 0.6% 0.5% 0.4% 0.3% 0.2% 0.1% 0.0% Expansion (%) 1 e ε() t = εhyd + ε 1 β L c 1+ e?? ε (t) = function of T(t) t τc t α e ( t τ ) τ?? ε,τ L,τ C, α, β = functions of T(t) Experimental data Proposed model improved from Brunetaud, 2005 Time (days) Example of parameters calibration (R1_J_81_14) ε (%) 0,89 τ L (days) 53 τ C (days) 15 α (days) 98 β 1,00 ε Hyd (%) 0,001 Deviation 1,66E-06 11

12 T(t): Which integrated indicator could make sense? Total supplied energy (T 0 =20 C) «DEF-effective» thermal energy (T 0 =65 C) Effective Thermal Energy (ETE) = < T(t)-T 0 > dt T 0 optimized for the reference R1 concrete to obtain master curves = 65 C Simplified assumption as compared to an Arrhenius-type activation of ettringite dissolution 12

13 R1, early age heating: ε vs. effective thermal energy Ultimate expansion (%) 2.0% 1.8% 1.6% 1.4% 1.2% 1.0% 0.8% 0.6% 0.4% 0.2% 0.0% 81 C 71 C 81 C_3 jr 81 C_5 jr 71 C_12 jr Martin C_7 jr 71 C_7 jr 71 C_14 jr 81 C_1 jr V_79 71 C_28 jr 81 C_14 jr 71 C_2 jr < T(t)-T 0 > dt T 0 =65 C Time (days) Effective Thermal Energy ( C.h) 13

14 Confirmed sense: R1 & R0 mixes + results from the literature (early age) Similar master curve for R1, Baghdadi & Al Shamaa R0 lower expansion 14

15 Results: τ L vs. effective thermal energy (early age) τ L vs. ETE: Relevance of a single master curve? 15

16 Results: τ C vs. effective thermal energy (early age) Single curve for R1 and similar mixes, higher τ C for R0 mix (less reactive)? 16

17 Early age vs. late heating: ε vs. ETE The pessimum effect with respect to final expansion seems to be shifted from thermal treatment at early age to treatment at a mature stage: Higher energy required for sulphates extraction / ettringite destabilization? 17

18 Early age vs. late heating: τ L vs. ETE τ L shorter than for early age heating in both cases, almost constant for R1 τ L (for R1, early age heating) tends to the value for late heat treatment for longlasting treatments τ L very high for R0 early age heating 1 day (650 days) 18

19 Early age vs. late heating: τ C vs. ETE For R0 badly conditioned S-shaped curves for late heating, indeterminate τ C For R1 τ c shorter than for early age heating, almost constant 19

20 Analysis: ultimate expansion, pessimum effect Some specimens observed using X-Ray Diffraction: confirms the origin of observed expansions as ettringite (DEF) consistent with the possible explanation of the pessimum effect on final expansion, by stable combination of aluminates (hydrogarnets) yet to be confirmed in a more quantitative way Shots Number R1_J_81_1_1 R1_J_81_3_1 R1_J_81_7_1 R1_J_81_14_1 AFt Q= Quartz P= Portlandite B=Brownmillerite AFt= Ettringite K=Katoite B AFt P + AFt K Q B + AFt Q 4 11 Angle (2θ) Proposal of a quantitative estimation of ε based on available reactants, accounting for the influence of alkali content 20

21 Proposed interpretation (early age) 21

22 Proposed model applied to R1 heated at early age 22

23 Proposed model extended to other mixes heated at early age Promising possible prediction of ultimate expansion, to be further validated 23

24 Effects of early / late heating Kinetics controlled by accessibility of sulphates? Early age heating Short duration SO4 2- hidden in LD CSH Semi-fine capillary pores τ L and τ C hardly influenced by the late treatment duration: the mature cement paste «host» of DEF might keep High τ L, τ c Early age heating Long duration similar, conversely to the early age SO4 2- at border of HD CSH heating case where size of capillary Wide capillary pores pores is critical Low τ L, τ c Low expansion amplitude for reduced late heating durations: the energy No early age heating required to destabilize ettringite might be Heating at mature stage higher than when heating is applied during setting, because ettringite might have a stronger structure and be better Short duration SO4 2- at the place of AFt Fine capillary pores Low τ L, τ c if any linked to other hydrates? Sulphates No early age heating might have kept on site? Heating at mature stage Long duration Low latency time in case of late SO4 2- at border of LD CSH heating due to the low porosity available Fine capillary pores for hosting delayed ettringite formed? Low τ L, τ c 24

25 Conclusions Results on concrete heated at early age help concluding that: The effective thermal energy makes sense as a global influent parameter on DEF expansion Master curves including a pessimum effect on ε can be quantitatively described consistently with probable mechanisms The concept of temperature threshold (around 65 C, mainly influenced by alkali content) is relevant (high non-linearity) For concrete heated at a mature stage: The risk of significant DEF-induced expansion is quantitatively confirmed in case of late heating + water A pessimum effect with respect to final expansion has been observed with possible similar origin as for early-age heated concrete Characteristic and latency times are shorter than for early-age heated concrete exhibiting DEF, and the treatment duration has negligible influence on the kinetics Future tasks: Complete chemical analyses to clarify where expansive species are formed and whether they migrate in the pores (role of water and alkalis) Confirm the predictive capability of the developed model with further data of ultimate expansion Implement ETE-dependence of ε in the model for re-assessment 25