Thoughts on Concrete Durability. Performance Specifications

Transcription

1 Thoughts on Concrete Durability Performance Specifications Jason Weiss, Purdue University Professor and Director of the Pankow Materials Laboratory Salt Reaction NCC Meeting April 21st, 2015 Slide 1 of 52

2 Durability in Concrete A large part of good concreting is doing what we already know how to do We can control the capillary pores by controlling the w/c (SCM and WRA good) Excess water leads to pores and increased transport Low Hanging Fruit all can reach Lower w/c, SCM move in right direction NCC Meeting April 21 st, 2015 Slide 2 of 52

3 Summary of Findings Our 4 Step to PRS Shrinkage Specification Free shrinkage, probability of cracking Transport (Diffusion) to Freeze Thaw Resistance Based Criteria; LTDSC reactivity NCC Meeting April 21 st, 2015 Slide 3 of 52

4 Can We Write Durability Equations 1970 s deicing salts become widely used for clear pavements App. US: Diffusion 8-12 mil. annual tons of salt Ions travels through concrete, depassivates steel, & corrodes Pore Can Solution we write an equation to predict when chloride reaches a critical level at the bar? Photo Credit Dolch NCC Meeting April 21 st, 2015 Slide 4 of 52

5 Fick s Second Law For Diffusion In 1855 Adolf Fick introduced an equation (2 nd law) regarding diffusion C t = D 2 C 2 x C is the concentration, t is time, x is position, D is diffusion coefficient Chloride At the Bar Critical Value Time NCC Meeting April 21 st, 2015 Slide 5 of 52

6 Ficks Second Law and Time to slow down and ask some questions. Is this perfect... not even close Despite a super-cool equation it has quite a bit of empiricism Is this better than the American Bandstand approach to durability. Probably Allows us to discuss other factors ( x, t) C C s C C o o = 1 erf 2 x Dt NCC Meeting April 21 st, 2015 Slide 6 of 52

7 Ficks Second Law ASTM C 1556 % Chloride Epoxy Coating Fitting with Depth (mm) 0 Immersion Cxt, C0 x = erfc( ) C C 2 D t s 91 d curing Plain mix 28 d Plain mix 91 d IC mix 28 d IC mix 91 d APP C s and D APP Grinding Titration NCC Meeting April 21 st, 2015 Slide 7 of 52

8 Concentrations of NaCl D APP need to obtained for the same concentration of field exposure ( 0.5 mol/l). Non-linear increase of total chloride with free chloride concentration. NCC Meeting April 21 st, 2015 Slide 8 of 52

9 Co-present Anions D APP greatly depends on the co-present anions C s (% Weight of Specimen) OH - SO Copresent Anions (mol/l) D APP (e -11 m2/s) OH - SO Copresent Anions (mol/l) Less chloride binding when NaOH and Na 2 SO 4 is present. NCC Meeting April 21 st, 2015 Slide 9 of 52

10 Where Do We Find Tests Much like Indiana on the search for the holy grail We want a test for transport (or durability) that is fast, accurate, inexpensive easy to interpret but also scientifically valid Diffusion is lengthy but also has pitfalls (binding, units, leaching, duration, age, lack of fit, exposure dependent, etc) We think that electrical measurements can be a significant part of this approach NCC Meeting April 21 st, 2015 Slide 10 of 52

11 Electrical Methods Food For Thought NCC Meeting April 21 st, 2015 Slide 11 of 52

12 Electrical Methods Food For Thought NCC Meeting April 21 st, 2015 Slide 12 of 52

13 Rapid Test Methods Uniaxial, surface, embedded, and RCPT all yield results that can be directly compared if done properly 3.0% NaCl To - Lead Epoxy Coating 0.3 M NaOH To + Lead Concrete Copper Mesh Electrodes ρ = RA L NCC Meeting April 21 st, 2015 Slide 13 of 52 V I A L RCPT Current Voltage Resistance Conductivity Coulombs Amps V K-Ohms kohm-cm S/cm RCPT Direct =

14 Comparison of Different Manufacturers Spragg et al NCC Meeting April 21 st, 2015 Slide 14 of 52

15 Lets Start with Notation and Archie Using resistivity, while I prefer conductivity, tests in practice that have 1 discussion in ρ ρ = Assume the only conductive phase is the fluid and the resistivity of the concrete is the product 1 1 ρ = ρo = ρ of resistivity of solution O F φ β and the formation factor (inverse porosity and connectivity) (solutions exist for other conductive phases Weiss et al.) σ NCC Meeting April 21 st, 2015 Slide 15 of 52

16 Maybe We Should Look at the Maybe it makes sense to look at the formation factor instead for specifications ASTM C1202 Classification (1) Charge Passed (Coulombs) (1) (kohm cm) (2) High >4,000 < ? Moderate 2,000-4, ? Low 1,000-2, ? Very Low 100-1, ? Negligible < 100 > ? (1) from ASTM C (2) calculated using first principles These numbers are just place holders however they illustrate how to get to the most fundamental value With this one can to go in two directions 1) This relates to service life 2) This enables various constituents NCC Meeting April 21 st, 2015 Slide 16 of 52

17 From to Diffusivity Nernst Einstein Relationship Walther Nernst ( ) German physical chemist/physicist Won1920 Nobel Prize NCC Meeting April 21 st, 2015 Slide 17 of 52 ρ ρ D D i i = = D D µ i µ i D D Pore i 1 = = µ Bulk i F ρ ρ Pore Bulk φβ = D µ i 1 F

18 Again Important So We Should Go Slow and Ask Questions is not a unique descriptor of microstructure ρpore Di 1 = = µ ρ D F Bulk factor is a unique descriptor i F = ρ ρ Bulk Pore = σ σ Pore Bulk NCC Meeting April 21 st, 2015 Slide 18 of 52

19 s Determine the Determine the correct value for the pore solution resistivity (obtain in the trial batch or mixture acceptance, know for QC/QA) Three approaches to obtain ρ O (for const.) 1) Extraction Doable 2) Sensor Promising (Rajabipour et al) 3) Calculation Are trying for state input binder to be sent Research to confirm approach NCC Meeting April 21 st, 2015 Slide 19 of 52

20 Components of Variation Machine/Operator/Material Traditionally estimated in a single lab as 3-4% (Purdue, LaDOT) Production Important when used as a QC/QA tool Dependent on contractor quality 10% is a typical value Data shown is from a central mix plant with one mixture run frequently, low variation Spragg et al NCC Meeting April 21 st, 2015 Slide 20 of 52

21 Components of Variation Attention to Curing is Critical AASHTO RR (12) Within-lab: 4.36% Machine/Operator/ Material Multi-lab: 13.22% Machine/Operator/ Material and curing Believed Curing Variation: 12.5% State Variation Shown (top young, bottom old samples) Spragg et al NCC Meeting April 21 st, 2015 Slide 21 of 52

22 Incorporating Aspects of Curing Spragg et al NCC Meeting April 21 st, 2015 Slide 22 of 52

23 Degree of Instead of correcting each term independently it may be possible to treat all the terms with a single function Works well for paste and mortar ongoing for air entrainment Function, f(s) w/c = 0.50 Experimental Data VCCTL Simulation Degree of, (S) NCC Meeting April 21 st, 2015 Slide 23 of 52

24 Testing Activation Energy of Conduction (test temp) Rajabipour et al. 2007, Sant et al.2007 In the past we noticed differences between Varied the solutions : 9-12 kj/mol Bulk Sample: kj/mol Spragg et al NCC Meeting April 21 st, 2015 Slide 24 of 52

25 Conductivity is a function of Maturity (hydration) and (conductivity) σ ( t) = f ( M ) f ( T f ( M ) = f ( T) = t 0 E exp R exp ar σ ( T) E ac 1 R T Maturity and Electrical Conductivity 1 T E ar = KJ/mole 1 T REF 1 T REF. ) dt Equivalent Specimen Age (Hours) NCC Meeting April 21 st, 2015 Slide 25 of 52 Paste Conductivity (S/m) W/C = C 23 C 36 C

26 Relationship Between Q (Columbs) and (ρ) Many relationships have been developed over the years (the black - theory) While all have a reasonable shape, details are very important when one tries to use this in spec s We will come back to this Spragg et al NCC Meeting April 21 st, 2015 Slide 26 of 52

27 Leaching During Storage + NCC Meeting April 21 st, 2015 Slide 27 of Spragg et al

28 Importance of Accounting for Several s 420 Top figure shows direct measurement Bottom has corrections applied for temperature, ionic strength, saturation, and leaching Spragg et al NCC Meeting April 21 st, 2015 Slide 28 of 52

29 Accelerating Curing Time Many materials we test take a long time to show benefits (91 d) We frequently want to speed this time up VTRC/NRMCA method Lime water 7d, 23C followed by 21d, 38C T equivalent 56d Application on the right shows difference ~25% Bu et al NCC Meeting April 21 st, 2015 Slide 29 of 52

30 Accelerated Curing Effects Transport testing and service life prediction usually performed on specimens of later age (91 days). Ea 1 1 ( ) R T Tr t = e t Same maturity (DOH) could be achieved with shorter time using a higher curing temperature. 91 days 23 days NCC Meeting April 21 st, 2015 Slide 30 of 52 e E a = 37 kj/mol Bu et al. 2014

31 Role of Fluid Expansion Examined normal and accelerated curing with samples at different temperatures that were sealed/saturated Porosity (%) D Cl - (10-11 m 2 /s) NA-Wet AA- Sealed AA- Wet Saturated Samples 24% Bu et al NCC Meeting April 21 st, 2015 Slide 31 of 52

32 Applications Acceptance Phase Mixture Acceptance Before construction to qualify mixture Time to corrosion Absolute value of D Development of master curve data strength v time resistivity v time Quality Control Measurements during construction Test with good repeatability Easy tests allow for large sample size, statistical information as well NCC Meeting April 21 st, 2015 Slide 32 of 52

33 Thoughts on Electrical Measures Measurements during construction, fast (minute) Use a comparison to the original mixture that was qualified (avoids complexities and makes this very accurate) Is this the same mixture we qualified? Test for repeatability Easy tests allow for large sample size in place with statistical information Relative Frequency Average = 2.2 SD = 0.24 COV = 11 % Mixture 1 Mixture 2 Average = 6.7 SD = 0.89 COV = 13 % Electrical (kohm cm) NCC Meeting April 21 st, 2015 Slide 33 of 52

34 Suggestion Quality RCPT F High 1000 F1 Medium 2000 F2 Low 4000 F3 NCC Meeting April 21 st, 2015 Slide 34 of 52

35 Thoughts on Concrete Durability Performance Specifications Jason Weiss, Purdue University Professor and Director of the Pankow Materials Laboratory Salt Reaction NCC Meeting April 21st, 2015 Slide 35 of 52

36 Durability in Concrete A large part of good concreting is doing what we already know how to do We can control the capillary pores by controlling the w/c (SCM and WRA good) Excess water leads to pores and increased transport Low Hanging Fruit all can reach Lower w/c, SCM move in right direction NCC Meeting April 21 st, 2015 Slide 36 of 52

37 Summary of Findings Our 4 Step to PRS Shrinkage Specification Free shrinkage, probability of cracking Transport (Diffusion) to Freeze Thaw Resistance Based Criteria; LTDSC reactivity NCC Meeting April 21 st, 2015 Slide 37 of 52

38 Conventional Air Good general approach: 1) measures limited (SAM etc) 2) sorption, DOS, rxn salt NCC Meeting April 21st, 2015 Slide 38 of 52

39 General Freeze Thaw Model (Ice - ) Fagerlund pioneered this approach, followed by Li et al. 1-D (shown) here and Farnam, Lucero adding details When concrete reaches a critical degree of saturation (S CRIT ) it will become damaged Damage occurs fast Rate is related to the S and fracture NCC Meeting April 21 st, 2015 Slide 39 of 52

40 ASTM C Absorption Test method used to determine the rate of absorption (sorptivity) of water by measuring the increase in the mass of a specimen as a function of time when only one surface of the specimen is immersed in water NCC Meeting April 21 st, 2015 Slide 40 of 52

41 Absorption and the Role of Air Just measuring mass absorbed (ASTM C1585) is not sufficient Need to know the sample degree of saturation Link to the critical degree of saturation and to the service life S Sat S Nick NCC Meeting April 21 st, 2015 Slide 41 of 52

42 Water Absorption and Pores Degree of S Int S Nick = S Mat S Sat Time Lucero et al. NCC Meeting April 21 st, 2015 Slide 42 of 52

43 Water Absorption and Pores Degree of S Sat 5 3 S Nick = S Mat 2 1 S Int Time Lucero et al. NCC Meeting April 21 st, 2015 Slide 43 of 52

44 What We Can Control Role of NCC Meeting April 21 st, 2015 Slide 44 of 52

45 Phase Diagram of NaCl Solution NaCl salt most common deicers used on concrete. Typical NaCl Water Phase Diagram Farnam et al. 2012b NCC Meeting April 21 st, 2015 Slide 45 of 52

46 Motivation: Observation of an Unexpected Phase Change in Samples Exposed to NaCl One cycle low temp freeze-thaw tests: +24 ºC to -40 ºC An unusual exothermic behavior at higher concentration Low Concentration High Concentration (Farnam et al., 2012b) NCC Meeting April 21 st, 2015 Slide 46 of 52

47 Hydrated Cement and Calcium Chloride/Magnesium Chloride Similar but more damage is observed for salts like CaCl 2 MgCl 2, more complex but similar (Farnam et al., 2014a,b) NCC Meeting April 21 st, 2015 Slide 47 of 52

48 Identification Tests and Mitigation Low Differential Scanning Calorimeter (LT-DSC) Uses cement powder with a Solution of CaCl 2 Good Screening Test Related to the Chemistry of Cement (C 3 A, CH) Role of SCM (good, watch scale in high slump mixes) Can use sealers also (Villani et al., 2015) NCC Meeting April 21 st, 2015 Slide 48 of 52

49 Test and Materials Longitudinal Guarded Comparative Calorimeter (LGCC) A new test method Two mechanisms proposed during F/T cycles depending on the concentration 1) low concentration hydraulic, osmotic 2) high concentration chemical reactions NCC Meeting April 21 st, 2015 Slide 49 of 52

50 General NCC Meeting April 21 st, 2015 Slide 50 of 52

51 Suggestion Freezing-Thawing NCC Meeting April 21 st, 2015 Slide 51 of 52

52 Summary Shrinkage Specification Can relate free shrinkage to probability of cracking Transport (Diffusion) is a good test is the way to go Thinking about the F in concrete Working Through ps conductivity (6 m) Freeze Thaw Resistance Based Criteria Air (Nick Point) S 2 is a paste quality, LTDSC NCC Meeting April 21 st, 2015 Slide 52 of 52