Dale P. Bentz (dale.bentz@nist) National Institute of Standards and Technology International Congress on the Chemistry o

1. Verification, Validation, and Variability of Virtual Standards Dale P. Bentz (dale.bentz@nist.gov) National Institute of Standards and Technology International Congress on the Chemistry of Cement July 10, 2007

2. Background • Abundance of Computer Models for Predicting Performance of Cement-Based Materials • HIPERPAV, FEMMASSE, DuCoM, Life-365, CIKS, VCCTL • Such models could form the basis for the development of virtual standards • Just as with the development of a physical test method, virtual test methods must be verified and validated, and their variability considered

3. Outline • Some definitions • Verification • Validation • Calibration • Variability • Example of a virtual test method for heat of hydration • Conventionally measured by ASTM C186 • Summary and Prospectus

4. Verification • “The process of determining that a model implementation accurately represents the developer’s conceptual description of the model and the solution to the model” fromhttp://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html • Answers the question “Are we building the model right?” • Are our equations correct? • Do we have the correct (best) values for all parameters? • Is our computer code bug-free?

5. Validation • “The process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended users of the model” fromhttp://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html • Answers the question “Are we building the right model?” • Are our predictions accurate and useful for our intended audience? • Does the model contribute to new technical insights and innovations?

6. Calibration • “The process of adjusting numerical or physical modeling parameters in the computational model for the purpose of improving agreement with experimental data” fromhttp://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html

7. Variability • Assessment of the change in model predictions when one or more input parameters are modified in a controlled manner • For a simulation, could be the random number seed • Could be an input parameter characterizing the system being modeled • Phase fractions and/or phase perimeters (surface fractions) • Phase correlation functions • Particle size distribution (PSD) • Activation energies

8. A Physical Testing Analogy • Compressive strength of high performance concrete (HPC) • Verification – Are we building the test method right? • Capping materials, strain rates, consolidation • Carino et al. references in conference paper • Validation – Are we building the right test method? • Is compressive strength the best measure to characterize the performance of HPC? • Early-age cracking, durability and transport measures may be more appropriate • Goodspeed, Vanikar, and Cook, Concrete International, 1996.

9. Virtual Test Method Example • Virtual Heat of Hydration Test • ASTM C186 is the only current physical test method for heat of hydration within ASTM • Few laboratories have the necessary equipment • Results only available after waiting 7 d or 28 d • Cumbersome- acid dissolution of samples, etc. • w/c=0.4, sealed hydration at 23 °C • Virtual test method is based on CEMHYD3D v3.0 model (freely available via Internet download at ftp://ftp.nist.gov/pub/bfrl/bentz/CEMHYD3D/version30) • Validation performed using a set of 5 CCRL cements • Two variants: • Complete PSD, SEM/X-ray image characterization • PSD and X-ray diffraction (XRD) volumetric phase analysis only

10. Enthalpy of Hydration of Cement Phases A For C3A hydration, values are for conversion to C3AH6, ettringite, and monosulfate (Afm) phase, respectively. B For C4AF hydration, values are for conversion to C3AH6 and ettringite, respectively.

11. Virtual Test Method Procedure 1) obtain a physical sample of the cement of interest and characterize it with respect to PSD and volumetric phase composition based on SEM/X-ray image analysis or X-ray diffraction (standards for the PSD and phase characterization methods are currently being pursued in the ASTM C01.25 and ASTM C01.23 subcommittees, respectively), 2) prepare a w/c=0.4 (23 °C) cement paste specimen and measure its chemical shrinkage according to the ASTM C 1608 test method, during at least the first 8 h of hydration; use the measured response to calibrate the kinetics factor, β, that connects model hydration cycles to time in the CEMHYD3D v3.0 computer model, for this virtual cement hydration (w/c=0.4, saturated hydration at 23 °C) , 3) using the same calibrated kinetics factor, conduct a virtual heat of hydration experiment (w/c=0.4, sealed hydration at 23 °C) with CEMHYD3D v3.0 to obtain the 7 d and 28 d (and other) heat of hydration values for comparison to the experimentally measured values from the ASTM C 186 test method, 4) optionally, conduct virtual (semi)adiabatic hydrations, etc. to estimate the (semi)adiabatic temperature rise of concrete mixtures of interest produced with this cement

12. C3S=red, C2S=blue, C3A=green, C4AF=orange, gypsum=olive, CaO=yellow, K2SO4=white

13. Measurement of Chemical Shrinkage • Chemical shrinkage assesses the imbibition of external water into a hydrating cement paste due to the fact that the hydration products occupy less volume than the reactants • Standardized in 2005 as ASTM C1608 by subcommittee C01.31 • Burrows has advocated that the 12-h chemical shrinkage be less than or equal to 0.0105 mL/g cement for a crack resistant cement (Burrows, et al., Three Simple Tests for Selecting Low-Crack Cement, Cement and Concrete Composites,26 (5), 509-519, 2004.) 311 380 m2/kg

14. CCRL Cement Compositions Volume fractions

15. Chemical Shrinkage Results CCRL Cement 141, w/c=0.4, saturated, 20 °C

16. Heat of Hydration Results CCRL Cement 141, w/c=0.4, sealed, 23 °C

17. Heat of Hydration Results

18. Variability – Random Number Seed CCRL cement 152

19. Summary and Prospectus • Virtual testing has shifted the emphasis from a later age physical measurement to a detailed starting material characterization and an accompanying early-age (8 h) chemical shrinkage measurement • Results demonstrate the feasibility of a virtual heat of hydration test method to predict 7 d and 28 d heats of hydration (actually the complete heat of hydration vs. time curve) • Cement characterization can be based on detailed SEM/X-ray image analysis or on more commonly available XRD volume fractions, along with a measured PSD (of course) • Methodology now being considered by ASTM C01.26