# Presentation: 438697

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

- National Institute of Standards and Technology
- International Congress on the Chemistry of Cement
- July 10, 2007

- Verification, Validation, and
- Variability of Virtual Standards

## 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

## 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

## 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” from _[http://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html](http://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?

## 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**” from _[http://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html](http://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?

## Calibration

- “The process of adjusting numerical or physical modeling parameters in the computational model for the purpose of improving agreement with experimental data” from _[http://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html](http://www.grc.nasa.gov/WWW/wind/valid/tutorial/glossary.html)__ _

## 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

## 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.

## 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](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

## Enthalpy of Hydration of Cement Phases

| Phase | Enthalpy (kJ/kg phase) |
| --- | --- |
| C3S | 517 |
| C2S | 262 |
| C3A | 908, 1672, 1144A |
| C4AF | 418, 725B |
| Anhydrite (to gypsum) | 187 |
| Hemihydrate (to gypsum) | 132 |

- 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.

## 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

## C3S=red, C2S=blue, C3A=green, C4AF=orange,

**C****3****S=red, C****2****S=blue, C****3****A=green, C****4****AF=orange,**
**gypsum=olive, CaO=yellow, K****2****SO****4****=white**

## 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.)

- 380 m2/kg

- 311

## CCRL Cement Compositions

| Phase | CCRL 115 | CCRL 116 | CCRL 135 | CCRL 141 | CCRL 152 |
| --- | --- | --- | --- | --- | --- |
| C3S | 0.596 | 0.627 | 0.634 | 0.632 | 0.690 |
| C2S | 0.218 | 0.207 | 0.162 | 0.106 | 0.088 |
| C3A | 0.031 | 0.067 | 0.066 | 0.115 | 0.123 |
| C4AF | 0.095 | 0.034 | 0.078 | 0.073 | 0.038 |
| Gypsum | 0.060 | 0.065 | 0.060 | 0.026 | 0.027 |
| Hemihydrate | Not meas. | Not meas. | Not meas. | 0.048 | 0.031 |
| Anhydrite | Not meas. | Not meas. | Not meas. | 0.000 | 0.003 |

- Volume fractions

## Chemical Shrinkage Results

- CCRL Cement 141, *w/c*=0.4, saturated, 20 °C

## Heat of Hydration Results

- CCRL Cement 141, *w/c*=0.4, sealed, 23 °C

## Heat of Hydration Results

| CCRL cement | Age (d) (# of labs) | CCRL C186 heat of hyd. (J/g) | CCRL std. dev. (J/g) | CEMHYD3D
 heat of hyd. (J/g) | |Model-Meas.|/
(Meas. dev.) |
| --- | --- | --- | --- | --- | --- |
| 115 | 7 (27) | 310.9 | 27.6 | 305.3 | 0.20 |
| 115 | 28 (16) | 368.6 | 21.8 | 346.3 | 1.02 |
| 116 | 7 (27) | 359.8 | 25.9 | 339.6 | 0.78 |
| 116 | 28 (16) | 402.1 | 17.2 | 383.0 | 1.11 |
| 135 | 7 (22) | 326.4 | 21.8 | 327.4 | 0.05 |
| 135 | 28 (15) | 360.2 | 19.2 | 375.0 | 0.77 |
| 141 | 7 (18) | 351.1 | 30.96 | 344.2 | 0.22 |
| 141 | 28 (11) | 380.7 | 36.4 | 399.6 | 0.52 |
| 152 | 7 (22) | 362.8 | 30.96 | 373.6 | 0.35 |
| 152 | 28 (18) | 415 | 23.85 | 419.0 | 0.17 |
| 152-1 | 7 (22) | 362.8 | 30.96 | 374.2 | 0.37 |
| 152-1 | 28 (18) | 415 | 23.85 | 416.6 | 0.07 |
| 152-2 | 7 (22) | 362.8 | 30.96 | 369.2 | 0.21 |
| 152-2 | 28 (18) | 415 | 23.85 | 416.4 | 0.06 |

## Variability – Random Number Seed

- CCRL cement 152

## 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