Dale P. Bentz (dale.bentz@nist.gov)National Institute of Standards and Technology

International Congress on the Chemistry of CementJuly 10, 2007

0

50

100

150

200

250

300

350

0 40 80 120 160 200 240

Time (h)

Hea

t R

elea

se (

J/g

cem

ent)

Coarse-1 Coarse-2

Fine-1 Fine-2

Type II limit Type IV limit

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

• 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

• 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

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)

• 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,gypsum=olive, CaO=yellow, K2SO4=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.)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 24 48 72 96 120 144 168

Time (h)

Ch

emic

al s

hri

nka

ge

(mL

/g c

emen

t)

Coarse Fine380 m2/kg311

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

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 5 10 15 20 25

Time (h)

Ch

emic

al s

hri

nka

ge

(mL

/g c

em)

Measured

CEMHYD3D

CEMHYD3D (vol only)

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

Heat of Hydration Results

0

100

200

300

400

500

0.01 0.1 1 10 100

Time (d)

Hea

t re

leas

e (J

/g) CCRL average

CEMYHD3D

CEMHYD3D (vol only)

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

Heat of Hydration ResultsCCRL

cementAge (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

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 5 10 15 20 25

Time (h)

Ch

emic

al s

hri

nka

ge

(mL

/g c

em)

Measured average

CEMHYD3D

CEMHYD3D-rep1

CEMHYD3D-rep2

0

100

200

300

400

500

0.01 0.1 1 10 100

Time (d)

Hea

t re

leas

e (J

/g)

CCRL average

CEMHYD3D

CEMHYD3D-rep1

CEMHYD3D-rep2

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