# Presentation: 328285

## Dale P. Bentz (dale.bentz@nist.gov)
National Institute of Standards and Technology
Japanese Cement Association
May 30, 2008

- Dale P. Bentz (dale.bentz@nist.gov)
- National Institute of Standards and Technology
- Japanese Cement Association
- May 30, 2008

- Modeling of Concrete Performance
- Based on Cement Chemistry: Efforts
- at NIST

## ???

- Cement
- Chemistry

- Concrete
- Performance

**MICROSTRUCTURE**

- A detailed quantitative understanding of microstructure is
- critical to linking cement chemistry to concrete performance

- CaO   C3S
- SiO2    C3A

- Strength
- Durability

## From Cement Chemistry to Concrete Performance

- Basic principles (according to NIST)
- To quantify the microstructure of a hydrating cement paste, one must first **quantitatively characterize the microstructure of the starting cement powder**
- A **three-dimensional model** of microstructure is necessary to appropriately account for large differences in **percolation properties** between 2-D and 3-D

## SEM/X-ray Characterization of Cement Powders

## Decision tree for image segmentation into phases

## Measurements- Cement Phase Characterization

**X-ray diffraction**

**SEM/X-ray analysis**

- paul.stutzman@nist.gov

- Clinker characterization
- Cement and fly ash (powder)
-  ***Area and surface fractions***
- Microstructure development
- Durability studies
- ASTM test methods

- Phase fractions
-   ***Sulfate forms***
- Rietveld analysis
- ASTM test methods

## Creation of a Starting 3-D Microstructure

- What would we like to preserve in going from a 2-D SEM/X-ray image set to a 3-D model starting microstructure image?
- Phase volume fractions
- Phase surface fractions
- Phase ***correlation*** properties (this gives the first two automatically)
- Particle size distribution (PSD)

## Autocorrelation function

- Measures degree of correlation within a phase (or phases) within a microstructure
- Sr(τ) =    f(t)*f(t+)dt
- Every function (image, microstructure) has a unique autocorrelation function, but the converse is not true.
- Equivalent to overlaying a displaced image on an original and observing the common porosity (or phase(s) of interest) for various displacements
- Can also be used to compare real and model microstructures

****

**-******

## Slide 9

## Measurements- Particle Size Distribution

- Laser diffraction – wet method in isopropanol

- chiara.ferraris@nist.gov

## CEMHYD3D

**Three-Dimensional Starting Microstructure Generation**

**Segmented SEM image**

- Phase fractions
- Surface fractions
- Phase correlations

- Particle Size Distribution

**3-D reconstruction**
**with flocculation, **
**random placement,**
**or dispersion**

## Cementitious Materials

## Slide 13

## Hydration Stoichiometry

- Silicate Reactions
- C3S + 5.3 H --> C1.7SH4 + 1.3 CH
-     1     1.34           1.521	0.61
- C2S + 4.3 H --> C1.7SH4 + 0.3 CH
-    1      1.49           2.077       0.19
- (numbers below reactions indicate volume stoichiometries)
- Note a chemical shrinkage as the sums of the volumes of the hydration products are less than the sums of the volumes of the reactants

## Principles of CEMHYD3D

- Cellular automata approach at voxel level
- Random events for dissolution, diffusion, and reaction (nucleation and surface precipitation)
- Volume conservation including chemical shrinkage
- Lattice-based (one micrometer per voxel)
- Finite element and finite difference schemes for property (e.g., ***D, E***) computation
- Version 3.0 includes four major clinker phases, 3 forms of calcium sulfates, silica fume, fly ash, slag, and limestone
-     (_[ftp://ftp.nist.gov/pub/bfrl/bentz/CEMHYD3D/version30](ftp://ftp.nist.gov/pub/bfrl/bentz/CEMHYD3D/version30)__)_
- Kinetics calibrated via experimental data
- Isothermal calorimetry or chemical shrinkage

## Slide 16

## Cellular Automata Model of Cement Hydration

**initial/dissolution/diffusion/early/late**

## Modeling - Percolation

_**Modeling - Percolation**_

- Connectivity of a phase(s) within a microstructure
- Porosity, solids, etc.
- 3-D burning algorithm for digital images
- Overlap algorithms for hard core/soft shell model

## Percolation Phenomena in Cement-Based Materials

- Setting/strength development-
- Percolation of a “solid” backbone
- Curing/transport-
- De-percolation of capillary porosity in cement paste
- Assessed using low temperature calorimetry (LTC)
- Percolation of C-S-H phase(s)
- Percolation of ITZ regions (aggregates, air voids, and fibers) in concrete
- Degradation-
- Percolation of CH/capillary porosity system (leaching)
- Percolation of micro-crack/macro-crack networks
- ASR (percolation of “shells” around reactive aggregates)

## Measurements – Low Temperature Calorimetry

***w/c*****=0.35, saturated, 20 ****o****C**

**Capillary **
**pores**

**Open gel**
**pores**

**Dense gel**
**pores**

_**Measurements – Low Temperature Calorimetry**_

- ∙ Assesses
- freezable
- water content
- ∙Percolation
- state of
- capillary pores
- ∙50 to 100 mg
- samples

## Percolation Phenomena in Cement-Based Materials

_**Percolation Phenomena in Cement-Based Materials**_

- Experiment

- CEMHYD3D Modeling

**SETTING**

**CAPILLARY POROSITY DEPERCOLATION**

## Real  and Model Microstructures CCRL Cement 115, w/c=0.3

_**Real  and Model Microstructures CCRL Cement 115, **__***w/c***__**=0.3**_

** ****saturated****	****	****	****	****	****sealed**

## Experimental and Model Degrees of Hydration (w/c=0.35)

_**Experimental and Model Degrees of Hydration**_** (*****w/c*****=0.35)**

## Direct Comparison of 3-D Microstructures

- In September 2000, experiments performed at European Synchrotron Radiation Facility (ESRF) in France to acquire 3-D microtomographic images of hydrating cement paste (and Plaster of Paris) with a voxel dimension of 0.95 μm
- Image sets made available through NIST ATP-funded project at _[http://visiblecement.nist.gov/](http://visiblecement.nist.gov/)__ _
- Simulate hydration of equivalent CCRL Cement 133 systems with CEMHYD3D codes to compare 3-D microstructures
- Visual and correlation-based comparisons are favorable
*    **Cem. Conc. Res.*, **36** (2), 259-263, 2006.

## Comparison of 3-D microstructures(w/c=0.47, α=0.62)

**	****Model****	****	****	****	****Real****	**

## What to do with microstructures?

- Compute properties
- Setting
- Diffusion coefficients and conductivities
- Elastic moduli and strength predictions
- Simulate degradation (durability studies)
- Leaching of calcium hydroxide
- Sulfate attack
- Chloride ingress and reaction

## Modeling- Conductivity and Diffusivity

**Virtual Test Methods**
**VIRTUAL RAPID CHLORIDE PERMEABILITY TEST**
- _[http://ciks.cbt.nist.gov/VirtualRCPT.html](http://ciks.cbt.nist.gov/VirtualRCPT.html)__ _

**Cement Paste Level**
- Resistor analogy on CEMHYD3D
- 	microstructures
- Diffusivity a function of *w/cm*, α, silica fume

**Concrete Level**
- Continuum random walkers in HCSS model
- 	 D = <r2/t> * (cement paste fraction)
- Aggregate volume fraction and ITZ also
- important at this scale

## Model Diffusivity vs. Capillary Porosity & Silica Fume Addition

_**Model Diffusivity vs. Capillary Porosity & Silica Fume Addition**_

## Modeling - Permeability

**3-D** Stokes solver code available at:
- _[ftp://ftp.nist.gov/pub/bfrl/bentz/permsolver](ftp://ftp.nist.gov/pub/bfrl/bentz/permsolver)__ _
- Example application – **Pervious Concrete**

- Model vs. Experimental Permeability

- Original 2-D Image

- Image from 3-D Reconstruction

## Modeling- Elastic Properties

_**Modeling- Elastic Properties**_

- edward.garboczi@nist.gov

## Modeling- Chloride Ingress

- Pixel-based models in VCCTL using CEMHYD3D microstructures
- Computer Integrated Knowledge System at
- _[http://ciks.cbt.nist.gov/welcome.html](http://ciks.cbt.nist.gov/welcome.html)_
- One-dimensional finite difference model
- _[http://ciks.cbt.nist.gov/~bentz/clpennew.html](http://ciks.cbt.nist.gov/~bentz/clpennew.html)_

## Modeling- Sulfate Attack

- CONCLIFE software – (sorption-based) service life of pavements and bridge decks subjected to sulfate attack and freeze-thaw
- available at _[http://ciks.cbt.nist.gov/cmml.html](http://ciks.cbt.nist.gov/cmml.html)__ _
- VCCTL module for pixel-based simulations of sulfate attack on CEMHYD3D microstructures

- White- ettringite, magenta- brucite, tan- C-S-H, blue- CH
- 	Hydrated			“Degraded”

## Modeling – VCCTL Consortium

- degree of hydration
- chemical shrinkage
- pore percolation
- pore solution pH
- ion concentrations
- concrete diffusivity
- set point
- adiabatic heat signature
- strength development
- interfacial transition zone
- rheology* (yield stress, *viscosity*)*
*workability*
- elastic moduli
- hydrated microstructures

**VIRTUAL CEMENT**
** ****AND CONCRETE**
** ****TESTING **
**LABORATORY**
**(VCCTL)**
**http://vcctl.cbt.nist.gov**

_**CURING CONDITIONS**_
- adiabatic, isothermal, T-programmed
- sealed, saturated,  saturated/sealed
- variable evaporation rate

_**SUPPLEMENTARY CEMENTITIOUS**_
_**MATERIALS**_
- PSD, composition
- silica fume, fly ash, slag*, kaolin,*limestone

_**AGGREGATES**_
- gradation
- volume fraction
- saturation
- shape

_**MIXTURE PROPERTIES**_
*w*/*cm* ratio
- chemical admixtures
- air content

** **_**CEMENT**_
- PSD
- phase distribution
- chemistry
- alkali content
- shape

_**Modeling – VCCTL Consortium**_

## VCCTL Consortium:  8th Year

## VCCTL Project Vision

***Computer-design concrete just like structural engineers computer-design structures***
**Make the VCCTL an effective tool for cement, aggregate, admixture, and concrete companies to optimize the use of existing and new materials for existing and new requirements**
**Widespread educational tool – every civil engineer will have used eVCCTL in their materials courses, just like they all learn finite element packages**
**New dues-paying members are always welcome**
- _[edward.garboczi@nist.gov](mailto:edward.garboczi@nist.gov)__ _

## VCCTL- Current Focus

- New hydration model (HYDRATICA)
- Rheology modeling
- User interface
- Software currently at version 7.1
- Version 1.1 is available on the Internet
- _[http://vcctl.cbt.nist.gov/](http://vcctl.cbt.nist.gov/)__ _
- Educational version (eVCCTL)
- Should be available sometime in 2008

## Modeling – Cement Hydration

- HYDRATICA (2004-present)
- Underlying 3-D lattice
- Stochastic models for diffusion and
- reaction
- 	Transition state theory
- 	Algorithms -> to standard PDE rate equations
- 	Object-oriented, scalable, and extensible
- 	Applies to general aqueous, mineral systems

- jeffrey.bullard@nist.gov

## HYDRATICA- Microstructure Development

- Seeded Nucleation

- Heterogeneous Nucleation on Surface

**Notes:**
HydratiCA is more fundamental in terms of chemical kinetics, but it also retains the power of a 3-D microstructure representation of a system, which can have profound effects on the evolution of the system.  Even so simple a variation as the energetic barrier for nucleation of solid reaction products can alter the entire evolution sequence.  In the top series, the nucleation barrier is lowest on the surface of the dissolving particles.  In the lower series, the nucleation barrier is lowest on an invisible seed located in the solution.

## Modeling - Rheology

_**Modeling - Rheology**_

- nicos.martys@nist.gov

**Modeling Concrete Rheometers**

**Flow of Real-Shaped Aggregates**

- Three-dimensional model with spheres or real-shaped particles
- Dissipative Particle Dynamics (DPD) Method
- 	Molecular dynamics of particles with Brownian motion + velocity
- 	   dependent dissipation
- Parallel computational codes executing on supercomputers
- Applicable to pastes, mortars, and concretes

## VCCTL Uses

- VCCTL software can be used to
- Streamline testing and save $$
- Two industry projects which realized cost savings
** ****~ $1 M** in one month (Dyckerhoff)
** ****~ $700 K** in several weeks (Cemex)
- Expedite design of new materials and admixtures
- Educate students and professionals (eVCCTL)
- Drive changes in standard test methods

## Specialized NIST Capabilities (X-rays)

- X-ray diffraction and X-ray imaging on SEM already mentioned
- X-ray absorption system for studying water movement in cement-based materials
- 1 of 4 in the world (Denmark, Scotland, Purdue)
- 3-D X-ray microtomography unit
- X-ray microfluorescence

## Measurements- X-Ray Absorption

- Assesses water movement during drying/curing
- Installed in Dec. 2001 (GNI)
- Four units in the world
- Upgraded in Dec. 2004
- Tomography capable
- CZT (cadmium-zinc-telluride) crystal detector upgraded in 2004 to a 256x256 pixel Cd-Te X-ray camera with an active area of 25 mm x 25 mm (100 μm pixel size)
- CCD camera for alignment
- XYZ positioning and rotating turntable for tomographic studies

## Example- X-ray Absorption

*In bilayer composite specimens, water always first moves preferentially from a coarser pore structure to a finer one during either drying or sealed hydration.  Example – variable w/c*

**w/c=0.3**

**w/c=0.45**

## Measurements- X-ray Microtomography

_**Measurements- X-ray Microtomography**_

**Porosity in a fire resistive material**

**Water movement during internal curing**

- Visible Cement Dataset- _[http://visiblecement.nist.gov](http://visiblecement.nist.gov/)__ _

- Assesses (changing) 3-D microstructure of materials

## Measurements- X-ray Microfluorescence

_**Measurements- X-ray Microfluorescence**_

- Assesses 2-D spatial elemental distribution on mm scale

- Chloride ingress into mortar
- Three color overlay, Si = red, Ca = blue, Cl = green

- _[jeff.davis@nist.gov](mailto:jeff.davis@nist.gov)__ _

- Exposure to deicing chemical
- causes sulfate depletion

## My Current Research Focus

- REACT: Reducing Early-Age Cracking Today (with Purdue University- Prof. Weiss)
- Shrinkage-reducing admixtures
- Internal curing (lightweight aggregates)
- Mixture proportioning
- Coarser cements
- Coarse limestone powder replacement for cement
- Doubling Concrete Service Life
- VERDiCT: Viscosity Enhancers Reducing Diffusion in Concrete Technology
- Nano-sized viscosity modifiers to half diffusion rates and thus double service life

## Concrete Solutions for Early-Age Cracking

_**Concrete Solutions for Early-Age Cracking**_

**Limestone (coarse) replacements for cement**
- Reduce **temperature rise** and decrease rate of temperature decrease during semi-adiabatic curing due to reduced cement content
- Reduce **autogenous shrinkage** due to increased interparticle spacing (D50 of 50 to 100 μm)
- Strength reduction:    7 % at 28 d

**Mortars**

**Pastes**

## Doubling Concrete Service Life – A New Nanotechnology Approach

- What’s the basic idea?
- Many degradation processes in concrete are controlled by diffusion rates
- Chloride ingress to induce corrosion of steel
- Sulfate ingress and attack
- Ionic diffusion will occur in the water-filled porosity of the concrete, whatever its porosity level
- Diffusion rates ***may*** be in direct inverse proportion to pore solution viscosity
- Nanoscale viscosity modifiers to reduce diffusion
**VERDiCT ****(****V****iscosity**** E****nhancers**** R****educing**** D****iffusion**** i****n**** C****oncrete**** T****echnology)**

## Doubling Service Life

- NIST research efforts (Sept. 2007-present)
- Concept feasibility study
- Viscosity and electrical conductivity measurements for ionic solutions
- D0 = (kBT)/(6πη0r)		(D/D0) = (σ/σ0)
- Chloride ingress into mortars
- Ongoing study of w/c=0.4 mortars with three “treatment” methods
- Curing solution containing viscosity modifier
- Viscosity modifier directly in the mixture
- Viscosity modifier delivered via fine lightweight aggregates (**FLAIR** – **F**ine **L**ightweight **A**ggregates as **I**nternal **R**eservoirs)

## It works well in bulk solutions for 

**Solution electrical conductivities and viscosities**

**It works well in bulk solutions for **
**nano-sized molecules!**

**Symbol size provides relative indication of molecular weight**

## Quantifying Chloride Ingress

- Silver nitrate spray on split cylinders

- Micro x-ray fluorescence (CSTL)

- Three color overlay, Si = red, Ca = blue, Cl = green

## Quantifying Chloride Ingress

_**Quantifying Chloride Ingress**_

**Specimens cured for 7 d prior to Cl****-**** exposure**

**Polyoxyalkylene alkyl ether**

**1.5x viscosity**
**effect**

## Summary

- NIST is interested and active in both **experimental** and **modeling **research to predict the performance of concrete
- Guest researchers are always welcome
- Industrial participation in ongoing consortia (VCCTL, REACT) is possible
- Concrete technology being driven by major global concerns and changes
- Sustainability (raw materials, emissions, recycling, durability, repair)
- Energy (production, placement, repair, demolition)