Dale P. Bentz ([e-mail])

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An Introduction to Internal Curing

• Dale P. Bentz (dale.bentz_at_nist.gov)
• National Institute of Standards and Technology
• NIST/ACBM Computer Modeling Workshop
• June 29, 2006

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Collaborators

• NIST - Mr. Max Peltz and Dr. Kenneth A. Snyder
• Mr. Thomas A. Holm (ESCSI)
• Mr. John W. Roberts (Northeast Solite)
• Pennsylvania State University
• Profs. Phillip M. Halleck and Abraham S. Grader
• Technical University of Denmark
• Profs. Mette Geiker, Ole M. Jensen, and Pietro
  Lura

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• Question What is internal curing (IC)?
• Answer As being considered by ACI-308, internal
  curing refers to the process by which the
  hydration of cement occurs because of the
  availability of additional internal water that is
  not part of the mixing water.
• For many years, we have been curing concrete from
  the outside in, internal curing is for curing
  from the inside out. Internal water is generally
  supplied via internal reservoirs, such as
  saturated lightweight fine aggregates,
  superabsorbent polymers, or saturated wood
  fibers.

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• Question What are the main benefits that IC can
  provide?
• Answers
• - Reduced autogenous deformation and less
  early-age cracking
• - Maintenance of a higher internal RH, enhanced
  (long term) hydration and strength development

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Early-Age Cracking Contributors

• Thermal Effects
• Hydration heat can raise concrete temperature
  significantly (causing expansion), subsequent
  thermal contraction during cooling can lead to
  early-age (global or local) cracking if
  restrained (globally or locally)
• Autogenous Shrinkage
• In lower w/cm concretes, if sufficient curing
  water can not be supplied externally, the
  chemical shrinkage that accompanies the hydration
  reactions will lead to self-desiccation and
  significant autogenous shrinkage (and possibly
  cracking)

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Some Terminology(from RILEM ICC committee)

• Chemical shrinkage
• An internal volume reduction that is the result
  of the fact that the absolute volume of the
  hydration products is less than that of the
  reactants (cement and water) can be on the order
  of 10 by volume new (2005) ASTM standard test
  method is C1608
• Self-desiccation
• The reduction in the internal relative humidity
  (RH) of a sealed system when empty pores are
  generated.
• Autogenous shrinkage
• The external (macroscopic) dimensional reduction
  of the cementitious system under isothermal,
  sealed curing conditions can be 100 to 1000
  microstrains

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Example of Chemical Shrinkage (CS)

• Hydration of tricalcium silicate
• C3S 5.3 H ? C1.7SH4 1.3 CH
• Molar volumes
• 71.1 95.8 ? 107.8 43
• CS (150.8 166.9) / 166.9 -0.096 mL/mL or
• -0.0704 mL/g cement
• For each lb (g) of tricalcium silicate that
  reacts completely, we need to supply 0.07 lb (g)
  of extra curing water to maintain saturated
  conditions (In 1935, Powers measured a value of
  0.053 for 28 d hydration 75 )

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From Chemical Shrinkage to Autogenous Shrinkage

• CS creates empty pores within
• hydrating paste
• During self-desiccation, internal RH and
  capillary stresses are both regulated by the size
  of the empty pores being created
• These stresses result in a physical autogenous
  deformation (shrinkage strain) of the specimen
• Analogous to drying shrinkage, but drying is
  internal

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• Question Why do we need IC?
• Answer Particularly in HPC, it is not easily
  possible to provide curing water from the top
  surface (for example) at the rate that is
  required to satisfy the ongoing chemical
  shrinkage, due to the extremely low
  permeabilities that are often achieved in the
  concrete as the capillary pores depercolate.
• Capillary pore percolation/depercolation first
  noted by Powers, Copeland and Mann (PCA-1959).

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• Question How does IC work?
• Answer IC distributes the extra curing water
  throughout the entire 3-D concrete microstructure
  so that it is more readily available to maintain
  saturation of the cement paste during hydration,
  avoiding self-desiccation (in the paste) and
  reducing autogenous shrinkage.
• Because the autogenous stresses are inversely
  proportional to the diameter of the pores being
  emptied, for IC to do its job, the individual
  pores in the internal reservoirs should be much
  larger than the typical sizes of the capillary
  pores (micrometers) in hydrating cement paste and
  should also be well connected (percolated).

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Cement paste
Water reservoir
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• Question How can the effectiveness of IC be
  quantified?
• Answer By direct and indirect experimental
  measurements including
• internal relative humidity (RH)
• autogenous deformation
• compressive strength development
• degree of hydration
• restrained shrinkage or ring tests
• 3-D X-ray microtomography

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Internal RH, How to measure it?

• On small volumes of mortar in sealed containers
  using Rotronic Hygroscope DT RH stations (at
• Technical University of Denmark)

Grasley, Lange, and DAmbrosia at UIUC have
recently developed a small insitu temperature/RH
sensor for use in (field) concrete
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Internal RH ResultsMortars with Internal
CuringLWA saturated lightweight aggregatesSAP
superabsorbent polymerFSF control with fine
silica fume
w/cm 0.35
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Autogenous Deformation, How to measure it?

• One way is with custom-built digital dilatometers
  (Prof. O.M. Jensen - DTU)
• Specimens sealed in corrugated polymer tubes
  and stored at
• constant
• temperature

Method under consideration for standardization by
ASTM C09.68
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Autogenous Deformation Results
w/cm 0.35
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Autogenous Deformation Results
IC added via fine LWA to increase total w/c
from 0.30 to 0.38 or 0.40 Note chemical
shrinkage of pozzolanic reaction of silica fume
with CH is 0.22 g water/g silica fume or about
3.2 times that of cement
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Autogenous Deformation Results
IC added via fine LWA to increase total w/c
from 0.30 to 0.38 Note chemical shrinkage of
slag hydraulic reactions is 0.18 g water/g slag
or about 2.6 times that of cement
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Degree of Hydration and Strength
w/cm 0.35 mortars, sealed curing
w/cm 0.3 HPM with silica fume blended cement
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Three-Dimensional X-ray Microtomography

• X-ray microtomography allows direct observation
  of the 3-D microstructure of cement-based
  materials
• Example Visible Cement Data Set
  http//visiblecement.nist.gov
• In October 2005, experiments were conducted at
  Pennsylvania State University to monitor
  three-dimensional water movement during internal
  curing of a high-performance mortar over the
  course of two days

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After mixing
1 d hydration
2 d hydration
All images are 13 mm by 13 mm
Aqua indicates drying Red indicates wetting
Subtraction 1 d after mixing
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Three-Dimensional X-ray Microtomography
2-D image with water evacuated regions (pores)
overlaid on original microstructure (4.6 mm by
4.6 mm)
Three-dimensional subtracted image of 1 d
hydration initial microstructure showing
water-filled pores that have emptied during
internal curing (4.6 mm on a side)
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Three-Dimensional X-ray Microtomography
Empty porosity within LWA from analysis of 3-D
microtomography data sets scales exactly with
measured chemical shrinkage of the cement for
first 36 h of curing
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Questions to Consider When Using IC

• How much water (or LWA) do I need to supply for
  internal curing?
• How far can the water travel from the surfaces of
  the internal reservoirs?
• How are the internal reservoirs distributed
  within the 3-D concrete microstructure?
• Answers
• May be found at the NIST internal curing web
  site http//ciks.cbt.nist.gov/lwagg.html

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http//ciks.cbt.nist.gov/lwagg.html

• Menu for Internal Curing with Lightweight
  Aggregates
• 1)Calculate Lightweight Aggregates Needed for
  Internal Curing
• 2)Estimation of Travel Distance of Internal
  Curing Water
• 3)Simulate Mixture Proportions to View Water
  Availability Distribution
• 4)View Water Availability Distribution Simulation
  Results
• 5)Learn more about FLAIR Fine Lightweight
  Aggregates as Internal Reservoirs for the
  autogenous distribution of chemical admixtures
• 6)View presentation on internal curing made at
  2006 Mid-Atlantic Region Quality Assurance
  Workshop
• Link to Workshop homepage
• 7)Internal Curing Bibliography
• 8)Direct Observation of Water Movement during
  Internal Curing Using X-ray Microtomography

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Question How much water (or LWA) do I need to
supply for internal curing?

• Answer Equation for mixture proportioning
• (Menu selection 1)
• MLWA mass of (dry) LWA needed per unit volume of
  concrete
• Cf cement factor (content) for concrete mixture
• CS (measured via ASTM C 1608-05 or computed)
  chemical shrinkage of cement
• amax maximum expected degree of hydration of
  cement, (w/c)/0.36 or 1
• S degree of saturation of LWA (0 to 1 when
  added to mixture
• øLWA (measured) absorption of lightweight
  aggregate (use desorption measured at 93 RH
  (potassium nitrate saturated salt solution) via
  ASTM C 149804a)

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Question How far can the water travel from the
surfaces of the LWA?

• Answer Equation balancing water needed
  (hydration) vs. water available (flow) (Menu
  selection 2)
• Reasonable estimates ---
• early hydration ---- 20 mm
• middle hydration --- 5 mm
• late hydration --- 1 mm or less
• worst case --- 0.25 mm (250 µm)
• Early and middle hydration estimates in agreement
  with x-ray absorption-based observations on
  mortars during curing

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Question How are the internal reservoirs
distributed within the 3-D concrete
microstructure?
30 mm by 30 mm

• Answer Simulation using NIST Hard Core/Soft
  Shell (HCSS) Computer Model (Menu selections 3
  and 4)
• Returns a table of protected paste
• fraction as a function of distance
• from LWA surface

Yellow Saturated LWA Red Normal weight
sand Blues Pastes within various distances of
an LWA
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• Question What does the future hold?
• Answer
• - Likely that more state DOTs and concrete
• ready-mix producers will begin to evaluate
• internal curing
• - Investigation of possible air void system
• provided by empty internal reservoirs
• - Investigation of possible enhanced workability
  (retention) in systems with internal curing
• - Usage of internal reservoirs to distribute
  chemical admixtures as well as water (SRAs,
  corrosion inhibitors, ASR mitigation admixtures,
  and phase change materials)
• FLAIR Fine Lightweight Aggregates as
  Internal Reservoirs
• (Menu selection 5 on NIST internal curing web
  site
• being investigated by W.R. Grace, Sika, and
  Maxit)

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• Question Who (else) is active in this area of
  research/application?
• Answer
• - NIST
• - NRC/Canada (Dr. Daniel Cusson)
• - Cleveland State University (Prof. Delatte for
  Ohio DOT)
• - RILEM-ICC (Dr. Kosta Kovler Technion,
  Israel)
• - Tennessee Tech Univ. (Prof. Ben Mohr)
• - Technical University of Denmark (Profs. Ole
  Jensen and Pietro Lura superabsorbent polymers)
• - University of Toronto (Prof. Hooton and Hoa
  Lam)
• - TXI (job in Texas, spring 2005 238,000 yd3
  concrete)
• - U.S. Concrete (specified density concrete)

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Thank you !More Questions ????