Dale P. Bentz

dale.bentz@nist.gov

5 cm

4.6 mm on a side

Question: What is internal curing (IC)?

Answer: As defined by ACI in 2010, IC is “supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation.”

For many years, we have cured concrete from the outside in; internal curing is for curing concrete from the inside out. Internal water is generally supplied via internal reservoirs, such as pre-wetted lightweight aggregates (LWA), superabsorbent polymers (SAPs- baby diapers), saturated wood fibers, or pre-wetted crushed (returned) concrete aggregates (CCA).

Question: Why do we need IC?

Answer: In practice, IC is being used mainly to reduce early-age cracking by maintaining a high relative humidity within the hydrating cement paste! This can be particularly important in lower w/cm (≤ 0.4) concretes when capillary pores depercolate within just a few days. If your concrete isn’t cracking at early ages, you may not need internal curing (may help with curling/warping).

Capillary pore percolation/depercolation first noted by Powers, Copeland and Mann (PCA-1959).

Question: How does IC work?

Answer: IC distributes the extra curing water (uniformly) 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 thereby reducing autogenous shrinkage.

Because the generated capillary 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.

Internal curing is not a substitute for external curing. At a minimum, evaporative moisture loss (after set) must be prevented using conventional external measures (misting, fogging, curing membrane or compound).

Cement paste

Water reservoir

Larger “sacrificial” pores within the reservoirs to minimize stress/strain

Question: What are the documented benefits that IC can provide?

Answers:- Reduced autogenous deformation and less early-age cracking

• Early-age deck cracking identified as #1 distress in 2003

FHWA Nationwide High Performance Concrete Survey Results

- Maintenance of a higher internal RH, reduced plastic shrinkage (cracking) and settlement, enhanced (long term) hydration and strength development, reductions in creep, improved interfacial transition zone (ITZ) microstructure, reduced transport coefficients, increased sulfate attack resistance

A Brief (57 year) Timeline• 1957- Paul Klieger writes “Lightweight aggregates absorb considerable water during mixing which apparently can

transfer to the paste during hydration.” in Klieger, P., Early High Strength Concrete for Prestressing, Proceedings World Conference on Prestressed Concrete, San Francisco, CA, July 1957, A5-1 to A5-14.

• 1991- Bob Philleo writes “..a way must be found to get curing water into the interior of high-strength structural members….A partial replacement of fine aggregate with saturated lightweight fines might offer a promising solution.”

• Mid 1990s – Research groups such as Weber and Reinhardt in Germany and Bentur et al. in Israel begin to actively investigate internal curing

• 1999 – NIST enters the arena with the publication of Bentz, D.P., and Snyder, K.A., Protected Paste Volume in Concrete: Extension to Internal Curing Using Saturated Lightweight Fine Aggregate, Cement and Concrete Research, 29 (11), 1863-1867, 1999.

• 2000 – In Denmark, Jensen and Hansen conceive and demonstrate the idea of using superabsorbent polymers (SAPs) as internal curing agents

• 2005 – TXI places 238,000 yd3 of concrete with internal curing (mid-range LWA) in a commercial paving project (railway transit yard) -- Feb. 2007 issue of Concrete International

• 2006 – Continuously reinforced concrete pavement with internal curing placed in Texas

• 2007 – Full-day session on internal curing held at Fall ACI convention in Puerto Rico; bridge deck with internal curing placed in Ohio

• 2010 – Bridge decks with internal curing placed in New York and Indiana

• 2011 – Bentz and Weiss publish NISTIR 7765- Internal Curing: A 2010 State-of-the-Art Review

• 2012 – Special session on internal curing held at TRB Annual Meeting in D.C.; bridge decks with internal curing placed in Utah; three ACI sessions on internal curing held in Toronto; ASTM issues ASTM C1761-12 Standard Specification for Lightweight Aggregate for Internal Curing of Concrete

• 2013 – ACI 308/213 publishes R-13 Report on Internally Cured Concrete Using Prewetted Absorptive Lightweight Aggregate; ACI webinar on Internal Curing delivered in English and Spanish in September

A Brief Dictionary(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; ASTM standard test method C1608-12, first approved in 2005

• Self-desiccation (internal drying) – 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 microstrain; along with thermal strains can be a significant contributor to early-age cracking; ASTM standard test method C1698-09 for pastes and mortars

Example of Chemical Shrinkage (CS)Hydration of tricalcium silicate(major component of portland cement)

C3S + 5.3 H C1.7SH4 + 1.3 CH

Molar volumes71.1 + 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 of tricalcium silicate that reacts completely, we need to supply 0.07 lb of extra curing water to maintain saturated conditions (In 1935, T.C. Powers measured a value of 0.053 for 28 d of hydration – 75 %)

Chemical shrinkage of blended cements with fly ash and/or slag can be significantly higher (2X to 3X) than that of ordinary portland cement by itself

C=CaOS=SiO2

H=H2O

10 % by volume

From Chemical 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; larger empty pores mean lower stresses and higher internal RH

• These stresses result in a physical autogenous deformation (shrinkage strain) of the specimen

• Analogous to drying shrinkage, but drying is internal

• Autogenous shrinkage is a strong function of both w/c and cement fineness; trends towards increasing fineness and lower w/c have both substantially increased autogenous shrinkage in recent years

mporecap V

RT

r

)RHln(2

IC Agent Characterization• Need to assess

– Total water (pre-wetted condition)

– Available curing water (desorption isotherms)

– Particle size distribution (PSD)

In final conditions (expanded SAPs, saturated wood fibers)

• “Primum non nocere” – in addition to supplying internal curing water, a worthy goal for the IC agent is that it “First, do no harm” to the desirable properties of the control concrete– Physical and chemical stability during mixing, etc.

• In 2012, ASTM committee C09 published ASTM C1761/C1761M-12 (now 13b) Standard Specification for Lightweight Aggregate for Internal Curing of Concrete

– Provides instructions on measuring physical properties and absorption and desorption of LWA for internal curing applications

Sample Desorption Isotherms

0.00

0.04

0.08

0.12

0.16

0.20

0.24

70 75 80 85 90 95 100

Relative Humidity (%)

Wat

er M

ass

(g/g

IC

A) LWAS

CCA-1000 psi

CCA-3000 psi

CCA-5000 psi

Saturated salt solutions of K2SO4, KNO3, and KClRef: Greenspan, L., Journal of Research of the NationalBureau of Standards, 81 (1), 89-96, 1977, see also ASTM C1498-04 and ASTM C1761-13b.

Concrete Mixture Proportioning

MLWA =mass of (dry) LWA needed per unit volume of concreteCf =cementitious factor (content) for concrete mixtureCS =(measured via ASTM C 1608-12 or computed) chemical shrinkage of

cementitious binderαmax =maximum expected degree of hydration of cementitious materials, for OPC = min{[(w/c)/0.36],1}S =degree of saturation of LWA (0 to 1] when added to mixtureΦLWA = (measured) sorption of lightweight aggregate [use desorption

measured at 93 % RH (potassium nitrate saturated salt solution) via ASTM C 1498–04a; see also ASTM C1761-12]

Nomograph available at http://concrete.nist.gov/ICnomographEnglishunits.pdf (SI units also)Calculator available at Expanded Shale, Clay, and Slate Institute (ESCSI) web site

Question of how uniformly water is distributed throughout the 3-D concrete microstructure remains ---- we will cover this soon

For lightweight aggregate (LWA)

demandsupply

MIXTURE PROPORTIONING WITH INTERNAL CURING

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

Water demand (lb/yd3)

w /c>=0.36

w /c=0.33

w /c=0.3

w /c=0.27

w /c=0.24

w /c=0.21

w /c=0.18

10

20

30

40

50

60

70

80

400 500 600 700 800 900

Cement content (lb/yd3)

CS=0.05

CS=0.06

CS=0.07

CS=0.08

Starting with the cement content in the graph on the upper right, find the chemical shrinkage of the mixture (a good default value is 0.07). Proceed to the value on the y-axis and starting with this same value in the graph on the upper left, find the line for the mixture’s w/c ratio. (Note that there is a single (thick) line for all w/c ratios greater than or equal to 0.36 as for these w/c ratio values, it is assumed that complete hydration of the cement powder can be achieved.) Proceed to the value on the x-axis and starting with this same value in the graph on the lower left, find the line for the absorption (dry mass of aggregate basis) of the lightweight aggregate. Finally, proceed to the value on the y-axis to obtain the recommended level of lightweight aggregate (dry mass basis) to be added to the concrete mixture. This replacement should then be conducted on a volumetric basis, replacing an equal volume of normal weight aggregates with pre-wetted (SSD) lightweight aggregates.

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60 70 80

LW

A a

dd

itio

n (

lb/y

d3 )

abs= 5 %

abs= 10 %

abs= 15 %

abs= 20 %

abs= 25 %

abs= 30 %

abs= 35 %

abs= 40 %

Performance EvaluationQuestion: How can the effectiveness of IC be

quantified?

Answer: By direct and indirect experimental measurements of performance including –

internal relative humidity (RH)

autogenous deformation (ASTM C1698)

plastic shrinkage cracking and settlement

compressive strength development

drying shrinkage

creep

degree of hydration

restrained shrinkage or ring tests (ASTM C1581)

sorptivity and diffusion coefficients

3-D X-ray microtomography

Scanning Electron Microscopy (SEM) observations

Autogenous Deformation Results

-200

-150

-100

-50

0

50

0 5 10 15 20

Time (days)

Def

orm

atio

n (

mic

rost

rain

)

SAP

LWA20

LWA08

FSFMortars

w/cm = 0.35, 8 % SF

Courtesy of P. Stutzman (NIST)

w/cm=0.30, 8 % silica fume

w/cm=0.30, 20 % slag

IC Control

SEM Observations

Web site for more informationhttp://concrete.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) Internal Curing and Reductions in Settlement and Plastic Shrinkage Cracking

6)Learn more about FLAIR: Fine Lightweight Aggregates as Internal Reservoirs for the autogenous distribution of chemical admixtures

7)View presentation on internal curing made at 2006 Mid-Atlantic Region Quality Assurance Workshop

Link to Workshop homepage

8)Internal Curing Bibliography

9)Direct Observation of Water Movement during Internal Curing Using X-ray Microtomography

Question: How are the internal reservoirs distributed within the 3-D concrete

microstructure?

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 LWARed – Normal weight sandBlues – Pastes within various

distances of an LWA

30 mm by 30 mm

Future Visions• Blending of LWA with crushed returned concrete fine

aggregate (CCA) and other underutilized porous materials to optimize economics and performance

• Utilization of pre-wetted LWA to distribute chemical admixtures as well as IC water throughout the concrete– Particularly for those admixtures that boost long term

performance but may sometimes negatively impact fresh concrete properties (such as workability and air void stability)

– Shrinkage-reducing admixtures and viscosity modifiers

• NIST has published extensively on this (VERDiCT)

– Self-healing agents --- Ongoing research in Europe

– Lithium admixtures --- Purdue and USBR studies ongoing

– Phase change materials – research at NIST

Autogenous Deformation Results (LWA/CCA)

-600

-500

-400

-300

-200

-100

0

100

200

0 7 14 21 28 35 42 49 56

Time (d)

Mic

rost

rain

Control IC-LWAS (1)IC-LWAS(2) CCA-1000CCA-3000 CCA-5000LWA-CCA1000 blend

IC added via fine LWA/CCA to increase total “w/cm” from 0.30 to 0.38 (0.36)

Note – chemical shrinkage of slag hydraulic reactionsis ~0.18 lb water/lb slag or about 2.6 times that of cement

Mortars with slag (20 %) blended cement w/cm=0.3

(60:40)

LWA for admixture distribution

WLW = water in LWAWLT= VERDiCT admixture solution (50:50) in LWATxx = VERDiCT in mix water (10 % solution)

Improvement in chloride penetration resistance via addition of a viscosity modifier (VERDiCT)

w/c=0.4 OPC mortars

Snyder, K.A., Bentz, D.P., and Davis, J.M., “Using Viscosity Modifiers to ReduceEffective Diffusivity in Mortars,” ASCE J Mat Civil Eng, 24(8), 1017-1024, 2012.

Potential Benefit – Resistance to Sulfate AttackASTM C1012 Testing of Mortar BarsASTM C1012 Testing of Mortar Bars

Measured average expansion vs. exposure time in replenished sulfate solution. Internal curing used pre-wetted fine LWA to replace a portion of the mortar sand. IC-VERDiCT used a 50:50 solution of SRA in water to pre-wet the same LWA. In both cases, expansion rates are dramatically decreased. (Bentz et al., Materials and Structures, 2013).

X-ray microtomography imaging

Control IC VERDiCT-IC Control VERDiCT-IC

X-ray microfluorescence imaging S map

Internal Curing - Prospectus• Practices and procedures are in place for utilizing

IC in infrastructure concrete

• IC slowly being adopted by the U.S. construction industry– Pavements and railway transit yard in Texas

– Bridge decks in Ohio, Indiana, New York, Georgia, Virginia, North Carolina, and Utah

– Water tanks in Colorado

• Natural extensions are– to blend LWA with CCA and perhaps other porous “waste” materials

to optimize economics and performance

– to use the LWA to distribute chemical admixtures in addition to/instead of water