Mixture Proportioning for Internal Curing

by

Dale P. BentzBuilding and Fire Research Laboratory

National Institute of Standards and TechnologyGaithersburg, MD 20899 USA

Pietro LuraDepartment of Civil EngineeringTechnical University of Denmark

DK-2800 Lyngby DENMARK

and

John W. RobertsNortheast Solite Corporation

Richmond, VA 23222

Reprinted from the Concrete International, Vol. 27, No. 2, pp. 35-40, February 2005.

NOTE: This paper is a contribution of the National Institute of Standards andTechnology and is not subject to copyright.

BY DALE P. BENTZ, PIETRO LURA, AND JOHN W. ROBERTS

transfer of water from the external surface to theconcrete interior.6 This is one justification for internalcuring.

Additional water that can be distributed somewhatuniformly throughout the concrete will be more readilyable

to reach unhydrated cement.

REQUIRED INTERNAL CURING WATERHow much lightweight aggregate is needed to supply

water for internal curing of any given concrete mixture?Bentz and Snyder6 have previously published anequation for calculating this:

(1)

c, X CS x amaxMLWA= S.X cl>LWA

where:

MLWA

c,

cs

(Xrnax

s

T he concept of internal concrete curing is steadilyprogressing from the laboratory to field practice.1.2

In terminology currently being considered by ACICommittee 308, Curing Concrete, "internal curing refersto

the process by which the hydration of cement occursbecause of the availability of additional internal waterthat is not part of the mixing water." The additionalinternal water is typically supplied by using relativelysmall amounts of saturated, lightweight, fine aggregates(LWA) or superabsorbent polymer (SAP)3particles in theconcrete.4 Benefits of internal curing include increasedhydration and strength development, reduced autogenousshrinkage and cracking, reduced permeability, andincreased durability.2.4 The impact of internal curingbegins immediately with the initial hydration of thecement, with benefits that are observed at ages as earlyas 2 days.2

Internal curing is especially beneficial in low water-cement ratio (w/c) concretes because of the chemicalshrinkage that accompanies portland cement hydrationand the low permeability of the calcium-silicate hydrates.Because the water that is chemically bound and adsorbedby the cement hydration products has a specific volumeless than that of bulk water, a hydrating cement paste willimbibe water (about 0.07 g water/g cement) from anavailable source.5 While in higher w/c concretes, thiswater can be, and often is, supplied by external (surface)curing, in low w/c concretes, the permeability of theconcrete quickly becomes too low to allow the effective

<l>LWA

= mass of (dry) fine LWA needed per unit

volume of concrete (kgjm3 or Ib/yd3);= cement factor (content) for concrete mixture

(kg/m3 or Ib/yd3);= chemical shrinkage of cement (g of water/g of

cement or lb/lb);= maximum expected degree of hydration

of cement;= degree of saturation of aggregate (0 to 1)

(Note: Eq. (1) is only valid for non-zero valuesof 5, as otherwise the amount of L W A neededdiverges to infinity.); and

= absorption of lightweight aggregate

(kg water/kg dry LWA or lb/lb).

/ FEBRUARY 2005 35

_0.07tIO'"~0.06

cQJ 0.05

SQJ() 0.04-0

'0 0.03

~ IS 0.02QJ

'0

J.. 0.01Q):

00 0.2 0.3

w/c ratio0.4 0.5

Fig.

1: Internal water needed to maintain saturated conditions incement paste (CS = 0.065)

the hydration of the cement, while Eq. (1) proposesadding enough water to keep the pores in the cement pastecompletely saturated. These represent two extreme viewsand the actual optimum in terms of performance may liesomewhere between the two lines shown in Fig. 1.

Equation (1) always estimates internal curing waterneeded to maintain complete saturation within thehydrating cement paste by exactly compensating for thechemical shrinkage of the hydrating cement paste in theconcrete mixture at the maximum expected degree ofhydration. While it can be applied to higher wjc (> 0.45)concretes with degrees of saturation (5) of less than one,it does not address the potential problems in rheologyand bleeding that might result in such an application dueto having an extremely wet mixture. For these higher wjcconcretes, it may be much more efficient and practical tosupply curing water via conventional means such asmisting or the use of wet burlap. Even when internalcuring is used, however, loss of water from the surfacemust be minimized to allow for dense cover concrete tobe obtained. This applies for all types of concrete.

This article presents refinements for estimating theparameters in Eq. (1) that will provide a readily recognizedmeans of choosing the proper amount of LWA andimprove mixture proportioning via this method. The twomajor factors to be considered are: 1) the variation of CSwith portland cement phase composition and curingtemperature, and 2) the relevant value for the absorption(or more appropriately desorption) of the lightweightaggregate. Taken together, expected variations in thesefactors could result in an underestimation of the quantityof internal curing water by over 30% according to Eq. (1).After addressing these concerns, a procedure for mixtureproportioning for internal curing is recommended.

CHEMICAL SHRINKAGEWhile no standard U.S. method exists for evaluating

chemical shrinkage, it can be directly measured usingeither a gravimetrics,s or a volumetric method.s.9.lo Arelatively simple standard test method is currently beingballoted by the ASTM CO1.31 Volume Change subcommittee(Japan already has one).11 Because a value for thelong-term chemical shrinkage is desirable for internalcuring calculations, an alternative to measurement is tocalculate it based on the phase composition of the cement.

Knowing the molar volumes of all relevant cementphases (as provided in Table 1)9.12.13 and the expectedcement hydration reactions,9.14 the chemical shrinkagedue to the hydration of each of the principal cementclinker phases can be calculated.s A volume balance isperformed on each (molar) balanced hydration reaction,and the chemical shrinkage is computed as the differencein volumes between the hydration products and the

For w/c below 0.36, the maximum expected degree ofhydration of the cement under saturated conditions canbe estimated as ([w/c]/0.36) and should not vary significantlywith curing temperature.7 For w/c higher than 0.36, themaximum expected degree of hydration of the cementcan be estimated as 1. Because the densities of the drylightweight aggregates and the conventional aggregatesare substantially different, the ultimate substitution in theconcrete mixture should be performed on a volume basiswith the determined mass of LWA from Eq. (1) replacingthe same volume of conventional aggregates. Knowingthe dry densities of the two types of aggregates, a simplecalculation can be employed to determine the mass ofconventional aggregates that must be removed from themixture (which will be more than the mass of the LWAdetermined by Eq. (1)). As an example of applying Eq. (1),a concrete mixture with a cement factor of 450 kg/m3(760 Ib/yd3), a chemical shrinkage of 0.07 g water/g cement,and an aggregate absorption of 15% at complete saturationwould require 193 and 210 kg/m3 (325 and 350 Ib/yd3) ofLWA for w/c of 0.33 and 0.40, respectively.

Substituting the relationship between w/c and maximumexpected degree of hydration into Eq. (1) yields a linearrelation between internal water demand of the cement(CS x amux in Eq. (1)) and w/c up to a w/c of 0.36 (Fig. 1).For a w/c less than 0.36, the relation is the same as thatproposed by Jensen and Hansen.3 For w/c greater than-0.36, the internal water demand reaches a plateau valueequivalent to the chemical shrinkage of the cement(CS = 0.065 in Fig. 1). For a w/c greater than 0.36, thisrelation differs substantially from that proposed byJensen and Hansen.3 For a w/c between 0.36 and 0.42,they proposed adding sufficient water only to complete

36 FEBRUARY 2005 / Concrel. Inlernallonal

TABLE 2:CALCULATED COEFFICIENTS FOR CHEMICAL SHRINKAGE DUE TO

CEMENT HYDRATION

TABLE 1:DENSITIES AND MOLAR VOLUMES OF CEMENTITIOUS

MATERIALS AT 25 °C (77 oF)

.Assuming sufficient sulfate to convert all of the aluminate phasesto ettringitetAssuming total conversion of the aluminate phases to monosulfate

TABLE 3:CALCULATED CHEMICAL SHRINKAGE FOR CCRL PROFICIENCYCEMENT SAMPLES

Note: 1 Mgjm3 = 1700 Ibjyd3; 1 cm3 = 0.061 in.3Typical cement chemistry notation is used thro!,!ghout thisarticle C = CaO, S = SiOz' A = AIZ03' F = Fez03, S = S03' and H = HzO.

.Assuming total conversion of the aluminate phases to monosulfate1 Assuming sufficient sulfate to convert all of the aluminate phasesto ettringite

of the cement and the chemic'al shrinkage coefficients inTable 2. Table 3 illustrates the results of applying thiscalculation procedure to two recent Cement and ConcreteReference Laboratory (CCRL) proficiency cement samples}7,18There is about a 10% difference in the calculated chemicalshrinkage for these two cements.

A further complication with respect to predictingchenlical shrinkage is the expected curing temperature.GeikerS first observed that the ultimate chemical shrinkageis significantly reduced at elevated curing temperatures.The observed magnitude of this effect was on the order of0.0005 (g waterjg cement) per degree Celsius in thetemperature

range of 12 to 50 °C (54 to 120 °F).5 Forcomparison, data for chemical shrinkage versus degree ofhydration at temperatures between 10 and 50 °C (50 and120 oF) present~d by Mounanga et al.8 yield a coefficientof approximately 0.0008 (g waterjg cement)jOC.

reactants (including water), normalized by the molarmass of the specific cement clinker phase being considered.By assuming a density of 1.0 g/cm3 for water, the finalunits of g water/g cement clinker phase are obtained.Table 2 lists typical chemical shrinkage coefficients forfour cement phases and silica fume. For the aluminatephases, the exact value depends strongly on the sulfatecontent of the cement and the resulting balance betweenthe formation of ettringite (high sulfate content) and themonosulfoaluminate phase Oower sulfate content).

The Table 2 coefficients are strongly sensitive to thevalues chosen for the densities of the different phasesgiven in Table 1, and other authors have thus calculatedvalues differing from those in Table 2.8,15 The values givenhere, however, are those historically and currently usedin the Virtual Cement and Concrete Testing Laboratory(VCCTL) system,7 whose prediction of measured chemicalshrinkage has been verified on a wide variety of portlandcements.9,IO,16

The expected chemical shrinkage of any portlandcement can be calculated based on the mass composition

/ FEBRUARY 2005

37

0.0

TABLE 4:"PROTECTED PASTE" VOLUME AS A FUNCTION OF DISTANCE FROM

THE LWA SURFACES --;::0...'0

~",0..0-rn17J~ 0...Q)

ta 0~

0.25,

Fig. 2: Desorption isotherm versus relative humidity (RH) andexposure time for Hydrocure@ LWA. RH values of 97.93. and85% were obtained using saturated salt solutions of potassiumsulfate.

potassium nitrate, and potassium chloride. respectively.RH of 0% was obtained with 40 o( (100 oF) oven drying followedby placement in a desiccator with a desiccant

Note:

1 in. = 25.4 mm.

Accepting

the values given in Tables 1 through 3 as beingthose for a nominal curing temperature of 25 °C (77 oF)and

taking a conservative approach to the influence oftemperature, calculated values for chemical shrinkage

(such as those in Table 3) should be reduced by 0.005 gwater/g cement for each 10 °C (18 oF) that the averageexpected curing temperature is above 25 °C (77 oF).Conversely, they should be increased by 0.005 g water/g cement for each 10 °C (18 oF) that the average curingtemperature

is below 25 OC (77 oF). For curing temperaturesin the range of 5 to 35 °C (40 to 95 oF), the magnitude ofthis

effect is on the order of 25%. For steam curing andlarger field structures (where temperatures can reach upto

60 °C [140 OF]), the effect would be even more significant.

To characterize the absorption of the aggregates «!>LW)at complete saturation (5 = 1), saturated LWAs can beconveniently exposed to environments with knownequilibrium RH that are maintained via saturated saltsolutions. The water desorbed from the LWA in theseexposures provides the value for <!>LWA (at 5 = 1) thatshould be used in Eq. (1). Exposures to saturated saltsolutions of potassium sulfate and potassium nitrateprovide equilibrium RH of about 97 and 92% at 25 °C (77 of),respectively. 10 Thus, use of the potassium sulfate solution

could be viewed as a conservative approach to internalcuring, while a potassium nitrate solution should providea more liberal estimate.

Figure 2 shows representative desorption isothermsfor an expanded shale lightweight aggregate,. measuredat a temperature of 20 °C (70 of) at the National Instituteof Standards and Technology (NIST). From Fig. 2, it canbe observed that a minimum of a 1- to 2-week exposure tothe saturated salt solution may be necessary to obtain anequilibrium mass for the "saturated" LWA, based onexposing 2 to 3 g (0.07 to 0.11 oz.) samples of saturatedLWA to the salt solutions. This particular LWA loses over95%'of its absorbed water at an RH as high as 93%, abeneficial characteristic for its application in internalcuring. Not all candidate L W A materials have this desirableproperty, with some LWAs losing as little as 50 to 80% oftheir absorbed water at an RH as low as 84%}9 An alternative

How much of the water in lightweight aggregates isreadily available to migrate to the hydrating cementpaste during curing? It is incorrect to assume that all ofthe

water in the LWA will be available.1O.19 A more reasonableapproach is to prewet the aggregates so the moisturecondition is similar to that of aggregates used in trialbatches, and then measure how much water is releasedat a reduced relative humidity. As low wjc cement pastehydrates under sealed conditions, the internal relativehumidity (RH) can drop to the range of 85 to 90%.10 Aswater surrounding an initially saturated LWA migrates toa nearby hydrating cement particle, LWA-absorbed waterreplaces the migrating water, maintaining saturated conditionswithin the hydrating cement paste, and developingunsaturated conditions within the LWA particles.1O Forthe water in the L W A to effectively participate in hydration,it must surely be released from the L W A before theseRH levels of 85 to 95% are reached internally.

* Certain commercial products are identified in this article to specify

the materials used and procedures employed. In no case does such

identification imply endorsement by NIST or ACI, nor does it indicate

that the products are necessarily the best available for the purpose.

FEBRUARY 2005 /38

1.05-1.10

1..151,20

to measuring desorption isotherms for the LWA may beto measure the cumulative absorption over different timeintervals, as the rate of absorption may relate to the easeof desorption.2

Lightweight agg.Normal

weight agg.

Unprotected pastePaste

within 0.02 mmPaste

within 0.05 mmPaste

within 0.1 mmPaste

within 0.2 mm

Paste within 0.5 mmPaste

within 1.0 mm

Fig. 3: Example two-dimensional image (]o x 30 mm) frominternal curing simulation. Note: 1 in. = 25.4mm

DETERMINING AMOUNT OF IWA NEEDED1. Obtain the mass composition of the cement clinker

from either a detailed scanning electron microscope(SEM)jX-ray image analysis7 or the Bogue calculation.

2. Calculate the expected chemical shrinkage (CS) of thecement at 25 °C (77 oF) using the coefficients providedin Table 2 (and considering the availability of sulfatefor the aluminate phase reactions).3. If the expected average curing temperature is above25 °C (77 oF), decrease the calculated value by 0.0005per °C above 25 °C (77 oF).4. If the expected average curing temperature is below25 °C (77 oF), increase the calculated value by 0.0005per °C below 25 °C (77 oF).

5. Measure the desorption of the LWA from a saturatedstate down to an RH of relevance for the internalcuring of concrete. Saturated salt solutions of potassiumsulfate and potassium nitrate, fo~ example, can beused to obtain equilibrium RH values of about 97 and92% (at 25 °C [77 OF]), respectively. If desired, a safetyfactor can be applied to account for the variability in theabsorption of the LWA, if multiple measurements ofdesorption are performed to estimate this factor.6.

Substitute the determined values for C5 and <l>LwA in Eq. (1)to obtain the desired mass of lightweight fine aggregatein the concrete mixture.

(HCSS) microstructural model developed at NIST;o canbe computed and viewed using a website available athttp://ciks.cbt.nist.gov/lwagg.html. At this site, the userprovides the aggregate gradation, lightweight aggregatereplacement fraction, and overall aggregate volumefraction and the system returns a representative color-<:odedtwo-dimensional image from the "virtual" concrete alongwith a table indicating the protected paste volumefraction6 as a function of distance from the surfaces of theLWA particles.

An example of the model output for a concrete with70% aggregates by volume and replacement of 20% of thefine aggregates by LWA is provided in Fig. 3 and Table 4.In this example, while 100% of the cement paste is within1.0 mm (40 mil) of a LWA surface (a relevant distance forearly age curing), only 56% of the cement paste is within0.2 mm (8 mil) of a LWA surface (a more relevant distancefor later age curing).21

This previously stated methodology has been developedfor concrete mixtures based on ordinary portlandcement. The use of blended cements (with silica fume,slag,

fly ash, and the like) will require further modificationsto the computation of chemical shrinkage and "internal

water demand." These blending components influenceboth the kinetics of hydration and the absolute volume ofchemical shrinkage.1O Further research on these materialsis needed to provide a quantitative basis for extendingthe

presented methodology to cover them.

The authors thank Mette Geiker of the Technical University of

Denmark for a careful review of the manuscript.

ADDITIONAL CONCERNSNaturally, other physical properties of the LWA such asmechanical strength, particle shape, and particle gradingwill influence properties of both the fresh and hardenedconcrete. To optimize the performance of the LWA in theconcrete system, these properties should be as similar aspossible to those of the normalweight sand being replacedby the LWA}

Internal curing water must be distributed uniformlythroughout the concrete. This is especially important forcuring at later ages, when the distance the water cantravel may become limited to hundreds of micrometersdue to the ever-decreasing permeability of the hydratingcement paste.6 A more uniform distribution of internalcuring water is best achieved by the use of fine (asopposed to coarse) lightweight aggregates (or SAPparticles, which are on the order of hundreds of micrometersin diameter3). The actual projected distribution of wateravailability for a given concrete mixture with saturatedlightweight aggregates, based on a hard core/soft shell

References1. Mather, B., "Self-Curing Concrete, Why Not? ," Concrete

International, V. 23, No. I, Jan. 2001, pp. 46-47.

2. Roberts, J., "Internal Curing in Pavements, Bridge Decks and

Parking Structures, Using Absorptive Aggregates to Provide Water toHydrate Cement not Hydrated by Mixing Water," 83rd Annual

/ FEBRUARY 2005 39

18. Cement and Concrete Reference Laboratory, "Cement and

Concrete Reference Laboratory Proficiency Sample Program: Final

Report on Portland Cement Proficiency Samples Number 139 and

140," Gaithersburg, MD, Mar. 2001.

19. Lura, P.; Bentz, D. P.; Lange, D. A.; Kovler, K.; and Bentur, A.,

"Pumice Aggregates for Internal Water Curing," PRO 36: Proceedings,

International Symposium on Concrete Science and Engineering, RILEM

Publications S.A.R.L., 2004, pp. 137-151.

20. Bentz, D. P.; Garboczi, E. J.; and Snyder, K. A., "A Hard Corel

Soft Shell Microstructural Model for Studying Percolation and

Transport in Three-Dimensional Composite Media," NIST Internal

Report 6265, U.S. Department of Commerce, 1999.

21. Bentz, D. P.; Koenders, E. A. B.; Monnig, S.; Reinhardt, H.-W.,

Van Breugel, K.; and Ye, G., "Materials Science-Based Models in

Support of Internal Curing," to appear in RILEM State-of-the-Art

Report, Internal Curing of Concrete, 2005.

Received and reviewed under Institute publication policies.

Dale P. Bentz is a Chemical Engineer in the

Materials and Construction Research

Division, National Institute of Standards and

Technology (NISl), Gaithersburg, MD. His

research interests include experimental and

computer modeling studies of the microstructure

and performance of cement-based materials.

ACI member John W. Roberts is Chairman of

Northeast Solite Corp. He is an active

participant in ACI Committees 224. Cracking;

308, Curing Concrete; 325. Concrete Pavements;

and 362, Parking Structures.

Meeting of the Transportation Research Board, Washington, DC,

Jan. 2004.

3. Jensen, O. M., and Hansen, P. F., "Water-Entrained Cement-

Based Materials: I. Principle and Theoretical Background," Cement

and Concrete Research, V. 31, No.4, 2001, pp. 647-654; and "Water-

Entrained Cement-Based Materials: II. Experimental Observations,"

Cement and Concrete Research, V. 32, No.6, 2002, pp. 973-978.

4. Geiker, M.; Bentz, D. P.; and Jensen, O. M., "Mitigating Autogenous

Shrinkage by Internal Curing," High-Perfomlance Structural

Lightweight Concrete, SP-218, J. P. Ries and T. A. Holm, eds.,

American Concrete Institute, Farmington Hills, MI, 2004, pp. 143-154.

5. Geiker, M., "Studies of Portland Cement Hydration: Measurement

of Chemical Shrinkage and a Systematic Evaluation of Hydration

Curves by Means of the Dispersion Model," PhD thesis, Technical

University of Denmark, Lyngby, Denmark, 1983.

6. 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, V. 29,1999,

pp.1863-1867.7. Virtual Cement and Concrete Testing Laboratory, Version 1.0,

available at http://vcctl.cbt.nist.gov. (access verified August 2004).

8. Mounanga, P.; Khelidj, A.; Loukili, A.; and Baroghel-Bouny, V.,

"Predicting Ca(OH)2 Content and Chemical Shrinkage of Hydrating

Cement Pastes Using Analytical Approach," Cement and Concrete

Research, V. 34, No.2, 2004, pp. 255-265.

9. Bentz, D.P., "Three-Dimensional Computer Simulation of

Cement Hydration and Microstructure Development," Journal of the

American Ceramic Society, V. 80, No. I, 1997, pp. 3-21.

10. Lura, P., "Autogenous Deformation and Internal Curing of

Concrete," PhDthesis, Delft University, Delft, The Netherlands, 2003.

11. Tazawa, E., ed., Autogenous Shrinkage of Concrete, E&FN Spon,

London, 1999, pp. 53-55.

12. CRC Handbook of Chemistry and Physics, 68th Edition, CRC

Press, Inc., Boca Raton, FL, 1987, pp. B68-BI46.

13. Young, J. F., and Hansen, W., "Volume Relationships for C-S-H

Formation Based on Hydration Stoichiometries," MRS Symposium

Proceedings, V. 85,1987, pp. 313-322.

14. Bentz, D. P., "CEMHYD3D: A Three-Dimensional Cement

Hydration and Microstructure Development Modelling Package,

Version 2.0," NIST Internal Report 6485, U.S. Department of

Commerce, 2000.

15. Justnes, H.; Sellevold, E. J.; Reyniers, B.; Van Loo, D.; Van

Gemert, A.; Verboven, F.; and Van Gemert, D., "The Influence of

Cement Characteristics on Chemical Shrinkage," Autogenous

Shrinkage of Concrete, E. Tazawa, ed., E&FN Spon, London, 1999,

pp. 71-80.

16. Haecker, C. J.; Bentz, D. P.; Feng, X.; and Stutzman, P. E.,

"Prediction of Cement Physical Properties by Virtual Testing,"

Cement International, V. 1, No.3, 2003, pp. 86-92.

17. Cement and Concrete Reference Laboratory, "Cement and

Concrete Reference Laboratory Proficiency Sample Program: Final

Report on Portland Cement Proficiency Samples Number 135 and

136," Gaithersburg, MD, Mar. 2000.

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