Expanded Sha/eC/ay & S/ateInstitute

Publication # 9340February 2004

Moisture Dynamics in Lightweight Aggregate and Concrete

T.A. Holm, P.E., FACI .0.8. Ooi, P. Eng. .T. w. Bremner, Ph.D., P.Eng, FACI

Synonsis: This paper evaluates the moisture dynamics in lightweight aggregates and structurallight-

weight concrete. The time related changes in the moisture contents of aggregate and matrix fractionsin the lightweight concrete were investigated while exposed to moist-curing and air-drying.

Methodology and terminology essential for the determination of the particle density, the porosity and

the absorption characteristics of structural lightweight aggregate are presented. The effects of these

properties on the batching and freezing and thawing resistance of lightweight concrete are reported.

When tested by the procedures of ASTM C666, concretes exposed to seven days of moist-curing and

provided with five days of air-drying at 23°C and 50% RH developed a relative durability factor of

over 100% after 300 cycles of freezing and thawing.

Ke~ords: absorption, air-drying, freezing and thawing, lightweight aggregate,

lightweight concrete, moisture, pore structure, saturation

AUTHORS' BIOGRAPHIES

Thomas A. Holm, P.E., FACI is the Director of Engineering of the Expanded Shale, Clay and SlateInstitute, Richmond Virginia. He is also the past-chainnan of ACI Committees 213 and 122. He has pub-

lished more than 40 papers on concrete, masonry, thenno-physical and geotechnical issues, and is the co-

author of the Corps of Engineers' State-of-the-Art Report on High-Strength. High-Durability. Low-

Density Concrete for Applications in Severe Environments .

Oon-Soo Ooi, P.Eng. is Materials Engineer with Levelton Engineering, Ltd., Vancouver, Canada. He has

a total of 15 years of experience in concrete design and technology, and repair and rehabilitation of con-crete structures. He earned his B.Sc.E. and M.Sc.E. at the University of New Brunswick, Canada. He is

a member of ACI Committees 121 and 515.

T. W. Bremner, Ph.D., P.Eng, FACI is an Honorary Research Professor of Civil Engineering at theUniversity of New Brunswick, New Brunswick, Canada, and is the Past President of the Atlantic Chapterof ACI. He received the ACI Cedric Willson Award for Research on Lightweight Concrete, and is the pastChair of ACI Committee 122, Energy Conservation, and 213, Lightweight Aggregate and Concrete

This paper was presented at the Professor 7: W Bremner Symposium on High Performance Lightweight

Concrete at the Sixth Internationa/ Conference on the Durabi/ity ofConcrete,Thessa/oniki, Greece, June 2003

Expanded Shale, Clay and Slate Institute (ESCSI)2225 E. MUITay Holladay Road, Suite 102 .Salt Lake City, Utah 84117

801-272-7070/ FAX: 801-272-3377www.escsi.org .e-mail to: info@escsi.org

INTRODUCTION

Physical inspection of mature lightweight concretes which have been exposed to severe environments have

shown that structural lightweight concrete has performed extremely well in marine and freezing and thaw-

ing environments (1-4). This successful performance of lightweight concrete has been attributed to the

elastic compatibility between the lightweight aggregate and the cement paste, the improved "contact zone"

between the aggregate and cement paste, and the extended curing derived from the internal moisture in the

lightweight aggregates (5- 7).

To fully understand the role of absorbed water in the enhancement of hydration, it is essential to use terms

that unambiguously define the amount and location of the water. As with all densities of concrete,absorbed water is useful for extended internal curing and the reduction of autogenous and plastic shrink-age. For precise determination of the W /Cm ratio, it is essential to evaluate the amount of adsorbed water

on the surface of the aggregate. Technical papers should use caution when using the expression "saturat-

ed" lightweight aggregates, to avoid the lack of precision existing in those cases where the lightweight

aggregate used, in fact, has a moderate degree of saturation. Additionally, the aggregate may also have car-

ried an unknown volume of adsorbed surface water that contributed to the "net" mixing water.

This paper presents the results of experimental studies that investigate the moisture dynamics between the

lightweight aggregates and cementitious matrix during moist-curing and air-drying, and the effects of air-

drying on the freezing and thawing durability of lightweight aggregate concrete produced with aggregates

batched at a high degree of saturation.

STRUCTURAL LIGHTWEIGHT AGGREGATES

Structural grade lightweight aggregates of nominal 20 mm maximum size, manufactured by the Solite

Corporation, Richmond, Virginia, were used in this research. Expanded shales, clays and slates (ESCS)aggregates have been used for more than 80 years in the United States in structural lightweight concrete,

structural concrete masonry units.Table 1

and geotechnlcal fill. ESCS aggre-gates are produced by heating Aggregate Absorption a-~~De~~~~~ration~"C

selected raw materials at high tem-

peratures in a rotary kiln.

Aggregates are produced to meet therequirements of ASTM C330, !

ASTM C331, andAASHTO M195. !I

Immersion

Time

Water Absorption

(% Mass)

Degree of

Saturation

% of 24-

Hour Soak

0

5.76

6.15

6.75

7.74

8.32

10.5

12.11

18.4

23.4

30

30

0

.17

.18

.20

.23

.24

.31

.35

.54

.69

.88

.88

0 mins

2 mins

5 mins

15 mins

60 mins

2 hours

1 day

3 days

28 days

4 months

1 year

2 years

0

55

59

64

74

79

100

115

175

223

285

285

TEST PROGRAM

Aggregate AbsorptionStarting from an oven-dried condi-

tion, lightweight coarse aggregates

were immersed in water at atmos-pheric pressure for a period of two

years. The moisture content andparticle density of the aggregates

were measured at various ages ofwater immersion.

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.The coarse aggregates were manually separated from the remaining concrete pieces using a14 oz. hammer;

The matrix and coarse aggregate fractions and the remaining concrete mixture were each

weighed and oven-dried separately to determine their moisture contents.Any free water present on the surface of the concrete cylinders was towel-dried prior to crushing. The

entire crushing and separation procedure took approximately 15 minutes to complete. Precautions were

taken to prevent the specimens from drying during this time.

PHYSICAL PROPERTIES OFSTRUCTURAL LIGHTWEIGHT AGGREGATE

Particle Density

Structural Lightweight Aggregates (LA) have a low particle density due to their internal cellular pore sys-tern. The cellular structure within the particles is developed by heating certain raw materials to high tem-

peratures to the point of incipient fusion, at which time gases are evolved within the pyroplastic mass,

causing expansion that is retained upon cooling. Strong, durable, ceramic lightweight aggregates containa relatively uniformly distributed system of pores that have a size range of approximately 5 to 300 ~

enveloped in a high-strength vitreous phase. Pores close to the surface are readily permeable and fill with-in the first few hours of exposure to moisture. Interior pores, however, fill extremely slowly, with many

months of submersion necessary for complete saturation. A fraction of the interior pores are essentially

non interconnected and may remain unfilled after years of immersion.I

The particle density of an aggregate is the ratio between the mass of the particle material and the volumeoccupied by the individual particles. This volume includes the pores within the particle, but does not

include voids between the particles. In general, the volume of the particles is determined from the volume

displaced while submerged in water. Penetration of water into the aggregate particles during the test is

limited by the aggregate's previous degree of saturation.

~

The oven-dry density of an individ-

ual particle depends both on the

density of the solid vitreous mater-

ial and the pore volume within the

particles, and generally increaseswhen particle size decreases. Afterpulverizing in a jar mill over an

extended period, the relative densi-

ty of the poreless, solid ceramic

material was determined to be 2.60

by methods similar to those used in

measuring the relative density of

cement.

~

Fig 2. Schematic of Dry Structural Lightweight Aggregate

'

1!,

Absorption CharacteristicsDue to their cellular structure, lightweight aggregates absorb more water than their ordinary aggregate

counterparts. Based upon a 24-hour absorption test conducted in accordance with the procedures ofASTM C 127 and ASTM C 128, structural-grade lightweight aggregates will absorb from 5 to more than

25 percent moisture by mass of dry aggregate. By contrast, ordinary aggregates generally absorb less than2 percent of moisture. The important distinction in stockpile moisture content is that with lightweight I

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ASTM DEFINITION OF ABSORPTION AFTER SUBMERSION FOR 24 HRS"SATURATED" SURFACE DRY, "SSD"

-IDEGREE OF

-SATURATION

Fig. 4. "Saturated" Surface Dry as defined by ASTM C 127 and C ]28 -

after 24-hour submersion

ASTM "absorption" of 10.5% by mass. Then, the oven-dry particle density (PDOD ) may be back cal-

culated to be as follows:1.52 I 38PDOD = = .

(I + .105)

It follows then that the fractional volume of ceramic solids,1.38 = .53

Vs= 2:"60"

Fraction Volume of pores, Vp = 1.00- .53 = .47

The degree of saturation (DS: the extent to which the pores are filled)

DS = .105 x 2.60 x .53 (Volume of absorbed water) = .31.47 (Fractional volume of pores)

Following the prescribed ASTM procedures the DS for ESCS LA will generally be in the range ofapproximately 25 to 35% of the theoretical saturation. The use of the ASTM expression "saturated sur-face dry" is therefore, inappropriate for LA, theoretically inaccurate and analytically misleading.

From a practical perspective and considering the fact that most LC is placed by pumping, the usual prac-

tice is to batch the LA at a moisture condition greater than the "Absorption Value" defined by ASTM pro-cedures (24-hour immersion). In this condition the absorbed (internal) moisture content will be in excess

of the arbitrarily defined ASTM "absorption" value. The degree of saturation (DS) necessary for adequate

pumping characteristics, as determined by practical field experience, may be obtained from the ESCS

supplier. Assume for this hypothetical LA that experience has shown that the LC will pump efficiently

when the LA used has an absorption of at least 17% by mass.

.17(2.60 x .53)At that condition the DS = = .50. 1

I.47

Due to the continuous pre-wetting, and because of the very slow further tendency to absorb water into theaggregate, there will invariably be a film of surface (adsorbed) water on the surface of the LA. It is essen-tial to evaluate this quantity of surface water for an accurate determination of the "net" mixing water that

influences workability and determines the effective W /Cm ratio.

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for an LA sample. When moisture absorption-versus-time relationships are extrapolated or theoretical

calculations used to estimate the total filling of all the LA pores, it can be shown that for this particularLA, the absorbed moisture content at total saturation (M@TS) after an infinite immersion will approach

34% by mass with a totally saturated particle density of 1.85.

Complete filling of pores in a structural grade LA is unlikely because the non-interconnected pores are

enveloped by a very dense ceramic matrix. However, these calculations do reveal a conservative upper

limit for the density in submerged design considerations.

THEORETICAL TOTAL SATURATION:(UNDERWATER FOR MORE THAN 10 YEARS)

DEGREE OFSATURATION

100%

Fig. 7. Total Saturation (TS), theoretically all pores filled

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In his 1965 report, "Concrete Strength Measurement -Cores vs. Cy/inders, " presented to the National Sand

and Gravel Association and the National Ready Mixed Concrete Association, Bloem (9) states, "Measured

strength for lightweight concrete cylinders was not reduced by simulated field curing methods employed.

This would tend to support the suggestion that the high absorption of lightweight aggregate may have the

beneficial effect of supplying curing water internally." This was confinned by R. Campbell and Bob Tobin

(10)(1967) in their comprehensive program which compared strengths of cores taken from field curedexposed slabs with test results obtained from laboratory specimens cured strictly in accordance with

ASTM procedures. Their tests confinned that the availability of absorbed moisture in LA produced a moreforgiving concrete that was less sensitive to poor field curing conditions.

.

While providing technical support to a New York City contractor building several twenty-story lightweight

concrete frame apartment houses, the first author of this paper had direct field experience that empirically

confirmed the findings of the Bloem and Tobin investigations. Discussions with a second contractor who

was building eight other normalweight multi-story concrete frames visible from our fifteenth floor vantage

point, focussed on the extensive plastic shrinkage cracking on his project, and the relative absence of the

problem on our building. Both projects were exposed to the same ambient conditions that promote plas-

tic shrinkage: high temperatures, /ow re/ative humidity and high wind ve/ocities. Both projects were fur-nished from one readymix concrete supplier with essentially similar mixture ingredients (cement, admix-

tures, natural sand) with only one differing component: His project used a crushed stone, while ours used

a lightweight aggregate batched with a high degree of saturation.

In a 1980 paper addressing the long term service performance ofLC, Holm (11) cited the improved integri-ty of the LAImatrix interface, attributing the improved quality to internal curing, pozzolanic activity at the

contact zone, and reduction in stress concentrations resulting from elastic compatibility of the concrete

phases. In another paper Holm (12)(1980) documented the long term increase in strength of high strength

LC incorporating pozzolons.

The benefits of "internal curing" go far beyond any improvements in long-term strength gain, which from

some combinations of materials may be minimal or non-existent. The principal contribution of "internal

curing" results in the reduction of permeability that develops from a significant extension in the time of

curing. Powers (13) showed that extending the time of curing increased the volume of cementitious prod-

ucts formed which caused the capillaries to become segmented and discontinuous.

It appears that in 1991, Philleo (14) was the first to recognize the potential benefits to high perfomlance

NC possible with the addition of LA containing high volumes of absorbed moisture. Weber and Reinhardt

( 15)( 1995) have also conclusively demonstrated reduced sensitivity to poor curing conditions in high

strength nomlalweight concrete containing an adequate volume of high moisture content LA. Since 1995a large number of papers addressing the role of water entrainment's influence on internal curing and auto-

genous shrinkage have been published of which Bentz, et al, is typical (16).

,The benefits of "internal curing" are increasingly important when pozzolans (silica fume, fly ash,

metokoalin, calcined shales, clays and slates, as well as the fines of LA) are included in the mixture. It iswell known that the pozzolanic reaction of finely divided alumina-silicates with calcium hydroxide liber-

ated as cement hydrates is contingent upon the availability of moisture. Additionally, "internal curing"provided by absorbed water minimizes the "plastic" (early) shrinkage due to rapid drying of concretes

exposed to unfavorable drying conditions.

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2225 East Murray-HolladayRoad, SuIte 102

In this study, lightweight concrete batched with aggregates having 24% moisture content developed

durability factors in excess of 100% when tested by the procedures of ASTM C666 after seven daysof moist-curing followed by five days of air-drying at 23°C and 50% RH.

References

I. Holm and Bremner, 2000, "State-of-the-Art Report on High-Strength, High-Dmability, Structural Low-Density Concrete forApplications in Severe Marine Environments," U.S. Army Corps of Engineers, Engineering Research andDevelopment Center.

2. Stunn, R.D., McAskill, N., Burg. R.G., and Morgan, D.R., "Evaluation of Lightweight Concrete Perfonnance in 22 to80-Year-Old Ships," High Perfonnance Concrete -Research to Practice, ACI SP189, ACI, Fannington Hills, MI, 1999.

3. Holm, Bremner, and Newman, 1984, "Lightweight Aggregate Concrete Subject to Severe Weathering," ACI ConcreteInternational, V.6 No.6

4. Malhotra, V.M., and Bremner, T. W., ( 1996) "Performance of Concrete at Treat Island, USA: CANMET Investigations,"Proceedings: Third CANMET/ACI International Conference. Concrete in the Marine Environment, SP163, V.MMalhotra, ed., St. Andrews-By- The-Sea, N.B., Canada

5. Bremner, T. W., and Holm, T.A., 1986, "Elastic Compatibility and the Behavior of Concrete," ACI Journal Proceedings.V. 83, No.2, pp.244-50

6. Bremner, T. W., Holm, T.A., and deSouza, J. 1984, "Aggregate-Matrix Interaction in Concrete Subject to Severe Exposure,'FIP-CPCllnternational Symposium on Concrete Sea Structures in Arctic Regions, Calgary, Canada

Hoff, 1992, "High Strength Lightweight Aggregate Concrete for Arctic Applications," T.A. Holm and A.M. Vaysburd, eds.,ACI SP 136, American Concrete Institute, Detroit, Mich., 1-245, Parts 1-3.

8. Klieger, P., "Early High Strength Concrete for Prestressing," Proceedings: World Conference on Prestressed Concrete,San Francisco, CA, July 1957

9. Bloem, D.L., "Concrete Strength Measurement -Cores an<l: Cylinders," ASTM Proceedings. Vo/65., 1965, Philadelphia, PA

10. Campbell, R.H. and Tobin, R.E., "Core and Cylinder Strengths of Natural and Lightweight Concrete," AC1 Journal, April /967

11. Holm, T.A., 1980, "Perfonnance of Structural Lightweight Concrete in a Marine Environment;' V.M. Malhotra, ed., ACI SP-65,American Concrete Institute, Detroit, Mich.

12. Holm, T.A., 1980 "Physical Properties of High-Strength Lightweight Aggregate Concrete," Second Intemationa/ Congress ofLightweight Concrete, London, UK

13. Powers, T.C., Copeland, L.E. , and Mann, H.M., "Capillary Continuity or Discontinuity in Cement Pastes," J. PCA Researchand Development Labs, May, 1959

14. Philleo, R., "Concrete Science and Reality," Skalny, J.P. and Mindess, S., eds., Materials Science ofConcrete II AmericanCeramic Society , Westervil1e, OH, 1991.

15. Weber, S., Reinhardt, "A Blend of Aggregates to Support Curing of Concrete," Proceedings: Internationa/ SymposiumStructura/ Lightweight Aggregate Concrete, Holland, Hamme and Fluge, eds., Sandefejord, Norway, 1995.

16. Bentz, D.P., and Snyder, K.A., "Protected Paste Volume in Concrete -Extension to Internal Curing Using Saturated LightweightFine Aggregate," Cement and Concrete Research 29 (1999)

2