Moisture Dynamics in Lightweight Aggregate and Concrete

Expanded Sha/eC/ay & S/ateInstitutePublication # 9340February 2004Moisture Dynamics in Lightweight Aggregate and ConcreteT.A. Holm, P.E., FACI .0.8. Ooi, P. Eng. .T. w. Bremner, Ph.D., P.Eng, FACISynonsis: This paper evaluates the moisture dynamics in lightweight aggregates and structurallightweightconcrete. 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 andthe absorption characteristics of structural lightweight aggregate are presented. The effects of theseproperties 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 andprovided with five days of air-drying at 23°C and 50% RH developed a relative durability factor ofover 100% after 300 cycles of freezing and thawing.Ke~ords:absorption, air-drying, freezing and thawing, lightweight aggregate,lightweight concrete, moisture, pore structure, saturationAUTHORS' BIOGRAPHIESThomas 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 publishedmore than 40 papers on concrete, masonry, thenno-physical and geotechnical issues, and is the coauthorof 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 hasa total of 15 years of experience in concrete design and technology, and repair and rehabilitation of concretestructures. He earned his B.Sc.E. and M.Sc.E. at the University of New Brunswick, Canada. He isa 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 ConcreteThis paper was presented at the Professor 7: W Bremner Symposium on High Performance LightweightConcrete at the Sixth Internationa/ Conference on the Durabi/ity ofConcrete,Thessa/oniki, Greece, June 2003Expanded Shale, Clay and Slate Institute (ESCSI)2225 E. MUITay Holladay Road, Suite 102 .Salt Lake City, Utah 84117801-272-7070/ FAX: 801-272-3377www.escsi.org .e-mail to: info@escsi.org

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.Thecoarse 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 eachweighed 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. Theentire crushing and separation procedure took approximately 15 minutes to complete. Precautions weretaken to prevent the specimens from drying during this time.PHYSICAL PROPERTIES OFSTRUCTURAL LIGHTWEIGHT AGGREGATEParticle DensityStructural Lightweight Aggregates (LA) have a low particle density due to their internal cellular pore systern.The cellular structure within the particles is developed by heating certain raw materials to high temperaturesto 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 withinthe first few hours of exposure to moisture. Interior pores, however, fill extremely slowly, with manymonths of submersion necessary for complete saturation. A fraction of the interior pores are essentiallynon interconnected and may remain unfilled after years of immersion.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 notinclude voids between the particles. In general, the volume of the particles is determined from the volumedisplaced while submerged in water. Penetration of water into the aggregate particles during the test islimited by the aggregate's previous degree of saturation.The oven-dry density of an individualparticle depends both on thedensity of the solid vitreous materialand the pore volume within theparticles, and generally increaseswhen particle size decreases. Afterpulverizing in a jar mill over anextended period, the relative densityof the poreless, solid ceramicmaterial was determined to be 2.60by methods similar to those used inmeasuring the relative density ofcement.Fig 2. Schematic of Dry Structural Lightweight AggregateAbsorption CharacteristicsDue to their cellular structure, lightweight aggregates absorb more water than their ordinary aggregatecounterparts. 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 than25 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

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In his 1965 report, "Concrete Strength Measurement -Cores vs. Cy/inders, " presented to the National Sandand Gravel Association and the National Ready Mixed Concrete Association, Bloem (9) states, "Measuredstrength 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 thebeneficial 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 withASTM 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 lightweightconcrete frame apartment houses, the first author of this paper had direct field experience that empiricallyconfirmed the findings of the Bloem and Tobin investigations. Discussions with a second contractor whowas building eight other normalweight multi-story concrete frames visible from our fifteenth floor vantagepoint, focussed on the extensive plastic shrinkage cracking on his project, and the relative absence of theproblem on our building.Both projects were exposed to the same ambient conditions that promote plasticshrinkage: high temperatures, /ow re/ative humidity and high wind ve/ocities. Both projects were furnishedfrom one readymix concrete supplier with essentially similar mixture ingredients (cement, admixtures,natural sand) with only one differing component: His project used a crushed stone, while ours useda 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 integrityof the LAImatrix interface, attributing the improved quality to internal curing, pozzolanic activity at thecontact zone, and reduction in stress concentrations resulting from elastic compatibilityof the concretephases. In another paper Holm (12)(1980) documented the long term increase in strength of high strengthLC incorporating pozzolons.The benefits of "internal curing" go far beyond any improvements in long-term strength gain, which fromsome combinations of materials may be minimal or non-existent. The principal contribution of "internalcuring" results in the reduction of permeability that develops from a significant extension in the time ofcuring. Powers (13) showed that extending the time of curing increased the volume of cementitious productsformed 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 perfomlanceNC 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 highstrength 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 autogenousshrinkage have been published of which Bentz, et al, is typical (16).The benefits of "internalcuring" 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 liberatedas 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 concretesexposed to unfavorable drying conditions.,~

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In this study, lightweight concrete batched with aggregates having 24% moisture content developeddurability 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.ReferencesI. 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.64. 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., Canada5. Bremner, T. W., and Holm, T.A., 1986, "Elastic Compatibility and the Behavior of Concrete," ACI Journal Proceedings.V. 83, No.2, pp.244-506. 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, CanadaHoff, 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 19579. Bloem, D.L., "Concrete Strength Measurement -Cores an