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Portland Cement Association

Contents

Treat Island, Maine Army Corps Test Facility

Specifing a Burnished FloorFinish

Mechanized CementTesting

The Influence of ConcreteCasting and CuringTemperatures

Ettringite and ConcreteDurability

Volume 17/Number 3

December 1996

5420 Old Orchard Road

Skokie, Illinois 60077-1083

Phone: (847) 966-6200

Fax: (847) 966-8389

Treat Island, MaineThe Army CorpsOutdoor Durability Test Facilityby Jamie Farny, Concrete Technologist, Portland Cement Association

Fig. 1. Treat Island, Maine. Both the dock and beach area are part of thelong-term test facility.

Treat Island, Maine, is the home of along-term natural weathering facilityused to study the durability of con-crete. The U.S. Army Corps of Engi-

neers maintains the test facility andprovides inspections of samples.Treat Island has provided valuableinformation about concrete durabilityfor over 50 years. This article reviewssome of the highlights of the programand the August 7, 1996 tour.

The Test Facility

Treat Island is located at the easterntip of Maine. The facility can be vis-ited only at selected times becausethe tides vary as much as 6.7 meters

(22 feet) (see Fig.1). Test specimensare subjected to repeatedinnundations of seawater and a rela-tively large number of annual freeze-

thaw cycles (120-140), perfect for du-rability testing in a severe marine en-vironment.

Long-Term Natural Weathering

Most concrete properties are deter-mined through physical testing.Laboratory durability tests com-monly attempt to simulate and accel-erate severe environments to deter-mine how a particular concrete re-sponds. Unfortunately, these testsnecessarily entail conditions that aredifferent than usually occur in na-ture. Also, most tests investigateonly one aspect of durability at atime, such as frost resistance. This

leaves question as to the durability inan environment of multiple attackmechanisms (frost, sulfates, chlo-rides, wetting and drying, etc.). Only

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long-term field studies can truly an-swer how a concrete will perform innatures, often severe, environment.By correlating concretes response inlong-term test sites to its response inlaboratory tests, the concrete industrygains confidence in the acceleratedtests and the variety of materials ituses.

Treat Island, with its combination oftides and relatively numerous freeze-thaw cycles, is well suited to naturalfield testing. Concrete samples of alltypes and sizes are exposed to tidalseawater, freezing and thawing, ero-sion/abrasion, and wetting and dry-ing. During the coldest part of the win-ter, temperatures average around-10C (15F). Depending on specimenplacement and the severity of the win-ter, samples may undergo anywherefrom fewer than 100 to about 160

cycles of freezing and thawing in ayear. It may take ten years or more be-fore trends can be established. Some ofthe specimens are over fifty years old.Natural exposure testing is advanta-geous because conditions are notsimulated, nor are they artificially se-vere to speed up the deteriorationmechanisms.

Although the focus of the test facil-ity is to study long-term durability ofconcrete in a marine environment,the results can be extrapolated toother conditions. For example, if a

concrete performs well in this envi-ronment, it can be expected to per-form well in a nonseawater, freeze-thaw environment where deicers areused.

Areas of Investigation

Today approximately 40 test pro-grams are active at Treat Island. Ar-eas of investigation include normalportland cement concrete, high-per-formance concrete, lightweight con-crete, prestressed concrete, fiber con-

crete, polymer concrete, sulfur/flyash concrete, and concrete with a va-riety of chemical and mineral admix-tures, such as high-range water re-ducers. Some of the variables studiedare aggregate type, supplementarycementitious materials, and blendedcements. Concrete test samples con-tain fly ash, slag, silica fume, andvarious chemical admixtures. In addi-tion, some samples are used to studythe effect of various bonding agents,

Fig. 3. Fibrous concrete prisms.

polymer impregnation,and repair or rehabilita-tion methods. Reinforcedsamples in-clude epoxy-coated rein-forcement.Each research project is in-valuable in providing an-swers to im-prove thequality of con-crete placedin the field.

Concrete specimens atTreat Island are a varietyof shapes and sizes.

Samples may be smallprisms; for example, speci-mens for one of the fibrous concreteprograms measure about 90 x 115 x405 mm (3-1/2 x 4-1/2 x 16 in.). Mostof the samples at Treat Island arelarger, such as prisms about 460 x 460x 915 mm (18 x 18 x 36 in.), or largecubes of concrete, 0.23 m3 (8 cu ft).Larger samples are primarily neces-sary because the physical action oftides and waves can destroy smallerspecimens. The size of these fieldsamples can be compared to concretespecimens used for laboratory freeze-thaw testing, which are small, beingonly about 75 to 125 mm (3 to 5 in.) inwidth, depth, or diameter, and be-tween 280 and 400 mm (11 and 16 in.)in length. Some of the Treat Islandprograms are listed in the table abovealong with the purpose of the investi-gation and specimen sizes.

Programs are administered by theUnited States Army Corps of Engi-neers, Waterways Experiment Sta-

Fig. 2. Durability tests of high-strength concrete.

tion. Sponsors of programs includethe U. S. Bureau of Reclamation, theU. S. Army Corps of Engi

Concrete Technology Today

2

Program name(Figure No.)

Area(s) ofinvestigation

Sample size inmm (approx.)

Sample size inin.

Mobil Research andDevelopment CorporationHigh-strength LightweightConcrete Program

Durability of high-strengthspecimens containingstructural lightweightaggregates and silica fume

305 x 305 x 915 12 x 12 x 36

Cold Regions Researchand EngineeringLaboratory/VTTCooperative Study ofDurability of High-strength

Concrete(Fig. no. 2)

Durability of non-air-entrained high-strengthconcrete, containing slag,silica fume, and rock dust

150 x 150 x 535 6 x 6 x 21

Specimen SizeFrostEffect Investigation

Effect of specimen size ondurability

90 x 115 x 405150 x 150 x 760460 x 460 x 915610 x 610 x 610

3-1/2 x 4-1/2 x 166 x 6 x 30

18 x 18 x 3624 x 24 x 24

CERL Fibrous ConcreteProgram(Fig. no. 3)

Effects of seawater andfreezing and thawing onthe flexural strength andother properties ofconcrete containingstainless steel fibers

90 x 115 x 405 3-1/2 x 4-1/2 x 16

Prestressed ConcreteProgram, Post-tensionedPhase

Test the performance ofend anchorages and end-anchorage protection forvarious types of post-tensioning

255 x 405 x 2440 10 x 16 x 96

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neers, the Canada Department of En-ergy, Mines, and Resources (CanadaCenter for Mineral and Energy Tech-nologyCANMET), the ConstructionProductivity Advancement ResearchProgram (CPAR), and private indus-try. Canadian agencies are currentlyresponsible for about 40% of the speci-mens at the Treat Island test facility.

Some of the agencies that requestprojects also provide materials fortesting at Treat Island. This is usefulwhen it is desired to evaluate the per-formance of specific materials. For in-stance, Kansas City District has pro-vided aggregates from its region foruse in Treat Island test programs.Other organizations interested in con-crete durability are encouraged toprovide materials or test specimens tobe used at the test facility.

Sample ProjectsAn evaluation of samples was per-formed on the beach while our groupvisited Treat Island. The project, en-titled "Corrosion of Intentionally Dam-aged Epoxy-Coated Reinforcing Bar,"compared the performance of blacksteel and epoxy-coated steel speci-mens. In 1990, seventy-two small con-crete slabs with embedded steel werelaid along the beach. The concrete hada water-cement ratio of 0.6, an air con-tent of 5% and a slump of 75 25 mm

(3

1 in.). The cover was small, typi-cally only about 20 mm (0.75 in.). Theconcrete slabs were exposed to twice-daily tide reversals and to severe win-ter conditions on the island. The corro-sion rate was measured on a yearly ba-sis using linear polarization tech-niques. Today, six years into the test,when a slab containing uncoated steelwas broken apart, the exposed barshowed rust and signs of disintegra-tion. Samples containing epoxy-coatedrebar were broken apart and revealedvery little or no damage. A side-by-

side comparison of the two slabsshowed that a concrete containing ep-oxy-coated rebar outperforms thesame concrete containing an uncoatedbar (see Fig. 4). Bars with purposelydamaged epoxy coating exhibited rustspots, showing that pinholes in thecoating accelerate deterioration. Bothqualitative and quantitative resultsprove the importance of both the coat-ing and its uniformity to ensure long-term service of concrete in a marineenvironment.

Fig. 5. Tests provide durability information for concretes containing a variety ofingredients.

Fig. 4. Side-by-side comparison of slabscontaining black (uncoated)reinforcing bars (L) and epoxy-coatedbars (R) after six years of exposure.

Similarly, most test programs in-clude qualitative results, which areusually visual observations, as wellas quantitative results, such as dy-namic modulus of elasticity andpulse velocity (see Reference). Pulsevelocity is a well established non-de-structive technique that allows evalu-

ating the uniformity or relative qual-ity of concrete specimens and con-crete in place.

Conclusion

Long-term field testing, such as thatat Treat Island, provides important

Reference

1996 Review of Programs, Natural Weather-ing Exposure Station, Treat Island, Maine,USA, USAE Waterways Experiment Sta-tion, Vicksburg, Mississippi, August 1996.

data for the concrete industry. Itprovides normal scale, real-timetesting, to determine durablecombinations of materials andeffective placing and curingtechniques, used to design long-lasting concrete structures efficiently.

In general, lower water-cement ra-

tios, higher portland cement content,higher air entrainment, and lowerslumps resulted in improved durabil-ity for concrete specimens exposed tonatural weathering at Treat Island.

For more information about TreatIsland, the test programs, or estab-lishing an investigation program,contact Ed O'Neil at the USAE Water-ways Experiment Station at 3909Halls Ferry Road, Vicksburg, Missis-sippi 39180-6199, or call (601)634-3387, or fax (601) 634-2873.

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of the following passes, the tilt angleand rotation speed of the blade areincreased.

How many troweling passes does ittake? Its hard to say. A ringingsound made by the trowel blades asthey slide over the surface is the bestindicator that the desired conditionhas been reached. But the sound of atrowel blade is pretty subjective. Be-cause all contractors may not havethe same finishing procedures andend point in mind when they readthe term burnished finish, specifi-ers must make their expectations clear.

How Should You Specify theSurface?

When specifying a burnished finish,tell the contractor what you know,but also what you expect to see. The

result may be a combination of aprescription and performancespecification.

The prescription portion may con-tain proportioning requirements suchas fine aggregate grading limits,minimum cement content, and mini-mum and maximum slump. The sec-tion on finishing methods might readsomething like this:

The contractor shall produce a bur-nished floor finish by repeated steel trow-eling with a power trowel until thetrowel blades make a ringing sound and

the floor surface has a glossy appearance.Because repeated steel troweling,ringing sound, and glossy appear-ance are all subjective terms, youstill might not get the look you want.So you need to take two other stepsto help ensure that your expectationswill be met.

First, before the bidding process,use language in the bid documentsthat requires prequalifying the con-crete contractor. Do this by havingbidders either show the architect anexample of a burnished floor their

firm has placed or build a smallmock-up of a floor section to demon-strate their ability to produce the de-sired finish. Getting an acceptable re-sult is much more likely when thecontractors finishers can show thatthey have the experience needed toproduce a burnished finish.

Also require placement of a smallinitial floor area on the project, afterthe contractor has been chosen. Uponapproval by the architect, this refer-ence sample becomes the standard of

acceptance for the remainder of thefloors placed.

A Few Cautions

Even if a floor looks smooth andglossy immediately after finishing, ef-florescence may produce a temporarydulling of the surface sheen. This iscaused by concrete porewater migrat-ing to the surface where it evaporatesand deposits calcium.

Cleaning the floor will remove theefflorescence and restore the sheen.However, this should be donepromptly, before the calcium hydrox-ide reacts with carbon dioxide andforms an insoluble calcium carbon-ate. Acidic cleaners can remove thecalcium carbonate but they may etchthe surface and dull the glossy look.If you want a glossy look to be

present from the beginning, you mayneed to specify use of an acryliccuring compound or a liquid surfacesealer.

Burnished surfaces are darker incolor than conventionally troweledsurfaces. But the color wont be per-fectly uniform. Also, craze cracking,other surface blemishes, and circularpatterns from power troweling willbe more noticeable on a burnishedsurface than on a surface that is notas smooth. While its not realistic toexpect uniform color, the contractor

and concrete supplier can take somesteps to avoid gross discoloration ormottling. These include avoiding cal-cium chloride admixtures, keepingthe water-cement ratio of each con-crete truckload as uniform as pos-sible, and ensuring that there are nochanges in source of cement orsupplementary cementing materialsfor the floor concrete.

Finally, although youll savemoney on floor coverings, the flooritself will cost more than one with anormal troweled surface. Thats be-

cause the larger number of trowelpasses for burnishing requires addedtime for the finishers. Despite this,burnished floors are a concrete alter-native that can often add value andesthetic appeal to commerical or in-stitutional buildings.

Note: Thanks to Bill Henry, John RohrerContracting Co., (913) 236-5005, and PatHarrison, Face Consultants, (913) 362-0675for their help in preparing this article.

Specifying aBurnished FloorFinish

Although budget limitations some-times put specifiers and builders in a

box, they can also create opportuni-ties for innovative construction ap-proaches that save money. Leavingconcrete floors uncovered in commer-cial buildings is one such approach.This saves the cost of covering thefloor, allows earlier occupancy, andavoids the possibility of floor cover-ing failures related to water vaportransmission. Some owners may fur-ther require exposed concrete floorsto have a pleasing finish. Using a bur-nished finish is one way to achieve this.

What Is a Burnished Finish?

A burnished finish is produced by re-peatedly power troweling the con-crete floor until it has a mirror-likeappearance (see photo). Often usedon industrial floors to improve wearresistance, the procedure densifies,strengthens, and darkens the surfacewhile removing pinholes and othersurface irregularities. On the firsttroweling pass, the finisher keeps thetrowel blades almost level. On each

Often used on industrial floors, aburnished finish can be aneconomical, functional, andattractive alterative for commercialor institutional buildings.Photo: Courtesy of John RohrerContracting Co.

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The Influence ofConcrete Castingand CuringTemperatures

by R. G. Burg, Principal Engineer/Group Manager, Materials TechnologyDepartment, CTL, Inc., and Jamie

Farny, Concrete Technologist, PCA

Concrete is usually placed and curedat temperatures above and below"normal" (23C (73F)). Seasons, aswell as geographic region, can affectcasting and curing temperatures. Howdo these temperatures affect concrete?The research discussed here providesdata to help predict the performanceof concrete at temperatures encoun-

tered in construction practice. To iso-late the effects of temperature, no ad-justments were made to the concretemixes to offset the change in work-ability due to temperature effects.

Casting/Curing Temperatures

Three temperatures were chosen forcasting and curing. These are 10C(50F), 23C (73F), and 32C (90F).Materials were temperature-condi-tioned and mixed in temperature-con-trolled laboratories.

Four casting and curing temperatureregimens were chosen. They are cast/cure: 23C /23C, 32C/32C, 10C/10C, and 23C/10C (73F/73F,90F/90F, 50F/50F, and 73F/50F).When demolded at 24 hours, speci-mens were cured under water at thedesired temperature until time of test.Specimens were brought to room tem-perature (23C/73F) before compres-sive strength testing.

Concrete Workability, SettingTimes, and Compressive

StrengthWhen working with concrete, threequestions are: Is the concrete workable? How long does the concrete re-

main workable and how muchtime is available for finishing?

What is the strength developmentof the concrete?

These questions can be answered,respectively, by looking at slump, set-ting times, and strength gain of plain

MechanizedCement Testing

Prior to industrialization, manufac-turing was a slow process. Now,modern factories contain equipmentdesigned to make production faster,

more efficient, and more uniform,thus improving mass production.That same type of thinking is nowchanging the way cements are testedfor quality control. Good candidatesfor mechanized testing proceduresare jobs that involve tedious or re-petitive activities. Cement strengthtesting is such a process.

Reason for Testing

Cement monitoring assures quality ofmaterials. This testing saves time and

money because materials that do notmeet performance standards are notshipped to a jobsite. Testing is there-fore an ongoing and important, buttime-consuming process.

The Texas Department of Transpor-tation (TxDOT) decided to investi-gate the possibility of mechanizedtesting as a means of improving effi-ciency while allowing employees tobe free to perform other, less tedioustasks. The Materials and Tests Divi-sion (MAT) of TxDOT performs on-going quality monitoring on a variety

of materials, including cement. MATtakes samples directly from cementplants for testing. Mechanized testingcould further simplify this process.

Project Development

Several universities were invited tovisit MAT to determine what testswere the most feasible for applicationof mechanization technology. TexasState Technical College (TSTC) waschosen for the project. A team of twoprofessors and four students at TSTC

created a robot and delivered it toMAT in January of 1996. The creatorsset up the robot and trained TxDOTemployees to operate it.

Test Procedure

Strength testing of cement in NorthAmerica is generally performed ac-cording to ASTM C 109, the StandardTest Method for Compressive Strength ofHydraulic Cement Mortars (using 2-in.or 50-mm Cube Specimens). Specified

amounts of cement, sand, and waterare mixed during a 1-1/2 minute pe-

riod, left to stand for 1-1/2 minutes,and then remixed for an additionalminute (ASTM C 305). The resultingmortar is then placed into 50-mm (2-in.) cube molds, consolidated, and al-lowed to set under conditions of con-trolled temperature and humidity.Cured for one day, the cubes are re-moved from their molds and testedfor compressive strength at the pre-scribed age. All the mixing and mold-ing steps are performed by the robot.

AccuracyMAT employees have compiled datafrom the robots tests and have deter-mined that the robot gives uniformand reliable test results. As a result ofthe data accumulated, TxDOT is con-sidering presenting this piece ofequipment to the American Societyfor Testing and Materials (ASTM) asa viable alternative for cementstrength testing. The project aims toimprove cost-effectiveness, safety,energy consumption, precision,

accuracy, and repeatability.For more information, contactGerald Lankes at TxDOT, MAT Divi-sion, 125 E. 11th Street, Austin TX78701, telephone (512) 465-7331, fax(512) 465-7999.

Reference

Etheredge, Tommy, Cement testing ben-efits from robotics, Transportation News,Texas Department of Transportation, Aus-tin, Texas, May 1996.

Mechanized testing simplifies qualitycontrol of cement for the TexasDepartment of Transportation.

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concrete "base" mixeshaving no ad-mixtures or other mix adjustments. Itis often helpful to discuss results interms of a reference temperature,taken here to be 23C (73F).

In actual construction, adjustmentschemical admixtures or additionalwaterare often made to a concretemix to maintain its workability at theanticipated casting temperature. Thereader can use this report to makejudgments as to the most appropriatemeans to account for casting and cur-

ing temperatures.

Workability

Workability is typically measured byslump. Fig. 1 shows the slump of con-cretes containing Cements A or B(Concretes A or B) as a percentage ofthe slump at 23C (73F). Cement Ahad a nearly linear response to tem-perature. The slump of Concrete Bshowed a nonlinear response to tem-peraturedue in part, perhaps, to aslightly higher air contentbut agreed

well at the higher temperature withConcrete A. For Concrete B, decreas-ing the temperature had a more sig-nificant effect on slump than increas-ing the temperature. In general, slumpdecreased about 20 mm for each 10Cincrease in temperature (0.8 in. de-crease for each 20F increase).

Setting Times

Setting times are measured followingASTM C 403, Test Method for Time of

Setting of Concrete Mixtures by Penetra-tion Resistance. Initial set is defined asthe time at which concrete has at-tained 3.5 MPa (500 psi) penetrationresistance; final set is defined as thetime it reaches 27.6 MPa (4000 psi)penetration resistance. A graphshowing time versus penetration re-

sistance for Concrete A at three tem-peratures is shown in Fig. 2 withhorizontal lines representing initialand final set. Lower temperatures in-crease set times, as indicated by theshift of the curves to the right.

Times for initial set at 32C (90F)and 10C (50F) are shown as a per-centage of time for initial set at 23C(73F) in Fig. 3. The impact of tempera-

Fig. 1. Effect of temperature on slumpfor two concretes with different Type Icements. Concretes A and B hadcement contents of 356 and335 kg/m3, respectively (600 and564 lb/yd3). Nominal w/c = 0.45.

Fig. 2. Set time curves for concrete as

influenced by temperature.

Fig. 3. Initial set characteristics as afunction of casting temperature.

appears that initial set times are af-fected in the same relative manner byan increase or decrease in temperature;

the same holds true for final set times.As a first order approximation, settime can be anticipated to change ap-proximately 50% for each 10C change(30% for each 10F change) in tempera-ture from a reference temperature of23C (73F). Lower temperatures in-crease set time; higher temperaturesdecrease set time. Two cautions on thisgeneral rule should be noted. The datacover only the common placementtemperature range of 10C-32C (50F-90F). Also, it should not be assumedthat cement setting time, given on mill

test certificates, is equivalent to con-crete setting time.

Compressive Strength

Compressive strength data for eachtemperature regimen are presented intwo formats for Concrete A. Fig. 4

Fig. 4. Strength gain for concreteswith varying casting and curingtemperatures.

ture on the relative change (percentchange) in setting characteristics wassimilar for both cements. It should benoted, however, that there was signifi-cant difference in the absolute settimes between Concretes A and B(about 3 hours for initial set of Con-crete A at 23C (70F); about 5 hoursfor Concrete B).

The low temperature caused an in-crease of 70% and 77% to the initial

set times for Concretes A and B, re-spectively (Fig. 3). Final set times in-creased by a similar magnitude to195% and 200% of the setting time at23C (73F).

The high temperature caused ini-tial set time to decrease by 19% forConcrete A and by 32% for ConcreteB (Fig. 3). Final set time was de-creased by 18% and 33% for the sametwo concretes.

From the somewhat linear curvesin Fig. 3, and data in the literature, it

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gives strength results for concrete ateach age. Lower temperature castingand curing, compared to the reference

the early age strength are reversedafter seven days when absolutestrength of concrete cast and cured at32C (90F) is lower than concrete castand cured at 23C (73F).

For the concretes cast and cured at23C (73F), 7-day strength will beabout 75% of 28-day strength.

Three-day strengths for the concretecast and cured at 32C (90F) isroughly equal to 7-day strengths forconcrete cast and cured at 23C (73F).

At 32C (90F), 3-day strength is ap-proximately 70% of the 28-day com-pressive strength (at 32C (90F)).

Using these observations, constructionpractices, such as finishing, can be bettercoordinated with the demands of hotand cold weather temperatures.

The complete report (RD113) of tem-perature effects on fresh and hardenedconcrete properties can be obtained fromPCAs Order Processing.

Ettringite andConcrete DurabilityQ: Petrographic reports often noteettringite or other sulfate related ma-terials present in voids or cracks ofdeteriorated cast-in-place concretestructures such as retaining walls andpavements. What is ettringite anddoes it or the sulfate in cement con-tribute to expansion and disintegra-tion of portland cement concrete?

A: Ettringite, calciumsulfoaluminate, is found in all port-land cement concretes and is com-monly referenced in petrographic re-ports. Calcium sulfate sources, suchas gypsum, are added to portland ce-ment to prevent rapid setting and im-prove strength development. Sulfateis also present in supplementarycementitious materials and admix-tures. Gypsum and other sulfate com-pounds react with calcium aluminatein the cement to form ettringitewithin the first few hours after mix-ing with water. Essentially all of thesulfur in the cement is normally con-sumed to form ettringite within 24hours. The formation of ettringite re-sults in a volume increase in thefresh, plastic concrete. Due to theconcretes plastic condition, this ex-pansion is harmless and unnoticed.In fact, the slight transient expansionof fresh cement paste is followed by a

small amount of contraction as thecement hydrates and consumes wa-ter, and as water evaporates into theair. At this stage ettringite is uni-formly and discretely dispersedthroughout the cement paste at a sub-microscopic level (less than a mi-crometer in cross-section).

If concrete is exposed to water forlong periods of time (many years), theettringite can slowly dissolve and re-form in less confined locations. Uponmicroscopic examination, harmlesswhite needle-like crystals of ettringitecan be observed lining air voids.

Any form of attack or disintegra-tion of concrete by freeze-thaw ac-tion, alkali-silica reactivity (ASR), orother means, accelerates the rate atwhich ettringite leaves its original lo-cation in the paste to go into solutionand recrystallize in larger spaces suchas voids or cracks. Both water andspace must be present for the crystalsto form. The space is often providedby cracks that form due to damagecaused by frost action, ASR, dryingshrinkage, or other mechanisms.Ettringite crystals in air voids andcracks are typically two to four mi-crometers in cross section and 20 to30 micrometers long. Under condi-tions of extreme deterioration, thewhite ettringite crystals appear tocompletely fill voids or cracks. How-ever, ettringite, found in its preferredstate as large needle-like crystals,should not be interpreted as causing

the expansion of deteriorating con-crete.

To determine if ettringite contrib-utes to expansion of deteriorating(non-heat treated) concrete, a PCAstudy (Reference 1) investigated ex-pansion caused by alkali-silica reactionand freeze-thaw action. By using ce-ments of different sulfate contents(higher sulfate contents forming moreettringite) it would be possible to de-termine if the solution and recrystalli-zation of the calcium sulfoaluminatecontributes to expansion.

The ASR study used a reactive ag-gregate with cement alkalies rangingfrom 0.53 to 1.05% Na

20 equivalent.

Mortar and concrete prisms weretested beyond three years. Concreteprisms were exposed to field andlaboratory conditions. Specimenswith the 3.5% sulfate cements usuallyhad about the same, and often less,expansion than those with the 1.5%sulfate cements. This indicates thatexpansion in the specimens resultedfrom ASR and that recrystallization ofthe ettringite, occuring in spaces cre-ated by the ASR, did not contribute tothe expansion.

The freeze-thaw study tested con-cretes with cements having sulfatecontents ranging from 1.7% to 4.0%.The specimens were exposed to 160freeze-thaw cycles, followed by 28days of drying in air, followed by oneyear in water. This testing regimentheoretically would disrupt the paste

Fig. 5. Concrete strength gain inrelation to temperature exposures asa percent of concrete strength at

23C (73F).

temperature, gives nearly equal or bet-ter results at all test ages beyond threedays; higher temperature gives lowerresults beyond three days. Fig. 5 showsthe relative change in strength due totemperature differences in casting andcuring. In this figure, data for 23C(73F) are defined to be 100% at eachage, and concrete strengths for othercasting/curing regimens are shown asa percentage of the 23C (73F) com-pressive strength at the same age. Thisformat clearly shows the relative effectof temperature at each age. Completedata for both Concrete A and ConcreteB can be found in the report listed atthe end of this article.

Later age strength of the concretecast and cured at 10C (50F) is es-sentially equal to or greater thanthat of concrete cast and cured at23C (73F).

The effects of high temperature on

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Portland Cement Association5420 Old Orchard RoadSkokie, Illinois 60077-1083

Route to

Intended for decision makers associ-ated with design, management, andconstruction of building projects, Con-crete Technology Today is publishedtriannually by the Construction Infor-mation Services Department of thePortland Cement Association.

PUBLISHER'S NOTE: Our purpose is to show various waysof using concrete technology to youradvantage and avoiding problems.If there are innovations or ideas youwould like discussed in future issues,please let us know. Items from thisnewsletter may be reprinted in otherpublications subject to prior permis-sion from the Association.

Direct all correspondence toSteve Kosmatka, EditorJamie Farny, Assistant EditorConcrete Technology TodayPortland Cement Association5420 Old Orchard RoadSkokie, Illinois 60077-1083Phone: 847/966-6200 Fax: 847/966-8389E-mail: steve_kosmatka@portcement.orgE-mail: jamie_farny@portcement.org

Printed in U.S.A.

This publication is intended SOLELY for use by PROFESSIONAL PERSONNEL who are competent to evaluate thesignificance and limitations of the information provided herein, and who will accept total responsibility for the applicationof this information. The Portland Cement Association DISCLAIMS any and all RESPONSIBILITY and LIABILITY for the

accuracy of and the application of the information contained in this publication to the full extent permitted by law.

and provide a dry and then wet environ-ment ideal for the recrystallization ofettringite. The specimens prepared withthe cements with 4.0% sulfate had lessexpansion and a smaller decrease in

dynamic modulus than those with thelower sulfate content. No abnormal ex-pansion was observed after the waterstorage. This indicates that the expan-sion in the specimens resulted from frostdamage and not from recrystallizationof ettringite. This research did not inves-tigate the hypothesis that partial or totalvoid filling by ettringite could reducethe protection the air void system af-fords against freeze-thaw damage. Ifconcrete is already failing due to frostaction, presumably this mechanismcould accelerate the deterioration. How-ever, this has not been verified, particu-larly for properly air-entrained con-cretes, and research is currently under-way on this issue.

Another term used in petrographicreports is delayed ettringite formation(DEF). This refers to a condition usuallyassociated with heat-treated concrete.As discussed in Reference 2, certain con-cretes of particular chemical makeupwhich have been exposed to tempera-

tures over about 70C (158F) duringcuring can undergo expansion andcracking caused by later ettringite for-mation. This can occur because the hightemperature decomposes any initial

ettringite formed and holds the sulfateand alumina tightly in the calcium sili-cate hydrate (C-S-H) gel of the cementpaste. The normal formation of ettringiteis thus impeded. In the presence of mois-ture, sulfate and alumina desorb fromthe confines of the C-S-H to formettringite in cooled and hardened con-crete. After months or years of desorp-tion, ettringite forms in confined loca-tions within the paste. Since the con-crete is rigid and if there are insufficientvoids to accommodate the ettringite vol-ume increase, expansion and cracks canoccur. In addition, some of the initialettringite formed before heating may beconverted to monosulfoaluminate athigh temperatures and upon cooling,revert back to ettringite. Becauseettringite takes up more space thanmonosulfoaluminate from which itforms, the transformation is an expan-sive reaction.

As a result of the increase in pastevolume, separation of the paste from

the aggregates is usually observed withDEF. It is characterized by the develop-ment of even rims of ettringite aroundthe aggregates, with larger aggregateshaving broader rims and smaller aggre-

gates having narrower rims. It shouldbe noted that concrete can sustain asmall amount of DEF without harm.Only extreme cases of DEF result incracking, and often DEF is associatedwith other deterioration mechanisms.Air voids can help relieve the stress byproviding a location for the delayedettringite to form.

Finally, some petrographers or con-crete technologists use the term sec-ondary ettringite to refer to both DEFand harmless ettringite found liningvoids (often listed under secondary de-posits in petrographic reports).

References

1. Lerch, William, Effect of SO3

Content ofCement on Durability of Concrete, R&D Se-rial No. 0285, Portland Cement Associa-tion, 1945.

2. Day, Robert L., The Effect of SecondaryEttringite Formation on the Durability ofConcrete: A Literature Analysis, RD108,Portland Cement Association, 1992.

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