ACI 207.4R - Cooling Insulting Systems for Mass Concrete

22
Cooling and Insulating Systems for Mass ACI 207.4R-93 (Reapproved 1998) Concrete Reported by ACI Committee 207 John M. Scanlon Chairman Terry W. West* Task Group Chairman Fred A. Anderson Howard L Boggs Dan A. Bonikowsky Richard A.J. Bradshaw Edward G.W. Bush Robert W. Cannon James L. Cope Luis H. Diaz Timothy P. Dolen James R. Graham Michael I. Hammons Kenneth D. Hansen Meng K. Lee Gary R. Mass James E. Oliverson Robert F. Oury Ernest K. Schrader* Stephen B. Tatro* l Task group member The need to control volume change induced primarily by temperature change in mass concrete has led to the development of cooling and in- sulating systems for use in mass concrete construction. This report reviews the development of these system the need for temperature control; pre- cooling post-cooling and insulating systems currently being used; and expected trens. A simplified method for computing the temperature of freshly mixed concrete cooled by various systems is also presented. 2.5-Heat generation 2.6-Climate 2.7-Concrete thermal characteristics 2.8-Concrete elastic properties 2.9-Strain capacity 2.10-Thermal shock Keywords: admixtures; cement content; cement types; coarse aggregate; cooling pipes; creep; formwork (construction); heat of hydration; ice; insulation; mass concrete; modulus of elasticity; precooling; post-cooling; pozzolans; restraints; specific heat; strains; stresses; temperature rise (in concrete); tensile strain capacity; tensile strength; thermal conductivity; thermal diffusivity; thermal expansion; thermal gradient; thermal shock; thermal transmittance. Chapter 3-Precooling systems, pg. 207.4R-9 3.1-General 3.2-Heat exchange 3.3-Batch water CONTENTS Chapter l-Introduction, pg. 207.4R-2 l.l-Scope and objective 1.2-Historical background 1.3-Types of structures 1.4-Normal construction practices 1.5-Instrumentation 3.4-Aggregate cooling 3.5-Cementitious materials 3.6-Heat gains during concreting operations 3.7-Refrigeration plant capacity 3.8-Placement area Chapter 2-Need for temperature control, pg. 207.4R-3 2.l-General Chapter 4-Post-cooling systems, pg. 207.4R-14 4.1-General 4.2-Embedded pipe 4.3-Refrigeration and pumping facilities 4.4-Operational flow control 4.5-Surface cooling 2.2-Structural requirements 2.3-Structure dimensions Chapter 5-Surface insulation, pg. 207.4R-16 5.l-General 2.4-Restraint 5.2-Materials 5.3-Horizontal surfaces 5.4-Formed surfaces ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in designing, plan- ning, executing, or inspecting construction and in preparing specifications. References to these documents shall not be made in the Project Documents. If items found in these documents are desired to be a part of the Project Docu- ments, they should be phrased in mandatory language and incorporated into the Project Documents. ACI 207.4R-93 supersedes ACI 207.4R-80 (Revised 1986) and became effective September 1,1993. Copyright 8 1993 American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any elec- tronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. 207.4R-1 Provided by IHS under license with ACI Not for Resale No reproduction or networking permitted without license from IHS --``,`,-`-`,,`,,`,`,,`---

description

Cooling Insulting Systems for Mass Concrete

Transcript of ACI 207.4R - Cooling Insulting Systems for Mass Concrete

ACI 207.4R-93(Reapproved 1998)

ACnispmdmin

Copyright American CProvided by IHS undeNo reproduction or ne

Cooling and Insulating Systems for Mass Concrete

Reported by ACI Committee 207

John M. ScanlonChairman

Terry W. West*Task Group Chairman

Fred A. AndersonHoward L BoggsDan A. BonikowskyRichard A.J. BradshawEdward G.W. BushRobert W. Cannon

James L. CopeLuis H. DiazTimothy P. DolenJames R. GrahamMichael I. HammonsKenneth D. Hansen

Meng K. LeeGary R. MassJames E. OliversonRobert F. OuryErnest K. Schrader*Stephen B. Tatro*

l Task group member

The need to control volume change induced primarily by temperaturechange in mass concrete has led to the development of cooling and in-sulating systems for use in mass concrete construction. This report reviewsthe development of these system the need for temperature control; pre-cooling post-cooling and insulating systems currently being used; andexpected trens. A simplified method for computing the temperature offreshly mixed concrete cooled by various systems is also presented.

Keywords: admixtures; cement content; cement types; coarse aggregate; coolingpipes; creep; formwork (construction); heat of hydration; ice; insulation; massconcrete; modulus of elasticity; precooling; post-cooling; pozzolans; restraints;specific heat; strains; stresses; temperature rise (in concrete); tensile straincapacity; tensile strength; thermal conductivity; thermal diffusivity; thermalexpansion; thermal gradient; thermal shock; thermal transmittance.

CONTENTS

Chapter l-Introduction, pg. 207.4R-2l.l-Scope and objective1.2-Historical background1.3-Types of structures1.4-Normal construction practices1.5-Instrumentation

Chapter 2-Need for temperature control, pg. 207.4R-32.l-General2.2-Structural requirements2.3-Structure dimensions2.4-Restraint

CI Committee Reports, Guides, Standard Practices, andommentaries are intended for guidance in designing, plan-ng, executing, or inspecting construction and in preparingecifications. References to these documents shall not beade in the Project Documents. If items found in theseocuments are desired to be a part of the Project Docu-ents, they should be phrased in mandatory language andcorporated into the Project Documents.

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2.5-Heat generation2.6-Climate2.7-Concrete thermal characteristics2.8-Concrete elastic properties2.9-Strain capacity2.10-Thermal shock

Chapter 3-Precooling systems, pg. 207.4R-93.1-General3.2-Heat exchange3.3-Batch water3.4-Aggregate cooling3.5-Cementitious materials3.6-Heat gains during concreting operations3.7-Refrigeration plant capacity3.8-Placement area

Chapter 4-Post-cooling systems, pg. 207.4R-144.1-General4.2-Embedded pipe4.3-Refrigeration and pumping facilities4.4-Operational flow control4.5-Surface cooling

Chapter 5-Surface insulation, pg. 207.4R-165.l-General5.2-Materials5.3-Horizontal surfaces5.4-Formed surfaces

ACI 207.4R-93 supersedes ACI 207.4R-80 (Revised 1986) and became effectiveSeptember 1,1993.

Copyright 8 1993 American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by

any means, including the making of copies by any photo process, or by any elec-tronic or mechanical device, printed or written or oral, or recording for sound orvisual reproduction or for use in any knowledge or retrieval system or device,unless permission in writing is obtained from the copyright proprietors.

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5.5-Edges and comers5.6-Heat absorption from light energy penetration5.7-Geographical requirements

Chapter 6-Expected trends, pg. 207.4R-206.1-Effects of aggregate quality6.2-Lightweight aggregates6.3-Blended cements6.4-Admixtures6.5-Temperature control practices6.6-Permanent insulation and precast stay-in-place

forms6.7-Roller-compacted concrete

Chapter 7-References, pg. 207.4R-217.1-Recommended references7.2-Cited references

CHAPTER l-INTRODUCTION

1.1-Scope and objectiveThis report presents a discussion of special construc-

tion procedures which can be used to control the temper-ature changes which occur in concrete structures. Theprincipal construction practices covered are precooling ofmaterials, post-cooling of in-place concrete by embeddedpipes, and surface insulation. Other design and construc-tion practices, including the selection of cementingmaterials, aggregates, chemical admixtures, cement con-tent, and strength requirements are not within the scopeof this report.

The objective of this report is to summarize experi-ences with cooling and insulating systems, and to offerguidance on the selection and application of these proce-dures in design and construction for controlling thermalcracking in all types of concrete structures.

1.2 - Historical backgroundThe first major use of artificial cooling (post-cooling)

of mass concrete was in the construction of the Bureauof Reclamation’s Hoover Dam in the early 1930’s. In thiscase the primary objective of the post-cooling was to ac-celerate thermal contraction of the columns of concretecomposing the dam so that the contraction joints couldbe filled with grout to insure monolithic action of thedam. The cooling was achieved by circulating cold waterthrough pipes embedded in the concrete. Circulation ofwater through the pipes was usually started several weeksor more after the concrete had been placed. Since Hoo-ver Dam, post-cooling has been used in construction ofmany large dams. Generally the practices followed wereessentially identical to those followed at Hoover Dam,except that circulation of cooling water was initiatedsimultaneously with the placement of concrete.

In the early 1940’s the Tennessee Valley Authorityutilized post-cooling in the construction of Fontana Damfor two purposes: (a) to control the temperature rise par-

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ticularly in the vulnerable base of the dam where crack-ing of the concrete could be induced by the restrainingeffect of the foundation, and (b) to accelerate thermalcontraction of the columns so that the contraction jointsbetween columns could be filled with grout to ensuremonolithic action. Post-cooling was started coincidentlywith the placing of each new lift of concrete on the pre-viously placed lift and on foundation rock. The pipespacing and lift thickness were varied to limit the max-imum temperature to a pre-designed level in all seasons.In summer with naturally high (unregulated) placing tem-peratures, the pipe spacing and lift thickness for thecritical foundation zone was 2.5 ft (0.76 m); in winterwhen placing temperatures were naturally low the pipespacing and lift thickness for this zone was 5.0 ft (1.5 m).Above the critical zone, the lift thickness was increasedto 5.0 ft (1.5 m) and the pipe spacing was increased to6.25 ft (1.9 m). Cooling was also started in this latterzone coincidently with the placing of concrete in eachnew lift.

In the 1960’s the Corps of Engineers began the prac-tice of starting, stopping, and restarting the coolingprocess based on the results of embedded resistance ther-mometers. At Dworshak Dam and the Ice Harbor Addi-tional Power House Units, the cooling water was stoppedwhen the temperature of the concrete near the pipesbegan to drop rapidly after reaching a peak. Within 1 to3 days, when the temperature would rise again to theprevious peak temperature, cooling would be startedagain to produce controlled safe cooling.

First use of precooling of concrete materials to reducethe maximum temperature of mass concrete was by theCorps of Engineers during the construction of NorforkDam (1941-1945). A part of the batch water was intro-duced into the mixture as crushed ice. The placing tem-perature of the concrete was reduced about 10 F (6 C).Precooling has become very common for mass concreteplacements. It also is used for placements of relativelysmall dimensions such as for bridge piers and founda-tions where there is sufficient concern for minimizingthermal stresses. For precooling applications variouscombinations of crushed ice, cold batch water, liquidnitrogen, and cooled aggregate were used to achieve aplacing temperature of 50 F (10 C) and in some dams toas low as 40 F (4.5 C).

Roller-compacted concrete (RCC) projects have effec-tively used “natural” precooling of aggregate. Large quan-tities of aggregate (sometimes all of the aggregate for adam) are produced during cold winter months and placedinto stockpiles. In the warm summer months the exteriorof the piles warms but the interior stays cold. At MiddleFork, Monkesville, and Stagecoach Dams it was not unu-sual to find frost in the aggregate stockpiles during pro-duction of RCC in the summer at ambient temperaturesabout 75-95 F (24-35 C).

Precooling and post-cooling have been used in com-bination in the construction of some massive structuressuch as Glen Canyon Dam, completed in 1963, Dworshak

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Dam, completed in 1975, and the Lower Granite DamPowerhouse addition, completed in 1978.

Insulation has been used on lift surfaces and concretefaces which are exposed to severe winter temperatures toprevent or minimize the tendency to crack under suddendrops in ambient temperatures. This method of control-ling temperature changes and the consequent crackinghas been used since 1950. It has become an effectivepractice where needed. The first extensive use of in-sulation was during the construction of Table Rock Dam,built during 1955-57. Insulation of exposed surfaces, forthe purpose of avoiding the development of cracking,supplements other construction control measures, such asprecooling materials and post-cooling of in-place con-crete.

Injection of cold nitrogen gas into the mixer has beenused to precool concrete in recent years. Practical andeconomical considerations must be evaluated, but it iseffective. As with ice, additional mixing time may berequired.

1.3-Types of structuresThese special construction practices have evolved to

meet engineering requirements of massive concrete struc-tures such as concrete gravity dams, arch dams, naviga-tion locks, nuclear reactors, powerhouses, large footings,mat foundations, and bridge piers. They are also appli-cable to smaller structures where high levels of internallydeveloped thermal stresses and potential cracks resultingfrom volume changes cannot be tolerated or would behighly objectionable (Tuthill and Adams 1972, and Schra-der 1987).

l.4-Normal construction practicesIn addition to controlling thermal stresses, mixing and

placing concrete at temperatures as low as feasible with-out adversely affecting the desired early strength gain willenhance its long-term durability and strength. It will alsoresult in improved consistency and will allow a longerplacing time. The improved workability can, at times, beused to reduce the water requirement. Cooler concreteis also more responsive to vibration during consolidation.Construction operations can be conducted to achievethese nominal cooling benefits with only modest extraeffort, and concurrently provide a start toward satisfyingspecific cooling objectives. Typical construction practicesused to control temperature changes within concretestructures include:

l Cooling batch waterl Replacing a portion of the batch water with icel Shading aggregates in storage9 Shading aggregate conveyors9 Spraying aggregate stockpiles for evaporative cooling

effectl Immersion of coarse aggregatesl Vacuum evaporation of coarse aggregate moisturel Nitrogen injection into the mix

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l Using light-colored mixing and hauling equipmentl Placing at nightl Prompt application of curing waterl Post-cooling with embedded cooling pipesl Controlled surface coolingl Avoiding thermal shock at form removall Protecting exposed edges and comers from excessive

heat loss

1.5-InstrumentationTemperature monitoring of concrete components dur-

ing handling and batching, and of the fresh concretebefore and after its discharge into the forms, can beadequately accomplished with ordinary portable ther-mometers capable of 1 F (0.5 C) resolution. Post-coolingsystems require embedded temperature-sensing devices(thermocouples or resistance thermometers) to provideinformation for the control of concrete cooling rates.Similar instruments will serve to evaluate the degree ofprotection afforded by insulation. Other instruments tomeasure internal volume change, stress, strain, and jointmovement have been described (Carlson 1970).

CHAPTER 2-NEED FORTEMPERATURE CONTROL

2.1-GeneralIf cement and pozzolans did not generate heat as the

concrete hardens, there would be little need for temper-ature control.

In the majority of instances this heat generation andaccompanying temperature rise will occur rapidly enoughto result in the hardening of the concrete in an expandedcondition. Further, concurrent with the increase in elasticmodulus (rigidity) is a continuing rise in temperature forseveral days or more. Even these circumstances would beof little concern if the entire mass of the placement couldbe:

a) limited in maximum temperature to a value close toits final cooled stable temperature;

b) maintained at the same temperature throughout itsvolume, including exposed surfaces; and,

c) supported without restraint (or supported on foun-dations expanding and contracting in the same manner asthe concrete).

Obviously none of these three conditions can beachieved completely; nor simultaneously. The first andsecond can be realized to some extent in most construc-tion. The third condition is the most difficult to obtain,but has been accomplished on a limited scale for ex-tremely critical structures by preheating the previously-placed concrete to limit the differential between olderconcrete and the maximum temperature expected in thecovering concrete. Many details of crack developmentand control are also discussed in ACI 207.1R, 207.2R

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and 224R, by Townsend (1965), Mead (1963), Tuthill andAdams (1972), Tatro and Schrader (1985), and Ditcheyand Schrader (1988).

2.2 - Structural requirementsThe size, type, and function of the structure, the

climatological environment, and the degree of internal orexternal restraint imposed on it dictate the extent of thetemperature control necessary. Gravity structures whichdepend upon structural integrity for safety and stabilitycan usually tolerate no cracks in certain plane orienta-tions. The number of joints should be a minimum, consis-tent with designers’ requirements and construction prac-ticality. The designer should establish a design strengththat is consistent with requirements for structuralperformance, construction loads, form removal, and dura-bility. Consideration should be given to specifyingstrength requirements at an age greater than 28 days.Concrete with an early (28-day) strength higher than isnecessary to resist later age loading will require excessiveamounts of cements, thus introducing additional heatinto the concrete and aggravating the temperature con-trol problem. Where cracks, including those resultingfrom thermal stress, permit the entry of water, subse-quent corrosion of reinforcement, leaching, and/orfreezing and thawing may result in spalling or otherdisruptive action.

The construction schedule, relating to rate of place-ment and the season of the year, should be considered bythe designer. The highest peak concrete temperature willoccur in concrete placed during the hot summer months;concrete placed in the late summer or early autumn willalso attain a high peak temperature and will likely beexposed to abrupt air temperature drops. Winter-placedconcrete will be exposed to severe low temperature con-ditions. These circumstances contribute to the need fortemperature control consideration.

Late spring is the most suitable time for placing massconcrete because the ambient air temperature tends toincrease daily, thus coinciding with the temperature riseof the concrete. The concrete thus neither absorbs much

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heat from the air, nor is it subjected to rapid changes intemperature at the surfaces.

2.3-Structure dimensionsWhere the least dimension of a concrete unit is not

large, the concrete mixture is low in heat evolution, andthe heat of hydration can escape readily from the twoboundary surfaces (forms not insulated), the maximumtemperature rise will not be great. However, in all in-stances some internal temperature rise is necessary inorder to create a thermal gradient for conducting theheat to the surface. Table 2.1 shows typical maximumtemperatures achieved. Two factors tend to lessen thedetrimental effects of heat generation: (a) the concretebegins to cool from its peak temperature while the mod-ulus of elasticity is still low, or the creep rate is high, orboth; and, (b) the total tensile force (opposed and bal-anced by an equal compressive force) is distributed overa significant proportion of the section, thus tending toavoid a high unit tensile stress.

Table 2.1-Temperature rise in walls

Wall thickness,ft (m) (ot3) (076) (oT9) (lY2) (1:) (~.O)

Infinite(Infinite)

Maximumtemperature rise

deg F 1.3

(deg C) (1.2)

Moderate heat (Type II) cement

3.2 5.2 7.0 8.6 13.7 17.8

(3-O) (4.9) (6.6) (8.1) (12.8) (16.7)

Placing temperature equal to exposure temperatureTwo sides exposedThermal diffusivity: 1.0 ft2/day (0.093 m2/day)Temperature rise: deg F per 100 lbs cement per cu yd concrete

deg C per 100 kg cement per cu m concrete

A foundation slab may be considered a wall of largedimensions cast on its side, such that heat is lost prin-cipally from a single exposed surface. For this case Table2.2 shows the typical maximum temperatures expected,which are not substantially higher than those for a ver-tically-cast wall. However, the maxima do occur at laterages and over large portions of the concrete mass. Sincea static tension-compression force balance must exist, thecompressive unit stress across the center portion is smalland essentially uniform, whereas very high tensile stressexists at the exposed sides.

Proof that massive concrete structures can be pro-duced, with modest precautions and aided by favorableclimate conditions, free of cracks is illustrated by adocumented construction example in Great Britain (Fitz-gibbon 1973). A heavily reinforced footing, 5200 ft2 (480m2) in area and 8.2 ft (2.5 m) in depth, and with a ce-ment content of 705 lb/yd3 (418 kg/m3), was placed as asingle unit. A maximum concrete temperature of 150 F(65 C) was attained, with side surfaces protected by 3/4 in.(19 mm) plywood forms and top surface by a plastic

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Table 2.2-Temperature rise in slabs on ground

Slab thickness, ft (m) (oY9) (A) (& (t ) (Ei)Infinite

(Infinite)

Maximum temperaturerise

deg F 6.0 9.3 14.0 16.0 16.8 17.3 17.8

(deg C) (5.6) (8.7) (13.1) (15.0) (16.7) (16.2) (16.7)

Moderate heat (Type II) cementPlacing temperature equal to exposure temperatureExposed top onlyThermal diffusivity: 1.0 ft2/day (0.093 m 2/day)Temperature rise: deg F per 100 lbs cement per cu yd concrete

deg C per kg cement per cu m concrete

sheet under a 1 in. (25 mm) layer of sand. Plywood andsand were removed at 7-day age, exposing surfaces to theambient January air temperature and humidity condi-tions.

2.4-RestraintNo tensile strain or stress would develop if the length

or volume changes associated with decreasing tempera-ture within a concrete mass or element could take placefreely. When these potential contractions, either betweena massive concrete structure and its rock foundation,

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between contiguous structural elements, or internallywithin a concrete member are prevented (restrained)from occurring wholly or in part, tensile strain and stresswill result. Concrete placed on an unjointed rigid rockfoundation will be essentially restrained at the concrete-rock interface, but the degree of restraint will decreaseconsiderably at locations above the rock, as shown in Fig.2.1. Yielding foundations will cause less than 100 percentrestraint. Total restraint at the rock plane is mitigatedbecause the concrete temperature rise (and subsequentdecline) in the vicinity of the rock foundation is reducedas a result of the flow of heat into the foundation itself.Discussions of restraint and analytical procedures to eval-uate its magnitude and effect appear in ACI 207.1R,207.2R and 224R, Wilson (1968), and Gamer and Ham-mons (1991).

=iCONTINUOUS BASE RESTRAINT

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1.0 0.9 0.6 0 7 0 6 0.5 0.4 0 .3 0 2 0 1

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Fig. 2.1-Degree of tensile restraint at center section

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2.5-Heat generationDesign strength requirements, durability, and the char-

acteristics of the available aggregates largely dictate thecement content of the mixture to be used for a particularjob. Options open to the engineer seeking to limit heatgeneration include: (a) use of Type II, moderate heatportland cement, with specific maximum heat of hydra-tion limit options if necessary; (b) use of blended hy-draulic cements (Type IS, Type IP, or Type P) which ex-hibit favorable heat of hydration characteristics whichmay be more firmly achieved by imposing heat of hydra-tion limit options for the portland cement clinker; and,(c) reduction of the cement content by using a pozzolanicmaterial, either fly ash or a natural pozzolan, to providea reduction in maximum temperatures produced withoutsacrificing the long-term strength development. In someinstances advantage can be taken of the cement reduc-tion benefit of a water-reducing admixture. RCC usuallyallows cement reduction by maintaining a low water/cement ratio while lowering the water content to a pointwhere the mixture has no slump. RCC also may use non-pozzolanic fines to permit cement reductions. From theseoptions, selections can be made which will serve to mini-mize the total heat generated. However, such lower heat-producing options may be offset by their slower strength

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gain which may require an extended design age. In somecases construction needs, such as obtaining sufficientearly strength to allow for form stripping, setting offorms, and lift-joint preparation, may not permit a re-duction in cement (and the corresponding early heat gen-eration) to the extent that could otherwise be achievable.Fig. 2.2, which shows typical adiabatic temperaturemaxima expected in mass concrete, is adapted from ACI207.1R.

TIME IN DAYS

Fineness

Cement type

IIIIIIIV

ASTM C 115 Heat of hydrationcm2/gm Calories per gm

1790 871890 762030 1061910 60

Fig. 2.2-Temperature rise of mass concrete containing 376lb/yd3 of various types of cement

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At early ages (up to 3 days) the temperature rise ofthe mixture containing the pozzolan replacement resultsprincipally from hydration of the cement, with little if anyheat contributed by the pozzolan. At later ages (after 7days) the pozzolan does participate in the hydrationprocess, and may contribute about 50 percent of theamount of heat which would have been generated by thecement it replaced. ASTM C 618 Class C fly ash general-ly produces more heat than Classes F or N pozzolans.

2.6-ClimateAs a general rule, when no special precautions are

taken, the temperature of the concrete when placed inthe forms will be slightly above the ambient air tem-perature. The final stable temperature in the interior of

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a massive concrete structure will approximate the averageannual air temperature at its geographical location.

Except for tropical climates, deep reservoir impound-ments will maintain the concrete in the vicinity of theheel of the dam at the temperature of water at its maxi-mum density, or about 39 F (4 C). Thus, the extremetemperature excursion experienced by interior concreteis determined from the initial placing temperature plusthe adiabatic temperature rise minus the heat lost to theair and minus the final stable temperature. Mathematicalprocedures are available to determine the net tempera-tures attained in massive placements. Lifts of 5 ft maylose as much as 25 percent of the heat generated if ex-posed for enough time (about 5 days) prior to placing thesubsequent lift, if the ambient temperature is below theinternal concrete temperature. Lifts greater than 5 ft andplacements with little or no difference between the airtemperature and internal concrete temperature will loselittle or no heat (ACI 207.1R and 207.2R).

At least of equal importance is the temperature gra-dient between the interior temperature and the exposedsurface temperature. This can create a serious conditionwhen the surface and near-surface temperatures declineat night, with the falling autumn and winter air temper-atures, or from cold water filling the reservoir, while theinterior concrete temperatures remain high. The decreas-ing daily air temperatures, augmented by abrupt cold per-iods of several days duration characteristic of changingseasons, may create tensile strains approaching, if notexceeding, the strain capacity of the concrete.

2.7-Concrete thermal characteristics2.7.1 Coefficient of thermal expansion-The mineral

composition of aggregates, which comprise 70-85 percentof the concrete volume, is the major factor affecting thelinear coefficient of expansion of concrete. Hardenedcement paste exhibits a higher coefficient than aggregate,and is particularly influenced by its moisture content. Thecoefficient of hardened cement paste in an air-dry condi-tion may be twice that under either oven-dry or saturatedconditions. The expansion coefficient for concrete isessentially constant over the normal temperature range,and tends to increase with increasing cement content anddecrease with age. The typical range of values given inTable 2.3 represents concrete mixtures with about a 30:70fine to coarse aggregate ratio, high degree of saturation,and a nominal cement content of 400 lb/yd3 (237 kg/m3).

2.7.2 Specific heat-The heat capacity per unit of tem-perature, or specific heat, of normal weight concrete var-ies only slightly with aggregate characteristics, tempera-ture, and other parameters. Values from 0.20 to 0.25Btu/lb F (cal/gm C) are representative over a wide rangeof conditions and materials.

2.7.3 Thermal conductivity - Thermal conductivity is ameasure of the capability of concrete to conduct heat,and may be defined as the rate of heat flow per unit tem-perature gradient causing that heat movement. Minera-logical characteristics of the aggregate, and the moisture

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Table 2.3-Linear thermal coefficient of expansion ofconcrete

Coarse aggregate

Thermal coefficient of expansion

Millionths/deg F Millions/deg C

Quartzite 7.5 13.5SiiCCOUS 5.2-6.5 9.4-11.7Basalt 4.6 83Limestone 3.0-4.8 5.4-8.6

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content, density, and temperature of the concrete allinfluence the conductivity. Within the normal concretetemperatures experienced in mass concrete construction,and for the high moisture content existing in concrete atearly ages, thermal conductivity values shown in Table2.4 are typical (ACI 207.1R).

Table 2.4-Typical thermal conductivity values forconcrete

Aggregate type

QuartxiteDolomiteLimestoneGraniteRhyoliteBasalt

Thermal conductivity

Btu h/h. ft2 F W/m.K

24 3.522 3.2

18-U 2.6-3318-19 2.6-2.7

15 2.213-15 1.9-2.2

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2.7.4 Themal diffusivity -As discussed in ACI 207.1R,thermal diffusivity is an index of the ease or difficultywith which concrete undergoes temperature change, andnumerically is the thermal conductivity divided by theproduct of density and specific heat. For normal weightconcrete, where density and specific heat values varywithin relatively narrow ranges, thermal diffusivity re-flects the conductivity value. High conductivity indicatesgreater ease in gaining or losing heat. Table 2.5, takenfrom the same reference, is reproduced here for conven-ience. Values for concrete containing quartzite aggregatehave been reported up to 0.065 ft2/hr (0.0060 m2/hr).

2.4-Concrete elastic propertiesPrior to achieving a “set” and measurable modulus of

elasticity, volume changes occur with no accompanying

Table 2.LThermal diffusivity and rock type

Coarse aggregate

Diffusivity

ft% m2b

Quartzite 0.058 0.0054Limestone 0.051 0.0047Dolomite 0.050 0.0046Granite 0.043 0.0040Rhyolite 0.035 0.0033Basalt 0.032 0.0030

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development of stress. At some time after placement, theconcrete will begin to behave elastically. For highercement content mixtures without retarders and placed at“warm” temperatures (in excess of about 75 F (24 C)) thismay occur within a few hours. For low cement contentmixtures with retarders and placed at very cold temper-atures this may not occur for 1 to 2 days. Primarily forconvenience, a one-&y age is frequently taken to be theearliest age at which thermally-caused stress will occur.The exact age is not critical, because the elastic moduluswill initially be low and the strain-to-stress conversionresult is further mitigated by high creep at early ages.

Typical instantaneous and sustained (long-term) elasticmodulusvalues for four conventional mass concretes (dif-ferent coarse aggregates) are given in Table 2.6. Table2.7 shows values for some low cement content RCC mix-tures. The lower modulus of elasticity values after one-year sustained loading reflect the increases in strainresulting from the time-dependent characteristic (creep)of the concrete. At intermediate dates, the unit strainincrease is directly proportional to the logarithm of theduration of loading. For example, with initial loading at90 days and basalt aggregate concrete, the initial unitstrain is 0.244 millionths per psi (35.7 millionths perMPa). After one-year load duration, the unit strain valueis 0.400 millionths per psi (58.8 millionths per MPa). At100-day age, or 10 days after initial loading, the unitstrain value in millionths per psi is given by the equation:

0.244 + (0.400 - 0.244) log lo/log 365

(in millionths per MPa: 35.7 + (58.8 - 35.7)log 10/log 365)

The resulting modulus of elasticity is 3.3 x 106 psi (22GPa).

Elastic properties given in Tables 2.6 and 2.7 were in-fluenced by conditions other than aggregate type, and formajor work laboratory-derived creep data based on ag-gregates and concrete mixtures to be used is probablywarranted.

fo

2.9-Strain capacityDesigns based on tensile strain capacity rather than

tensile strength are more convenient and simpler wherecriteria are expressed in terms of linear or volumetricchanges. Examples are temperature and drying shrinkagephenomena. The Corps of Engineers employs a modulusof rupture test as a measure of the capability of massconcrete to resist tensile strains (Hook et al. 1970)(Houghton 1976).

The tensile strain test beams are 12 x 12 x 64 in. (300x 300 x 1600 mm), nonreinforced, tested to failure underthird-point loading. Strains of the extreme fiber in ten-sion are measured directly on the test specimen. At the7-day initial loading age, one specimen is loaded to fail-ure over a period of a few minutes (rapid test). Concur-rently, loading of a companion test beam is started, with

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Table 2.6-Typical instantaneous and sustained modulus of elasticity for conventional mass concrete

Million psi (GPa)

Age at time ofBasalt Andesite & Slate Sandstone Sandstone & Quartz

loading (days) E E E E E E E E’

2(z)

0.83(Z)

054(5.7) (3.7) (i-i) (iz) (;;)

0.63(4.3)

7(2) &) (Y) (ki) (z) (E) (2)

0.94(6.5)

28(2) (i-i) (E) (if)

4.5

(31) (i-i) (it) (if)

90(Z) ;) $) (z) (2) $) (li) (?i)

365(ii) (2) ;‘;I, (ii)

All concrete mass mixed, wet screened to 1?4 in. (38 mm) maximum size aggregateE = instantaneous modulus of elasticity at time of loadingE’ = sustained modulus after 365 days under loadBased on ACI 207.1R

Table 2.7-Typical instantaneous and sustained modulus of elasticity for roller-compacted concrete

Million psi (GPa)

Ignimbrite1 Ignimbrite1 Basalt2 Basalt3 Basalt’Age at timeof loading PJrr, mffl

(&ys) (internal gauges) (external gauges)

E E E E E E E E E E

7 0.7 0.8 0.7(5) (6) (5)

28(i-i) ;; (if) i; ;; ;;

90(t-t) (it) (if) (ii) (i-i) 1

(1) Cement content of 151 lbs/cy (90 kg/m3), no pozzolan.(2) Cement content of 100 lbs/cy (59 kg/m3), no pozzolan.(3) Cement content of 175 lbs/cy (104 kg/m3), pozzolan content of 80 lbs/cy (47 kg/m3).(4) Cement content of 80 Ibs/cy (47 kg/m3), pozzolan content of 32 lbs (19 ks/m3).All mixes contained 3-in (76-mm) maximun size aggregateE = instantaneous modulus of elasticity at time of loadingE = sustained modulus after 365 days under load

05 03(4) (2)

(z) 6)0.9(6) ;;

weekly loading additions, 25 psi/week (0.17 MPa/week),of a magnitude which will result in beam failure at about90 days (slow test). Upon failure of the slow test beams,a third specimen is sometimes loaded to failure under therapid test procedure to provide a measure of the changein elastic properties over the duration of the test period.

Tensile strain capacity results (Table 2.8 shows typical values) aid in establishing concrete crack control proce- dures. For example, assuming the first concrete in Table 2.8 has a coefficient of thermal expansion of 5.5 mil- lionths/F (9.9 millionths/C) from Table 2.3, sufficient insulation must be used to avoid sudden surface tem- perature drops greater than 64/5.5 = 11.6 F (6.4 C) at early ages, and 88/5.5 = 16 F (8.9 C) at 3-month or later ages, In the event embedded pipe cooling is used, the total temperature drop should not exceed 118/5.5 = 21 F (12 C) over the initial 3-month period. --``,`,-`-`,,`,,`,`,,`---

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An abbreviated tensile strain capacity prediction pro-cedure has been reported (Liu 1978), but the system isempirical, approximate, and promises no more than amoderate correlation with measured values.

2.10 - Thermal shockThe interior of most concrete structures, with a mini-

mum dimension greater than about 2 ft (0.6 m) will be ata temperature above the ambient air temperature at thetime forms are removed. At the boundary between theconcrete and the forms, the concrete temperature will bebelow that in the interior, but above that of the air. Withsteel forms, the latter difference may be small, but withinsulated steel or wood forms the difference may be sub-stantial. When the forms are removed in that instance,the concrete is subjected to a sudden steepening of thethermalgradient immediately behind the concrete surface.

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Table 2.8-Tensile strain capacity

Tensile strains (Millionths)(a)(b)

Concrete componentsRapid test Rapid test(Initial) Slow test (Final)

Quartz diorite (natural)w/c = 0.66(c)

64 (89) 118 (102) 88 (78)

Quartz diorite (natural)w/(c + p) = 0.63(c)

52 (65) 88 (80) 73 (74)

Granite gneiss (crushed)w/(c + p) = 0.60

Limestone (crushed)Quartz sand (natural)w/(c + p) = 0.63

Limestone (crushed)Quartz sand (natural)w/(c + p) = 0.47

86 245 110

45 (70) 95 (89) 73 (75)

62 (66) 107 (83) 8JJ (71)

(a) At 90 percent of failure loading(b) Strain values not in parentheses are from beams initiallyloaded at 7-days age. Values in parentheses are from testsstarted at 28-days or later(c) w/c is water-cement ratio

w/(c + p) is water-cement plus pozzolan ratio

Sn

This sudden thermal shock can cause surface cracking.Identical circumstances will arise with the approach of

the cooler autumn months or the filling of a reservoirwith cold runoff. Abrupt and substantial drops in air tem-perature will cause the near-surface gradient to suddenlysteepen, resulting in tensile strains that are nearly 100percent restrained. Exposed unformed concrete surfacesare also vulnerable.

These critical conditions are mostly avoided during thesecond and subsequent cold seasons because much of theheat has been lost from the interior concrete and thetemperature gradient in the vicinity of the surface ismuch less severe.

CHAPTER 3 - PRECOOLING SYSTEMS

3.1-GeneralThe possibility of cracking from thermal stresses

should be considered both at the surface and within themass. One of the strongest influences on the avoidanceof thermal cracking is the control of concrete placingtemperatures. Generally, the lower the temperature ofthe concrete when it passes from a plastic or as-placedcondition to an elastic state upon hardening, the less willbe the tendency toward cracking. In massive structures,each 10 F (6 C) lowering of the placing temperature be-low the average air temperature will result in a loweringby about 6 F (3 C) of the maximum temperature the con-crete will reach.

Under most conditions of restraint, little significantstress (or strain) will be developed during and for a shorttime after the setting of the concrete. The compressiveeffects of the initial high temperature rise are reduced tonear zero stress conditions due to lower modulus of elas-

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ticity and high creep rates of the early age concrete. Thezero-stress condition occurs at some period in time nearthe peak temperature. A concrete placing temperaturemay be selected such that the potential tensile strainresulting from the temperature decline from the initialpeak value to the final stable temperature does not ex-ceed the strain capacity of the concrete. The procedureis described by the following relationship:

where

I;: =T =c! =et =

R =At =

- At

placing temperature of concretefinal stable temperature of concretestrain capacity (in millionths)coefficient of thermal expansion per deg of tem-perature (in millionths)degree of restraint (in percent)initial temperature rise of concrete

The object of the precooling program is to impose adegree of control over crack-producing influences of con-crete temperature changes. The designer should knowthe type and extent of cracking that can be tolerated inthe structure. Proper design can accommodate antici-pated cracking. In most circumstances it is unrealistic toexpect cracking not to occur, so provisions must be im-plemented to deal with cracking. The benefits of temper-ature control and other crack control measures havebeen demonstrated during the construction of large con-crete dams and similar massive structures.

3.2 - Heat exchange3.2.1 Heat capacities - The heat capacity of concrete is

defined as the quantity of heat required to raise a unitmass of concrete 1 degree in temperature. In those sys-tems of units where the heat capacity of water is estab-lished as unity, heat capacity and specific heat arenumerically the same. The specific heat of concrete isapproximately 0.23 Btu/lb deg F (0.963 kJ/ kg K); valuesfor components of the mixture range from a low of about0.16 (0.67) for some cements and aggregates to 1.00(4.18) for water. The temperature of the mixed concreteis influenced by each component of the mixture and thedegree of influence depends upon the individual compo-nent’s temperature, specific heat, and proportion of themixture. Because aggregates comprise the greatest partof a concrete mixture, a change in the temperature of theaggregates will effect the greatest change (except whereice is used) in the temperature of the concrete. Since theamount of cement in a typically lean mass concrete mix-ture is relatively small its cooling may not be significantto a temperature control program.

For convenience, the concrete batch and the compo-nents of the concrete batch can be considered in terms ofa water equivalent, or the weight of water having an--``,`,-`-`,,`,,`,`,,`---

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equivalent heat capacity. An example of 1 cu yd of massconcrete and its water equivalent follows:

Ingredient

Specific Batch WaterBatch heat heat equiv-weight capacity content aient

lb Btu/lb-deg F Btu/deg F lb

aggregate1 percentmoisture

Fme aggregate5 percentmoisture

Cement

Fly ash

Batched water

2817 0.18 507 507

28 1.00 28 28

890 0.18 160 160

45 1.00 45 4.5

197 0.21 41 41

85 0.20 17 17

139 1.00 139 139

4201 937 937

An example of a 1 m3 mass concrete mixture and itswater equivalent follows:

Ingredient

Specific WaterBatch heat Batch heat equiv-weight capacity content aient

kg kI/kg-deg K kJ/deg K kg

aggregate1 percentmoisture

Fine aggregate5 percentmoisture

Cement

Fly ash

Batch water

1672 0.75 1254 300

17 4.18 71 17

528 0.75 396 95

26 4.18 109 26

117 0.88 103 25

50 0.84 42 10

82 4.18 343 82

2492 2318 5.55

In other words, 1 cu yd of this concrete would requirethe same amount of cooling to reduce (or heating toraise) its temperature 1 F as would be required by 937lbs of water. Similarly, 1 m3 of this concrete wouldrequire the same amount of cooling (or heating) tochange its temperature 1 C as would be required by 555kg of water.

3.2.2 Computing the cooling requirement-Assume thata 50 F (10 C) placing temperature will satisfy the designcriteria that have been established. From the tempera-tures of the concrete ingredients as they would be re-ceived under the most severe conditions, a computationcan be made of the refrigeration capacity that would berequired to reduce the temperature of the mixture to 50F (10 C). Using the same mass concrete mixture, the re-frigeration requirement per cu yd can be computed asfollows:

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Ingredient

Moist coarse aggMoist fine aggCementFly ashBatched waterHeat of mixing

(est)

Initial Degrees Water Btu’s totemp to 50 F equivalent 50 F

deg F deg F lb Btu

75 25 535 1337573 23 205 4,715

120 70 41 2,87073 23 17 39170 20 139 2,780

1,000

937 25,131

Refrigeration required for a 1 m3 mixture as fol-lows:

IngredientInitial Degrees Water kJ(a) totemp to 10 C equivalent 10 c

deg C deg C kg kJ

Moist coarse agg 24 14 300 17,556Moist fine agg 23 13 121 6,575Cement 49 39 25 4,076Fly ash 23 13 10 543Batched water 21 11 82 3,770Heat of mixing

(est) 1390

538 33,910

(a) Product of (deg to 10 C) x (water equivalent) x (4.18)

It will be observed that if this concrete is mixed underthe initial temperature conditions as set forth, the mixedtemperature of the concrete will be:

US uunitsnits(a):

5OF+ 25,131 Btu =50 F + 27 F = 77 F937 Btu/deg F

SI units @):

10 C + 33,910 hJ =lOC+15C=25C2,318 kJ./deg K

(1) U.S. cus10maly uaits@) sysleme Inlenulionrk unils

To lower the temperature of the concrete to 50 F(10 C), it would be necessary to remove 25,131 Btu(33,910 kJ) from the system. The temperature of mixedconcrete can be lowered by replacing all or a portion ofthe batch water with ice, or by precooling the compo-nents of the concrete. In this example a combination ofthese practices would be required.

3.2.3 Methods of precooling concrete components - Theconstruction of mass concrete structures, primarily dams,has led to improved procedures for reducing the temper-ature of the concrete while plastic with a resultantlessening of cracking in the concrete when it is hardened.

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Concrete components can be precooled in severalways. The batch water can be chilled or ice can besubstituted for part of the batch water. In this event,attention should be given to addition of admixtures andadjusting mixing times. Aggregate stockpiles can beshaded. Aggregates can be processed and stockpiledduring cold weather. If the piles are large, only theoutside exposed portion will heat up any significantamount when warm weather occurs, preserving the colderinterior for initial placements. Fine aggregates can beprocessed in a classifier using chilled water. Methods forcooling coarse aggregates, which provide the greatestpotential for removing heat from the mixture, can rangefrom sprinkling stockpiles with water to provide forevaporative cooling, spraying chilled water on aggregateson slow-moving transfer belts, immersing coarse aggre-gates in tanks of chilled water, blowing chilled airthrough the batching bins, to forcing evaporative chillingof coarse aggregate by vacuum. While the most commonuse of nitrogen is to cool the concrete in the mixer,successful mixture cooling has resulted from nitrogencooling of aggregates and cooling at concrete transferpoints. Introduction of liquid nitrogen into cement andfly ash during transfer of the materials from the tankersto the storage silos has also been effective (Forbes,Gillon, and Dunstun, 1991). Combinations of several ofthese practices are frequently necessary.

3.3 - Batch waterThe moisture condition of the aggregates must be

considered not only for batching the designed concretemixture, but also in the heat balance calculations forcontrol of the placing temperature. The limited amountof water normally required for a mass concrete mixturedoes not always provide the capacity by itself to ade-quately lower the temperature of the concrete even if iceis used for nearly all of the batch water.

3.3.1 Chilled batch water-One lb (one kg) of waterabsorbs one Btu (4.18 kJ) when its temperature is raised1 F (1 C). A unit change in the temperature of the batchwater has approximately five times the effect on the tem-perature of the concrete as a unit change in the temper-ature of the cement or aggregates. This is due to thehigher specific heat of water with respect to the othermaterials. Equipment for chilling water is less compli-cated than ice-making equipment. Its consideration isalways indicated whether solely for chilling batch wateror in combination with other aspects of a comprehensivetemperature control program, i.e., inundation cooling ofcoarse aggregates, cold classifying of fine aggregate, orpost-cooling of hardened concrete with embeddedcooling coils.

It is practical to produce batch water consistently at35 F (2 C) or slightly lower. Using the mass concretemixture discussed above, chilling the 139 lb (82 kg) ofbatch water from 70 F (21 C) to 35 F (2 C) will reducethe concrete temperature about 5 F (3 C).

This can be readily computed by multiplying the

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pounds of batch water by the number of degrees thewater temperature is reduced and dividing the whole bythe water equivalent of the concrete. For the illustrationmix, this would be as follows:

US units:

139 (lb) x (70 F - 35 F) = 5.2 F*937 water equivalent (lb)

SI units:

82 (kg) x (21 C - 2 C) = 2.8 C*555 water equivalent (kg)

3.3.2 Using ice as batch water-One lb (kg) of iceabsorbs 144 Btu’s (334 kJ) when it changes from ice towater; thus, the use of ice is one of the basic and mostefficient methods to lower concrete placing temperatures.

The earliest method involved the use of block ice thatwas crushed or chipped immediately before it wasbatched. Later methods utilized either ice flaking equip-ment, where ice is formed on and scraped from a refri-gerated drum that revolves through a source of water; orequipment where ice is formed and extruded from re-frigerated tubes and is clipped into small biscuit-shapedpieces as it is extruded.

It is important that all of the ice melts prior to theconclusion of mixing and that sufficient mixing time isallowed to adequately blend the last of the melted iceinto the mix. Where aggregates are processed dry, thismay mean adding no more than 3/4 of the batch water asice. Where aggregates are processed wet, there willnormally be enough moisture on the aggregates to permitalmost all of the batch water to be added as ice with justenough water to effectively introduce any admixtures. Ifthe entire 139 lb (82 kg) of batch water in the illustrationmixture is added as ice, the effect of the melting of theice would lower the temperature of the concrete by 21 F(12 C), computed as follows:

Us units:

139 (lb) x 144 (Btu/lb) = 21.4 F937 water equivalent(lb) x (1.0 Btu/lb deg F)

SI units:

82 (kg) x 334 (kJ/kg) = 11.8C555 water equivalent (kg) x (4.18 kT,,kgdeg K)

3.4- Aggregate coolingAlthough most rock minerals have a comparatively low

unit heat capacity, aggregates comprise the greatest pro-portion of concrete mixtures. Therefore, the temperatureof the aggregates has the greatest influence on the tem-perature of the concrete. Under the most severe temper-ature conditions of construction, the objectives of acomprehensive temperature control program cannot be

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achieved without some cooling of the concrete aggre-gates.

3.4.1 Cold weather aggregate processing-Generally onlarge conventional mass concrete projects, aggregate pro-cessing occurs concurrently with concrete production andplacement, Projects constructed of RCC, have requiredlarge proportions of the concrete aggregate to be pro-cessed and stockpiled prior to the commencement ofplacement operations. The RCC placement occurs at amuch faster rate than does aggregate production. Sig-nificant aggregate temperature reductions can be realizedby processing such aggregate during the colder winterseason at locations where a marked winter season occurs.The use of large stockpiles and selective withdrawal ofaggregates from the stockpiles can reduce the in-placetemperatures of the concrete significantly. Additionalthermal considerations for RCC are discussed by Tatroand Schrader (1985).

3.4.2 Processing fine aggregate in chilled water-Prob-ably the most efficient way to cool fine aggregate is touse chilled water in the final classification of the fineaggregate. The effluent water from the classifier isdirected to a settling tank to drop out excess fines andthe water is returned to the cooler and then back to theclassifier. Fine aggregate is readily cooled by this method,it gains heat quite slowly following wet classificationbecause of the moisture it carries and the possibility forevaporation. By this method, fine aggregate can be con-sistently produced at temperatures between 40 F (4 C)and 45 F (7 C).

If the entire 935 lb/cu yd (554 kg/m3) of fine aggregate(including moisture) of the illustration mixtures isreduced to 45 F (7 C) from its 73 F (23 C) temperature,the result would be a lowering of the concrete temper-ature by about 6 F (4 C), computed as follows:

US units:

205 moist F.A water equiv (lb) x (73 F - 45F) =6.1F937 concrete water equiv (lb)

SI units:

121 moist FA. water equiv (kg) x (23C-7C) =3.5C*555 concrete water equiv (kg)

Hollow-screw heat exchangers for contact cooling offine aggregate have not proven as effective as has clas-sification with chilled water.

3.4.3 Sprinkling of coarse aggregate stockpiles-Mistingor sprinkling water onto coarse aggregate stockpiles is aninexpensive but limited means of reducing coarse aggre-gate temperatures. The amount of cooling that can beobtained depends upon the cooling effect of natural eva-poration, which in turn, depends upon the ambient con-ditions of temperature, wind, and relative humidity.Adequate drainage should be provided beneath thestockpiles. Only enough water to meet evaporation ratesis necessary. In very large stockpiles only the areas from

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which material is being withdrawn need to be sprinkled.3.4.4 Immersion cooling of coarse aggregate-one of

the most effective ways to cool coarse aggregate is im-mersion in holding tanks through which chilled water iscirculated. The tanks are open at the top with a conicalbottom leading to a watertight aggregate discharge gate.The water is piped into and out of the tanks for fillingand circulation. The cooling cycle consists of filling thetank with chilled water, dumping the coarse aggregateinto the tank, circulating the chilled water through theaggregate, draining water from the tank, and dischargingthe aggregate from the bottom gate. The aggregate is dis-charged onto a conveyor belt and fed over a vibratingscreen to remove excessive moisture. With this methodusing 35 F (2 C) water, even the larger 6 in. (152 mm)top size cobbles for mass concrete can be cooled to about38 F (3 C) with a circulating time of 45 min. However,the complete cycle including filling and discharging wouldbe about 2 hr. Separate tanks, up to 125 tons (113 Mg)capacity, have been used for each size of coarse aggre-gate.

If the entire 2845 lb (1689 kg) of coarse aggregate (in-cluding moisture) of the illustration mixture is reduced to38 F (3 C) from its 75 F (24 C) temperature, it wouldresult in a lowering of the concrete temperature by about20 F (12 C), computed as follows:

US units:

507 moist C.A. water equiv (lb) x (75F-38F) =20.0F937 concrete water equiv(lb)

SI units:

317 moist C.A. water equiv(kg) x (24C-3C)555 concrete water equiv(kg)

=12.0C

3.4.5 Chilled water spray - Cooling the coarse aggregatewhile on the belt conveyor enroute to the batch bins byspraying with 40 F (4 C) water may be necessary to sup-plement the use of ice in the batch water. For practicalreasons, the duration of the spray application is limitedto a few minutes (possibly 2 to 5) while on the belt,resulting in removal of heat only from near the surfacesof the individual pieces of aggregate. Data on the exactamount of heat removed under specific conditions of beltspeed, temperature, and rate of water application are notreadily available. On one large mass concrete project, 150gal. of chilled water per ton of coarse aggregate wasrequired (in addition to other precooling techniques) toproduce 45 F (7 C) concrete. Waddell (1978) gives dataon cooling rates of large size aggregate.

A system for removing excess water before dischargeinto the batch bins is essential. Blowing chilled airthrough the cool and damp aggregate in the bins willfurther lower its temperature, but careful control isrequired to avoid freezing the free water.

3.5.6 Vacuum cooling of aggregates-Vacuum coolingof aggregates utilizes (a) the lower boiling point of water

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when under less than atmospheric pressure, and (b) thelarge heat absorptive capacity of water when it changesfrom liquid to vapor. Fine aggregates and all sizes ofcoarse aggregates can be effectively cooled by thismethod. The aggregates must be processed moist, or con-tain sufficient water to absorb the amount of heat it isdesired to extract from the aggregates. Steel silos or bins,with capacities from 100 to 300 tons each (91 to 272 Mg)of aggregate exposed to a vacuum of 0.25 in. (6 mm) ofmercury, will usually provide for a reduction of initialtemperatures of 110 F (43 C) to a final average tempera-ture of 50 F (10 C) over a 45-min operational cycle.

This method utilizes the free moisture on the aggre-gates for the evaporative cooling. The moist aggregatesare fed into a pressure vessel that can be sealed at boththe top inlet and the bottom outlet. Vacuum is appliedfrom a side chamber by steam-fed diffusion pumps.

Again using the illustration mixture, if the 1 percentsurface moisture carried by the coarse aggregates is eva-porated, the temperature of the coarse aggregates will belowered by about 54 F (31 C), or down to about 20 F(-8 C), computed as follows:

us units:

28 (lb) x 1040(BzU/zb)535 muist CA. water equivaienr(lb) x l.O(Btu/l&.dcg I;)

=54.4F

SI units:

17wx242Owk)317 moist CA. water equivaknt(kg) x 4.18 (k$kgdcg K)

=31.oc

In this illustration, the heat of vaporization of water at0.25 in. (6 mm) of mercury is approximately 1040 btu/lb(2420 kJ/kg).

As a result of this cooling of the coarse aggregate, thetemperature of the concrete would be lowered by about31 F (18 C), computed as follows:

us units:

28 (Zb) x 1040 (Br@)937 concrete wafer equf3yyfb) x l.O(B&$bdeg F)

SI units:

555 concrete wazer equivalent(kg) x 4.18 (ki,,kgdeg K)= 17.7c

3.4.7 Liquid nitrogen-An alternate method for coolingbatch water and creating an ice/water mixture employsliquid nitrogen, an inert cryogenic fluid with a temper-ature of -320 F (-196 C) (Concrete Construction, May1977, p. 257).

From a cryogenic storage tank located at the batchplant, the liquid nitrogen is injected through lancesdirectly into the batch water storage tank to bring the

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water temperature down to 33 F (1 C). To promotegreater cooling of the concrete, liquid nitrogen is injectedinto the water in a specially designed mixer just prior tothe water entering the concrete mixer, whereby the liquidnitrogen causes a portion of the water to freeze. Theamount of ice produced can be varied to meet differenttemperature requirements.

Liquid nitrogen systems have proven successful on anumber of construction projects, particularly where auto-matic or flexible operation control is beneficial. Localavailability must be considered, and cooling to tempera-tures below about 60 F (16 C) is currently not feasible.Liquid nitrogen has also been injected directly into mixerdrums. This approach may require that the mix time beprolonged from several minutes to 10 minutes before sig-nificant cooling results.

3.5 - Cementitious materialsCementitious materials used in concrete must be

handled dry. If the temperature of the cement is broughtdown below the dew point of the surrounding atmo-sphere, moisture can condense and adversely affect theultimate quality of the cement. As a general rule, theconcrete mixture heat balance does not require coolingof the cement in order to meet the placing temperaturerequirements. Normally, cement is delivered at a tem-perature of about 130-155 F (54-68 C). The cement tem-perature can be increased or decreased a few degreesdepending on the cement handling equipment and pro-cedures used. However, since cement is such a relativelysmall portion of mass concrete mixtures, its initial tem-perature has little effect on the concrete temperature.Also, cooling the cement is not very practical or eco-nomical.

3.6-Heat gains during concreting operationsConsiderations for temperature control should recog-

nize the heat gains (or losses) of the concrete or theconcrete ingredients during batch plant storage, duringconcrete mixing, and during the transportation and place-ment of the concrete. The placement of a large massconcrete structure can be visualized as a rapid sequenceof procedures all of which are guided by several overallobjectives not the least of which is to protect the con-crete from any avoidable heat gain.

Ingredients can be protected against heat gain at thebatch plant by means such as insulation, reflective siding,air conditioning, and circulation of chilled air throughcoarse aggregates. The energy required for the mixing ofconcrete imparts about 1000 Btu (1390 kJ) of heat per cuyd (m3) of concrete. Where a project plant is convenient-ly set up to permit rapid bucket transport and placement,there will be an insignificant temperature gain betweenmixing and placement of the concrete. However, to ac-count for delays and inconvenient placements, it may bejudicious to include in the heat balance computation asmall contingency for heat gain between mixing andplacement.

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3.7 - Refrigeration plant capacityThe size of the cooling plant required, expressed in

tons of refrigeration, is given by

Maximum concrete placing rate (yd3/h) l heat to beremoved (Btu/yd )

12,000 Btu/h

Because the temperatures of the aggregates will generallyfollow the annual cycle of ambient air temperatures, arefrigeration plant capacity requirement should be deter-mined for specific segments of time, such as a week or amonth. The refrigeration plant may be designed for thecooling of only one material, such as production of ice,or may be divided into various cooling systems for pro-duction of ice, chilled water and/or cooled air accordingto heat balance needs. A trial procedure for deriving theamount of ice required to satisfy a given initial tem-perature of 60 F (16 C) is shown in the accompanyingTrial Heat Balance illustration, for the mixture pro-portions cited in Paragraph 3.2.1, with 79 lbs (47 kg) ofice and 60 lbs (35 kg) of chilled water.

Trial Heat Balance

Tempera- Tempera-ture as ture after Water Heat

Ingredient batched mixing equivalent exchanged

deg F deg F lb Btu

aggregate 75 60 535 8,025(moist)

Fineaggregate 73 60 205 2,665(moist)

Cement 120 60 41 2,460

W=h 73 60 17 221

Heat of 1,000mixing

14571

Batched waterIce 32 144(a) 79 -11,376Ice (melted) 32 60 79 -2,212Water

(chilled) 35 60 60 -1,500

-15,088(a) Units of heat required to change one lb of ice at 32 F towater at same temperature.

Trial Heat Balance

Ingredient

aggregate(moist)

Tempera- Tempera-ture as ture after Water Heatbatched mixing equivalent exchanged

deg C deg C kg kJ

24 16 300 10,032

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Fineaggregate(moist)

Cement

Fly ash

Heat ofmixing

Batched waterIceIce (melted)Water

(chilled)

23 16 121 3,520

49 16 25 3,448

23 16 10 293

1,390

18,703

0 334(a) 47 -15,6980 16 47 -3,143

2 16 35 -2,048

-20,889(a) Units of heat required to change one kg of ice at 0 C to waterat the same temperature.

3.8 - Placement areaDuring hot weather, precooled concrete can absorb

ambient heat and solar radiation during placement, whichwill increase the effective placing temperature and theresulting peak temperature. This increase in temperaturecan be minimized or eliminated by reducing the tempera-ture in the immediate placing area with fog spray and/orshading. Placing at night will also reduce the effects ofhot weather and radiant heat.

CHAPTER 4 -POST-COOLING SYSTEMS

4.1-GeneralControl of concrete temperatures may be effectively

accomplished by circulating a cool liquid (usually water)through thin-walled pipes embedded in the concrete. De-pending on the size of the pipe, volume of fluid circu-lated, and the temperature of the fluid, the heat removedduring the first several days following placement canreduce the peak temperature by a significant amount.The post-cooling system also accelerates the subsequentheat removal (and accompanying volume decrease) dur-ing early ages when the elastic modulus is relatively low.

The radial temperature isotherms developed aroundeach cooling pipe create a complex, nonuniform, andchanging thermal pattern. Smaller pipes with colder fluidcreate a more severe local condition than larger pipeswith a less cold fluid. Under conditions of rapid andintense cooling, this could result in localized radial orcircumferential cracks. Up to the age at which the max-imum concrete temperature in the vicinity of the pipeoccurs, no restriction on cooling rate is needed. After aninitial peak concrete temperature has been experienced,cooling is usually continued until the first of theseconditions occur:

a) The concrete cooling rate reaches the maximumthat can be tolerated without cracking (see Paragraph4.2); or

b) The temperature of the concrete decreases to about30 F (17 C) below the initial peak value. This is an em-

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pirically-derived value (Paragraph 4.4.2) generallysubstantiated by slow strain capacity tests (Table 2.8).

c) The concrete has been cooled to its final stabletemperature or an intermediate temperature prescribedby the designer.

The duration of this initial cooling period may be asshort as several days or as long as one month. Subse-quently the concrete temperature usually will increaseagain. If the increase is significant, one or moreadditional cooling periods will be necessary. Experiencehas shown that supplementary cooling operations cansafely reduce the concrete temperatures to below a finalstable value, or to a point that creates joint openings ofample width to permit grouting, if required. Coolingrates, in degrees per day, for these later periods shouldbe lower than that permitted during the initial periodbecause of the higher modulus of elasticity at later ages.Other methods such as evaporative cooling with a finewater spray, cool curing water, and shading may provebeneficial, but the results are variable and do not greatlyaffect the interior of massive placements, when the ratioof exposed surface area to volume is less than about0.3 ft-1 or 1.0 m-1. For example, a 3 ft (0.9 m) thick liftwith only the top surface exposed for evaporative coolingwould have a surface area to volume ratio of 0.33 ft-(1.1 m-l).

4.2 - Embedded pipe4.2.1 Materials-Aluminum or thin-wall steel tubing, 1

in. (25.4 mm) nominal outside diameter and 0.06 in. (1.5mm) wall thickness, has been used successfully for em-bedded cooling coils. Plastic and PVC pipe may also beused. Couplings of the compression type used to join sec-tions of aluminum or steel tubing should be of the samematerial or nonconductor sleeves and gaskets should beprovided to avoid the galvanic effect of dissimilar metals.Aluminum tubing has an advantage of light weight andeasy installation, but is subject to breakdown and leakagedue to reaction with alkalies in the cement. When theexpected active cooling period exceeds three months, alu-minum should not be used. In mass concrete dam con-struction there have been no well-documented instancesreported where the long-term effects of aluminum tubingdeterioration has caused distress in the concrete; butshould be avoided where absolute integrity over the lifeof the structure is imperative and possible leakage couldbe critical.

4.2.2 Spacing-For practical reasons, pipe coils areusually placed directly on and tied to the top of a hard-ened concrete lift. Thus, the vertical pipe spacing typi-cally corresponds to the lift height. A horizontal spacingthe same as the vertical spacing will result in the mostuniform cooling pattern, but variations may be utilized.

Figure 5.4.2(a) through 5.4.2(c) of ACI 207.1R can beused as a guide to establish pipe spacings and amount ofcooling necessary for the temperature control desired.

4.2.3 Pipe loop layout-Individual pipe runs may rangefrom 600 ft to about 1200 ft in length (183 m to 366 m),

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with 800 ft (244 m) being a target value for design pur-poses. Splices within the pipe runs should be minimizedas much as practical.

Pipe loops served by the same coolant distributionmanifold should be approximately the same length so asto equalize the flow and cooling effect.

Tie-down wires should be embedded in the lift sur-faces prior to final set. Additional tie-down wires shouldbe placed on either side of splices to help prevent thecouplings from working loose. Each pipe loop should beleak-tested before the covering concrete is placed. Eachpipe run should include a visual flow indicator on theloop side of the supply or return manifold. To assure theinitiation of cooling at the earliest age, to minimizedamage to the emplaced pipe, and to help keep the pipefrom floating in the fresh concrete, water circulationshould be in progress at the time concrete placementbegins in the covering lift.

4.3 - Refrigeration and pumping facilities4.3.1 Pumping plant-Pumping plant requirements are

determined from the number of pipe coils or loops inoperation, which in turn may be established from thelift-by-lift construction schedule. The flow rate for eachcoil typically is from 4 to 4.5 gallons (15 to 17 liters) perminute, which for a 1 in (25 mm) diameter pipe, will re-sult in a fluid velocity about four times the minimumnecessary to insure turbulent flow conditions. Turbulentflow within the coil increases the rate of energy transferbetween the fluid particles, therefore increasing the rateof heat flow by convection. Cooling water used in the sys-tem, particularly if from a river or similar untreatedsource, should be filtered to remove sediment so as toreduce the possibility of system stoppages at bends, con-strictions, or control valves. Unless the length of thecooling run is short, flows through the coils should bereversed at least daily, automatically or manually, byvalved cross-connections at the pumping plant or at thesupply/return manifolds serving each bank of individualpipe coils. Insulating the exposed supply and return piperuns will help ensure the desired water temperatures atthe manifold locations. Sizing of the distribution systemsegments and head loss allowances should follow cus-tomary design procedures.

4.3.2 Refrigeration plant-The size and number of refri-geration plant components should be based on the maxi-mum requirements (number of coils in operation at thesame time and a coil inlet temperature established by thedesign criteria) and the need for flexibility over theexpected duration of the concrete cooling operations.Chilled water as low as 37 F (3 C) has been used forpost-cooling. Where lower coolant temperatures are re-quired, a chilled mixture of 70 percent water and 30 per-cent antifreeze (Propylene glycol) has been effectivelyused at 33 F (1 C). Details of compressors, condensers,surge tanks, valves, meters, and other production andcontrol items, for either portable or stationary plant, aremechanical engineering functions not covered in this

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report.4.3.3 Alternate water cooling practices-Cool water

from natural sources, such as wells or flowing streams,may be used as the coolant, providing the supply is ade-quate, the temperature is reasonably constant, and thewater contains only a slight amount of suspended sedi-ment. Manifolds, valves, gages, and loop bends are par-ticularly vulnerable to stoppages if dirty water is used.

Water discharged back into a stream or well will beslightly warmer after circulation through the coolingpipes, and may require permits in conformance with localor national environmental codes. In favorable climatesconventional cooling towers utilizing the evaporativecooling effect rather than refrigeration could be feasible.

4.4-Operational flow control4.4.1 Manifolds-Supply and return headers or riser

pipes should be tapped at convenient intervals with fit-tings for attaching manifolds to serve each bank of cool-ing coils. Flexible connectors with adapters and universaltype hose couplings are recommended.

4.4.2 Cooling rates-Charts or isothermal diagrams,showing the expected final stable temperature distri-bution within the fully-cooled structure, provide tem-perature objectives for scheduling coil flow operations.During the period of early rapid heat generation andtemperature rise, pipe cooling may be carried out asvigorously as the capacity of the system will permit. Ingeneral, when the concrete has reached its peak temper-ature, cooling should be continued for a period of about1 to 2 weeks at a rate such that the concrete temperaturedrop generally does not exceed 1 F (0.6 C) per day (themaximum rate that does not exceed the early age tensilestrain capacity with creep). A cooling rate of 2 F (1 C)per day can sometimes be tolerated, but only for a shortperiod of time.

When the desired rate of temperature drop is exceed-ed, post-cooling operations in that bank of pipe coilsshould be interrupted until the temperature rises again.Cooling should be resumed when the concrete tempera-ture exceeds the initial peak temperature and is predictedto continue to increase to unacceptable levels.

Experience has shown that most mass concretes havingaverage elastic and thermal expansion properties can re-sist cracking if the temperature drop is restricted to 15 Fto 30 F (8 C to 17 C) over approximately a 30-day periodimmediately following its initial peak.

4.4.3 Repairs during placement-The pipes can be dam-aged during concrete placement by the concrete itself,concrete buckets being dropped on it, vibrators, etc.Extra sections of pipe and couplings should be readilyavailable in the event that a pipe is damaged duringplacement. The freshly placed concrete around the dam-aged pipe should be removed, the damaged section cutout, and a new section spliced onto the existing undam-aged pipe. It is important that all of the damaged pipe beremoved so that it will not restrict flow through the pipe.

4.4.4 Temperature monitoring-A history of concrete

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cooling should be maintained, using resistance thermo-meters or thermocouples installed at representative loca-tions within the concrete including locations close to eachpipe and midway between the pipes. Vertical standpipesembedded in the concrete and filled with water can alsobe used to measure the concrete temperature withinselected zones. A thermometer is lowered into the stand-pipe to the desired elevation and held there until thereading stabilizes. Water temperatures at supply and re-turn manifolds will also serve as a check on the amountof heat being removed from the concrete.

4.5- Surface coolingCooling the surfaces of a thick or massive concrete

structure can be a useful crack-control practice (Carlsonand Thayer, 1959). The objective of surface cooling is tocreate a steep, but tolerable, thermal gradient adjacent tothe exposed vertical surfaces concurrently with the plac-ing of the concrete, and to maintain the cooling a mini-mum of 2 weeks. The optimum period determined theo-retically for a “typical” mass concrete placement is about3 weeks. By developing an initial zero-stress condition ata low temperature, subsequent tensile strains (andstresses) due to further ambient temperature drops arereduced.

Three construction methods are: (a) circulating refrig-erated water within double-walled steel forms left inplace for the 3-week period, (b) discharging used coolingwater from the bottom of hollow forms for the balanceof the 3-week period after being raised for the next lift ofconcrete; and (c) a near-surface embedded pipe coolingsystem. The surface must not be cooled at a rate causingsurface cracks that may later propagate into the massconcrete.

4.5.1 Forms-Where noninsulated metal forms areused, some beneficial effects can be achieved by sprayingwith cold water and by shading.

4.5.2 Curing water-Shading and water curing offormed and finished surfaces can be conducted to cooldirectly, with the added benefit of evaporative cooling, insome regions. On horizontal surfaces the curing shouldbe controlled so that no water remains on the surfacelong enough to become warm.

CHAPTER 5 - SURFACE INSULATION

5.1-GeneralAs mass concrete is deposited in the forms, its tem-

perature begins to rise as a result of the hydration of thecementitious materials. With lifts of 5 ft (1.5 m) orgreater and lateral form dimensions of about 8 to 10 ft(2.4 to 3 m) the temperature rise is essentially adiabaticin the central part of the mass of fresh concrete. At theexposed surfaces (formed or unformed) the heat gener-ated is dissipated into the surrounding air at a ratedependent upon the temperature differential; therefore,

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the net temperature rise in the concrete adjacent to thesurface (or forms) is less than in the interior. While thisresults in a gradually increasing temperature gradientfrom the surface to the interior, little or no stress (andstrain) is developed because the concrete is not yet elas-tic. Generally lean mass concrete exhibits only a slightdegree of rigidity from 4 to 8 hrs after placing dependingupon the placing temperature, initial heat control, andcement characteristics. During the next 16 to 20 hrs thecement hydration rate normally increases substantiallywith the concrete passing from a plastic state to plas-tic-elastic state until at about 24 hr age the concretebegins to act in an elastic manner. During this first daythe modulus of elasticity of the concrete is low and itscreep is high, with the net result that the stresses (andstrains) are essentially zero.

5.1.1 Stress development-Before initial set of concrete,the temperature gradient or thermal change causes littleor no stress or strain. As the concrete begins to acquirestrength and elasticity, changes in the temperature gradi-ent result in length and volume changes which are par-tially restrained within the structure itself. Staticallybalanced tensile and compressive stresses are developed.

5.1.2 Strength development-The rate of strength gainis closely related to both the time and the temperature atwhich the hydration is taking place. Measured by com-pression, tensile splitting, and penetration tests, mostconventional mass concrete will exhibit a slight degree ofrigidity 4 and 8 hrs after placing when mean hydrationtemperature conditions of 90 F and 50 F (32 C and10 C), respectively, exist. High pozzolan, low cement,highly retarded, and/or very cold mixtures may not de-velop any rigidity or significant hydration heat for anextended time (12 to 50 hrs). After acquiring this initialelasticity, the rate of change in strength developmentincreases substantially over the next 16 to 20 hrs. Thus ata nominal 24-hr age, mass concrete begins to respondelastically in a generally predictable manner.

5.1.3 Temperature gradients-Development of steepthermal gradients near exposed surfaces during early ageswhile the modulus of elasticity is very low is usually nota serious condition. Low ambient temperatures duringthe initial 24 hrs may indeed be helpful by establishing asteep gradient at an early age before the concrete re-sponds to stress-strain relationships. After the concretehardens and acquires elasticity, decreasing ambient tem-peratures and rising internal temperatures work togetherto steepen the temperature gradient, and widen the stressdifference between the interior and the surface. On anysectional plane the summation of internal forces must bezero. This results in a high unit tensile stress in theregion of the exposed surfaces and a comparatively lowunit compressive stress over the extensive interior areas.Hence a small temperature increase in the interior willcause a slight increase in compression over a large area,but will also cause a corresponding large increase in ten-sion stress near the surface in order to maintain a zerototal net force system.

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As the rate of hydration slows, ambient temperaturealone becomes the significant parameter, and becomesmost serious at the downward sloping part of the annualtemperature cycle. The effect is further intensified byshort abrupt drops in ambient temperatures, which tendto be more prevalent at night and during the fall season.

5.2--MaterialsThe degree of protection required to avoid or reduce

significantly the thermal tensile stresses at concretesurfaces can be determined theoretically and has beendemonstrated in practice. A thermal resistance (R-value)of 4.0 hr*sq ft*deg F/Btu (0.70 sq m*deg K/W) has beenfound to be effective for moderate climates. This is pro-vided by a 1 in. thickness of expanded synthetic materialsuch as polystyrene or urethane, whose thermal conducti-vity is of the order of 0.20 and 0.30 Btu in/h sq ft deg F(0.03 to 0.04 W/m-K). The closed-cell structure of thistype material is advantageous in minimizing water ab-sorption and capillarity. Mineral wool blankets are not aseffective, usually requiring additional thickness to achievethe same amount of protection. Single-ply polyethyleneenclosures that remain sealed and provide a static airspace between the outside environment and the concretesurface are usually less effective but more economical.

5.2.3 Effect of sudden air temperature drop-The cal-culated effects on concrete temperatures adjacent toexposed and to insulation-protected surfaces when sub-jected to rapid air temperature drops are compared inTables 5.1 and 5.2. Concrete thermal properties used inthe calculations were:

Density 4160 lb/yd3 (2470 kg/m3)

Conductivity 15.8 Btu in/hr ft2 deg F (2.28 W/m-K)

Specific Heat 0.22 Btu/lb deg F (0.920 J/kg-K)

Diffusivity 0.039 ft2/hr (0.0036 m2/hr)

It was assumed that no heat was being generated withinthe concrete.

5.3 - Horizontal surfacesUnformed surfaces of concrete lifts are difficult to

insulate effectively because of damage from and inter-ference with construction activities. Maximum efficiencyof the insulation requires close contact with the concrete,a condition not usually attainable on rough lift surfaces.Neither ponding of water nor layers of sand have provento be practical systems for intermediate lifts in multi-ple-lift construction. Mineral or glass wool blankets orbatting, 2 to 4 in. (50 to 100 mm) in thickness, and anumber of roll-on flexible rubber-type materials nowcommercially available provide considerable protectionand have been widely used.

Until the next layer of concrete is placed, the need for

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Table 5.1 - Effect of insulation protection of concrete exposed to a 28 F (16 C) rapid air temperature drop

Elapsed time Air temp change Concrete temperature changes, deg F (deg C), at various depths, ft (m)

hr deg F (deg C) 0 1 (0.3) 2 (0.6) 3 (0.9) 6 (1.8)

12 -14 (-8)24 -21 (-12)48 -28 (-16)72 -28 (-16)96 -28 (-16)

12 -14 (-8)24 -21 (-12)48 -28 (-16)72 -28 (-16)96 -28 (-16)

12 -14 (-8)24 -21 (-12)48 -28 (-16)72 -28 (-16)96 -28 (-16)

12 -14 (-8)24 -21 (-12)48 -28 (-16)72 -28 (-16)96 -28 (-16)

-9 (-5)-16 (-9)-24 (-13)-25 (-14)-27 (-15)

-1 (-1)-3 (-2)-9 (-5)

-13 (-7)-15 (-8)

0-1 (-1)-2 (-1)-5 (-3)-7 (-4)

000

-1 y-1,

No insulation

(a) R-value is the thermal resistance of insulation in hwsq ftdeg F/Btu.

-2 (-1) 0-5 (-3) -1 (-1)-11 (-6) -4 (-2)-1s (-8) -7 (-4)-17 (-9) -10 (-6)

R-factor l.OO@)

0 0-1 (-1) 0-3 (-2) -1 (-1)-6 (-3) -2 (-1)-8 (-4) -4 (-2)

R-factor 2.0@)

0 0

-1 y-1,00

-2 (-1) -1 (-1)-3 (-2) -2 (-1)

R-factor 4.d’)

0 00 00 00 0

-1 (-1) 0

00

-1 (-1)-4 (-2)-5 (-3)

000

-1 (-1)-2 (-1)

0000

-1 (-1)

0000

-1 (-1)

Table 5.2-Effect of insulation protection of concrete exposed to a 46 F (22 C) rapid air temperature drop

Elapsed time Air temp change Concrete temperature changes, dcg F (deg C), at various depths, ft (m)

hr deg F (deg C) 0 1 (0.3) 2 (0.6) 3 (0.9) 6 (1.8)

12 -26 (-14)24 -39 (-22)48 -40 (-22)72 -40 (-22)96 -40 (-22)

12 -26 (-14) -2 (-1)24 -39 (-22) -6 (-3)48 -40 (-22) -15 (-8)72 -40 (-22) -19 (-11)96 -40 (-22) -22 (-12)

1224487296

12 -26 (-14)24 -39 (-22)48 -40 (-22)72 -40 (-22)96 -40 (-22)

-26 (-14) 0-39 (-22) -1 (-1)-40 (-22) -4 (-2)-40 (-22) -8 (-4)-40 (-22) -11 (-6)

-17 (-9)-30 (-17)-35 (-19)-37 (-21)-37 (-21)

00

-1 (-1)-2 (-1)-2 (-1)

No insulation

-4 (-2) 0-9 (-5) -2 (-1)

-18 (-10) -4 (-4)-22 (-12) -11 (-6)-25 (-14) -14 (-8)

R-factor l.OO@)

0 0-1 (-1) 0-5 (-3) -1 (-1)-9 (-5) -4 (-2)

-13 (-7) -6 (-3)

R-factor 2.0@)

0 00 0

-1 (-1) 0-3 (-2) -1 (-1)-5 (-3) -2 (-1)

R-factor 4.0(s)

0 00 00 00 0

-1 (-1) 0

0

-2 y-1,-5 (-3)-8 (-4)

000

-1 (-1)-3 (-2)

0000

-1 (-1)

0000

-1 (-1)

(a) R-value is the thermal resistance of insulation in hraq ftdeg F/Ettu.

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insulation protection of horizontal surfaces is as great asfor formed surfaces, and the insulation should be appliedas soon as the concrete has hardened sufficiently to per-mit access by workmen.

In severe climates application of insulation, by work-men in special shoes, may be required as soon as theconcrete has been placed to desired elevation.

The insulation must be removed to permit lift surfaceclean-up, but should be replaced promptly unless the cov-ering lift is to be placed within a few hours.

5.3.1 Insulation rating-Acceptable temperature gradi-ents can be maintained during the winter season in mod-erate climates by the application of insulation with an R-value of 4.0 hr*ft**deg F/Btu (0.70 m**K/W). In severeclimates insulation with a R-value of 10.0 (1.76) isrecommended. Applying several layers of blankets withlower insulative value is recommended instead of onelayer of high insulation material. This has the advantageof overlapping the blankets at joints to improve the uni-formity of insulation, and it allows gradual removal ofthe insulation (for example one layer removed every 10days). Gradual removal minimizes the problems of ther-mal shock to the surface when the material is removed.

The provisions of ACI 306R, which cautions againstplacing concrete on frozen foundations, apply equally tohorizontal lift surfaces which are at or below 32 F (0 C).Prior to placing new concrete, such surfaces should beallowed to warm to 40 F (5 C) or higher so that the max-imum differential temperature between the old concreteand the maximum temperature of the fresh concrete, dueto heat of hydration, will not exceed 40 F (5 C). Underrigidly controlled conditions, embedded pipe may be usedto circulate warm water to prevent an abrupt change inthe concrete temperature gradient and to assure ade-quate hydration of the cement in the freshly-placed con-crete. The amount of heat introduced into the concreteshould be the minimum necessary to develop a tempera-ture gradient such that strains will not exceed the straincapacity of the concrete.

5.4-Formed surfacesRigid synthetic cellular material in sprayed, board, or

sheet form as well as blankets containing closed-cellmaterial are practical methods of insulating. The closed-cell structure of the material results in very low absorp-tion characteristics, and its strength and elastic propertiesprovide adequate rigidity. Generally, foamed materials ofthis type are somewhat sensitive to heat at about 175 F(80 C), but temperatures of this level are not normallyencountered after installation. Most synthetic materialswill bum when exposed to open flame, and this is ahazard which should not be overlooked.

5.4.1 Integral form insulation-A minimum wood formthickness of 3 in. (75 mm) is necessary to provide thedesired level of protection while the forms remain inplace. Steel forms offer virtually no insulation protection,and should be supplemented with suitable insulationmaterials prior to placing concrete. A practical solution

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is to coat the exterior of reusable steel forms with aspray-on synthetic foam of the necessary thickness.

5.4.2 Form removal-Upon removing the forms, eitherwood or steel, insulation should be promptly installedagainst the exposed concrete surface. For unexposedformed surfaces, an alternate procedure is to installinsulation on the inside of the forms, prior to concreteplacement, with wire anchors which will project into theconcrete when placed. The insulation then is held inplace against the concrete surface when the forms areremoved. This method has not been successful on ex-posed concrete because of surface imperfections causedby the relatively flexible insulation. In no event shouldthe gradual surface temperature drop exceed the valuesrecommended in ACI 306R when protection is removed.

5.5-Edges and cornersWhere heat can flow concurrently in two or more

directions, rapid temperature drops can occur. Thisresults in the development of tensile strains more quicklyat edges and comers than on the sides or tops of thestructure. Interior concrete in the vicinity of edges andcomers will also be subjected to larger tensile strainssooner than in other portions of the structure.

Increased insulation along the edges and at comers ofmassive concrete structures has effectively reduced therate and magnitude of the temperature decline duringthe cold-weather season. Doubling the insulation thick-ness (reducing conductance by 1/2 over a distance of from2 to 4 ft (0.6 to 1.2 m) from the concrete edges andcomers is a reasonable provision for a structure ofmoderate size.

5.6 - Heat absorption from light energy penetrationDuring construction of Libby Dam by the Corps of

Engineers, significantly higher temperatures were mea-sured at the top surfaces of lifts protected by a urethanefoam insulation exposed to direct sunlight than whenshaded. This phenomena did not occur at DworshakDam when such surfaces were protected by layers ofblack sponge rubber. A limited series of experimentsconfirmed the conclusion that urethane foam insulationpermitted light of some wavelength, probably ultraviolet,to pass through to the concrete surface, where it wasconverted to heat energy. The tests indicated that a bar-rier of some type (black polyethylene or aluminum foil)was required to block out the potential light-sourceenergy and avoid augmenting the heat being generatedwithin the concrete.

5.7-Geographical requirementsThe period during which insulation is required for

protection against thermal cracking depends on the cli-mate and geography. In most of the United States andCanada, and especially in mountainous regions, thisperiod extends from a more or less arbitrary date inautumn, through the winter months, to a spring datewhen the average ambient air temperature is rising and--``,`,-`-`,,`,,`,`,,`---

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comparable to that of the selected autumn date. Thedates in column A below reflect the seasonal decline inaverage air temperatures and the probability of abrupttemperature drops associated with short duration coldsnaps. The column B dates recognize the beneficial ef-fects of the seasonal air temperature trends and thelessened probability of severe and damaging temperaturedrops. At very high elevations and at other locationswhere severe climatic conditions are expected, earlierapplication dates may be advisable. Suggested dates forgeneral locations in the United States are:

Northern U.S.Middle U.S.Southern U.S.

A B

Earliest date noEarliest date needed longer needed

15 September 31 March1 October 15 March15 October 1 March

Surfaces of concrete placed prior to the earliest datesneeded in the preceding tabulation are to receive insul-ation protection no later than the date specified. Formedsurfaces of concrete placed during the autumn-to-springperiods should be insulated from the time of placement,and unformed surfaces as soon as practicable followingcompletion of placement.

CHAPTER 6 - EXPECTED TRENDS

6.1-Effects of aggregate qualityIt is expected that environmental factors and lack of

availability will force the increased use of marginalquality aggregates in concrete. To compensate for aggre-gate quality, additional cement is usually added which inturn increases the expected temperature rise. For somestructures this temperature increase would have little orno effect; for others, it could significantly affect structuralproperties as well as costs.

6.2 - Lightweight aggregatesLightweight aggregates have been used in structural

concrete specifically for the thermal insulation benefitsresulting from low thermal conductivity properties. Therehas been little if any use of such aggregates to modify thethermal conductivity of mass concrete. The technical andeconomic feasibility of using a zone of lightweight aggre-gate concrete adjacent to vertical surfaces, or in precaststay-in-place forms, to control temperatures and tempera-ture gradients, could lead to significant benefits. Thestrength requirements of concrete for arch dams, and thedensity needed for gravity locks and dams, would likelypreclude the use of these aggregates for entire massconcrete structures.

6.3 - Blended cementsMore extensive use of portland-pozzolan and other

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blended cements in mass concrete for improvement ofproperties including temperature reduction is occurringwhere these materials are available and/or their impor-tation is justified. A factor to be considered when usingblended cements, in mass concrete as well as for generalstructural use, is the limited opportunity to vary the ratioof cement and pozzolan. Most demands for structuralconcrete are for faster strength development, hencegreater temperature rise, whereas for mass concrete theopposite concrete properties are usually desired, and areattained by manipulating the proportions of cement andpozzolan. Along with the development of other blendedcements which could reduce temperature rise and easethe requirements for temperature control in mass con-crete, consideration should be given to imposing chemicaland physical limits for the purpose of modifying heatgeneration without sacrificing long-term strength gain.

6.4-AdmixturesChemical admixtures that permit reductions in cement

content have become common in mass concrete. High-range water-reducing admixtures have not become com-mon in typical mass concrete mixtures because of costand the fact that they generally are less effective in low-cement content mixtures. However, they can be very ef-fective in reducing the cement content and subsequentheat problems in structural mixtures that have sufficientvolume to develop significant thermal stresses. Use ofhigh-range water-reducing admixtures should follow therecommendations in ACI 212.2R.

6.5-Temperature control practicesControl measures to minimize thermal distress or

cracking discussed in earlier chapters include precoolingof the concrete components, post-cooling of the concreteby systems of embedded pipes, and insulation of forms orexposed surfaces. Improved techniques for precooling thedry components, including cement and smaller aggregatesizes, may be beneficial when a large reduction in placingtemperature is necessary. More effective and rugged in-sulation materials may provide cost benefits.

6.6-Permanent insulation and precast stay-in-placeforms

An insulation system which could be used on the facesof mass concrete to effectively reduce the thermal dif-ferential from the interior to the exterior can provideextremely beneficial temperature control. Such temper-ature control would reduce susceptibility to crackingcaused by thermal stresses and would most certainlyreduce costs for temperature control. One concept is tohave a thermally adequate insulation system with suf-ficient structural capability so that the system can serveas the form for the placement of the concrete as well asthe permanent insulator. It is considered that insulation,precasting, and waterproofing technology is sufficient topermit development of such an all-purpose insulation andforming system.

t for Resale

COOLING AND INSULATING SYSTEMS 207.4R-21

Copyright AmeProvided by IHNo reproductio

Stay-in-place precast concrete panels have been used onseveral RCC dams to form both upstream and down-stream faces, and also to form spillway training walls.The primary purpose has been for speed and simplifi-cation of construction, but a secondary reason could beto provide permanent surface insulation. The conceptshould be applicable to conventional concrete as well asRCC.

6.7-Roller-compacted concreteRCC has used very low cement contents and/or high

quantities of pozzolan that result in mass concrete witha low temperature rise. At low cement contents, RCCcan have low elastic modulus values and high creep rateswhich combine with the low temperature rise to allowlarge placements with minimal thermal cracking. ACI207.5R, “Roller-compacted Mass Concrete,” discusses thissubject further.

CHAPTER 7 -REFERENCES

7.1 - Recommended referencesThe documents of the various standards-producing or-

ganizations referred to in this document are listed below.

American Concrete Institute207.1R Mass Concrete207.2R Effect of Restraint, Volume Change, and Rein-

forcement on Cracking of Mass Concrete207.5R Roller-compacted Mass Concrete212.2R Guide for Use of Admixtures in Concrete305R Hot Weather Concreting306R Cold Weather Concreting

American Society for Testing and Materials (ASTM)C 150 Standard Specification for Portland CementC 494 Standard Specification for Chemical Admixtures

for ConcreteC 512 Standard Test Method for Creep of Concrete in

CompressionC 595 Standard Specification for Blended Hydraulic

CementsC 618 Standard Specification for Fly Ash and Raw or

Calcined Natural Pozzolan for Use as a MineralAdmixture in Portland Cement Concrete

U.S. Army Corps of EngineersCRD-C 36 Method of Test for Thermal Diffusivity of

ConcreteCRD-C 38 Test Method for Temperature Rise in Con-

creteCRD-C 39 Method of Test for Coefficient of Linear

Thermal Expansion of ConcreteCRD-C 44 Method for Calculation of Thermal Con-

ductivity of Concrete

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rican Concrete Institute S under license with ACI

Notn or networking permitted without license from IHS

Bureau of ReclamationConcrete Manual, Eighth Edition-Revised, Denver, 1981.

The above publications may be obtained from the fol-lowing organizations:

American Concrete InstituteP.O. Box 19150Detroit, MI 48219

ASTM1916 Race St.Philadelphia, PA 19103

U.S. Army Corps of EngineerWaterways Experiment Station3909 Halls Ferry RoadVicksburg, MS 39180

Bureau of ReclamationAttn: D-7923AP.O. Box 25007Denver, CO 80225-0007

7.2-Cited referencesAnderson, Arthur R., “Precast Concrete Panels for

Cladding on Mass Concrete,” Rapid Construction ofConcrete Dams, American Society of Civil Engineers,New York, 1971, pp. 309-311.

Cannon, R. Williams, “Concrete Dam ConstructionUsing Earth Compaction Methods,” Economical Con-struction of Concrete Dams, American Society of CivilEngineers, New York, 1972, pp. 143-152.

Carlson, Roy W., and Thayer, Donald P., “SurfaceCooling of Mass Concrete to Prevent Cracking,” ACIJOURNAL, Proceedings V. 56, No. 2, Aug. 1959, pp. 107.120.

Carlson, Roy W., “Manual for the Use of StrainMeters and Other Instruments for Embedment in Con-crete Structures,” Carlson Instruments, Campbell, Cali-fornia, 1970, 24 pp.

“Cooling Concrete Mixes with Liquid Nitrogen,” Con-crete Construction, V. 22, No. 5, May 1977, pp. 257-258.

Ditchey, E. and Schrader, E., “Monkesville Dam Tem-perature Studies,” International Congress on Large Dams,Par 15, Q.62, R.21, 1988, pp. 379-396.

Fitzgibbon, Michael E., “Thermal Controls for LargePours,” Civil Engineering and Public Works Review (Lon-don), V. 68, No. 806, Sept. 1973, pp. 784-785.

“The Fontana Project, Tennessee Valley AuthorityTechnical Report No. 12, U.S. Government PrintingOffice, Washington, D.C., 1950.

Forbes, B.A., Gillon, B.R., and Dunstun, T.G.,“Cooling of RCC and Construction Techniques for NewVictoria Dam, Australia,” Proceedings, InternationalSymposium on Roller-compacted Concrete Dams, Bei-jing, 1991, pp. 401-408.

for Resale

207.4R-22 ACI COMMlTTEE REPORT

Copyright AmProvided by No reproduc

Gamer, S., and Hammons, M., “Development andImplementation of Tune-Dependent Cracking MaterialModel for Concrete,” Technical Report SL-91-7, USCAEWaterways Experiment Station, Vicksburg, MS, 1991, 44pp.

Houghton, D.L., “Determining Tensile Strain Capacityof Mass Concrete, ACI JOURNAL, Proceedings V. 73, No.12, Dec. 1976, pp. 691-700.

Houk, Ivan E., Jr.; Paxton, James A; and Houghton,Donald L., “Prediction of Thermal Stress and StrainCapacity of Concrete by Tests on Small Beams,” ACIJOURNAL, Proceeding V. 67, No. 3, Mar. 1970, pp.253-261.

Liu, Tony C., and McDonald, James E., “Prediction ofTensile Strain Capacity of Mass Concrete,” ACIJOURNAL, Proceedings V. 75, No. 5, May 1978, pp.192-197.

Mead, AR., “Temperature-Instrumentation Observa-tions at Pine Flat and Folsom Dams,” Symposium onMass Concrete, SP-6, American Concrete Institute,Detroit, 1963, pp. 151-178.

Price, Walter H., “Admixtures and How They Devel-oped,” Concrete Construction, V. 21, No. 4, Apr. 1976, pp.159-162.

erican Concrete Institute IHS under license with ACItion or networking permitted without license from IHS

Schrader, Ernest K, “Control Heat for Better Con-crete,” Concrete Construction, Sept. 1987, pp. 767-770.

Tatro, Stephen B., and Schrader, Ernest K., “ThermalConsiderations for Roller-Compacted Concrete,” ACIJOURNAL, Proceedings V. 82, No. 2, Mar.-Apr. 1985, pp.119-128.

Townsend, C.L., “Control of Cracking in Mass Con-crete Structures,” Engineering Monograph No. 34, U.S.Bureau of Reclamation, Denver, 1965.

Tuthill, Lewis, and Adams, Robert F., “Cracking Con-trolled in Massive, Reinforced Structural Concrete byApplication of Mass Concrete Practices,” ACI JOURNAL,Proceedings V. 69, No. 8, Aug. 1972, pp. 481-491.

Waddell, Joseph J., Concrete Construction Handbook,2nd Edition, McGraw-Hill Book Co., New York, 1974,Chapter 20.

Wallace, G.B., “Insulation Facilitates Winter Con-crete,” Engineering Monograph No. 22, U.S. Bureau ofReclamation, Denver, Oct. 1955.

Wilson, Edward L., “The Determination of Temper-ature Within Mass Concrete Structures,” Structures andMaterial Research Report No. 68-17, Department of CivilEngineering, University of California, Berkeley, Dec.1968, 33 pp.

ACI 207.4R-93 was submitted to letter ballot of the committee and processedin accordance with ACI standardization procedures.

Not for Resale

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