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BY JOHN GAJDA AND MARTHA VANGEEM

pecifications generally limit temperatures in massconcrete to prevent cracking and durability prob-

lems. Temperature limits are specified to seeminglyarbitrary values of 135 F (57 C) for the maximumallowable concrete temperature and 35 F (19 C) for themaximum allowable temperature difference betweenthe center and surface of the mass concrete section.Typically, the contractor must meet all of the specificationrequirements, but without a good understanding of massconcrete, keeping concrete temperatures within limitscan be a difficult task.

Often, well-established temperature control measuresare sufficient to meet project specifications. However, ifthese are overlooked or poorly understood, they couldresult in concrete temperatures and temperature differ-ences that greatly exceed specified limits, causing delaysto the construction schedule or damage to the concrete.Additionally, recent trends, such as increasing the size ofconcrete sections and requiring high minimum cementcontents or low water-cementitious materials ratios,make temperature control even more difficult.

Understanding mass concrete is the key to controllingtemperatures and ultimately saving time, effort, and money.

����������Temperatures

�� �����������Understanding mass concrete is the key to controlling temperatures

and ultimately saving time, effort, and money

�������������������A question often arises as to exactly what is considered

to be mass concrete. According to ACI 116R,1 mass concreteis defined as “any volume of concrete with dimensionslarge enough to require that measures be taken to copewith generation of heat from hydration of the cement andattendant volume change, to minimize cracking.”

Because this definition doesn’t provide a specificmeasure, many agencies have developed their owndefinitions of mass concrete. For example, mass con-crete is defined by some agencies as “any concreteelement having a least dimension greater than 3 ft(0.9 m).” Under this definition, a large mat foundationwith a thickness of 3 ft (0.9 m) would not be consideredmass concrete, but a large mat foundation with a thick-ness of 3.25 ft (1 m) would be considered mass concrete.

Other agencies use different minimum dimensions,ranging from 1.5 to 6.5 ft (0.46 to 2.0 m), depending onpast experience. Note that none of these definitionsconsiders the cementitious material content of theconcrete. Temperatures within a concrete element will bemuch different if high-performance or high-early-strengthconcrete is used rather than typical structural concrete.

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For lack of a standard definition, we consider massconcrete to be any element with a minimum dimensionequal to or greater than 3 ft (0.9 m). Similar consider-ations should be given to other concrete elements thatdo not meet this definition but contain Type III cement orcementitious materials in excess of 564 lb/yd3 (335 kg/m3)of concrete. In many cases, these non-mass elements willalso generate significant amounts of heat.

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Maximum allowable concrete temperatures andtemperature differences are often specified to ensurethat proper planning occurs prior to concrete place-ment. In many cases, the specified limits are seeminglyarbitrary and do not consider project specifics. As anexample of this, certain project specifications limit themaximum concrete temperature to 135 F (57 C), and limitthe maximum concrete temperature difference to 35 F(19 C). Other restrictions are often included, such aslimits on the maximum and minimum temperatures ofdelivered concrete.

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The maximum concrete temperature is limited for avariety of reasons. The primary reason is to preventdamage to the concrete. Studies have shown that thelong-term durability of certain concretes can be compro-mised if the maximum temperature after placementexceeds the range of 155 to 165 F (68 to 74 C). Theprimary damage mechanism is delayed ettringite forma-tion (DEF). DEF can cause internal expansion andcracking of concrete, which may not be evident forseveral years after placement.

Other reasons to limit the maximum concretetemperature include reducing cooling times andassociated delays, and minimizing the potential forcracking due to thermal expansion and contraction.Temperatures over 190 F (88 C) can also reduceexpected compressive strengths.

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A maximum allowable concrete temperature differenceis often specified to minimize the potential for thermal

cracking. This temperature difference is the differencebetween the temperature at the hottest portion of theconcrete and that at the surface. Thermal cracking willoccur when contraction due to cooling at the surfacecauses tensile stresses that exceed the tensile strengthof the concrete.

A maximum allowable temperature difference of 35 F(19 C) is often specified in contract documents. Thistemperature difference is a general guideline based onexperience with unreinforced mass concrete placed inEurope more than 50 years ago.

In many situations, limiting the temperature differenceto 35 F (19 C) is overly restrictive; thermal cracking maynot occur even at higher temperature differences. In othercases, significant thermal cracking may still occur evenwhen the temperature difference is less than 35 F (19 C).

The maximum allowable temperature difference is afunction of concrete mechanical properties such asthermal expansion, tensile strength, and elastic modulus,as well as the size and restraints of the concrete element.ACI 207.2R2 provides guidance on calculating the maxi-mum allowable temperature difference to preventthermal cracking based on the properties of the concreteand for a specific structure.

Figure 1 compares the typically specified 35 F (19 C)maximum allowable temperature difference with that ofthe calculated maximum allowable temperature differ-ence for a specific mat foundation. As the concretereaches its design strength, the calculated maximumallowable temperature difference is much greater than 35F (19 C). Use of the calculated maximum allowabletemperature difference can significantly reduce theamount of time that protective measures, such as surfaceinsulation, must be kept in place.

�����������������������������Specifications for mass concrete often require particular

cement types, minimum cement contents, and maximumsupplementary cementitious material contents. Oncethis information is available, the process of predictingmaximum concrete temperatures and temperaturedifferences can begin. Several options are available topredict maximum concrete temperatures.

Fig. 1: Comparison of maximum allowable temperature differences

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All concretes generate heat as the cementitiousmaterials hydrate. Most of this heat generationoccurs in the first days after placement.

For thin items such as pavements, heat dissipatesalmost as quickly as it is generated. For thickerconcrete sections (mass concrete), heat dissipatesmore slowly than it is generated. The net result isthat mass concrete can get hot.

Management of these temperatures is necessaryto prevent damage, minimize delays, and meetproject specifications.

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A simplistic method is briefly described in a PCAdocument.3 This method is useful if the concrete containsbetween 500 and 1000 lbs of cement per cubic yard ofconcrete (297 and 594 kg/m3) and the minimum dimensionis greater than 6 ft (1.8 m). For this approximation, every100 lbs (45 kg) of cement increases the temperature of theconcrete by 12.8 F (7 C). Using this method, the maximumconcrete temperature of a concrete element that contains900 lb of cement per cubic yard (534 kg/m3) and is cast at60 F (16 C) is approximately 175 F (79 C). This is above thesafe limit for controlling DEF formation. If such a concretewas used, the initial concrete temperature would have tobe reduced to 45 F (7 C) to ensure that the maximumconcrete temperatures do not exceed 160 F (71 C). ThisPCA method also does not consider surface temperaturesor supplementary cementitious materials.

A more precise method, known as Schmidt’s methodand described in ACI 207.1R,4 can be used to predictmaximum temperatures and temperature differences formany concrete mix designs and a variety of conditions.CTL staff developed and utilize software based on thismethod, and have validated it using field calibrationsand 12 years of experience. The software can be used topredict maximum concrete temperatures and tempera-ture differences for any concrete mix proportion underalmost any placement condition with any commonlyutilized means of temperature control. The software iscapable of 1-, 2-, and 3-dimensional analyses, dependingon the geometry of the structure being analyzed.

����������������������������Methods of controlling mass concrete temperatures

range from relatively simple to complex, and from inexpen-sive to costly. Depending on a particular situation, it maybe advantageous to use one or more methods over another.

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Different types of cement (and cements within eachtype) generate varying amounts of heat. Figure 2 presentsthe typical heats of hydration of different cement types.Type IV cement is not shown because it is rarely available.

Low-heat generating concrete mixtures are always awise choice for mass concrete to minimize potentialthermal problems. Low-heat generating concrete mixesuse the maximum allowable level of low-heat pozzolans—such as Class F fly ash or slag—as cement replacements,and the minimum amount of total cementitious materialsthat achieves the project requirements. Class F fly ashgenerates about half as much heat as the cement that itreplaces and is often used at a replacement rate of 15 to25%. Ground granulated blast-furnace slag is often usedat a replacement rate of 65 to 80% to reduce heat. Thereduction in heat generation achieved depends on theconcrete temperature, and should be evaluated on acase-by-case basis.

Figure 3 illustrates the effect of Class F fly ash anddifferent cement types on the adiabatic temperature riseof concrete. This is the theoretical increase in tempera-ture of the concrete above the placement temperature,

if the concrete is not allowed to cool. In Fig. 3, the totalquantity of cementitious materials for all mixes is525 lb/yd3 (311 kg/m3) of concrete.

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The concrete temperature at the time of placementhas a great impact on the maximum concrete tempera-ture. Typically, for every 1 F (0.6 C) reduction or increasein the initial concrete temperature, the maximumconcrete temperature is changed by approximately 1 F(0.6 C). As an example, to reduce the maximum concretetemperature by approximately 10 F (6 C), the concretetemperature at the time of placement should generallybe reduced by 10 F (6 C).

Methods to precool concrete include shading andsprinkling of aggregate piles (as appropriate), use ofchilled mix water, and replacement of mix water by ice.Efforts to cool aggregates have the most pronouncedeffects on the concrete temperature because theyrepresent 70 to 85% of the weight of the concrete.

Liquid nitrogen can also be used to precool concreteor concrete constituents. This option can significantlyincrease the cost of concrete; however, it has been usedto successfully precool concrete to 34 F (1 C) for highlyspecialized mass concrete placements.

Fig. 2: Heat of hydration for typical cements

Fig. 3: Concrete temperatures of mixes with 525 lb/yd3 ofcementitious materials

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ACI member John GJohn GJohn GJohn GJohn Gajdajdajdajdajdaaaaa is a licensedprofessional engineer and senior engineerfor Construction Technology Laboratories(CTL) in Skokie, Ill. He received hismaster’s and bachelor’s degrees ofengineering from Iowa State University.Gajda has been involved with more than75 mass concrete projects throughout theU.S. over the past nine years.

ACI member MMMMMarararararthththththa a a a a VVVVVanGeemanGeemanGeemanGeemanGeem is a licensedprofessional engineer and principal engineerfor Construction Technology Laboratories(CTL) in Skokie, Ill. She received herbachelor’s degree of engineering from theUniversity of Illinois and her MBA from theUniversity of Chicago. In her 19 years at CTL,VanGeem has worked in the areas of heattransfer and thermal properties of concrete.During the past 12 years, she has beeninvolved with most major mass concretebridge projects in the U.S.

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Cooling pipes in mass concrete are sometimes used toreduce maximum concrete temperatures and to quicklyreduce interior temperatures. This method can havehigh initial and operating costs, but benefits can oftenoutweigh these costs if cooling pipe size, spacing, andtemperatures are optimized properly.

Figure 4 illustrates the reduction in the averagetemperature of a mass concrete pour with and withoutinternal cooling pipes. Note the reduction in the maxi-mum concrete temperature and the increased rate ofcooling. It is important to emphasize again that signifi-cant internal and surface thermal cracking can result ifpost-cooling is improperly designed or performed.However, if properly designed, a post-cooling system cansignificantly reduce concrete temperatures and theamount of time required for cooling.

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Insulation or insulated formwork is often used towarm the concrete surface and reduce the temperaturedifference, which in turn minimizes the potential forthermal cracking. For most mass pours, surface insula-tion does not appreciably increase the maximum concretetemperature, but it can significantly decrease the rate ofcooling. Insulation is inexpensive, but resulting delaysfrom the reduced cooling rate can be costly. Insulationoften has to remain in place for several weeks or longer.Removing it too soon can cause the surface to coolquickly and crack.

Many types of insulation materials are available, andinsulation levels can be optimized to meet requiredtemperature differences and maximize the rate of cooling.

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Thermal properties of the coarse aggregate can have asignificant effect on mass concrete. Concretes containinglow-thermal-expansion aggregates such as granite andlimestone generally permit higher maximum allowabletemperature differences than concretes made using high-thermal-expansion aggregates, as shown in Fig. 5 (Fig. 5is similar to Fig. 1, except a second calculated maximumallowable temperature difference is added for concrete

with a high-thermal-expansion aggregate). This meansthat selecting an aggregate with a low thermal expansionwill reduce the potential for thermal cracking.

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1. ACI Committee 116, “Cement and Concrete Terminology (ACI116R-00),” American Concrete Institute, Farmington Hills, Mich.,2000, 73 pp.

2. ACI Committee 207, “Effect of Restraint, Volume Change, andReinforcement on Cracking of Mass Concrete (ACI 207.2R-95),”American Concrete Institute, Farmington Hills, Mich., 2000, 26 pp.

3. Portland Cement Association, Design and Control of ConcreteMixtures, 13th Edition, Skokie, Ill., 1988, 212 pp.

4. ACI Committee 207, “Mass Concrete (ACI 207.1R-96),” AmericanConcrete Institute, Farmington Hills, Mich., 1996, 42 pp.

Selected for reader interest by the editors after independent expertevaluation and recommendation.

Fig. 5: Effect of aggregate on the maximum allowable temperaturedifference

Fig. 4: Effects of internal cooling pipes