guide to hot weather concreting · 2020. 5. 12. · aci 207.1r, 207.2r, and 224r. chapter...

ACI 305R-10

Reported by ACI Committee 305

Guide to Hot Weather Concreting

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First PrintingOctober 2010

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ACI 305R-10 supersedes ACI 305R-99 and was adopted and published October 2010.Copyright © 2010, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

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Guide to Hot Weather ConcretingReported by ACI Committee 305

ACI 305R-10

Environmental factors, such as high ambient temperature, low humidity,high wind, or both low humidity and high wind, affect concrete propertiesand the construction operations of mixing, transporting, and placing of theconcrete materials. This guide provides measures that can be taken tominimize the undesirable effects of these environmental factors and reducethe potential for serious problems.

This guide defines hot weather, discusses potential problems, and presentspractices intended to minimize them. These practices include selectingmaterials and proportions, precooling ingredients, and batching. Othertopics discussed include length of haul, consideration of concrete temperatureas placed, facilities for handling concrete at the site, and, during the earlycuring period, placing and curing techniques, and appropriate testing andinspection procedures in hot weather conditions.

The materials, processes, quality control measures, and inspectionsdescribed in this document should be tested, monitored, or performed asapplicable only by individuals holding the appropriate ACI certificationsor equivalent.

Keywords: air entrainment; cooling; curing; evaporation; high tempera-ture; hot weather construction; plastic shrinkage; production methods;retempering; slump tests; water content.

CONTENTSChapter 1—Introduction and scope, p. 2

1.1—Introduction1.2—Scope

Chapter 2—Notation and definitions, p. 22.1—Notation2.2—Definitions

Chapter 3—Potential problems and practices, p. 33.1—Potential problems in hot weather3.2—Potential problems related to other factors3.3—Practices for hot weather concreting

Chapter 4—Effects of hot weather on concrete properties, p. 3

4.1—General4.2—Estimating evaporation rate4.3—Temperature of concrete4.4—Ambient conditions4.5—Water4.6—Cement4.7—Supplementary cementitious materials4.8—Chemical admixtures4.9—Aggregates4.10—Proportioning

James M. Aldred Darrell F. Elliot Frank A. Kozeliski Bruce G. Smith

Godwin Q. Amekuedi Michael Faubel Darmawan Ludirdja Edward G. Sparks

Philip Brandt Richard D. Gaynor David R. Nau Boris Y. Stein

D. Gene Daniel Antonio J. Guerra Dan Ravina Louis R. Valenzuela

Kirk K. Deadrick Kenneth C. Hover Robert J. Ryan

James N. Cornell IIChair

G. Terry Harris, Sr.Secretary



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2 GUIDE TO HOT WEATHER CONCRETING (ACI 305R-10)

American Concrete Institute Copyrighted Material—www.concrete.org

Chapter 5—Production and delivery, p. 145.1—General5.2—Temperature control of concrete5.3—Batching and mixing5.4—Delivery5.5—Slump adjustment5.6—Properties of concrete mixtures5.7—Retempering

Chapter 6—Placing and curing, p. 166.1—General6.2—Preparations for placing and curing6.3—Placement and finishing6.4—Curing and protection

Chapter 7—Testing and inspection, p. 197.1—Testing7.2—Inspection

Chapter 8—References, p. 208.1—Referenced standards and reports8.2—Cited references

Appendix A—Estimating concrete temperature,pp. 22

A.1—Estimating temperature of freshly mixed concreteA.2—Estimating temperature of concrete with ice

Appendix B—Methods for cooling fresh concrete, p. 23

B.1—Cooling with chilled mixing waterB.2—Liquid nitrogen cooling of mixing waterB.3—Cooling concrete with iceB.4—Cooling mixed concrete with liquid nitrogenB.5—Cooling of coarse aggregates

CHAPTER 1—INTRODUCTION AND SCOPE1.1—Introduction

Hot weather can create problems in mixing, placing, andcuring hydraulic-cement concrete that adversely affect theproperties and serviceability of the concrete. Most of theseproblems relate to the increased rate of cement hydration athigher temperature and increased evaporation rate of moisturefrom the freshly mixed concrete. The rate of cement hydrationdepends on ambient and concrete temperature, cementcomposition and fineness, amount and type of supplementarycementitious materials, and admixtures used.

A maximum as-placed concrete temperature is oftenspecified in an effort to control rate of setting, strength,durability, plastic shrinkage cracking, thermal cracking, anddrying shrinkage. The placement of concrete in hot weather,however, is too complex to be dealt with by setting amaximum as-placed or as-delivered concrete temperature.Concrete durability is defined as the ability of concrete toresist weathering action, chemical attack, abrasion, or anyother process of deterioration (ACI 201.2R). Generally, ifconcrete strengths are satisfactory and curing practices aresufficient to avoid undesirable drying of surfaces, the durability

of hot weather concrete will not differ greatly from similarconcrete placed at normal temperatures.

Where an acceptable record of field tests is not available,concrete proportions can be determined by trial batches (ACI301 and 211.1). Trial batches should be made at temperaturesanticipated in the work and mixed following one of theprocedures described in Section 4.10, Proportioning. Theconcrete supplier is generally responsible for determiningconcrete proportions to produce the required quality ofconcrete unless specified otherwise.

If the initial 24-hour curing is at 100°F (38°C), the 28-daycompressive strength of the test specimens may be 10 to 15%lower than if cured at the required ASTM C31/C31M curingtemperature (Gaynor et al. 1985). If the cylinders areallowed to dry at early ages, strengths will be reduced evenfurther (Cebeci 1987). Therefore, proper curing of the testspecimens during hot weather is critical, and steps should betaken to ensure that the specified procedures are followed.

The effects of high air temperature and low relativehumidity are more pronounced with increases in wind speed.The potential problems of hot weather concreting can occurat any time of the year, but generally occur during thesummer season. Drying conditions can occur even at lowerambient temperatures, with slower set times, lower relativehumidity, and wind, all of which are conducive to higherevaporation. Precautionary measures required on a windy,sunny day will be stricter than those required on a calm,humid day, even if air temperatures are identical.

1.2—ScopeThis guide identifies problems associated with hot weather

concreting and describes practices that alleviate these potentialadverse effects. These practices include suggested preparationsand procedures for use in general types of hot weatherconstruction, such as pavements, bridges, and buildings.Temperature, volume changes, and cracking problemsassociated with mass concrete are treated more thoroughly inACI 207.1R, 207.2R, and 224R.

CHAPTER 2—NOTATION AND DEFNITIONS2.1—NotationE = evaporation rate, lb/ft2/h (kg/m2/h)ea = water vapor pressure in mmhg (psi) in the air

surrounding the concrete obtained by multiplyingthe saturation vapor pressure at the temperature ofthe air surrounding the concrete by the relativehumidity of the air. Air temperature and relativehumidity are measured approximately 1.2 to 1.8 m(4 to 6 ft) above the concrete surface on thewindward side and shielded from the sun’s rays

eo = saturation water vapor pressure in mmhg (psi) inthe air immediately over the concrete surface, at theconcrete temperature. Obtain eo from Table 4.2(a)or (b)

es = saturation vapor pressure, kPa (psi)r = (relative humidity percent)/100T = temperature, °C (°F)Ta = air temperature, °F (°C)



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GUIDE TO HOT WEATHER CONCRETING (ACI 305R-10) 3


Tc = concrete (water surface) temperature, °F (°C)V = average wind speed in km/h (mph), measured at

0.5 m (20 in.) above the concrete surfaceW = mass of water evaporated in kg (lb) per m2 (ft2) of

water-covered surface per hour

2.2—DefinitionsACI provides a comprehensive list of definitions through

an online resource, “ACI Concrete Terminology,” http://terminology.concrete.org. The definition provided hereincomplements that resource.

hot weather—one or a combination of the followingconditions that tends to impair the quality of freshly mixedor hardened concrete by accelerating the rate of moistureloss and rate of cement hydration, or otherwise causingdetrimental results: high ambient temperature; high concretetemperature; low relative humidity; and high wind speed.

CHAPTER 3—POTENTIAL PROBLEMSAND PRACTICES

3.1—Potential problems in hot weatherPotential problems for concrete in the freshly mixed state

include:• Increased water demand;• Increased rate of slump loss and corresponding

tendency to add water at the job site;• Increased rate of setting, resulting in greater difficulty

with handling, compacting, and finishing, and a greaterrisk of cold joints;

• Increased tendency for plastic shrinkage and thermalcracking; and

• Increased difficulty in controlling entrained air content.Damage to concrete caused by hot weather can never be

fully alleviated. Potential deficiencies to concrete in thehardened state can include:• Decreased strengths resulting from higher water demand;• Increased tendency for drying shrinkage and differential

thermal cracking from either cooling of the overallstructure, or from temperature differentials within thecross section of the member;

• Decreased durability resulting from cracking; and• Greater variability of surface appearance, such as cold

joints or color difference, due to different rates ofhydration or different water-cementitious materialratios (w/cm).

3.2—Potential problems related to other factorsOther factors that should be considered along with

climatic factors include:• Cements with different and increased rate of hydration;• High-early-compressive-strength concrete, which requires

higher cement contents;• Thin concrete sections with correspondingly greater

percentages of steel, which complicate placing andconsolidation of concrete;

• Economic necessity to continue work in extremely hotweather; and

• Use of shrinkage-compensating cement.

3.3—Practices for hot weather concretingGood judgment is necessary to select procedures that

appropriately blend quality, economy, and practicability.The procedures selected will depend on type of construction,characteristics of the materials being used, and the experienceof the local industry in dealing with high ambient temperature,high concrete temperatures, low relative humidity, and highwind speed.

The most serious difficulties occur when personnelplacing the concrete lack experience in constructing underhot weather conditions or in doing the particular type ofconstruction. Last-minute improvisations are rarelysuccessful. Early preventive measures should be appliedwith the emphasis on materials evaluation, advanced planningand purchasing, and coordination of all phases of work.Planning in advance for hot weather involves detailedprocedures for mixing, placing, protecting, curing, andtesting of concrete. Precautions to avoid plastic shrinkagecracking are important. The potential for thermal cracking,either from overall volume changes or from internalrestraint, should be anticipated to be properly assessed.

Typical methods to minimize and to limit crack size andspacing include: proper use and timely installation of joints,increased amounts of reinforcing steel, practical limits onconcrete temperature, reduced cement content, low-heat-of-hydration cement, and selection and dosage of appropriatechemical and mineral admixtures.

Developing a comprehensive plan and procedures for usein hot weather concreting conditions include the followingpractices and measures used to reduce or avoid the potentialproblems of hot weather concreting as discussed in detail inChapters 4, 5, and 6:• Selecting concrete materials and proportions with

satisfactory records in hot weather conditions;• Reducing and controlling the temperature of fresh

concrete;• Using a concrete consistency that permits rapid placement

and effective consolidation;• Minimizing the time to transport, place, consolidate,

and finish the concrete;• Scheduling of placing operations during times of the

day or night when weather conditions are favorable;• Protecting the concrete from moisture loss during

placing and curing periods; and• Scheduling a preplacement conference to discuss the

requirements of hot weather concreting.

CHAPTER 4—EFFECTS OF HOT WEATHER ON CONCRETE PROPERTIES

4.1—GeneralProperties of concrete that make it an excellent construction

material can be affected adversely by hot weather. Harmfuleffects are minimized by procedures outlined in this guide.Rate of setting, strength, permeability, dimensional stability,and resistance of the concrete to weathering, wear, andchemical attack all depend on the selection and propercontrol of materials and mixture proportioning; initial



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concrete temperature; wind speed; ambient temperature; andhumidity condition during the placing and curing periods.

4.1.1 Effect on strength—Concrete mixed, placed, andcured at elevated temperatures normally develops higherearly strengths than concrete produced and cured at lowertemperatures, but strengths are generally lower at 28 daysand later ages. Figure 4.1 shows that with increasing curingtemperatures, 1-day strength increases, and 28-day strengthdecreases (Klieger 1958; Verbeck and Helmuth 1968). Someresearchers conclude that a relatively more uniformmicrostructure of the hydrated cement paste can account forhigher strength of concrete mixtures cast and cured at lowertemperatures (Mehta 1986).

4.1.2 Effect of proper curing—Laboratory tests havedemonstrated the adverse effects of high temperatures with a

lack of proper curing on concrete strength (Bloem 1954).The longer the delay between casting of the cylinders andplacing them into standard moist storage, the greater thestrength reduction. The data illustrate that inadequate curingcombined with high placement temperatures impairs thehydration process and reduces strength. The tests were madeon concrete without admixtures or pozzolans that might haveimproved its performance at elevated temperatures. Otherresearchers determined that improper curing is moredetrimental than high temperatures (Cebeci 1986), and alsothat required strength levels can be maintained by the properuse of either chemical or mineral admixtures (Gaynor et al.1985; Mittelacher 1985, 1992).

4.1.3 Effect of surface drying—Plastic shrinkage crackingis frequently associated with hot weather concreting in aridclimates. It occurs in exposed concrete, primarily in flat-work, but also in beams and footings, and can develop inother climates when the surface of freshly cast concrete driesand subsequently shrinks. Surface drying is initiated when-ever the evaporation rate is greater than the rate at whichwater rises to the surface of recently placed concrete. Highconcrete temperatures, high wind speed, low humidity, or acombination of these, can cause rapid evaporation of surfacewater. The rate of bleeding, on the other hand, depends onconcrete mixture ingredients and proportions, on the depthof the member being cast, and on the type of consolidation.Because surface drying is initiated when the evaporation rateexceeds the bleeding rate, the probability of plasticshrinkage cracking increases whenever the environmentalconditions increase evaporation or when the concrete has areduced bleeding rate. For example, concrete mixtures thatincorporate fly ash, silica fume, or fine cements frequentlyhave a low to negligible bleeding rate, making suchmixtures highly sensitive to surface drying and plasticshrinkage, even under moderately evaporative conditions(ACI 234R).

4.1.4 Effect of evaporation—Plastic shrinkage cracking isseldom a problem in hot and humid climates where relativehumidity is rarely less than 80%. Table 4.1 shows, forvarious relative humidities, the concrete temperatures thatmay result in critical evaporation rate levels, and therefore

Fig. 4.1—Effects of curing temperature on compressivestrength of concrete (Verbeck and Helmuth 1968).

Table 4.1—Sample relationship between concrete temperatures and critical relative humidity

Concretetemperature, °F (°C)

Airtemperature, °F (°C)

Evaporation rate

0.2 lb/ft2/h(1.0 kg/m2/h)

0.15 lb/ft2/h (0.75 kg/m2/h)

0.1 lb/ft2/h(0.5 kg/m2/h)

0.05 lb/ft2/h(0.25 kg/m2/h)

Relative humidity, %*

105 (41) 95 (35) 85 100 100 100

100 (38) 90 (32) 80 95 100 100

95 (35) 85 (29) 75 90 100 100

90 (32) 80 (27) 60 85 100 100

85 (29) 75 (24) 55 80 95 100

80 (27) 70 (21) 35 60 85 100

75 (24) 65 (19) 20 55 80 100*Relative humidity, percent at which percent at evaporation rate will exceed the critical values shown, assuming air temperatureis 10°F (6°C) cooler than the concrete temperature and a constant wind speed of 10 mph (16 km/h), measured at 20 in. (0.5 m) abovethe evaporating surface. Note: Based on Fig. 4.2; results rounded to nearest 5%.



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increase the probability of plastic shrinkage cracking. Thetable is based on the assumption of a 10 mph (16 km/h) windspeed and an air temperature of 10°F (6°C) cooler than theconcrete temperature.

Figure 4.2 is the National Ready Mixed Concrete Association(NRMCA)-Portland Cement Association (PCA) nomographthat is based on common hydrological methods for estimatingthe rate of evaporation of water from lakes and reservoirs,

Fig. 4.2—Effect of concrete and air temperatures, relative humidity, and wind speed onthe rate of evaporation of surface moisture from concrete. This chart provides a graphicmethod of estimating the loss of surface moisture for various weather conditions. To usethis chart, follow the four steps outlined above. If the rate of evaporation approaches0.2 lb/ft2/h (1 kg/m2/h), precautions against plastic shrinkage cracking are necessary(Lerch 1957). Wind speed is the average horizontal air or wind speed in mph (km/h), andshould be measured at a level approximately 20 in. (0.5 m) higher than the evaporatingsurface. Air temperature and relative humidity should be measured at a level approximately4 to 6 ft (1.2 to 1.8 m) higher than the evaporating surface on its windward side shieldedfrom the sun’s rays (Lerch 1957).



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and is therefore the most accurate when estimating the rateof evaporation from the surface of concrete while thatsurface is covered with bleedwater. When the concretesurface is not covered with bleedwater, the nomograph andits underlying mathematical expression tends to overestimatethe actual rate of water loss from the concrete surface by asmuch as a factor of 2 or more (Al-Fadhala and Hover 2001).The method is therefore most useful in estimating theevaporation potential of the ambient conditions and not as anestimator of the actual rate of water loss from the concrete.Early in the bleeding process, however, and at rates of evap-oration less than or equal to 0.2 lb/ft2/h (1.0 kg/m2/h), themethod has been shown to be in good agreement with waterloss measurements as long as the temperature, humidity, andwind speed were measured as described in the text belowFig. 4.2.

It is critical that wind speed be monitored at 20 in. (0.5 m)above the evaporating surface because wind speed increasesrapidly with height above the surface, and wind measurementstaken from higher than the prescribed height used in developingthe nomograph will overestimate evaporation rate. It shouldalso be noted that wind speed varies tremendously over time,and estimates should not be based on transient gusts of wind.

Figure 4.2 provides evaporation rate estimates based onenvironmental factors of temperature, humidity, and windspeed that contribute to plastic shrinkage cracking. Thegraphic method of the chart also yields ready information onthe effect of changes in one or more of these factors. Forexample, it shows that concrete at a temperature of 70°F(21°C) placed at an air temperature of 70°F (21°C), with arelative humidity of 50% and a moderate wind speed of 10mph (16 km/h), will have six times the evaporation rate ofthe same concrete placed when there is no wind.

4.1.5 Effect of bleeding—When evaporation rate isexpected to approach the bleeding rate of the concrete,precautions should be taken, as explained in detail in Chapter 6.Because bleeding rates vary from zero to over 0.2 lb/ft2/h(1.0 kg/m2/h) over time and are not normally measured, it iscommon to assume a value for the critical rate of evaporation.The most commonly quoted value is 0.2 lb/ft2/h (1.0 kg/m2/h).More recent experience with bridge deck overlays containingsilica fume has led to specified allowable evaporation ratesof only 0.05 lb/ft2/h (0.25 kg/m2/h) (Virginia Department ofTransportation 1997; Krauss and Rogalla 1996). Construc-tion specifications for the State of New York and the City ofCincinnati assume intermediate evaporation rates of 0.15and 0.10 lb/ft2/h (0.75 and 0.50 kg/m2/h), respectively. Theprobability for plastic shrinkage cracks to occur can beincreased if the setting time of the concrete is delayed due tothe use of slow-setting cement, an excessive dosage ofretarding admixture, fly ash as a cement replacement, orcooled concrete. Fly ash is also likely to reduce bleeding andcan thereby contribute to an increase in cracking tendency(ACI 232.2R). In such instances, it is prudent to extend thetime where countermeasures are employed to reduce evapora-tive conditions above the concrete. These countermeasuresmay include the use of monomolecular films, fogging, wind-breaks, shading, and others as discussed in subsequent

sections herein. Plastic shrinkage cracks are difficult to closeonce they have occurred (refer to Section 6.3.4).

4.2—Estimating evaporation rateIn Fig. 4.2, the Menzel equation is used to quantify the

severity of evaporative exposure. As documented by Uno(1998) and in more detail by Hover (2006), Menzel (1954)should be credited for finding the equation in the hydrologicliterature and bringing it (exactly as he found it) to the atten-tion of the concrete industry. Kohler (1952, 1954) andKohler et al. (1955) actually developed the equation on thebasis of studies of the evaporation of water from LakeHefner near Oklahoma City.

To use the Menzel equation, a value for the saturationvapor pressure of water at the temperature of the concrete isneeded. This is the pressure exerted by water vapor at 100%relative humidity at temperature T, when T is the same as thetemperature of the concrete. The bleedwater on the surfaceof the concrete evaporates, and the bleedwater temperature isassumed to be the same as the temperature of the concrete.

The value of saturation water vapor pressure has beentraditionally hard to compute with sufficient accuracy; thus,the saturation water vapor pressures for various temperaturesare given in Tables 4.2(a) and (b).

4.2.1 Precision of computed evaporation rate—Theevaporation rate of Fig. 4.2 includes a built-in plot of thesaturation water vapor pressure values. This is within theaccuracy of the approximate nature of the measurements andthe nature of the evaporation rate equation itself. When itcomes to specified values and issues of compliance versusnoncompliance, it is recognized that the precision of thenomograph could lead to problems in interpretation. Thus,the reference source in the ACI 305.1 specification is theequation and not the nomograph, and for that reason, thespecification includes tables of U.S. customary and SI unitvapor pressures. This leads to several problems:

1. Computing the evaporation rate is made morecumbersome by the need to manually look up orinterpolate table values;

2. It is difficult to develop a sufficiently precise mathematicalequation that would fit all tabular data. There are manyequations for saturation vapor pressure that are accurateover a narrow range of temperatures, but not accurateover the range experienced in concrete construction;

3. Standard tables of vapor pressure are available in SIunits. Conversion to U.S. customary units is a cumber-some process that requires the user to: a) convertdesired temperature °F to °C; b) look up vapor pressurevalues for converted temperature; c) convert interpo-lated values to psi; and d) round off the convertedvalues to number of significant figures in table. Printedvalues can be argued a digit or two either way in the lastdecimal place; and

4. Round-off and units-conversions can significantlyaffect the accuracy of the computed evaporation rate.

4.2.2 Menzel equation using vapor pressure tables—Siteconditions (air temperature, humidity, and wind speed)should be monitored to assess the need for evaporation



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control measures beginning no later than 1 hour before thestart of concrete placing operations. Site conditions shouldbe continually monitored at intervals of 30 minutes or lessuntil the specified curing procedures have been applied.

Equipment or instruments for measuring the rate ofevaporation of surface moisture should be certified by themanufacturer or checked to be accurate within 1.8°F (1°C),within 5% relative humidity, and within 1 mph (1.6 km/h)wind speed, and should be used in accordance with theproduct manufacturer recommendations.

The committee recommends initiating evaporation controlmeasures when concrete (air temperatures, relative humidityof the air, and the wind velocity) has the capacity to evaporate

water from a free water surface at a rate equal to or greaterthan 0.2 lb/ft2/h (1.0 kg/m2/h). To determine the evaporationrate of surface moisture by the Menzel equation

W = 0.315(eo – ea )(0.253 + 0.060V) (for pressure in SI unit of kPa)

W = 0.44(eo – ea )(0.253 + 0.096V) (in.-lb units)

4.2.3 Example of Menzel formula that uses vapor pressuretables—Assume the conditions in Table 4.3.

4.2.3.1 Example with SI units—Using data fromTable 4.2(b):Concrete temperature of 35°C: eo = 5.623 kPaSaturated water vapor pressure of air at 40°C = 7.374 kPaea = 7.374 × 0.45 = 3.3 kPa

(precision of relative humidity value limited to twosignificant digits)

Equation: W = 0.315(eo – ea)(0.253 + 0.060V)Calculation Step 1: W = 0.315(5.623 – (7.374 × 0.45)) (0.253

+ (0.060 × 16))(precision on wind speed is two significant digits)(precision on subtraction of vapor pressures limited totwo significant digits)

Table 4.2(a)—Saturation water vapor pressure (psi) over water (U.S. customary units)

Concretetemperature, °F

Saturationpressure, psi

Concretetemperature, °F

Saturationpressure, psi

40 0.121 81 0.523

41 0.126 82 0.542

42 0.132 83 0.558

43 0.137 84 0.578

44 0.142 85 0.595

45 0.147 86 0.615

46 0.154 87 0.637

47 0.159 88 0.655

48 0.165 89 0.678

49 0.171 90 0.697

50 0.178 91 0.721

51 0.185 92 0.742

52 0.192 93 0.767

53 0.199 94 0.780

54 0.206 95 0.816

55 0.214 96 0.843

56 0.221 97 0.866

57 0.230 98 0.895

58 0.238 99 0.920

59 0.247 100 0.951

60 0.257 101 0.977

61 0.265 102 1.009

62 0.276 103 1.036

63 0.285 104 1.070

64 0.290 105 1.104

65 0.296 106 1.134

66 0.317 107 1.171

67 0.327 108 1.202

68 0.339 109 1.240

69 0.352 110 1.273

70 0.363 111 1.313

71 0.376 112 1.348

72 0.388 113 1.390

73 0.387 114 1.433

74 0.402 115 1.470

75 0.430 116 1.516

76 0.443 117 1.555

77 0.459 118 1.611

78 0.476 119 1.643

79 0.490 120 1.694

80 0.508

Table 4.2(b)—Saturation water vapor pressure (kPa) over water (SI units) (from Weast [1987], converted to kPa)

Concretetemperature, °C

Saturationpressure, kPa

Concretetemperature, °C

Saturationpressure, kPa

4 0.81 25 3.167

5 0.872 26 3.36

6 0.934 27 3.564

7 1 28 3.779

8 1.07 29 4.004

9 1.15 30 4.242

10 1.23 31 4.492

11 1.31 32 4.753

12 1.402 33 5.029

13 1.497 34 5.319

14 1.598 35 5.623

15 1.705 36 5.94

16 1.817 37 6.274

17 1.937 38 6.624

18 2.063 39 6.99

19 2.197 40 7.374

20 2.338 41 7.777

21 2.486 42 8.198

22 2.643 43 8.198

23 2.809 44 9.099

24 2.983 45 9.582

Table 4.3—Example conditions for use with Menzel formulaAir temperature, °F (°C) 104 (40)

Relative humidity, % 45

Concrete temperature, °F (°C) 95 (35)

Wind speed V, mph (km/h) 10 (16)



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Calculation Step 2: W = 0.315(5.623 – 3.318)(0.253 + 0.96)(precision on addition of 0.253 + 0.96 limited to twodecimal places)

Calculation Step 3: W = 0.315(2.3)(1.21)(precision limited to two significant digits by 2.3)

Result: W = 0.88 kg/m2/hIn this example, an evaporation rate of 0.88 kg/m2/h is less

than the specified evaporation rate of free surface water of1.0 kg/m2/h, as listed in Section 4.2.2. This example indicatesthat, although the evaporation rate is approaching the specifiedlimit, measures to reduce the evaporation rate would not berequired by specification. For some mixtures, however, thisevaporation rate could result in plastic shrinkage cracking,which is why the architect/engineer may select a lowerspecified value (for example, 0.75 kg/m2/h) in accordancewith the optional requirements checklist.

4.2.3.2 Example with in.-lb units—Using data fromTable 4.2(a):Concrete temperature of 95°F: eo = 0.816 psiSaturated water vapor pressure of air at 104°F = 1.070 psiea = 1.070 × 0.45 = 0.48 psi

(precision of relative humidity value limited to twosignificant digits)

Equations: W = 0.44 (eo – ea)(0.253 + 0.096V)Calculation Step 1: W = 0.44(0.816 – (1.070 × 0.45))(0.253

+ (0.096 × 10))(precision on wind speed is two significant digits)(precision on subtraction of vapor pressures limited totwo significant digits)

Calculation Step 2: W = 0.44(0.82 – 0.48)(0.253 + 0.96)(precision on addition of 0.253 + 0.96 limited to twodecimal places)

Calculation Step 3: W = 0.44(0.34)(1.21)(precision limited to two significant digits by 0.34 andby empirical constant 0.44)

Result: W = 0.18 lb/ft2/hIn this example, an evaporation rate of 0.18 lb/ft2/h is less

than the value (0.2 lb/ft2/h) as listed in Section 4.2.2. Thisexample indicates that, although the evaporation rate isapproaching the specified limit, measures to reduce the evap-oration rate would not be required by specification. For somemixtures, however, this evaporation rate could result in plasticshrinkage cracking, which is why the architect/engineer mayselect a lower specified value (for example, 0.15 lb/ft2/h) inaccordance with the optional requirements checklist.

4.2.4 Equations for saturation vapor pressure—Uno(1998) reported a reliable equation for saturation vaporpressure as follows

es = 0.61exp (SI units)

es = 0.0885exp (in.-lb units)

These equations have been around for a long time, and areused by the World Meteorological Organization (Tetens

17.3T237.3 T+( )

----------------------------

17.3 T 32–( )

T 395.1+-------------------------------

1930; Murray 1967; Dilley 1968; Mills 1975). The resultsfrom the U.S. customary unit equation were compared withthe result from the present ACI vapor pressure table, asshown in Fig. 4.3. The two curves are indistinguishable, andon average, there is less than 0.3% difference between thetable values and the equation. Given that the table valueswere interpolated, converted, and rounded, it cannot be saidthat the table values are more accurate.

Uno (1998) identified these vapor pressure equations andcombined them with the old Menzel equation to produce aunified equation that takes vapor pressure into account

E = (Tc2.5 – r · Ta

2.5)(1 + 0.4V) × 10–6 (in.-lb. units)

E = 5([Tc + 18]2.5 – r · [Ta + 18]2.5)(V + 4) × 10–6 (SI units)

Uno (1998) presented a table that compares the final,predicted evaporation rate computed by the newer equationand by Menzel for a wide range of differing ambient conditions.The results are shown in Fig. 4.4, where the line x = y is shownfor reference. Given the much greater variability in the measuredinput data and the approximate nature of the calculation asdocumented by Al-Fadhala and Hover (2001), the Unoversion is quite satisfactory, and its use is recommended.

4.3—Temperature of concreteUnless measures are taken to control concrete performance at

elevated temperatures by the selection of suitable materials

Fig. 4.3—Comparison of table data and equation for vaporpressure (Al-Fadhala and Hover 2001). (Note: 1 psi =0.007 MPa; °C = (°F – 32)/59.)



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and proportions, as outlined in Sections 4.4 through 4.10,increases in concrete temperature will have the followingadverse effects:• The amount of the water required to produce a given

slump increases with the time between batch anddischarge at the project site. For a constant mixingtime, the amount of water required to produce a givenslump also increases with the temperature, as shown inFig. 4.5 and 4.6;

• Increased water content creates a decrease in rate ofsetting, strength, and durability if the quantity ofcementitious material is not increased proportionately;

• Slump loss is evident earlier after initial mixing and at amore rapid rate, and can cause difficulties withhandling and placing operations;

• In an arid climate, plastic shrinkage cracks are moreprobable;

• In sections of large dimensions, there is an increasedrate of hydration and heat evolution that increases thedifference in temperature between the interior and theexterior concrete. This can cause thermal cracking(ACI 207.1R); and

• Application of early curing methods is critical; a lackof proper curing is increasingly detrimental as temper-atures rise.

4.4—Ambient conditionsGenerally, in hot weather construction it is impractical to

recommend a single maximum ambient or concrete temper-ature; for example, if the humidity and wind speed are low,higher ambient and concrete temperatures are permitted. Amaximum ambient or concrete temperature that serves aspecific case may be unrealistic in others. Accordingly, thecommittee can only provide information about the effects ofhigher temperatures in concrete as mentioned in Sections 3.1and 4.3, and advise that at some concrete temperaturebetween approximately 75 and 100°F (24 and 38°C) there is

Fig. 4.4—Comparison of Menzel equation to Uno equation (Uno 1998).

Fig. 4.5—Effect of concrete temperature on slump and onwater required to change slump (average data for Type Iand Type II cements) (Klieger 1958).

Fig. 4.6—Effect of temperature increase on water require-ment of concrete (U.S. Bureau of Reclamation 1975).



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a limit that will be found to be most favorable for best resultsin each hot weather operation, and such a limit should bedetermined for the work. Hot weather concreting should bediscussed during the preplacement conference and writtenprocedures should be submitted by the contractor.

Trial batches of concrete for the job should be made at thelimiting temperature selected, or at the expected job site hightemperature, rather than the 68 to 86°F (20 to 30°C) rangegiven in ASTM C192/C192M. On critical projects withmixture designs containing ingredients that reduce bleeding,it has been prudent to complement preplacement conferenceswith a trial or mockup placement. This is especially beneficialfor finishing crews unfamiliar with specialty mixtures and todemonstrate the importance of countermeasures inprotecting the fresh concrete in hot weather and critical needfor immediate curing. Procedures for testing of concretebatches at temperatures higher than approximately 70°F(21°C) are given in Section 4.10.

4.5—WaterWater, as an ingredient of concrete, greatly influences

many of its significant properties, both in the freshly mixedand hardened state. High water temperatures cause higherconcrete temperatures, and as the concrete temperatureincreases, more water is needed to obtain the same slump.Figure 4.6 illustrates the effect of concrete temperature onwater requirements to maintain slump. The extra waterincreases the w/cm and will decrease the strength, durability,watertightness, and other related properties of the concrete;this should be accounted for during mixture proportioning.Although pertinent to concrete placed under all conditions,there is an increased need to limit the use of additional waterin concrete placed under hot weather conditions (Section 4.3).

4.5.1 Effect on slump—Figure 4.5 illustrates the generaleffects of increasing concrete temperature on slump ofconcrete when the amount of mixing water is held constant.It indicates that an increase of 20°F (11°C) in temperaturecan be expected to decrease the slump by about 1 in. (25 mm).Figure 4.5 also illustrates changes in water requirementsnecessary to produce a 1 in. (25 mm) increase in slump atvarious temperature levels. For 70°F (21°C) concrete, about2.5% more water is required to increase slump 1 in. (25 mm);for 120°F (49°C) concrete, 4.5% more water is needed forthe 1 in. (25 mm) slump increase. The mixing water requiredto change slump will be less when a water-reducing, mid-range water-reducing, or high-range water-reducing admixtureis properly used.

4.5.2 Effect on drying shrinkage—Drying shrinkagegenerally increases with total water content (PCA 1992).Rapid slump loss in hot weather often increases the demandfor water, increasing total water content and, therefore,increasing the potential for subsequent drying shrinkage.Concrete cast in hot weather is also susceptible to thermalshrinkage as it subsequently cools. The combined thermaland drying shrinkage can lead to more cracking than observedfor the same concrete placed under milder conditions.

4.5.3 Effect on temperature of concrete—Because waterhas a specific heat of about four to five times that of cement

or aggregates, the temperature of the mixing water has thegreatest effect per unit weight on the temperature ofconcrete, even though water is used in smaller quantitiesthan the other ingredients. For most concrete, chilled watercan reduce the concrete placing temperature, usually by amaximum of approximately 8°F (4.4°C) (Fig. 4.7). Ingeneral, lowering the temperature of the batch water by 3.5to 4°F (1.9 to 2.2°C) will reduce the concrete temperatureapproximately 1°F (0.5°C), but the quantity of cooled watershould not exceed the batch water requirement, which willdepend on the mixture proportions and the moisture contentof aggregates. Efforts should therefore be made to obtain coldwater. Water can be cooled to as low as 33°F (1°C) usingwater chillers, ice, heat pump technology, or liquid nitrogen.To keep it cold, tanks, pipes, and trucks used for storing ortransporting water should be insulated and painted white.

Using ice as part of the mixing water is a major means ofreducing concrete temperature. On melting, ice absorbs heatat the rate of 144 Btu/lb (335 J/g). To be most effective, theice should be crushed, shaved, or chipped when placeddirectly into the mixer as part of the mixing water; the iceshould not be allowed to melt before it is placed in the mixerin contact with other ingredients, but it should meltcompletely before concrete mixing is complete. Crushed iceshould be stored at a temperature that will prevent lumpsfrom forming by refreezing of particles.

For a more rapid blending of materials at the beginning ofmixing, not all of the available batch water should be addedin the form of ice. Its quantity is usually limited to approxi-mately 75% of the batch water requirement. To maximizeamounts of ice or cold mixing water, aggregates should bewell drained of free moisture, permitting a greater quantityof ice or cold mixing water to be used. Figure 4.8 illustratespotential reductions in concrete temperature by substitutingvarying amounts of ice at 32°F (0°C) for mixing water at the

Fig. 4.7—General effects of cooled mixing water onconcrete temperature (NRMCA 1962).



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temperatures shown. Mixing should be continued until theice has melted completely.

Temperature reduction by adding ice can also be estimatedby using Eq. (A-4) or (A-5) in Appendix A. For mostconcrete, the maximum temperature reduction with ice isapproximately 20°F (11°C).

When greater temperature reductions are required, coolingby injection of liquid nitrogen into the mixer holding mixedconcrete can be the most expedient means (Appendix B).Injected liquid nitrogen does not affect the mixing waterrequirement except by reducing concrete temperature.

4.6—CementHigh concrete temperature increases the rate of hydration

(Fig. 4.9). As a result, concrete stiffens more rapidly andrequires more water to produce or maintain the desiredslump unless offset by measures described in Sections 4.7and 4.8. The higher water content will cause strength lossand increase the tendency of the concrete to crack.

4.6.1 Effect of slow-setting cement—Selection of a particularcement can have a decided effect on the hot weatherperformance of concrete, as illustrated in Fig. 4.9. Althoughthe curves are based on limited data from mixtures that usedifferent cements in combination with a set-retarding admix-ture, they show, for example, that when tested at 100°F(38°C), the concrete with the slowest-setting cement reachestime of final setting 2-1/2 hours later than the concrete withthe fastest-setting cement. The concrete that sets slowest at100°F (38°C) was the fastest-setting cement when tested at50°F (10°C). Figure 4.9 illustrates the difficulty ofpredicting concrete performance at different temperatures.

The use of a slower-setting Type II portland cement(ASTM C150/C150M) or Type IP or IS blended cement(ASTM C595/C595M) can improve the handling character-istics of concrete in hot weather (ACI 225R). Concrete thatcontains the slower-setting cements is more likely to exhibitplastic shrinkage cracking, if not protected.

When using slower-hydrating cements, the slower rate ofheat development and the simultaneous dissipation of heatfrom the concrete result in lower peak temperatures. There isless thermal expansion, and the risk of thermal crackingupon cooling of the concrete is reduced. This is an importantconsideration for slabs, walls, and mass concretes, asdiscussed in ACI 207.1R and 207.2R. The temperatureincrease from hydration of cement in a given concretemixture is proportional to its cement content. Therefore, thecement content should be limited to the amount required toprovide strength and durability.

Fig. 4.8—General effects of ice in mixing water on concretetemperature. Temperatures are normal mixing watertemperatures (NRMCA 1962).

Fig 4.9—Effect of temperature and brand of cement on setting time characteristics ofconcrete mortars (Tuthill and Cordon 1955).



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Concrete mixtures that obtain high strength at an early agewill develop high concrete temperature during initial curing.Thermal protection should be provided for high-strengthconcrete mixtures to ensure gradual cooling at a rate that willnot cause them to crack (Section 6.4.1).

Cement is sometimes delivered at relatively hightemperatures. This is not unusual for newly manufacturedcement that has not had an opportunity to cool after grindingof the component materials. Concrete mixtures consist ofapproximately 10 to 15% cement; thus, the concretetemperature will increase approximately 1°F (0.5°C) foreach 8°F (4°C) increase in cement temperature.

4.7—Supplementary cementitious materialsMaterials in this category include fly ash and other pozzolans

(ASTM C618) and slag cement (ASTM C989). Each iswidely used as partial replacements for portland cement;they can impart a slower rate of setting and of early strengthgain to the concrete, as explained in Section 4.6.1. Faster-setting cements or cements that cause rapid slump loss in hotweather can perform satisfactorily in combination with thesematerials (Gaynor et al. 1985). The use of fly ash can reducethe rate of slump loss of concrete under hot weather conditions(Ravina 1984; Gaynor et al. 1985).

4.8—Chemical admixturesVarious types of chemical admixtures (ASTM C494/

C494M) were found to be beneficial in offsetting some of theundesirable characteristics of concrete placed during periodsof high ambient temperatures (refer also to ACI 212.3R).The benefits can include lower mixing water demand,extended periods of use, and strengths comparable with, orhigher than, concrete without admixtures placed at lowertemperatures. Their effectiveness depends on the chemicalreactions of the cement used in the concrete. Admixtureswithout a history of satisfactory performance at the expectedhot weather conditions should be evaluated before their use.

4.8.1 Water-reducing and retarding admixtures—Retarding admixtures meeting ASTM C494/C494M Type Drequirements have both water-reducing and set-retardingproperties, and are used widely under hot weather conditions.They can be included in concrete in varying proportions andin combination with other admixtures so that, as temperatureincreases, higher dosages of the admixture can be used toobtain a uniform time of setting. Their water-reducingproperties largely offset the higher water demand that resultsfrom increases in concrete temperature. Because water-reducing and retarding admixtures generally increaseconcrete strength, they can be used, with proper mixtureadjustments, to avoid strength losses that would otherwiseresult from high concrete temperatures (Gaynor et al 1985;Mittelacher 1985, 1992). Compared with concrete withoutadmixtures, a concrete mixture that uses a water-reducingand retarding admixture can have a higher rate of slump loss.The net water reduction and other benefits remain substantialeven after the initial slump is adjusted to compensate forslump loss.

Hydroxylated carboxylic acid (ACI 212.3R, Section 4.2.2,Category 3) and some admixtures meeting ASTM C494/C494M Type D requirements can increase the early bleedingand rate of bleeding of concrete. This admixture-inducedearly bleeding can be helpful in preventing drying of thesurface of concrete placed at high ambient temperature andlow humidity. Concrete that is prone to bleeding shouldgenerally be reconsolidated after most of the bleeding hastaken place. Otherwise, differential settling can occur, whichcan lead to cracks over reinforcing steel and other inserts innear-surface locations. This cracking is more likely in coolweather with slower-setting concretes than in hot weather. Ifthe admixture reduces the tensile strength and tensile straincapacity, however, plastic shrinkage tendencies can beincreased (Ravina and Shalon 1968a,b). Other admixtures(ACI 212.3R, Section 4.2.2, Categories 1 and 2) can reducebleeding rate. If drying conditions are such that surfacecrusting inhibits bleedwater from reaching the surface,trapped bleedwater accumulating beneath a crusted surfacecan contribute to surface delaminations. Under such conditions,fog sprays, evaporation retardants (materials that retard theevaporation of bleeding water of concrete), or both, shouldbe used to prevent crusting.

4.8.2 Flowing concrete—Some high-range, water-reducing and retarding admixtures (ASTM C494/C494M,Type G), and water-reducing and retarding admixtures(ASTM C1017/C1017M, Type II) can provide significantbenefits under hot weather conditions when used to produceflowing concrete. At higher slumps, heat gain from internalfriction during mixing of the concrete will be less (ASTMInternational 1994; ACI 207.4R). The improved handling char-acteristics of flowing concrete permit more rapid placementand consolidation, and the period between mixing and initialfinishing can therefore be reduced. The rate of slump loss offlowing concrete can also be less at higher temperatures thanin concrete using conventional retarders (Yamamoto andKobayashi 1986). Concrete strengths are generally found tobe substantially higher than those of comparable concretewithout admixtures and with the same cement content.Certain products can cause significant bleeding, which canbe beneficial, but can also require some precautions (Section4.8.1). Air-content tests are needed before placement toassure maintenance of specified air content. Assurance thatthe air-void system is not impaired can be determined by ahardened air analysis or ASTM C666/C666M freezing-and-thawing testing.

4.8.3 Extended slump—Some high-range water-reducingretarders can maintain the necessary slump for extendedperiods at elevated concrete temperatures (Collepardi et al.1979; Hampton 1981; Guennewig 1988). These will be ofparticular benefit in delayed placements or deliveries overgreater distances. Other high-range water-reducing admix-tures can greatly accelerate slump loss, particularly wheninitial slumps are less than 3 to 4 in. (75 to 100 mm). Somewater-reducing admixtures can cause the concrete to extendits working time by up to 2 hours, followed by acceleration ofstrength gain.



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4.8.4 Mid-range water reducing admixtures—Since theearly 1990s, the use of mid-range water-reducing admixturesin hot weather has increased. Mid-range water-reducingadmixtures provide up to 15% water reduction, which ishigher than conventional water-reducing admixtures, but haslower water reduction than high-range water-reducing admix-tures. Although at present there is no ASTM classification,mid-range water-reducing admixtures comply with therequirements of ASTM C494/C494M Type A admixturesand, in some cases, Type F admixtures. These admixturesshould not significantly delay the setting time of theconcrete. At higher dosages, conventional water-reducingadmixtures can achieve this water reduction, but with asignificant increase in the setting time of the concrete. Thepumping and finishing characteristics of concrete containingmid-range water-reducing admixtures are improved whencompared with concrete containing conventional Type Awater-reducing admixtures. The use of mid-range water-reducing admixtures is particularly beneficial in cases whereaggregate properties contribute to poor workability orfinishing difficulties. The surface appearance of concretecontaining a mid-range water-reducing admixture could bechanged, thereby requiring a change in the timing offinishing operations. Mid-range water-reducing andretarding admixtures that comply with ASTM C494/C494Mrequirements for Type D admixtures are also available.

4.8.5 Extended set-control—The use of extended set-control admixtures to stop the hydration process of freshlymixed concrete (freshly batched or returned fresh concretethat normally would be disposed) and concrete residue(washwater) in concrete truck drums has gained increasedacceptance in hot weather environments since their introductionin 1986. Some extended set-control admixtures comply withASTM C494/C494M requirements for Type B retardingadmixtures and Type D water-reducing and retardingadmixtures. Extended set-control admixtures differ fromconventional retarding admixtures in that they stop thehydration process of both the silicate and aluminate phasesin portland cement. Regular retarding admixtures act only onthe silicate phases, which extend, but do not stop, the hydrationprocess. The technology of extended set-control admixturescan also be used to stop the hydration process of freshlybatched concrete for hauls requiring extended time periodsor slow placement methods during transit. For this application,the extended set-control admixture is added during orimmediately after the batching process. Proper dosage ratesof extended set-control admixtures should be determined bytrial mixtures that incorporate project time requirements inthis way, ensuring that the concrete will achieve the requiredsetting time. Additional admixtures are not required torestart hydration.

4.8.6 Evaulation—The qualifying requirements of ASTMC494/C494M provide a valuable screening procedure for theselection of admixture products. Admixtures without aperformance history that pertain to the concrete materialselected for the work should first be evaluated in laboratorytrial batches at the expected high job temperature using oneof the procedures described in Section 4.9. Some high-range

water-reducing retarders cannot demonstrate their potentialbenefits when used in small laboratory batches. Furthertesting may then be required in production-size concretebatches. During preliminary field use, concrete containing anadmixture should be evaluated for consistency of performancein regard to the desired characteristics in hot weatherconstruction. When evaluating admixtures, properties suchas workability, pumpability, early strength development,placing and finishing characteristics, appearance, and effecton reuse of molds and forms should be considered in additionto the basic properties of slump retention, setting time, andstrength. These characteristics can influence selection of anadmixture and its dosage more than properties usuallycovered by most specifications.

4.9—AggregatesAggregates, the major constituent of concrete, usually

account for 60 to 80% of the volume of normalweightconcrete. Therefore, the properties of the aggregate affect thequality of concrete significantly. The three principal factorsthat affect the amount of water required to produce concreteat a given slump are gradation, particle shape, and theabsence of undersized material (ACI 221R). Crushed coarseaggregate contributes to higher water demand than roundedgravels, but is reported to provide better resistance tocracking (ACI 224R).

Coarse aggregate is the ingredient with the greatest massin concrete; changes in its temperature have a considerableeffect on concrete temperatures. For example, a moderate1.5 to 2°F (0.8 to 1.1°C) temperature reduction will lower theconcrete temperature by 1°F (0.5°C). Cooling the coarseaggregate can be an effective supplementary means toachieve desired lower concrete temperature (Appendix B).

4.10—ProportioningMixture proportions should be established or adjusted on

the basis of field performance records in accordance withACI 318, provided that the records indicate the effect ofexpected seasonal temperatures and delivery times.

The selection of ingredients and their proportions shouldbe guided by their contribution to satisfactory performanceof the concrete under hot weather conditions (ACI 211.1;ACI 211.2). The cement content should be kept as low aspossible, but sufficient to meet strength and durabilityrequirements. Inclusion of supplementary cementitiousmaterials, such as fly ash or slag cement, should be consideredto delay setting and to mitigate the temperature rise fromheat of hydration. The use of various types of water-reducingadmixtures can offset increased water demand and strengthloss that could otherwise be caused by higher concretetemperatures. High-range water-reducing retarders formulatedfor extended slump retention should be considered wherelonger delivery periods are anticipated. Unless requiredotherwise, concrete should be proportioned for a slump ofnot less than 3 in. (75 mm) to permit prompt placement andeffective consolidation in the form.

4.10.1 Trial batches—The performance of the concretemixtures should be verified under conditions approximating



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the delivery time and hot weather environment expected atthe project. Trial batches used to select proportions arenormally prepared in accordance with ASTM C192/C192M.The method requires concrete materials to be at roomtemperature (in the range of 68 to 86°F [20 to 30°C]). Trialbatches, however, should also be performed at the expectedmaximum placing temperature using a mixing and agitatingperiod longer than that required in ASTM C192/C192M tohelp define the performance to be expected.

When determining mixture proportions using laboratorytrial batches, estimate the slump loss during the periodbetween first mixing of the concrete and its placement in theform by use of Procedures A and B, adopted from ACI 223,Section 4.5.2. The procedures from ACI 223 produce a rateof slump loss similar to that expected for a 30- to 40-minutedelivery time.

4.10.1.1 Procedure A1. Prepare the batch using ASTM C192/C192M procedures,

but add 10% additional water over that normally required;2. Mix initially in accordance with ASTM C192/C192M

(3 minutes of mixing followed by a 3-minute rest and a2-minute remixing);

3. Determine the slump and record as initial slump;4. Continue mixing for 15 minutes;5. Determine the slump and record as estimated placement

slump. Experience has shown this slump correlates with thatexpected for 30- to 40-minute delivery time. If this slumpdoes not meet the specification limits, either discard andrepeat the procedure with an appropriate water adjustment,or add water to give the required slump and then test theconcrete; and

6. Determine other properties of fresh concrete (temperature,air content, unit weight), and mold strength test specimens.

4.10.1.2 Procedure B

1. Prepare the batch using ASTM C192/C192M proceduresfor the specified slump;

2. Mix in accordance with ASTM C192/C192M (3 minutesof mixing followed by a 3-minute rest and a 2-minuteremixing) and confirm the slump;

3. Stop the mixer and cover the batch with wet burlap;4. After 20 minutes, remix for 2 minutes, adding water to

produce the specified slump. The total water (initial water plusthe remixing water) can be expected to equal that required atthe batch plant to give the required job site slump; and

5. Determine other properties of fresh concrete (temperature,air content, and unit weight) and mold-strength test specimens.

4.10.1.3 Alternative to Procedures A or B—As analternative to Procedures A or B, the use of full-size productionbatches may be considered for verification of mixtureproportions, provided the expected high temperature levelsof the concrete can be attained. This may be the preferredmethod when admixtures selected for extended slump retentionare used. It requires careful recording of batch quantities atthe plant and of water added for slump adjustment beforesampling. Sampling procedures of ASTM C172 should bestrictly observed.

CHAPTER 5—PRODUCTION AND DELIVERY5.1—General

Production facilities and procedures should be capable ofproviding the required quality and quantity of concrete underhot weather conditions at production rates required by theproject. Satisfactory control of production and deliveryoperations should be assured. Concrete plant and deliveryunits should be inspected and in good operating condition.Intermittent stoppage of deliveries due to equipment break-down can be much more serious under hot weather conditionsthan in moderate weather. In hot weather concreting operations,concrete placements can be scheduled at times other thanduring daylight hours, such as during the coolest part of themorning. Night-time production requires additional planningand lighting.

5.2—Temperature control of concreteConcrete can be produced in hot weather without

maximum limits on placing temperature, and perform satis-factorily when proper precautions are observed in propor-tioning, production, delivery, placing, consolidating, finishing,and curing. As part of these precautions, an effort should bemade to keep the temperature of the fresh concrete as low aspractical. Using the relationships given in Appendix A, it canbe shown, for example, that the temperature of concrete cantypically be reduced by 1°F (0.5°C) if any of the followingreductions are made in material temperatures:• 8°F (4°C) reduction in cement temperature;• 4°F (2°C) reduction in water temperature; or• 2°F (1°C) reduction in the temperature of the aggregates.

5.2.1 Aggregate cooling—Figure 5.1 shows the influence ofthe temperature of concrete ingredients on concrete temperature.As the greatest portion of concrete is aggregate, reduction ofaggregate temperature brings about the greatest reduction inconcrete temperature. Therefore, all practical means shouldbe employed to keep the aggregates as cool as possible.Shaded storage of fine and coarse aggregates, and sprinklingor fog spraying of coarse aggregate stockpiles under aridconditions will help. Sprinkling coarse aggregates with coolwater reduces aggregate temperature by evaporation anddirect cooling (Lee 1987). Passing water through a properlysized evaporative cooling tower will chill the water to thewet bulb temperature. This procedure has greater effects inareas that have low relative humidity. Wetting of aggregatescan cause variations in surface moisture. Moisture tests orthe use of moisture probes are necessary to ensure that thecorrect batch adjustments are made. Above-ground storagetanks for mixing water should be provided with shade andthermal insulation. Silos and bins absorb less heat if coatedwith heat-reflective paints.

5.2.2 Mixer drum color—Painting mixer surfaces white tominimize solar heat gain also helps. Based on a 1-hourdelivery time on a hot, sunny day, concrete in a clean, whitemixer drum, should be 2 to 3°F (1 to 1.5°C) cooler than in ablack or red mixer drum, and 0.5°F (0.3°C) cooler than in acream-colored drum. When an empty mixer drum stands inthe sun for an extended period before concrete is batched, theheat stored in the white mixer drum will raise concrete



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temperatures 0.5 to 1°F (0.3 to 0.5°C) less than a yellow orred mixer drum. Spraying the exterior of the mixer drumwith water before batching or during delivery has beensuggested as a means of minimizing concrete temperature,but it provides only a marginal benefit.

5.2.3 Project plan—Setting up the means for cooling sizeableamounts of concrete production requires planning well inadvance of placement and installation of specialized equip-ment. This can include chilling of batch water by waterchillers or heat pump technology as well as other methods,such as substituting crushed or flaked ice for part of themixing water, or cooling by liquid nitrogen. Delivery of therequired quantity of cooling materials should be ensured foreach placement.

Details for estimating concrete temperatures are providedin Appendix A. Various cooling methods are described inAppendix B. The general influence of the temperature ofconcrete ingredients on concrete temperature is calculatedfrom the equations in Appendix A, and shown in Fig. 5.1.

5.3—Batching and mixingBatching and mixing is described in ACI 304R. Procedures

under hot weather conditions are not different from goodpractices under normal weather conditions. Producing

concrete with specified properties, such as slump, is essentialbecause an interruption in the concrete placement due to rejec-tion can cause the formation of cold joints or serious finishingproblems. Testing of concrete is discussed in Chapter 7.

For truck-mixed concrete, an initial mixing of approximately70 revolutions at the batch plant before transporting allowsfor an accurate verification of the condition of the concrete,primarily its slump and air content. Generally, centrallymixed concrete can be inspected visually as it is beingdischarged into the transportation unit.

5.3.1 Slump control—Slump can easily change due tominor changes in materials and concrete characteristics. Forexample, an undetected change of only 1.0% moisturecontent of the fine and coarse aggregates could changeslump by 1 to 2 in. (25 to 50 mm) (ACI 211.1). An errorrange of approximately 0.5% in the determination of aggregatemoisture complicates moisture control, even with advancedsystems. Plant operators often batch concrete in a driercondition than desired to avoid producing a slump higherthan desired or specified. Care should be taken to avoid with-holding excessive water from the batch, as this could resultin inadequate mixing, dry-packing that reduces the effectivenessof chemical admixtures, or delivery at a slump below thespecified minimum.

Fig. 5.1—Influence of temperature of concrete ingredients on concrete temperature(calculated from equations in Appendix A).



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5.3.2 Hydration control—Hot weather conditions andextended hauling time can indicate a need to split thebatching process by batching the cement at the job site, orlayering the materials in the mixer drum at the plant to keepsome of the cement dry and then mixing the concrete afterarrival at the job site. This, however, can decrease concreteuniformity between loads. These methods can, on occasion,offer the best solution under existing conditions. A better-controlled concrete can usually be provided when all materialsare batched at the concrete production facility.

By using some effective retarding admixtures at appropriatedosages, preferably in combination with cementitious materialof slow-setting characteristics, concrete can be maintained ina placeable condition for extended periods, even in hotweather (Section 4.8). Field experience indicates thatconcrete set retardation can be extended further by separatelybatching the retarding admixture with a small portion ofmixing water (1 to 2 gal/yd3 [5 to 10 L/m3]), after theconcrete has been mixed for several minutes. These admixtures,together with the cementitious materials and other ingredientsproposed for the project, should be evaluated in the field fordesired properties. Should the slump be lower than required,the use of mid-range or high-range water-reducing admixturesis recommended to increase the concrete slump.

5.3.3 Mixer control—Under hot weather conditions, mixerrevolutions at mixing speed should be held to a minimum toavoid unnecessary heat gain of the concrete (ACI 207.4R).For efficient mixing, mixers should be free of buildup ofhardened concrete and excessive wear of mixer blades. Assoon as the concrete has been mixed to a homogeneouscondition, all further drum rotation should be at the lowestagitating speed of the unit (generally one revolution perminute). The drum should not be stopped for extendedperiods of time. There is the potential for false setting problemsto cause the concrete to stiffen rapidly or set in the drum, orto flatten the mixer rollers.

Specifications that govern the total number of revolutionsof the drum usually set a limit of 300 revolutions for truckmixers. This limit should be waived for:• Concrete that retains its workability without the addition

of water;• Separate addition of high-range water-reducing

admixtures; or• Direct addition of liquid injected nitrogen into the

mixer as a means of lowering the concrete temperature.

5.4—DeliveryWhile the concrete is in the mixer, cement hydration,

temperature rise, slump loss, aggregate grinding, and changeof air content all occur with the passage of time; thus, theperiod between start of mixing to placement of the concreteshould be minimized. Coordination of mixer truckdispatching with the rate of concrete placement helps toavoid delays in arrival or waiting periods until discharge. Onmajor concrete placements, provisions should be made forgood communication between the job site and concreteproduction facility, and they should be scheduled duringperiods of lower urban traffic. When placement is slow,

consideration should be given to reducing load size, usingset-retarding admixture, or using cooled concrete.

5.5—Slump adjustmentFresh concrete is subject to slump loss with time, whether

it is used in moderate or hot weather. With given materialsand mixture proportions, the slump change characteristicsbetween plant and job site should be established. If, onarrival at the job site, the slump is less than the specifiedmaximum, additional water can be added if the maximumallowable water content is not exceeded. When water isadded to increase slump, the drum or blades should be turnedan additional 30 revolutions or more, if necessary, at mixingspeed. For expeditious placement and effective consolidation,structural concrete should have a minimum slump of 4 in.(100 mm). Slump increases should be allowed when chemicaladmixtures are used, provided that the admixture-treatedconcrete has the same or lower w/cm and does not exhibitsegregation potential.

5.6—Properties of concrete mixturesThe proposed mixtures should be suitable for expected job

conditions. This is particularly important when there are nolimits on ambient placing temperatures, as is the case in mostconstruction in warmer regions. Use of cements or cementitiousmaterials that perform well under hot weather conditions, incombination with water-reducing and retarding admixtures,can provide concrete with the required properties (Mittelacher1985). When using high-range water-reducing and retardingadmixtures, products should be selected that provideextended slump retention in hot weather (Collepardi et al.1979; Guennewig 1988). In dry and windy conditions, thesetting rate of concrete used in flatwork should be adjustedto minimize plastic shrinkage cracking or crusting of thesurface, whereas the lower layer remains in a plastic condition.The type of adjustment depends on local climatic conditions,timing of placements, and concrete temperatures. A changein quantity or type of admixture or cementitious materialscan often provide the desired setting time.

5.7—RetemperingLaboratory research, as well as field experience, shows

that strength reduction and other detrimental effects areproportional to the amount of retempering water added.Therefore, water additions in excess of the proportionedmaximum water content or w/cm to compensate for loss ofworkability should be prohibited. Adding chemical admixtures,particularly high-range water-reducing admixtures, can bevery effective to maintain workability.

CHAPTER 6—PLACING AND CURING6.1—General

6.1.1 The requirements for good results in hot weatherconcrete placing and curing are no different than in otherweather. They are:• Concrete should be handled and transported with a

minimum of segregation and slump loss;• Concrete should be placed where it is to remain;



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• Concrete should be placed in layers shallow enough toassure vibration well into the layer below and that theelapsed time between layers should be minimized toavoid cold joints;

• Construction joints should be made on sound, cleanconcrete (refer to ACI 224.3R);

• Finishing operations and timing should be guided onlyby the readiness of the concrete for them, and nothingelse; and

• Curing should be conducted so that at no time duringthe prescribed period will the concrete lack amplemoisture and temperature control to permit fulldevelopment of its potential strength and durability.

6.1.2 Details of placing, consolidation, and curing proceduresare described in ACI 304R, 308R, and 309R. This chapterpoints out hot weather factors that can affect these operationsand the resulting concrete and recommends what should bedone to prevent or offset their influence.

6.2—Preparations for placing and curing6.2.1 Planning hot weather placements—Before the start of

the project, plans should be made to minimize the exposure ofthe concrete to adverse conditions. Whenever possible, slabplacement should be scheduled after the roof structure andwalls are in place to minimize drying winds and directsunlight. A roof also reduces thermal shock from rapidtemperature drops caused by wide day and night temperaturedifferences or cool rain on concrete heated by the sun earlierin the day.

Under hot weather conditions, scheduling concrete place-ments at other-than-normal hours may be advisable. Pertinentconsiderations include ease of handling and placing, and mini-mizing the risk of plastic shrinkage and thermal cracking.

6.2.2 Preparing for ambient conditions—Personnel incharge of concrete construction should be aware of thedamaging combinations of high air temperature, directsunlight, drying winds, and high concrete temperature. Localweather reports should be monitored, and routine recordingsof site conditions should be made, including air temperature,sun exposure, relative humidity, and prevailing winds. Thesedata, together with projected or actual concrete temperatures,enable supervisory personnel, using Fig. 4.2, to determineand prepare the required protective measures. Equipmentshould also be available at the site to measure the evaporationrate (refer to Section 4.2.2).

6.2.3 Expediting placement—Preparations should bemade to transport, place, consolidate, and finish the concreteat the fastest possible rate. Concrete delivery to the jobshould be scheduled so that it is placed promptly on arrival,particularly the first batch. Many concrete placements get offto a bad start because the concrete was ordered before the jobwas ready, and slump control was lost at this most criticaltime. Traffic arrangements at the site should ensure easyaccess of delivery units to the unloading points over stableroadways. Site traffic should be coordinated for a quickturnaround of concrete mixer trucks. If possible, large orcritical placements should be scheduled during periods oflow urban traffic loads.

6.2.4 Placing equipment—Equipment for placing theconcrete should be of suitable design and have amplecapacity to perform efficiently. All equipment should haveadequate power for the work and be in first-class operatingcondition. Breakdowns or delays that stop or slow theplacement can seriously affect the quality and appearance ofthe work. Arrangements should be made for readily availablebackup equipment. Concrete pumps, where used, should becapable of pumping the specified class of concrete throughthe length of line and elevation at required rates per hour.Where placement is by crane and buckets, wide-mouthbuckets with steep-angled walls should be used to permitrapid and complete discharge of bucket contents. Adequatemeans of communication between bucket handlers andplacing crew should be provided to ensure that concrete ischarged into buckets only when the placing crew is ready touse the concrete without delay. Concrete should not beallowed to rest exposed to the sun and high temperaturebefore it is placed into the form. To minimize the heat gainof the concrete during placement, delivery units, conveyors,pumps, and pump lines should be kept in the shade wherepossible. In addition, pump lines should be painted white.Lines can also be cooled by being covered with damp burlapor kept wet with a soaker hose.

6.2.5 Consolidation equipment—There should be amplevibration equipment and workers to consolidate the concreteimmediately as it is received in the form. Procedures andequipment are described in ACI 309R. Provisions should bemade for an ample number of standby vibrators—at least onestandby for each three vibrators in use. Where a site is subjectto occasional power outages, portable generators should beavailable for uninterrupted vibrator operation. Apart fromthe unsightliness of poorly consolidated concrete, insufficientcompaction in the form can seriously impair the durabilityand structural performance of reinforced concrete.

6.2.6 Preparations for protecting and curing the concrete—Ample water should be available at the project site formoistening the subgrade, as well as for fogging forms andreinforcement before concrete placement. For moist curing,use water with a temperature no more than 20oF (11°C)cooler than the concrete temperature to avoid thermal shockwhere applicable. Fog nozzles should produce a fog blanket.They should not be confused with common garden-hosenozzles, which generate an excessive washing spray. Pressurewashers with a suitable nozzle attachment can be a practicalmeans for fogging on smaller jobs. Materials and meansshould be on hand for erecting temporary windbreaks andshades as needed to protect against drying winds and directsunlight. Plastic sheeting or sprayable compounds forapplying temporary moisture-retaining films should beavailable to reduce evaporation from flatwork betweenfinishing passes. Where concrete placed under hot weatherconditions is exposed to rapid temperature drops, thermalprotection should be provided to protect the concrete againstthermal shrinkage cracking. Finally, curing materials shouldbe readily available at the project site to permit promptprotection of all exposed surfaces from premature drying uponcompletion of the placement.



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6.2.7 Preparing incidental work—Due to faster settingand hardening of the concrete in hot weather, timing ofvarious final operations, such as saw cutting joints andapplying surface retarders, becomes more critical; therefore,these operations should be planned in advance. Plans shouldbe made for the timely sawing of contraction joints in flat-work to minimize cracking due to excessive tensile stress.Typically, joints that are cut using the conventional wet ordry process are made within 4 to 12 hours after the slab hasbeen finished; 4 hours in hot weather, to 12 hours in coldweather. For early entry dry-cut saws, the waiting period willtypically vary from 1 hour in hot weather to 4 hours in coldweather (ACI 302.1R).

6.3—Placement and finishing6.3.1 General—Expeditious placement and finishing

materially reduces hot weather difficulties. Delays increaseslump loss and invite the addition of water offsets to offsetthose losses. Each operation in finishing should be carriedout promptly when the concrete is ready. The concreteshould not be placed faster than it can be properly consolidatedand finished. When the placing rate is not coordinated withthe available work force and equipment, the quality of thework will be marred by cold joints, poor consolidation, anduneven surface finishes.

6.3.2 Placing formed concrete—In hot weather, it isusually necessary to place concrete in shallower layers thanthose placed in moderate weather to ensure coverage of thelower layer while it will still respond readily to vibration.The interval between monolithic wall and deck placementsbecomes very short in hot weather. This interval can beextended by the judicious use of set-retarding admixtures.

6.3.3 Placement of flatwork—When concrete is depositedfor flatwork on the ground, the subgrade should be moist, butfree of standing water and soft spots. When placing concreteslabs of any kind in hot weather, it may be necessary to keepthe operation confined to a small area and to proceed on afront with a minimum amount of exposed surface to whichconcrete is added. A fog nozzle should be used to cool theair, to cool any forms and steel immediately ahead, and tolessen rapid evaporation from the concrete surface beforeand after each finishing operation. Excessive fog application(which would wash the fresh concrete surface or causesurplus water to cling to reinforcement or stand on theconcrete surface during floating and troweling) should beavoided. Other means of reducing moisture loss includespreading and removing impervious sheeting or applyingsprayable moisture-retaining (monomolecular) films one ormore times as needed between the various finishing operations.Finishing of flatwork should begin after the surface sheen ofthe (monomolecular) film has disappeared. These productsshould not be used as finishing aids or worked into thesurface, as concrete durability can be reduced. The productmanufacturer should be contacted for information on properapplication and dosage. These procedures can cause a slightincrease in concrete temperature in place due to reducedevaporative cooling. Generally, the benefit from reduced

moisture evaporation is more important than the increase ofin-place concrete temperature (Berhane 1984).

6.3.4 Plastic shrinkage cracks—Without protectionagainst moisture loss, plastic shrinkage cracks can occur(refer to Section 4.1.4). In relatively massive placements,revibration before floating can sometimes close this type ofcracking. Before the concrete reaches final set, the crackscan frequently be closed by striking the surface on each sideof the crack with a float. The affected area is then retroweledto a level finish.

It serves no lasting purpose to merely trowel a slurry overthe cracks, because these are likely to reappear when notfirmly closed and immediately covered to avoid evaporation.

6.4—Curing and protection6.4.1 General—Immediately following completion of

finishing operations, efforts should be made to protect theconcrete from low humidity, drying winds, and extremeambient temperature differential. Whenever possible, theconcrete and surrounding formwork should be kept in auniform moisture and temperature condition to allow theconcrete to develop its maximum potential strength anddurability. High initial curing temperatures can negativelyaffect ultimate strength and durability to a greater degreethan high placement temperatures of fresh concrete (Bloem1954; Barnes et al. 1977; Gaynor et al. 1985). Procedures forkeeping exposed surfaces from drying should beginpromptly and continue without interruption. Failure to do socan result in excessive drying shrinkage and relatedcracking, which can impair the surface durability of theconcrete. An approved curing method should be continuedfor at least 7 days. If more than one curing method is usedduring this period, any changes in method should be doneafter a minimum of 3 days. In addition, concrete surfacesshould not be allowed to become surface-dry at any pointduring the transition. A variety of curing methods aredescribed in ACI 308R. ACI 308R addresses in detail theconcept of initial curing during the plastic stage of theconcrete. Initial curing techniques, such as fog spray, can beused to ensure timely replacement of bleedwater and avoid-ance of plastic shrinkage cracking. Concrete should also beprotected against thermal shrinkage cracking due to rapidtemperature drops, particularly during the first 24 hours.Thermal shrinkage cracking is associated with a cooling rateof more than 5°F (3°C) per hour, or more than 50°F (28°C)in a 24-hour period for concrete with a least dimension lessthan 12 in. (300 mm). Concrete exposed to rapid coolingdevelops lower tensile strain capacity, and is more susceptibleto other types of shrinkage cracking than concrete that coolsat a slower rate (ACI 207.4R). Hot weather patterns increasethe potential for thermal cracking due to vast day and nighttemperature differences. Additionally, seasonal weatherpatterns often include passing cold fronts that produce rain,which can induce thermal shock to exposed concretesections. Under these conditions, concrete should beprotected by placing an approved waterproof material overthe exposed concrete, or by using other insulating methodsand materials described in ACI 306R.



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6.4.2 Moist curing of flatwork—Moist curing is usuallythe best method for maximizing strength and durability andminimizing early-age drying shrinkage of concrete flatwork.Examples of moist curing methods are ponding, coveringexposed concrete surfaces with clean sand kept continuouslywet, fog-spraying, or continuous sprinkling. These methodsrequire a sufficient water supply and disposal of any runoff.Where sprinkling is used, care should be taken that surfaceerosion does not occur. A common and practical method ofmoist curing is to cover the concrete with impervioussheeting or fabric mats kept continuously wet with a soakerhose or similar means. Other suitable coverings aredescribed in ACI 308R. Curing materials should be kept incontact with the concrete surface at all times. Alternatingcycles of wetting and drying should be avoided, as it willresult in pattern cracking. The temperature of water used forinitial curing should be as close as possible to that of theconcrete to avoid thermal shock.

6.4.3 Membrane curing of flatwork—Where job conditionsare not favorable for moist curing, the most practical methodof curing is liquid membrane-forming compounds. Themembrane restricts the loss of moisture from the concrete,thereby allowing the development of strength, durability,and abrasion resistance of the surface. Concrete surfacesexposed to direct sunlight should use heat-reflecting, white-pigmented compounds where applicable. Note that themoisture-retention rate varies considerably between products.For use in hot weather conditions, a material should beselected that ensures equal or greater moisture retention thanrequired by ASTM C309, and limits the moisture loss in a72-hour period in excess of 9 lb/yd3 (0.55 kg/m3) whentested per ASTM C156. Some agencies have a more restrictivemoisture loss limit of 6.4 lb/yd3 (0.39 kg/m3) in a 72-hourperiod. Application of an approved moisture-retentive materialshould immediately follow the disappearance of surfacewater sheen after the final finishing pass. When a sprayapplication is required or approved, the spray nozzle(s)should be positioned sufficiently close to the surface toensure the correct application rate and prevent wind-blowndispersion. Manual spray application should be performed intwo passes, with the second pass perpendicular to the firstpass. Most curing compounds should not be used on anysurface against which additional concrete or other materialsare to be bonded. The curing compound material will notreduce bond strength if removal of the curing compound isassured before subsequent bonded construction.

6.4.4 Concrete in formwork—Forms should be coveredand kept continuously moist during the early curing period.Formwork should be loosened or removed at the earliestpractical age without damage to the concrete, and provisionsshould be made for an approved curing method to begin.Following formwork removal, tie holes and significantdefects can be filled and repairs made by exposing thesmallest practical section of concrete at one time to performthe work. All repairs should be completed within the firstfew days following form stripping so to the repaired areascure with the surrounding concrete. At the end of the curingperiod, the covering should be left in place without wetting

for several days (4 days is suggested) so that the concretesurface will dry slowly and be less prone to surfaceshrinkage cracking. Surface cracking due to drying can beminimized by applying a curing compound to exposedsurfaces at the end of the moist-curing period.

CHAPTER 7—TESTING AND INSPECTION7.1—Testing

Tests on the fresh concrete sample should be conductedand specimens prepared in accordance with ASTM C31/C31M, C138/C138M, C143/C143M, C172, ASTM C231/C231M, C232/C232M, C173/C173M, C1064/C1064M,C1611/C1611M, and C1621/C1621M, as appropriate. Testsshould be performed by an certified ACI Concrete FieldTesting Technician – Grade I. ASTM C31/C31M requiresthat the concrete samples be protected from exposure to sun,wind, rapid evaporation, and contamination. Failure to do sowill not provide valid test results. High temperature, lowrelative humidity, and drying winds affect the rate of evapo-ration of the concrete sample surface when not protectedproperly as recommended by ASTM C31/C31M.

It is desirable in hot weather to conduct tests, such asslump, air content, ambient and concrete temperature, relativehumidity, and density (unit weight), more frequently than innormal conditions.

7.1.1 Curing test specimens—Particular attention shouldbe given to the protection and curing of strength test specimensused as a basis for acceptance of concrete. Due to their smallsize, test specimens are quickly influenced by changes inambient temperatures. Extra care is needed in hot weather tomaintain strength test specimens at a temperature of 60 to80°F (16 to 27°C) for < 6000 psi (40 MPa), and 68 to 78°F (20to 26°C) for ≥ 6000 psi (40 MPa), and to prevent moisture lossduring the initial curing period, in accordance with ASTMC31/C31M, with the exception of C1611/C1611M andC1621/C1621M for self-consolidated concrete. The speci-mens should be provided with an impervious cover and placedin a temperature-controlled cylinder box or building immedi-ately after molding. When stored outside, exposure to the sunshould be avoided. Curing in a no-moisture-loss environmentwithin the prescribed temperature range is also required.

Molds should not be manufactured of a material thatexpands when in contact with moisture or when immersed inwater, and should meet the requirements of ASTM C470/C470M. Merely covering the top of the molded test cylinderwith a lid or plate is usually not sufficient in hot weather toprevent loss of moisture and to maintain the required initialcuring temperature. During the transfer to the testing facility,the specimens should be kept moist and also be protected andhandled carefully. They should then be stored in a moistcondition at 73 ± 2°F (23 ± 1.8°C) until the moment oftesting as per ASTM C31/C31M.

7.1.2 Additional test specimens—Specimens, in additionto those required for acceptance, can be made and cured atthe job site to assist in determining when formwork can beremoved, when shoring can be removed, and when the structurecan be placed in service. Unless the temperature and moistureconditions of concrete specimens used for these purposes



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match those of the concrete in the structure they are to represent,results of the tests can be misleading. Alternative testmethods for determining in-place concrete strength aredescribed in ASTM C900, C1074, and C918/C918M.

7.2—Inspection7.2.1 The numerous details to be looked after in concrete

construction are covered in ACI SP-2 (ACI Committee 3112007) and ACI 311.4R. Project inspection of concrete isnecessary to ensure and document compliance with previouslymentioned precautions and procedures. The need for suchmeasures, such as spraying of forms and subgrade, coolingconcrete, providing sunshades and windscreens, the use ofevaporation retarders or fogging, and minimizing delays inplacement, initial curing, and final curing procedures, shouldbe observed and documented when the rate of evaporation ishigher than the rate of bleedwater coming to the surface.

7.2.2 Air temperature, concrete temperature (ASTMC1064/C1064M), general weather conditions (clear orcloudy), wind speed, relative humidity, and evaporation rateshould be recorded at hourly intervals. The measurementsshould be taken per the instructions in Fig. 4.2. In addition,the following should be recorded and identified with thework in progress so that conditions relating to any part of theconcrete construction can be identified at a later date:• All water added to the concrete with corresponding

mixing times;• Time batched, time discharge started, and time

discharge completed;• Concrete temperature at time of delivery and after

concrete is placed;• Observations on the appearance of concrete as delivered

and after placing in forms;• Slump of concrete at point of delivery;• Protection methods;• Initial curing method used;• Final curing method used;• Where a curing membrane is used, the time and rate of

application and visual appearance of concrete; and• Duration and termination of curing.

These observations should be included in the permanentproject records.

CHAPTER 8—REFERENCES8.1—Referenced standards and reports

The standards and reports listed below were the latesteditions at the time this document was prepared. Becausethese documents are revised frequently, the reader is advisedto contact the proper sponsoring group if it is desired to referto the latest version.

American Concrete Institute201.2R Guide to Durable Concrete207.1R Guide to Mass Concrete207.2R Report on Thermal and Volume Change Effects

on Cracking of Mass Concrete207.4R Cooling and Insulating Systems for Mass Concrete211.1 Standard Practice for Selecting Proportions for

Normal, Heavyweight, and Mass Concrete

211.2 Standard Practice for Selecting Proportions forStructural Lightweight Concrete

212.3R Chemical Admixtures for Concrete221R Guide for Use of Normal Weight and Heavy-

weight Aggregates in Concrete223 Standard Practice for the Use of Shrinkage-

Compensating Concrete224R Control of Cracking in Concrete Structures224.3R Joints in Concrete Construction225R Guide to the Selection and Use of Hydraulic

Cements232.2R Use of Fly Ash in Concrete234R Guide for the Use of Silica Fume in Concrete301 Specifications for Structural Concrete302.1R Guide for Concrete Floor and Slab Construction304R Guide for Measuring, Mixing, Transporting, and

Placing Concrete305.1 Specification for Hot Weather Concreting306R Cold Weather Concreting308R Guide to Curing Concrete309R Guide for Consolidation of Concrete311.4R Guide for Concrete Inspection318 Building Code Requirements for Structural

Concrete and Commentary

ASTM InternationalC31/C31M Standard Practice for Making and Curing

Concrete Test Specimens in the FieldC138/C138M Standard Test Method for Density (Unit

Weight), Yield, and Air Content (Gravi-metric) of Concrete

C143/C143M Standard Test Method for Slump ofHydraulic-Cement Concrete

C150/C150M Standard Specification for Portland CementC156 Standard Test Method for Water Loss

[from a Mortar Specimen] through LiquidMembrane-Forming Curing Compoundsfor Concrete

C172 Standard Practice for Sampling FreshlyMixed Concrete

C173/C173M Standard Test Method for Air Content ofFreshly Mixed Concrete by the VolumetricMethod

C192/C192M Standard Practice for Making and CuringConcrete Test Specimens in the Laboratory

C231/C231M Standard Test Method for Air Content ofFreshly Mixed Concrete by the PressureMethod

C232/C232M Standard Test Methods for Bleeding ofConcrete

C309 Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete

C470/C470M Standard Specification for Molds forForming Concrete Test Cylinders Vertically

C494/C494M Standard Specifications for ChemicalAdmixtures for Concrete

C595/C595M Standard Specification for Blended HydraulicCements



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C618 Standard Specification for Coal Fly Ashand Raw or Calcined Natural Pozzolan forUse in Concrete

C666/C666M Standard Test Method for Resistance ofConcrete to Rapid Freezing and Thawing

C900 Standard Test Method for Pullout Strengthof Hardened Concrete

C918/C918M Standard Test Method for MeasuringEarly-Age Compressive Strength andProjecting Later-Age Strength

C989 Standard Specification for Slag Cement forUse in Concrete and Mortars

C1017/C1017M Standard Specification for ChemicalAdmixtures for Use in Producing FlowingConcrete

C1064/C1064MStandard Test Method for Temperature ofFreshly Mixed Hydraulic-Cement Concrete

C1074 Standard Practice for Estimating ConcreteStrength by the Maturity Method

C1611/C1611M Standard Test Method for Slump Flow ofSelf-Consolidating Concrete

C1621/C1621M Standard Test Method for Passing Abilityof Self-Consolidating Concrete by J-Ring

These publications may be obtained from these organizations:

American Concrete Institute38800 Country Club DriveFarmington Hills, MI 48331www.concrete.org

ASTM International100 Barr Harbor Drive West Conshohocken, PA 19428www.astm.org

8.2—Cited referencesACI Committee 311, 2007, ACI Manual of Concrete

Inspection, SP-2, American Concrete Institute, FarmingtonHills, MI, 199 pp.

Al-Fadhala, M., and Hover, K. C., 2001, “Rapid Evaporationfrom Freshly Cast Concrete and the Gulf Environment,”Construction and Building Materials (Edinburgh), V. 15,No. 1, Jan., pp. 1-7.

ASTM International, 1994, “Significance and Tests Proper-ties of Concrete and Concrete-Making Materials,” STP 169,ASTM International, West Conshohocken, PA, 571 pp.

Barnes, B. D.; Orndorff, R. L.; and Roten, J. E., 1977,“Low Initial Curing Temperature Improves the Strength ofConcrete Test Cylinders,” ACI JOURNAL, Proceedings V. 74,No. 12, Dec., pp. 612-615.

Berhane, Z., 1984, “Evaporation of Water from Fresh Mortarand Concrete at Different Environmental Conditions,” ACIJOURNAL, Proceedings V. 81, No. 6, Nov.-Dec., pp. 560-565.

Bloem, D., 1954, “Effect of Curing Conditions onCompressive Strengths of Concrete Cylinders,” PublicationNo. 53, NRMCA, Dec., 15 pp.

Cebeci, O. Z., 1986, “Hydration and Porosity of CementPaste in Warm and Dry Environment,” 8th International

Congress on the Chemistry of Cement, Rio de Janeiro, V. III,pp. 412-416, 423-424.

Cebeci, O. Z., 1987, “Strength of Concrete in Warm andDry Environment,” Materials and Structures, Research andTesting (RILEM, Paris), V. 20, No. 118, July, pp. 270-272.

Collepardi, M.; Corradi, M.; and Valente, M., 1979,“Low-Slump-Loss Superplasticized Concrete,” Transporta-tion Research Record 720, Transportation Research Board,Washington, DC, Jan., pp. 7-12.

Dilley, A. C., 1968, “On the Computer Calculation ofVapor Pressure and Specific Humidity Gradients fromPsychometric Data,” Journal of Applied Meteorology, V. 7, No.4, Aug., pp. 717-719.

Gaynor, R. D.; Meininger, R. C.; and Khan, T. S., 1985,“Effects of Temperature and Delivery Time on ConcreteProportions,” Temperature Effects on Concrete, STP-858,ASTM International, West Conshohocken, PA, pp. 68-87.

Guennewig, T., 1988, “Cost-Effective Use of Super-plasticizers,” Concrete International, V. 10, No. 3, Mar.,pp. 31-34.

Hampton, J. S., 1981, “Extended Workability of ConcreteContaining High-Range Water-Reducing Admixtures in HotWeather,” Developments in the Use of Superplasticizers,SP-68, V. M. Malhotra, ed., American Concrete Institute,Farmington Hills, MI, pp. 409-422.

Hover, K. C., 2006, “Evaporation of Water from ConcreteSurfaces,” ACI Materials Journal, V. 103, No. 5, Sept.-Oct.,pp. 384-389.

Klieger, P., 1958, “Effect of Mixing and Curing Temperatureon Concrete Strength,” ACI JOURNAL, Proceedings V. 54,No. 12, June, pp. 1063-1081. Also, Research DepartmentBulletin 103, PCA Association, Skokie, IL.

Kohler, M. A., 1952, “Lake and Pan Evaporation,” WaterLoss Investigations: Lake Hefner Studies, Geological SurveyCircular 229, U.S. Government Printing Office, Wash-ington, DC.

Kohler, M. A., 1954, “Lake and Pan Evaporation,” WaterLoss Investigations: Lake Hefner Studies, Technical Report,Geological Survey Professional Paper 269, U.S. Govern-ment Printing Office, Washington, DC, pp. 127-148.

Kohler, M. A.; Nordenson, T. J.; and Fox, W. E., 1955,“Evaporation from Pans and Lakes,” Research Paper No.38, U.S. Department of Commerce, Washington, DC, May.

Krauss, P. D., and Rogalla, E. A., 1996, “TransverseCracking in Newly Constructed Bridge Decks,” NCHRPReport 380, National Cooperative Highway ResearchProgram, Transportation Research Board, NationalAcademy Press, Washington, DC, 126 pp.

Lee, M., 1987, “New Technology in Concrete Cooling,”Concrete Products, V. 89, No. 7, July, pp. 24-26, 36.

Lerch, W., 1957, “Plastic Shrinkage,” ACI JOURNAL,Proceedings V. 53, No. 8, Feb., pp. 797-802.

Mehta, P. K., 1986, Concrete Structures: Properties andMaterials, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 56-57.

Menzel, C. A., 1954, “Causes and Prevention of CrackDevelopment in Plastic Concrete,” Proceedings, PCAAnnual Meeting, pp. 130-136.



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Mills, G. A., 1975, “A Comparison of Some Formulae forthe Calculation of Saturation Vapor Pressure Over Water,”Meteorological Note No. 82, Bureau of Meteorology,Australia, Nov.

Mittelacher, M., 1985, “Effect of Hot Weather Conditionson the Strength Performance of Set-Retarded FieldConcrete,” Temperature Effects on Concrete, STP 858,ASTM International, West Conshohocken, PA, pp. 88-106.

Mittelacher, M., 1992, “Compressive Strength and theRising Temperature of Field Concrete,” Concrete Interna-tional, V. 14, No. 12, Dec., pp. 29-33.

Murray, F. W., 1967, “On the Computation of SaturationVapor Pressure,” Journal of Applied Meteorology, V. 6, No. 1,Feb., pp. 203-204.

NRMCA, 1962, “Cooling Ready Mixed Concrete,” Publi-cation No. 106, Silver Spring, MD, June, 7 pp.

Portland Cement Association, 1992, Design and Controlof Concrete Mixtures, 13th edition, PCA, Skokie, IL, 212 pp.

Ravina, D., 1984, “Slump Loss of Fly Ash Concrete,”Concrete International, V. 6, No. 4, Apr., pp. 35-39.

Ravina, D., and Shalon, R., 1968a, “Shrinkage of FreshMortars Cast under and Exposed to Hot Dry ClimaticConditions,” Proceedings, Colloquium on Shrinkage ofHydraulic Concrete, RILEM/Cembureau, Paris, V. 2,Instituto Eduardo Torroja, Madrid.

Ravina, D., and Shalon, R., 1968b, “Plastic Shrinkage andCracking,” ACI JOURNAL, Proceedings V. 65, No. 4, Apr.,pp. 282-291.

Tetens, O., 1930, “Uber einige meteorologische Begriffe,”Zeitschrift fur Geophysik, V. 6, p. 297.

Tuthill, L. H., and Cordon, W. A., 1955, “Properties andUses of Initially Retarded Concrete,” ACI JOURNAL,Proceedings V. 52, No. 3, Nov., pp. 273-286.

Uno, P. J., 1998, “Plastic Shrinkage Cracking and Evapo-ration Formulas,” ACI Materials Journal, V. 95, No. 4, July-Aug., pp. 365-375.

U.S. Bureau of Reclamation, 1975, Concrete Manual,eighth edition, Denver, CO, 627 pp.

Verbeck, G. J., and Helmuth, R. H., 1968, “Structure andPhysical Properties of Cement Pastes,” Proceedings, FifthInternational Symposium on the Chemistry of Cement,Tokyo, V. III, pp. 1-32.

Virginia Department of Transportation, 1997, “Specifi-cations for Highway and Bridge Construction,” VDOT,Richmond, VA.

Weast, R. C., ed., 1987, CRC Handbook of Chemistry andPhysics, 68th edition, CRC Press, Boca Raton, FL.

Yamamoto, Y., and Kobayashi, S., 1986, “Effect ofTemperature on the Properties of SuperplasticizedConcrete,” ACI JOURNAL, Proceedings V. 83, No. 1, Jan.-Feb., pp. 80-87.

APPENDIX A—ESTIMATING CONCRETE TEMPERATURE

A1—Estimating temperature of freshly mixed concrete

Equations for estimating temperature T of freshly mixedconcrete are shown in Eq. (A-1) through (A-3).

Without ice (in.-lb and SI units)

(A-1)

With ice (in.-lb units)

(A-2)

With ice (SI units)

(A-3)

whereTa = temperature of aggregate;Tc = temperature of cement;Tw = temperature of batched mixing water from normal

supply excluding ice;Ti = temperature of ice, °F (°C) (Note: Temperature of

free and absorbed water on the aggregate is assumedto be the same temperature as the aggregate.);

Wa = dry mass of aggregate;Wc = mass of cement;Wi = mass of ice;Ww = mass of batched mixing water; andWwa = mass of free and absorbed moisture in aggregate

at Ta, lb (kg).

A2—Estimating temperature of concrete with iceEquations (A-2) and (A-3), for estimating the temperature

of concrete with ice in U.S. customary or SI units, assumethat the ice is at its melting point. A more exact approachwould be to use Eq. (A-4) or (A-5), which includes thetemperature of the ice.

With ice (in.-lb units)

(A-4)

+

With ice (SI units)

(A-5)

+

T0.22 TaWa TcWc+( ) TwWw TaWwa+ +

0.22 Wa Wc+( ) Ww Wwa+ +--------------------------------------------------------------------------------------------=

T0.22 TaWa TcWc+( ) TwWw TaWwa 112Wi–+ +

0.22 Wa Wc+( ) Ww Wi Wwa+ + +------------------------------------------------------------------------------------------------------------------=

T0.22 TaWa TcWc+( ) TwWw TaWwa 79.6Wi–+ +

0.22 Wa Wc+( ) Ww Wi Wwa+ + +-------------------------------------------------------------------------------------------------------------------=


0.22 Wa Wc+( ) Ww Wi Wwa+ + +--------------------------------------------------------------------------------------------=

TaWwa Wi 128 0.5Ti–( )–

0.22 Wa Wc+( ) Ww Wi Wwa+ + +--------------------------------------------------------------------------------


0.22 Wa Wc+( ) Ww Wi W+ wa+ +--------------------------------------------------------------------------------------------=

TaWwa Wi 79.6 0.5Ti–( )–

0.22 Wa Wc+( ) Ww Wi W+ wa+ +--------------------------------------------------------------------------------



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APPENDIX B—METHODS FOR COOLING FRESH CONCRETE

The summary is limited to a description of methods suitablefor most structural uses of concrete. Methods for the coolingof mass concrete are explained in ACI 207.4R.

B.1—Cooling with chilled mixing waterConcrete can be cooled to a moderate extent by using

chilled mixing water; the maximum reduction in concretetemperature that can be obtained is approximately 10°F(6°C). The quantity of cooled water cannot exceed themixing water requirement, which depends on the moisturecontent of aggregates and mixture proportions. The methodinvolves a significant investment in mechanical refrigerationequipment and insulated water storage large enough for theanticipated hourly and daily production rates of cooledconcrete. Available systems include one that is based onheat-pump technology, which is usable for both cooling andheating of concrete. Apart from its initial installation price,this system appears to offer cooling at the lowest price ofavailable systems for cooling mixing water.

B.2—Liquid nitrogen cooling of mixing waterMixing water can be chilled rapidly through injection of

liquid nitrogen into an insulated holding tank. This chilledwater is then dispensed into the batch. Alternatively, themixing water may be turned into ice slush by liquid nitrogeninjection into the mixing water stream as it is discharged intothe mixer. The system enables cooling by as much as 20°F(11°C). The ratio of ice-to-water in the slush should beadjusted to produce the temperature of concrete desired.Installation of this system requires insulated mixing waterstorage, a nitrogen supply vessel, batch controls, and auxiliaryequipment. Apart from the price of installation, there areoperating expenses from liquid nitrogen usage and rentalfees for the nitrogen supply vessel. The method differs fromthat by direct liquid nitrogen injection into mixed concretedescribed in Section B.4.

B.3—Cooling concrete with iceConcrete can be cooled by using ice for part of the mixing

water. The amount of cooling is limited by the amount ofmixing water available for ice substitution. For mostconcrete, the maximum temperature reduction is approxi-mately 20°F (11°C). For correct proportioning, the iceshould be weighed. Cooling with block ice involves the useof a crusher/slinger unit, which can finely crush a block ofice and blow it into the mixer. A major obstacle to the use ofblock ice in many areas is insufficient supply. The price ofusing block ice is: the price of ice including transportation,refrigerated storage, handling and crushing equipment,additional labor, and, if required, provisions for weighingthe ice. An alternative to using block ice is to set up an iceplant near the concrete plant. As the ice is produced, it is

weighed, crushed, and conveyed into the mixer. It can alsobe produced and used as flake ice. This system requires alarge capital investment.

B.4—Cooling mixed concrete with liquid nitrogenB.4.1 Injecting liquid nitrogen into freshly mixed concrete

is an effective method for reduction of concrete temperature.The practical lower limit of concrete temperature is reachedwhen concrete nearest the injection nozzle forms into afrozen lump; this is likely to occur when the desired concretetemperature is less than 50°F (10°C). The method has beensuccessfully used in a number of major concrete placements.The performance of concrete was not affected adversely byits exposure to large amounts of liquid nitrogen. The price ofthis method is relatively high, but it can be justified on thebasis of practical considerations and overall effectiveness.

B.4.2 Installation of the system consists of a nitrogensupply vessel and injection facility for central mixers, or oneor more injection stations for truck mixers. The system can beset up at the construction site for last-minute cooling of theconcrete before placement. This reduces temperature gains ofcooled concrete in transit between the concrete plant and jobsite. Coordination is required in the dispatching of liquidnitrogen tanker trucks to injection stations for the timelyreplenishing of gas consumed in the cooling operations. Thequantity of liquid nitrogen required will vary according tomixture proportions and constituents, and the amount oftemperature reduction. The use of 135 ft3 (48 m3) of liquidnitrogen will usually reduce concrete temperature 1°F (0.5°C).

B.5—Cooling of coarse aggregatesB.5.1 An effective method of lowering the temperature of

the coarse aggregate is by cool water spraying or inundation.Coarse aggregate has the greatest mass in a typical concretemixture. Reducing the temperature of the aggregateapproximately 2 ± 0.5°F (1 ± 0.5°C) lowers the finalconcrete temperature approximately 1°F (0.5°C). To use thismethod, the producer should have available large amounts ofchilled water and the necessary water-cooling equipment forproduction requirements. This method is most effective whenadequate amounts of coarse material are contained in a silo orbin so that cooling can be accomplished in a short period oftime. Care should be taken to evenly inundate the material sothat slump variation from load to load is minimized.

B.5.2 Cooling of coarse aggregate can also be accomplishedby blowing air through the moist aggregate. The air flowenhances evaporative cooling, and can bring the coarseaggregate temperature within 2°F (1°C) of wet bulbtemperature. The effectiveness of the method depends onambient temperature, relative humidity, and velocity of airflow. The added refinement of using chilled air instead of airat ambient temperature can reduce the coarse aggregatetemperature to as low as 45°F (7°C). This method, however,involves a relatively high installation price.



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As ACI begins its second century of advancing concrete knowledge, its original chartered purposeremains “to provide a comradeship in finding the best ways to do concrete work of all kinds and inspreading knowledge.” In keeping with this purpose, ACI supports the following activities:

· Technical committees that produce consensus reports, guides, specifications, and codes.

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· Student programs such as scholarships, internships, and competitions.

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· Periodicals: the ACI Structural Journal and the ACI Materials Journal, and Concrete International.

Benefits of membership include a subscription to Concrete International and to an ACI Journal. ACImembers receive discounts of up to 40% on all ACI products and services, including documents, seminarsand convention registration fees.

As a member of ACI, you join thousands of practitioners and professionals worldwide who share acommitment to maintain the highest industry standards for concrete technology, construction, andpractices. In addition, ACI chapters provide opportunities for interaction of professionals and practitionersat a local level.

American Concrete Institute38800 Country Club DriveFarmington Hills, MI 48331U.S.A.Phone: 248-848-3700Fax: 248-848-3701

www.concrete.org





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The AMERICAN CONCRETE INSTITUTE

was founded in 1904 as a nonprofit membership organization dedicated to publicservice and representing the user interest in the field of concrete. ACI gathers anddistributes information on the improvement of design, construction andmaintenance of concrete products and structures. The work of ACI is conducted byindividual ACI members and through volunteer committees composed of bothmembers and non-members.

The committees, as well as ACI as a whole, operate under a consensus format,which assures all participants the right to have their views considered. Committeeactivities include the development of building codes and specifications; analysis ofresearch and development results; presentation of construction and repairtechniques; and education.

Individuals interested in the activities of ACI are encouraged to become a member.There are no educational or employment requirements. ACI’s membership iscomposed of engineers, architects, scientists, contractors, educators, andrepresentatives from a variety of companies and organizations.

Members are encouraged to participate in committee activities that relate to theirspecific areas of interest. For more information, contact ACI.

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guide to hot weather concreting · 2020. 5. 12. · aci 207.1r, 207.2r, and 224r. chapter...

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