ACI 207.4R-05 superdes 207.4R-93 (Reapproved 1998) and became effective August 15, 2005.
Copyright © 2005, American Concrete Institute.
All rights rerved including rights of reproduction and u in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduction or for u in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.
订单号ACI Committee Reports, G uides, Standard Practices, and Commentaries are intended for guidance in planning,designing, executing, and inspecting construction. This document is intended for the u of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains.The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.
Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.
Cooling and Insulating Systems
for Mass Concrete
Reported by ACI Committee 207
质点系ACI 207.4R-05
The n eed to con trol volume chan ge in duced primarily by temperature chan ge in mass con crete often requires coolin g an d in sulatin g systems.This report reviews precooling, postcooling, and insulating systems. A sim-plified method for computin g the temperature of freshly mixed con crete cooled by various systems is also prented.
初中英语被动语态Keywords: cement content; coar aggregate; creep; formwork; heat of hydration; mass concrete; modulus of elasticity; precooling; postcooling;pozzolan; restraint; specific heat; strain; stress; temperature ri; tensile strength; thermal conductivity; thermal diffusivity; thermal expansion; thermal gradient; thermal shock.
CONTENTS
Chapter 1—Introduction, p. 207.4R-11.1—Scope and objective 1.2—Historical background
1.3—Types of structures and temperature controls 1.4—Construction practices for temperature control 1.5—Instrumentation
Chapter 2—Precooling systems, p. 207.4R-32.1—General
2.2—Heat exchange 2.3—Batch water
2.4—Aggregate cooling 2.5—Cementitious materials
2.6—Heat gains during concreting operations 2.7—Refrigeration plant capacity 2.8—Placement area
Chapter 3—Postcooling systems, p. 207.4R-93.1—General
3.2—Embedded pipe
3.3—Refrigeration and pumping facilities 3.4—Operational flow control 3.5—Surface cooling
Chapter 4—Surface insulation, p. 207.4R-114.1—General 4.2—Materials
4.3—Horizontal surfaces 4.4—Formed surfaces 4.5—Edges and corners
4.6—Heat absorption from light energy penetration 4.7—Geographical requirements Chapter 5—References, p. 207.4R-14
5.1—Referenced standards and reports 5.2—Cited references
CHAPTER 1—INTRODUCTION
1.1—Scope and objective
The need to control volume change induced primarily by temperature change in mass concrete often requires cooling
Jeffrey C. Allen Teck L. Chua David E. Kiefer Terrence E. Arnold Eric J. Ditchey Gary R. Mass Randall P. Bass Timothy P. Dolen Tibor J. Pataky J. Floyd Best Barry D. Fehl Ernest K. Schrader Anthony A. Bombich Rodney E. Holderbaum Gary P. Wilson
Robert W. Cannon
Allen J. Hulshizer
Stephen B. Tatro
Chair
Copyright American Concrete Institute --`,`,````,`,```,,,````,,,`,,,,`-`-`,,`,,`,`,,`---
207.4R-2ACI COMMITTEE REPORT
and insulating systems. This report discuss three construction procedures ud to control temperature changes in concrete structures: precooling of materials, postcooling of in-place concrete by embedded pipes, and surface insulation. Other design and construction practices, such as lection of cementing materials, aggregates, chemical admixtures, cement content, or strength requirements, are not within the scope of this report.
The objective of this report is to offer guidance on the lection and application of the procedures for reducing thermal cracking in all types of concrete structures.
1.2—Historical background
Major developments in cooling and insulating systems for concrete began with postcooling systems for dams. Later gains were made in developing precooling methods. The u of natural cooling methods has incread with the u of better analytical methods to compute thermal performance. S
imilarly, insulating systems expanded beyond just cold weather protection and into control of thermal gradients during other weather conditions.
The first major u of postcooling of in-place mass concrete was in the construction of the Bureau of Reclama-tion’s Hoover Dam in the early 1930s. The primary objective was to accelerate thermal contraction of the concrete mono-liths within the dam so that the contraction joints could be filled with grout to ensure monolithic action of the dam. Cooling was achieved by circulating cold water through pipes embedded in the concrete. Circulation of water was usually started veral weeks or more after the concrete had been placed. Since the construction of Hoover Dam, the same basic system of postcooling has been ud in the construction of many large dams and other massive structures, such as power-hous, except that circulation of cooling water is now typi-cally initiated immediately after placing the concrete.
In the early 1940s, the Tenne Valley Authority ud postcooling in the construction of Fontana Dam for two purpos: to control the temperature ri, particularly in the vulnerable ba of the dam where cracking of the concrete could be induced by the restraining effect of the foundation; and to accelerate thermal contraction of the columns so that the contraction joints between columns could be filled with grout to ensure monolithic action. Postcooling was started coincidently with the placing
of each lift of concrete. The pipe spacing and lift thickness were varied to limit the maximum temperature to a predesigned level in all asons. In summer, with naturally high (unregulated) placing temperatures, the pipe spacing and lift thickness for the critical foundation zone was 2.5 ft (0.76 m); in winter, when placing temperatures were naturally low, the pipe spacing and lift thickness for this zone was 5.0 ft (1.5 m). Above the critical zone, the lift thickness was incread to 5.0 ft (1.5 m), and the pipe spacing was incread to 6.25 ft (1.9 m). Cooling was also started in this latter zone coincidently with the placing of concrete in each new lift.
In the 1960s, the Corps of Engineers began the practice of starting, stopping, and restarting the cooling process bad on temperatures measured with embedded resistance thermometers. At Dworshak Dam and at the Ice Harbor Additional Power Hou Units, the cooling water was stopped when the temperature of the concrete near the pipes began to drop rapidly after reaching a peak. Within 1 to 3 days later, when the temperature would ri again to the previous peak temperature, cooling would be started again to produce controlled, safe cooling.
Generally, arch dams were constructed with postcooling systems to expedite the volume change of the mass concrete for joint grouting. The first roller-compacted concrete (RCC) arch dam was Knellpoort Dam in South Africa, completed in 1988. Due to the height and rapid construction of RCC
arch dams, design engineers paid clo attention to the heat-of-hydration issues due to their effect on the final stress state of the dam. In China, veral arch dams have been completed, including Shapai Dam near Chengdu, China, which was the world’s highest until 2004. At Shapai Dam, and others since, cooling pipes were embedded between some of the RCC lifts to circulate cool liquid to control the maximum internal temperature of the RCC. Testing showed that high-density polyethylene cooling pipes worked quite well with RCC. The controls and operation procedures for the RCC arch dams were the same as ud in conventional concrete dams in the past. By late 2003, 14 RCC arch dams had been completed or were under construction, mainly in China and South Africa.
The first reported u of precooling concrete materials to reduce the maximum temperature of mass concrete was by the Corps of Engineers during the construction of Norfork Dam from 1941 to 1945. A portion of the batch water was introduced into the mixture as crushed ice. Placement temperature of the concrete was reduced by approximately 10 °F (6 °C). The concrete was cooled as a result of the thermal energy (heat of fusion) required to convert ice to water and from the lowered temperature of the water after melting. Since then, precooling has become very common for mass concrete placements. It also is ud for placements of relatively small dimensions, such as for bridge piers and foundations where there is sufficient concern for minimizing thermal stress.
Injection of cold nitrogen gas into the mixer has been ud to precool concrete in recent years. Practical and economical considerations should be evaluated, but it can be effective. As with ice, additional mixing time may be required. Minor amounts of concrete cooling have been achieved by injecting it at transfer points on conveyor delivery systems, in gob hoppers, and in the mixing chamber. Nitrogen’s main inefficiency is losing gas to the atmosphere if the mixer or transfer is not well enclod.
Various combinations of crushed ice, cold batch water, liquid nitrogen, and cooled aggregate are ud to lower placement temperature to 50 °F (10 °C) and, when necessary, to as low as 40 °F (4.5 °C).
RCC projects have effectively ud “natural” precooling of aggregate during production. Large quantities of aggregate produced during cold winter months or during cold nighttime temperatures and stockpiled in naturally cold conditions can remain cold at the interior of the pile well into the warm
Copyright American Concrete Institute --`,`,````,`,```,,,````,,,`,,,,`-`-`,,`,,`,`,,`---
COOLING AND INSULATING SYSTEMS FOR MASS CONCRETE207.4R-3
summer months. At Middle Fork, Monksville, and Stage-coach Dams, it was not unusual to find frost in the stockpiles during production of RCC in the summer at ambient temperatures about 75 to 95 °F (24 to 35 °C). Ice was obrved in southern New Mexico’s rindstone Canyon’s coar aggregate stockpile as late as June. Precooling and postcooling have been ud in combination in the construction of massive structures such as Glen Canyon Dam, completed in 1963; Dworshak Dam, completed in 1975; and the Lower G ranite Dam Powerhou addition, completed in 1978.
Insulation has been ud on lift surfaces and concrete faces to prevent or minimize the potential for cracking under sudden drops in ambient temperatures. This method of minimizing cracking by controlling rapid cooling of the surface has been ud since 1950. It has become an effective practice where needed. The first extensive u of insulation was during the construction of Table Rock Dam, built during 1955 to 1957. More recent examples of mass concrete insulation include the Lock & Dam 26 (Bombich, Norman, and Jones 1987), McAlpine Lock Replacement, and Victoria RCC dam in northern Michigan, constructed in 1992. Insulation of expod surfaces, for the purpo of avoiding crack development, supplements other control measures during construction, such as precooling materials and post-cooling of in-place concrete. A uful practice is to apply surface insulation in layers, such as with multiple blan-kets, so that the insulation can be removed gradually i
n warmer weather. Removing all the insulation at once can cau cracking if the air temperature is much lower than the concrete temperature under the insulation.
In addition to reducing thermal stress, other benefits result from mixing and placing concrete at lower tempera-tures, such as enhanced long-term durability and strength, improved consistency, and longer placement time. The improved workability can, at times, be ud to reduce the water requirement. Cooler concrete is also more responsive to vibration during consolidation.
1.3—Types of structures and temperature controls Cooling and insulating systems have evolved to meet engineering and construction requirements for massive concrete structures, such as concrete gravity dams, arch dams, navigation locks, nuclear reactors, powerhous, large footings, mat foundations, and bridge piers. They are also applicable to smaller structures where high levels of internally developed thermal stress and potential cracks resulting from volume changes cannot be tolerated or would be highly objectionable (Tuthill and Adams 1972; Schrader 1987). 1.4—Construction practices for temperature control Practices that have evolved to control temperatures and conquently minimize thermal stress and cracking are listed below. Some of the require minimal effort, while others require substantial initial expen:
•Cooling batch water;
•Producing aggregate during cold asons or cool nights;•Replacing a portion of the batch water with ice;•Shading aggregates in storage;
•Shading aggregate conveyors;
•Spraying aggregate stockpiles for evaporative cooling;•Immersion in cool water or saturation of coar aggregates, including wet belt cooling;
•Vacuum evaporation of moisture in coar aggregate;•Nitrogen injection into the mixture and at transfer points during delivery;
•Using light-colored mixing and hauling equipment, and spraying the mixing, conveying, and delivery equipment with a water mist;
•Scheduling placements when ambient temperatures are lower, such as at night or during cooler times of the year;•Cooling cure water and the evaporative cooling of cure water;
•Postcooling with embedded cooling pipes;•Controlling surface cooling of the concrete with insulation;
•Avoiding thermal shock during form and insulation removal;
•Protecting expod edges and corners from excessive heat loss;
•Cooling aggregates with natural or manufactured chilled air; and
damn什么意思•Better monitoring of ambient and material temperatures.
1.5—Instrumentation
The monitoring of temperatures in concrete components and in fresh concrete can be adequately accomplished with ordi-nary portable thermometers capable of 1 °F (0.5 °C) resolution. Recent practice has ud thermocouples placed at various locations within large aggregate stockpiles to monitor temperatures in the piles, especially when the aggregate is procesd and stockpiled well in advance of when it is ud. Postcooling systems require embedded temperature-nsing devices (thermocouples or resistance thermometers) to provide special information for the control of concrete cooling rates. Similar instruments will provide the data to evaluate the degree of protection afforded by insulation. Other instruments ud to measure internal volume change, stress, strain, and joint movement have been described (Carlson 1970; USACE 1980).
CHAPTER 2—PRECOOLING SYSTEMS
2.1—General
confirmemailaddress
Minimizing the temperature of the fresh concrete at placement is one of the most important and effective ways to minimize thermal stress and cracking. G enerally, the lower the temperature of the concrete when it pass from a plastic or as-placed condition to an elastic state upon hardening, the lower the tendency toward cracking. In massive structures, each 10 °F (6 °C) reduction of the placing temperature below average air temperature will lower the peak temperature of the hardened concrete by approximately 4 to 6 °F (2 to
3 °C) (ACI 207.2R).
A simple example demonstrates how precooling can minimize thermal stress and cracking. Under most conditions
Copyright American Concrete Institute --` , ` , ` ` ` ` , ` , ` ` ` , , , ` ` ` ` , , , ` , , , , ` -` -` , , ` , , ` , ` , , ` ---
207.4R-4ACI COMMITTEE REPORT
of restraint in mass concrete structures, low levels of stress (or strain) will be developed during and f
or a short time after the tting of the concrete. The compressive stress caud by thermal expansion due to the initial high temperature ri are reduced to near zero as a result of a low modulus of elasticity and high creep rates of the early-age concrete. Assuming substantial relaxation continues for some time after final tting during the temperature ri, an idealized condition of zero compressive stress may result when peak temperature is finally reached. Of cour, under realistic conditions, the actual stresd state of the structure at peak temperature should be taken into account; however, assuming a state of zero compressive stress at peak temperature will immediately subject the concrete to tension when cooling begins. A concrete placing temperature may be lected to limit resulting tensile strain from exceeding the strain capacity of the concrete during subquent cooling from peak temperature to the final stable temperature. The procedure is described by the following relationship
(2-1)
where
T i =placing temperature of concrete;T f =final stable temperature of concrete;C =strain capacity (in millionths);
T i T f 100C ×e t R
×------------------∆t –+=e t =coefficient of thermal expansion per degree of
temperature (in millionths);
R =degree of restraint (in percent) (refer to ACI 207.2R);
and
∆t =initial temperature ri of concrete.
The object of the precooling program is to keep temperature T i low. The designer should know the type and extent of cracking that can be tolerated in the structure. Proper design can accommodate anticipated cracking. When circumstances favor the potential for cracking, provisions should be implemented to deal with cracking. In some cas, it may be more appropriate to allow thermal cracking to develop and then grout or al the cracks. This might occur, for example,when a large mass of concrete is placed at the ba of a deep excavation to act as a dead load to offt uplift pressures. The mass may also become a ba for footings and structural concrete that is sufficiently reinforced to resist potential problems due to the cracks.
2.2—Heat exchange
2.2.1 Heat capacities—The heat capacity of concrete is defined as the quantity of heat required to rai a unit mass of concrete one degree in temperature. In tho systems of units where the heat capacity of water is established as unity,heat capacity and specific heat are numerically the same. The specific heat of concrete is approximately 0.23 Btu/lb °F (0.963 kJ/ kg K); values for components of the mixture range from a low of approximately 0.16 (0.67) for some cements and aggregates to 1.00 (4.18) for water. The temperature of the mixed concrete is influenced by each component of the mixture and the degree of influence depends on the indi-vidual component’s temperature, specific heat, and propor-tion of the mixture. Becau aggregates compri the greatest part of a concrete mixture, a change in the tempera-ture of the aggregates will affect the greatest change (except where ice is ud) in the temperature of the concrete.Becau the amount of cement in a typically lean mass concrete mixture is relatively small, cooling it may not be significant in a temperature control program.
For convenience, the concrete batch and the components of the concrete batch can be considered in terms of a water equivalent, or the weight of water having an equivalent heat capacity. An example of 1 yd 3 of mass concrete and its water equivalent is shown in Table 2.1(a). An example of a 1 m 3
mass concrete mixture and its water equivalent shown in Table 2.1(b).
In other words, 1 yd 3 of this concrete would require the same amount of cooling to reduce (or heating to rai) its temperature 1 °F as would be required by 937 lb of water.Similarly, 1 m 3 of this concrete would require the same amount of cooling (or heating) to change its temperature 1 °C as would be required by 555 kg of water.
2.2.2 Computing the cooling requirement —Assume that a 50 °F (10 °C) placing temperature will satisfy the design criteria that have been established. From the temperatures of the concrete ingredients as they would be received under the most vere conditions, a computation can be made of the refrigeration capacity that would be required to reduce the
Table 2.1(a)—Water equivalent of 1 yd 3mass concrete
Ingredient Batch
weight, lb Specific heat capacity,Btu/lb ⋅ °F
Batch heat content,Btu/°F Water equivalent, lb Coar aggregate 28170.18哈密瓜英文
civil rights
5075071% moisture 28 1.002828Fine aggregate 8900.181601605% moisture 45 1.004545Cement 197
0.214141Fly ash 850.201717Batched water
139 1.00139139Totals
4201
—
937
937
Table 2.1(b)—Water equivalent of 1 m 3mass concrete
Ingredient Batch weight, kg Specific heat capacity, kJ/kg-K Batch heat content,kJ/K
Water
equivalent,
kg Coar aggregate 1672
0.7512543001% moisture 17 4.187117Fine aggregate 5280.75396955% moisture 26 4.1810926Cement 1170.8810325Fly ash 500.844210Batched water
82 4.1834382Totals
2492
—
2318
555
Copyright American Concrete Institute --`,`,````,`,```,,,````,,,`,,,,`-`-`,,`,,`,`,,`---
COOLING AND INSULATING SYSTEMS FOR MASS CONCRETE 207.4R-5
temperature of the mixture to 50 °F (10 °C). Using the same mass concrete mixture, the refrigeration requirement per cubic yard can be computed as shown in Table 2.2(a) and (b).The refrigeration requi
red for a 1 m 3 mixture is shown in Table 2.2(b).
If the concrete is mixed under initial temperature conditions in the example, the mixed temperature of the concrete will be inch-pound units:
= 50 °F + 27 °F = 77 °F
SI units:
= 10 °C + 15 °C = 25 °C To lower the temperature of the concrete to 50 °F (10 °C),it would be necessary to remove 25,131 Btu (26,514 kJ) from the system. The temperature of mixed concrete can be lowered by replacing all or a portion of the batch water with ice or by precooling the components of the concrete.Often one cooling method does not provide enough cooling capacity to meet required temperatures. Exam-ples of limitations that lead to multiple cooling methods include practical limitations to the amount of water can be reasonably replaced with ice, and aggregate stockpiles that have excessive free water that enters the mixture at the aggregate temperature.
2.2.3 Methods of precoolin g con crete compon en ts—The construction of mass concrete structures, primarily dams,has led to improved procedures for reducing the initial temperature of the concrete.
Concrete components can be precooled in veral ways.The batch water can be chilled, ice added, ice can be substituted for part of the batch water, or a combination of both. Sufficient water needs to be added as water rather than ice, allowing proper introduction of admixtures and reasonable mixing efficiency and times. Also, it is important to realize that ice added to batch water, thereby chilling the batch water,cannot also be considered in the calculation of replacement ice in the mixture. The energy spent melting does not cool the water and the mixture. If the ice is fully melted, its effect on cooling has fully been realized in achieving the batch water temperature. Aggregate stockpiles can be shaded.Aggregates can be procesd and stockpiled during cold weather. If the piles are large, only the outside expod portion will heat up a significant amount when warm weather occurs, prerving the colder interior for initial placements. Fine aggregates can be procesd with chilled water using a classifier, sand screw, or dewatering plate vibrator. Methods for cooling coar aggregates, which often provide the greatest potential for removing heat from the mixture, can include sprinkling stockpiles with water to provide for evaporative cooling, spraying chilled water on aggregates on slow-moving transfer belts, immersing coar
50°F 25,131 Btu
937Btu °⁄F
---------------------------+10°C 33,910 kJ 2318 kJ/K
------------------------+aggregates in tanks of chilled water, blowing chilled air through the batching bins or storage silos, and forcing evaporative chilling of coar aggregate by vacuum. While the most common and efficient u of nitrogen is to cool the concrete in the mixer, successful cooling of the mixture can result from cooling aggregates with nitrogen and cooling at concrete transfer points. Introduction of liquid nitrogen into cement and fly ash during transfer of the materials from the tankers to the storage silos has also been effective (Forbes et al.1991), but the overall benefit will be minimal if the amounts of cement and ash ud in the mixture are small. Combinations of veral of the practices are frequently necessary.
2.3—Batch water
The moisture condition of the aggregates should be considered not only for batching the designed concrete mixture, but also in the heat balance calculations for control of the placing temperature. The limited amount of water normally required for a mass concrete mixture does not often provide the capacity by itlf to adequately lower the temperature of the concrete even if ice is ud for nearly all of the batch water.
2.3.1 Chilled batch water —One lb of water absorbs 1 Btu when its temperature is raid 1 °F. Similarly, 1 kg of water absorbs 4.18 kJ when its temperature is raid 1 °C. A unit change in the temperature of the batch water has approximately five times the effect on the temperature of the concrete as a unit change in the temperature of the cement or aggregates.This is due to the higher specific heat of water with respect to the other materials. Equipment for chilling water is less complicated than ice-making equipment, and it avoids problems that can be encountered with handling and feeding ice to the
Table 2.2(a)—Refrigeration requirement per cubic yard
Ingredient Initial temperature,°F Degrees to 50 °F,°F Water equivalent,lb Btus to get to 50 °F,
Btu Moist coar aggregate 752553513,375Moist fine aggregate 73232054715
Cement
白马王子英文piab12070412870Fly ash
732317391Batched water
70
20
1392780Heat of mixing, estimated
1000Totals
937
25,131
Table 2.2(b)—Refrigeration requirement per cubic meter
Ingredient Initial
temperature,°C Degrees to 10 °C,°C Water equivalent,kg kJ * to get to 10 °C,
kJ
Moist coar aggregate 24
1430017,556Moist fine aggregaterdz
23131216575Cement 4939254076Fly ash
231310543Batched water
21
11
823770Heat of mixing, estimated
1390Totals
538
33,910
*Product of degrees to 10 °C × water equivalent × 4.18.
Copyright American Concrete Institute --`,`,````,`,```,,,````,,,`,,,,`-`-`,,`,,`,`,,`---
