《ACI-207.4R-2005.pdf》由会员分享,可在线阅读,更多相关《ACI-207.4R-2005.pdf(15页珍藏版)》请在三一文库上搜索。
1、ACI 207.4R-05 supersedes 207.4R-93 (Reapproved 1998) and became effective August 15, 2005. Copyright 2005, American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or m
2、echanical device, printed, written, or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. ACI Committee Reports, Guides, Standard Practices, and Commentaries are inten
3、ded for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use 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 materi
4、al 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
5、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. 207.4R-1 Cooling and Insulating Systems for Mass Concrete Reported by ACI Committee 207 ACI 207.4R-05 The need to control volume change induced
6、primarily by temperature change in mass concrete often requires cooling and insulating systems. This report reviews precooling, postcooling, and insulating systems. A sim- plified method for computing the temperature of freshly mixed concrete cooled by various systems is also presented. Keywords: ce
7、ment content; coarse aggregate; creep; formwork; heat of hydration; mass concrete; modulus of elasticity; precooling; postcooling; pozzolan; restraint; specific heat; strain; stress; temperature rise; tensile strength; thermal conductivity; thermal diffusivity; thermal expansion; thermal gradient; t
8、hermal shock. CONTENTS Chapter 1Introduction, p. 207.4R-1 1.1Scope and objective 1.2Historical background 1.3Types of structures and temperature controls 1.4Construction practices for temperature control 1.5Instrumentation Chapter 2Precooling systems, p. 207.4R-3 2.1General 2.2Heat exchange 2.3Batch
9、 water 2.4Aggregate cooling 2.5Cementitious materials 2.6Heat gains during concreting operations 2.7Refrigeration plant capacity 2.8Placement area Chapter 3Postcooling systems, p. 207.4R-9 3.1General 3.2Embedded pipe 3.3Refrigeration and pumping facilities 3.4Operational flow control 3.5Surface cool
10、ing Chapter 4Surface insulation, p. 207.4R-11 4.1General 4.2Materials 4.3Horizontal surfaces 4.4Formed surfaces 4.5Edges and corners 4.6Heat absorption from light energy penetration 4.7Geographical requirements Chapter 5References, p. 207.4R-14 5.1Referenced standards and reports 5.2Cited references
11、 CHAPTER 1INTRODUCTION 1.1Scope and objective The need to control volume change induced primarily by temperature change in mass concrete often requires cooling Jeffrey C. AllenTeck L. ChuaDavid E. Kiefer Terrence E. ArnoldEric J. DitcheyGary R. Mass Randall P. BassTimothy P. DolenTibor J. Pataky J.
12、Floyd BestBarry D. FehlErnest K. Schrader Anthony A. BombichRodney E. HolderbaumGary P. Wilson Robert W. CannonAllen J. Hulshizer Stephen B. Tatro Chair 207.4R-2ACI COMMITTEE REPORT and insulating systems. This report discusses three construction procedures used to control temperature changes in con
13、crete structures: precooling of materials, postcooling of in-place concrete by embedded pipes, and surface insulation. Other design and construction practices, such as selection of cementing materials, aggregates, chemical admixtures, cement content, or strength requirements, are not within the scop
14、e of this report. The objective of this report is to offer guidance on the selection and application of these procedures for reducing thermal cracking in all types of concrete structures. 1.2Historical background Major developments in cooling and insulating systems for concrete began with postcoolin
15、g systems for dams. Later gains were made in developing precooling methods. The use of natural cooling methods has increased with the use of better analytical methods to compute thermal performance. Similarly, insulating systems expanded beyond just cold weather protection and into control of therma
16、l gradients during other weather conditions. The first major use of postcooling of in-place mass concrete was in the construction of the Bureau of Reclama- tions Hoover Dam in the early 1930s. The primary objective was to accelerate thermal contraction of the concrete mono- liths within the dam so t
17、hat 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 several weeks or more after the concrete had been placed. Since the constructi
18、on of Hoover Dam, the same basic system of postcooling has been used in the construction of many large dams and other massive structures, such as power- houses, except that circulation of cooling water is now typi- cally initiated immediately after placing the concrete. In the early 1940s, the Tenne
19、ssee Valley Authority used postcooling in the construction of Fontana Dam for two purposes: to control the temperature rise, particularly in the vulnerable base of the dam where cracking of the concrete could be induced by the restraining effect of the foundation; and to accelerate thermal contracti
20、on 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 l
21、evel in all seasons. 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
22、 m). Above the critical zone, the lift thickness was increased to 5.0 ft (1.5 m), and the pipe spacing was increased 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
23、 starting, stopping, and restarting the cooling process based on temperatures measured with embedded resistance thermometers. At Dworshak Dam and at the Ice Harbor Additional Power House Units, the cooling water was stopped when the temperature of the concrete near the pipes began to drop rapidly af
24、ter reaching a peak. Within 1 to 3 days later, when the temperature would rise 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
25、 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 close attention to the heat-of- hydration issues due to their effect on the final stress sta
26、te of the dam. In China, several arch dams have been completed, including Shapai Dam near Chengdu, China, which was the worlds 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 tempe
27、rature 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 used in conventional concrete dams in the past. By late 2003, 14 RCC arch dams had been completed or were under cons
28、truction, mainly in China and South Africa. The first reported use 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
29、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 ve
30、ry common for mass concrete placements. It also is used for placements of relatively small dimensions, such as for bridge piers and foundations where there is sufficient concern for minimizing thermal stresses. Injection of cold nitrogen gas into the mixer has been used to precool concrete in recent
31、 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 c
32、hamber. Nitrogens main inefficiency is losing gas to the atmosphere if the mixer or transfer is not well enclosed. Various combinations of crushed ice, cold batch water, liquid nitrogen, and cooled aggregate are used to lower placement temperature to 50 F (10 C) and, when necessary, to as low as 40
33、F (4.5 C). RCC projects have effectively used “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
34、the warm 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 observed in sou
35、thern New Mexicos Grindstone Canyons coarse aggregate stockpile as late as June. Precooling and postcooling have been used in combination in the construction of massive structures such as Glen Canyon Dam, completed in 1963; Dworshak Dam, completed in 1975; and the Lower Granite Dam Powerhouse additi
36、on, completed in 1978. Insulation has been used 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 used since 1950. It has become an
37、effective practice where needed. The first extensive use 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 Schrader 1987). 1.4Construction practices for temperature control Practices that have evo
38、lved to control temperatures and consequently minimize thermal stress and cracking are listed below. Some of these require minimal effort, while others require substantial initial expense: Cooling batch water; Producing aggregate during cold seasons or cool nights; Replacing a portion of the batch w
39、ater with ice; Shading aggregates in storage; Shading aggregate conveyors; Spraying aggregate stockpiles for evaporative cooling; Immersion in cool water or saturation of coarse aggregates, including wet belt cooling; Vacuum evaporation of moisture in coarse aggregate; Nitrogen injection into the mi
40、xture 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 w
41、ater 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 exposed edges and corners from excessive heat loss; Cooling aggregates with natu
42、ral or manufactured chilled air; and Better monitoring of ambient and material temperatures. 1.5Instrumentation 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 p
43、ractice has used thermocouples placed at various locations within large aggregate stockpiles to monitor temperatures in the piles, especially when the aggregate is processed and stockpiled well in advance of when it is used. Postcooling systems require embedded temperature-sensing devices (thermocou
44、ples 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 used to measure internal volume change, stress, strain, and joint moveme
45、nt have been described (Carlson 1970; USACE 1980). CHAPTER 2PRECOOLING SYSTEMS 2.1General Minimizing the temperature of the fresh concrete at placement is one of the most important and effective ways to minimize thermal stresses and cracking. Generally, the lower the temperature of the concrete when
46、 it passes 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 approxima
47、tely 4 to 6 F (2 to 3 C) (ACI 207.2R). A simple example demonstrates how precooling can minimize thermal stresses and cracking. Under most conditions 207.4R-4ACI COMMITTEE REPORT of restraint in mass concrete structures, low levels of stress (or strain) will be developed during and for a short time
48、after the setting of the concrete. The compressive stresses caused by thermal expansion due to the initial high temperature rise 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
49、 after final setting during the temperature rise, an idealized condition of zero compressive stress may result when peak temperature is finally reached. Of course, under realistic conditions, the actual stressed 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 selected to limit resulting tensile strain from exceedin
链接地址:https://www.31doc.com/p-3728743.html