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1、This document has been approved for use by agen- cies of the Department of Defense and for listing in the DoD Index of Specifications and Standards. (Reapproved 2002) ACI 306R-88 Cold Weather Concreting Reported by ACI Committee 306 Nicholas J. Carino, Chairman* Fred A. Anderson* Peter Antonich Geor
2、ge R. U. Burg* Oleh B. Ciuk Douglas J. Haavik* Gilbert J. Haddad Don B. Hill Jules Houde David A. Hunt Robert A. Kelsey The general requirements for producing satisfactory concrete during cold weather are discussed, and methods for satisfying these require- ments are described. One of the objectives
3、 of cold weather concret- ing practice is to provide protection of the concrete at early ages to prevent damage from freezing. For many structural concretes, pro- tection considerably in excess of that required to prevent damage by early freezing is needed to assure development of adequate strength.
4、 The following items are discussed in the report: recommended temperature of concrete, temperature records, temperature of mate- rials, preparations prior to placement, duration of protection period, methods for determining in-place strength, form removal, protective insulating covers, heated enclos
5、ures, curing methods, and accelerat- ing admixtures. References are included that provide supplementary data on the effects of curing temperature on concrete strength. Keywords: accelerating admixtures; age; aggregates; calcium chloride; cold weather; compressive strength; concrete construction; con
6、cretes; curing; dura- bility; form removal; formwork (construction); freeze-thaw durability; heat- ing; in-place testing; insulation; materials handling; protection; subgrade prep- aration; temperature. CONTENTS Chapter 1 -Introduction, p. 306R-1 1.1 - Definition of cold weather 1.2 - Standard speci
7、fication 1.3 - Objectives 1.4 - Principles 1.5 - Economy Chapter 2 - General requirements, p. 306R-3 2.1 - Planning 2.2 - Protection during fall and spring 2.3 - Concrete temperature 2.4 - Temperature records 2.5 - Heated enclosures 2.6 - Exposure to freezing and thawing 2.7 - Concrete slump ACI Com
8、mittee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in designing, plan- ning, executing, or inspecting construction and in preparing specifications. Reference to these documents shall not be made in the Project Documents. If items found in these documents are desir
9、ed to be part of the Project Documents they should be phrased in mandatory language and incorporated into the Project Documents. Albert W. Knott Derle Thorpe* William F. Perenchio Valery Tokar* John M. Scanlon* Harry H. Tormey Michael L. Shydlowski* Lewis H. Tuthill* Bruce A. Suprenant Harold B. Wen
10、zel Chapter 3 - Temperature of concrete as mixed and placed and heating of materials, p. 306R-5 3.1 - Placement temperature 3.2 - Mixing temperature 3.3 - Heating mixing water 3.4 - Heating aggregates 3.5 - Steam heating of aggregates 3.6 - Overheating of aggregates 3.7 - Calculation of mixture temp
11、erature 3.8 - Temperature loss during delivery Chapter 4 - Preparation before concreting, p. 306R-7 4.1 - Temperature of surfaces in contact with fresh concrete 4.2 - Metallic embedments 4.3 - Removal of snow and ice 4.4 - Condition of subgrade Chapter 5 - Protection against freezing and protection
12、for concrete not requiring construction supports, p. 306R-7 5.1 - Protection to prevent early-age freezing 5.2 - Need for additional protection 5.3 - Length of protection period 5.4 - Stripping of forms 5.5 - Temperature drop after removal of protection 5.6 - Allowable temperature differential Chapt
13、er 6 - Protection for structural concrete requiring construction supports, p. 306R-9 6.1 - Introduction 6.2 - Tests of field-cured specimens 6.3 - In-place testing 6.4 - Maturity method 6.5 - Attainment of design strength 6.6 - Increasing early strength 6.7 - Cooling of concrete 6.8 - Estimating str
14、ength development 6.9 - Removal of forms and supports 6.10 - Requirements *Task force member. This report supercedes ACI 306R-78 (Revised 1983). Copyright 2002, American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by any means, including the making
15、 of copies by any photo process, or by any electronic or mechanical device. printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. 306R-1 306R-2 MANU
16、AL OF CONCRETE PRACTICE Chapter 7 - Materials and methods of protection, p. 306R-13 7.1 - Introduction 7.2 - Insulating materials 7.3 - Selection of insulation 7.4 - Enclosures 7.5 - Internal electric heating 7.6 - Covering after placement 7.7 - Temporary removal of protection 7.8 - Insulated forms
17、Chapter 8 - Curing requirements and methods, p. 306R-20 8.1 - Introduction 8.2 - Curing during the protection period 8.3 - Curing following the protection period Chapter 9 - Acceleration of setting and strength development, p. 306R-21 9.1 - Introduction 9.2 - Calcium chloride as an accelerating admi
18、xture 9.3 - Other accelerating admixtures Chapter 10 - References, p. 306R-22 10.1 - Recommended references 10.2 - Cited references 10.3 - Selected references CHAPTER 1 - INTRODUCTION 1.1 - Definition of cold weather This report describes construction procedures which, if properly followed, can resu
19、lt in concrete placed in cold weather of sufficient strength and durability to satisfy intended service requirements. Concrete placed during cold weather will develop these qualities only if it is properly produced, placed, and protected. The necessary degree of protection increases as the ambient t
20、emperature decreases. Cold weather is defined as a period when, for more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 40 F (5 C) and 2) the air temperature is not greater than 50 F (10 C) for more than one-half of any 24-hr period.* The a
21、verage daily air temperature is the average of the highest and the lowest temperatures occurring during the period from midnight to midnight. Cold weather, as defined in this report, usually starts during fall and usually continues until spring. 1.2 - Standard specification If requirements for cold
22、weather concreting are needed in specification form, ACI 306.1 should be ref- erenced; if necessary, appropriate modifications should be added to the contract documents after consulting the specification checklist. 1.3 - Objectives The objectives of cold weather concreting practices are to: 1.3.1 -
23、prevent damage to concrete due to freezing at early ages. When no external water is available, the degree of saturation of newly placed concrete decreases *The values in SI units are direct conversions of the in.-lb values. They do nor necessarily represent common metric ranges or sizes. For practic
24、al applica- tion. The user should adjust them to conform with local practice. as the concrete gains maturity and the mixing water combines with cement during hydration. Under such conditions, the degree of saturation falls below the critical level (the degree of water saturation where a single cycle
25、 of freezing would cause damage) at ap- proximately the time that the concrete attains a com- pressive strength of 500 psi (3.5 MPa) (Powers 1962). At 50 F (10 C), most well-proportioned concrete mix- tures reach this strength during the second day. 1.3.2 - assure that the concrete develops the re-
26、quired strength for safe removal of forms, for safe re- moval of shores and reshores, and for safe loading of the structure during and after construction. 1.3.3 - maintain curing conditions that foster nor- mal strength development without using excessive heat and without causing critical saturation
27、 of the concrete at the end of the protection period. 1.3.4 - limit rapid temperature changes, particu- larly before the concrete has developed sufficient strength to withstand induced thermal stresses. Rapid cooling of concrete surfaces or large temperature dif- ferences between exterior and interi
28、or members of the structure can cause cracking, which can be detrimental to strength and durability. At the end of the required period, insulation or other means of protection should be removed gradually so that the surface temperature decreases gradually during the subsequent 24-hr period (see Sect
29、ion 5.5). 1.3.5 - provide protection consistent with the in- tended serviceability of the structure. Concrete struc- tures are intended for a useful life of many years. The attainment of satisfactory strength for 28-day, stan- dard-cured cylinders is irrelevant if the structure has corners damaged b
30、y freezing; dehydrated areas; and cracking from overheating because of inadequate pro- tection, improper curing, or careless workmanship. Similarly, early concrete strength achieved by indis- criminate use of excessive calcium chloride is of no avail if the concrete becomes excessively cracked in la
31、ter years because of the likelihood of disruptive internal expansion due to alkali-aggregate reaction or of possi- ble corrosion of reinforcement (see Section 9.2). Short- term construction economy should not be obtained at the expense of long-term durability. 1.4 - Principles This report presents r
32、ecommendations to achieve the objectives listed in Section 1.3 (Schnarr and Young 1934a and 1934b). The practices and procedures de- scribed in this report stem from the following princi- ples concerning cold weather concreting: 1.4.1 - Concrete that is protected from freezing un- til it has attaine
33、d a compressive strength of at least 500 psi (3.5 MPa) will not be damaged by exposure to a single freezing cycle (Powers 1962). 1.4.2 - Concrete that is protected as in Section 1.4.1 will mature to its potential strength despite subsequent exposure to cold weather (Malhotra and Berwanger 1973). No
34、further protection is necessary unless a cer- tain strength must be attained in less time. COLD WEATHER CONCRETING 306R-3 Table 3.1 - Recommended concrete temperatures Line 1 Section size, minimum dimension, in. (mm) 72 in. Air temperature (300 mm) (300-900 mm) (900-1800 mm) (1800 mm) Minimum concre
35、te temperature as placed and maintained - 55 F (13 C) 50 F (10 C) 45 F (7 C) 40 F (5 C) Minimum concrete temperature as mixed for indicated air temperature* 2 3 4 Above 30 F ( - 1 C) 60 F (16 C) 55 F (13 C) 50 F (10 C) 45 F (7 C) 0 to 30 F 65 F (18 C) 60 F (16 C) 55 F (13 C) 50 F (10 C) (-18 to -1 C
36、) Below 0 F 70 F (21 C) 65 F (18 C) 60 F (16 C) 55 F (13 C) (- 18 C) Maximum allowable gradual temperature drop in first 24 hr after end of protection 5 - 50 F (28 C) 40 F (22 C) 30 F (17 C) 20 F (11 C) *For colder weather a greater margin in temperature is provided between concrete as mixed and req
37、uired minimum temperature of fresh concrete in place. 1.4.3 - Where a specified concrete strength must be attained in a few days or weeks, protection at temper- atures above 50 F (10 C) is required. See Chapters 5 and 6. 1.4.4 - Except within heated protective enclosures, little or no external suppl
38、y of moisture is required for curing during cold weather. See Chapter 8. 1.4.5 - Under certain conditions, calcium chloride should not be used to accelerate setting and hardening because of the increased chances of corrosion of metals embedded in concrete or other adverse effects. See Chapter 9. Tim
39、es and temperatures given in this report are not exact values for all situations and they should not be used as such. The user should keep in mind the pri- mary intent of these recommendations and should use discretion in deciding what is adequate for each partic- ular circumstance. 1.5 - Economy Ex
40、perience has shown that the overall costs of ade- quate protection for cold weather concreting are not excessive, considering what is required and the result- ing benefits. The owner must decide whether the extra costs involved in cold weather concreting operations are a profitable investment or if
41、it is more cost effective to wait for mild weather. Neglect of protection against early freezing can cause immediate destruction or per- manently weakened concrete. Therefore, if cold weather concreting is performed, adequate protection from low temperatures and proper curing are essential. CHAPTER
42、2 - GENERAL REQUIREMENTS 2.1 - Planning It is recommended that the concrete contractor, con- crete supplier, and owner (or architect/engineer) meet in a preconstruction conference to define in clear terms how cold weather concreting methods will be used. This report provides a basis for the contract
43、or to select spe- _ cific methods to satisfy the minimum requirements during cold weather concreting, Plans to protect fresh concrete from freezing and to maintain temperatures above the recommended mini- mum values should be made well before freezing tem- peratures are expected to occur. Necessary
44、equipment and materials should be at the work site before cold weather is likely to occur, not after concrete has been placed and its temperature begins to approach the freezing point. 2.2 - Protection during fall and spring During periods not defined as cold weather, such as in fall or spring, but
45、when heavy frost or freezing is forecast at the job site,* all concrete surfaces should be protected from freezing for at least the first 24 hr after placement. Concrete protected in this manner will be safe from damage by freezing at an early age. If the concrete is air entrained and properly cured
46、, the ulti- mate strength and durability of the concrete will be un- impaired. Protection from freezing during the first 24 hr does not assure a satisfactory rate of strength devel- opment, particularly when followed by considerably colder weather. Protection and curing should continue long enough -
47、 and at a temperature sufficiently above freezing - to produce the strength required for form removal or structural safety (see Chapters 5 and 6). 2.3 - Concrete temperature During cold weather, the concrete temperature at the time of placement should not be lower than the values given in Chapter 3.
48、 In action, to prevent freezing at early ages, the concrete temperature should be main- tained at not less than the recommended placement temperature for the length of time given in Chapter 5. This length of time depends on the type and amount of cement, whether an accelerating admixture is used, an
49、d the service category. The recommended minimum placement temperatures given in Table 3.1 in Chapter 3 apply to normal weight *Charts showing mean dates of freezing weather in the United States may be obtained from the National Climatic Center, Federal Building, Ashville, NC 28801 306R-4 MANUAL OF CONCRETE PRACTICE concrete. Experience indicates that freshly mixed light- weight concrete loses heat more slowly than freshly mixed normal-weight concrete. Lighter weight insulat- ing concretes lose heat even more slowly. However, when exposed to freezing tempe
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