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1、i04 4R 95 Obb2949 0520900 308 AC1 304.4R-95 Placing Concrete with Belt Conveyors Reported by AC1 Committee 304 . american concrete institute BOX 19150, REDFORD STATION DETROIT, MICHIGAN 48219 A C 1 304-4R 95 W 0662949 0520901 244 W First Printing, May 1995 AC1 Manual of Concrete Practice Most AC1 St
2、andards and committee reports are gathered together in the annually revised AC1 Manual of Concrete Practice. The several volumes are arranged to group related material together and may be purchased individually or in sets. AC1 Committees prepare standards and reports in the general areas of material
3、s and properites of concrete, construction practices and inspection, pavements and slabs, struc- tural design and analysis, structural specifica- tions, and special products and processes. A complete catalog of all AC1 publications is available without charge. American Concrete Institute Box 19150,
4、Redford Station Detroit, Mich. 48219-0150 AC1 Certification Programs The final quality of a concrete structure depends on qualified people to construct it. AC1 certification pro- grams identify craftsmen, technicians and inspectors who have demonstrated their qualifications. The follow- ing programs
5、 are administered by AC1 to fulfill the grow- ing demand in the industry for certified workers: Concrete Flatwork Finisher Concrete Field Testing Technician-Grade I Concrete Laboratory Testing Technician-Grade I Concrete Laboratory Testing Technician-Grade II Concrete Construction Inspector-In-Train
6、ing Concrete Construction Inspector-Level II This document may already contain reference to these AC1 certification programs, which can be incorporated into project specifications or quality control procedures. If not, suggested guide specifications are available on request from the AC1 Certificatio
7、n Department. Enhancement of AC1 Documents The technical committees responsible for AC1 commit- tee reports and standards strive to avoid ambiguities, omissions, and errors in these documents. In spite of these efforts, the users of AC1 documents occasionally find information or requirements that ma
8、y be subject to more than one interpretation or may be incomplete or incorrect. To assist in the effort for accuracy and clarity, the Technical Activities Committee solicits the help of indi- viduals using AC1 reports and standards in identifying and eliminating problems that may be associated with
9、their use. Users who have suggestions for the improvement of AC1 documents are requested to contact the AC1 Engineering Department in writing, with the following information: 1. Title and number of the document containing the problem and specific section in the document; 2. Concise description of th
10、e problem; 3. If possible, suggested revisions for mitigating the problem. The Institutes Engineering Staff will review and take appropriate action on all comments and suggestions re- ceived. Members as well as nonmembers of the Institute are encouraged to assist in enhancing the accuracy and useful
11、ness of AC1 documents. A C 1 304.4R 75 0662747 0520902 LBO M Placing Concrete With Belt Conveyors Reported by AC1 Committee 304 Members of Committee 304 voting on 1995 revisions: . Robert A. Kelsey C h a i r m a n David J. Akers James E. Bennett* Casimir Bognacki* Tianxia Cao Arthur C. Cheff Henri J
12、ean Decarbonel James D. Floreyt Daniel Green Neil R. Guptil Terence C. Holland James Hubbard Thomas A. Johnson Samuel A. Kalat John C. King William C. Krell G a r y R. Mass* Patrick McDowell James L. Cope Subcommittee Chairman Dipak T. Parekh* James S. Pierce Paul E. Reinhart Royce J. Rhdes Kenneth
13、L . Saucier Donald L. Schlegel* Paul R. Stodola William X. Sypher Robert E. Tobin *Members of subcommittee who reviscd this repon. T h e committee also wished to acknowledge the contribution of Associate Member Robert M. Eshbach as a member of the subcommittee revis- +Decased. ing his report. Member
14、s of Committee 304 who prepared the original report: Robert E. Tobin Chairman Joseph E Artus0 James E. Bennet, Jr. Alan C. Carter A l b e r t B. Crosby* Donald E. Graham Donald L. Hall Donald L. Houghton Ivan E. Houk, Jr.* Jerome G. Kiubail BN A. Lamberton H a r r y H. McLean James L. Cope Task Grou
15、p 1 Chairman J. Neil Mustard* James S. Pierce Orton W. Spanley James H. Sprouse* Paul R. Stodola Francis C. Wilson * Members of Task Gmup 1 who prepared original report This state-of-the-art report includes a short history on the early develop- ment o f conveyor belts for tramporring andpiacing conc
16、rete. The design of conveyor systems is discussed in relation to the properties o f the phtic concrete, the delivery rate and the job specijkations. Bell widths, speeds, and angles of inclination are considered as they apply to specijk site requirements. The three types of concrete conveyors are the
17、 portable, feeder; and spreader types and their particular applications are covered, Field practices i n the selectwn, use and maintenance o f conveyors are described The economics o f bell conveyor plncement is discussed The quag of the in-place concrete and inspection procedures are stressed Keywo
18、rds: belt conveyors; concrete construction; concretes, conveying; conveyors; economics; feeders; fresh concretes; inspection; maintenance; placing; quality control; workability. AC1 Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in designing, planning, exec
19、uting, or inspecting construction and in preparing specifica- tions. Reference to these documents shali not be made in the Project Documents. if items found in these documents are de- sired to be part of the Project Documents, they should be phrased in mandatory language and incorporated in the Proj
20、ect Documents. CONTENTS Chapter 1-Introduction, p. 304.4R-2 1. I-Generai 1.2-History 1.3-Concrete conveyor development Chapter 2-Design considerations, p. 34.4R-2 2. I-Generai requirements 2.2-Concrete ribbon parameters 2.3-Belt charging 2.4-B elt discharge 2.5-Belt conveyor design principles 2.fj-C
21、oncrete mixture proportioning for conveying 2.7-Specifications AC1 34.4R-95 supersedes AC 304.4R-89 and became effective April i, 1995. Copyright 8 1995, American Concree Institute. All rights reserved including righis of reproduction and use in any form or by any means, including the making of copi
22、es by any photo process, o r by any eiecbonic or mechanical device, printed o r written or oral, or recording for sound or visual repm duction or for use in any knowledge or reicval system or device, unless permission in writing is obtained from the copyright proprietors. 304.4u-1 -,-,- 304.4R-2 A C
23、 1 304.4R 95 = Obb2949 0520903 017 = AC1 COMMITTEE REPORT Chapter radial or side discharge. 1.2-History The earliest recorded use of belt conveyors in North America was to handie grain. A grain belt conveyor was de- scribed in the 1795 “Millers Guide.” The first recorded use of belt conveyors to han
24、dle material heavier than grain did not come until the early 1890s when belt conveyors were in- stalled at an ore processing plant in Edison, N.J. The com- mercial introduction of antifriction bearings in idler rollers paved the way for the modem belt conveyor. In 1923 con- veyors were first success
25、fully used in handiing coal. The first known successful use of concrete belt conveyors was in 1929 when Corbetta Construction Co., Inc., used a oO-ft (183-m) conveyor to place structural concrete for the East 238th Street Bridge, Bronx County, City of New York. The concrete mix (1:2:4) contained %-i
26、n. (19-mm) NMSA (nominal maximum size aggregate). Belt conveyors were used to transport concrete between the mixing plant and a centrai distribution point where the concrete was loaded into buckets for placement on several Corps of Engineers and TVA projects between 1935 and 1944. These projects use
27、d from 320 to 432 lb of cement/yd3 (190 to 256 kg/m3) and 4 to 6 in. (100 to 150 mm) NMSA. Segregation of the largest aggregate at the transfer points and hoppers gave considerable trouble and various baffles, chutes and hoppers were developed to reduce segregation to a minimum2 From 1941 to 1950 On
28、tario Hydro successfully used concrete belt conveyors to place concrete on seven dif- ferent dam rojects. . 1.3-Concrete conveyor development The almost universal availability of ready -mixed concrete for building projects in the United States in the early 1950s created a demand for equipment to bri
29、dge the gap between the area accessible to the truck mixer and the location where the concrete was to be placed. The fmt commercially avail- able portable concrete belt conveyors were marketed in the late 1950s.4 The transporting or feeder conveyor was developed in about 19625 and the first spreadin
30、g belt conveyor was a side discharge unit used in 1963 to place the deck concrete for the elevated East 46th Avenue Freeway in Denver, Colo. Radial spreaders were developed shortly thereafter. Modification and improvement of these conveyors have been rapid and significant. Early belt conveyors were
31、limited to capacities of 30 to 40 yd3h (23 to 3 1 m3/hr). Today place- ment rates of 120 yd3h(92 m3h) on 16-in. (0.41-m) wide belts and 300 yd3/hr (230 m3h) on 24-in. (0.61-m) belts make concrete belt conveyors applicable to massive concrete placements as well as to building construction. All concre
32、te conveyors require charge and discharge hop- pers, belt wipers, and proper combinations of belt support idlers and belt speed to prevent segregation of the concrete. Any normal or lightweight aggregate concrete that can be discharged by a truck mixer can be placed by a concrete belt conveyor. Also
33、 concrete containing 3- and especially the drive unit, support frame, and belt support idlers. Normal weight con- crete is about 50 percent heavier than commonly conveyed materials such as aggregates. 2. The conveyor itself, or at least the concrete discharge mechanism, must be capable of movement o
34、ver the entire placement area without significantly interrupting or delaying concrete placement. This is required because the concrete must be distributed uniformly over the entire placement area. When placement in lifts is required for proper consolidation of the concrete, the required movement is
35、greatly increased. 3. Concrete belt conveyors must be able to stop, hold the concrete on the belt, and restart the fully loaded belt. This is necessary because placement cannot progress faster than the concrete can be spread and consolidated. This requirement is especially important when conveyors p
36、lace concrete in wall and column forms because it is difficult to control filling of the form by varying only the rate of charging of concrete onto the conveyor. 4. Finally, concrete belt conveyors must be designed to op- erate dependably under capacity loads without mechanical failures. Once placem
37、ent begins, it should continue without interruptions which could result in cold joints. The require- ments of reliability and dependability cannot be achieved simply by making components larger and heavier because this conflicts with the requirements of mobility over the placement area. To meet the
38、requirements of mobility and dependability, the booms of most concrete belt conveyors are constructed of steel trusses or aluminum extrusions. Lightweight belt support idlers and drive components are used wherever possible. with belt conveyors can be traced directly to a failure to in- 2.2-Concrete
39、ribbon parameters The characteristics of the ribbon of concrete on a conveyor belt are determined by the angle of surcharge of the concrete, the required minimum edge distance, and the load cross sec- tion. 2.2.1 Angle o f surcharge-Each plastic concrete mix has its own angle of repose. This is the
40、angle which the surface of a normal, freely formed pile makes to the horizontal. The angle of repose for 2- to 6-in. (50- to 150-mm) slump con- crete will usually range from 20 to 30 deg. The angle of sur- charge is the angle to the horizontal which the surface of the same concrete assumes while it
41、is being carried on a moving (horizontal) belt conveyor. The angle of surcharge for most concrete falls in a range from O to 10 deg. A lower angle of surcharge results in a shallower ribbon of concrete. The an- gle of surcharge is influenced by aggregate characteristics and mixture proportions such
42、as: a. Size and shape of the aggregate b. Surface texture of the coarse aggregate c. Ratio of fine aggregate to coarse aggregate (FAKA) d. Ratio of aggregate to cementitious materals (dcm) e. Ratio of water to cementitious materals (wkm) f. Additives which affect cohesiveness Plastic concrete is non
43、homogenous material; its angle of surcharge is influenced by all of its components. Small ag- gregates, water, and smooth, rounded and uniform size ag- gregate tend to reduce the angle of surcharge. Irregular, rough aggregate, cement, and additives which make the mix- ture more cohesive or reduce th
44、e water requirement tend to increase the angle of surcharge. The angle of surcharge determines the cross section of the concrete ribbon which can be efficiently carried on the belt. It is also an indication of the maximum angle of incline or decline at which concrete can be handled by a belt conveyo
45、r. “Angle of incline” and “angle of decline” refer to the angle to the horizontal formed by the load-carrying belt of the conveyer. The many variables that influence the angle of surcharge of concrete make it difficult to predict the maximum permis- sible angle of incline or decline. A good rule of
46、thumb is that a concrete belt conveyor can operate with less than a 10 per- cent loss of transverse cross-sectional area at an angle of 20 to 25 deg when equipped with a smooth belt and up to an an- gle of 30 to 35 deg when the belt is equipped with small straight corrugations or ribs on the load-ca
47、rrying surface. Concrete has been successfully conveyed at greater angles of incline or decline with close control of factors which affect the angle of surcharge. As the belt passes successively over each belt-supporting idler, the concrete on the belt is disturbed. This tends to work pieces of coar
48、se aggregate to the surface of the concrete and to flatten the concrete ribbon. This is the primary reason that the angle of surcharge is less than the angle of repose. A proper combination of belt tension, belt speed, and idler spacing is necessary to prevent objectionable segregation (see Section
49、2.5.7). Belt speeds of 300 fpm (92 dmin) to 600 fpm (183 dmin) with 3-ft (0.9-m) idler spacing and-belt speed of 600 fpm on idlers spaced about 5 ft (1.5 m) apart have been used successfully on many projects. 2.2.2 Minimum edge distancexoncrete cannot be carried across the entire face of a belt. The ribbon of concrete should be centered on the belt with equal widths of clear belt or “edge distance” between it and each edge of the belt. The following equation is used to determine minimum edge distance minimum edge distance, in. = 0.05 bel
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