ACI 304.4R-1995 Placing Concrete with Belt Conveyors《有带式输送机的浇筑混凝土》.pdf

1、304.4R-1ACI 304.4R-95 supersedes ACI 304.4R-89 and became effective April 1, 1995.Copyright 1995, American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by anymeans, including the making of copies by any photo process, or by any electronic ormechanic

2、al device, printed or written or oral, or recording for sound or visual repro-duction or for use in any knowledge or retrieval system or device, unless permission inwriting is obtained from the copyright proprietors.ACI Committee Reports, Guides, and Commentaries areintended for guidance in planning

3、, designing, executing, andinspecting construction. This document is intended for the useof individuals who are competent to evaluate the significanceand limitations of its content and recommendations and whowill accept responsibility for the application of the material itcontains. The American Conc

4、rete Institute disclaims any andall responsibility for the stated principles. The Institute shallnot be liable for any loss or damage arising therefrom.Reference to this document shall not be made in contractdocuments. If items found in this document are desired by theArchitect/Engineer to be a part

5、 of the contract documents, theyshall be restated in mandatory language for incorporation bythe Architect/Engineer.This state-of-the-art report includes a short history on the early develop-ment of conveyor belts for transporting and placing concrete. The design ofconveyor systems is discussed in re

6、lation to the properties of the plasticconcrete, the delivery rate and the job specifications. Bell widths, speeds,and angles of inclination are considered as they apply to specific siterequirements. The three types of concrete conveyors are the portable,feeder, and spreader types and their particul

7、ar applications are covered.Field practices in the selection, use and maintenance of conveyors aredescribed. The economics of bell conveyor placement is discussed. Thequality of the in-place concrete and inspection procedures are stressed. Keywords: belt conveyors; concrete construction; concretes,

8、conveying;conveyors; economics; feeders; fresh concretes; inspection; maintenance;placing; quality control; workability.CONTENTSChapter 1Introduction, p. 304.4R-21.1General1.2History1.3Concrete conveyor developmentChapter 2Design considerations, p. 304.4R-22.1General requirements2.2Concrete ribbon p

9、arameters2.3Belt charging2.4Belt discharge2.5Belt conveyor design principles2.6Concrete mixture proportioning for conveying2.7SpecificationsChapter 3Types of conveyors and functions,p. 304.4R-73.1General3.2Portable conveyors3.3Feeder conveyors3.4Spreading conveyors3.5Conveyor combinations3.6Special

10、belt conveyors3.7Economics of conveyor placementChapter 4Field practice, p. 304.4R-134.1Selection of conveyors4.2Actual capacity4.3Conveyor charging4.4Discharge control4.5MaintenanceChapter 5Inspection and testing, p. 304.4R-145.1Concrete inspection5.2TestingChapter 6References, p. 304.4R-156.1 Spec

11、ified and/or recommended reference6.2Cited references6.3Additional referencesACI 304.4R-95(Reapproved 2008)Placing Concrete with Belt ConveyorsReported by ACI Committee 304Robert A. KelseyChairmanJames L. CopeSubcommittee ChairmanDavid J. Akers Terence C. Holland Dipak T. Parekh*James E. Bennett* Ja

12、mes Hubbard James S. PierceCasimir Bognacki* Thomas A. Johnson Paul E. ReinhartTianxia Cao Samuel A. Kalat Royce J. RhodesArthur C. Cheff John C. King Kenneth L. SaucierHenri Jean DeCarbonel William C. Krell Donald L. Schlegel*James D. FloreyGary R. Mass* Paul R. StodolaDaniel Green Patrick McDowell

13、 William X. SypherNeil R. Guptil Robert E. Tobin*Members of subcommittee who revised this report.Deceased.The committee also wishes to acknowledge the contribution of Associate Member Robert M. Eshbach as a member of the subcommittee revising thisreport.304.4R-2 ACI COMMITTEE REPORTCHAPTER 1INTRODUC

14、TION1.1GeneralBelt conveyors for handling concrete are special in thatthey transport plastic concrete which is about 48 percentheavier than aggregate or other commonly conveyed materi-als. They transport plastic concrete from a supply sourcesuch as a truck mixer or a batching and mixing plant to the

15、point of placement or to other equipment which is used toplace the concrete. Concrete placement by belt conveyorsshould be a continuous operation. Maximum success forconveyor placing requires a constant supply of properlymixed concrete for charging the belt conveyor and a provi-sion for moving the d

16、ischarge point during placement so thatthe plastic concrete is deposited over the entire placementarea without the need for rehandling or excessive vibration.Concrete belt conveyors are classified into three types: 1)portable or self contained, 2) feeder or series, and 3) spread-er; radial or side d

17、ischarge.1.2HistoryThe earliest recorded use of belt conveyors in NorthAmerica was to handle grain. A grain belt conveyor was de-scribed in the 1795 “Millers Guide.” The first recorded useof belt conveyors to handle material heavier than grain didnot come until the early 1890s when belt conveyors we

18、re in-stalled at an ore processing plant in Edison, N.J. The com-mercial introduction of antifriction bearings in idler rollerspaved the way for the modern belt conveyor. In 1923 con-veyors were first successfully used in handling coal.1The first known successful use of concrete belt conveyorswas in

19、 1929 when Corbetta Construction Co., Inc., used a600-ft (183-m) conveyor to place structural concrete for theEast 238th Street Bridge, Bronx County, City of New York.The concrete mix (1:2:4) contained 3/4-in. (19-mm) NMSA(nominal maximum size aggregate).Belt conveyors were used to transport concret

20、e betweenthe mixing plant and a central distribution point where theconcrete was loaded into buckets for placement on severalCorps of Engineers and TVA projects between 1935 and1944. These projects used from 320 to 432 lb of cement/yd3(190 to 256 kg/m3) and 4 to 6 in. (100 to 150 mm) NMSA.Segregatio

21、n of the largest aggregate at the transfer points andhoppers gave considerable trouble and various baffles,chutes and hoppers were developed to reduce segregation toa minimum.2 From 1941 to 1950 Ontario Hydro successfullyused concrete belt conveyors to place concrete on seven dif-ferent dam projects

22、.31.3Concrete conveyor developmentThe almost universal availability of ready-mixed concretefor building projects in the United States in the early 1950screated a demand for equipment to bridge the gap betweenthe area accessible to the truck mixer and the location wherethe concrete was to be placed.

23、The first commercially avail-able portable concrete belt conveyors were marketed in thelate 1950s.4The transporting or feeder conveyor was developed inabout 19625 and the first spreading belt conveyor was a sidedischarge unit used in 1963 to place the deck concrete for theelevated East 46th Avenue F

24、reeway in Denver, Colo. Radialspreaders were developed shortly thereafter.Modification and improvement of these conveyors havebeen rapid and significant. Early belt conveyors were limitedto capacities of 30 to 40 yd3/hr (23 to 31 m3/hr). Today place-ment rates of 120 yd3/hr(92 m3/hr) on 16-in. (0.41

25、-m) widebelts and 300 yd3/hr (230 m3/hr) on 24-in. (0.61-m) beltsmake concrete belt conveyors applicable to massive concreteplacements as well as to building construction.All concrete conveyors require charge and discharge hop-pers, belt wipers, and proper combinations of belt supportidlers and belt

26、 speed to prevent segregation of the concrete.Any normal or lightweight aggregate concrete that can bedischarged by a truck mixer can be placed by a concrete beltconveyor. Also concrete containing 3- and 6-in. (80- and150-mm) coarse aggregate has been transported successfullyon 16-in. (0.41-m) and 2

27、4-in. (0.61-m) wide belt conveyors,respectively.6Concrete belt conveyors are an excellent method for mov-ing the concrete from the batch plant to the lift surface on roll-er compacted concrete (RCC) projects because they eliminatetracking mud onto the lift surface and damage from turninghaul vehicle

28、s. Belt conveyors were used successfully on Up-per Stillwater, Elk Creek, and Middle Fork dams.7CHAPTER 2DESIGN CONSIDERATIONS2.1General requirementsNot all belt conveyors can successfully place concrete.Concrete conveyors should be designed specifically to dealwith the problems that concrete presen

29、ts.8 Concrete convey-ors running at the correct belt speed and with properly func-tioning charging hoppers, transfer devices, and belt wipershave only a minor effect on the strength, slump, or air con-tent of the concrete that they carry.9,10 Successful layout ofconcrete belt conveyors for specific

30、job applications dependsupon an understanding of the interaction of the many vari-ables involved. Most unsuccessful attempts to place concretewith belt conveyors can be traced directly to a failure to in-corporate in the design the capability of meeting the follow-ing handling and placing requiremen

31、ts:1. All components of the conveyor must be sized to ac-commodate the weight of concrete; especially the drive unit,support frame, and belt support idlers. Normal weight con-crete is about 50 percent heavier than commonly conveyedmaterials such as aggregates.2. The conveyor itself, or at least the

32、concrete dischargemechanism, must be capable of movement over the entireplacement area without significantly interrupting or delayingconcrete placement. This is required because the concretemust be distributed uniformly over the entire placement area.When placement in lifts is required for proper co

33、nsolidationof the concrete, the required movement is greatly increased. 3. Concrete belt conveyors must be able to stop, hold theconcrete on the belt, and restart the fully loaded belt. This isPLACING CONCRETE WITH BELT CONVEYORS 304.4R-3necessary because placement cannot progress faster than thecon

34、crete can be spread and consolidated. This requirement isespecially important when conveyors place concrete in walland column forms because it is difficult to control filling ofthe form by varying only the rate of charging of concreteonto the conveyor.4. Finally, concrete belt conveyors must be desi

35、gned to op-erate dependably under capacity loads without mechanicalfailures. Once placement begins, it should continue withoutinterruptions which could result in cold joints. The require-ments of reliability and dependability cannot be achievedsimply by making components larger and heavier becauseth

36、is conflicts with the requirements of mobility over theplacement area. To meet the requirements of mobility anddependability, the booms of most concrete belt conveyorsare constructed of steel trusses or aluminum extrusions.Lightweight belt support idlers and drive components areused wherever possibl

37、e.2.2Concrete ribbon parameters The characteristics of the ribbon of concrete on a convey-or belt are determined by the angle of surcharge of the con-crete, the required minimum edge distance, and the loadcross section.2.2.1 Angle of surchargeEach plastic concrete mix hasits own angle of repose. Thi

38、s is the angle which the surfaceof a normal, freely formed pile makes to the horizontal. Theangle 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 thesame concrete assumes while

39、 it is being carried on a moving(horizontal) belt conveyor. The angle of surcharge for mostconcrete falls in a range from 0 to 10 deg.1 A lower angle ofsurcharge results in a shallower ribbon of concrete. The an-gle of surcharge is influenced by aggregate characteristicsand mixture proportions such

40、as:a. Size and shape of the aggregateb. Surface texture of the coarse aggregatec. Ratio of fine aggregate to coarse aggregate (FA/CA)d. Ratio of aggregate to cementitious materials (a/cm)e. Ratio of water to cementitious materials (w/cm) f. Additives which affect cohesivenessPlastic concrete is nonh

41、omogeneous material; its angle ofsurcharge 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 the wat

42、er requirement tend toincrease the angle of surchargeThe angle of surcharge determines the cross section of theconcrete ribbon which can be efficiently carried on the belt. Itis also an indication of the maximum angle of incline or declineat which concrete can be handled by a belt conveyor. “Angle o

43、fincline” and “angle of decline” refer to the angle to the horizon-tal formed by the load-carrying belt of the conveyer.The many variables that influence the angle of surchargeof concrete make it difficult to predict the maximum permis-sible angle of incline or decline. A good rule of thumb is thata

44、 concrete belt conveyor can operate with less than a 10 per-cent loss of transverse cross-sectional area at an angle of 20to 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 smallstraight corrugations or ribs on the load-carrying surface.1Con

45、crete has been successfully conveyed at greater angles ofincline or decline with close control of factors which affectthe angle of surcharge.As the belt passes successively over each belt-supportingidler, the concrete on the belt is disturbed. This tends to workpieces of coarse aggregate to the surf

46、ace of the concrete andto flatten the concrete ribbon. This is the primary reason thatthe angle of surcharge is less than the angle of repose. Aproper combination of belt tension, belt speed, and idlerspacing is necessary to prevent objectionable segregation(see Section 2.5.7). Belt speeds of 300 fp

47、m (92 m/min) to600 fpm (183 m/min) with 3-ft (0.9-m) idler spacing and beltspeed of 600 fpm on idlers spaced about 5 ft (1.5 m) aparthave been used successfully on many projects.2.2.2 Minimum edge distanceConcrete cannot becarried across the entire face of a belt. The ribbon of concreteshould be cen

48、tered on the belt with equal widths of clear beltor “edge distance” between it and each edge of the belt. Thefollowing equation is used to determine minimum edgedistanceminimum edge distance, in. = 0.05 belt width + 0.9 in. i.e., for a 16-in. belt, minimum edge distance is(0.05 l6 in. + 0.9 in.) = 1

49、.7 in. (40 mm) Failure to observe the “minimum edge distance” require-ment will result in excessive spilling and loss of large aggre-gate off the edge of the belt.All concrete belt conveyors utilize idlers which trough orcup the belt, enabling it to carry a deeper ribbon of concretethan would be possible on a flat belt. The head pulley is usu-ally slightly crowned to provide belt alignment or training(see Section 2.5.3). Since the belt will flatten as it goes overthe head pulley, the ribbon of concrete will tend to flow to-ward the belt edges in