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304.6R-91

Guide for the Use ofVolumetric-Measuring and

Continuous-Mixing Concrete Equipment (Reapproved 1997)

reported by ACI Committee 304

Arthur C. CheffThomas R. Clapp*James L. CopeWayne J. CostaHenri Jean DeCarbonelRobert M. EshbachJames R. FloreyClifford Gordon

This guide includes a short history of and information on the basic

design and operation of equipment used to produce concrete by vol-

umetric measurement andcontinuous mixing (VMCM), frequentlycalled mobile mixers. Definitions, applications, and quality assur-

ance testing are discussed. The use of this equipment is compared to

weigh-batch-mix equipment for some of the li mited differences.

Keywords: admixtures; aggregates;batching; calibrating; cements; cold weatherconstruction; colored concrete; concrete construction; field tests; fresh con-cretes; grout; hot weather construction; material handling; measurement; mix-

ing; mixing plants; mixing time;mix proportioning; polymer concrete: precastconcrete; process control; production methods; shotcrete; slump; transit mix-ers.

CONTENTSChapter 1 -Introduction

1.l--General1.2-Discussion1.3-History

Chapter 2-Equipment2.l-Materials storage and measurement2.2-Mixers2.3-Equipment condition

Chapter 3-Operations3.1-General3.2-Production rates3.3-Planning

Gary R. MassKurt R. MelbyRichard W. NarvaJohn H. Skinner, IIIPaul R. Stodola*+William X. SypherLouis L. SzilandiRobert E. Tobin

Francis C. Wilson

James S. PierceChairman

Donald E. GrahamNeil R. GuptillTerence C. Holland*James HubbardThomas A. JohnsonRobert A. KelseyJohn C. KingWilliam C. Krell

3.4-Materials3.5-Personnel qualifications

Chapter 4-Applications4.l-General4.2-Mixtures with short working times4.3-Low-slump mixtures4.4-Long unloading times4.5-Concrete at remote sites4.6-Making small deliveries4.7-Precast operations4.8-Hot weather concreting4.9-Mining applications

4.10-Grouting and pile filling4.1l-Colored concretes4.12-Emergency applications

Chapter 5-Quality control and testing5.1 -General5.2-Calibration5.3-Production testing

Chapter 6-Operational precautions6.1-General6.2-Cold weather concrete6.3-Hot weather concrete6.4-Aggregate moisture6.5-Rapid slump loss6.6-Use of admixtures6.7-Fresh concrete properties

ACI Committee Reports, Guides, Standard Practices, andCommentaries are intended for guidance in designing, plan-ning, executing, or inspecting construction and in preparingspecifications. Reference to these documents shall not be madein the Project Documents. If items found in these documentsare desired to be part of the Project Documents they shouldbe phrased in mandatory language and incorporated into theProject Documents.

*The committee recognizes the special contributions of Paul Stodola, ThomasClapp, and Terry Holland.chairman of Committee 304 since March 1989.

AC1 304.6R-91 became effectiveMay 1, 1991.Copyright 0 1991, 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 anyelectronic or mechanical device, printed, written, or oral, or recording for soundor visual reproduction or for use in any knowledge or retrieval system or de-vice, unless permission in writing is obtained from the copyright proprietors.

304.6R-1

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304.6R-2 MANUAL OF CONCRETE PRACTICEChapter 7-References

7.1-Specified and/or recommended references7.2-Cited references

Appendix A-New York State DOT calibration

method

CHAPTER 1 -INTRODUCTION1.1-GeneralThe purpose of this document is to offer guidance on

volumetric-measurement and continuous-mixing(VMCM) concrete production. It contains backgroundinformation on this method and items to be consideredwhen using it. A discussion of other types of continu-ous-measurement equipment (i.e., conveyor belt scalesor weigh-in-motion scales) is outside the scope of thisreport.

1.2-DiscussionThe title uses the words volumetric measuring and

continuous mixing. The significance of these wordsin the context of this guide are discussed in the follow-ing paragraphs.

Volumetric measurement-Whenthe ingredients ofconcrete are flowing continuously and measured byvolume, by using a calibrated rotary opening, a cali-brated fixed-gate opening, or a combination of these,so that a known, predetermined volume of each ingre-dient is obtained in a designated time interval, themethod of measurement is volumetric. Continuous vol-umetric measurement with multiple ingredients requiresthat the proper relationship among those ingredients be

maintained.Continuous mixing-When the output of the mixer is

equivalent to the input of materials and the mixer canbe operated without interruption to charge or dischargematerial, the mixer can be considered continuous. Themixer may be started and stopped as required to meet

production requirements (provided that material inputis also started and stopped). Such a mixer is suitable for

both continuous or intermittent operation.

1.3-HistoryVolumetric measurement and continuous mixing

have a long history of producing concrete. For manyyears the concept of one shovel of cement, two shov-els of sand, and three shovels of stone was used to

produce concrete. Patents on continuous mixers dateback at least to 1913. It was not until these two tech-nologies were successfully combined in the early 1960sthat general field use of this type of equipment began.The first commercial unit was delivered in 1964. Be-cause of the detail of original patents, there was onlyone manufacturer of VMCM units until the early 198Os,when other manufacturers began to offer this type ofequipment for concrete production.

By the mid-1970s, there were over 4000 VMCM ma-

chines in operation in the U.S. Generally, they wereused to produce small volumes of concrete. During the

late 1970s and early 198Os, specialty concretes neededfor bridge-deck renovation and highway repair, whichwere difficult to produce in conventional transit mix-ers, were being produced successfully by VMCMequipment. This application gave the equipment cre-dence and showed it could produce close-tolerance,high-quality concrete consistently. VMCM equipmenthas been considered by some people to be limited toproducing special mixtures or small volumes; however,VMCM may be suitable for almost any concrete re-quirement .

Standards activities related to concrete produced byVMCM equipment have been limited. However, in 1971ASTM developed and now maintains ASTM C 685,Standard Specification for Concrete Made by Volu-metric Batching and Continuous Mixing.

CHAPTER 2-EQUIPMENT2.1-Materials storage and measurement

Measurement of material by volume can be accom-plished by a variety of means. Rotary vane feeders

(both horizontal and vertical axis), screw conveyors(both adjustable and fixed speed), drag chains, cali-

brated gate openings, variable-volume sliding compart-ments, and vibrating plate feeders all have been used tomeasure quantities of dry ingredients. Liquids may beintroduced by air pressure, pumps, or cylinders with theflow controlled by valves or timers and measured byflow meters. Readers are directed to the documents

produced by the equipment manufacturers for operat-ing details of the various types of equipment. Cement,water, and admixtures are stored in separate containersand measured separately. Fine and coarse aggregates

are stored either separately or combined. If aggregatesare stored and used in a combined state, they must beaccurately preblended, and particular care must betaken to avoid segregation.

In presently available equipment, a meter records therate of introduction of cement into the mixture and thisrate serves, directly or indirectly, to control the rate atwhich other ingredients are added. All systems are in-terconnected so that, once they are calibrated and set toproduce a specific concrete mixture, all ingredients aresimultaneously and continuously measured into themixer. This interconnecting allows either continuous orintermittent operation of the system to accommodate

the quantities of the concrete needed. These intercon-nections should not be confused with the interlockstypically found in weigh-type batch plants. VMCMequipment is designed to allow the relative proportionsof ingredients to be changed rapidly to vary the con-crete mixture as required. Because the mixing chamberonly holds about 2 ft, such changes can be made withlittle or no waste.

Typical VMCM units carry enough materials to pro-duce 6 to 10 yd3 of concrete (Fig. 2.1). This limitationis based upon axle loading limitations. Production oflarger volumes of concrete or high rates of production

will require special provisions for recharging the mate-rial storage compartments.

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CONTINUOUS MIXING EQUIPMENT 304.6R-3

crete on the mixer-auger surfaces. Belts must be prop-erly adjusted and kept in good repair. There should beno leaks in the hydraulic or air systems. There shouldbe no cut or damaged insulation on electric wires. Allcovers and guards should be securely in place.

CHAPTER 3-OPERATIONS3.1-General

Fig. 2. 1- Typical system

Volumetric measurement and continuous mixing aresuitable for producing almost any concrete with appro-priately sized aggregate, provided the equipment is op-erated with the same attention to detail as would be re-quired to produce concrete by any other means. Mostof the present equipment is truck- or trailer-mounted,or at least portable, and typically serves as its own ma-terial transport. The portability of the equipmentmakes it practical to bring the VMCM unit to the pointof use, which can be an advantage in many applica-tions. Having the unit at the placement site also allowsclose control of concrete quality at the site.

2.2-MixersFor mixing, most of the present continuous mixers

utilize an auger rotated in a sloped trough or tube. Ma-terials are introduced at or near the lower end, and themixed concrete is discharged at the other. This basicprinciple is the same for all VMCM equipment, al-though there are many individual variations. Augers areavailable in different lengths and diameters and oper-ate at different speeds and may have continuous or in-terrupted flighting. Troughs may have flexible or rigidbottoms and covered or open tops. The slope of themixer may be fixed or adjustable. Lowering the trough(they are normally set at about 15 deg) will reduce the

mixing time, while raising the trough will extend it. Apivot at the base of most mixers allows them to swingfrom side to side.

3.2-Production ratesMaximum production rates are dependent upon the

physical and mechanical characteristics of the VMCMunit. Discharge rates for a cubic foot of cement (about100 lb) range from about 48 to 28 sec. For a concretewith a cement content of 564 lb/yd3, these dischargerates imply production rates of about 12 to 21 yd3/hr.However, these rates can only be achieved if the unitsare resupplied with materials during production.

3.3-Planning

With this type of mixer, output is always equal to in-put, with a relatively small amount of material beingmixed at any one time. Thorough mixing is accom-plished in a very short time by applying high-shear,high-energy mixing to the material. Actual mixing timefrom input to output is usually less than 20 sec. Pro-duction capability of the unit is more dependent on thesupply of materials than on the type or capacity of themixer.

A review should be made of the job requirementsprior to concrete production. Depending on the appli-cation, this may be a review by the operator or a moredetailed formal meeting among all parties involved.Review points should include discussion of the follow-ing items, which are further covered in Section 5:

1. Current calibration of materials being used.2. Functional controls and settings proper for the

job.3. Production rates and delivery times.4. Required testing requirements and methods.5. Availability of testing equipment.6. Adequate access on site for operation.

2.3-Equipment condition 3.4-MaterialsAll proportioning and mixing equipment should bewell maintained in accordance with the manufacturersinstructions. This point cannot be overemphasized. Thefinished product is probably the best test of equipmentcondition.

There are certain areas to which particular attentionshould be paid. The cement dispenser must be cleanand free of any buildup. Valves must operate smoothlyand not leak. Any accumulation of materials on anycontrolling surface or opening in the system will alterthe calibrated flow of materials. Mixer augers shouldnot be allowed to wear beyond the manufacturers rec-

ommended limits. There should be no buildup of con-

Ingredients that are used to calibrate the unit shouldbe the same that will be used for production. All ma-terials should be stored and handled in accordance withgood concrete practices (ACI304R). The moisture con-tent of the fine aggregate must be carefully controlledto avoid undesirable variations in the mixture. Particu-lar care should be taken during loading to avoid spill-ing materials into the wrong compartments. Whenmoist aggregates are preloaded (6 to 8 hr in advance ofproduction), the operator will need to reduce the initialwater introduction slightly to maintain the properslump and compensate for water that has drained to the

bottom of the aggregates. Preloaded equipment should

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304.6R-4 MANUAL OF CONCRETE PRACTICE

Fig. 4.1-Trailer-mountedunit modified to producepolymer concrete

be stored inside or covered during inclement weather.Driving loaded equipment over rough roads may com-

pact aggregates, causing errors in flow rates. If this oc-curs, recalibration may be necessary at the productionsite.

3.5-Personnel qualificationsIt is essential that personnel responsible for controlbe knowledgeable in all phases of equipment use. Con-trol of material proportions is direct and immediate;therefore, operators must also understand the signifi-cance of any adjustments made. This also places addi-tional responsibilities on quality control personnel, asany change in the system could possibly adversely af-fect concrete quality and cost. Personnel involved inoperating this type of equipment should have a thor-ough understanding of the controls and should be ac-quainted with concrete technology. Personnel author-ized to make adjustments of the proportioning controlsshould have received training and/or certification fromthe equipment manufacturer or have at least 4 weeks ofon-the-job training with qualified personnel.

CHAPTER 4-APPLICATION4.1 -General

VMCM equipment lends itself to many different ap-plications. While many of these applications involverelatively low-volume production of concrete, large jobshave also been done with this equipment. In addition to

producing conventional concrete, VMCM equipment iswell suited for a variety of special applications (Fig.4.1). Some of these applications are discussed in thefollowing sections.

4.2-Mixtures with short working timesConcretes made with rapid-setting cements, special

rapid-setting admixtures, or polymeric materials have arelatively short working life. Applications include re-pairs to hydraulic and highway structures and precastconcrete products. Since VMCM equipment propor-

tions and mixes at the jobsite, maximum possibleworking time is obtained.

Fig. 4.3.1-Production of concrete for a low-slubridge overlay with a VMCM unit

Fig. 4.3.2-Slipforming a bridge parapet with concmade by a VMCM

applications include slipform placing (Fig. 4.3.2) shotcrete mixtures. The efficient mixing action ofcontinuous mixer is capable of handling all of theseplications.

4.4-Long unloading timesSome applications require relatively small amoun

concrete on a constant basis. Shotcrete and verslipforming are good examples. Changes in the crete properties could occur if a large volume of ccrete is held at the jobsite and discharged over a lperiod.

4.5-Concrete at remote sitesA VMCM unit is a complete proportioning and m

ing system. It can be used as a plant at the jobthereby eliminating long haul times for ready-mconcrete (Fig. 4.5.1 and 4.5.2). In remote areas, can be very cost effective from both a production

quality standpoint.

4 6 Making small deliveries

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CONTINUOUS MIXING E Q U I P M E N T 304.6R.5

Fig. 4.5.1-Transportable plant providing concrete di-rectly and continuously to a concrete pump

Fig. 4.5.2--Self-loading unit premixing concrete fordelivery into an agitator truck

4.7-Precast operationsVMCM units in precast plants can provide uninter-

rupted delivery throughout a large area with rapid con-trol of consistency and workability. Waste can be sig-nificantly reduced when casting architectural panels,

block, and molded items.

4.8-Hot weather concretingThe concrete is discharged as it is mixed; therefore,

most hydration takes place after the discharge. Theconcrete can be in place in the forms very quickly after

mixing so there is very little chance for the concrete toheat up after mixing, but before placing. No temperingwater is required to maintain workability, therefore, thewater-cement ratio can be controlled more easily.

4.9-Mining applicationsBecause of their compact size, VMCM units have

been customized to fit into a mine shaft. Typically,these units have been reduced in height. Units also havebeen designed in components that bolt together so theycould be reassembled in the mine after entering via astandard hoist.

4.10-Grouting and pile fillingThese applications also often require small volumes

of grout or concrete over an extended period, Both

Fig. 4.6-VMCM unit supplying concrete for a resi-dential foundation

placement types require that a suitable material beavailable when the application is ready, and both mayrequire indefinite volumes of material. Retemperingmay be required if large volumes of ready-mixed con-

crete or grout are held waiting at the jobsite.4.11 -Colored concretes

Many precast operations require colored concrete.The small mixing auger can be cleaned much morequickly and more thoroughly than batch-type mixers.The vigorous mixing action of the auger-type mixerthoroughly homogenizes the mixture for uniform col-oring.

4.12-Emergency applicationsVMCM units may be used as emergency sources of

concrete to handle repair situations. A preloaded unit

could be held in standby for emergency situations thatarise when there is no other source of concrete.

CHAPTER 5-QUALITY CONCRETE ANDTESTING

5.1 -GeneralThe production of concrete by volumetric measure-

ment and continuous mixing is subject to the same rulesof quality control as any other concrete productionmethod. The equipment should be clean, well main-tained, and operated by experienced personnel. ASTMC 685 (AASHTO M 241) is the standard specificationfor concrete made by these methods and is similar toASTM C 94. As with any type of batching equipment,common sense, experienced personnel, and trained in-spectors are the best quality assurance tools.

5.2-CalibrationTo insure production of quality concrete, each volu-

metric-measuring unit must be calibrated for each re-spective concrete ingredient, following the manufactur-ers recommendations and ASTM C 685. These ingre-dients must be the same as those to be used in actualconcrete production. The measuring devices for aggre-gates, cement, and dry admixtures are calibrated by

weighing the discharged ingredient. Devices for water,latex modifier (if required), and liquid admixtures suchas air-entraining and water-reducing admixtures gener-

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304.6R-6 MANUAL OF CONCRETE PRACTICEally are calibrated by weighing or measuring the vol-ume of the discharged ingredient. The objective of cal-ibration is to coordinate the discharge of all concreteingredients to produce the proper mixture.

A complete calibration procedure should be con-ducted: 1) for all new equipment; 2) when test data in-dicate that the concrete is not meeting specified per-formance levels; 3) when requested by the purchaser orengineer; or 4) when a change is made in materials or

mixture proportions for which previous calibration dataare unavailable. Complete calibrations should also beaccomplished on a periodic basis depending upon in-tervening time since the unit was calibrated for anotherreason and the volume of concrete being produced.

An abbreviated calibration to verify cement dis-charge or a volumetric yield check will verify the accu-racy of previous control settings. Such abbreviated cal-ibrations are useful and economical when small quan-tities of concrete (under 50 yd3) are to be producedusing the same control settings with similar ingredients.Project specifications should clearly define concrete

performance requirements, and equipment should becalibrated to meet those requirements.

The New York State Department of Transportationhas developed a detailed method for calibrating VMCMunits.1 A copy of this calibration procedure is includedasAppendix A to this report.

5.2.1 Equipment required-The following equipmentis required to perform a full calibration: a scale with aminimum capacity of 300 lb, a clean container to catchcement and aggregate discharge, a container calibratedin fluid ounces to catch admixture discharge, a con-tainer to catch water or other liquid discharge, a stopwatch accurate to one-tenth sec, and a container to

check volumetric yield (normally al

/4yd3 box). Toler-ances as stated in ASTM C 685 are:

Cement, percent by weight 0 to + 4Fine aggregate, percent by weight + /- 2

Coarse aggregate, percent by weight + /- 2Admixtures, percent by weight + /- 3

Water, percent by weight + /- 1

5.2.2Cement-The cement discharge system is nor-mally connected directly to the indicator used to deter-mine concrete production quantity. This system also

determines the rate at which all other discharge systemsmust provide materials to the mixer to produce the re-quired mixture. It is necessary to establish the weight ofcement discharged for a given register or counter read-ing as well as the amount discharged in a given time.

5.2.5Admixtures-Wet or dry admixture dischargeshould be calibrated for indicated flow rate versusmeasured delivery. The flow of each admixture beingcalibrated should be caught in a calibrated receptaclefor at least as long as the discharge time of 94 lb of ce-ment. A chart of flow indicator position versus actualflow can be established. As many calibration runs asnecessary to meet the specified tolerances should bemade.

When calibrating cement, precautions should be 5.2.6Post-calibration volumetric-yield test-All con-taken to insure that the aggregate bins are empty (or trols should be set to produce the desired mixture. All

separated from the system) and that all of the dis- controls should be engaged and all systems charged. Allcharge is collected. Any carrying mechanisms for the controls should be stopped simultaneously and thecement should be primed by operating the system until meter register or counter reset. A container of knownany surface between the storage bin and the collection volume with rigid sides is then placed under the dis-container which might attract cement becomes coated. charge of the mixer. All controls are then engaged si-The meter register or counter should then be reset to multaneously and the proper count is run on the meter

zero and a minimum of five calibration runs should be register. The count is determined based on the known

made. These runs should each use at least 94 lb of ce-ment.

When calibrating rotary discharge systems, it is pref-erable to stop the run at a whole number of revolutionsrather than attempt to stop at a fixed time or weight.At the conclusion of each run, the meter reading, time(in seconds), and the gross and tare weights are re-corded. The net weight and weight per meter unit arethen calculated. The weight of cement discharged in 1

min is also calculated.5.2.3Aggregates-Aggregate discharge controls must

be calibrated to provide the correct proportions in re-lation to the cement. This can be accomplished by es-tablishing discharge rate in weight per unit time orweight per cement meter unit. The required weight ofaggregate discharge for either of these units may becalculated and trials made at various control settingsuntil the desired weight is collected. Aggregates must becalibrated individually. This method of trial and erroris best used when working with a familiar mix design

and similar aggregates.

When the system is being calibrated for severalmixtures and with unfamiliar aggregates, it may beuseful to plot the weight per unit time versus controlsettings for a minimum of five control settings. Thegraph developed can then be used to interpolate the re-quired settings for the various concrete mixtures. Veri-fication runs should be made after any such chart isdeveloped.

5.2.4 Water-Normally, the control setting for themaximum permitted water is determined for a giventime or meter unit on the cement register. The dis-charge is then collected and weighed or measured in agraduated container to verify the setting. The accuracy

of flow meters (gal./min) and/or recording flow me-ters (total gal.), if present, should be verified at thistime. For each calibration run, the system should beoperated at least as long as the discharge time for 94 lbof cement.

Because there are fewer mechanical operating com-ponents involved with the water discharge than with thecement, fewer calibration runs will be necessary.

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CONTINUOUS MIXING EQUIPMENT 304.6R-7

Fig. 5.2.6 Volumetric yield test

volume of the container; for example, one-fourth of the1-yd count should be used with a 1/4yd3 container (Fig.5.2.6).

Another method for a yield check by weighting is de-tailed in ASTM C 685.

5.2.7 Preproduction tests-After calibration, pre-production tests may be made to confirm whether theproduction mixture proportions meet the requirementsof the laboratory mixture proportions and provide areference for production testing. The following mini-

mum tests should be made at this time: air content(ASTM C 231 or C 173), slump (ASTM C 143), andunit weight (ASTM C 138). It is also advisable to castcylinders for compressive strength testing at this time.

5.3-Production testingParameters for testing should be established to meet

jobsite requirements. Generally, testing for concreteproduced using VMCM equipment should follow thesame guidelines as for concrete produced by othermethods. Suggested tests include: air content (ASTMC 173 or C 231), slump (ASTM C 143), and unit weight(ASTM C 138). Project specifications should includethe frequency interval for these tests. This frequencymay vary from one set of tests per unit per cubic yardto one set for each load. As with weigh-batched con-crete, these tests serve as a quick check for quality con-trol.

It is also good practice to perform a volumetric-yieldtest on each mixer at least once per day or at intervalsof at least 50yd3 of production. The concrete producedfor this yield test can often be incorporated directly intothe work. The previously mentioned air, slump, andunit weight tests should also be made at this time. Cyl-inders or beams for strength tests should be cast fromconcrete obtained at point of discharge at the same timeas the other testing. Any other suitable tests may be

used at the discretion of the specifier; however, experi-ence and economics dictate that such testing need notbe more stringent than that required for weigh-batchedconcrete.

CHAPTER 6-OPERATIONAL PRECAUTIONS6.1 -General

The volumetric-measurement and continuous-mixingequipment should be in good condition. All shields andcovers should be in place. All controls should operatesmoothly and be connected according to the manufac-turers recommendations. All material-feed operationsmust start and stop simultaneously. The cement-meas-uring device must be inspected and cleaned regularly.Indicating meters and dials should be operational andreadable. Admixture systems should be checked for

proper flow and operation. All filters should be cleanand allow full flow of water. Aggregate feed systemsshould be free of any blockage. Checks of the variousfeeding systems should be carried out according to themanufacturers recommendations and as job experi-ence indicates.

6.2.Cold weather concreteAll aggregates must be free of frozen material, as

frozen lumps may effect the metering accuracy. All liq-uid lines must be protected from freezing and drainedwhen not in use. Flow meters must be checked for

proper operation and protected from damage by freez-ing liquids. Additional information on cold weatherconcreting may be found in the report of ACI Com-mittee 306.

6.3-Hot weather concreteUsing VMCM under hot weather concreting condi-tions is not greatly different from conventional con-crete practice. The general principles as outlined byACI Committee 305 for maintaining concrete tempera-tures below specified limits will still apply.

6.4-Aggregate moistureSince proportioning is done on a continuous basis, it

is desirable to supply the machine with aggregates of auniform moisture content. Bulking of fine aggregate isnot normally a consideration since the usual moisturecontent covers a small range where bulking is fairly

constant. A yield check is recommended when there isa wide swing in moisture content (2 percent or more).This check will indicate if recalibration is required. Ag-gregate stockpiles being used to charge VMCM unitsshould be covered to minimize variations in moisturecontent. It may be necessary to limit the free moisturein aggregate by drying and/or covering to meet the loww/c requirements when high volumes of liquid addi-tives, such as latex, are used.

6.5-Rapid slump lossIt has been noted that with some cements a rapid

slump loss occurs after discharge from the mixer (ACI

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304.6R-8 MANUAL OF CONCRETE PRACTICE225). The cause is believed to be related to the shortmixing time typical with this type of equipment. The

problem does not occur often, and a change of cementwill normally correct it.

6.6-Use of admixturesContinuous mixers are high-shear, high-speed mix-

ers. Some admixtures perform differently than might beexpected when used with conventional mixers. For this

reason, the performance of admixtures should be veri-fied by testing for the desired result before actual pro-

ject placement begins. Experience has shown that theseresults will remain consistent once the desired result hasbeen verified on a particular piece of equipment. Ifdeemed necessary to improve the performance of anadmixture, a limited increase in mixing time may beachieved by increasing the angle of the mixing equip-ment .

6.7-Fresh concrete propertiesFresh concrete produced by VMCM equipment be-

haves slightly differently than ready-mixed concrete.

Elapsed hydration time at discharge is measured in sec-onds rather than in minutes. This means that, while theactual setting time (from start of hydration) is thesame, the apparent setting time (from time in place)may seem longer. Finally, the apparent slump at dis-charge is often higher than the measured slump 3 to 5min after discharge. Finishers and inspectors should bemade aware of these differences.

CHAPTER 7--REFERENCES7.1-Specified and/or recommended references

The documents of the various standards-producing

organizations referred to in this document are listedbelow with their serial designations.

The preceding publications may be obtained from thefollowing organizations:

American Association of State Highway and Trans-portation Officials444 N. Capitol Street NW, Suite 225Washington, DC 20001

American Concrete InstituteP. O. Box 19150Detroit, MI 48219

American Society for Testing and Materials1916 Race StreetPhiladelphia, PA 19103

American Association of State Highway and Transpor-

tation OfficialsM 241-86 Concrete Made by Volumetric Batching

and Continuous MixingAmerican Concrete Institute225 R Guide to the Selection and Use of Hy-

draulic Cements304 R Guide for Measuring, Mixing, Trans-

porting, and Placing Concrete

305 Hot Weather Concreting

306 Cold Weather ConcretingAmerican Society for Testing and MaterialsC 94 Standard Specification for Ready-Mixed

ConcreteC 138 Standard Test Method for Unit Weight,

Yield, and Air Content (Gravimetric) ofConcrete

C 143 Standard Test Method for Slump ofPortland Cement Concrete

C 173 Standard Test Method for Air Contentof Freshly Mixed Concrete by the Volu-metric Method

C 231 Standard Test Method for Air Contentof Freshly Mixed Concrete by the Pres-sure Method

C 685 Standard Specification for Concrete

Made by Volumetric Batching and Con-tinuous Mixing

7.2-Cited reference1. Calibration of Mobile Mixers (Concrete Mobiles) to Produce

Portland Cement Concrete,Material Method NY9.1, New YorkState Department of Transportation, Albany, 8 pp.

This report was submitted to letter ballot of the committee and approved inaccordance with AC1 balloting procedures.

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APPENDIX A

INEW YORK STATE DEPARTMENT OF TRANSPORTATIONMATERI ALS BUREAU ALBANY, NY 12232

I

MATERIALS METHOD - NY9.4

I

MATERI ALS METHOD ISSUE DATE - May, 1979

CALI BRATI ON OF MOBI LE MI XERS (CONCRETE- MOBI LES) 3.

MAP CODE 7.42-l - 9.4J AMES J . MURPHY DI RECTOR: MATERI ALS BUREAU

V

4.

Thi s method prescri bes the procedure t o be fol l owed when checki ng the cal i brati onof the mobi l e mi xers t o produce port l and cement concret e. The purpose for cal i -brati on i s to set the control s of the mobi l e mi xers. using materi al s proposed forthe part i cul ar j ob, so that i t produces a cubi c yard of port l and cement concretecontai ning those rel ati ve quanti ti es establ i shed i n a mi x desi gn.

Each mobi l e mi xi ng uni t shal l be inspected and approved by the Engineer. I f i nthe opi non of t he Engi neer, i mproper condi ti ons exist , the condit i ons shall becorrected to the sati sfacti on of t he Engineer, or the mi xer shall be repl aced.I mproper condi ti ons shall i nclude, but not be li mi ted to, hydrated cement deposi tsand mi xi ng paddl es which are l oose, broken, bent scal l oped, worn 20 percent i nany di mension, or heavil y caked wi th mortar.

Each mobi l e mi xi ng uni t shal l be cal i brated by the contractor and checked by theEngineer i nit i all y usi ng project materi als t o set the contr ols so that materi alsare proporti oned to t hose rel ati ve quanti ti es establ i shed i n the proj ect mi xdesign. After this i ni ti al cali bration, addi ti onal f ul l or partial cal i brationsmay be requi red by the engi neer as f ol l ows: whenever maj or mai ntenance operati onsoccur i n the mobi l e mi xi ngsi te, or whenever materi al

CALI BRATION PROCEDURES

A. Precai l brati on CHECKS

uni t, whenever the uni t l eaves and returns to the jobproport i oning becomes suspect .

I n order for the mobi l e mi xing unit to batch accurately several key points l i stedi n the current edi ti on of the Concrete-Mobi l e Handbook found under "MechanicalFactors t hat af fect concrete produced by a Concrete Mobi l e Uni t" , must beperi odi call y checked.

A few of t hese key poi nts are l i sted below:

1. Check cl eanli ness of cement bin. The bi n must be dry and f ree of anyhardened cement. The cross auger must be clean and the steel f i ngerswelded to i t must al l be in pl ace and str aight. The aerators must beoperati ve and the vent must be open and free of debri s.

2. Check ground strap. Uni t must be properl y grounded to prevent cementfr omcl i nging to si des of bi ns due to stati c electri ci ty accumul ati on.

Manual Materi al s Method, NY 9. 4 Code 7. 42-l -9.4 Dateay, 19792 2Subj ect:

Cal i brati on Of Mobi l e Mi xers (Concret e-Mobi l es) To Produce Qort l anaCement Concr ete

5.

6.

7.

Check cl eanl i ness of cement meter- feeder. The pockets i n the mi ter drummust be f ree frcm any cement bui l dup; and the hammers at the end of the

spri ng ti nes should be properl y str i ki ng the meter drum as i t r otates.

Check the cement meter r egi ster for proper operati ng condi ti on. The dri vecabl e shoul d be ti ght and free from kinks.

Check the mai n conveyor bel t f or cl eanl i ness and tensi on. The bel t shal l

not show excessi ve sag.

Check al l the bin vi brators f or proper worki ng order.

Check the operati onal speed speci f i cati on (RPM). I n order to achi eveuni form fl ow of material s, i t i s essenti al t o maintai n consi stentoperati onal speed wi thi n the desi gned operat i onal speed range for t heuni t. Mechanical uni ts have a tachometer for moni tor i ng operati onal

speed. I f the unit i s functi oni ng properl y the foll owi ng tachometer(RPM) readings shoul d resul t i n the proper operat i onal speed.

UNI T TACH. (RPM).

Seri al #' s 708 and l ower 1250

Seri al #' s 709 and higher 1670

Magnum 2250

Hydraul i c uni ts are not equipped wi th t achometers. The operat i ng speedof these uni ts shoul d be checked by ti mi ng the main conveyor bel t dri veshaft. The main conveyor belt dri ve shaf t shoul d operate between 39 to43 revol uti ons per mi nute. The only excepti on bei ng the newl y devel oped

Model #60 Concrete-Mobi l e; t his should operate at 56 RPM. A secondarycheck on operat i ng speed i s the cement meter counter , i t should operateat a rate of 142 counts per mi nute.

B. CEMENT CALI BRATI ON

Aggregate bi ns mus t be empty and cl ean.

Charge the cement hopper wit h at l east 36 bags or 3400 pounds of thesame type and brand of cement to be used on proj ect. Conti nuous feedi ngof cement i s not permi tt ed.

Pri me the conveyor belt wi th cement for i ts enti re l ength. Bypass themi x auger by leaving it i n the travel posi ti on. Run out at l east twobags of cement. I t i s not necessary to weigh thi s sample.

Obtai n the cement contai ner tare wei ght to nearest 0. 5 pound. Thi star e wei ght shal l be determi ned pri or to taki ng each cement sample.

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Subject Cal i brati on Of Mobi l e Mi xe rs( Concret e-Mobi l es) To Produce Port l and

5. Reset the cement meter counter t o zero.

6. Set t he mi xi ng uni t at the proper operati ng speed and obtai n cement samplesto determi ne the exact t i me (t o nearest 0. 1 second) and meter count ( tonearest 0. 5 count) required to discharge one bag plus 2% by wei ght or 96pounds of port l and cement. *

eit her by (a) Tri al and error, or

(b) Averaging 5 two bag samples or 10 one bag sampl es

*The addit i onal 2% cement i s to al l ow for a 0 to +4 percent t olerance oncement del i very, which i s consistent wi th ASTM and manufacturers gui del i nes.

(a) Tri al and Error. Obtai n and record cement wei ghts f or several metercounts, and discharge ti mes to determi ne the cement meter count anddischarge ti me that deli vers 96 pounds of cement. Record thi s dataon the cement cal i brati on worksheet; i f additi onal space i s requi red,use back of worksheet. Usi ng thi s establ i shed count and ti me obtai nthree additi onal samples and record each wei ght. These cement samplesmust meet the fol l owi ng tolerance: 96 pounds + 2%.

NOTE: I f the cement samples do not meet the 2% tolerance, take threeadditi onal samples to recheck del i very tol erance. I f theseresul ts al so fall outsi de the 2% tol erance, the uni t shal l notbe acceptable for project use.

(b-l ) Averaging 5 two bag samples. Obtai n fi ve cement samples of approxi matel y2 bag si ze ( 188 Pounds) . Record the cement wei ght, meter count anddischarge t i me on the Cement Cal i brati on Worksheet. Compute thecement meter count and discharge ti me to del i ver 96 pounds of cement.Steps 2 thru 5 of t he worksheet detai l the computati on procedure. Usi ngthe establ i shed count and ti me obtai n one addi ti onal sample to checkcomputati on accuracy. Record t hi s sample on worksheet i n space provi dedfor Check Run. This sample must be wi thin +2%of the desi red 96 pounds.

NOTE: I f the cement sampl e does not meet t he 2% tol erance checkfor error i n mathemati cal computat i ons. I f no errors are foundrepeat the cal i brati on procedure. I f r etest also fai l s, t he

uni t shall not be acceptabl e for project use.

(b-2) Averaging 10 one bag sampl es. Obtai n ten cement samples of approxi matel y1 bag size ( 94 pounds). Record the cement wei ght, meter count anddischarge ti me on the Cemnt Cal i brat i on Worksheet. Compute the cementmeter count and discharge ti me to del i ver 96 pounds of cement. Steps2 thru 5 of the worksheet detai l the computati on procedure. Usi ng theestabl i shed count and ti me obtai n one addi ti onal sample to checkcomputati on accuracy. Record t hi s sample on worksheet i n space provi dedfor Check Run. Thi s sample must be wi thin + 2%of the desi red 96 pounds.

ManualMateri al s Method, NY 9.4 Code 7. 42-l -9.4 Date May, 1979

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Subi ect :Cal i brati on of Mobi l e Mi xers (Concrete-Mobi l es) TO Produce Port l and

Cement Concrete

(NOTE:) I f the cement sample does not meet the 2% tolerance, checkfor error i n mathemati cal computat i ons. I f no errors arefound, repeat t he above procedure. I fthe retest al so fail s,the unit shal l not be acceptable for proj ect use.

ALL REMAI NI NG INGREDI ENTS ARE CALI BRATED T O THE TI ME CYCLE OR CEMENT METERFEEDER COUNT ESTABLI SHED ABOVE. WHEN THE MOBI LE- MI XI NG UNI T I S EQUIPPEDWI TH A CEMENT METER- FEEDER BYPASS SHAFT, THE COUNT MODE MAY BE USED.

C. (FI NE AND COARSE AGGREGATE CALI BRATION). - ~-

1. Obtai n the mi x design proport i ons based on a one bag mi x. These can beobtai ned from your Materi al s Engineer.

2. Obtai n the Fi ne and Coarse Aggregate Absorpti on percentages fromei therthe NYSDOT Approved source l i st i ng or Regional Materi al s Engineer.

3. Determi ne the Fi ne and Coarse Aggregate oven dry moisture content at t hestockpi l es immedi ately pri or to cal i brati on.- This may bTYThis may be done pri or t o thecement cal i brati on i f deemed necessary.

4. Cal cul ate the Proj ect Fi ne and Coarse Aggregate Wei ghts ( per 1 bag mi x)as fol l ows:

PROJ . AGG. WGT. = 1 Bag Agg. Mi x Desi gn Wgt. (SSD)

(Moist, (%) - Abs. %

l - - 100

NOTE: Resul ts to nearest 0. 5 pound.

5. Usingproject aggregates l oad eit her the coarse or f i ne aggregate bin atl east 2/3 f i l l . (Note - Only the bi n being cal i brated shal l have materi alin it. I f the rubber di vider i s defl ected toward one bi n, f i l l the binthat t he rubber divi der proj acts i nto fi rst So as to prevent fl owOfmaterial s i nto the adj acent bi n).

6. Di sengage the cement di scharge mechanism.

i . Pri me the conveyor bel t wi th aggregate for i ts enti re l ength. Thi s idone each ti me the gate setti ng is changed.

s to be

NOTE: At t his t i me, note f l ow patt ern of aggregate at t he end ofconveyor bel t. An overf l ow may occur due to deformati on o

the

f the

rubber divder at the bottom of t he bi n separator. I f overfl owi s present stop i t by ei ther adjusti ng or changing the rubberdivi der. I f the overfl ow sti l l occurs, the di vider can berestr ained by bl ocking wi th l umber or si mi l ar materi al on theempty si de of the bin di vi der.

8. Set the mi xi ng uni t at the proper operati ng speed.

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Manual Materi al s Method, NY 9.4 Code 7.42-1-9.4 Date May, 1979 Page6

Subi ect:Cal i brati on of Mobi l e Mi xers (Concrete-Mobi l es) To Produce Port l and

Cement ConcreteSubi ect:

Cal i brati on Of Mobi l e Mi xers ( Concrete-Mobi l es) To Produce Port l andCement Concrete

9. Usi ng ei ther the t i me cycl e or cement meter count est abli shed i n thecement cal i brati on, vary the aggregate gate setti ngs to establi sh asett i ng that di scharges the proj ect aggregate wei ght (Step 4). Recordthi s data on the aggregate cal i brati on worksheet.

6. Repeat t he above procedure at l east 2 more t i mes at the same fl owmetersett i ng and di scharge t i me. I f each i ndi vi dual test has no more than + 2%vari ati on f rom the average wei ght t he unit water di scharge system i sacceptabl e.

10. Af ter establi shing the gate sett i ng run at l east three more samples atthi s gate setti ng. For the aggregate discharge system to be acceptabl e:

(a) The average of t he three samples must be wi thin + 2%of the calcul atedproj ect aggregate wei ght

(b) Each sample must be wi thin + 2%of the average.

E. ADMI XTURE I NJ ECTION SYSTEM CALI BRATION

These systems provi de a means of i njecti ng predet ermi ned amount s of admi xture i nsoluti on i nto the concrete mi x. I n order for t he aff ected concrete properti es tobe uni f orml y maintai ned these systems must deli ver materi al quanti ti es consi stentl yand repeatedly. Admi xtures are batched vol umetri cail y and f i owmeters are used tomoni tor batching quanti ti es. These systems shoul d be cal i brated as f ol l ows:

11. Upon completi on completel y cl ean the aggregate bi ns and conveyor belt anddetermi ne the gate setti ng for the remai ning aggregate by fol l owi ng thesame procedure as stated previ ousl y; or r ather than emptyi ng the initi alaggregate bin, set i ts gate in the closed posi ti on. A sl i ght overfl ow ofmateri al may resul t when running wi th t he gate cl osed. Determi ne thi s amountof materi al before l oading the other bin by operati ng the conveyor belt atthe proper operati ng speed and for the ti me cycl e determi ned to del i ver 1bag of cement, measure and record the actual overfl ow. Thi s shoul d be avery smal l amount of mater i al , approxi matel y 3 to 5 pounds. The measuredoverf l ow should be taken into account by tari ng out when cal i brati ng theremai ning aggregate gate sett i ng.

Calcul ate admi xture sol uti on fl ow rate f or ( 1) LO-FLOW and (2) HI -FLOW

systems as fol l ows:

(1)LO- FLOW: FLOWRATE (oz/mi n) =

60 (sec/mi n) x Dosage Rate (oz/ bag)x6 (p art solCement Di scharge Ti me (sec/bag)

HI-FLOW: FLOWRATE (qt/ mi n) =60 (sec/mi n) x Dosage Rate (oz/bag)xl O (part so

Cement Di scharge Ti me (set/ bag) x 32 (oz/qt)

D. WATER CALI BRATI ON

The total water content i n the concrete mi x is contr ol l ed i ndi rectl y, by concrete sl umpvalues, because of vari ati on i n aggregate moisture content. However, i t i s essenti althat t he water deli very system discharge a constant rate of water so that excessivesl ump vari ati ons do not occur.

Fi l l the admi xture systems wi th t he proper part soluti ons to be used on thej ob. Pl ain water may be used for cal i brati on, however, the correct soluti onsmust be used for the yi el d test.

Set the ai r pressure regulator gauge at 15 psi for st andard uni ts or 25 psifor magnumuni ts.

Usi ng a cali brated vial , eit her ounces or mi l l i l i ters (29. 5 ml = 1 oz. ), anda discharge, t i me of one mi nute, establi sh a f l owmeter setti ng_ t hat wi l l delthe cal cul ated fl ow rate (step 1). I f a Concrete-Mobi l e Handbook i s avai l abl ethe Fl owmet er Di agrams for LO-FLOWsyst ems (pg. 44, 45, or 46) or HI -FLOWsystems (pg. 38 or 39) can be used to obtain an i nit i al f l owmeter sett i ng.

NOTE - The f l owmeter sett i ng should be wi thi n the worki ng range of the scal e

These mobil e mi xers batch water volumetr i cal l y through fl owmeters which moni tor thewater fl ow rate i n gal l ons per mi nute. The water del i very system, i ncludi ng fl owmeter,shall be checked for repeatabil i ty of water f l ow rate as fol l ows:

1.

2.

3.

4.

5.

Fi l l the water t ank (when water reduci ng admi xtures are added di rect l y tothe water system, i t need not be added for the cal i brati on; however i t must

be added for t he yi eld test) .

Obtai n the water contai ner tare wei ght t o nearest 0. 1 pound. (Contai ner shal lhol d at l east 5 gal. ).

Set the mi xi ng uni t at the proper operati ng speed.

Pri me the water system by al l owi ng water to fl ow out f or approximatel y 15seconds.

Set the fl owmeter to di scharge approxi matel y 5 gal . / mi n. Obtai n sample byi nterrupti ng water f l ow and record the di scharge ti me (to nearest 0. 1 second)to approximately f i l l the 5 gal l on contai ner. Wei gh thi s amount of waterand subtract tare weight of contai ner to obtai n the actual weight of water.

5.

to all ow for adj ustments due to vari ati ons i n the fi eld air content.The part sol uti on may be changed to accompl i sh thi s.

Having establ i shed a f l owmeter sett i ng, obtai n three (3) one mi nute samplesand record each volume. I f each indivi dual test has no morethan + 3% vari atif rom the average fl ow rate, the unit admi xture system i s acceptabl e.

F. LATEX CALIBRATION

Latex i s batched vol umetri call y through f l owmet er s which moni tor batching quanti ti esi n GALLONS PER MI NUTE. The latex del i very system i s essenti al l y the same as the waterdel i very system and shoul d be cal i brated to accuratel y deli ver 3. 5 gall ons of l atexper bag of cement as f ol l ows:

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Cal i brati on Of Mobi l e Mi xers (Concrete-Mobi l es) To Produce Portl and

Code 7.42- l -9. 4 Date May, 1979 Page 8

Subj ect: Cement Concrete Subiect:Cal i brati on Of Mobi l e Mi xers (Concrete-Mobi l es) To Produce Port l and

Cement Concrete

1.

2.

3.

4.

5.

6.

Calculate the Latex Flow Rate in gal / mi n as fol l ows:

FLOWRATE (gal/ mi n) =60 (sec/ mi n) x 3.5 (gal / bag, l atex dosage rate)

Cement Di scharge Time (set/ bag)

Obtain the l atex contai ner tare weight to nearest 0.1 pound. The contai nershal l have a mi nimumcapaci ty of f i ve gal l ons.

Fi l l the holding tank wi th l atex and setspeed.

Pri me the l atex system by al l owi ng l atexseconds.

the mi xi ng unit at t he proper operati ng

to f l ow out for approxi mately 15

Set t he f l owmeter at t he cal culated f l ow rate ( step 1) and record dischargeti me (to nearest 0.1 second) t o approxi mately fi l l the 5 gal l on contai ner.Wei gh this amount of l atex and subtract t are wei ght of contai ner to obtai nthe actual weight of l atex. Usi ng the same ti me, obtai n two (2) moresamples of l atex, and cal cul ate an average wei ght.

The latex system i s acceptabl e i f the fol l owi ng repeatabi l i ty ( 1) andaccuracy.( 2) cri teri a are met:

(1) I f each indi vi dual test has no more than + 1%variati on fr om theaverage wei ght

-

(2) The average fl ow rate is wi thin + 1%of the cal cul ated fl ow rate(st ep 1). Average fl ow rate i s calcul ated as foll ows:

AVG. FLOWRATE (gal/ mi n) =8 Avg sampl e Wgt* (l bs) x 60(sec/m!n)

8.5(l bs/gal) x Latex Sample Di scharge Ti me (sec)

G. YI ELD TEST

After establ i shing and checki ng the vari ous setti ngs that contr ol batchi ng quanti ti esof al l i ngredients as outl i ned i n the calebrati on steps, i t i s necessary to check theyield of the integrated mi xi ng system to i nsure that the proporti ons set i n the mi xdesign actual l y produce a cubi c yard of port l and cement concrete.

Steps that should be fol l owed to perf orm the yiel d test are:

1. Fil l the mi xing uni t wi th project materials, i ncludi ng admi xtures.

2. Check all gate, valve, and fl owmeter setti ngs f or conformance wi th t hoseestabli shed i n the cal i brati on steps.

3. Determi ne the Cement Meter Count t o del i ver a l / 4 cubi c yard of concreteas fol l ows:

4.

5.

6.

Place a l / 4 cubi c yard box (36"x36"x9") beneath t he mi x conveyor to catchal l the concrete di scharged by the unit. Be sure t hat l / 4 yard box isri gi d, l evel , cl ean, and wel l supported.

Set t he mi x conveyor at an angl e of at l east 15' , and swi ng i t to t he si deso concrete wi l l not di scharge into the box.

Set uni t at proper operati ng speed and di scharge suf fi ci ent concrete toperf orm sl ump, ai r content, and uni t weight t ests.

NOTE - Stop the mi xi ng acti on and mai n conveyor si multaneousl y.. -

After achi eving speci f i ed sl ump and ai r content restart uni t and dischargeunti l f resh concrete i s produced. Stop mi x conveyor and mai n conveyor

si multaneousl y dnd reset the cement meter regi ster to "0".

Swi ng the mi x conveyor over t he l / 4 yard box. Engage the mi x conveyorand the mai n conveyor si mul taneousl y to di scharge concrete unti l the metercount equal s that for a l / 4 yard or the box becomes ful l . Be sure toconsol i date the concrete wi th mechanical vi bratory equipment duri ng andi mmedi ately aft er f i l l i ng the box.

Stri ke-off the concrete i n box and record the exact count. I f box i s not

completel y ful l , re- engage the uni t si multaneousl y to di scharge smal lquanti ti es of concrete and record the exact count needed to f i l l thecontai ner. The count must be wi thin + 2% of the count calculated i n Step for the system to be acceptable.

NOTE - Concrete yiel d is di rectl y proporti onal to the ai r content of theconcrete. I n order f or a concrete mi x to achieve 100 percent yieldthe air content must be i denti cal t o that specif i ed. Any devi ati onfound i n the actual ai r content ( Step 6) wi th that speci f i ed mustbe taken into account when checki ng yi eld tol erance. (See workshee

l / 4 C.Y. CEMENT COUNT =(One Bag Cement Count) /\ (Mi x Desi gn Bags per C. Y.)

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