304.6R-91

Guide for the Use of Volumetric-Measuring andContinuous-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 basicdesign and operation of equipment used to produce concrete by vol-umetric measurement and continuous mixing (VMCM), frequentlycalled mobile mixers. Definitions, applications, and quality assur-ance testing are discussed. The use of this equipment is compared toweigh-batch-mix equipment for some of the limited 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.

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

James S. PierceChairman

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

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

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.

304.

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 applications4.10-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

*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 effective May 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.

6R-1

304.6R-2 MANUAL OF CONCRETE PRACTICE

Chapter 7-References7.1-Specified and/or recommended references7.2-Cited references

Appendix A-New York State DOT calibrationmethod

CHAPTER 1 -INTRODUCTION1.1-General

The purpose of this document is to offer guidance onvolumetric-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-When the 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 bemaintained.

Continuous mixing-When the output of the mixer isequivalent 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 meetproduction requirements (provided that material inputis also started and stopped). Such a mixer is suitable forboth 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 toproduce 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 documentsproduced 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 aggregatesare 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 accommodatethe 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 limitation

is based upon axle loading limitations. Production oflarger volumes of concrete or high rates of productionwill require special provisions for recharging the mate-rial storage compartments.

CONTINUOUS MIXING EQUIPMENT 304.6R-3

Fig. 2. 1- Typical system

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

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.

2.3-Equipment condition All proportioning and mixing equipment should be

well maintained in accordance with the manufacturer’sinstructions. 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 manufacturer’s rec-ommended limits. There should be no buildup of con-

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

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.

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-PlanningA review should be made of the job requirements

prior 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.

3.4-MaterialsIngredients that are used to calibrate the unit should

be the same that will be used for production. All ma-terials should be stored and handled in accordance withgood concrete practices (ACI 304R). 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 thebottom of the aggregates. Preloaded equipment should

304.6R-4 MANUAL OF CONCRETE PRACTICE

Fig. 4.1-Trailer-mounted unit 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 control

be 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 toproducing 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.

4.3-Low-slump mixturesA well-known application of this type is the low-

slump Iowa DOT high-density overlay (Fig. 4.3.1). In

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

this case, a l-in. maximum slump is allowed and noadditional water may be added to the concrete. Other

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

applications include slipform placing (Fig. 4.3.2) andshotcrete mixtures. The efficient mixing action of thecontinuous mixer is capable of handling all of these ap-plications.

4.4-Long unloading timesSome applications require relatively small amounts of

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

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

ing system. It can be used as a plant at the jobsite,thereby eliminating long haul times for ready-mixedconcrete (Fig. 4.5.1 and 4.5.2). In remote areas, this

can be very cost effective from both a production andquality standpoint.

4.6-Making small deliveriesSmall orders of ready-mixed concrete require indi-

vidual trips for each order. These small orders can beconsolidated into one trip with a VMCM unit. The unitcan go out full and does not need to return until empty(Fig. 4.6).

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

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

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 aftermixing 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

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 concretesMany 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 unitcould 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-er’s 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 byweighing the discharged ingredient. Devices for water,latex modifier (if required), and liquid admixtures suchas air-entraining and water-reducing admixtures gener-

304.6R-6 MANUAL OF CONCRETE PRACTICE

ally 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 ormixture 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 concreteperformance 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 includedas Appendix 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 tocheck volumetric yield (normally a l/4 yd3 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

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

When calibrating cement, precautions should be taken to insure that the aggregate bins are empty (or separated from the system) and that all of the dis- charge is collected. Any carrying mechanisms for the cement should be primed by operating the system until any surface between the storage bin and the collection container which might attract cement becomes coated. The meter register or counter should then be reset to zero and a minimum of five calibration runs should be

5.2.5 Admixtures-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.

5.2.6 Post-calibration volumetric-yield test-All con-trols should be set to produce the desired mixture. Allcontrols should be engaged and all systems charged. Allcontrols should be stopped simultaneously and themeter register or counter reset. A container of knownvolume with rigid sides is then placed under the dis-charge of the mixer. All controls are then engaged si-multaneously and the proper count is run on the meterregister. 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 1min is also calculated.

5.2.3 Aggregates-Aggregate discharge controls mustbe 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 designand 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 accuracyof 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.

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 1/4 yd3 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 50 yd3 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-turer’s 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 forproper 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 themanufacturer’s 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 forproper 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 fairlyconstant. 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

304.6R-8 MANUAL OF CONCRETE PRACTICE

225). The cause is believed to be related to the shortmixing time typical with this type of equipment. Theproblem 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 thisreason, 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-producingorganizations 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 Concrete305 Hot Weather Concreting306 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 ConcreteMade by Volumetric Batching and Con-tinuous Mixing

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

Portland Cement Concrete,” Material Method NY 9.1, 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.

APPENDIX A

I NEW YORK STATE DEPARTMENT OF TRANSPORTATIONMATERIALS BUREAU ALBANY, NY 12232

I

MATERIALS METHOD - NY 9.4

IMATERIALS METHOD ISSUE DATE - May, 1979

CALIBRATION OF MOBILE MIXERS (CONCRETE-MOBILES) 3.

MAP CODE 7.42-l-9.4JAMES J. MURPHY DIRECTOR: MATERIALS BUREAU

V

4.

This method prescribes the procedure to be followed when checking the calibrationof the mobile mixers to produce portland cement concrete. The purpose for cali-bration is to set the controls of the mobile mixers. using materials proposed forthe particular job, so that it produces a cubic yard of portland cement concretecontaining those relative quantities established in a mix design.

Each mobile mixing unit shall be inspected and approved by the Engineer. If inthe opinon of the Engineer, improper conditions exist, the conditions shall becorrected to the satisfaction of the Engineer, or the mixer shall be replaced.Improper conditions shall include, but not be limited to, hydrated cement depositsand mixing paddles which are loose, broken, bent scalloped, worn 20 percent inany dimension, or heavily caked with mortar.

Each mobile mixing unit shall be calibrated by the contractor and checked by theEngineer initially using project materials to set the controls so that materialsare proportioned to those relative quantities established in the project mixdesign. After this initial calibration, additional full or partial calibrationsmay be required by the engineer as follows: whenever major maintenance operationsoccur in the mobile mixingsite, or whenever material

CALIBRATION PROCEDURES

A. Precailbration CHECKS

unit, whenever the unit leaves and returns to the jobproportioning becomes suspect.

In order for the mobile mixing unit to batch accurately several key points listedin the current edition of the Concrete-Mobile Handbook found under "MechanicalFactors that affect concrete produced by a Concrete Mobile Unit", must beperiodically checked.

A few of these key points are listed below:

1. Check cleanliness of cement bin. The bin must be dry and free of anyhardened cement. The cross auger must be clean and the steel fingerswelded to it must all be in place and straight. The aerators must beoperative and the vent must be open and free of debris.

2. Check ground strap. Unit must be properly grounded to prevent cementfrom clinging to sides of bins due to static electricity accumulation.

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5.

6.

7.

Check cleanliness of cement meter-feeder. The pockets in the miter drummust be free frcm any cement buildup; anda the hammers at the end of thespring tines should be properly striking the meter drum as it rotates.

Check the cement meter register for proper operating condition. The drivecable should be tight and free from kinks.

Check the main conveyor belt for cleanliness and tension. The belt shallnot show excessive sag.

Check all the bin vibrators for proper working order.

Check the operational speed specification (RPM). In order to achieveuniform flow of materials, it is essential to maintain consistentoperational speed within the designed operational speed range for theunit. Mechanical units have a tachometer for monitoring operationalspeed. If the unit is functioning properly the following tachometer(RPM) readings should result in the proper operational speed.

UNIT TACH. (RPM).

Serial #'s 708 and lower 1250Serial #'s 709 and higher 1670Magnum 2250

Hydraulic units are not equipped with tachometers. The operating speedof these units should be checked by timing the main conveyor belt driveshaft. The main conveyor belt drive shaft should operate between 39 to43 revolutions per minute. The only exception being the newly developedModel #60 Concrete-Mobile; this should operate at 56 RPM. A secondarycheck on operating speed is the cement meter counter, it should operateat a rate of 142 counts per minute.

B. CEMENT CALIBRATION

Aggregate bins must be empty and clean.

Charge the cement hopper with at least 36 bags or 3400 pounds of thesame type and brand of cement to be used on project. Continuous feedingof cement is not permitted.

Prime the conveyor belt with cement for its entire length. Bypass themix auger by leaving it in the travel position. Run out at least twobags of cement. It is not necessary to weigh this sample.

Obtain the cement container tare weight to nearest 0.5 pound. Thistare weight shall be determined prior to taking each cement sample.

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Subject Calibration Of Mobile Mixe rs (Concrete-Mobiles) To Produce Portland

5. Reset the cement meter counter to zero.

6. Set the mixing unit at the proper operating speed and obtain cement samplesto determine the exact time (to nearest 0.1 second) and meter count (tonearest 0.5 count) required to discharge one bag plus 2% by weight or 96pounds of portland cement.*

either by (a) Trial and error, or

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

*The additional 2% cement is to allow for a 0 to +4 percent tolerance oncement delivery, which is consistent with ASTM and manufacturers guidelines.

(a) Trial and Error. Obtain and record cement weights for several metercounts, and discharge times to determine the cement meter count anddischarge time that delivers 96 pounds of cement. Record this dataon the cement calibration worksheet; if additional space is required,use back of worksheet. Using this established count and time obtainthree additional samples and record each weight. These cement samplesmust meet the following tolerance: 96 pounds + 2%.

NOTE: If the cement samples do not meet the 2% tolerance, take threeadditional samples to recheck delivery tolerance. If theseresults also fall outside the 2% tolerance, the unit shall notbe acceptable for project use.

(b-l) Averaging 5 two bag samples. Obtain five cement samples of approximately2 bag size (188 Pounds). Record the cement weight, meter count anddischarge time on the Cement Calibration Worksheet. Compute thecement meter count and discharge time to deliver 96 pounds of cement.Steps 2 thru 5 of the worksheet detail the computation procedure. Usingthe established count and time obtain one additional sample to checkcomputation accuracy. Record this sample on worksheet in space providedfor Check Run. This sample must be within +2% of the desired 96 pounds.

NOTE: If the cement sample does not meet the 2% tolerance checkfor error in mathematical computations. If no errors are foundrepeat the calibration procedure. If retest also fails, theunit shall not be acceptable for project use.

(b-2) Averaging 10 one bag samples. Obtain ten cement samples of approximately1 bag size (94 pounds). Record the cement weight, meter count anddischarge time on the Cemnt Calibration Worksheet. Compute the cementmeter count and discharge time to deliver 96 pounds of cement. Steps2 thru 5 of the worksheet detail the computation procedure. Using theestablished count and time obtain one additional sample to checkcomputation accuracy. Record this sample on worksheet in space providedfor Check Run. This sample must be within + 2% of the desired 96 pounds.

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(NOTE:) If the cement sample does not meet the 2% tolerance, checkfor error in mathematical computations. If no errors arefound, repeat the above procedure. If the retest also fails,the unit shall not be acceptable for project use.

ALL REMAINING INGREDIENTS ARE CALIBRATED T O THE TIME CYCLE OR CEMENT METERFEEDER COUNT ESTABLISHED ABOVE. WHEN T H E MOBILE-MIXING UNIT IS EQUIPPEDWITH A CEMENT METER-FEEDER BYPASS SHAFT, THE COUNT MODE MAY BE USED.

C. (FINE AND COARSE AGGREGATE CALIBRATION).-~-

1. Obtain the mix design proportions based on a one bag mix. These can beobtained from your Materials Engineer.

2. Obtain the Fine and Coarse Aggregate Absorption percentages from eitherthe NYSDOT Approved source listing or Regional Materials Engineer.

3. Determine the Fine and Coarse Aggregate oven dry moisture content at thestockpiles immediately prior to calibration.- This may bTYThis may be done prior to thecement calibration if deemed necessary.

4. Calculate the Project Fine and Coarse Aggregate Weights (per 1 bag mix)as follows:

PROJ. AGG. WGT. = 1 Bag Agg. Mix Design Wgt. (SSD)

(Moist,(%) - Abs. %l- - 100

NOTE: Results to nearest 0.5 pound.

5. Using project aggregates load either the coarse or fine aggregate bin atleast 2/3 fill. (Note - Only the bin being calibrated shall have materialin it. If the rubber divider is deflected toward one bin, fill the binthat the rubber divider projacts into first So as to prevent flow Ofmaterials into the adjacent bin).

6. Disengage the cement discharge mechanism.

i. Prime the conveyor belt with aggregate for its entire length. This idone each time the gate setting is changed.

s to be

NOTE: At this time, note flow pattern of aggregate at the end ofconveyor belt. An overflow may occur due to deformation o

thef the

rubber divder at the bottom of the bin separator. If overflowis present stop it by either adjusting or changing the rubberdivider. If the overflow still occurs, the divider can berestrained by blocking with lumber or similar material on theempty side of the bin divider.

8. Set the mixing unit at the proper operating speed.

Z

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9. Using either the time cycle or cement meter count established in thecement calibration, vary the aggregate gate settings to establish asetting that discharges the project aggregate weight (Step 4). Recordthis data on the aggregate calibration worksheet.

6. Repeat the above procedure at least 2 more times at the same flowmetersetting and discharge time. If each individual test has no more than + 2 %variation from the average weight the unit water discharge system isacceptable.

10. After establishing the gate setting run at least three more samples atthis gate setting. For the aggregate discharge system to be acceptable:

(a) The average of the three samples must be within + 2% of the calculatedproject aggregate weight

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

E. ADMIXTURE INJECTION SYSTEM CALIBRATION

These systems provide a means of injecting predetermined amounts of admixture insolution into the concrete mix. In order for the affected concrete properties tobe uniformly maintained these systems must deliver material quantities consistentlyand repeatedly. Admixtures are batched volumetricaily and fiowmeters are used tomonitor batching quantities. These systems should be calibrated as follows:

11. Upon completion completely clean the aggregate bins and conveyor belt anddetermine the gate setting for the remaining aggregate by following thesame procedure as stated previously; or rather than emptying the initialaggregate bin, set its gate in the closed position. A slight overflow ofmaterial may result when running with the gate closed. Determine this amountof material before loading the other bin by operating the conveyor belt atthe proper operating speed and for the time cycle determined to deliver 1bag of cement, measure and record the actual overflow. This should be avery small amount of material, approximately 3 to 5 pounds. The measuredoverflow should be taken into account by taring out when calibrating theremaining aggregate gate setting.

Calculate admixture solution flow rate for (1) LO-FLOW and (2) HI-FLOWsystems as follows:

(1)LO-FLOW: FLOW RATE (oz/min) =

60 (sec/min) x Dosage Rate (oz/bag)x6 (part sol.)Cement Discharge Time (sec/bag)

HI-FLOW: FLOW RATE (qt/min) =60 (sec/min) x Dosage Rate (oz/bag)xlO (part sol.) Cement Discharge Time (set/bag) x 32 (oz/qt)

D. WATER CALIBRATION

The total water content in the concrete mix is controlled indirectly, by concrete slumpvalues, because of variation in aggregate moisture content. However, it is essentialthat the water delivery system discharge a constant rate of water so that excessiveslump variations do not occur.

Fill the admixture systems with the proper part solutions to be used on thejob. Plain water may be used for calibration, however, the correct solutions must be used for the yield test.

Set the air pressure regulator gauge at 15 psi for standard units or 25 psi for magnum units.

Using a calibrated vial, either ounces or milliliters (29.5 ml = 1 oz.), and a discharge, time of one minute, establish a flowmeter setting_ that will deliverthe calculated flow rate (step 1). If a Concrete-Mobile Handbook is availablethe Flowmeter Diagrams for LO-FLOW systems (pg. 44, 45, or 46) or HI-FLOWsystems (pg. 38 or 39) can be used to ob tain an initial flowmeter setting.

NOTE - The flowmeter setting should be within the working range of the scale

These mobile mixers batch water volumetrically through flowmeters which monitor thewater flow rate in gallons per minute. The water delivery system, including flowmeter,shall be checked for repeatability of water flow rate as follows:

1.

2.

3.

4.

5.

Fill the water tank (when water reducing admixtures are added directly tothe water system, it need not be added for the calibration; however it mustbe added for the yield test).

Obtain the water container tare weight to nearest 0.1 pound. (Container shallhold at least 5 gal.).

Set the mixing unit at the proper operating speed.

Prime the water system by allowing water to flow out for approximately 15seconds.

Set the flowmeter to discharge approximately 5 gal./min. Obtain sample byinterrupting water flow and record the discharge time (to nearest 0.1 second)to approximately fill the 5 gallon container. Weigh this amount of waterand subtract tare weight of container to obtain the actual weight of water.

5.

to allow for adjustments due to variations in the field air content.The part solution may be changed to accomplish this.

Having established a flowmeter setting, obtain three (3) one minute samplesand record each volume. If each individual test has no morethan + 3% variationfrom the average flow rate, the unit admixture system is acceptable.

F. LATEX CALIBRATION

Latex is batched volumetrically through flowmeters which monitor batching quantitiesin GALLONS PER MINUTE. The latex delivery system is essentially the same as the waterdelivery system and should be calibrated to accurately deliver 3.5 gallons of latexper bag of cement as follows:

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1.

2.

3.

4.

5.

6.

Calculate the Latex Flow Rate in gal/min as follows:

FLOW RATE (gal/min) =60 (sec/min) x 3.5 (gal/bag, latex dosage rate)

Cement Discharge Time (set/bag)

Obtain the latex container tare weight to nearest 0.1 pound. The containershall have a minimum capacity of five gallons.

Fill the holding tank with latex and setspeed.

Prime the latex system by allowing latexseconds.

the mixing unit at the proper operating

to flow out for approximately 15

Set the flowmeter at the calculated flow rate (step 1) and record dischargetime (to nearest 0.1 second) to approximately fill the 5 gallon container.Weigh this amount of latex and subtract tare weight of container to obtainthe actual weight of latex. Using the same time, obtain two (2) moresamples of latex, and calculate an average weight.

The latex system is acceptable if the following repeatability (1) andaccuracy.(2) criteria are met:

(1) If each individual test has no more than + 1% variation from theaverage weight

-

(2) The average flow rate is within + 1% of the calculated flow rate(step 1). Average flow rate is calculated as follows:

AVG. FLOW RATE (gal/min) = 8 Avg sample Wgt* (lbs) x 60 (sec/m!n)8.5 (lbs/gal) x Latex Sample Discharge Time (sec)

G. YIELD TEST

After establishing and checking the various settings that control batching quantitiesof a l l ingredients as outlined in the calebration steps, it is necessary to check theyield of the integrated mixing system to insure that the proportions set in the mixdesign actually produce a cubic yard of portland cement concrete.

Steps that should be followed to perform the yield test are:

1. Fill the mixing unit with project materials, including admixtures.

2. Check all gate, valve, and flowmeter settings for conformance with thoseestablished in the calibration steps.

3. Determine the Cement Meter Count to deliver a l/4 cubic yard of concreteas follows:

4.

5.

6.

Place a l/4 cubic yard box (36"x36"x9") beneath the mix conveyor to catchall the concrete discharged by the unit. Be sure that l/4 yard box isrigid, level, clean, and well supported.

Set the mix conveyor at an angle of at least 15', and swing it to the sideso concrete will not discharge into the box.

Set unit at proper operating speed and discharge sufficient concrete toperform slump, air content, and unit weight tests.

NOTE - Stop the mixing action and main conveyor simultaneously..-

After achieving specified slump and air content restart unit and dischargeuntil fresh concrete is produced. Stop mix conveyor and main conveyorsimultaneously dnd reset the cement meter register to "0".

Swing the mix conveyor over the l/4 yard box. Engage the mix conveyorand the main conveyor simultaneously to discharge concrete until the metercount equals that for a l/4 yard or the box becomes full. Be sure toconsolidate the concrete with mechanical vibratory equipment during andimmediately after filling the box.

Strike-off the concrete in box and record the exact count. If box is notcompletely full, re-engage the unit simultaneously to discharge smallquantities of concrete and record the exact count needed to fill thecontainer. The count must be within + 2% of the count calculated in Step 3for the system to be acceptable.

NOTE - Concrete yield is directly proportional to the air content of theconcrete. In order for a concrete mix to achieve 100 percent yieldthe air content must be identical to that specified. Any deviationfound in the actual air content (Step 6) with that specified mustbe taken into account when checking yield tolerance. (See worksheet.

l/4

C.Y. CEMENT COUNT = (One Bag Cement Count) /\ (Mix Design Bags per C.Y.)