THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATIFASCICLE XIV MECHANICAL ENGINEERING, ISSN 1224-5615

2004

ABOUT CONCRETE CONSOLIDATION ANDVIBRATION

drd.ing. Roza PAIOVICI,ICECON S.A. Bucuresti

s.l. dr.ing.Stefania IONESCU,Universitatea “Dunarea de Jos” din Galati

ABSTRACT

Proper vibration of fresh concrete is an important process in order toensure optimum strength, durability, and appearance of quality precastproducts. Accepted practices in the precast industry are unique in somerespects due to the industry's production environment. The following is asummary of some fundamental consolidation principles and recommendedpractices.

1. INTRODUCTIONFresh concrete must be properly vibrated so thatonce hardened its strength and durabilitypotential are fully realized. Studies haverevealed a marked influence of reducedconsolidation of fresh concrete on its ultimatecompressive, tensile, and flexural strength. Forexample, a 5% reduction in consolidation canresult in a 30% reduction in compressivestrength. Reduced density, due to under-consolidation, can result in increasedpermeability and consequently less resistance todeterioration. Other important characteristicssuch as rebar bond capacity, as well asappearance are likewise affected. Over-consolidation is a lesser concern.Newly placed concrete must be worked toeliminate voids, honeycombs, entrapped air,and to fully encase all rebars and otherembedments. Vibration of fresh concretereduces its internal shear strength and enablesthe concrete to temporarily liquefy, facilitatingthe consolidation process. Once the vibrationstops, its liquid flow subsides.Adequate consolidation in congested areas dueto blockouts and embedments warrantsadditional vibration efforts, etc, to ensure aquality product.The selection of the type of consolidationmethod is largely a matter of experience.Factors to consider include the product'sconfiguration, mix design, size and rate of

concrete placement, aggregate size, rebarconfiguration and desired finish.

2.THE WHAT AND WHY OFCONSOLIDATION

Because finishing of quality precast concreteproducts depends upon consolidation andcompaction, it is important to understand thesetwo terms. Consolidation is the evendistribution of all ingredients throughout themix, while compaction refers to the packing ofconcrete and removal of entrapped air.Vibration or compaction is the principle methodof consolidation of concrete. Several methodsof vibration are employed in the precastindustry, including; external and internalvibration, tamping, centrifugation, vacuums,and pressure. Internal vibration is vibrating theconcrete from within the formwork, whileexternal vibration is vibrating the formworkfrom the outside. Who hasn't seen a precastpatio block and wished for that kind of finishon the next concrete job? If you were toinvestigate, you would find that the block wasexternally vibrated, and you might concludethat the best method of vibration is external.But the method of vibration is not the onlyconsideration.

3. TECHNIQUESVibration or compaction is the principle methodof consolidation of concrete. Several methodsof vibration are employed in the precast

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industry, including; external and internalvibration, tamping, centrifugation, vacuums,and pressure. An American Concrete InstituteCommittee on Consolidation of t Concrete hasdefined several of these methods in a manual ofpractice, a portion of which included asAttachment A of this bulletin.

3.1. INTERNAL VIBRATIONInternally vibrated, vertically cast concreterequires craftsmanship and must be done withcare. Many European countries require avibrator operator to be licensed, which canmean a 10-year apprenticeship. In the UnitedStates, no license is required and there are fewconsolidation courses available.

A vibrator operator for vertically cast,internally vibrated concrete should ask twoquestions:Ø How far apart should I insert the

vibrator?Ø How deep should I penetrate the

subsequent lift?To answer these two questions a

contractor needs to know the rheology of themix (discussed later) and the radius of influenceof the vibrators that are to be used.Knowing the radius of influence, multiply by1.5 to attain the center-to-center insertionspacing. Internal vibrators never influencedownward when used vertically. The vibrationrays curve upward at about a 30-degree angle *when immersed in low-slump concrete, and to alesser upward angle in high slump concrete.The shape of the area of influence is an invertedcone, with the cone's vertex being the nose ofthe vibrator.When you place concrete into vertical formswith a bucket and use internal vibration, eachbucket load will be "melted down" prior tovibrating the lift (a 2-foot lift is recommendedfor internal vibration). But note that the meltingdown process is simply to get an even lift, andyou have to avoid moving the concretehorizontally, which could lead to segregation.Internal vibration is very effective for wet-castconcrete. Surface finishes, however, can oftenbe enhanced by utilizing both stingers andform-mounted vibrators. The followingrecommendations will help ensure effectivestinger vibration.

• Vibration time depends on frequency - thehigher the frequency, the less vibration timeneeded.

• Frequency will be reduced by about 20-25percent, when the stinger is immersed in theconcrete.

• The diameter of the stick should be thewall thickness of the product being poured,divided by four.

• Overlap the "field of action" (vibratingradius) throughout the pour. Doing this willbond the batches and lifts of concrete togetherinto a monolithic pour.

• Completely immerse the stinger into theconcrete.

• Immerse vertically and quickly (aboutone foot per second), but withdraw slowly(about three seconds per foot).

• Put the stinger into each area of concrete,only once.

• When concrete is poured in layers, placethe stinger about six inches into the previouslayer.

• Start vibration when the stinger iscompletely submerged into theconcrete.

• Stop vibration when the surface becomesshiny and there are no more breaking airbubbles. . These vibrators use eccentric(unbalanced) weights to generate the vibratoryforces. Generally, they can be adjusted for bothamplitude and frequency. Form vibration isfaster than stinger vibration, but the formsusually need to be stronger.

Some tips for form vibration:• Don't fasten the vibrators directly onto

the form. Mounting brackets should be weldedonto a form stiffener with the vibrator attachedto the mounting bracket.

• Vibrator location is critical. They must bemounted on the form at locations where theirpotential will be maximized (confirm withvibration supplier).

• Check with your vibrator supplier todetermine the size of vibrator required for yourproduct. The vibratory force required can varyfrom the total weight of the form and concretedivided by two; for walls, to as much as two orthree times the total form and concrete weightto achieve zero-slump concrete.

• Start vibration when the concrete is aboutsix inches above the vibrator.

• Stop vibration when the concrete has alevel, glossy surface and there are no morebreaking air bubbles.

• Fully tie rebar cages to ensure that theirpositions are maintained during theconsolidation effort and to reduce the potentialfor adverse vibrations that could compromisethe concrete-rebar bond.

The primary factors that influence theeffectiveness of internal vibrators are thevibrator`s head diameter, frequency andamplitude. Head diameter for range from %" to

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T dia. and are available in varied shapes.However, larger diameter heads are notemployed in the precast industry. Internalvibrator frequencies range from 4,000 to 15,000vpm and many have variable frequencies andamplitudes to accommodate a wide range ofuse.Internal vibrators should be vertically droppedinto the concrete, allowed to vibrate in place(e.g., 5-15 seconds for wet mixes or up to 2-3minutes for stiff mixes) and then removed. Thewith drawl should be at a some what quickerrate than its placement. Vibrators should not beused to transport concrete laterally. An internalvibrator should slightly penetrate into theprevious lift to ensure an adequate bond, notesketch below.

Correct Incorrect

Figure 1. Internal Vibration of Top LiftCare should be taken to avoid touching ordamaging the formwork.

3.2. EXTERNAL VIBRATIONExternal vibration also requires a qualifiedoperator, but more important is the position ofthe vibrators or vibrator brackets on the form.External vibrators also have radii of influence,and positioning the vibrator locations shouldfollow the 1.5-times-the-radius rule. The shapeof the area of influence is a hemispherecentered on the location of the vibrator. Aconcrete element that is thicker than the radiusof influence of the vibrator will requirevibrators on both sides of the formwork. Ifvibrators are required on both sides, neverposition them directly opposite one another--typically the backside or second-side vibratorlocations should be positioned at the halfwaypoints of the front side.If the height of the element is greater than theradius of influence, a second set of vibrators ormounting brackets is needed if the vibrators areto be moved from location to location. If avibrator track is used, the track should beinstalled vertically, since entrapped air migratesvertically, not horizontally. Unlike an internalvibrator that can easily be moved vertically,external vibration is usually started when thefirst concrete is discharged into the form andoperated until the form is filled. The lift depth

should be 1.3 times the radius of influence andmay be greater than the 2 feet recommended forinternal vibration. The time duration should notexceed the 2-inches-per-second or 3-inches-per-second rule, or the vibrators must be activatedintermittently to allow the vibrator to densitythe concrete and assist in the migration ofentrapped air.If the vibrators operate continually withoutobserving timing constraints, the formwork willbe overstressed, and the entrapped air willappear in pockets at the outer extremities of theradii of influence.With both internal and external vibration, theweight of concrete helps consolidate the lowerlifts, but gravity's effect in the upper parts ofthe lift is negligible. The top lift should berevibrated for aesthetically pleasing concrete orfor any high-quality concrete, and this can bedone well after placement is complete. Aninternal vibrator, properly equipped to allow thevibrator to remain in a horizontal position, canbe pulled horizontally through the top sectionof concrete. This step can often be delayed byan hour or more, allowing the concrete todensity by gravity, and it greatly improves boththe density and appearance of the concrete inthe top area of a vertical placement by reducingbug holes. The technique requires practice andwill not be feasible for every application,especially in areas of reinforcing congestion.This technique is possible only for externalvibration when multiple vertical vibratorlocations are used.Formwork for externally vibrated concreteconstruction must be watertight to preventwater or grout leakage and to achieve the bestpossible finishes. Water tightness in eitherconventional or custom-built forms adds to thecost, and conventional formwork is rarelywatertight. To produce a watertight form,compressible gasketing between each form jointand tie hole is necessary, and the compressedgasket must be able to expand in the joint whenthe face sheet deflects due to differentialpressure. Externally vibrated formwork is alsovery susceptible to uplift due to the formworkdeveloping a slight batter during vibration, ordue to increased lateral stresses on the baseanchorage if the vibrators are operatedsequentially, although sequentially operatedexternal vibration is a necessity for most jobs.Vibration tables, external form vibrators, droptables and other specialized equipments arecommon in precast plants. Specializedequipment can offer product specific benefits,ofering more uniform control and greateroverall economy.Vibration tables are unique to the precast

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industry. Vibration tables are rigid decksmounted on flexible supports which operate at3,000 to 6,000 vibrations per minute (vpm).External form vibrators should be mounted justbelow poured concrete surfaces and havefrequencies ranging from 2,000 to 6,000 vpm.Precautions should be exercised to ensure that apumping action is not created that couldintroduce air into the fresh concrete.Drop tables are also unique to the precastindustry. This equipment employs a low-frequency, high amplitude method of shockcompaction.All consolidation equipment should beadequately secured and the formwork should besufficiently study to resist the repeatedvibration and/or shock loads. Lockingmechanisms are recommended on connectors asneeded.

4. MIX CONSIDERATIONSMany people in our industry have themisconception that external vibration canproduce aesthetically pleasing concrete withany mix. With the heavy use of admixturestoday, very low-slump mixes are not ascommon as they once were. But whether slumphas been increased by water or by admixture,the potential for segregation is great withpoorly proportioned mixes. For aestheticallypleasing concrete, the rheology of the mix ismuch more important than slump. Rheology isdefined in ACI 309 as "the science that dealswith the flow of materials," and this includesthe deformation of hardened concrete, thehandling and placing of freshly mixed concrete,and the behavior of slurries and pastes. Therheology of concrete is characterized by threeproperties: stability, compactability, andmobility.Stability is fresh concrete's ability to resistsegregation as it flows into forms.Compactability is a measure of how easily theconcrete is compacted and is the most-abusedcharacteristic of vertical concrete's rheology.Highly sanded mixes are the norm, and theresults they produce are not aestheticallypleasing, durable, or generally acceptable.Compactability can be checked in the field byplacing some of the mix in a pile in awheelbarrow and then vibrating the concretewith the internal vibrator that is to be used onthe job, or vibrating the wheelbarrow forexternal vibration. In either case, observe thereaction of the concrete. If consolidation occursalmost instantaneously, the mix is compactible.This is not a highly scientific method, but ittells us a lot about how the concrete will behavein the formwork.

6. EXCESSIVE VIBRATIONOver-vibrated concrete has a very wet surfaceand a layer of mortar without coarse aggregate.When over-vibration is evident, the slumpshould be reduced. Over-consolidation,however, is normally not a significant concernand rarely occurs in the precast industry. Infact, in cases in which the appearance of theconcrete surface is important, it is common todouble the normal vibration time to ensure asmooth, defect-free finished surface. Thisdepends upon the type of vibration system, socheck with the vibrator supplier.

5.HOW DO VIBRATORS WORK?Vibrators impart a vibratory force into theconcrete through a combination of frequencyand amplitude. Frequency is the number ofvibration cycles per minute, and is expressed as"rpm" or "vpm." It is noisier than amplitude andmore effective with lighter mass. It moves thesand and slurry around the rock, and governsliquefaction.Vibration of fresh concrete reduces its internalshear strength and enables the concrete totemporarily liquefy, facilitating theconsolidation process. Once the vibration stops,its liquid flow subsides.Amplitude is more effective with a heaviermass of concrete. It moves the rocks anddetermines the radius of action. It may besimpler to think of frequency as the number oftimes that the vibratory forces occur, whileamplitude is the distance that the force is"thrown." A light, thin section, for example,would be vibrated at a high frequency and lowamplitude, because high amplitude would throwthe concrete out of the form. Heavier, thickersections, on the other hand, are more effectivelyvibrated with higher amplitude and lowerfrequency.Vibrators, either internal or external, arecharacterized by their frequency and amplitudeof vibration. Optimum frequencies andamplitude will vary with mix design, formconfigurations, and other factors. For examplelower water-cement ratios, greater cementcontents, greater angularity of coarse aggregateall require greater compactive efforts. Also,superplasticized concretes typically require 20-50% of the compactive effort of conventionalconcretes and can effectively employ smallerdiameter internal vibrators.The ideal vibrator is one in which the frequencyand amplitude can be varied ranging from low-frequency, high-amplitude during initialconsolidation to high-frequency, low-amplitudeat final consolidation,Fresh concrete should be vibrated until all voids

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and entrapped air pockets are released. Theconsolidation of the concrete will be evidentwhen escaping air bubbles cease.Full consolidation can be judged by theformation of a mortar rich appearance on theconcrete surface and, in cases, a noticeabledifference in the sounds emanating from thevibrator. If a concrete is over-vibrated itssurface will appear very wet and have a layer ofmortar without coarse aggregate. When over-vibration is evident the slump rather than theamount of vibration should be reduced. Over-consolidation is normally not significantconcern. In fact, in cases in which theappearance of the concrete surface is important,it is common to double the normal vibrationtime to ensure a smooth defect free finishedsurface.Re-vibration of concrete after initialconsolidation is an accepted practice asnecessary to weld successive lifts together.Fully tied rebar cages are urged to ensure thattheir positions are maintained during theconsolidation effort and to reduce the potentialfor adverse vibrations that could compromisethe concrete-rebar bond.

7. CONSOLIDATION METHODSConsolidation of precast concrete products canbe accomplished using a variety of vibrationmethods. An optimal vibration system dependsupon the concrete mix design and whether theconcrete to be consolidated is wet-cast or dry-cast. External vibration can be accomplishedwith forms and tables, while stingers (spuds,pencils, sticks, pokers) are used for internalvibration. Dry-cast concrete can be vibratedwith vibrators mounted on external and/orinternal forms, while some systems utilize avibrating table that imparts vertical vibratoryforces to the concrete.

8. TYPES OF VIBRATORSSeveral different types of vibrators are used inthe precast concrete industry. Each has itsadvantages for a given application.A variety of features are offered with each typeof vibrator. These features include adjustablespeed and force (frequency and amplitude),remote converters, and various types ofmounting brackets. Vibrator types are:

• electric (115 volt / 220 volt)• pneumatic• operating pressure• turbine models• hydraulic• compaction tables• foot pedal, on-off switch

Some high-end features can result in greatbenefits for a precaster. For example, aproducer of box culverts installed radio controlof an inverter-driven vibrator, with greatbenefits. The casting process included avibrating system with a variable-speed drive.The inverter was usually moved away from theproduct to allow for more room during casting.However, this set-up usually required a workerto be assigned to running the inverter.Radio control of the variable-speed drive wasinstalled. This has saved lots of footsteps (25-30 each way) for workers and often completelyfreed up a worker. The crew likes it becausethey can run the vibrators without leaving theproduct during casting. An additional feature isthat they can vary the frequency with the radiocontroller, as needed.

9. CONCLUSIONSurface imperfections can be caused by eitherunder-vibration or over-vibration. Under-vibration generally results in honeycombing,excessive entrapped air, and sand streaks. Theresults of over-vibration can be segregation,form deflection, form damage, and sand streaks.It may take a few trial runs to determine whichcombination of frequency and amplitude, aswell as vibration time, are correct for a givenproduct.It is important to recognize that the vibration ofdry-cast products is very different from wet-cast vibration. Suppliers of dry-cast systemsdesign their forms to work with their specificvibration system. It is not, therefore, a goodidea to "mix and match" forms that weredesigned for one system with a vibration systemsupplied by another manufacturer.Dry-cast forms are specially designed towithstand the high vibratory forces that arerequired for zero-slump concrete. As a result,they are generally more expensive than wet-castforms.

REFERINCE[1] Bartos, P. Fresh Concrete: Properties and Tests, Elsevier.(1992);[2] Ford, J. Internal or external vibration? Done correctly, eitherwill produce a high-quality finish - of concrete, ConcreteConstruction, 2003;[3] Shane Tymkowicz ,S. , Steffes R. F. Vibration Study forConsolidation of Portland Cement Concrete, 1996Semisesquicentennial Transportation Conference,1996.

THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATIFASCICLE XIV MECHANICAL ENGINEERING, ISSN 1224-5615

2004

Attachment AConsolidation Methods for Precast Concrete Products

N

Notes:a. Very stiff mixes, with water-cement ratios of 0.30 by weight or lessb. Stiff mixes having a water-cement ratio exceeding 0.30 but with less than 1 in. slumpc. Uniformly gap graded mixes with slump in the 1-4 in. ranged.Mixes with over 4 in. slump, which flow readily and segregate if mechanical vibration is apllied.

Product MixClassification

FormingMaterial

Conveying and Placementmethod

Consolidation method

Concrete Pipe a to d Steel Pumping or bucket(thin layers)

Tamping; internal or external vibration;centrifugation; vacuum; pressure

Concrete piles and poles c,d Steel Pumping or conveyed bymixer trucks Centrifugation; internal or external highfrequency, low amplitude vibration; rollerpacked

Concrete block b Steel Machine hopper Low frequency, high amplitude vibrationplus pressure

Slab and beam sections b,c Steel Traveling hopper, mixer trucks, beltconveyors

External vibration with or without rollercompactions; internal vibration with surfacevibration screed

Wall panelsa to c Reinforced concrete

steel or woodBuckets and belt conveyors (continuous

ribbon feed) Tampers; internal and external vibration