CA2161454A1 - Method and apparatus for staged vibration of concrete - Google Patents

Abstract

Introduction of vibration forces into plastic concrete structures, such as concrete slabs (M), decks and similar or related concrete structures controls the consolidation of the concrete mass. Vibrating apparatus (3) imparts controlled vibrations either onto the surface (1) or beneath the surface of the concrete mass in sequential stages. The number of stages, the amplitude and frequency of the vibrations, the physical orientation of the vibration producing apparatus (3), the time duration in each stage, and the thickness of each stage of vibration is variable depending upon the physical characteristics of the concrete mass, including the physical characteristics of the concrete being used, the thickness of the concrete slab and the specific materials incorporated into the concrete during formation of the concrete mass.

Description

WO94/~681 PCTIGB941~859 - 2161~

Title: METHOD AND ~PP~R~TUS OF ST~GED VIBR~TION OF CONCRETE

The present invention generally relates to a method and apparatus for introducing vibrational energy into plastic concrete structures which are generally oriented horizontally, (such as concrete slabs, decks, roadways ~nd similar or related concrete structures), in successive stages or increments.

The term "Staged Vibration", as herein used means, sub~equent to lO the pourins of a concrete mass, the introduction of vibrations into the concrete mass in such a manner that ths lower portions of the mass are first consolidated, and the sequential modification of the character of the vibrations so as to cause successively higher portions of the concrete mass to become 15 consolidated~ untiL all or nearly all of the concrete mass is consolidated into a single mass of uniform density.

Collateral with the consolidation of the concrete mass into a uniform density, "Staged Vibration" also produces a concrete 20 structure whose final ~exposed top) surface is uniform and which may be produced at a uniform rate.

SUBSTITUTE SHEET (RULE ~6 2~6~

Essentially, the Present in~ention is a msthod of placing concret~ u~ing sta~ed vibration of th~ concreto by employing a vibration-producing apparatus which is in contact with a concrete mass, ~either by submerged devices, or by devices in contact with 5 the surface of the concrete mass~ or both), in a staged, stepped or phased soquence. In the present in~ention ~ibrations are introduced to a concrete mass in sequential timed ~sta~es"~ with the number of stages, the amplitude and frequency of the vibrations, the time-duration of each sta~s, and the relat~ve lOorientation of th~ vibration-producin~ devices each being variable, depending upon the physical characteristics of the concrote mass. The Pertinent physical charac~eristics of the concrete mass include, but are not necessarily limited to: The physical characteristics of the concrste being usedi the 15 thickness of the slab ~or other structure) being placed; and the specific matsrials incorporated into the concrete during formation of the concrete m~ss~

The determination of which variables to alter, ths amount of 20 alteration, and the implementation of the desirod alterations are made and monitored bY sensors and associated controls which determine the locationvof the boundary layer between that portion of ths concrets mass which has been satisfactorily consolidated by vibration and that portion which has not been satisfactorily 25 consolidated.

SUBSTITUTE SHEET (RULF 26) WO g4125681 PCTIGB94/00859 DES~RIPTION OF THE P~IOR f~RT
t In constructing concrete structures, such as concrete slabs and the like, certain conventional Procedures involve simplY placing the concrete mass in 8 form and finishing the top surface in various well known manners and permitting the concrete to harden with no vibration whatsoever~ Other Procedures involve the use of vibrators placed temporarily into or upon the concrote mass at various locations, with the surface being finished by using 10 various combinations of striking off the surface and~or trowelling operations, includin~ the use of hand trowels, Powored rotary trowels and the like.

~ problem with prior methods of placing concrete using 15 vibrators is associstsd with the lack of control of the vibrators. when anY one secti~n of a poured concrete slab is vibr~ted too much, it causes hard spots in the concrete slab approximatelY at the locstion of the contact with the vibrator.
In addition, over-vibration of the concrete can also cause 20 aggregate separation in the vicinity of the vibrator. ~ggregate separation and "hard spots" both results in a non-uniform and weakened final slab. for these reasons, prior concrete placing operations typic~lly cautiously "under vibrate" the concrete mass or may not vibrste the concrete mass at all~

~nother known procedure involves tho use of sliP forms in which SUBSTITUTE SHEET (RULE 26) W O 94/25681 E~CTIGB94/008S9 ~,~6~ 4 the concrete mass placed in the forms may or may not be vibrated by a continuously moving form into which, or in front of which the concrete is poured and provided with a specific shape which is maintained after the form moves pro~ressively, with the 5 concrete then being finished using conventional procedures.

The principal purpose of vibrating plastic concrete in this context is to exPeditiously consolidate the concrete mass to its maximum and at as nearly a uniform density as possible by lOencouraging and assisting the upward migration of water and air which would otherwise mi~rate slowly or not at all. Entrapment of air and water weakens the concrste and the ~low migration of these materials extends the time required to place and finish the mass. Existing procedures for the application of vibrations to 15the concrete mass provide virtually no means to control or to modify the vibrational characteris~ics of the ~ibrators (other than by manually turning the ~ibrator off and on), and no mesns to control or modify the length of time the vibrators act upon the concrete mass in whi~h the control means i5 based upon the 20 degree of accomplishment of the end result desired. Therefore, the existing procedures produce a concrete mass in which the degree of consolidation varies from one location to the other and in whiçh the time required for water to evaporate from the surface varies from one location to the other.

Another phenomenon associated with natural (i.e. non-vibrated) SUBSR~UTE SHEET (RULE ~26~`

WO94/25681 PCT~GB94/00859 2161~5~

consolidation and curing of concrete is the entrapment of moisture inside of the curing maæs. As poured, concrete mixtures commonly comprise an amount of water far exceeding the quantity which is actually necessary to effect proper curing and maximum 5 strength of the concrete mass. The excess water is intentionally added to the concrete mixture in order to facilitate transporting, pouring, forming, and finishing operations~ If left stagnant (i~e. un-vibrated), Pressurs from the weight of the concrete mass initially slowly prssses some of the excess water 10 upward through the concrete mass~ thus initiallY ~nducing migration of some of the excess water towards the surface of the slab and, at the same time, effecting the consolidation of the concrete mass near the bottom of ths slab. ~s this concrete mass bscomes dry, the concrete begins to cure~ even while the concrete 15 mass may not yet be optimally consolidated. rhis curing of the concrete mass retards the migration of the water towards the surface of the slab. At the same time J in many instances (particularly when the slab is poured in sunlight, on a windY daY
of low humidity), water may evaporate so quickly from the top 20 surface of the slab that the concrete at the top prematurely dries out and begins to cure~ This results in the setting of the concrete at or near the surface of the slab~ which further retards migration of excessive water from ths concrete mass below to the surface~ Ultimately J this phenomenon results in the 25 entrapment of the moisture inside of the concrete slab. Over time the moisture bubbles dry out J leaving small air Pockets SUBSTITUTE SHEET (RULE 26) throughout ths solid concrete slab~ Such air poekets reduce the final strength of the concrete slab.

In prior concrete slab placing operstions, de-watering techniques are used wherein the concrete mass is Poured and formed into a structure having an ~pper surface, and th~ mass is then de-watersd by applying a vacuum water extracting system o~er the wet concrete surface. ~lternativelY. the surface of the concrete mass is de-watered by placing absorbent material (such as burlap or the like) over the wet concrete surface~ and then spreading a 10 desiccant ~such as dry cement~ on the burlaP. The sur~ace is convsntionally finished after the de-wsterin~ process has been completed, and the burlaP or ths vacuum water extraction syst~m is removed. Existing concrete finishing procedures sre labor intensive and require extensive use of skillsd labor and 35 considerable time expenditure in properly carrying them out Efforts which have been made to automatically control mechanical finishin~ devices hav~ been less than satis~actory due to the lack of uniformity of the physical characteristics of the 20 concrete mass at the time the automaticallY controlled finishing devices are introduced to the mass, and due to the inabilitY to control the physical characteristics of the concrete mass iust prior to the ~inishing operation. Thus, human operators have been necessarY to make decisions and adjustments to the finishing 25equipment. Adjustments generally must b8 mads on a continuing SUBSTITUTE SHEET (RULE 26~

W094f25681 PCT/GB94/~859 ..

basis for the entire time of the finishing operation since the variations in the character of the wetness or lack of consolidation of the concrete surface are prevalent in the concrete surface immediately prior to and remain during the finishing operation and no mechanical element of the finishing operation has the capacity to moderate or reduce these variations either in numbsr or intensity.

In hi~hway construction, the surface is generally not required to lO be smoothly finished, with vibration and strike-off being all that is usually required. While this is essentially a machine operation, the end Product (the concrete slab) is not uniform because the delivered materi~ls forming the concrete mass are not uniform and the vibration and strike-off equipment do not sdjust 15 in any way to compensate for this lack of uniformity. TyPicallY, highway concrete is de-watered only by gr~vity (and evaporation~.
Accordingly, water removal from highwaY slab is tyPically a slow, uneven and uncontrolled process. Water removal from highway slabs in the described a slow, uneven and uncontrolled manner 20 results in uneven shrinkage of the slab as it cures, and ultimately resulting in cracks and a weakened structure The following U.5. Patents relate to developments in the introduction of vibration into a concrete mass by the use of 25 vibrating devices that are immersed in or otherwise associated with the concrete mass: 2,015,217. 2,223,734; 2,269,109;

SUBSTITUTE SHEET (RULE 26~

WO 94/25681 PCT/GB94tO0859 2,293,962; and 2,332,687.

While the above patents relate to the vibration of a concrete msss, none of them suggests a staged or step~bY-step vibration of a concrete slab, deck or the like in which each stage of the vibration introduced onto or into the concrete mass affects the mass of concrete to specific desired depths in the concr~te mass;
nor do any of them describe a means of determinin~ the depth to which the concrete mass hss been consolidated or should be 10 vibrated~

SUMM~RY OF THE INVENTION

Accordingly, it is a primary obiective of the present invention 15 to provide a method and apparatu~ for placing concrete slabs~ or similar structures, bY use of machine operationC, in which the concrete is Placed using staged vibration of the uncured, plastic concrete mass.

20 It is another object of the present invention to provide a method and aPparatus of Placing concrete slsbs of the character described wherein, during each stage of vibration, water and ~ir are caused to migrate upward throu~h the uncured concrete mass, thus forming a definable, and substantially hori~ontal~ boundary SUBSTITUTE SHEET (RULE 26~

WO 94/2~;681 PCTtGB94100859 2 1 ~

lay~r, below which boundary laysr the concrste mass may be defined ~s being sufficiently consolidated, and above which boundary layer the concrete mass may be defined as not yet being sufficiently consolidated.

Another objsctive of the present invention i5 to provide a method and apparatus of placing concrete slsbs, or similsr structures, of the character described in which the uncured concrote mass is sequentislly consolidated from the bottom upward toward the top 10 surfacs, so as to effsct a placed structure of substantially uniform density from the bottom to ~or nearly to) the top, wherein the consolidation and integration of adjacent horizontaL
laYers of the concrete mass is effected bY a vibrator aPParatus which advantageously imparts vibrations into the uncured concrete 15 mass.

It is another object of the present invention to provide a method and apparatus for placins concrete slabs, or similar structures, of the chsracter described, wherein the vibrationAl 20 characteristics of frequency, amplitude and time in contact with the uncured concrete mass are each monitored and controlled, ~o that the elevation of said substantially-horizontal boundary layer (between the sufficicntly consolidated and the not-yet sufficiently consolidated concrete mass) maY be closely regulated 25 and adjusted bY said apparatus.

SUBSTITUTE SHEET (RUL 26) WO 9412~681 PC:T/GB94/00859 It is another objective of the present invention to Provide a method and apparatus for plscing concrete slabs having the character described~ wherein the characteristics of said vibrations advantageously imparted into the uncured concrete mass 5 are controlled by sensors located in front of, under and~or behind the vibrator apparatus as it progresses across ths concrete mass.

It i5 another obiect of the Present invention to provide a method 10 and apparatus for placing concrete slabs having the character described, wherein the sensors determine the elevation location of said boundary layer in front ofl behind, and/or under the vibrator apparatus, relative to the elevation location of either the sub-base upon which the concrete mass is being placed or the 15 top surface of the concrete mass.

~nother obJective of the present invention is to provide a method and apparatus of placing concrete slabs, or similar structures, by the use of machine operations in which the rate of hardening 20 O~ ths concrete mass is somewhat controlled by a plurality of "stages" ~or series of vibrations of the concrete mass), with each "stage" affecting only a portion of the total thickness of the concrete mass~

25 It is another object of the prssent invention to provide a method and apparatus for placing concrete slabs of ths character SUB~TITUTE SHET ~ULE 26) 2 1 ~

described wherein the final "stage" of vibration produces a surface of the concrete mass which is substantially uniform as to wstness and othsr characteristics which are critical to the finishing process.

Another objectivs of the invention is to provide a method and apparatus of staged vibration of concrete in which variably controlled vibrations are introduced into or upon the concrete mass in successive stages so as to cause the elevation of said 10 boundary layer, ~or more specifically, the height of the sufficiently-consolidsted, relatively drier, uncured concrete mass) to rise at each "stage", with the number of vibration stages applied to a particular concrete mass being determined by the thickness of the mass and other physical chsracteristics 15 thereof.

It is another object of the Present invention to provide a method and apParatus for placing a concrete slab of the character described wherein the first vibratin~ "stage" causes the 20 formation of a layer of relatively drier, relativsly stiffer, and relatively more consolidated concrete which extends from adjacent the sub-base of the concrete mass to a finite, definable elevation above said sub-base; wherein the second vibration "stage", if required, causes the thickness of said layer of 25 relatively drisr, relatively stiffer, and relatively more consolidated concrete to increase, thereby movins the boundarY

SUE~STITUTE SHEET (RtJLE 26) WO g4J25681 PCT/GB94/00859 p~45 ~

layer upwards toward the surface of the concrete mass, and correspondingly decreasing the thickness of tho relatively 18ss dry, relatively less stiff, and relativelY less consolidated concrete between the boundsry layer and the surface of the S concrete mass; and wherein additional successive "stages", i~
re~uired, cause a further thickenin~ of the layer of sufficiently consolidated concreteJ until nearly the entire mass of concrete becomes essentially one consolidated, homogeneous mass.

10 It is another object of the present invention to provide a method and apparatus of the character described in which the number of "stagesN of vibrstion required to effect a single consolidated, homogeneous concrete mass is dependant upon the thickness and the phYsical characteristics of ths slab being constructed, with 15 thicker slabs ~enerally re~uiring more "stages" of vibration than less thick slabs.

~ further objective of the invention is to provide a method and apParatUs of determining the location of the boundarY layer 20 betwecn the relatively dry, firm, consolidated concr~te in the lower portion of the concrete mass and the relatively wet, soft concrete in the upPer portion of the concrete mass, so that the vibrational characteris~ics such as ~requency, amplitude, and duration can be adjusted in order to cause the boundary layer to 25 sssume a uniform depth beneath the sur~ace of the concrete mass.

SUBSTITUTE SHE~T (I~ULE 26~

21Sl~

~ further objective of the invention is to provide a method and aPParatus of staged vibration of concrete in accordance with the preceding objectivss in which the staged, stepped or phased vibration procedure described in the preceding objects rssults in migration of water to the surface of the concrete mass, and wherein ths accumulated water on the surface of the concrete mass can be removed by mechanical means (such as by vacuuming), thus providing a uniform surface to the slab which enables surface-finishin~ operations to be advantageousLy performed automatically 10 bY machine Another object of the invention is to provide a method andapparatus of the character described which is oPerationally e~ectivs for use in any concrete slab of conventional thickness 15 in various conditions, including interior slabs for buildings;
slabs that maY be either on ~rade or elevatedi slabs for hi~hways, bridges, sidewalks and the like; and slabs either of single thickne~s or integrally or monolithicallY topped, thus enabling the unique method to be utilized with many concrete 20 strUctures~

The~e together with other objectives and advantages which will become subsequentlY apparent reside in the details of con~truction and operation as more fullY hereinafter described 25 and claimed, reference being made to ths accompanying drawings forming a part hereof, wherein like numerals refer to like parts SUBSTITUTE SHEET (RULE 26~

WO g4/25681 PCT/GB941~859 ~,~G~ 14 throughout.

~RIEF DESCRIPTION OF THE DR~WINGS

5 Figure 1 is a schematic sectional view of a concrete slab under construction immediatelY after the concrete mass has been poured;

Figure 2 is a schematic sectional view of the concrete slab of figure 1 shown a short time after the concrete mass has been 10 POured;

Figure 3 is a schematic sectional view of the concrete slab of fisure 1 shown during the first sta~e of vibration using the Present invention;

Figure 4 is a s~hematic sectional view of the concrete slab of figure 1 shown im~ediately after the first stage of vlbration using the present invention;

20 Figure 5 i~ a schematic sectional view of the con~te slab of figure 1 shown during ths second stage of ~ibration using the present invention;

Figure 6 is a schematic sectional view of the concrete slab of 25 figure 1 sho~n during the final stage of vibration using the present invention;

SUBSTITUTE SHEET ~RULE 26) . WO94/25681 PCT/GB94/008S9 2~61 ~4 Figure 7 is a perspective Vi8W showing a plate vibrator apparatus used in the present invention;

Figure 8 is side elevation showing a mechanical probe used in a S modification of the present invention;

Figure 9 is a side elevation showing a sled probe used in a modification of the present invention;

10 Figure 10 is a perspective view of the apparatus of the present invention showing the vibrstor apPsratus supported from above;

Figure 11 is a side elevation of a modification of the vibrator apparatus of the present invention showing a submerged vibratin~
15 plate; and Figure 12 is a side elevation of a modi~ication of the present invention showing an adjustable vibrator structure.

Figure 1 of the drawings illustrates a concrete mass (senerallY
indicsted "M" in the figures) which may be in the form of a slab as the concrete has-bePn Poured into a form ~not shown) or the 25 like from anY suitable source onto a slab sub-base B. The 5~JBSTITUTE SHEET (RULE 26) ~oncrete mass M typically includes aggregate, cement, water and ~ther additives which may conventionally be employed in concrete 31abs~

When the concrete mass ~ is initially poured, as illustrated in Figure 1, the aggregate, cement, water and other materials incorporated into the concrete are typically randomly distributed throughout the thickness of the concrste mass M between the sub-base B and the exposed top surface 1 of the concrete slab. At lothe instant at which ths concrste is mass ~ is first poured 7 virtually none of the concrete mass is sufficiently consolidated and drY enou~h for purposes of finishing ths top sur~acs 1 o~ the slab~ Rlso, at the ins~ant at which the concrete mass M is flrst pouredJ there typically exist~ variations in the moisture content 15 and degree of consolidation ~i~e~ density~ of the concrete mase M
from one point to another o~er the entire volume of the concrete mass M. Such variation in consistency of Poured concrete is not crucial to the operation of the present invention, but, as will be appreciated by those skilled in the art, is an inherent 20 property of randomlY mixed concrete.

Referring now to figure 2: After the concrete mass ~ has been poured onto the sub-base B into the form of a slab, the wei~ht of the aggregates which comprise the concrete mass naturally push 25 downward toward the sub-base ~. The aggregate~ J bein~ of relatively high density, be~in to squee~e water and entrapped air SUBSTITUTE SHEET (RULE 26) ~ 216;1 4~

out of the concrete mas~ M~ Because there is more pressure near the bot~om 2 of ~he slab than near the top 1 of the slab~ more of the water and entrapped air is initially squee~ed out of the concrete mass near the bottom of the slab than near the top of 5 the sla~, thus resulting in relatively more consolidated, relatively more firm and relatively drier concrete M1 near the bottom 2 of the slab, and rel~tively less consolidated, relatively less firm snd relativelY less dry concrete M~ nearer the top 1 of the slab.
At the surfacs 1 of the con~rete slab there develops a finishing zone 7 which prefsrably is no more than 1~4 inch thick. In the finishinq zone 7 migrated water may collect throughout th~
placing operation~ ~lso, finishing oPerationS (which will be 15 desçribed in more detail later) may be used which effc~t a relatively higher concentration of "fines" anci "superfines", and a relatively lower conoentration of aggre~ates in the finishing zone 7 th~n in the rest of the concrste mass M.

20 Between the relatively more consolidsted, relatively more firm and relatively drier concrete M1 near the bottom 2 of the sl~
~nd the relatively less consolidated, relativelY less firm and r~latively less dry concrete M2 nearer th~ toP 1 of the ælab, is . a ~oundary layer L. For purposes of understanding the Present 25 disc~osure, the boundary layer L may be interPreted a~
representins the line (or ~one) below which the concrete mass M1 SlJBSTITUTE SHEET (Rl.J~E 26) WO Y4/2~681 PCT/GB94/~85g is sufficiently consolidated~ firm and dry to effect the desired concrete curing rate and properties, and immediately above which line the concrete mass M2 is not sufficiently consolidated, firm and drY to effect the desired concrete curing rate and properties. Further, it should be understood that the boundary layer L repressnts a line ~or ~one) throu~h the concrete mass M
wherein the concrete mass ~t all points along ths boundary laYer L is of substantially similar, consolidation, firmness and dryness. In Practice the boundary layer L may be either a very 10 narrow line (for exsmple as exists after a plastic concr~te mass is vibrated) or a zone hsvin~ a measurable vertical width (for example as exists at the instant a conerete slab is initially poured~.

15 As will b~ appreciated by those skilled in the art, because of the inconsistencies inherent in the mixing and pourin~ of concrete, the boundary l~yer L which naturallY occur~ in a newly poured slab is notoriously uneven, ~s illustrated in figure 2.
The unevenness of the boundary layer L maY varY due to uneven 20 concentrations of ag~regateJ or Pockets of water, ~tc. in tho poured concrete mass M. It will further be ~PPreciated ~y those skillsd in the art that the curing rate (andy there~ore~ the strength and consistency) of the concrete mass M will normally vary dependin~ upon ~he depth o~ the boundary laye~ L b~low the 25 top surfacs 1 of the slab~ More specifically, in a yivon vertical ~egment of the concrete slab, the grsater the depth SUBSTITUTE SHEET (RULE 26) WO94/~681 PCTI~B94100859 21~14~4 below the surface 1 to the bottom of the insufficiently consolidated, insufficiently firm and insufficiently dry concrete mas~ M2, the longer the curing time for that particular vertical segment-of thc concrete slab~
Referring now to figure 3: A vibrator aPparatus 3 capable of introducing vibrations into the concr~te m~ss M moves across the top surface 1 of the slab in the forward dire~tion ~indicated by arrow 4 in the figures)~ ~s the vibrator apparatus 3 is lOactivated, it introduces vibr~tions into the concrete mass beneath the v~ra~or ap~aratus~ which causes water and air ent.rapped inside of the concrete mass M to migrate upwards towards the top surface 1 of the slab.

~s ths water and air migrate upward due to the vibra~ions, the dePth of the relatively more consolidated, relatively more firm and rela~ively drier concrete Ml near the bottom 2 of the slab rises, and, correspondingly the elevation of the boundary layer La rises. As illustrated in figure 3~ the boundary layer L
ahead of th~ vibrator apparatus 3 remains substantially unchanged ~i~e. uneven and at a relativelY lower elevation~. It has been found that, due to internal friction within the concrete mass M
ss well ~s the geometric diffusion of the vibrational energy, the farther away from the vibrator apparatus, the less thk vibrations 25are felt and the less the effect of the vibrator apparatus~
~ccordingly~ the vibrations more profoundly affoct the concrete SUBSTI-rUTE SHECT (RULE 26) WO 9~ 81 PCT/GB94/00859 mass beneath the vibrator apparatus 3 than concrete mass ~istant to the vibrator apparatus 3~ By advantagsouslY adjustin~ the frequency of the vibration, the amplitude of the vibr~tion, and/or the duration of the vibration ~i.e~ by varying the forward 5 speed of the vibrator apparatus 3) so as to selectively effect the consolidation of the concrete mass, the depth of the boundary layer La beneath the vibrator apparatus 3 can be correspondin~ly adjusted.

10 Sensors S in communication with the ~ibrator apparatus 3 monitor the instantansous depth of the boundary l~Yer L beneath the vibrator apparatus ~ In operation the sensors 5 provide feedback data to ~ Processins unit 6 which determines the depth and Profile of the boundary layer L and which adjusts the 15 frequency of the vibratlon, the amplitude of the vibration, and/or the duration of the vibration ~i.e~ by varyin~ the forw~rd speed of the vibrator aPParatus 3~ as necessary to sffect the desired pro~ile of the boundary layer La.

20 Figure 4 illustrates the condition of the concrete sl~b after the vibr~tor apparatus has comPleted a first pass or first "st~e" of vibration of the ~oncrete mass M. As contrasted with the charaeteristic of the sl~b prior to the first sta~e (as illustrated in figure 2~, th~ volume of the sufficiently ~5 consolidated, su~ficientlY firm and sufficiently dry con~rote mass M1 is ~reater~ and the profile of the boundary lsyer La i~

SUBSTIT~JTE SHEET (RI~IEE 26) WO94125~1 PCT/GB94/0085g 2~61~

more flat J a~ter the fir~t sta~ of vibration is complet~d ~a~
illustrated in fi~ure 4).

Rsferring now to figure 5: ~fter the boundary layer La has boen raised and somewhat e~ened out by the first pass or "stage" of vibration~ ~he vibrator apparatus 3 maY thsn bs used for a second pass or ~stage~ of ~ibration, as shown in figure 5, to further raise the boundary layer Lb. ~s will be ~ppr~ciated by thos~
skilled in the art, by introducing controlled vibrations into the 10 concrete m~s~ M in the manner described above, the consolidation and drying of the concrste mass M is expedited relati~e to that which would naturally occur from stagnant settling of the concrete mass. In addition to more rapidly consolidating and drying the concrete mass M, the structural integritY of the slab 15 is improved. The struetural integrity of the slab is improved by use of the pre~ent invsntion due to the improved consistency of consolidation, ~represented by the substantiallY horizontal orientation of the boundary layer ~b), and due to th~ expedited migration (and subsequont rsmoval) of water from the concrste 20 mass whioh advantageously results in los~ entrapped watcr and air pockets in the concrete slab.

Referrin~ now to figur~ 6: In the preferred embodimant of th~
invention, upon completion of a final "stage" of vibration or 8 25 final p~ss of the vibrator apparatus across the surfaco 1 of the concrete slab, the depth of the sufficiently consolidated, SUBSTITUTE SHEET (RULE 26) WO 94/2S681 PC:TIGB94100859 .

. ~2 sufficiently firm and sufficiently dry concrete ~1 oxtend~ from the bottom of the slab ~ to ~or nearly to) the finishing zone 7 at top surface of the concrete slab 1. Typically, the water which had migrated toward the top of the slab 1 may accumulate in 5 the finishin~ zone 7, and may subsequently simply evaporate, run off the slab due to gravity, be pushed off the slab by thc vibrator apparatus 3~ be vacuumed, or otherwise removed~

It maY be appreciatsd from an understanding of the foregoins that 10 by usin~ a method and apparatus of placing concrete in accordance with the disclosed invention, the boundary laycr L, (or more specifically the top of the suf~icientlY consolidated, firm and dry concrete mass M1), is eve'~ly brought uP towards the top surface of the concrete slab 1. Because the boundarY laYer L, lS(or more specificallY the top of the sufficientlY consolidated, firm and dry concrete mass M1, is evenly brought up towards the top surface of ths concrete slab 1~, th~ entir~ top of th~ slab 1 (or more sp~cifically, the finishin~ zone 7~ becomes in condition for finishing operations at substantially the same tims. In the 20 preferred embodiment of the invention, the boundary layer L is so evenly brought upwards toward the top surface of the concrete slab 1 ~-hat the depth of the boundary layer Lc does not vary by more than 1 inch after completion of the final stage of vibration.
Ths present invention not only expedites the consolidation and SUBSTITUTE SHEET ~RULE ,~

WO ~4/2S681 PCT/GB~41008~

2 1 ~

drying of the relativelY desp pockets o~ moist and unconsolidated concrete by spplying vibrational ~ner~y directly abov~ such areas~ but it also restricts the premature drying and hardening of relatively shal~ow areas o~ moist and unconsolidated concrete 5 by reducin~ the vibration~l energy i~parted in~o such shallow areas~ It maY be appreciated by those skillsd in ths art that if constant vibrational forces wsre equally imparted into all arcas o~ a hetsrogenous concrete mass J the boundary layer would approach the surface of the slab earlier in some areas than in lOother areas~ thus having the undssirable sffect of causing "hard spotsa in the concrete mass. Hard spots in concrete typically cause uneven curing, cause crackin~ of ths ~lab~ increase the difficulty of finishing operationsl virtually Precludes the u5e of automatic finishing equipment, and significantly reduce the l5~tructural strength of the slab. By adJusting the vibrational energy imparted into ~arious aress of the concrete mass M so as to evenly bring the boundary layer L towards the surface of the slab 1, a concrete slab made with the m~thod and appar~tus of the present invention hss fewer (or no) hsrd spots~ is more ~asily 20f1niShed, has fewer cracks~ and is structurally more strons ~han concrete slabs produced using either uncontrolled vibrations or using no ~ibration~l input.

A vacuum water removal system ~not shown) maY be provided 25comprising a vacuuming device which emploYs a rolling or track-like dsvics attached to the vibra~in~ apparatus. Such a SUBSTIT~JTE SHEET (RULE ~26) WO g412S681 PCT/GB94/00859 vacuuming device preferablY includes a means of imPosing a vacuum within a rolling cylinder, the exterior surface of ~-he cylinder being porous and composed of a matsrial throu~h which water, but not the other materials composing concrete, could freely pass.
The exterior surfacc of the vacuum enclosing cylinder is kept clear of accumulated materials by a scraper which is in contact with the surface at some time durin~ each rotation of the cyllnder. Th~ v~cuum is applied to the porou~ surfacs only when that surface is in contact with thc surface of the concrete mass~
10 The surface speed of the rollins cylinder is preferably made to match the &peed of the vibrator apparatus relative to the sur~ace of the concrete mass~

It will be appreciated ~rom an understanding of the above 15 ~isclosure thst a concrete slab placed in accordance with the method and apparatus of the present invention produces ~ top surface 1, and finishin~ zone 7, of uniform physical character o~er the entirs area of the slab. Fur~hermore, because of the consistency of the physical char~cter of the entire area of th~
20surface of the slab, finishing operations may bc Porformed automatically by machins. Thus, this method and apparatu~ for placins concrete by staged vibration uniquelY produccs a uniform surface condition which allows the finishin~ of the top surfacQ
to be perform&d automatically by machine without ~he problems 25which typically hinder prior automatic finishin~ effortæ in concrete slabs placed by prior methods and apparatuses.

SUBSTlTUTE SH~T ~r~ULE 261i WO94/2S681 PCT/&B94/00~59 2i6~ f~ .

The disclosed sta~ed vibration method and apparatus for placin~
concrete is effective due to the reaction of concrste to vibration. During vibration, the water, air and csrtain finer and lighter materials migrate upward, with the materials' 5 mi~rstion being affected by the characteristics of the vibration including the amplitude~ frequency and duration of the vibration.
The characteristics of the vibration are adjusted in the present invention to consolidate th~ relatively lsss consolidate~, relatively less firm and relatively less drY concrets M2 near ths 10 top 1 of the slab at a controlled rate.

In addition to controlling the profile of the bound~ry layer La and Lb by adjustments to the ~rsquency, amplitude and duration of ths vibration, the effect of the vibration is also dePendent uPon 15th~ shape, orientation and configuratlon of a surface (or surfaces~ of the vibrator apparatus 3 which is in contact with the concr~te mass M and whlch imparts the vibrations to the concrete. - -20Using this invention, when a horizontally place~ ~oncr~tc m~ss Mis d~posited on a prepared sur~aoe ~i.e. sub-b~se B), consolidation of.ths concrete mass M occurs in æta~es to bring water to tho surfacs for removal in a manner that finishins operations, if necessary, (or curin~ if no finishin~ oParationS
25are required), maY immsdiatsly follow behind the concrete placins SUE;5TITUTE SHEET (~ULE 26~

operations at a uniform rat~

8ecauss the consolidation of the concrete mass M is exPedited, and because the slab is of substantially uniform consolidstion and dryness from the top of the sub-base B to the bottom of the finishing ~one 7, using the method and apparatus in accordance with the present invention a concrete sl~b can be plsced without using wire mesh (which is commonly imbedded inside of concre~o slabs so as to reduce the undesirable affects of unev~n drying 10 and curing rates between the top and bottom of the slabs~.

The vibration produces an boundary layer La and Lb bet~een the relatively consolidated and relatively unconsolidated portions of the mass ~1 and M2, respectively, with the boundary layer 15 pr~fersbly being maintained as nearly ~s possible at a uniform depth below the top surface 1 of ths ~lab.

The vibrator apparatus used in the preferred embodiment of this invention differs from prior vibrators by inoludin~ a structure 20 ti.e. sensor~ 5~ which enables the location of the boundary layer L relative to the surface of the slab 1 to be d~termined with an associ~ted feedback control system (i.e. proces~or unit 6~.
Sensors 5 may be mounted on or adjacent to the ~ibrator apparatus 3. The sens~rs S sense the depth of the boundary lay~r L, La or 25 Lb, and~ through suitable feedback data processing equipment (i.e. proecssor unit 6~ the controllabLe f~atures of the SUBSTITUTE SHE~T (R~LE 26~

WO 94/~5681 PCT/~B94/OOB~

~ 1 6~

~ibrator ~ppar~tus 3 aro adjustsd as necess~ry to l~ve bchind a boundAry layer La and Lb at a substantially uniform depth below the top surface 1 of the slab.

5 The sensors 5 may be advantageously positioned to determine the vertic~l location of thc boundary laysr L, La and Lb at locations in front of, bshind or directly bsneath the vibrator. The sensors 5 are prefsrsbly mounted in a manner such that the vibration$ will not adveræely affect performsnce of the sensors 105. Further, the sensors 5 are preferably provided in ~ufficient numbers and at sufficient locations to sense the loc~tion of the boundary layer L at as many siteæ relative to the vibra~or apparstus 3 as may be necessary to produce the dosirsd location and profile of the boundary layer La and Lb.

The characteristics of the vibr~tor ~PParatus 3 which ar~
controlled include ths frequency, amplitude and focus or direction of vibrating energy. In sddition the forward sp~d of the entirs vibrator spparatus 3 maY.be controlled~ ~he vibrator 20 spparatus 3 may comprise means for adiusting the character of the vibrations to ensble vibrstions to be focused to a p~rticular dspth either by independent adjustment of individual vibrstors or by adjustment of a plurality of vibrators in concert with each other, thus providins additional control of the depth of the 25 consolidation of the concrste bY the vibrator appar~tus.

SUBSflTUTE SHEET (R~LE 26) WO94/2~681 PCT/GB94/00859 ~,~6~

~8 One type of vibrator apparatus that can be used is a plate vibrstor 3a (as shown in figurs 7) having either one or a plurality of moving pistons or rotating eccentric vibrators 8 mounted thereon, with each vibrator 8 being individually 5 controlled by its own sensor 5a and proccssor unit 6a (or by a single multifunctional processor unit, not shown) which reads the conditions in front of the plate ~. ~s the vibrators 8 vibr~te~
th~ unbalanced dynamic forces of the vibrators 8 are imposed upon the Plate g~ which transmits the forces directly into the 10 ~oncrete mass ~ as vibrations. Additiona} sensors 5b may be mounted behind the plate vibrator 3a in order to ssnse the results of the vibr~tion~ The plate vibrator ~ can be pulled or moved by ~ winch which moves at a sPeed that i~ al~o controlled by the sensor's ~5) data.
The sensor 5 may rely on mechanical probss, submsrged~slsds or skis, or a~oustic characteristics, penetrating radar or similar technology ~or ~etsrmining the dePth of the boundary layer.
Typically, mechanical sensors are less expensi~e than more 20 sophi~ticatsd probes.

several alternative arrangements sre suitable for supportin~ the vibrator apparatus abo~e or on th~ toP surface of the conorets slab 1, including~ but not limited to the following: support from 25 concrete forms; supPort from support arms mounted upon ~arious types of peripheral squiPment; support from skis riding on SUBSTITUTE SHEET ~RULE 26~

WO94/~681 PCT/GB94/00859 2 1 ~

trscks; support from skis or sleds submerged in the eoncrete mass and riding on the boundary layer L between the top and bottom concrete portions M2 and Ml, respectively; or the vibrator apparatus ~ may be supported by any msan~ which will allow the 5 vibrator apparatus 3 to move in a manner to produce the desired sSaged vibration.

One type o~ sensor which may bo used in the present invention is a mechsnical probe 5c, ~s illustrated in figure 8. A piston 11 10is pivotally connected to a pivot arm 12, with the bottom of the pivot arm being provided with a flat plate 13 which comPrise~ the sensing surface of the Probe 5c. The flat plate 13 determines the location of ths boundarY layer L between the relativelY more consolidated, relstively more firm and rclatively drier concrete 15M1 near the bottom 2 of the slab and relatively less consolidated, relatively less ~irm and rclatively less drY
concrete M2 near the top 1 of the slab. The force required to push the flat plate 13 downward to the location of the boundary layer L is measured. Cslibration of the force indicstive of the 20 boundary layer L maY preferably be based on 8 determination of the force required to push against a sufficiently ~onsolidated, sufficiently firm and sufficiently dry concrete mass, and this data preferably forms the bssis of the feedb~ck control system~
.

25 ~n alternative sensor which m~y be used with the pres~nt invention i5 a sl~d Probe 5d, as illustraSed in figure 9 The SUBSTITUTE SHEET (~ULE 26) WO 94/23681 PCT/GBg4/0085~
~4~4 sled 14 is mounted on a pivoting suPPOrt ~rm 15 extending ~rom the beam 16. The sled has a substantially flat bot~om surface 14 that is dragged along by the forwsrd moving beam 16~ The force rsquired to keep the sled 14 at the oorrect penetration into the boundary layer L is determined and forms the reference point for the feedback control system.

It will be appreciatsd that ths by emploYing staged vibration in accordanco with the above doscribed in~ention, a system i5 10 pro~ided that induces water from the concrete mass to the surface of the slsb in such a way that dryin~ of the surface advances at uniform r~te~ Those skilled in ths art will aPpreciate that such uniform rate of drying of the slab is an essential first step tow~rd the facilitation of automatic or robotic finishing of 15 concrete slabs~

In addition, this enables concrete placement to be completed much faster than prior methods of concrete placement, dePending on t~mperature and weather conditions.
As will be appr~ciated from a review of the above disclosure, the present invention providss a mothod and apparatus for plscing concrete slabs Which eliminates the need for various additives ~such as drying a~ents, accelerators, pl~sticizers, etc); which 2 results in mors uniform slab den~ity; which has a flattsr finished surface~ less shrinkage~ less curling and fewer cracks;

SUBSTITUTE SHEET (RULE 26) WO9412S~1 PCT/GB94/008~

5 ~

and which re~uires the use of les5 manpower, than is typically necessary with prior concrete placing methods and apparatuses.
This disclosed method And aPparatus for placing concrete slabs can be ussd in conjunction with common form systems or la~er 5 screeding~

.

The vibrating apParatus 3 may ride on reinforcing barss on an in~ep~ndent base, on metal forms ~s illustrated in figure 10), or other supporting means.

While the preferred embodiment of the invention comprises a vibrating apparatus 3 which applies vibrational forces directly to the surface 1 of the concrete slab (for example, by plate 9 of Pl~te vibrator 3a~ as shown in figure 7~ in cases where the 15 concrete mass M is PartiGularly thick it may be desir~ble to impose the vibrational forces directly to the concrete mass M at a finite distance below the surface 1 of the concrete slab.
Figure 11 illustrates a modified vibrator apparatus 3b which is capable of aPplYing vibrations directlY to the concrete mass M
zo beneath the surface 1 of the concrete slab. The modified vibrator aPParatus 3b is provided with wheels 18 which ride upon rail~s) 19~ ~ vibratin~ arm 20 is pivotally connected ~o an eccentric drive motor ~1 on one end, and attached to a ~-amping rod 22 which extends below the surface 1 of the concrete slab at 25 its opposite end~ ~ sub~tantially horizontally oriented vibrating plate 23 is attached to the bottom of the tamping rod SUBsTlTl5~E SHEET ~RUL~ 26) WO 94/~681 PCT/GB94/00859 3~
22. When the eccentric drive motor ~1 is ~ctivated, the vibrating plate ~3 vibrates, thereby applying vibrational forces directly to the concrete mass M bene~th the surface 1 of ths.
concr~te slab.
s It will be ~ppreci~ted from and understanding of the above disclosure that, resardlsss of whether the vib~ational forces are applied directly to the surface 1 of the concrets slab ~as illustrated in figures 3, 5, 6 and 7) or directly to the concrete 10 mass M beneath the surface 1 of the concrete slab ~as illustrated In figure 11), the disclosed method and apparatus o~ applying vibrations to the concrete mass ~ must occur while the concrete mass ~ is plastlc ~i.e~ while the concrste mass M is uncured).

15 In the prefsrred embodiment of the invention, the final finished concrete surface of the slab is related dimensionally to a reference device or system. Such a reference system may comprise either the fixed rails ~such as rails 19 in figurs 11~ or fixed forms (such ~s metal form 17 in figure 10), or a laser system in 20 fixed relationship to the sub-base B ~not shown~, or similar means.

A modified vibrating apparatus 3c is illustratsd in figure 12.
The vibrating apparatus 3c ~hown in fi~ure 12 compri~es a 25 flexible structure 30 which moves vertically and~or hori~ontaLlY
relativs to the surface upon which it rests, the structurs . .

SUBSTITU~E SHE~T ~l3LE 26~

WO94/~681 PCT/GB94/0085~
- 2 ~

typically being supported by wheels 81 which roll upon the sub-base B, or by skids ~not shown~ which rest upon the structural steel or formwork ~1 of the installation. The vertical locations of the sensors 5 ~nd the ~ibrating surfac~ 3~ (or surfsces) relative tc the structure 30 are preferably fixed. The entirs structure 30 adiusts vertically in response to dats supplied to the Processor unit ~ by ~n optical sensor 40 or othsr instrumsnt which detects the vertical location of the reference de~ice or system relative (for sxample the formwork 31) to the structure lO 30, maintaining a predetermined ~ertical relationship with that device or system.

Ths processor unit 6 recei~es data from the sensor~ 5 re~ardin~
the elevation of the boundary laYer L~ as well as data pertainin~
15to the relative elevation of the r~fer~nce device ~i.e. formwork 31) from the opticsl sensor 40~ and adjusts the vibratin~
characteristics of the vibrating apparatus 3c, the forw~rd speed of the ~tructure 30 and the relati~e locations of ths vibrating surface~s) 32 so as to produce a boundary layer L which is as 20nearly parallel to the desired surface 1 of the concrete slab as possible.

While the foresoing describes the U~8 0~ the apparatus and me~hod of the present invention in placin~ a hori~ontal concr~tc slab o~
5substantially constant thickness using a single pour o~ concreteS
it should be understood that the application of this method and SUBSTITU~E SHEET (RULE ~26) WO 94/~5681 PCT/GB94/0085g ~pparatus to the placing of slabs having substantiallY flat inclined top surfaces, and to the placing of slabs on top of un~ven or inclined sub-bases are within the scope of the present invention. In addition, the described ~ethod and apparatus also has appli~tion to the placing of concrete slabs which have integrslly bonded toppings, wherein a second pour of concrete i .8 . toppin~ may be introduced on top of a first concrete pour~
In the placing of a concrete slab having an int~grally bonded topping, the second concrete pour is prefarably mads after 10 initial series of sta~od vibrations hsve bcen introduced to the first-poured concrete in accordancs with the pressnt invention, but before the boundary layer reaches too close to ths top of the first-pourod concrete mass~

15The fore~oin~ is considered as illustrstive onlY of the principles of the invention. ManY other variations are possible, for examp~e: .

A single vibr~tor apparatus 3 may be provided with both ~urface-vibrating plates (as illustrated in figure 7) and with submerged vibrating plates ~as illustratod in fi~ure 11 );

singlo vibrator apparatus 3 may be provided with cither a sin~le sensor S or with multiple sensors S;
The Vibrating Surface 9 or 23 may comprise a flat plate of SuBsTlruTE SHEET (RULE 26) WO941~681 PCTIGB94/00859 2 1 ~

any shape which is either in contact with th~ ~urface of the concrete mass or is submerged therein;

The vibrating surface ~ or 23 may comprises a plate havin~ a single ~ace in contact with the surface of the concrete mass, or msy comprise an object of any shaPe which i~
submerged withi n the concrete mass;

When multiple vibratins surfaces 9 or 23 are used in a single vibrating aPparatus 3, they may be constructed in such a waY as to permit indspsndent adjustment relative to each other;

The vibrator apparatus 3 msy be of anY type, Provided the characteristics of either smplitude, ~requency, or duration o~ vibration, or any combination thereof, can be controlled ovsr the ranges which are necessary ~or the proper control of the concrete consolidation The vibrator aPparatus 3 may be either electric, hydraulic or air powered;

When a pi~ton type ~ensor Sc is employed, the scnsing Plate 13 may be either a flat or curved ~urface, snd the piston maY be either electric, mechanic~l, air, or hydraulic powered;

.

SUBSTITUTE SHF~ ~RUEE 26~

WO91~'681 PCT/GB~4/00859 ?,~6~S4 When a sled or ski type ssnsor Sd is employed, the probe can be either electric, mechanical~ air or hydraulic Powered;

Ultrasonic J acoustic or ground penetrating radar or other similar electronic systems may be employed as sensors 5;

The vibratin~ appsratus 3 may be supported by wheel~ on the sub-grade, formwork or screeds; or it may be supported directly by snd slide upon the formwork or screeds; and The reference device or system may be any device or system which can provide the vibrator appsratus with a means of dstermining the verticsl location of the desired surface o~
thc concrete slab; Such a reference device or sYstem maY
lS comprise either a taut strin~, ~ laser beam, ~ wooden or metal form or a pipe screed or anY other device or system which can provide similar information.

Further, since numerous modifications and changes will readily 20 occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and, accordingly, ~11 suitable modifications and equivalents may be resorted to J falling within the SGOpe of the invention. Accordln~ly, the scoPe of the invention should be 25 determined not by the embodiment illustrated, but bY the appended claims and their legal e~uivslents.

SUBSTITUTE SHEET (~IJLE 26~

Claims (9)

1. A method of placing concrete comprising the following steps:
depositing a plastic concrete mass to form a concrete structure (M), said concrete structure having a bottom surface (B) and a substantially flat top surface (1 );
introducing into said concrete structure a first series of vibrations so as to cause a first segment of said plastic concrete mass (M1) to become relatively more dense than a second segment of said plastic concrete mass (M2), said first segment (Ml) extending from said bottom surface of said concrete structure (B) to a manipulatable definable boundary layer (L), said second segment (M2) extending from said manipulatable definable boundary layer (L) to said top surface of saidconcrete structure (1), wherein said first series of vibrations causes said manipulatable definable boundary layer to become located at a first position (La) between said first segment (M1) and said second segment (M2);
determining the location of said first position of said manipulatable definable boundary layer (La);
introducing into said concrete structure a second series of vibrations subsequent to said first series of vibrations so as to cause said manipulatable definable boundary layer to become located at a second position (Lb) closer to the top surface (1) than said first position (La);
and controlling a physical characteristic of said second series of vibrations so as to cause said manipulatable definable boundary layer to be more closely parallel to said top surface of said concrete structure (1) at said second position (Lb) than at said first position (La).

2. A method according to claim 1, wherein said step of introducing said first series of vibrations into said concrete structure comprises the steps of vibrating a first tamping member (9) disposed in contact with said plastic concrete mass (M) and above said manipulatable definable boundary layer (L): and moving said firsttamping member (9) along said concrete structure during vibration of said concrete structure.

3. A method according to claim 2, wherein said steps of determining the location of said first position of said manipulatable definable boundary layer (La) and of adjusting said physical characteristic of said second series of vibrations are each performed concurrently with said step of moving said first tamping member (9)

4. A method according to claim 2 or 3, comprising the step of immersing at least a portion of said first tamping member within said second segment of said plastic concrete mass (M2).

5. A method according to any preceding claim. wherein the step of determining the location of said first position (La) of said manipulatable definable boundary layer comprises generating feedback data; and said step of controlling a physical characteristic of said second series of vibrations comprises the steps of processing said feedback data and adjusting said physical characteristic of said second series of vibrations in response to said feedback data.

6. A method according to any preceding claim, wherein said controlled physical characteristic of said second series of vibrations comprises frequency, amplitude or duration.

7. Apparatus for placing a plastic concrete mass for forming a concrete structure (M) having a bottom surface (B), a substantially flat top surface (1) and an area within a definable perimeter, said apparatus comprising:
a tamping member (9) operative to vibrate at a first frequency and at a first amplitude for introducing into said concrete structure a first series of vibrations so as to cause a first segment of said plastic concrete mass (M1) to become relatively more dense than a second segment of said plastic concrete mass (M2), said first segment (M1) extending from said bottom surface of said concrete structure (B) to a manipulatable definable boundary layer (L), said second segment (M2) extendingfrom said manipulatable definable boundary layer (L) to said top surface of saidconcrete structure (1), wherein said first series of vibrations is adapted to cause said manipulatable definable boundary layer to become located at a first elevation (La) such that said first segment (M1) is below said first elevation (La) and said second segment (M2) is above said first position (La);
said tamping member (9) being operative to vibrate at a second frequency or at a second amplitude for introducing into said concrete structure a second series of vibrations subsequent to said first series of vibrations so as to cause said manipulatable definable boundary layer to become located at a second elevation (Lb) closer to said top surface than said first elevation (La);
a sensor (5) for determining the location of said first elevation of said manipulatable definable boundary layer (La);
and processor means (6) coupled to said sensor (5) and said tamping member (9) for controlling the frequency, the amplitude or the duration of said second series of vibrations.

8. Apparatus according to claim 7, comprising moving means (10) for moving said tamping member (9) along said concrete structure during vibration of said concrete structure.

9. Apparatus according to claim 8, wherein said processor means (6) is adapted to process signals from said sensor during movement of said tamping member and to adjust said first amplitude or said first frequency to said second amplitude or said second frequency, respectively, in response to said sensor signals.