ACI 309.3R-92.

Guide to Consolidation ofConcrete in Congested Areas (Reapproved 1997)

Reported by ACI Committee 309

Dan A. Bonikowsky*Neil A. CummingTimothy P. DolenJerome H. FordJoseph J. Fratianni

Mikael P. J. Olsen, ChairmanSteven H. G e bGary R. MassRichard E. Miller Jr.Roger A. MinnichH. Celik Ozyildirim

Sandor PopovicsThomas J. ReadingDonald L. SchlegelBradley K. Violetta

*Subcommittee chairman.Originating committee chairman.*Subcommittee members.Steven A. Ragan actively contributed to the development of this document

and served as chairman of the editorial committee.

This guide is primarily directed toward architects/engineers and con-structors. It describes various situations where design requirementsresult in highly congested forms that impede consolidation of con-crete. Techniques to overcome these difficulties are presented. Theguide also identifies for constructors various difficult placing andconsolidation conditions and proposes solutions such as special pro-cedures and mix proportions. In addition, the guide alerts construc-tors to review design drawings closely where congested areas are ex-pected to insure that appropriate allowances have been included intheir bids.

Keywords: admixtures; concrete construction; consolidation; embedment;formwork (construction); mix proportioning; parting agents: placing; pre-placed aggregate concrete; reinforced concrete; reinforcing steel; splicing;structural design; surface defects; tunnel linings.

CONTENTSChapter l-Introduction, pg. 309.3R-1

Chapter 2-Criteria for designation as a congested area,pg. 3093R-2

2.l-Reinforcing steel2.2-Embedments and boxouts2.3-Formwork2.4-Definitions

Chapter 3-Factors contributing to congestion problems,pg. 309.3R-3

3.l-Reinforcing steel arrangement3.2-Embedded parts/boxouts3.3-Formwork

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.

309.3R-

3.4-Mix proportioning3.5-Concrete placing methods3.6-Construction considerations3.7-Tunnel linings

Chapter 4-Consequences of congested areas in concreteconstruction, pg. 309.3R-4

4. l-Honeycombed concrete4.2-Reduced density4.3-Increased cleaning costs4.4--Increased formwork costs4.5-Increased placing costs

Chapter 5-Recommended practices, pg. 309.3R-65.1 -Design considerations5.2-Construction considerations5.3-Summary

Chapter 6-References, pg. 309.3R-96.1 -Specified and/or recommended references6.2-Cited reference

CHAPTER 1--INTRODUCTIONMany concrete structures such as those with seismic

provisions, post-tensioning, and high-strength concreteare difficult to consolidate because of congested areaswithin the formwork. This congestion can result in

ACI 309.3R-92 became effective December 1,1992.Copyright 0 1992, American Concrete Institute.AU rights reserved, including rights of reproduction and use in any form or by

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1

ACI COMMlTTEE REPORT

structural inadequacy and time-consuming and expen-sive remedial work.

Various techniques have been employed to alleviatethis type of problem. This document presents an over-view of the factors contributing to the problem, theconsequences of inappropriate concrete procedures inthese areas, and recommended practices to minimizethe problem.

As a prerequisite to successful concreting in con-gested areas or difficult placing conditions, it is impor-tant that architects/engineers become more aware ofhow their designs will be constructed, and that con-structors become more aware of special procedures andnecessary precautions. Most importantly, communica-tion between the architect/engineer and constructor isessential to insure that the design details, constructionmaterials, and procedures are compatible.

CHAPTER 22-CRITERIA FOR DESIGNATION ASA CONGESTED AREA

Congested areas are those in which the reinforcingsteel, embedments, bboxouts, prestress ducts and an-chorages, or configurations and form shape make con-crete placement and consolidation difficult to achieve.To obtain the desired placement results and degree ofconsolidation, access for inspection and consolidation,special concrete mixtures, special formwork, additionalconsolidation effort, and specific placing methods arefrequently used.

2.1-Reinforcing steelCongestion causes problems when the clear spacing

between reinforcing bars or between a bar and the formis less than 1 l1/3 times the maximum size of coarse ag-gregate used in the concrete mixture. This condition ismore likely to occur at splices and bends in reinforce-ment and at beam-column connections. Sometimes,congestion is caused by multiple layers of reinforce-ment in which the bars in the lower layers are not di-rectly below those in the upper layers, as shown in Fig.1 and 2. See AACI 117 for tolerances for concrete con-struction and materials.

Fig. 1 -Dense reinforcing steel

Fig. 2-Dense reinforcing steel

2.2-Embedments and boxouts

Embedments consist of items such as plumbing,prestress hardware, ducts, connection inserts, and an-chorages for handling devices that are cast into theconcrete (see Fig. 3). Boxouts are used to form open-ings, keyways, or pockets in the concrete. When theseitems restrict the placement and consolidation of theconcrete, they cause congestion. The spacing betweenembedments, boxouts, and the form must be at least1% times the nominal maximum size of the coarse ag-gregate to avoid this problem. Frequently, these itemscause congestion because the concrete cannot be placedand consolidated easily underneath them (see Fig. 4).The architect/engineer must be alert to such condi-tions, and construction procedures must provide forproper placement and consolidation of concrete on theundersides of these embedments.

Fig. 3-Closely spaced embedments

2.2.1 Embedments may be anchors, weld plates, me-

GUIDE TO CONSOLIDATION OF CONCRETE

Fig.4--Stacked boxouts

chanical and electrical boxes, threaded inserts, or otherdevices used to attach or incorporate items after form-work is removed.

2.2.2 Sleeves are openings that generally go all theway through a wall or slab to allow piping or otherpenetrations to pass. Formwork for these openings isoften completely removed prior to piping placement.

2.2.3 Boxouts are either removable or stay-in-place.Removable boxouts are similar to sleeves but generallyare much larger, as in door and window boxouts orhatch boxouts in slabs. Most removable boxout formscan be adapted readily for placement and vibrationtubes, as described in Section 5.1.2. If these removableforms are not provided, the constructor must move theconcrete horizontally under the formed boxout, whichusually results in segregation of the concrete, adds ad-ditional stress to the boxout form, allows the buildupof frictional resistance along the formwork, and slowsdown the entire placement. Without placement and vi-bration tubes, the degree of consolidation is generallyreduced under a boxout. Boxouts often are designedwith open bottoms.

Stay-in-place boxout forms, such as hollow metaldoor and window frames, often require bracing and donot allow placement and vibration tubes to be cutthrough them. This increases the chances of voids orincomplete consolidation.

2.2.4 Formwork accessories, architectural items suchas cast-in numbers and letters, form liners, rustication,chamfers, and keyways can cause simple to complexconsolidation problems in one manner or another. Thisis especially true of horizontal rustication or keyways ina wall.

2.3- FormworkThe surface texture, shape, type, and orientation of

the formwork may restrict concrete placement. Consid-eration needs to be given to form release agents that arecompatible with form texture, particularly if intricateshapes are to be cast into the concrete at the formedsurface. These release agents may also serve to some-what reduce the frictional resistance between the plasticconcrete and the form, thereby improving the ease ofremoving entrapped air. The forms must also be de-signed for easy removal.

Used or poorly oiled wood forms are more likely tohinder consolidation than steel or plastic-lined forms.The frictional resistance of a wood form impedes theflow of concrete and can create difficulty when used i nconjunction with congested embedments.

2.4-Definitions2.4.1 External form tie rods-External form tie rods

are installed on the outside of narrow wall forms in thelongitudinal direction. The tie rods are attached to thebulkhead walers at the ends of the wall.

2.4.2 Lie-flat hose-Lie-flat hose is a very pliablepolyvinyl chloride reinforced discharge hose, typically

purchased in 4- or 5-m. (100- or 125-mm) diameter by300-ft (92-m) long rolls.

2.4.3 Side ports-Side ports are temporary openingsin the form on one side of narrow walls. The purposeof the side ports is to allow insertion and extraction ofvibrators and to observe consolidation of the concrete.

2.4.4 Slide valve-A slide valve is a short piece ofsteel pipe with a slide plate mounted in it. The pipe isthe same diameter as the concrete discharge hose andthe other end is bolted to the form. The purpose is toallow pumping of concrete through the open slide valveto completely fill a form to the underside of a horizon-tal structural steel beam. When the form is full, theslide plate is closed, preventing the concrete from seep-ing back through the valve.

2.4.5 Steel reinforced hose-Steel reinforced hose isa rubber concrete-discharge hose reinforced withstrands of steel wire between the tube and outer cover.

CHAPTER 3-FACTORS CONTRIBUTING TOCONGESTION PROBLEMS

3.1 -Reinforcing steel arrangementThe reinforcing steel arrangement must take into ac-

count the factors that contribute to congestion. Seismicand strength design requirements often result in a rein-forcing steel layout that inhibits access for preplace-ment cleanup and concrete placement and consolida-tion. Recommended practices are described in Chapter5.

3.1.1 Splices-The density of reinforcing steel result-ing from current design procedures often makes it dif-ficult to provide continuity of reinforcing bars by thetraditional method of lap splices. The various methods

309.3R-4 ACI COMMITTEE REPORT

of splicing reinforcing bars, as discussed in Section5.1.1.1, need to be considered by the architect/engi-neer .

3.2-Embedded parts/boxoutsThere is increasing use of embedments and boxouts

to incorporate piping and electrical and mechanicalsystems into placements. The use of embedments andboxouts, in conjunction with dense reinforcement, of-ten results in congestion that inhibits acceptable plac-ing and consolidation practices.

3.2.1 Tolerances for placement of concrete aroundembedments and boxouts should be considered at thedesign stage. Frequently, mechanical and electrical em-bedments are located adjacent to doors and windows.These areas usually require additional reinforcing dueto stress concentrations around the boxout. The corearea in buildings is another example where additionalreinforcement, embedments, and boxouts cause con-gestion.

3.3- FormworkThe design of formwork can contribute significantly

to congestion in placements if the design does not takeinto account other factors, e.g., location of embed-ments and boxouts, reinforcing steel arrangements,placing equipment, and form-tie spacing.

The design should consider the number, location,and size of form-tie rods; location of embedments andblockouts; location of trunks or concrete hose; heightof forms; and possible use of side ports. In narrow,congested walls, external form-tie rods should be con-sidered. Reduced spacing of wales leads to an increasednumber of form ties, resulting in added congestion. In-creased spacing of load-bearing members with highercapacity ties and form sheathing can ease congestion.

3.3.1 More concentrated vibration may be needed incongested areas. Since this may result in increased hy-drostatic head during placement, this should be takeninto account in the formwork design.

3.4-Mix proportioningThe advantages of a large maximum size aggregate

concrete can quickly be lost if the mix proportioningdoes not take into account the congestion existing in theproposed placement.

The use of modified mix proportions with smallermaximum size aggregate is becoming a necessary tool toachieve proper consolidation in certain congested areasof a placement. The modified mixture may also includeadmixtures, increased cement content, and fly ash.

The modified mixture need only replace the originalmix proportions in the zones of extreme congestion,e.g., around multiple embedments, boxouts, or densereinforcement configurations.

3.5-Concrete placing methodsThe constructor must assess whether traditional con-

crete placing methods will be adequate in congested ar-eas. The conditions of the placement must be consid-

ered in selecting the best method for getting the con-crete to its final consolidated state (see ACI 304R).

3.6-Construction considerationsDesign considerations should include construction

methods and should not be solely limited to the re-quirements in the design code and specifications. Thedesign of heavily congested areas can have serious im-pact on quality, construction costs, and constructabil-ity.

Best results are achieved when the architect/engineerworks closely with the constructor to insure that the in-tent of the design can be met under field conditions.

3.7-Tunnel liningsThe concrete lining of tunnels is a difficult operation

due to the logistics of concrete transportation and lim-ited access for concrete placement and consolidation.Congestion can be caused by temporary support mem-bers, reinforcing steel requirements, and grouting pipes.Heavily reinforced concrete tunnel linings have becomemore common in the 1980s.

Best results are obtained with a plastic concrete mix-ture that has been proportioned to flow readily alongform sidewalls, yet remain cohesive. Ample openings ofsufficient size must be provided in the formwork foraccess by workers to consolidate concrete with immer-sion-type vibrators and for inspection as the work pro-gresses. Larger reinforcing bars at increased spacing ispreferred to smaller, more closely spaced bars to pro-vide maximum access. Where heavily reinforced sec-tions are essential, the concrete lining thickness shouldbe increased to allow room behind the form for work-ers. The cost of the additional concrete volume due toincreased thickness often can be offset by a higherquality lining. In general, 14 to 16 in. (356 to 406 mm)clear distance is required between the reinforcementand ground excavation lines.

Allowance must be made for temporary steel sup-ports that may interfere with access. The placement ofconcrete in heavily reinforced sections can also be im-proved by bundling reinforcing bars into groups of twoor three bars to increase spacing. When encasing per-manent steel plate liners in underground work, it is es-sential to provide adequate concrete thickness for ac-cess by workers during concreting.

CHAPTER 4-CONSEQUENCES OFCONGESTED AREAS IN CONCRETE

CONSTRUCTION4.1 -Honeycombed concrete

Honeycombed concrete can occur in congested areasdue to the inability of vibrators to consolidate the con-crete around and through the congestion and out to theform face. There are several primary reasons for hon-eycombed concrete.l The nominal maximum size aggregate may be too

large to pass through the clearances provided, result-

GUIDE TO CONSOLIDATION OF CONCRETE 309.3%5

ing in bridging of aggregate particles and blockage offlow. A harsh mix may also cause bridging and thusblock flow.The densely placed reinforcing steel or embeddedparts may prevent access for the vibrator to completeconsolidation in these congested areas.Extension of vertical reinforcement above the form-work in heavily congested forms can restrict thelateral movement of workers. This restriction ofmovement can lead to operator fatigue and result inincomplete consolidation of the concrete.

4.2-Reduced densityProper density of in-place concrete is dependent

upon adequate consolidation. Incomplete consolida-tion will lead to excessive amounts of entrapped air.This entrapped air causes reduced strength and in-creased permeability and can also decrease bond ofconcrete to the reinforcement.

Large or numerous embedded parts can result in un-der-consolidation on the undersides of these parts, cre-ating air pockets. Unless corrective action is taken, ad-equate consolidation may not be achieved.

4.3-Increased cleaning costsCongestion within forms can lead to significant ad-

ditional costs for clearing the formed space of debris.Construction materials left behind during form build-ing; reinforcing steel installation; and setting of em-bedded parts, boxouts, cableways, and pipes create se-rious cleaning problems.

Debris in the bottom of the placement area cannot beblown across from one end to the other due to block-age by the reinforcement; therefore, cleaning costs areincreased due to the need to clear the form in severalisolated cells. The time required for hand removal ofdebris is substantially increased because workers mustcontinuously climb in and out to cover the total area ofthe placement.

4.4-Increased formwork costsCongested placements can lead to additional form-

work costs when form design changes are required tominimize consolidation problems.

It may be necessary to increase the form-tie spacingto reduce the number of form ties passing through aform. Stronger form faces, walers, and strongbacks arerequired to accomplish this.

In short narrow walls, bulkhead ties may need to beplaced outside the form to prevent the longitudinalform ties from interfering with concrete consolidation.

It also may be necessary to install side ports, asshown in Fig. 5, for observation and consolidationpurposes. If lie-flat hose is used for placement, it canbe conveniently cut off and removed through the sideports (see Fig.6). As the concrete reaches the level ofthe side ports, the ports are closed and secured by bolt-ing or nailing them to the main form walers. Congestedareas within forms may require that embedded parts besupported from a framework spanning the top of theformwork. This reduces the need to install stiffenersand positioning supports in an already congested form.

Fig. 5-Side ports in wall form

Fig. 6-Lie-flat hose viewed through side port

4.5-Increased placing costsCongested regions of reinforcing steel, primarily due

to increased seismic requirements, have resulted insteadily increasing concrete placing costs.

Placing methods are being modified due to increasedform congestion and the reduction of clearances avail-able to get concrete to its final location.

The use of cranes and buckets in conjunction withhoppers and trunks is often not possible due to thespace restrictions in forms. Placing methods for con-crete are now frequently planned as independent oper-ations to avoid using crane time for such activities asplacement of forms, reinforcement, and embeddedparts.

Concrete pumps are available with boom lengths ex-ceeding 200 ft (60 m). Concrete also can be pumpedthrough stationary pipelines hundreds of feet long andthen placed with a placing boom at the end of the line.

The constructor can attach steel reinforced rubber

309.3R-6 ACI COMMITTEE REPORT

hose up to 5 in. (125 mm) in diameter and 30 ft (9 m)long to the end of the pump boom to get concrete tothe point of deposition. Fragile lie-flat hose is often re-quired at the end of the rubber hose to get past extremecongestion (see Fig. 7).

Fig. 7-Lie-flat hose coupled to concrete line

Where it is not possible for the vibrator operator toinsert the vibrator all the way to the bottom of wallforms, the constructor should install side ports in theform to allow lowering the vibrators through theseports.

CHAPTER 5-RECOMMENDED PRACTICES5.1 -Design considerations

5.1.1 Reinforcing steel arrangement-Arrangementof reinforcing steel should provide enough space to al-low concrete placement into the form. The architect/engineer may have to increase the member size overthat required. by the design calculations so that suffi-

cient room is provided for placement.In extreme cases, it may be necessary for the ar-

rangement to include accessways through the reinforc-ing steel.

5.1.1.1 Reinforcement splicing methods-Until thelate 197Os, most reinforcing steel arrangements pro-vided for lapping reinforcing steel bars without causingundue congestion problems. More stringent seismic re-quirements have resulted in a dramatic increase in theamount of reinforcing steel per unit area, especially atbeam and column connections.

Lapping the bars would cause such severe congestionthat space between bars would almost disappear, re-quiring a change to splicing.

Sometimes this congestion problem associated withsplicing can be solved by mechanically connecting thereinforcing bars, as described by ACI 439.3R. In spe-cial cases, the reinforcing bars may be spliced bywelded connections, provided that proper welding pro-cedures are used considering the metallurgy of the re-inforcing steels being joined. However, with either amechanical or welded connection, there will be somelocalized increase in the reinforcement diameter, whichshould be considered in detailing clearances and barspacing.

5.1.2 Embedded parts/boxouts-Embedment, sleeve,and boxout configuration should consider reinforcingdetails, concrete mix proportions, and especially thenominal maximum size of aggregate. If possible, em-bedments should be spread out.

Void forms should be used to eliminate form pene-trations, but if they are large [more than 2 ft (0.6 m) ineither direction], a placement and vibration tube (seeFig. 8) should be provided.

Fig. 8-Placement and vibration tubes: Large blockoutwithin a wall with pipes through the formed blockoutto permit access for concrete placement and vibration

Boxouts that remain in place should have tolerancesto allow them to be shifted and placement and vibra-tion tubes should be provided. Boxouts that are to beremoved and exceed 2 ft (0.6 m) in either direction alsoshould provide placement and vibration tubes.

In situations where the boxout spans from one formface to the other, access should be provided through thebottom of the boxout. As the concrete reaches the bot-tom of the boxout, the access can be closed off with apreformed insert, which is then bolted to the boxoutform.

5.1.3 Placing-The constructor must assess whetherhis traditional placing methods will be adequate for thejob. Bidding merely on the total volume, averageplacement size, and known project access conditionscan result in reduced profit margins. The constructormust review reinforcing steel, embedment, and form-work drawings to tailor the placing methods to suit theconditions. The constructor may need to requestchanges in the design of the placement or formwork toobtain a quality product at a reasonable cost.

Increased cooperation between the architect/engi-neer and constructor prior to beginning work will facil-itate quality construction. Prebid and preconcretingmeetings to discuss all phases of the concrete work areencouraged.

5.2-Construction considerations5.2.1 Use of admixtures-Proper placement of con-

crete in congested areas usually requires the concrete tohave flowing characteristics. Flowing concrete is gen-erally considered to have a slump of 7% in. (190 mm)or more, while remaining cohesive without excessivebleeding or segregation (ACI 309R). The use of suchmaterial permits placement and consolidation in areaswhere less workable concrete mixtures cannot be prop-erly placed and consolidated due to lack of mobilityand vibrator access.

Flowing concrete is commonly used in congested ar-eas where the member itself is unusual in shape or sizeor a large amount of reinforcement is present.

Since producing flowing concrete only by adding ex-tra water results in lower quality concrete, such con-crete should be obtained through the use of chemicaladmixtures. Admixtures used to achieve flowing con-crete should meet the requirements of ASTM C 494 andASTM C 1017. Commonly used materials for produc-ing flowing concrete include:

1. High-range water-reducing admixtures (superplas-ticizers), ASTM C 494, Types F or G.

2. A combination of high-range water-reducing ad-mixtures plus a water-reducing and retarding admix-ture, ASTM C 494, Type D, or water-reducing and ac-celerating admixture, ASTM C 494, Type E.

3. High dosages of a water-reducing normal-settingadmixture, ASTM C 494, Type A, plus a water-reduc-

ing and accelerating admixture, ASTM C 494, Type E. Where flowing concrete is required, trial mixturesshould be tested with materials representative of thoseto be used in the project and under the environmentalconditions expected on the project. Trial mixturesshould be made using the initial slump resulting fromthe maximum allowable specified water-cement ratio.Chemical admixture dosages can be varied to achievethe desired slump range. If necessary, the initial slumpscan be reduced by lowering the water-cement ratio andthus improving the hardened properties of the con-crete. Excessive retardation and loss of air contentshould be avoided.

5.2.2 Use of modified mixtures (ACI 211.1 and211.2)-Normally, architects/engineers will specify thelargest nominal maximum size aggregate mixtures thatare readily available and can be consolidated by con-ventional placing methods. However, the need to meetstringent seismic requirements has led architects/engi-neers to make provisions in the specifications to usesmaller maximum size aggregate for some placementsor portions of placements. The architect/engineershould consider this substitution based on the degree ofcongestion of reinforcing steel or embedded parts.

As an example, when the concrete is specified with anominal maximum size aggregate of 1 l/2 in. (40 mm),the architect/engineer may allow for substitution of aportion of the concrete (in practice about 20 to 30 per-cent) with %-in. (20-mm) nominal maximum size ag-gregate (Bonikowsky).

Where the design mixture specifies nominal maxi-

mum size aggregate of 3/4 in. (20 mm) for extremelycongested areas, the architect/engineer ‘may allow sub-stitution of a portion of the placement with nominalmaximum sized aggregate of 1/2 in. (13 mm). When themaximum aggregate size of a specified mix is reduced,the mix has to be modified by adjusting the water andcement content to maintain the water-cement ratio anddesign strength. Some specifications also allow the ad-dition of fly ash to enhance workability. Typically, anaddition of fly ash equal to 5 percent of the cementweight will provide excellent lubrication. At times, upto 30 percent is allowed.

5.2.3 Formwork-Formwork design should be basedon full hydrostatic head conditions wherever practical.Form-tie locations need to be considered when choos-ing a form system and are often fixed in liquid headforms. Full hydrostatic head forms often have large ties[l-in. (25-mm) diameter or greater] and require place-ment, pockets and cleanouts. Bulkhead design shouldalso consider full hydrostatic head. If longitudinal tiesor special corner ties are required, external ties shouldbe considered. Formwork accessories such as rustica-tion, chamfers, and keyways should be considered inreinforcement details, mix proportions, and placement,as well as consolidation.

In general, formwork design should follow the prac-tices and guidelines presented in ACI 303R and ACI347R. Careful consideration should be given to areas ofcongested reinforcement or other embedments. In ar-eas of heavy congestion, concentrated vibration is likelyto occur that can increase the hydrostatic pressure onthe forms. When ‘needed, the spacing of load-bearingmembers should be increased and combined with higher capacity ties and sheathing. The use of external tie rodsin narrow congested walls also can help reduce the con-gestion. When vertical access to the forms from the topis limited and internal chutes cannot be used, side portsshould be incorporated to allow for the placement ofthe concrete and consolidation by internal vibration.

Battered form faces or counterforts generally resultin areas of poor consolidation due to the problems ofplacement, vibrator access, and restricted air migrationduring vibration. The additional concrete required fora vertical rather than sloped face may be highly cost-effective if required repair of the formed surface is sig-nificantly reduced.

Corbels and haunches generally are areas of conges-tion. Similarity of shape and position can reduce form-work costs.

5.2.4 Consolidation methods--Congestion is forcingarchitects/engineers to take into consideration the con-struction aspects of placing and consolidating qualityconcrete. Some reinforcing steel arrangements are in-corporating openings to provide access for cleaning,placing, and consolidation. Fig. 5 shows designed-inaccesses in a heavily reinforced wall section.

All aspects of the consolidation operation in con-gested forms should be well planned prior to start ofthe concrete placement. Smaller size vibrators may beused in the lower areas within the forms when a high-

309.3R-8 ACI COMMITTEE REPORT

range water-reducing admixture is used with a modi-fied concrete mixture as described in Section 5.2.2.When it is reasonable to return to the normal concretemixture using larger maximum size aggregate, biggerand more effective vibrators [typically up to 3 in. (75mm) in diameter] should be used.

When access into the form by the placing crew islimited due to reinforcing steel, additional vibrators

should be lowered down through the upper reinforcing mat from the top of the placement. This practice willreduce the tendency of operators to try to throw the vi-brators horizontally past interferences and will encour-age them to operate vibrators in a nearly vertical posi-tion.

If the constructor is using pneumatic vibrators, heshould insure a good supply of compressed air withheaders located near the form. Oilers should bemounted on each line coming off the air header. Heshould also provide sufficient spare vibrators in theevent of a vibrator breakdown. Adequate power shouldalso be provided for electric vibrators.

In congested narrow wall forms, it may be necessaryto place side ports in one of the wall forms. The sideports are typically 2 ft (0.6 m) square with a spacing of6 ft (1.8 m). The side ports are used to lower the vibra-tors into the form and to observe the concrete placingwithin the form. This is necessary to insure that bridg-ing of the concrete during placement has not occurred.It may also be possible to lower small-diameter vibra-tors between the outer layer of reinforcement and theform face, except in the case of architectural faces,where external form vibrators should be used. Externalform vibrators are discussed in ACI 309R, Chapter 5,and formwork considerations are discussed in SP-4,Chapter 5.

Due to the increased time required for congestedplacements, it may be necessary to use high-range wa-ter-reducing and retarding admixtures or a high-range

water-reducing admixture with extended slump reten-tion. Great care must be taken by the operator not tolodge or snag the vibrator within the placement be-cause it can become virtually impossible to extract.

The constructor and inspector must be aware that itis a far lesser evil to overvibrate than to undervibratedue to the risk of honeycombed concrete, air pockets,and lack of density in congested areas.

5.2.5 Placing methods-Congested forms and diffi-cult placing conditions have resulted in drastic changesin placing methods. Concrete pumping or conveyors areused more frequently than crane and bucket under suchconditions. The prime means of insuring good consoli-dation continues to be the ability to place concrete asclose to its final position as possible.

The majority of concrete placed in congested formsis placed by pump booms or placing booms using 4- or5-in. (100- or 125-mm) diameter steel reinforced hose.To insure good pumpability, the architect/engineer isusually restricted to a maximum of l 1/2-in. (40-mm)nominal maximum size aggregate.

The majority of concrete in congested forms can beplaced by lowering the concrete hose through the rein-forcement to within 6 ft (1.8 m) of the surface, dis-charging the lift thickness, then raising and reinsertingthe hose at typically 10-ft (3-m) centers.

In narrow wall forms where it is not possible to lowerthe concrete hose through the reinforcement, lie-flathose coupled to the steel reinforced hose has been usedsuccessfully. The lie-flat hose is very pliable and cantransfer concrete vertically through very narrow spaces.The hose is relatively inexpensive, making it economi-cal to cut off for removal from the placement if it be-comes caught on reinforcement or embedments (seeFig. 7).

Where wall placements extend up to the underside ofstructural steel members or concrete beams, concreteshould be placed under pressure through slide valves, asdepicted in Fig. 9. When the form is full, the slide valveis closed and the line disconnected. After the concretehas set, the slide valve and supporting form are re-moved. The remaining concrete stub is removed bychipping and the wall is ground smooth.

Fig. 9-Slide valves for pressure pumping of narrowcongested walls to the underside of horizontal struc-tural steel beams

Pumping concrete from the bottom of the form canoffer a solution to congestion in some instances. Flow-ing concrete is recommended for use with this method.The shape of the element has a great deal to do withwhether or not the technique is viable. Rectangles,squares, and other polygons require special design offormwork because pressure concentrates at the cornersof angles or point loading is developed. Circular unres-tricted structures lend themselves best to pumping fromthe bottom. Unrestricted means that the concrete mustbe unimpeded all the way around the inside of the col-umn and there are no baffles that restrict upwardmovement.

If there is a vertical steel “H” section within the col-umn, the concrete will not pump if the concrete entersat a point perpendicular to one of the flanges of the“H.” If concrete is discharged directly into the web of

GUIDE TO CONSOLIDATION OF CONCRETE 309.3R-9

the “H,” the concrete will pump. This effect is dimin-ished when at least 12 in. (300 mm) of concrete coverover the “H” is present. Successful pumping has beenachieved with less than 4-in. (lOO-mm) cover if pumpedinto the web.

When pumping from the bottom, there should be re-strictions on the number and size of embedments orboxouts and their position in the form. Also, if thereare large numbers of dowels, the flow of concrete maybe restricted, causing pump and/or form failures. Atleast a 4-in. (lOO-mm) clearance should be provided be-tween the embedment and the reinforcement or 4 in.(100 mm) free at the top of the placement below thestructural steel or turned out reinforcement.

Preplaced aggregate (PA) is another placing methodthat has been used effectively in congested areas. Toproduce concrete by the PA method, coarse aggregateis first placed in the prepared form. Then the voids inthe preplaced aggregate are filled with a fluid groutconsisting of cement, sand, water, and sometimes anadmixture, which is pumped into the forms from thebottom through form inserts or pipes. Materials re-quirements, procedures, and properties are described inACI 304.1R.

The PA method has been used to advantage forplacing concrete around congested reinforcement.Where the reinforcing steel and forms are already inplace, grout pipes are inserted from the top or sides tothe bottom of the space to be filled. Coarse aggregateis then dropped into place or shoved in from the sides,and assisted by rodding and/or blowing with the helpof air lances.

After the form has been completely filled with aggre-gate, the grout is pumped into the forms. Alterna-tively, the coarse aggregate may be placed in lifts as thereinforcement and forms are erected. Fig. 10 shows aportion of a boxout left in the side of a nuclear con-tainment structure 50 ft long by 35 ft wide by 6 ft thick(15.2 x 10.6 x 1.8 m). The reinforcement placed duringinitial construction was too congested to permit the useof vibrators, especially because the rear wall was a steelmembrane that could not be cut to receive ports. Theboxout was filled with PA concrete in 7-ft (2.1-m) lifts.

Fig. 10-Preplaced aggregate method: Close-up ofcongested reinforcement in blockout in side of a con-tainment structure. Grout inserts [l-in. (25mm) diam-eter pipes] are shown in right center (one uncapped)and near bottom (two, temporarily capped). Top offirst lift of coarse aggregate [approximately 7 ft (2.1 m)deep] is visible at bottom of photo. Grout will bepumped to a few inches below surface of the coarse ag-gregate to provide a keyed joint with the succeeding lift

The preplaced aggregate method provides three plac-ing advantages:

1. There is no time limit on placing the coarse aggre-gate.

2. Areas that do not contain aggregates due to bridg-ing are not critical because all spaces are filled withgrout having approximately the same strength as thesurrounding concrete. The PA method can signifi-cantly reduce the chances of honeycomb.

3. Continuous pumping of the grout eliminates coldjoints. However, if pumping is interrupted for any rea-son, the effect of the cold joint that forms is negligiblebecause coarse aggregate particles extending throughthe grout surface provide structural continuity acrossthe interface between the two grout placements with ahigh probability that the negative effects of the coldjoint can be minimized or eliminated.

Disadvantages of the PA method include the diffi-culty of isolating congested sections to be placed mon-olithically from less heavily reinforced concrete. PAconcrete may be somewhat time-consuming and labor-intensive.

5.3-SummarySuccessful concreting under difficult conditions or in

highly congested sections requires an effective combi-nation of design, placement, and consolidation tech-niques. While this document has presented a number ofoptions that can be considered by architects/engineersand constructors, it must be recognized that each situ-ation may be unique. The architect/engineer and con-structor, in consultation with each other, must assesseach situation and agree on the most appropriate ap-proach for the situation in question. However, it is ofutmost importance that situations requiring special at-tention be identified with sufficient lead time to allowproper planning.

CHAPTER 6-REFERENCES6.1 -Specified and/or recommended references

The documents of the various standards-producingorganizations referred to in this document are listedwith their serial designations.

These publicationsing organizations:

may be obtained from the follow-

309.3R-10 ACI COMMITTEE REPORT

American Concrete Institute P.O. Box 19150Detroit, MI 48219

ASTM1916 Race StreetPhiladelphia, PA 19103

American Concrete Institute117

211.1

211.2

212.3R303R

Standard Specifications for Tolerances forConcrete Construction and MaterialsStandard Practice for Selecting Proportions forNormal, Heavyweight and Mass ConcreteStandard Practice for Selecting Proportions forStructural Lightweight ConcreteChemical Admixtures for ConcreteGuide to Cast-in-Place Architectural ConcretePractice

304R

304.1R

309R Guide for Consolidation of Concrete347R Guide to Formwork for Concrete439.3RR Mechanical Connections of Reinforcing BarsSP-4 Formwork for Concrete

ASTMc 494

c 1017

6.2-Cited referenceBonikowsky, Dan, “Consolidation of Concrete in Congested Ar-

eas at Darlington NGS,"” Consolidation of Concrete, SP-96, Ameri-can Concrete Institute, Detroit, 1987, pp. 10-18.

Guide for Measuring, Mixing, Transportingand Placing ConcreteGuide for the Use of Preplaced AggregateConcrete for Structural and Mass ConcreteApplications

Standard Specification for Chemical Admix-tures for ConcreteChemical Admixtures for Use in ProducingFlowing Concrete

ACI 309.3R-92 was submitted to letter ballot of thein accordance with ACI standardization procedures.

committee and processed