guide to consolidation of concrete in congested areas

3.4-Mix proportioning 3.5-Concrete placing methods 3.6-Construction considerations 3.7-Tunnel linings Keywords: admixtures; concrete construction; consolidation; embedment; formwork cons

ACI 309.3R-92

.

Guide to Consolidation of Concrete in Congested Areas (Reapproved 1997)

Dan A Bonikowsky*

Neil A Cumming

Timothy P Dolen

Jerome H Ford

Joseph J Fratianni

Mikael P J Olsen, Chairman Steven H G e b Gary R Mass Richard E Miller Jr.

Roger A Minnich

H Celik Ozyildirim

*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 requirements

result in highly congested forms that impede consolidation of

con-crete Techniques to overcome these difficulties are presented The

guide also identifies for constructors various difficult placing and

consolidation 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 in

their bids.

3.4-Mix proportioning 3.5-Concrete placing methods 3.6-Construction considerations 3.7-Tunnel linings

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.

Chapter 4-Consequences of congested areas in concrete construction, pg 309.3R-4

4 l-Honeycombed concrete 4.2-Reduced density 4.3-Increased cleaning costs 4.4 Increased formwork costs 4.5-Increased placing costs

CONTENTS Chapter l-Introduction, pg 309.3R-1

Chapter 2-Criteria for designation as a congested area,

pg 3093R-2

Chapter 5-Recommended practices, pg 309.3R-6

5.1 -Design considerations 5.2-Construction considerations 5.3-Summary

2.l-Reinforcing steel

2.2-Embedments and boxouts

2.3-Formwork

2.4-Definitions

Chapter 6-References, pg 309.3R-9

6.1 -Specified and/or recommended references 6.2-Cited reference

Chapter 3-Factors contributing to congestion problems,

pg 309.3R-3

CHAPTER 1 INTRODUCTION

Many concrete structures such as those with seismic provisions, post-tensioning, and high-strength concrete are difficult to consolidate because of congested areas within the formwork This congestion can result in

3.l-Reinforcing steel arrangement

3.2-Embedded parts/boxouts

3.3-Formwork

ACI Committee Reports, Guides, Standard Practices, and

Commentaries are intended for guidance in designing,

plan-ning, executing, or inspecting construction and in preparing

specifications Reference to these documents shall not be made

in the Project Documents If items found in these documents

are desired to be part of the Project Documents, they should

be phrased in mandatory language and incorporated into the

Project Documents.

Sandor Popovics Thomas J Reading Donald L Schlegel Bradley K Violetta

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 any means, including the making of copies by any photo process, or by any elec-tronic or mechanical device, printed, written, or oral, or recording for sound or use in any knowledge retrieval system or device, unless permission is obtained in writing from the copyright proprietors.

309.3R-1

ACI COMMlTTEE REPORT

Fig 1 -Dense reinforcing steel

Fig 2-Dense reinforcing steel

structural inadequacy and time-consuming and

expen-sive remedial work

Various techniques have been employed to alleviate

this type of problem This document presents an

over-view of the factors contributing to the problem, the

consequences of inappropriate concrete procedures in

these areas, and recommended practices to minimize

the 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 of

how their designs will be constructed, and that

con-structors become more aware of special procedures and

necessary precautions Most importantly,

communica-tion between the architect/engineer and constructor is

essential to insure that the design details, construction

materials, and procedures are compatible

Fig 3-Closely spaced embedments

CHAPTER 2 2-CRITERIA FOR DESIGNATION AS

A CONGESTED AREA

Congested areas are those in which the reinforcing steel, 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 of consolidation, access for inspection and consolidation, special concrete mixtures, special formwork, additional consolidation effort, and specific placing methods are frequently used

2.1 -Reinforcing steel

Congestion causes problems when the clear spacing between reinforcing bars or between a bar and the form

is less than 1 l1/3times the maximum size of coarse ag-gregate used in the concrete mixture This condition is more 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

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 the concrete (see Fig 3) Boxouts are used to form open-ings, keyways, or pockets in the concrete When these items restrict the placement and consolidation of the concrete, they cause congestion The spacing between embedments, boxouts, and the form must be at least 1% times the nominal maximum size of the coarse ag-gregate to avoid this problem Frequently, these items cause congestion because the concrete cannot be placed and consolidated easily underneath them (see Fig 4) The architect/engineer must be alert to such condi-tions, and construction procedures must provide for proper placement and consolidation of concrete on the undersides of these embedments

2.2.1 Embedments may be anchors, weld plates,

me-GUIDE TO CONSOLIDATION OF CONCRETE

chanical and electrical boxes, threaded inserts, or other

devices used to attach or incorporate items after

form-work is removed.

2.2.2 Sleeves are openings that generally go all the

way through a wall or slab to allow piping or other

penetrations to pass Formwork for these openings is

often completely removed prior to piping placement.

2.2.3 Boxouts are either removable or stay-in-place.

Removable boxouts are similar to sleeves but generally

are much larger, as in door and window boxouts or

hatch boxouts in slabs Most removable boxout forms

can be adapted readily for placement and vibration

tubes, as described in Section 5.1.2 If these removable

forms are not provided, the constructor must move the

concrete horizontally under the formed boxout, which

usually results in segregation of the concrete, adds

ad-ditional stress to the boxout form, allows the buildup

of frictional resistance along the formwork, and slows

down the entire placement Without placement and

vi-bration tubes, the degree of consolidation is generally

reduced under a boxout Boxouts often are designed

with open bottoms.

Stay-in-place boxout forms, such as hollow metal

door and window frames, often require bracing and do

not allow placement and vibration tubes to be cut

through them This increases the chances of voids or

incomplete consolidation.

2.2.4 Formwork accessories, architectural items such

as cast-in numbers and letters, form liners, rustication,

chamfers, and keyways can cause simple to complex

consolidation problems in one manner or another This

is especially true of horizontal rustication or keyways in

a wall.

2.3- Formwork

The surface texture, shape, type, and orientation of

the formwork may restrict concrete placement

Consid-eration needs to be given to form release agents that are

compatible with form texture, particularly if intricate

shapes are to be cast into the concrete at the formed

surface These release agents may also serve to

some-what reduce the frictional resistance between the plastic

concrete and the form, thereby improving the ease of

removing entrapped air The forms must also be

de-signed for easy removal.

Used or poorly oiled wood forms are more likely to

hinder consolidation than steel or plastic-lined forms.

The frictional resistance of a wood form impedes the

flow of concrete and can create difficulty when used i n

conjunction with congested embedments.

2 4 - D e f i n i t i o n s

2.4.1 External form tie rods-External form tie rods

are installed on the outside of narrow wall forms in the

longitudinal direction The tie rods are attached to the

bulkhead walers at the ends of the wall.

2.4.2 Lie-flat hose-Lie-flat hose is a very pliable

polyvinyl chloride reinforced discharge hose, typically

Fig.4 Stacked boxouts

purchased in 4- or 5-m (100- or 125-mm) diameter by 300-ft (92-m) long rolls.

2.4.3 Side ports-Side ports are temporary openings

in the form on one side of narrow walls The purpose

of the side ports is to allow insertion and extraction of vibrators and to observe consolidation of the concrete.

2.4.4 Slide valve-A slide valve is a short piece of steel pipe with a slide plate mounted in it The pipe is the same diameter as the concrete discharge hose and the other end is bolted to the form The purpose is to allow pumping of concrete through the open slide valve

to completely fill a form to the underside of a horizon-tal structural steel beam When the form is full, the slide plate is closed, preventing the concrete from seep-ing back through the valve.

2.4.5 Steel reinforced hose-Steel reinforced hose is

a rubber concrete-discharge hose reinforced with strands of steel wire between the tube and outer cover.

CHAPTER 3-FACTORS CONTRIBUTING TO

CONGESTION PROBLEMS 3.1 -Reinforcing steel arrangement

The reinforcing steel arrangement must take into ac-count the factors that contribute to congestion Seismic and strength design requirements often result in a rein-forcing steel layout that inhibits access for preplace-ment cleanup and concrete placepreplace-ment and consolida-tion Recommended practices are described in Chapter 5.

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 the traditional method of lap splices The various methods

309.3R-4 ACI COMMITTEE REPORT

of splicing reinforcing bars, as discussed in Section

5.1.1.1, need to be considered by the

architect/engi-neer

3.2-Embedded parts/boxouts

There is increasing use of embedments and boxouts

to incorporate piping and electrical and mechanical

systems into placements The use of embedments and

boxouts, 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 around

embedments and boxouts should be considered at the

design stage Frequently, mechanical and electrical

em-bedments are located adjacent to doors and windows

These areas usually require additional reinforcing due

to stress concentrations around the boxout The core

area in buildings is another example where additional

reinforcement, embedments, and boxouts cause

con-gestion

3.3- Formwork

The design of formwork can contribute significantly

to congestion in placements if the design does not take

into account other factors, e.g., location of

embed-ments and boxouts, reinforcing steel arrangeembed-ments,

placing equipment, and form-tie spacing

The design should consider the number, location,

and size of form-tie rods; location of embedments and

blockouts; location of trunks or concrete hose; height

of 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 increased

number of form ties, resulting in added congestion

In-creased spacing of load-bearing members with higher

capacity ties and form sheathing can ease congestion

3.3.1 More concentrated vibration may be needed in

congested areas Since this may result in increased

hy-drostatic head during placement, this should be taken

into account in the formwork design

3.4-Mix proportioning

The advantages of a large maximum size aggregate

concrete can quickly be lost if the mix proportioning

does not take into account the congestion existing in the

proposed placement

The use of modified mix proportions with smaller

maximum size aggregate is becoming a necessary tool to

achieve proper consolidation in certain congested areas

of a placement The modified mixture may also include

admixtures, increased cement content, and fly ash

The modified mixture need only replace the original

mix proportions in the zones of extreme congestion,

e.g., around multiple embedments, boxouts, or dense

reinforcement configurations

3.5-Concrete placing methods

The 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 considerations

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

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

3.7-Tunnel linings

The 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 become more common in the 1980s

Best results are obtained with a plastic concrete mix-ture that has been proportioned to flow readily along form sidewalls, yet remain cohesive Ample openings of sufficient size must be provided in the formwork for access by workers to consolidate concrete with immer-sion-type vibrators and for inspection as the work pro-gresses Larger reinforcing bars at increased spacing is preferred to smaller, more closely spaced bars to pro-vide maximum access Where heavily reinforced sec-tions are essential, the concrete lining thickness should

be increased to allow room behind the form for work-ers The cost of the additional concrete volume due to increased thickness often can be offset by a higher quality lining In general, 14 to 16 in (356 to 406 mm) clear distance is required between the reinforcement and ground excavation lines

Allowance must be made for temporary steel sup-ports that may interfere with access The placement of concrete in heavily reinforced sections can also be im-proved by bundling reinforcing bars into groups of two

or 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 OF CONGESTED AREAS IN CONCRETE

CONSTRUCTION 4.1 -Honeycombed concrete

Honeycombed concrete can occur in congested areas due to the inability of vibrators to consolidate the con-crete around and through the congestion and out to the form 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

Fig 5-Side ports in wall form

ing in bridging of aggregate particles and blockage of

flow A harsh mix may also cause bridging and thus

block flow

The densely placed reinforcing steel or embedded

parts may prevent access for the vibrator to complete

consolidation in these congested areas

Extension of vertical reinforcement above the

form-work in heavily congested forms can restrict the

lateral movement of workers This restriction of

movement can lead to operator fatigue and result in

incomplete consolidation of the concrete

4.2-Reduced density

Proper 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 of

concrete 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 costs

Congestion 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 be

blown across from one end to the other due to

block-age by the reinforcement; therefore, cleaning costs are

increased due to the need to clear the form in several

isolated cells The time required for hand removal of

debris is substantially increased because workers must

continuously climb in and out to cover the total area of

the placement

Fig 6-Lie-flat hose viewed through side port

4.4-Increased formwork costs

Congested placements can lead to additional form-work costs when form design changes are required to minimize consolidation problems

It may be necessary to increase the form-tie spacing

to reduce the number of form ties passing through a form Stronger form faces, walers, and strongbacks are required to accomplish this

In short narrow walls, bulkhead ties may need to be placed outside the form to prevent the longitudinal form ties from interfering with concrete consolidation

It also may be necessary to install side ports, as shown in Fig 5, for observation and consolidation purposes If lie-flat hose is used for placement, it can

be conveniently cut off and removed through the side ports (see Fig.6) As the concrete reaches the level of the side ports, the ports are closed and secured by bolt-ing or nailbolt-ing them to the main form walers Congested areas within forms may require that embedded parts be supported from a framework spanning the top of the formwork This reduces the need to install stiffeners and positioning supports in an already congested form

4.5-Increased placing costs

Congested regions of reinforcing steel, primarily due

to increased seismic requirements, have resulted in steadily increasing concrete placing costs

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

The use of cranes and buckets in conjunction with hoppers and trunks is often not possible due to the space restrictions in forms Placing methods for con-crete are now frequently planned as independent oper-ations to avoid using crane time for such activities as placement of forms, reinforcement, and embedded parts

Concrete pumps are available with boom lengths ex-ceeding 200 ft (60 m) Concrete also can be pumped through stationary pipelines hundreds of feet long and then placed with a placing boom at the end of the line The constructor can attach steel reinforced rubber

309.3R-6 ACI COMMITTEE REPORT

Fig 7-Lie-flat hose coupled to concrete line

Fig 8-Placement and vibration tubes: Large blockout

within a wall with pipes through the formed blockout

to permit access for concrete placement and vibration

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 to

the point of deposition Fragile lie-flat hose is often

re-quired at the end of the rubber hose to get past extreme

congestion (see Fig 7)

Where it is not possible for the vibrator operator to

insert the vibrator all the way to the bottom of wall

forms, the constructor should install side ports in the

form to allow lowering the vibrators through these

ports

CHAPTER 5-RECOMMENDED PRACTICES

5.1 -Design considerations

5.1.1 Reinforcing steel arrangement-Arrangement

of reinforcing steel should provide enough space to

al-low concrete placement into the form The architect/

engineer may have to increase the member size over

that 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 the

late 197Os, most reinforcing steel arrangements pro-vided for lapping reinforcing steel bars without causing undue congestion problems More stringent seismic re-quirements have resulted in a dramatic increase in the amount of reinforcing steel per unit area, especially at beam and column connections

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

Sometimes this congestion problem associated with splicing can be solved by mechanically connecting the reinforcing bars, as described by ACI 439.3R In spe-cial cases, the reinforcing bars may be spliced by welded connections, provided that proper welding pro-cedures are used considering the metallurgy of the re-inforcing steels being joined However, with either a mechanical or welded connection, there will be some localized increase in the reinforcement diameter, which should be considered in detailing clearances and bar spacing

5.1.2 Embedded parts/boxouts-Embedment, sleeve,

and boxout configuration should consider reinforcing details, concrete mix proportions, and especially the nominal 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) in either direction], a placement and vibration tube (see Fig 8) should be provided

Boxouts that remain in place should have tolerances

to allow them to be shifted and placement and vibra-tion tubes should be provided Boxouts that are to be removed and exceed 2 ft (0.6 m) in either direction also should provide placement and vibration tubes

In situations where the boxout spans from one form face to the other, access should be provided through the bottom of the boxout As the concrete reaches the bot-tom of the boxout, the access can be closed off with a preformed insert, which is then bolted to the boxout form

5.1.3 Placing-The constructor must assess whether

his traditional placing methods will be adequate for the job Bidding merely on the total volume, average placement size, and known project access conditions can result in reduced profit margins The constructor must review reinforcing steel, embedment, and form-work drawings to tailor the placing methods to suit the conditions The constructor may need to request changes in the design of the placement or formwork to obtain 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 preconcreting meetings to discuss all phases of the concrete work are encouraged

5.2-Construction considerations

5.2.1 Use of admixtures-Proper placement of

con-crete in congested areas usually requires the concon-crete to

have flowing characteristics Flowing concrete is

gen-erally considered to have a slump of 7% in (190 mm)

or more, while remaining cohesive without excessive

bleeding or segregation (ACI 309R) The use of such

material permits placement and consolidation in areas

where less workable concrete mixtures cannot be

prop-erly placed and consolidated due to lack of mobility

and vibrator access

Flowing concrete is commonly used in congested

ar-eas where the member itself is unusual in shape or size

or 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 chemical

admixtures Admixtures used to achieve flowing

con-crete should meet the requirements of ASTM C 494 and

ASTM C 1017 Commonly used materials for

produc-ing flowproduc-ing 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-setting

admixture, ASTM C 494, Type A, plus a

water-reduc-ing and acceleratwater-reduc-ing admixture, ASTM C 494, Type E

Where flowing concrete is required, trial mixtures

should be tested with materials representative of those

to be used in the project and under the environmental

conditions expected on the project Trial mixtures

should be made using the initial slump resulting from

the maximum allowable specified water-cement ratio

Chemical admixture dosages can be varied to achieve

the desired slump range If necessary, the initial slumps

can be reduced by lowering the water-cement ratio and

thus improving the hardened properties of the

con-crete Excessive retardation and loss of air content

should be avoided

5.2.2 Use of modified mixtures (ACI 211.1 and

211.2)-Normally, architects/engineers will specify the

largest nominal maximum size aggregate mixtures that

are readily available and can be consolidated by

con-ventional placing methods However, the need to meet

stringent seismic requirements has led

architects/engi-neers to make provisions in the specifications to use

smaller maximum size aggregate for some placements

or portions of placements The architect/engineer

should consider this substitution based on the degree of

congestion of reinforcing steel or embedded parts

As an example, when the concrete is specified with a

nominal maximum size aggregate of 1 l/2 in (40 mm),

the architect/engineer may allow for substitution of a

portion 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 extremely congested areas, the architect/engineer ‘may allow sub-stitution of a portion of the placement with nominal maximum sized aggregate of 1/2 in (13 mm) When the maximum aggregate size of a specified mix is reduced, the mix has to be modified by adjusting the water and cement content to maintain the water-cement ratio and design strength Some specifications also allow the ad-dition of fly ash to enhance workability Typically, an addition of fly ash equal to 5 percent of the cement weight will provide excellent lubrication At times, up

to 30 percent is allowed

5.2.3 Formwork-Formwork design should be based

on 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 head forms Full hydrostatic head forms often have large ties [l-in (25-mm) diameter or greater] and require place-ment, pockets and cleanouts Bulkhead design should also consider full hydrostatic head If longitudinal ties

or special corner ties are required, external ties should

be considered Formwork accessories such as rustica-tion, chamfers, and keyways should be considered in reinforcement 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 ACI 347R Careful consideration should be given to areas of congested reinforcement or other embedments In ar-eas of heavy congestion, concentrated vibration is likely

to occur that can increase the hydrostatic pressure on the forms When ‘needed, the spacing of load-bearing members should be increased and combined with higher capacity ties and sheathing The use of external tie rods

in narrow congested walls also can help reduce the con-gestion When vertical access to the forms from the top

is limited and internal chutes cannot be used, side ports should be incorporated to allow for the placement of the concrete and consolidation by internal vibration Battered form faces or counterforts generally result

in areas of poor consolidation due to the problems of placement, vibrator access, and restricted air migration during vibration The additional concrete required for

a 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 forcing

architects/engineers to take into consideration the con-struction aspects of placing and consolidating quality concrete Some reinforcing steel arrangements are in-corporating openings to provide access for cleaning, placing, and consolidation Fig 5 shows designed-in accesses in a heavily reinforced wall section

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

high-309.3R-8 ACI COMMITTEE REPORT

Fig 9-Slide valves for pressure pumping of narrow

congested walls to the underside of horizontal

struc-tural steel beams

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 concrete

mixture using larger maximum size aggregate, bigger

and more effective vibrators [typically up to 3 in (75

mm) in diameter] should be used

When access into the form by the placing crew is

limited due to reinforcing steel, additional vibrators

should be lowered down through the upper reinforcing

mat from the top of the placement This practice will

reduce 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, he

should insure a good supply of compressed air with

headers located near the form Oilers should be

mounted on each line coming off the air header He

should also provide sufficient spare vibrators in the

event of a vibrator breakdown Adequate power should

also be provided for electric vibrators

In congested narrow wall forms, it may be necessary

to place side ports in one of the wall forms The side

ports are typically 2 ft (0.6 m) square with a spacing of

6 ft (1.8 m) The side ports are used to lower the

vibra-tors into the form and to observe the concrete placing

within the form This is necessary to insure that

bridg-ing of the concrete durbridg-ing placement has not occurred

It may also be possible to lower small-diameter

vibra-tors between the outer layer of reinforcement and the

form face, except in the case of architectural faces,

where external form vibrators should be used External

form 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 congested

placements, 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 to lodge or snag the vibrator within the placement be-cause it can become virtually impossible to extract The constructor and inspector must be aware that it

is a far lesser evil to overvibrate than to undervibrate due 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 changes

in placing methods Concrete pumping or conveyors are used more frequently than crane and bucket under such conditions The prime means of insuring good consoli-dation continues to be the ability to place concrete as close to its final position as possible

The majority of concrete placed in congested forms

is placed by pump booms or placing booms using 4- or 5-in (100- or 125-mm) diameter steel reinforced hose

To insure good pumpability, the architect/engineer is usually restricted to a maximum of l 1/2-in (40-mm) nominal maximum size aggregate

The majority of concrete in congested forms can be placed 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 reinserting the hose at typically 10-ft (3-m) centers

In narrow wall forms where it is not possible to lower the concrete hose through the reinforcement, lie-flat hose coupled to the steel reinforced hose has been used successfully The lie-flat hose is very pliable and can transfer 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 (see

Fig 7)

Where wall placements extend up to the underside of structural steel members or concrete beams, concrete should be placed under pressure through slide valves, as depicted in Fig 9 When the form is full, the slide valve

is closed and the line disconnected After the concrete has set, the slide valve and supporting form are re-moved The remaining concrete stub is removed by chipping and the wall is ground smooth

Pumping concrete from the bottom of the form can offer 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 with whether or not the technique is viable Rectangles, squares, and other polygons require special design of formwork because pressure concentrates at the corners

of angles or point loading is developed Circular unres-tricted structures lend themselves best to pumping from the bottom Unrestricted means that the concrete must

be unimpeded all the way around the inside of the col-umn and there are no baffles that restrict upward movement

If there is a vertical steel “H” section within the col-umn, the concrete will not pump if the concrete enters

at 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 cover

over the “H” is present Successful pumping has been

achieved with less than 4-in (lOO-mm) cover if pumped

into the web

When pumping from the bottom, there should be

re-strictions on the number and size of embedments or

boxouts and their position in the form Also, if there

are large numbers of dowels, the flow of concrete may

be restricted, causing pump and/or form failures At

least 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 the

structural steel or turned out reinforcement

Preplaced aggregate (PA) is another placing method

that has been used effectively in congested areas To

produce concrete by the PA method, coarse aggregate

is first placed in the prepared form Then the voids in

the preplaced aggregate are filled with a fluid grout

consisting of cement, sand, water, and sometimes an

admixture, which is pumped into the forms from the

bottom through form inserts or pipes Materials

re-quirements, procedures, and properties are described in

ACI 304.1R

The PA method has been used to advantage for

placing concrete around congested reinforcement

Where the reinforcing steel and forms are already in

place, grout pipes are inserted from the top or sides to

the bottom of the space to be filled Coarse aggregate

is then dropped into place or shoved in from the sides,

and assisted by rodding and/or blowing with the help

of 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 the

reinforcement and forms are erected Fig 10 shows a

portion 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 during

initial construction was too congested to permit the use

of vibrators, especially because the rear wall was a steel

membrane that could not be cut to receive ports The

boxout was filled with PA concrete in 7-ft (2.1-m) lifts

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 with

grout having approximately the same strength as the

surrounding concrete The PA method can

signifi-cantly reduce the chances of honeycomb

3 Continuous pumping of the grout eliminates cold

joints However, if pumping is interrupted for any

rea-son, the effect of the cold joint that forms is negligible

because coarse aggregate particles extending through

the grout surface provide structural continuity across

the interface between the two grout placements with a

high probability that the negative effects of the cold

joint can be minimized or eliminated

Fig 10-Preplaced aggregate method: Close-up of congested 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 of first lift of coarse aggregate [approximately 7 ft (2.1 m) deep] is visible at bottom of photo Grout will be pumped to a few inches below surface of the coarse ag-gregate to provide a keyed joint with the succeeding lift

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

5.3-Summary

Successful 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 of options that can be considered by architects/engineers and constructors, it must be recognized that each situ-ation may be unique The architect/engineer and con-structor, in consultation with each other, must assess each situation and agree on the most appropriate ap-proach for the situation in question However, it is of utmost importance that situations requiring special at-tention be identified with sufficient lead time to allow proper planning

CHAPTER 6-REFERENCES 6.1 -Specified and/or recommended references

The documents of the various standards-producing organizations referred to in this document are listed with their serial designations

These publications ing organizations:

may be obtained from the

follow-309.3R-10 ACI COMMITTEE REPORT

American Concrete Institute 304R

P.O Box 19150

Detroit, MI 48219 304.1R

ASTM

1916 Race Street

Philadelphia, PA 19103

309R Guide for Consolidation of Concrete 347R Guide to Formwork for Concrete 439.3RR Mechanical Connections of Reinforcing Bars SP-4 Formwork for Concrete

American Concrete Institute

ASTM

c 494

117

211.1

211.2

212.3R

303R

Standard Specifications for Tolerances for

Concrete Construction and Materials

Standard Practice for Selecting Proportions for

Normal, Heavyweight and Mass Concrete

Standard Practice for Selecting Proportions for

Structural Lightweight Concrete

Chemical Admixtures for Concrete

Guide to Cast-in-Place Architectural Concrete

Practice

c 1017

6.2-Cited reference

Bonikowsky, 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, Transporting and Placing Concrete

Guide for the Use of Preplaced Aggregate Concrete for Structural and Mass Concrete Applications

Standard Specification for Chemical Admix-tures for Concrete

Chemical Admixtures for Use in Producing Flowing Concrete

ACI 309.3R-92 was submitted to letter ballot of the

in accordance with ACI standardization procedures.

committee and processed