This document provides guidance on consolidating concrete in congested construction areas. It defines criteria for what constitutes a congested area, such as when reinforcement is spaced less than 1.5 times the maximum aggregate size. Factors contributing to congestion include dense reinforcement, embedded items like pipes and boxes, and formwork design. Congested areas can result in honeycombed or low-density concrete with increased cleaning, formwork and placement costs. The document recommends practices to address these issues, such as modified mix designs, placement methods, and design considerations that facilitate consolidation.

1. ACI 309.3R-92
.
Guide to Consolidation of
Concrete in Congested Areas (Reapproved 1997)
Reported by ACI Committee 309
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 I--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

2. 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 22-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 l1/3 times 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-

3. 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-Definitions
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 placement 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

4. 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 arrangements,
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-

5. 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 nailing 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

6. 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.

7. 5.2-Construction considerations
5.2.1 Use of admixtures-Proper placement of con-
crete in congested areas usually requires the concrete 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 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-setting
admixture, ASTM C 494, Type A, plus a water-reduc-
ing and accelerating 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-

8. 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 during 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 l1/2-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

9. 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. IO-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-

10. 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