Consolidating Concrete
in Congested Areas
Special design considerations and construction
techniques can ease congestion problems
ome design requirements
for concrete construction,
such as seismic provisions
and post-tensioning, can
result in congested areas within the
formwork that are difficult to consolidate. Reinforcing steel, embedments, boxouts, prestress ducts,
and anchorages all contribute to
congestion. Certain form shapes also can make concrete difficult to
place and consolidate.
Incomplete consolidation in
congested areas can lead to structural inadequacy and time-consuming, expensive remedial work.
Honeycombed concrete can occur
because vibrators can’t consolidate
concrete around and through the
congestion and out to the form
face. Incomplete consolidation also
S
can result in excessive amounts of
entrapped air, which reduces concrete strength and increases permeability.
Successful concreting in highly
congested areas requires special
design considerations, concrete
mixtures, and placement and consolidation techniques. Here are
some factors that contribute to
congestion and ways to minimize
congestion problems.
Causes of Congestion
Reinforcing steel. Congestion
can be a problem when the clear
spacing between rebar or between
a bar and the form is less than 11⁄3
times the maximum size of coarse
aggregate used in the concrete mixture. This condition is more likely
Place concrete in congested forms by pump, using 4- or 5-inch-diameter steelreinforced discharge hose. To ensure good pumpability, restrict the nominal
maximum aggregate size to 112⁄ inches.
to occur at splices and bends in reinforcement and at beam-column
connections. Sometimes multiple
layers of rebar cause congestion
when the bars in the lower layers
are not directly below those in the
upper layers.
Embedments and boxouts. The
use of embedments and boxouts
along with dense reinforcement often results in congestion. Embedments are items cast into concrete,
such as plumbing, prestress hardware, ducts, mechanical and electrical boxes, connection inserts,
and anchorages. Boxouts are used
to form openings, keyways, or
pockets in concrete. They are either
removable or stay-in-place.
Frequently, these items cause
congestion because concrete cannot be placed and consolidated
easily underneath them. As with reinforcing steel, the spacing between embedments, boxouts, and
the form must be at least 11⁄3 times
the maximum size of coarse aggregate to avoid congestion.
Tolerances for placing concrete
around embedments and boxouts
should be considered at the design
stage. Often, mechanical and electrical embedments are located
near 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 re i n f o rc ement, embedments, and boxouts
cause congestion.
Fo rm wo rk . The surface texture,
shape, type, and orientation of
f o rm w o rk can restrict concrete
placement. Used or poorly oiled
wood forms are more likely to
cause problems than steel or plastic-lined forms. The frictional resistance of wood forms can impede
concrete flow.
Battered form faces or counterforts can result in areas of poor consolidation. Their sloped profile restricts concrete placement, vibrator
access, and air migration during vibration. Corbels and haunches also
are common areas of congestion.
Design Considerations
Before work begins, carefully review design drawings with the engineer to ensure that appro p ri a t e
allowances have been included in
congested areas. Best results are
achieved when the intent of the design can be met under field conditions. Pay careful attention to the
following factors:
Reinforcing steel arrangement
and splicing. Reinforcing steel
should be arranged to provide
enough space for concrete placement into the form. The engineer
may have to increase the member
size over that required by design
calculations to provide sufficient
space. In extreme cases, it may be
necessary to include access ways
through the reinforcing steel.
Lap splicing of rebar can cause
congestion problems in areas with
high densities of reinforcing steel.
Sometimes this problem can be
solved by mechanically connecting
rebar, as described in Reference 1. In
special cases, rebar may be spliced
by welded connections. However, a
mechanical or welded connection
will produce some localized increase in rebar diameter. This
should be considered when detailing clearances and bar spacing.
Embedment
and
boxout
a r ra n g e m e n t. Embedment and
boxout configuration should consider reinforcing details, concrete
mix pro p o rt i o n s, and nominal
maximum aggregate size.
To eliminate form penetrations,
use void forms. If the forms are
large (more than 2 feet in either direction), placement and vibration
tubes should be provided. These are pipes
through the formed
blockout that permit access for concrete placement and vibration.
Boxouts that are to be
removed and exceed 2
feet in either direction
also should have placement and vibration
tubes. Stay-in-place boxout forms, such as hol- Use flowing concrete, with a slump of 71⁄2 inches or
low metal door and win- more, in highly congested areas where less workable
dow
frames,
often concrete mixtures can’t be properly placed and
require bracing and can- consolidated.
not be cut to accommosuch as around multiple embeddate placement and vibration
ments or dense reinforcement. The
tubes. Where possible, use stay-inmodified mixture may include adplace boxouts having tolerances
justments in aggregate size and cethat allow them to be shifted.
ment content and the use of adWhere the boxout spans from
mixtures and fly ash.
one form face to the other, provide
No rm a l l y, engineers will specify
access through the bottom of the
the largest nominal maximumboxout. As the concrete reaches the
s i ze - a g g regate mixtures that are
bottom of the boxout, the access
readily available and can be consolcan be closed off with a preformed
idated by conventional methods.
insert, which is then bolted to the
For congested areas, howe ve r, engiboxout form.
neers may specify smaller maxiFo rm wo rk design. Fo rm w o rk
mum-size aggregate depending on
design can contribute significantly
the degree of congestion.
to congestion if it does not take
For example, when concrete with
other factors into account. These
a nominal maximum-size aggreinclude:
gate of 1 1⁄2 inches is specified, the
• Number, location, and size of tie
engineer may allow substitution of
rods
3
⁄4-inch maximum-size aggregate in
• Location of embedments and
a portion of the concrete. Where
blockouts
the design mixture specifies nomi• Location of trunks or concrete
nal maximum-size aggregate of 3⁄4
hose
inch for extremely congested areas,
• Form height
the engineer may allow substitu• Use of side ports
tion of 1⁄2-inch maximum aggregate
In narrow, congested walls, exterin a portion of the placement.
nal tie rods can help reduce congesWhen the maximum aggregate size
tion. The tie rods are attached to the
of a specified mix is reduced, the
bulkhead walers at the ends of the
mix has to be modified to maintain
wall. Another way to ease congesthe specified water-cement ratio
tion is to increase the spacing of
and design strength. This can be
loadbearing members and use highdone by adjusting cement and waer capacity ties and form sheathing.
ter contents.
Mix Proportioning
Proper placement of concrete in
congested areas usually requires a
The use of modified mix propormaterial with flowing chara c t e ri stions may be necessary to achieve
tics. Flowing concrete typically has
proper consolidation in congested
a slump of 71⁄2 inches or more and
a re a s. This mixture need only reremains cohesive without excessive
place the original mix proportions
bleeding or segregation (Ref. 2).
in zones of extreme congestion,
Use flowing concrete in areas
where less workable concrete mixtures cannot be properly placed
and consolidated due to lack of
mobility and vibrator access.
Producing flowing concrete only
by adding extra water results in
lower-quality concrete. Instead, use
chemical admixtures that meet
ASTM C 494 and ASTM C 1017 requirements. These include:
• High-range water-reducing admixtures (superplasticizers),
ASTM C 494, Types F or G
• A high-range water-reducing
admixture and a water-reducing
and retarding admixture, ASTM
C 494, Type D, or water-reducing and accelerating admixture,
ASTM C 494, Type E
• High dosages of a water-reducing normal-set admixture,
ASTM C 494, Type A, plus a water-reducing and accelerating
admixture, ASTM C 494, Type E
Where flowing concrete is required, prepare trial mixtures made
with the materials to be used in the
project. Test each mixture under
the environmental conditions expected on the project. Trial mixtures should have initial slumps resulting from the maximum
allowable specified water-cement
ratio. Vary chemical admixture
dosages to achieve the desired
slump range.
Some specifications also allow
the addition of fly ash to enhance
workability. Typically, an addition
of fly ash equal to 5% of the cement
weight will significantly increase
concrete flow.
Placing Methods
Review reinforcing steel, embedment, and formwork drawings and
select placing meth-ods to suit conditions. Using cranes and buckets
along with hoppers and trunks to
place concrete often is not possible
if forms are congested. Most concrete is placed in congested forms
by pump or placing booms using 4or 5-inch-diameter steel-reinforced
hose. To ensure good pumpability,
restrict the nominal maximum aggregate size to 11⁄2 inches.
Most concrete can
be placed in congested forms by lowe ring the discharg e
hose through the rei n fo rcement
to
within 6 feet of the surface, then disc h a rg i n g
concrete for that lift
thickness. Raise and
re i n s e rt the hose a t
10-foot centers. In
n a r row wall form s
w h e re it is not possible to lower the
hose through the
re i n f o rcement, attach lie-flat hose to
the end of the d i sc h a rge hose. Ma d e
of
a
pliable,
p o l y v i n y l - c h l o ri d e reinforced material,
lie-flat hose can
t ransfer concre t e
ve rtically thro u g h
very narrow spaces.
Where wall placements extend up to Often, it’s difficult to adequately place and consolidate
the underside of concrete underneath a boxout. As this project shows,
s t ru c t u ral
steel gaps can result. The problem could have been avoided
members or concrete by the use of placement and vibration tubes through
the boxout to permit access.
beams, place concrete under pressure
Un re s t ricted means that the conthrough slide valves. A slide valve is
crete is unimpeded all the way
a short piece of steel pipe with a
a
round the inside of the column
slide plate mounted in it. One end
and
there are no baffles restricting
of the pipe is bolted to the form and
u
pw
a
rd movement. If there is a verthe other end receives concrete
tical
steel
“H” section within a colfrom the discharge hose. Pump
umn,
concrete
will not pump if it
concrete through the open slide
enters
at
a
point
perpendicular to a
valve to fill the form. When the
flange
of
the
“H
.” It’s best to disform is full, close the slide plate and
c
h
a
rge
concrete
directly into the
disconnect the line. After concrete
web
of
the
“H
.”
has set, the slide valve and supportWhen pumping from the boting form are removed.
tom,
restrict the number and size of
Sometimes pumping concrete
embedments
or boxouts in the
from the bottom of the form works
f
o
rm.
They
can
restrict concrete
well. The shape of the element def
l
ow.
A
large
number
of dowels also
termines whether the technique is
can
re
s
t
rict
flow.
Provide
at least a
viable. Rectangles, squares, and
4-inch
clearance
between
an emother polygons require special
bedment
and
re
i
n
f
o
rcement
or 4
f o rm w o rk designs because presinches
free
at
the
top
of
the
placesure concentrates at the corners of
ment below the structural steel or
angles or point loading develops.
turned out reinforcement.
C i rc u l a r, unrestricted stru c t u re s,
such as columns, lend themselves
best to pumping from the bottom.
Consolidation
Plan all aspects of consolidation
before placing concrete. If used
p ro p e r l y, internal vibrators do the
best job of consolidation. Howe ve r,
external vibration may be needed
to supplement internal vibration in
congested areas.
Use small-diameter internal vibrators in the lower areas within
the forms when a high-range water-reducing admixture is used with
a modified concrete mixture. When
placing a normal concrete mixture
having larger maximum-size aggregate, use bigger vibrators (up to 3
inches in diameter).
When reinforcing steel limits access into a form, lower additional
vibrators down through the upper
reinforcing mat from the top of the
placement. This will encourage operators to keep vibrators in a nearly
vertical position instead of trying to
throw the vibrators hori zo n t a l l y
past interferences. Be careful not to
lodge or snag a vibrator within the
placement; it can be impossible to
extract.
In congested, narrow wall forms,
it may be necessary to place side
ports in one form. Side ports are
temporary openings in one side of
the form that allow workers to lower vibrators into the form and observe concrete placement. Typically, they are 2 feet square with a
spacing of 6 feet.
It’s also possible to lower smalldiameter vibrators between the
outer layer of reinforcement and
the form face. Howe ve r, in the case
of architectural faces, use external
vibrators.
Whatever consolidation method
is used, do not underv i b ra t e. Und e rv i b ration can result in honeycombed concrete, air pockets, and
lack of density in congested areas.
See Reference 2 for more information on consolidating and vibrating
concrete.
References
1. ACI 439.3R-83, “Mechanical Connections of Reinforcing Bars,” American Concrete Institute (ACI), Detroit.
2. ACI 309R-87, “Guide for Consolidation of Concrete,” ACI.
Acknowledgment
This article is adapted from “Guide to
Consolidation of Concrete in Congested Areas,” ACI Structural Journal,
September-October 1992.
PUBLICATION #C940228
Copyright © 1994, The Aberdeen Group
All rights reserved