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BY FRANK A. RANDALL, JR.STRUCTURAL ENGINEER
If it slows the flow of water but doesnt stop it, is it a
wa-terstop? If it is made of the best material to the bestdesign
but is not installed properly is it still a waterstop?When
waterstops are required in concrete constructionthey must be
carefully planned and installed.
The waterstop is a long, thin barrier against waterl e a k a g
e. Like the links in a chain, eve ry part of its lengthmust do the
job. There are many different waterstopsand there are many
different situations that re q u i re wa-t e r s t o p s.
Design
The state of the art of waterstop design lags behind
thetechniques of manufacture and installation. The con-c rete
industry has no detailed guidelines or standards for
what size and which shape to use in a given situation.The
catalogs give only vague advice on the size andshape to use. Be s i
d e s, it is often difficult to determ i n eh ow much movement will
occur at a joint. So it is im-p o rtant for all invo l ved to do
their part. Whoever speci-fies the waterstop should give careful
thought to its sizeand shape, and the contractor should use the
best pra c-tices when installing whatever is specified.
Some large engineering offices and many publicagencies have had
enough experience to develop theirown waterstop designs and
specifications. The U.S. Bu-reau of Reclamation and the Hyd ro - El
e c t ric Power Co m-mission of On t a rio have conducted detailed
inve s t i g a-tions and established standards for waterstops for
damsand water re s o u rce stru c t u re s. The rest of the
industryrelies on common sense, its own limited experi e n c e,
andthe advice of re p re s e n t a t i ves of waterstop
manufactur-e r s.
When selecting any waterstop, the size should be inkeeping with
the mass of the concre t e. Catalogs list
To achieve watertightness at joints, waterstops must
bepositioned correctly and care must be taken to avoidhoneycombing
near the waterstop. Forms have beenremoved at this construction
joint and before the nextconcrete placement dirt, oil or hardened
concrete must beremoved from the exposed portion of the
waterstop.
WaterstopsChoose them wisely and install them with care
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widths from 4 to 9 inches. The thickness will determ i n ethe
stiffness and how well the unit will stay in place dur-ing
construction. Catalogs list center thicknesses fro m31 6 to 12 i n
c h .
Catalogs rate the maximum head of water for water-s t o p s.
Water heads range from 65 feet for the smallestunits up to 260
feet. Howe ve r, such ratings are offered forre f e rence only, and
the bro c h u res state that the actualcapability to resist head
pre s s u re depends on quality ofc o n c rete and placement. This
capability also dependson the amount and directions of joint
movement thatd e ve l o p.
Use
Waterstops are used in walls, in floors and in roofs ofu n d e
rg round stru c t u re s. They are used in basements,t a n k s,
swimming pools, dams, tunnels, canal linings,b ri d g e s, locks,
sewage treatment plants, water re s e r-vo i r s, mine shafts,
retaining walls, aqueducts, under-g round vaults and parking stru c
t u re s.
On ra re occasions waterstops are used in concre t es t ru c t u
res having no re b a r s. In these cases the amount ofm ovement at
the joints may be appre c i a b l e, and move-ment might take place
in one, two or three dire c t i o n s.
Choosing the size and shape
In concrete with re b a r s, the size and shape of a water-stop
would depend on the type of jointany of three ba-sic kinds:
construction joint, which has practically no move-
m e n t
control joint, which has a limited move m e n t
expansion joint, which has a greater move m e n tAgain, if no re
i n f o rcing bars cross the joint, then
m ovement may increase the width of the joint and inaddition,
the concrete on one side may shift in either di-rection, lengthwise
or crosswise of the joint. For contro ljoints and expansion joints,
somebody has to know orestimate how much movement the concrete may
deve l-o p.
Waterstops pri m a rily accommodate movement thatwidens the
joint. Some waterstops are made to also ac-commodate movements
perpendicular to the plane ofthe waterstop. When movement occurs
longitudinally,or parallel to the axis of the waterstop, it
develops a tear-ing action which could cause a leak to deve l o
p.
Materials
The earliest types of waterstops we re made of rigid
orsemiflexible metal, usually copper, nickel-steel or galva-n i zed
iron. A crimp was often provided in the metal atthe center of the
joint. For their effectiveness these typesrelied solely on a good
bond between the metal and thec o n c re t e. Their main
applications we re where only ve rylimited movements would
occur.
Ru b b e r. The rubber chosen for a waterstop should bea l k a l
i - resistant and should be able to resist the effects of
aging. It should not become brittle and crack when ex-posed to
sunlight and continuous flexing. For manykinds of installations,
the rubber should also be re s i s t a n tto chemicals, petroleum
products and sewage. Ru b b e rwaterstops are manufactured in
accordance with stan-d a rds of the Rubber Ma n u f a c t u rers
Association.
Plastics. Synthetic materials we re first used for water-stops
about 40 years ago. Po l y v i n y l c h l o ride (PVC), thefirst
plastic to be developed for the purpose, is still ve ryp o p u l a
r. PVC waterstops have elongations as much as350 percent, tensile
strengths over 2000 psi and re s i s-tance to brittleness at
tempera t u res down to minus 35d e g rees F. They are rated as to
alkali re s i s t a n c e, tear re-s i s t a n c e, volatility loss
and water absorption in accor-dance with ASTM tests.
P VC, when pure and uncompounded, forms art i c l e sthat are
hard and rigid such as pipe, chairs, boxes andautomobile bodies.
When compounded with cert a i np l a s t i c i ze r s, the PVC
becomes flexible and tough. Ori g i-nally some plasticizers we re
used that had a tendency tom i g rate into any liquid in which the
compound was sub-
m e rged. Howe ve r, special polymeric plasticizers are nowused
which are virtually unextractable by water.
ACI 504R-77, Guide to Joint Sealants for Co n c re t eSt ru c t
u re s, also notes that PVC does not have quite asgood re c ove ry
and fatigue resistance as va rious ru b b e r sand it is
susceptible to oils. Ne ve rt h e l e s s, being therm o-plastic,
PVC can easily be spliced on the jobsite and it isthe material most
widely used. Competition in the in-d u s t ry has caused a few
manufacturers to use re c l a i m e dP VC compounds in order to
lower material costs. Ma n yspecifications prohibit the use of
reclaimed compoundsand permit only virgin materi a l .
Moldable plastic. Plastic strips that are moldable at or-d i n a
ry tempera t u res are ava i l a b l e. These are inserted in-to
keyways of construction joints. This type of material is
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designed to be applied after the keyway form has beens t ripped
rather than cast into the concre t e.
Shapes
Early flexible waterstops we re shaped, in cross sec-tion, like
a conventional dumbbell. They we re intendedto be used where the
joint moved mainly in the sameplane as the waterstop, that is,
where pri m a rily tensionand little shear was expected.
The next development was a hollow center bulb in thedumbbell so
that it could absorb shear movement. Thisalso lessened the pull on
the end bulbs.
When plastics came along, waterstops we re given ac o r rugated
or ribbed profile and then a hollow centerb u l b. The ribs
increased the effective mechanical seala rea of the waterstop. They
also concentrated the bondat intervals along the unit, so that the
ribs closer to thejoint resisted the greater amount of pull. The
ribs far-ther from the joint we re under less stress; thus they
de-f o rmed less and so we re in more intimate contact withthe
concre t e, effecting a tighter seal. With smooth water-stops the
same effect is obtained by simply using ag reater width. A ribbed
profile may be somewhat morei m p o rtant in PVC than in rubber
because PVC does noth a ve the same degree of elasticity.
In a later development, half of the waterstop was splitso that
the unit could be more easily installed in the
f o rms (see split-ribbed waterstop among the illustra t e
dshapes). With the usual solid waterstop, it is necessary tosplit
the bulkhead in the form, placing one half the wa-terstop in the
first pour of the concrete and half in thesecond pour. Since one
end of the unit is already split,the parts can be spread so that
the whole waterstop iscontained by an ord i n a ry bulkhead. After
the bulkheadis re m oved, the two split parts are straightened out
andstitched together before the second pour is made.
The laby rinth waterstop was developed for massives t ru c t u
re s. Like the split-ribbed waterstop, it also avo i d se x t ra
carpentry work because it is placed within thebulkhead of the first
pour. When the bulkhead is re-m oved, the concave openings on only
one side of thewaterstop have been filled with concre t e. Openings
onthe opposite side are filled when concrete is placed onthat side.
Laby rinth waterstops are usually used only in a
ve rtical position because of the difficulty of
successfullycompletely filling the concave openings when the strip
isrun hori zo n t a l l y. The cellular laby rinth waterstop
willaccommodate lateral shear move m e n t .
Co n t raction joint forming waterstops we re deve l o p e dfor
use in canal linings which may be only about 5 inch-es thick. The
unit is placed with the stem of the T in ave rtical position; it
then will induce a shri n k a g e / t e m p e r-a t u re crack just
as a tooled control joint would. The hor-i zontal dumbbell portion
then serves as the waterstop.The cellular type of contraction joint
former will accom-modate lateral or shear move m e n t .
Ex p a n d e r-type waterstops are useful in situationsw h e re
larger movements are expected, for example atexpansion joints. A
U-shaped loop (see diagram) opensto accommodate up to 112 inches of
joint movement. Athin membrane at the base of the loop pre vents it
fro mfilling up when concrete is placed, but the membra n etears
easily when the waterstop is tensioned. Pu l l i n gs t ress on the
unit is reduced so that the ribs deform ve rylittle and remain in
intimate contact with the concre t e.Also available is a split unit
of this kind which facilitatesinstallation entirely on one side of
a bulkhead, as withthe usual split-ribbed units.
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Some waterstops are thick and short and are conse-quently stiff
enough to resist being bent out of positionwhen the concrete is
dumped into the form s. But mostwaterstops are too flexible to stay
in position withouts u p p o rt during concreting. It is usually
necessary to wirethem to the forms or re i n f o rcing bars. Small
holes mustbe drilled or punched near the extremities of the units
sothat tie wire can be threaded through. These holes arecostly and
they are obviously not desirable in a water-s t o p. To avoid the
holes one manufacturer provides acontinuous wire looping at the
edges for inserting the tiew i re s. The looping also provides
extra bond to the con-c re t e, thus enhancing the waterproofing
effective n e s sof the pro f i l e.
The last type of waterstop among the shapes illustra t-ed is the
simple strip of moldable plastic. A keyway isused to form a
construction joint without casting anywaterstop in place. After the
form is stripped, the con-c rete in the keyway is brushed clean of
all foreign mate-rial and dust, and a primer is brushed onto the
base ofthe keyway. The moldable strip is then simply pre s s e
dinto position at the base of the keyway, and fresh con-c rete is
cast against it. It is re p o rted to maintain the sealby remaining
plastic perm a n e n t l y. Since part of thedepth of the keyway is
taken up by the waterstop, thekeyway may have to be made deeper
than usual to main-tain its stru c t u ral function.
Installation
T h e re are seve ral important re q u i rements for achiev-ing
watert i g h t n e s s :
Position the waterstop corre c t l y. The waterstop mustbe
located accurately and possibly braced or lashedf i rmly to pre
vent movement during placing of concre t e.The center bulb or loop
that accommodates joint move-ment must be placed directly at the
joint; otherwise itsvalue will be lost.
The formwork must be tight-fitting. It must not allowa leakage
path for the cement mort a r, leakage whichcould lead to
honeycombing. A joint is always the mostv u l n e rable point of a
stru c t u re.
The waterstop must be clean. If it is dirty or gre a s y, it
will not seal out water. Di rt and splattered concrete mustbe
cleaned from the ribs and corrugations prior to plac-ing concrete
in each side of the joint.
C o n c rete must be carefully consolidated. This is ve ryi m p
o rtant; the efficiency depends on good compactionof the concre t
e. The waterstop must be embedded inc o n c rete of sufficient
quality to hold it in place whenm ovement occurs at the joint.
Waterstops are meant top rovide a barrier across construction,
contraction andexpansion joints. They are not intended as a remedy
forp o rous concre t e. Intimate contact with the concrete
isessential over the entire surface of the waterstop; en-t rapped
air and honeycombing near the joint will nulli-fy its va l u e.
Splices must be correctly made. A poor splice would bea weak
link in the water barri e r. Splices should be avo i d-
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ed if possible. Fo rtunately this is made easy to plan be-cause
waterstops come in rolls 50 to 125 feet long. Sp l i c-ing is a
subject that merits considerable discussion.
Splicing procedures
Rubber waterstops can be lap-spliced cold. The endsa re
overlapped about 3 inches with flat surfaces pre s s e dt o g e t h
e r. Coatings of rubber cement and uncured gumrubber are placed on
the surfaces that will be in contactand the pieces are held
together by clamping betwe e nflat stainless steel plates. Cold lap
splices are re l a t i ve l y
easy to make and do not re q u i re any electri c i t y, but
thetensile strength achieved is low. A much stronger splicecan be
made by vulcanizing. Small, portable vulcanize r sa re used,
commonly heated electri c a l l y. The buttededges are beveled to
45 degrees or less. Rubber cementand uncured gum rubber are then
applied to the endsand vulcanized, usually at around 290 F.
Splicing PVC is simply a matter of butting together thetwo ends
after they have been melted by means of ani n d i rect source of
heat. The ends should be accura t e l yt rimmed square before
exposing them to the heat. Me l t-ing is necessary to a state of
only tacky plasticity or soft-n e s s. The melting must be uniform
across the whole sec-tion. After the ends have become molten, they
are forc e dtogether and held firmly until they have
completelycooled and fused. They must cool naturally and not beq u
e n c h e d .
With PVC, an indirect source of heat is essential be-cause
direct exposure to a flame will change the chemi-cal composition of
the plastic. A common method is toheat a 14 -inch steel plate by
means of a plumbers torc h .An electric hot plate can otherwise be
used, and somem a n u f a c t u rers have special tools for
splicing.
Ribbed designs depend on the continuity of the ri b sfor their
ability to stop the passage of water. Leakagetests have shown that
it is important to pre s e rve the ri b st h roughout the splice
area. A soldering iron can beequipped with a gro oved tip for
remolding the PVC ri b s
that are lost during splicing. The continuity of the ribs ismost
apt to be lost where corners and intersections arem a d e. For this
reason it is especially economical to ord e rp re f a b ricated
intersections and corn e r s. These ares t ronger than
intersections made in the field and betterable to withstand water
pre s s u re and seepage. In any
Interferences between waterstops and reinforcing steel
cansometimes be avoided by advance planning. Here, aninverted
keyway was designed to raise the waterstop abovehorizontal
steel.
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e vent, the corners and intersections should be made byuse of
miters in order to maintain the continuity of theribbing and center
bulbs.
A corner splice in a wall can sometimes be avoided byc u rving
the waterstop around the corner (see photo). Aslong as the concrete
walls are thick enough there and there i n f o rcing steel does not
interf e re, this would seem likethe pre f e r red way to form a
corn e r.
To be sure that the integrity of the waterstop can bemaintained
at the corners the designer should considerall corners and
intersections of split waterstops andthose of other special shapes
ahead of time. It will be
t ri c k y, difficult or impossible to make proper corn e r sand
intersections with some special types of waterstops.
Planning to avoid interferences
Installation of waterstops in some re i n f o rced stru c-t u
res becomes even more a matter of advance planning.The drawing
illustrates the congestion at the base of awall in a water
treatment plant. Because of this conges-tion an upturned keyway has
been created in which toplace the waterstop so it will clear the
hori zontal bars inthe concrete mat. Diagonal rebars at the haunch
have tobe detailed and fabricated to clear the waterstop, and top e
rmit the stripping of the 2x4-inch forms used to formthe key. This
is an example of an integrated layout thate vo l ved from seve ral
jobs where things had just not fit-ted and worked together. Ca
reful planning like this willn ow make for smooth sailing on the
job.
References1. Kellam, B. and Loughborough, M. T., Waterstops
forJoints in Concrete, Journal of the American Concrete Insti-tute,
June 1959, page 1269.2. ACI Committee 504, Guide to Joint Sealants
for Con-crete Structures, ACI Manual of Concrete Practice.
Corner splices can be avoided if there is enough room tocurve
the waterstop around a corner. Prefabricatedintersections and
corners are also available; these arestronger than field-spliced
intersections.
SOURCES OF WATERSTOPS
A total of 20 manufacturers of waterstops, withtheir address, is
given in Concrete 84 SourceBook.It is available from Concrete
Construction Publica-tions, Inc., 426 South Westgate, Addison,
Illinois60101.
PUBLICATION #C840569Co py right 1984, The Ab e rdeen Gro u p
All rights re s e rve d