How to Seal Joints in Concrete Structures_tcm45-342478

8/7/2019 How to Seal Joints in Concrete Structures_tcm45-342478

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Often underestimated, joint sealants inconcrete structures play a vital role inprotecting the structure from premature de-

terioration and interruptions in its eff e c t i v e

service life. Sealants now available and pre-

sent knowledge of joint sealing criteria are

adequate to provide successful joint sealant

installations at least 90% of the time. Since

the cost of providing well-sealed joints usu-ally is only a small fraction of the total cost

of the structure, there is little justification

for poor sealing practices.

Use of an appropriate type of sealant

and proper sealant installation, however,

are not a guarantee of successful perfor-

mance. The location and design of the

joint itself may be to blame if sealing

problems develop. Also, the shape of the

joint and the amount and type of move-

ment that occur at the joint will aff e c t

sealant behavior. The best way to ensure

that sealing efforts will pay off is to com-

bine the right type of sealant with the ap-

propriate joint detail for the application,

then properly design, specify, install, and

maintain the joint sealant system.

Why sealants are needed

Most concrete structures have joints as

part of their design. Joints accommodate

movements of the concrete units caused

by contraction (from drying, shrinkage,

and creep), expansion (from sulfate attack

and alkali-aggregate reactions), and cycli-

cal volume changes (from exposure to the

environment or the application of loads).If these movements are restrained, the

concrete can distort, crush, or crack.

Though joints serve a necessary func-

tion in controlling movement of the con-

crete, they also create openings that need to

be sealed to maintain the integrity of the

structure. The primary function of a joint

sealant is to prevent the intrusion of un-

wanted substances into or through the

joint. Joint sealants help keep wind and

rain out of buildings and foreign solid mat-

t e r, such as dirt and ice, out of joints. They

also keep the contents of tanks, pipes,

canals, and dams in. Sealants can help pre-

vent damage to floor joints from concen-

trated loads, improve thermal and acousti-

cal installations, and dampen vibrations.

Joint types

Determining an appropriate joint

sealant for the job is easier with an under-

standing of typical joint configurations.

The main types of joints are:

Contraction (control) joints P u r p o s e-

ly made planes of weakness designed to

regulate cracking from contraction of

concrete structural units. These joints

most frequently are used to divide larg e ,

relatively thin units such as floors, pave-

ments, and retaining walls into smaller

units. Contraction joints are only appro-

priate where contraction and expansion

leaves abutting units shorter than when

the concrete was placed.

Expansion (isolation) joints D e-

signed to isolate structural units that be-

have diff e r e n t l y, expansion joints prevent

crushing and distortion caused by com-

pressive forces produced by applied

loads, settlement, or volume changes. Ex-

pansion joints are used to isolate walls

from floors or roofs, columns from floors

or cladding, and pavement slabs anddecks from bridge abutments or piers.

C o n s t ruction jointsJoints created by

interruptions in concrete placement or

from the positioning of precast units. The

locations of construction joints usually are

predetermined as much as possible. They

may be required to function monolithical-

ly with the previous placement through

complete bonding, or as expansion or

contraction joints.

Combined and special purpose

j o i n t sConstruction joints also can be

designed to function as contraction or ex-

pansion joints. Hinge joints that permit

rotation are found primarily in pave-

ments. Sliding joints are useful where one

How to seal jointsin concr et e st r uctur es

By Marilyn Palmer

ACI 50 4R-90 , Guide to Sealing Joints in Concrete Structures, providesa comprehensive guide to joint sealant materials, design, and installation

F i g u re 1. Function of a bond breaker and backup material in field-molded sealants.


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unit of a structure moves in a plane at

right angles to the plane of another unit.

From the viewpoint of how the sealant

functions, the two basic joint configura-

tions are butt joints and lap joints. Butt

joints occur where the structural units

abut each other and movement occurs in

the plane of the cross section of the joint.

In the less common lap joint, the units

slide over each other.

Joint types and their corresponding

sealant requirements can be grouped into

three main structural applications:

Structures not under fluid pressure

(most buildings, bridges, storage bins,

retaining walls)

Containers under fluid pressure (dams,

reservoirs, tanks, pipelines)

Pavements (highways and airports)

Sealant typesJoint sealants fall into two categories:

field-molded and preformed. Field-mold-

ed sealants are applied in liquid or semi-

liquid form and take on their required

shape from the mold provided at the joint

opening. They usually cycle between

compression and tension and change their

shape without changing their volume.

Preformed sealants are preshaped in rela-

tively solid form, usually are in compres-

sion, and change their shape as their

width changes.Examples of field-molded sealants in-

clude mastics, thermoplastics (hot-applied

and cold-applied), thermosetting chemi-

cally curing, and thermosetting solvent re-

lease. Primers, bond breakers, and backup

materials often are needed with field-

molded sealants to economize on the

sealant and ensure its successful perfor-

mance. Figure 1 shows the function of a

bond breaker and backup material. Pre-

formed sealants include rigid waterstops,

flexible waterstops, gaskets, strip or gland

seals, compression seals, and flexible

foam (impregnated and nonimpregnated).

ACI 504R-90 contains a comprehen-

sive system of tables cross-referencing

sealant material type and properties, com-

pression seal type and uses, and specific

applications for these materials. A series

of specific joint details for most commonstructural applications also is provided in

the guide along with information on ex-

posure and service environment and tips

for better performance.

Requirements for satisfacto ryperformance

Selection of a joint sealant for a partic-

ular application is affected by the limita-

tions of the material, the configuration of

the joint, how the joint is constructed, and

access restrictions for sealant installation.

G e n e r a l l y, for satisfactory performance asealant must:

Be an impermeable material

Deform to accommodate the move-

ment and rate of movement occurring

at the joint

S u fficiently retain its original proper-

ties and shape if subjected to cyclical

d e f o r m a t i o n s

Adhere to concrete without failing in

a d h e s i o n

Not fail in cohesion (internally rupture) Resist flow due to gravity, fluid pres-

sures, or softening at higher service

t e m p e r a t u r e s

Not harden or become brittle at lower

service temperatures

Withstand aging, weathering, and other

service factors for a reasonable service

life under the existing environmental

c o n d i t i o n s .

Depending on the specific application,

a sealant also may be required to resist in-

trusion of foreign material, wear, indenta-

tion, pickup by traffic, fire, or chemical

attack. Specifications may require the

sealant to be a specific color, to resist col-

or change, and be nonstaining to the sub-

strate. Sealants must not deteriorate under

normal storage conditions, should be easy

to handle and install, and should be free

of substances that may be harmful to the

Figure 2. Temperature effects on field-molded sealants.


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u s e r, the concrete, or any of the materials

they may contact when installed.

Temperature effects

D i fferences between the temperature at

installation and the temperatures experi-

enced in service affect sealant behavior.

The magnitude of joint movements and

the rate of temperature change aff e c t i n g

those movements are equally important.

Sealants generally perform better at high-er temperatures than at lower tempera-

tures, and when movement at a joint oc-

curs at a slow and uniform rate.

ACI 504R-90 assumes a service range

of temperatures from -20 to +130 F. The

closer the installation temperature is to the

mean of that range (55 F), the less the

strain placed on the sealant. Installation at

the low end of the service temperature

range can cause excessive sealant com-

pression when the temperature rises, and

more sealant is required at installation. In-

stallation at the high end of the servicetemperature range causes excessive sealant

extension and a high likelihood of adhe-

sion, cohesion, or peeling failure. Figure 2

shows behavior of the sealant in these situ-

ations. Whenever possible, therefore, ACI

504R-90 recommends an installation tem-

perature range from 40 to 90 F.

It is important to note that the service

range of temperature affecting the

s e a l a n t s behavior is notthe same as the

ambient air temperature range. It is the ac-

tual temperature of the units being joined

by the sealant. In massive sections orcomplex structural units it may be neces-

sary to measure internal temperatures to

adequately assess joint movement. In any

case, an attempt should be made to collect

appropriate temperature information, in-

cluding a thermometer reading of ambient

temperature in the shade. Take particular

care if the units to be joined are not the

same material and have significantly dif-

ferent surface temperatures due to diff e r-

ences in their material properties.

Preparation of jo int surfaces

Joint surfaces must be clean, dry, and

free of defects, such as loose aggregate,

embedded foreign material, and spalls, that

could impair the sealant bond. Debris from

sawing, wire brushing, routing, and sand-

blasting should be washed out thoroughly.

Solvents are useful in cleaning non-

porous surfaces such as glass or metal

frames, but can carry the contaminants

further into the pores of porous surfaces

such as concrete. Final cleanup to remove

dust is usually best accomplished with the

use of oil-free compressed air or a vacu-

um cleaner.

Avoiding defects

Malfunction of a sealant usually is

caused by cohesive or adhesive failure.

Preformed sealants commonly malfunc-

tion by failing to generate enough con-

tact pressure with the joint faces in cold

w e a t h e r, or by extruding from overcom-

pression in hot weather, as shown in Fig-

ure 3. Field-molded sealants can fail

from repetitive cycles of stress reversal

and suffer one or more of the defects

shown in Figure 4.

Other defects caused by improper joint

construction and the possible causes of

these defects are shown in Figure 5. Rea-

sons for joint sealant failures include:

Design of the joint geometry was in-

s u fficient to accommodate movement

Unanticipated service conditions re-

sulted in greater joint movements than

those allowed for when the joint design

and type of sealant were determined

The wrong type of sealant for the par-

ticular conditions was selected, often

on the false grounds of economy infirst cost

Poor workmanship occurred during

joint construction and preparation to

receive the sealant or sealant installa-

t i o n

Many of these defects can be avoided

by paying attention to the following

g u i d e l i n e s :

Saw or form the joint to the required

uniform depth, width, and location

shown on the plans

Align the joint with any connectingjoints to avoid blockage to free

m o v e m e n t

Correctly position dowels and other

joint hardware, fillers, waterstops, and

bulkheads, and rigidly support them to

avoid displacement during concreting

Remove any temporary material or

filler used to form the sealant reservoir

Figure 3. Defects in preformed compression seals are shown at top; methods toimprove performance for each defect are listed at bottom.


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by raking out or rotary-cutting to the

specified depth

Keep curing compound from contami-

nating joint faces, and apply supple-

mental curing where the original cur-

ing is broken by construction

operations before the joint edges and

faces have fully cured

Typical difficulties with field-molded

sealants often can be avoided with prop-

er use of a bond breaker or backup ma-

terial. Select a bond breaker that doesnt

adhere to the sealant so the sealant is

free to change shape to suit conditions.

A bond breaker isnt necessary where

the backup materials dont adhere to the

sealant. In those cases, backup materials

can be used alone to help support the

sealant, control sealant shape (depth to

width ratio), and allow the sealant to

achieve maximum extension without

peeling at the edges.

MaintenanceFew exposed sealants will last as long

as the structure whose joints they are seal-

ing. Most field-molded or preformed

sealants will eventually require renewal to

maintain an effective seal and prevent de-

terioration of the structure. Exactly when

renewal becomes necessary depends on

service conditions, the type of material

used, and whether any defects were built

in at the time of the original sealing. Re-

sealing often is postponed beyond the

time when it truly is needed due to a lack

of knowledge that it is needed or failure tobudget ahead.

The industry still needs to improve

the working life of joint sealants. The

performance of sealants is continually

compromised by the need to store, ap-

p l y, and use them in less than optimal

circumstances. The result of this is an

expected life of 1 to 5 years for most

current products. Designers looking to

minimize maintenance would like to see

high-performance sealants with life cy-

cles of 10 to 20 years.

Reference

Guide to Sealing Joints in Concre t eStructures, (ACI 504R-90), ACI Manu-al of Concrete Practice, 1992, Ameri-can Concrete Institute, Detroit.

Marilyn Palmer is a contributing editor

and re g i s t e red professional engineer. She

holds B.S. and M.S. degrees in civil engi-

neering and has been an engineering edi-

tor with the American Concrete Institute.Figure 5. Random cracks in joint construction and possible causes. A, B, and Care cross-sect ional views; D, E, F, and G are plan views. Note that movement willoccur at the crack, not the intended joint, so if the joint is not repaired, it is thecrack that should be sealed.

Figure 4. Defects with elastic behavior in field-molded sealants are shown at top;methods to improve performance for each defect are listed at bottom.

P U B L I C ATION #C920799

Copyright 1992, The Aberdeen Group

All rights reserved

How to Seal Joints in Concrete Structures_tcm45-342478

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