WHY JOINTS ARE NEEDED

This paper focuses on the need for and how to install control (relief) joints in concrete. To a

limited extent, other joint types (expansion, isolation and construction) are discussed in

order to describe the difference between a control joint and other common joints employed

in the concrete industry.

There is movement in structures; regardless of size, height, width, the structure moves. To

accommodate or cushion structural movement, there is need for elastic joints at varying

strategic locations throughout the exterior of a building. In addition to the problem of

potential torsion, seismic, or vibration stresses, the dimension and location of joints are

directly related to the tolerances and thermal movement characteristics of various substrates

that make up the structure, potential shrinkage, and design aesthetics.

Concrete is normally subject to changes in length, width, depth or volume caused by

changes in its moisture content and/or temperature, reaction with atmospheric carbon

dioxide, loads (dynamic and/or static) and other forces. Joints are a designed feature,

because of the dimensional changes concrete goes through to allow for movement due to the

following:

I. Control

1. Drying Shrinkage

2. Carbonation

3. Irreversible creep

II. Cyclical Contraction

1. Environmental Differences (Humidity, Moisture Content and

Temperature)

2. Application Loads (Expansion or Contraction)

III. Abnormal Volume Changes

1. Permanent Expansion

a. Sulfate Attack

b. Alkali Reaction (between cement and certain aggregate)

The results of these changes are movement (permanent and/or transient) of the concrete

element.

STRUCTURAL DESIGN REQUIRING JOINTS

I. Structures not under fluid pressure (most civil-engineered projects)

II. Containers subject to fluid pressure (dams, reservoirs, tanks, pipe

linings)

III. Pavement highway and airfield

TYPES OF JOINTS AND FUNCTIONS

Control: When contraction forces associated with curing shrinkage and movement

associated with thermal actions or mechanical loads are restrained, then

cracking will occur within concrete when the tensile stresses exceed the

strength of the concrete. Joints and cracks will open up and become wider as

the concrete contracts (shrinks). Concrete is cut during the placement

process to help “control” where these cracks will occur.

Expansion: If expansion movement is restrained, it may result in distortion and cracking

within the unit or crushing of its ends and transmission of unanticipated

forces to the abutting elements. Joints and cracks will be closed and the

forces will cause spalling if objects preclude the closing. Expansion joints

are placed throughout the structure to accommodate this planned and

continuous dimensional change.

Deflection: When deflection (torsion, flexural, etc.) movement stress is anticipated that

may exceed the materials structural design strength limitations, isolation

joints are employed.

I. CONTROL (RELIEF) JOINTS

Control joints are saw cut, tooled, formed; or a bond breaker (plastic or metal strip) is added

to provide a weakened plane. They are designed to regulate and control shrinkage crack

locations that normally occur in concrete segments. Since the joint is expected to control the

location of crack, these joints are often referred to as control or (stress) relief joints. Without

the control joint, tensile stress induced cracks would occur at unpredictable locations,

thereby relieving the concrete of build up internal stresses.

They are frequently used to divide large, relatively thin, structural units or sections, for

example:

• Pavement

• Floor slabs

• Canal linings

• Retaining walls

Control joints form a complete break, which in the case of floor slab, the joint is designed to

go completely through the unit, allowing each floor slab to function independent of the other.

They can also be designed to not act isolated from the adjoining floor slab. If the control

joint is saw cut or tooled to one quarter of the floor slab thickness (and the joint is not wide)

there may be aggregate interlock, perhaps coupled with wire mesh restraint. Where greater

continuity is desired from floor slab to floor slab, dowel (usually slip bars), stepped or keyed

joints may be employed. To protect the floor slab contraction joint from the deleterious

effect of hammer loads (impact from small wheeled carts or vehicles) it is necessary to fill

the joint with a semi-rigid stress relieving epoxy or polyurea material expressly designed to

reinforce joint nosing to prevent spalling and raveling.

NOTE: Semi-rigid resin system should comply with ACI 302.1R-15

II. EXPANSION (ISOLATION) JOINTS

Expansion joints (also referred to as expansion contraction joints) are used to isolate one

structural element from another to prevent crushing and distortion, such as displacement,

buckling and warping. They are sometimes called isolation joints because they are used to

isolate structures that behave in different manners. For example: they are used to isolate

abutting concrete structural units that might otherwise cause distress in one or both of the

units due to transmission of compressive forces that develop during expansion, under

applied loads or differential settlement.

Isolation joints are used primarily to isolate walls from floors or roofs, columns from floors

or cladding, and pavement slabs and decks from bridge abutments - thus the name "isolation

joint".

Where greater continuity is desired from one structural unit to the next (floor slab to floor

slab or floor slab to stem wall) reinforcing bars or dowels, stepped or keyed joints may be

employed.

To protect and fluid proof the joints (prevent egress of fluids in or out of the structure) when

movement will occur require the use of a flexible joint filler (sealant or assemblage). This

material must be capable of accommodating the anticipated movement between the

structural units. High elongation, Elastomeric urethane, silicone or polyurea materials are

frequently used for this application.

NOTE: Elastomeric (urethane, silicone, etc) joint sealants should comply with ACI 302.1R-

15 and ACI 504.

III. CONSTRUCTION (INTERRUPTION) JOINTS

Construction joints may be planned or unplanned. Planned construction joints are

incorporated into the structural units for several reasons, such as precast elements, length

restriction or during a concrete pour due to configuration or "trick" form placement

requirements. Planned construction joints can be called upon to function as expansion joints

to accommodate the normal or even radical movement of a structure. Planned construction

joints are treated in a similar fashion to expansion joints listed above.

Unplanned construction joints usually occur due to unforeseen concrete placement

difficulties or forming restrictions. In the case of unplanned and unwanted construction

joints due to unforeseen interruption of concrete placement, an epoxy injection adhesive can

be used to bond the units together,, providing a monolithic structural unit as originally

designed, by permanently welding the unit together at the construction joint.

NOTE: Epoxy injection adhesive should comply with ACI 503 and ASTM C 881-87 Type

IV.

1.0 CONTROL JOINT DESIGN CONSIDERATION

Joint fillers are formulated to reduce or prevent the deterioration of industrial

floor joints subjected to impact and point loading from steel and hardwheeled

vehicular traffic. The semi-rigid epoxy or polyurea joint grouts are

designed to support traffic while providing a low degree of flexibility for

joint movement.

1.1 Joint Fillers: Semi-rigid epoxy or polyurea joint grouts were specifically

developed to fill control (relief) joints and inductive loops in concrete floor

slabs.

Caution: A semi-rigid epoxy or polyurea joint fillers, in most cases, should

not be used if the joint to be repaired is an engineered expansion and/or

isolation joint, or is otherwise working or moving. The designer or owner

may waive this caution. The benefits of reinforcing the joint, out weigh the

effects of a small stress crack which may develop between the epoxy joint

filler and one side of the concrete joint or as a cohesive failure within the

joint filler itself.

Semi-rigid epoxy and polyurea joint fillers are formulated to provide a joint

grout material with a tough resilient wearing surface capable of

accommodating limited joint movement. Separation of the joint filler from

either side of the joint or internal cohesive hairline cracking does not

necessarily indicate failure of the semi-rigid epoxy joint filler application.

Further, curing shrinkage after the joint has been filled, or other contraction

movement may exceed the stress-relieving capabilities of the joint filler,

leading to cracking or splitting. When separation does occur, actual inservice

conditions will determine whether or not further treatment is required.

1.2 Temperature Changes: The upper and lower service temperature limits must

be considered. If the slab will be exposed to thermal cycling, freeze/thaw or

extreme seasonal variations in temperature, or if there are other special

conditions (freeze rooms, etc.), the concrete will be expected to realize

greater movement. Consult with the joint filler manufacturer to determine

the best material for use in these situations..

1.3 Construction Sequence: Construction sequence or joint filler installation

sequence will require a compromise between working and curing time. A

fast curing product, such as polyurea, has a short working time, the

advantage is that the floor can be put back into service sooner than a product

that is slow to cure, such as epoxy. Corresponding longer working time

products may be easier to work with, but they are slower to cure. Sufficient

cure prior to exposure to traffic is necessary to insure against costly repairs

and additional downtime in the future.

2.0 MATERIAL CONSIDERATION

2.1 Application Characteristics: All epoxy joint fillers change their handling

characteristics when they are conditioned to the prevailing ambient

temperature fluctuation. At low temperature they become more viscous (less

fluid) and, unless they are heated, often time more difficult to apply. High

temperature causes a decrease in viscosity and a reduction in non-sag

properties. Polyurea joint fillers have a broader range of cure temperatures

and can be placed at temperatures well below freezing.

It is important to determine the application temperature range and select a

product with handling characteristics suitable for that range. Use of more

than one product may be required to accommodate a wide temperature range

associated with year-round work.

2.2 Curing Characteristics: Working time and cure times are affected by the

ambient and substrate temperatures.

Working Time: Pot life and open time are the two elements, which make up

working time.

Pot Life: Pot life is the time a predetermined quantity of mixed product is

workable in the mixing vessel just prior to gel. Elevating the material's

temperature and/or increasing the volume of the material mixed will

decrease its pot life.

Cure Time: Cure time or cure rate accelerates with an increase in ambient

and surface temperature. There is a minimum temperature below which

formulations will cease to cure and/or cure at a rate that is too slow for the

intended use.

2.3 Cure Characteristics: The bond strength of a joint filler to the concrete

surface is dependent on the surface preparation, substrate durability at the

interface, the bonding ability of the joint filler material itself and the sand

loading, if any. The resin’s ability to bond can be formulated to tolerate both

dry and damp surface conditions, however, for best results the substrate

should be clean and dry.

2.4 Toughness: To maintain integrity in use, joint fillers must be tough and

impact resistant to prevent gouging, chipping and spalling from steel and

hard rubber wheeled vehicular traffic and other abusive conditions.

2.5 Chemical Resistance: The resistance to chemicals is dependent on the

inherent resistance of the resin and hardener formulation, the temperature

and duration of exposure and the integrity of the application. If chemical

resistance is an important design consideration, the installed epoxy joint

filler should be free of pinholes, holidays, dishing and other defects, as well

as having resistance to the specific chemical(s) that is required to withstand

(consult the material manufacturer for chemical resistance details).

Polysulfide and fluroelastomeric resins can be used for high chemical

exposure applications.

3.0 SURFACE PREPARATION

3.1 General: Joints to be filled must be clean and sound, if a bond is desirable.

In all cases this will require some form of surface preparation.

3.2 Contaminants: The presence of grease, wax, or oil may be detected by

dropping a small amount of muriatic acid onto the surface. No reaction or a

little reaction ind icates that the surface is contaminated. Oil penetration of

the concrete surface can also be detected by raising the temperature of a

an oil film appears or if the surface becomes greasy to the touch.

3.3 Cleaning Procedures:

1. Grease, wax and oil contaminants can be removed by scrubbing with

an industrial grade detergent or degreasing compounds, followed

with mechanical cleaning. Severely contaminated joints may be saw

cut with a blade slightly oversized.

2. Weak or deteriorated concrete must be removed to sound concrete by

bush hammering, needle scaler, abrasive grit blasting, vacuum shot

blasting, scarifying, water blasting or other suitable mechanical

means.

3. Dirt, dust, laitance, form release agents and curing compounds

should be removed by water cutting or abrasive grit blasting.

Caution: Acid etching (15% solution of hydrochloric acid) is

recommended only when there is no practical alternative. Etched

surfaces must be thoroughly scrubbed and flushed with a large

volume of potable water. A moist pH paper reading of 10 or more

will indicate that the acid salts have been removed.

4. Dust residue from mechanical cleaning may be removed by

vacuuming, water jet or by clean, oil free high pressure air.

4.0 APPLICATION TECHNIQUES

4.1 General Installation Techniques of Joint Fillers: Prior to installing the

joint filler, the joint surface must be dry and free of all substances

detrimental to the bonding of the polymeric compound. Concrete

should be prepared in accordance with the procedures outlined in

Section 3. (If "cannot dry" conditions exist, contact the manufacturer

for product and/or procedural recommendations, including special

installation techniques, if any.)

4.2 Installation Contractor: Only installation contractors who are

experienced specialty contractors should install the joint filler.

Contractors with control joint grouting experience and competent

trained application personnel will provide installation services that

meet the owner’s needs. Contractors with limited experience or no

previous experience in placing joint fillers should contact the

manufacturer for application assistance or subcontract the work.

Contractors with limited experience should start out in a small area,

mixing only enough material to complete a few feet of joint.

The installed material should then be allowed to cure before

attempting any further installation work. If the test is successful, the

contractor can proceed at an increasing application rate until reaching

maximum productivity. If the trial application fails for any reason,

contact the manufacturer for additional application assistance.

4.3 Rebuilding Joint: In

cases where a joint has

been exposed to traffic

resulting in deterioration

of the edges, the nosing

must be rebuilt prior to

placing the joint filler.

Clean and prepare the

concrete and fill with a

rapid setting polymer

modified concrete or

epoxy mortar to repair

the nosing.

4.4 Material Preconditioning:

To facilitate speed of

cure, mixing and

application of the epoxy

joint filler, it may be

preconditioned above

ambient temperature.

Uniform preconditioning will require approximately 24 hours (in

CAUTION: While preconditioned material may facilitate ease of

installation, higher material temperature accelerates gel time and

shortens pot life and corresponding working life.

4.5 Mixing: Joint filler materials must be thoroughly mixed to disperse

pigments and fillers, which may have settle during storage. To ensure

proper cure, it is important that all components of the joint filler are

carefully measured and adequately mixed per the product technical

data sheet.

CAUTION: Do not add solvents unless that procedure is specifically

recommended on the technical data sheet.

4.6 Method of application: Epoxy joint fillers can be installed with a

wide range of application tools, such as a deformed or "v" bent tin

can. Both epoxy and polyurea joint fillers are most efficiently

applied using state-of-the-art plural component metered mixing

equipment.

4.7 Installation Procedure: Installation procedures vary based on job

conditions and equipment available.

Prepare the joint: Prepare the joint per the instructions listed above

and per the manufacturer's recommendations.

Always thoroughly mix each component separately to ensure proper

pigment and filler dispersion, before mixing the components together.

If sand is to be added as a third component (for slower setting epoxy

only), it must be added only after a complete and uniform mix of the

resin and hardener has been obtained.

Joints with exceedingly wide "through cracks" at the base should be

filled to 1/8" of #70-90 mesh U.S. Sieve size fine aggregate poured

directly into the control joint just prior to placement if the material to

reduce epoxy joint filler material loss.

If the crack is wide enough to allow fluid outflow, a second epoxy

grout pass may be required. Allow the first application to gel prior to

placing the second application.

Fill the joint to the top and crown if conditions permit. The overfilled

or crowned grout will allow for shrinkage to occur and any

remaining crown material will be worn flush by vehicular traffic or

“shaved” to a flush level.

Uneven or Sloped Slabs: Most joint filler materials will attempt to

self- level. When a self- leveling material is not desired, a high build

series of joint fillers should be specified to reduce the materials

tendency to self-level. Under severe sloped conditions, the use of

high build material alone may not be sufficient to avoid excessive

flow of the epoxy joint filler from the joint. Under these conditions,

it may be necessary to use the high build series in conjunction with

multi-pass applications to accomplish full-depth, level grouting,

while avoiding excessive material loss.

5.0 OPEN TO TRAFFIC

5.1 Open to Traffic: Ideally, traffic should be kept off of the grout

material until full cure is reached. However, access can usually be

granted to moderate traffic prior to full cure. The actual time when

the slab can be "opened to traffic”: will vary based on reactivity of

the grout installed and the temperature of the host substrate. Open to

traffic time, in most cases, will run between 4-24 hours for epoxies

and less than one hour for polyurea materials.

5.2 Early Access: If access must be granted to the owner or operator

prior to the joint filler reaching a tack free state, broadcast a 70-90

mesh aggregate, to excess, on the exposed joint filler to reduce

tracking of the material. In most cases, premature access will,

however, void the warranties and guarantees offered by the

manufacturer for that area where the access is required.

6.0 PHYSICAL PROPERTIES

6.1 Semi-Rigid Epoxy and Polyurea Joint Fillers: semi-rigid joint fillers

should possess these physical properties:

1. Long term adhesion to the faces of the joint

2. Resistance to creep, slump or cold flow

3. Resistance to shrinkage

4. Non-bleeding and non-staining properties

5. Adequate elastic properties to accommodate movement

without splitting

6. Resistance to aging

7. Compatibility with floor and overlay design

8. Resistance to specified chemicals

9. Adequate hardness and abrasion resistance

10. Retention of physical properties

11. Stability in storage

12. Ease of mixing

13. Ease of installation

6.2 Reasons for Failure: Prior to replacing semi-rigid joint filler the

reason for a pervious joint failure should be determined.

1. Unsuitable joint filler specified and installed

2. Joint dimensions that do not match specification and/or

product capability

3. Joint movement is greater than anticipated

4. Incorrect mixing of multi-component material

5. Improper application of material

6. Lack of chemical resistance