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CHAPTER 9:PRINCIPLES OF JOINTS AND SEALANTS

Expansion and Contraction Control Expansion & Contraction Control – Introduction

Principles of Joints and Sealants

Joints in Buildings

(a) Joints are a common feature in buildings, particularly on building facades. Some of these joints are aesthetic joints to provide surface relief, rhythm, patterns, and so on, while others are provided for performance reasons related to building design and (or) construction. (b) The fasteners used in the joint in the copper cladding of Burton Barr Central Library, Phoenix, Arizona, contribute to the rhythm and pattern in the cladding. (c) The textured surface and the bold horizontal lines in the exterior concrete walls of the same library building relieve the monotony of a large concrete surface.

Static and Movement Joints

Static joint – a nonmoving joint (connection) between the components. In the case of the window glass and frame, a static joint

would most likely result in broken glass, broken frame, or both.

Dynamic joint or movement joint – a connection that allows both materials to move independently and at the same time hold the window glass securely in the frame.

Expansion & Contraction Control – Types of Movement Joints

Principles of Joints and Sealants

Types of Movement Joints

Based on the purpose they serve, movement joints are classified as Building joints —joints between different parts of the

building as a whole

Component joints —joints between individual components of an assembly

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Types of Movement Joints – cont’d

A building joint divides the entire building into two or more separate buildings.

Based on the purpose for which it is provided, it is classified as either a building separation joint or a seismic joint .

A joint between two adjacent building components can be an expansion joint , a contraction joint (commonly called a control joint), or an isolation joint .

Types of movement joints in a building.

Expansion & Contraction Control – Building Separation Joints and Seismic Joints

Principles of Joints and Sealants

Building Separation Joints

A building separation joint prevents the stresses created in one part of the building from affecting the integrity of the other part. Needed in large buildings.

Rough guide: Provided at 250-ft (75-m) intervals

Provide building separation joints at the following discontinuities:

Where a low building mass meets a tall mass.

Where the building changes direction, such as in an L-shaped or T-shaped building.

Where the building’s structural material changes, such as where a steel frame building meets a concrete frame.

Building Separation Joints – cont’d

A building separation joint should divide the building into smaller, structurally independent sections (as if they are separate buildings) so that each section is geometrically simple.

Building Separation Joints – cont’d

A building separation joint runs through the entire building from the ground floor (or the basement, if provided) up to the roof. Two columns are often used at the separation joint. However, a combined footing may be used for the two adjacent columns.

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Building Separation Joints – cont’d

Building separation joint with two columns in a reinforced concrete building. Because the building is under construction, daylight is showing through the joint between the columns. In a completed building, the joint is sealed and covered with a joint cover, as is the case here for the separation joint in the floor.

Building Separation Joints – cont’d

Building separation joint with two columns and two beams in a steel frame building. Because the building is under construction, daylight is showing through the joint between the columns. In a completed building, the joint is sealed and covered with a joint cover.

Building Separation Joints – cont’d

Building separation joint using a single column in a reinforced concrete building. Observe the provision of a beam on each side of the separation joint.

Building Separation Joints – cont’d

Building separation joint in a steel frame building using a single column. Because the building is under construction, daylight is showing through the separation joint. In a completed building, the joint is sealed and covered with a joint cover. Observe the provision of a steel joist on each side of the separation joint.

Seismic Joints vs. Building Separation Joints

A seismic joint is similar to but also different from a building separation joint. Building separation joints accommodate moderate building

movements caused by temperature and moisture changes, creep, and foundation settlements.

Building separation joints are designed to accommodate mainly horizontal movement, that is, in the direction perpendicular to the joint.

Seismic Joints

The purpose of seismic joints is to ensure that one section of the building does not collide with the adjacent section during an earthquake.

A seismic joint must accommodate simultaneous movements in horizontal as well as vertical directions, that is, in all three principal directions.

A seismic joint is generally much wider than a building separation joint.

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Covers Over Building Separation or Seismic Joints

A typical detail of a building separation joint cover at the floor level. Similar covers are provided in interior and exterior walls and roofs. Joint cover manufacturers’ details and specifications should be strictly followed for structural, fire rating, and other performance reasons.

Expansion & Contraction Control – Movement Joints in Building Components

Principles of Joints and Sealants

Movement Joints in Building Components

Movement in a building component can be caused by several phenomena.

Factors that must be considered in sizing and detailing movement joints in building components: Thermal movement

Moisture movement

Elastic deformation and creep

Construction tolerances and other considerationsX indicates the presence of particular movement in the material and—indicates the absence of such movement.

Importance of Movement Joints in Building Components

Importance of Movement Joints in Building Components – cont’d

(a) An acrylic sheet in an un-deformed state. (b) Deformation of the same sheet (nearly 4 ft long) shortened by approximately1/16 in. at each end by placing 25-cent coins. Note that the (lateral) deformation of the sheet (approximately 2 in.) is much largerthan the longitudinal shortening (approximately 18 in.).

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Expansion & Contraction Control – Thermal Movement

Principles of Joints and Sealants

Thermal Movement

Eq. (1)

Building Construction: Principles, Materials, & Systems, 2e

Mehta, Scarborough, and Armpriest

© 2013, 2010, 2008 by Pearson Higher Education,

Upper Saddle River, NJ 07458. All Rights Reserved.

Thermal Movement – cont’d Coefficient of Thermal Expansion ( )

Annual Temperature Change (∆t)

Annual temperature change (∆t) to which a building component is subjected in the United States.

Estimating Thermal Movement– Example 1

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Estimating Thermal Movement– Example 1 - cont’d

Estimating Thermal Movement– Example 2

Expansion & Contraction Control – Moisture Movement

Principles of Joints and Sealants

Moisture Movement

Portland cement–based materials shrink due to moisture loss.

Because the moisture loss continues for several weeks after construction, portland cement–based components keep shrinking long after they are placed in a wall or slab.

Therefore, concrete slabs, concrete and concrete masonry walls, and stucco require shrinkage joints—more commonly referred to as control joints .

Moisture Movement in Concrete and Masonry

Clay (brick) masonry units expand on absorption of water or water vapor. This expansion, which occurs during and after construction, is irreversible; that is, clay masonry does not shrink on drying.

In fact, a brick unit is smallest in size when it comes out of the kiln, after which it grows in size.

Thus, brick masonry walls require expansion joints .

The amount of moisture movement in concrete and masonry is given by

Eq. (2)

Moisture Movement in Concrete and Masonry

The amount of moisture movement in concrete and masonry is given by

= moisture expansion or contraction of a unit of length L and µ is the moisture coefficient of the material. The values of µ are given in Table 9.2 . Note that µ has no units. It is simply a number.

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Building Construction: Principles, Materials, & Systems, 2e

Mehta, Scarborough, and Armpriest

© 2013, 2010, 2008 by Pearson Higher Education,

Upper Saddle River, NJ 07458. All Rights Reserved.

Moisture Movement in Wood

The microstructure of wood consists of hollow cellular tubes bundled together.

The walls of the tubes shrink and swell, depending on the ambient air’s humidity and temperature, changing the dimensions of a wood member.

Estimating Moisture Movement– Example 3

Estimating Moisture Movement– Example 4

Expansion & Contraction Control – Elastic Deformation and Creep

Principles of Joints and Sealants

Elastic Deformation and Creep

Instantaneous (elastic) deflection (A) and creep (inelastic) deflection (B) of a beam. The dashed lines show the unloaded condition of the beam. Creep is the incremental deformation of a component caused by sustained loading.

Building Construction: Principles, Materials, & Systems, 2e

Mehta, Scarborough, and Armpriest

© 2013, 2010, 2008 by Pearson Higher Education,

Upper Saddle River, NJ 07458. All Rights Reserved.

Deflection of Spandrel Beam

If a non-load-bearing wall is placed directly under a spandrel beam, an adequate space between the beam and the top of the wall should be provided to allow the beam to deflect under the load.

If adequate space is not provided, the load from the beam will be transferred to the wall, damaging it.

The connection between the spandrel beam and the non-load-bearing wall below must account for the beam’s deflection.

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Expansion & Contraction Control – Total Joint Dimension

Principles of Joints and Sealants

Total Joint Dimension

Factors to be considered when determining joint dimensions include:

Thermal, moisture, elastic, and creep deformations

Material tolerances (i.e., building components vary from their specified dimensions).

Construction tolerances

Construction tolerances are a function of quality control during construction.

Eq. (3)

Width of joint =with based on temperature,

moisture, and other movements

+ tolerances

Effect of a Sealant on Joint Dimension

Most joints in building envelope components are filled with elastomeric sealants.

Sealants have a finite ability to cyclically stretch and compress, which is given as the plus-minus movement ability of the sealant.

The plus value denotes the maximum stretch the sealant can withstand, and the minus value denotes the maximum compression it can sustain.

Sealants are generally classified based on their movement ability.

A sealant with a movement ability of ±25 is called a class 25 sealant . Such a sealant is able to stretch and compress by 25% of its installed dimension. Similarly, a sealant with a movement ability of ± 50 is called a class 50 sealant.

If the width of a joint at the time the sealant is applied to it is 1 in., a class 50 sealant will safely stretch to 1.5 in. or compress to 0.5 in.

The movement ability of sealants must be included in determining the total joint width of a sealed joint. Therefore, the total joint width of a sealed joint is given by

Eq. (4)

Joint width of sealed joint =100

[joint movement] + tolerancesSealant class

Estimating Total Joint Width Between Components – Example 5

Expansion & Contraction Control – Principles of Joint Detailing

Principles of Joints and Sealants

Principles of Joint Detailing

In detailing a movement joint in a component, it is important to distinguish between an expansion joint and a control (shrinkage) joint .

Because the joint width in an expansion joint will become smaller over time, the filler in an expansion joint (if used) must be elastomeric to allow unrestrained movement of the components.

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Detail of an Expansion Joint

Detail of an expansion joint in a brick veneer wall. The joint is filled with a compres-sible backer rod and an elastomeric sealant.

Detail of a Control Joint

Detail of a control joint in a concrete masonry wall.

Expansion & Contraction Control – Components of a Sealed Joint

Principles of Joints and Sealants

Components of a Sealant Joint

The primary purpose of sealing a joint is to prevent water penetration.

However, control of air leakage, dust penetration, and noise transmission are additional benefits of sealing a joint.

An unsealed joint may also get filled with incompressible materials over time, resulting in joint failure.

Although joint sealant is the most important component of a sealed joint, these other components must also be carefully selected:

Substrate

Primer

Sealant backup

Bond breaker

Un-tooled and Tooled Sealants

Un-tooled and tooled sealants in joints. Note that the depth of the sealant must be at least half the width of the joint.

Bond Breaker

For the sealant to function effectively, it should be bonded to only two opposite surfaces of the substrate so that it comes under axial tension or axial compression when the joint moves.

A bond breaker is required only if the third surface is hard and unyielding, such as concrete, metal, masonry, or any other inflexible backup.

A bond breaker is not required with a flexible backup that will not significantly restrict the freedom of sealant movement.

A bond breaker is not required in a conventional joint with a foam backer rod.

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Use of a Bond Breaker

An ideal sealed joint should only be bonded to two opposite surfaces so that the sealant is subjected to either tension or compression. A bond between the sealant and the third (in this case, the rear) surface may cause sealant failure, as shown in (b), because of the adhesion of sealant to the third surface. Therefore, where a third surface exists in a sealed joint, a bond breaker should be used, as shown in (c).

Expansion & Contraction Control – Types and Properties of Joint Sealants

Principles of Joints and Sealants

Types and Properties of Joint Sealants

Joint sealants may be divided into three categories: Preformed tapes

Caulks

Elastomeric sealants

Types of Elastomeric Sealants

Elastomeric sealants are synthetic materials (polymers) and are the ones most commonly used in contemporary construction.

The five commonly used synthetic sealants are1. Polyisobutylene

2. Acrylics

3. Polyurethane

4. Polysulfide

5. Silicone