1 Introduction

Well detailed and constructed joints play a vital partin maintaining the integrity of the external envelopeof the building, ensuring it is weatherproof andmeeting any other requirements such as fire-resistance and acoustic performance.

This Practice Note provides guidance on planningjoint locations, gives requirements for joint types,widths, the choice of sealant and discusses the firerating of joints.

For the purpose of this Note, a joint is an intentionalgap between adjoining elements (typically cladding)or between an element and some other portion ofthe structure. Joints may be horizontal, vertical orinclined.

The function of a joint between precast elements isto provide physical separation between the units and,in conjunction with joint sealants, prevent the ingressof water and air into the building; and, if required, fireresistance.

Two aspects of joint selection need to beemphasised:

■ The positioning of joints in relation to windowsand to the structure can affect the serviceability,construction and maintenance of the buildingenvelope. Poor joint location will lead to problemswhich cannot be overcome by joint detailing (seeFigure 1)

■ Careful control of construction tolerances isnecessary to ensure the integrity of the claddingsystem.

2 Design2.1 General

It is recommended that joints be treated as a strongvisual feature of architectural wall design. Recessing ofjoints and/or sealants will help diminish the visualimpact of possible variations between adjacentsurfaces.

The following general aspects need to be addressed

■ Buildability and minimum sizeSelect details that are simple to fabricate andinstall on site. Proven details should be usedwherever possible.

■ Maintenance and repairAlthough modern sealants have a long service lifethey, if exposed to sunlight, will eventually needreplacement or repair. Access for repair andreplacement must be taken into consideration inthe design of the building.The positioning ofservices or other features in front of joints willmake future access difficult. Consideration must begiven to the fact that inspection and repair willusually have to be made from the exterior of thebuilding.

2.2 Number, location and width of joints

The key points are (see also Figure 1):

■ For maximum economy in manufacture anderection, panels should be as large as practical.

■ If architectural requirements dictate more-closely-spaced joints, false joints can be used to achieve asimilar visual effect.

Precast

Practice

NOTES

NATIONAL PRECAST CONCRETE ASSOCIATION AUSTRALIA

PPN 1JUNE 2003

1 This Note is a summary of information from the Precast Concrete Handbook, Z48.For more information or purchase, refer to National Precast Concrete Association Australia web site: www.npcaa.com.au

Joints in PrecastConcrete Buildings1

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PPN 1 Joints in Precast Concrete Buildings

■ Weathering of the building facade can becontrolled to a large extent by careful jointlocation.

■ Recessing the sealant in the joint, or use of anopen-drained system, will minimise concentratedrainwater runoff and water-stain patterns.

■ A nominal joint width of 20 mm will usually besatisfactory for most conditions and is therecommended design starting point.

3 Types of Joints3.1 General

The most common types of joint between precastconcrete cladding and/or wall panels are:

■ open-drained;

■ face-sealed; and

■ compression-seal.

Their advantages and disadvantages are summarisedin Table 1.

Dummy joint

DO NOT curtail vertical dummyjoint as this will lead to staining

DO NOT stagger vertical(or horizontal) joints as thisleads to staining and stresson sealants.In some cases it can lead tostress on the panel withpossible cracking of thepanel

AVOID this configuration as itcan lead to potential waterstaining and leakage problems

Vertical jointbetween panels

Vertical jointbetween panels

Figure 1

Location of Joints

Table 1

Advantages and Disadvantages of Joint Types

Joint Type Advantages Disadvantages

Open-drained Can tolerate relatively large movements.The rear sealant is protected from UV lightand weather.Can be installed from inside the building(no scaffold required).Long maintenance-free life.Best for medium- and high-rise construction.

Face-sealed Panel edges can have simple profile,no grooves required.Can be used for complex panel shapes(angled or curved).Can have a rear seal as a second line of defence.Lowest first cost.Can be readily inspected, repaired or replaced.Best for low-rise construction.

Compression-seal Simple and quick to install.Panel edges can be plain or simple profile.Economical.

Careful supervision is required during installation as itis difficult to remedy defects due to poor workmanship.

Not suitable for tall vertical panels (> 9.0 m in height).

Cannot accommodate joint gap tolerances > 5 mm.

Must be applied from external scaffolding or other formof access.Sealant is exposed to UV light and weather –needs more maintenance.In a single-seal system even a small failure may allowwater penetration due to capillary effects and pressuredifferentials.

Cannot be fully weatherproof, so limited to low-riseindustrial buildings.Joint width is critical.Maintaining compression on seal at intersection ofhorizontal and vertical joints is difficult.Difficult to maintain and/or replace.Time-consuming while erecting.

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PPN 1 Joints in Precast Concrete Buildings

3.2 Open-drained joints

The open-drained joint is recommended for high-riseconstruction. It consists of a rain barrier in the formof an expansion chamber with a loose-fitting baffleand an air-seal at the interior face of the panel (seeFigure 2).

The baffle prevents direct entry of the wind-drivenrainwater.The pressure in the chamber between thebaffle and the internal air seal is at external airpressure.There is, therefore, no pressure differentialto drive rain past the baffle.The air-seal is thedemarcation barrier between outside and internal airpressures.

Water that enters the joint in front of the baffle isdrained downwards. At every intersection betweenthe vertical and horizontal joints, a short length offlashing (300 mm) is used to ensure water isdischarged to the outside.

Figure 2

Design and Construction of Open-Drained Joints

Install neoprene baffle strip after erection of next level of panels.See lapping detail on right for treatment at horizontal joints

Top baffle

Bottom bafflelapped underflashing

Air-seal

Minimum20

Minimum20

Horizontal air-seal‡

‡ Air-seal may be asealant with backing rod,closed-cell sponge orsquare neoprene strip

Vertical air-seal‡

Flashing

Drainage zone, 50 mm minimum

300

Upstand*

Minimum 120Preferred 150

* 50 mm typical,75 mm in exposedlocations

Fix flashing in placewith flexible seal under back and sides

Open-Drained Joints are the RecommendedType for most Medium- to High-RiseConstruction.

Open-Drained Joints can Tolerate LargeMovements and the Rear Sealant is Protectedfrom UV Light

Open-Drained Joints can be Installed fromInside the Building.

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PPN 1 Joints in Precast Concrete Buildings

3.3 Face-sealed joints

These joints are simple, economical and are mostsuited to low-rise construction (see Figure 3).Theyare sealed by a single run of gun-applied sealant closeto the exterior surface of the joint. A backing-rodforms the rear of the sealant.The external face sealshould, where practical, be supplemented by a sealnear the inside face of the panel.

3.4 Compression-seal joints

This type of joint utilises a compressible impregnatedpolyethylene or polyurethane foam strip.The strip ispre-compressed and inserted into the joint after thepanels are erected or it is glued in position beforeplacement of the second panel, Figure 4. It thenexpands to fill the joint.

The use of this type of joint seal is usually limited tolow-rise buildings such as factories and warehouseswhere wind pressures are low. It can be used wherespandrel beams, downturns or columns restrict theaccess required for placement of gun-applied sealants.

3.5 Hollowcore wall joints

Hollowcore wall units are primarily used on low-risecommercial and industrial buildings.The jointsbetween panels are normally 10 mm wide and aresealed with a two-part polyurethane sealant placedagainst a closed-cell backing rod.

Figure 4

Compression-seal Joints

A A

B

B

VERTICAL FORMAT

SECTION A-A SECTION B-B

HORIZONTAL FORMAT

Compression seals gluedto edge of one panel

Figure 3

Face-Sealed Joints

Minimum 15Preferred 20Maximum 30

Minimum 15Preferred 20Maximum 30

Sealant withbacking-rod

Sealant withbacking-rod

Rear sealant optional

Rearsealantoptional

50

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PPN 1 Joints in Precast Concrete Buildings

4 Joint Sealants4.1 General

The choice of sealant and its specification, should bediscussed with sealant manufacturers.

Factors to be considered by the designer whenchoosing a suitable sealant material include:

■ The sealant should be impermeable to water.

■ It should have a low elastic modulus toaccommodate strain due to joint movementwithout significant stress, with the shape of thesealant influencing the stress in the sealant.

■ It should be able to recover its original shape aftercyclic deformation.

■ It must bond firmly to the joint face without failingin adhesion nor splitting or peeling under theanticipated joint movements.

■ It must not soften or flow at higher servicetemperatures and should not harden and becomebrittle at low temperatures.

■ It should not be adversely affected by ageing orweathering and should be stable when exposed toUV light.

■ For face-sealed joints the sealant should have astable colour, be non-staining and resistant topickup of dirt.

4.2 Sealant types

Field-moulded sealants are available in the followingtypes:

■ Polysulphide sealants (two-part)

■ Polyurethane sealants (one- or two-part)

■ Acrylic sealants

■ Butyl sealants

■ Silicone sealants.

Note, silicone sealants should be avoided wherepossible as they stain the concrete surface andcannot be painted.

4.3 Joint design and sealant application

To ensure the joint and sealant give satisfactoryperformance, the following points should be noted(see also Figure 5):

■ Correct joint preparation

■ Correct sealant-backing systems

■ Correct joint geometry

■ Sufficient curing time.

5 Fire Rating of JointsExternal cladding may be required to have a specifiedFire Resistance Level (FRL). Cladding panels willusually be designed or tested to meet theserequirements in accordance with Section 5 ofAS 3600.

Most sealant manufacturers produce sealants that aredesigned to provide resistance to fire.The jointdetails and sealants should be designed and applied inaccordance with the manufacturer’s recommen-dations to give the required level of fire resistance.

Good width/depth profile

Concave sealant surface

Width greater than depth

Sealant too deep

No backup rod

Poor geometry for movement

Chamfers set sealant backand protect concrete

Concrete arrises subject tohandling damage

Correct Joint Design Incorrect Joint Design

12 x 12chamferstypical

20 typical

Figure 5

Joint Design Principles

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Disclaimer: Since the information provided in this publication is intended for guidance only and in no way replaces the services of professional consultants on particularprojects, no legal liability can be accepted by the National Precast Concrete Association Australia for its use.