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Sealing and resealing of joints in buildings
Construction ml Euilcfing Materials, Vol. 9. No. 6, pp. 379-387, 1995 Copyright 0 1996 Else&r Science Ltd Printed in Great Britain. All rights reserved 0950-0618/95/$10.00+0.00
0950-0618(95)MO50-X
Sealing and resealing of joints in buildings A . R. Hutchinson*‘,
A. Pagliuca*
*Joining Technology ‘Consultant
Research
Received
1994; accepted
11 October
and R. Woolman’
Centre, Oxford
30 March
Brookes
University,
Oxford,
UK
1995
Sealants play a vital role in joints in maintaining the weather-tightness of buildings and engineering structures. This paper outlines the nature of joint movements and the requirements of sealants to fulfil this role. Aspects of economics, joint design, sealant system selection and joint preparation are then discussed, both in the context of new sealing and resealing. Important new guidance is also given in terms of resealing of joints. Finally, the requirements of a detailed contract specification are outlined. Keywords:
buildings; cost-in-service;
curing
Historical perspective The exterior of a building must be weatherproofed to eliminate drafts and to prevent wind-driven rain from entering the structure. Water penetration usually leads to damp and unsightly damage, and may additionally cause structural deterioration. It is the movement of a building or its component parts which often leads to sealed joint failure and resulting leaks sooner or later. Movement occurs in all buildings and structures. In traditional forms of construction involving massive component parts, movement was accommodated by minor cracks and fissures in thick sections or by design features such as overhangs and flashings. Oil-based caulks such as bitumen or putty were sometimes used to fill cavities to prevent leakage. The modern building sealant industry is a post-Second World War phenomenon because modern structural frame buildings are characterised by lighter, thinner and often larger exernal sections such as the elements of a curtain wall. The often frequent cyclic movements at joints must be allowed for and can only be accommodated by modern synthetic sealant materials. Satisfactory performance can no longer be achieved by just filling holes with a sticky mastic. Joints represent discontinuities in a structure, located in positions between either similar or dissimilar materials and they must be designed to enable movements to take place between its component parts. Joints should not be regarded as convenient discontinuities for a contractor to accommodate tolerances, or as unfortunate gaps to be made as small as possible or even invisible. Joints represent vital parts of buildings and need to be
designed, constructed and sealed in a professional manner if they are not to become the weak link in the performance of the building structure. Sealant materials have an onerous role to play in modern building structures in as much as they need to be able to: adhere to a wide variety of substrates; multi-directional movements; resist accommodate degrading environmental factors; retain appropriate aesthetic qualities, and last for as long as possible. Current knowledge, in terms of materials requirements and performance needs, has been accumulated during a time when many buildings have been sealed with varying degrees of success. The building industry is therefore faced with an increasingly large number of buildings which require resealing, some of which may have inherent defects varying from incorrect sealant or poor application to more serious design faults. Even a well-sealed joint has a life expectancy somewhat less than that of a typical building, perhaps 25 years compared to 60 years or more. Thus resealing of joints in building structures needs to take place, perhaps many times, within the lifetime of a building. Currently the repair of sealed joints in the UK accounts for perhaps 75% of all gunnable sealants used, an amount which equates to resealing some 100000 km of joints per year’. The purpose of this paper is to explore the aspects of economics, joint design, sealant system selection and joint preparation, both in the context of new sealing and in resealing. Important guidance is also provided from the RESEAL projectl, particularly in terms of resealing joints. Economics and life expectancy
‘Correspondence to Dr A.R. Hutchinson, Joining Technology Research Centre, School of Engineering, Oxford Brookes University, Gipsy Lane Campus, Headington, Oxford OX3 OBP, UK Construction
One of the major parameters that should be considered in building design is cost-in-service (e.g. see BS 75432). and Building
Materials
1995 Volume
9 Number
6
379
Sealing and resealing of joints: A. R. Hutchinson
et al
Maintenance of the weather-tightness of the external envelope is an important part of the cost-in-service, and this includes the repair and resealing of joints. It has already been stated that the life expectancy of joints subject to movement is usually less than that of the building facade. It is important, therefore, that such joints should be designed to allow resealing to be carried out. The cost of sealing or resealing joints in buildings is made up of three major partsi: 0 access; cleaning (more onerous in resealing than in new sealing); sealant and back-up system (primer, 0 sealant material).
etical structural movements of the materials involved, together with experience of the type of construction and anticipated construction tolerances. The movement experienced in different types of joint is depicted in Figure 1. Settlement
Generally slow, urn-directional movement, depending upon the nature of the land, the mass of the structure and its foundations. Less common sources of settlement include subsidence, changes in water table, and clay shrinkage and swelling.
l
The price of the sealant itself is only a minor part of the total and hence financial economies in sealant material have a very small influence on the overall cost but could have a major effect on performance. A large proportion of the cost, perhaps up to 75% of the total, is represented by labour. The largest part of the labour cost is usually joint preparation, particularly in resealing where it is necessary to remove the old sealant material. However, economies in joint preparation may result in early failure, giving rise to a large increase in the effects on cost-in-service. The cost of access is generally less than 20% of the total cost and generally comprises two parts: that of erecting or moving the equipment, and the rental of the equipment. Any extra time taken in proper joint preparation or sealant application has a very modest effect on the cost of access. Joint sealing is often one of the last tasks in new building construction. All too often the time and cost requirements, access needs and weather conditions necessary for proper sealant application are ignored, despite the critical role that sealed joints play in the performance of the structure. However assuming the proper application of sealant material, the life of a sealed joint depends upon the: l l l
l
l l
Moisture
Initial shrinkage of materials such as concrete, blockwork and plaster is inevitable; initial expansion of (dry) brickwork is common. Movement tends to be unidirectional, slow and permanent. Timber and, to a lesser extent, concrete, brick and stone may expand when wet and shrink when dry. Exposure to weathering leads to cyclic changes in moisture content and consequential dimensional changes (Table 1). Absolute movement depends upon section dimensions. Creep
The effects of creep are associated with concrete-framed structures. Unreinforced and reinforced concrete elements creep under load; columns become shorter whilst beams sag. Movement is very stow, load-directional and load-sensitive due to compression.
nature of the sealant; exposure of the joint to weather; amount and frequency of movement affecting the seal; degrading or damaging influences such as UV, ozone, bacteriological attack, vandalism, etc.; geometry of the sealed joint; nature of the joint faces.
Proper joint design and sealant application are primary requirements to ensure suitable joint dimensions, preparation, cleaning, priming, back-up and application of sealant. Sources and nature of joint movement The movement potential of a joint is difficult to determine accurately. Estimates may be based upon theor-
Figure 1 Movement in different types of joint: (a) butt joint sion/compression; (b) butt joint in shear; (c) lap joint in shear
380
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1995 Volume
in ten-
Sealing Table I Reversible
moisture
movement
of building
materials
0.02-0.06 0.03HI.06 0.14.2 0.1-0.25 0.01 0.05W3.08 0.003-0.02 O.OlH).O5 0.15-0.25 0.05-O. I 2.0-8.0
Dense concrete Light aggregate concrete Ultra-lightweight concrete Fibreicement board Limestone (average) Sandstone Brick (clay) Brick (sand-lime) Glass reinforced cement Timber (with grain) Timber (across grain)
of joints:
A. R. Hutchinson
et al.
Colour, surface finish, mass, heat capacity and insulation affect the rate of response of components to temperature change (Figure 2). For example, matt black finish aluminium or plastics react very rapidly to heating from the sun, and heavily insulated panels retain this heat. Sealant materials in butt joints tend to be in compression when it is hot, and in tension when cold. It is a common observation that joint failures predominate on the south-facing elevations of buildings where overall joint movements are greatest due to thermal cycling. Further, the range of joint movement is greatest in spring and in autumn because of large daily temperature changes; cyclic movement due to shading of buildings may also be significant. Stick/slip, rather than gradual, movement is frequently associated with curtain walls whilst ‘pattern-effect’ movements of building facades reflect construction details.
Expansion Dry to saturated Over 3m length % (mm)
Material
and resealing
0.6-1.8 0.9-l .8 3.0-6.0 3.0-7.5 0.3
1.5-2.4 0.09-0.6 0.3-1.5 4.5-1.5
1.5-3.0 Not applicable
Columns may compress 3-4 mm per floor height. The British Standard Code of Practice for Brick and Concrete Cladding requires a 13 mm compression joint at each floor level on concrete-framed buildings to allow for this effect.
Loading
Externally applied loads such as equipment and machinery represent semi-permanent loads resulting in semi-permanent deformations. In contrast, bridge deck joints are subject to rapid transient deformations arising from traffic.
Thermul expansion
Constructional materials expand when heated and contract on cooling. The coefficient of linear thermal expansion (CLTE) represents the amount a material expands for a temperature change of one degree (Tuble 2). The actual movement of a building component depends upon the CLTE, its size and the change in temperature of that component; through-thickness temperature variations can be very high so that surface temperatures are likely to reflect the movement of thin sections rather than thick ones. Table 2 Typical
linear thermal
Material
Clay bricks Concrete (light aggr.) Concrete’(grave1 aggr.) Concrete (limest. aggr.) Aluminium Stainless steel Structural steel Copper Bronze Glass Limestone Granite Sandstone Marble Timber (with grain) Timber (across grain) Glass reinforced cement Acrylic sheet Polyester GRP Vinyl sheet (PVC)
expansion
Coefficient of expansion per “C x 10m6
5.0 8.3 11.7 6.0 23.5 18.0 12.1 18.0 19.8 9.1 2.5-9.0 8.5-11.0 7.0-16.0 13.2 3.8-6.5 50-60 7.0~12.0 70-90 18-50 40-75
of building
Wind
Wind results in non-uniform pressure or suction on building components, giving rise to cyclic movements of Time lag due to thermal mass
materials
Expansion of 3 m length over temperature differential of: 70°C 85°C 100°C (mm) (mm) (mm)
1.05
1.3 1.5 2.1 2.5 3.0 3.5 1.26 1.53 1.8 5.0 6.1 7.15 3.78 4.6 5.4 2.54 3.1 3.63 3.8 4.6 5.4 4.16 5.05 5.94 1.91 2.32 2.13 0.15-2.7 0.5-1.9 0.65-2.3 1.8-2.3 2.1-2.8 2.5-3.3 1.5-3.3 1.84.1 2.148 2.77 3.37 4.0 1.141.95 0.8-1.4 1.0-1.67 15.0-18.0 10.5-12.6 12.7-15.3 1.5-2.5 1.8-3.1 2.1-3.6 14.7-18.9 17.9-23.0 21.0-27.0 3.8-10.5 4.6-12.7 5.4-15.0 8.4-15.8 10.2-19.2 12.0-22.5 1.75 2.45
70°C differential is appropriate for light coloured uninsulated or heavy cladding. 85°C differential is appropriate for light coloured insulated or dark coloured uninsulated cladding. 100°C differential is appropriate for dark coloured insulated cladding. Note: Even greater temperature differentials may occur with high standards of insulation used with high absorption finishes.
Construction
1200
0000
l
Sun dunng normal sunny day (alumlnium)
A
Sun emerges late mornmg, resulting I” rapid temperature nse (alummiumb
l
Sun during normal sunny day (concrete) Note time lag due to thermal mass and total heat capacity
Figure2 Typical movement temperature
2400
of a 3 m length
of material
over a 70°C
change
and Building Materials
1995 Volume
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Sealing and resealing of joints: A. R. Hutchinson
et al.
short periodicity. Tall buildings are subject to the greatest deflections, theoretically giving rise to the largest joint movements at ground level. Vibration Sources of vibration include internal heavy machinery or external traffic. The effect of vibration is frequently to relieve friction between components resulting in large, sudden movements.
Joint design Butt-joints accept movement by deforming in extension and compression; lap-joints can accommodate greater movement than butt-joints by deforming in shear (e.g. see Figure I). However, the latter are often more difficult to incorporate and to seal, and particularly difficult to reseal. The section of sealant in butt- and lap-joints is rectangular, and two-sided adhesion only is essential. Joints rarely fail because they are too large, but they frequently fail because they are too small. A sealant section size less than 6 mm square is out of the question both practically and theoretically. Other sealant shapes such as triangular fillets and Vjoints are widely used but suited only to very low movement joints. This is because movement causes an uneven distribution of strain, leading to overstressing in narrow Table 3 Typical properties
Sealant
type
parts of the joint and consequential adhesion loss.
tearing, splitting or
Design width: butt-joints The maximum and minimum width of joint are determined from the anticipated movement, the sealant performance and the joint tolerances. Different sealant materials possess different movement accommodation factors (MAFs); for example, see Table 3. The MAF is not the same as the extension at failure measured in a short-term tensile adhesion test, but rather a safe indication of relative performance for joint design width calculation purposes. Design joint width, ~ movement x 100 w= + movement MAF Movement refers to the total joint movement between minimum and maximum. Design depth: butt-joints Depth must be sufficient to accommodate an adequate depth of sealant and back-up or bond-breaker material. Sealant section depends on the function of the joint and the elastic properties of the sealant material. For most applications the recommended seal depths are shown in Figure 3.
of sealants Movement accommodation factor (%)
Character
Life expectancy
Joint suitability
(years)
Oil-based Butyl-based
10 10
Plastic Plastic
l-10 15-20
Perimeter pointing Concealed joints (not UV resistant)
Acrylic Water-based
15
Plastic
lo-15
20
Plastic
15-20
Internal joints, plaster cracks, etc. Perimeter pointing, concrete, stone cladding, etc.
Solvent-based
Polysulphide One-part
20-25
Elasto-plastic and elastic
20-25
Two-part
25-30
Elasto-plastic and elastic
20-25
1O-20
Elasto-plastic and elastic
20-25
IO-30
Elastic and elasto-plastic
20-25
20-30
Elastic
20-25
Silicone Low modulus
50-70
Elastic and elasto-plastic
25-30
High modulus
20-30
Elastic
25-30
Flexible epoxy
5-15
Elasto-plastic
lo-20
Two-part high modulus Polyurethane One-part
Two-part
382
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Materials
1995 Volume 9 Number 6
Perimeter pointing, structural joints, stone cladding, etc. Structural joints, stone and cladding, joints subject to early high movement Paving, traffic, floor joints, etc. Light cladding, curtain walling, structural joints, stone cladding, etc. Light cladding, curtain walling, paving, etc. Perimeter pointing, curtain walling, stone and concrete cladding, structural joints, etc. Glazing, sanitary ware, etc. Floor joints, traffic areas, etc.
Sealing
and resealing
of joints:
A. R. Hutchinson
et al.
Main generic types
Elasto-plasw
Depth = wdth
sealants
Depth = y
Elastic sealants
Depth s y
Figure 3 Joint depths for different sealant types
Elastic sealants perform best in thin sections and are usually applied 6-12mm deep, often in an hour-glass shape. Sealants are frequently used in deeper sections when subjected to pressure, loading or traffic; this reduces the MAF slightly which means that an increase in joint width is required to accommodate the joint movement.
Sealant materials Charucter und general suitubility Elastic sealants, for example polyurethane and silicone materials, are suitable for joints affected by large, frequent, cyclic movement. They are appropriate for lightweight and well-insulated cladding systems and curtain walling. Prolonged distortion increases the risk of adhesive failure, tearing or splitting from minor defects, and degradation. Elastoplastic sealants, such as polysulphides and some acrylics, epoxies and silicones, are best suited to joints affected by slow, cyclic movement and to permanent deformation such as occurs in heavy cladding systems and structural joints. Typical applications include precast concrete cladding, stone cladding and brickwork joints. The stress-relaxing feature reduces the risk of adhesive failure whilst the more elastic versions can accommodate cyclic movements. Plastic sealants, such as non-curing oil-, butyl- and acrylic-based materials, are suitable for joints subject to low movement and permanent compressive deformation. They have been used successfully in joints in traditional construction and in heavy cladding systems. Plastic seals stress-relax under deformation, minimizing the risk of adhesion failure. Large, frequent cycles of movement lead to creasing, folding and eventual splitting. Non-curing sealants also become stiffer with age, reducing their ability to accommodate movement or to deform plastically. Table 3 indicates the typical character of various generic types of sealants and their suitability for different applications. In the absence of loading or other factors (for example, damage- or vandal-resistance), low modulus sealants are always preferred as they impose less strain on adhesion and on the substrate surfaces, reducing the risk of failure. Construction
Sealants are available based on a variety of materials and polymers. A wide range of physical and mechanical properties is available within each generic type, and individual formulations based upon the same generic type may vary markedly between manufacturers. Silicones. These sealants are probably the most durable of all the materials available and tend to be very popular with users. Building sealants are generally based upon one-component systems which cure fairly rapidly on exposure to air. A wide variety of different curing mechanisms leads to characteristic odours and a range of properties from tough, elastic materials to very low modulus materials which can accommodate high movement accommodation. Silicones possess low surface tensions, enabling them to wet out and flow over a wide variety of surfaces. They have a particular affinity for glass, and two-component silicone materials are sometimes used in insulating glass manufacture. Polyurethunes. Polyurethane sealants encompass a huge range of materials with widely varying characteristics and are available as one- or two-component systems. The sealants are generally tough and very elastic, ranging from low modulus to high modulus types. Primers are frequently required to promote adhesion to many substrates. Polysufphides. Polysulphide materials have a long pedigree as sealants and have been used in building and civil engineering applications for many years. The sealants are generally tough and elasto-plastic in nature, and both one- and two-part products are available; slow through-cure is generally associated with the one-part materials. Because of their relatively fast cure, two-part products are particularly suitable for joints subject to large movements soon after sealant application. Acrylics. The two types of sealants in common usage are solvent- and water-based materials. Solvent-based sealants are thermoplastic materials with plasto-elastic properties. They possess good weathering properties and exhibit excellent adhesion to a wide variety of surfaces; for the latter reason they are sometimes used in resealing operations where adequate surface cleaning is difficult. Water-based materials are used primarily for internal sealing. Butyl mastics. These materials, which date back to the 1960s are essentially plastic in nature and are susceptible to UV attack. This type of sealant is seldom used externally today but its residues on joint faces make it very difficult to obtain adequate adhesion with modern sealants. Oil-bused mustics. These are the oldest type of sealants available and are plastic in nature. They are based upon blends of drying- and non-drying oils and plasticisers reinforced with polymers and fillers. The skin of such materials thickens and toughens with age, reducing the flexibility of the seal. Again, the residues of such materials in joints provide difficulties for resealing with modern sealants. and Building
Materials
1995 Volume
9 Number 6
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Sealing and resealing of joints: A. R. Hutchinson
et al.
Curing Development of full cure may take anything between a week and several months depending upon generic type, cure mechanism, sealant mass, joint geometry and atmospheric conditionss. Sealant formulators strike a balance between speed of cure, pot-life, tooling requirements and long-term retention of properties such as elasticity and movement accommodation. The curing of a two-part sealant occurs on mixing two components, the base polymer and a curing agent. Once thoroughly mixed, the sealant will cure in a homogeneous fashion throughout the bulk of the polymer. Two-part products generally achieve a high degree of cure in a shorter time than an equivalent one-part system, assuming proper and complete mixing of the two components. Moisture is required, and the presence of oxygen and elevated temperatures may also accelerate cure development of some materials. One-part sealants rely upon atmospheric moisture and oxygen to initiate cure reactions; moisture often acts as the catalyst for the action of an internal curing system. Cure begins on the surface with the formation of a skin; full cure of the bulk of the polymer then follows from the outside, moving inwards. An increase in temperature, humidity and air flow can accelerate curing.
Joint preparation - new sealing Sealants form a seal by bonding to both sides of a joint and forming a flexible link between the two substrates. Maintaining a seal is dependent on the strength of the bond to the substrates being greater than the forces required to stretch or compress the sealant. Joint performance therefore relies upon the sealant gaining adhesion, and remaining bonded, to the joint faces. Adhesion between sealant and substrate (or primer material) relies upon interfacial forces of attraction, requiring fundamentally: l l
intimate contact between the materials involved; absence of weak layers or contamination at the interfaces such as dirt, dust, grease and existing sealant residues.
affinity to the sealant and to the substrate concerned; substitution of primers can therefore lead to catastrophic failure. Experience has shown that with proper joint design and sealant application, the use of appropriate primer materials results in joints of the greatest durability - particularly under prolonged damp and wet conditions.
Joint preparation - resealing When replacing existing seals, there are essentially three cleaning and preparation options: Option 1: Complete removal of the old seal Option 2: Removal of most of the old sealant Option 3: Oversealing. Option I In most cases the first option of complete removal is intended, particularly if the original seal has failed by loss of adhesion. However, it may be very difficult or even impossible to remove all traces of the existing sealant (and primer, if present) without removing a surface layer of substrate material. Option 2 Where it is impossible to completely remove all traces of existing sealant, and where seal failure was cohesive, it may be acceptable to leave a film of residue, say ~0.5 mm thick, although this will aimost always be at the expense of performance 19. A reduction in joint extension and performance is almost inevitable, in proportion to the type and amount of residue. In the course of the RESEAL project’ a number of important findings were made regarding ‘contaminated’ surfaces: l
l
l
Primers, being relatively low viscosity materials, assist adhesion either by penetrating the pores of a porous surface or by forming a chemical link between the surface and the high viscosity sealant. Primers may also bind and reinforce weak surface layers of certain substrates such as concrete or stone. Where painted surfaces are involved, care must be taken to ensure that the paint will remain securely bonded to the substrate and to ensure that there is no detrimental interaction between the paint and sealant, sealant cleaners or primers. Two main types of primer are associated with the majority of sealant materials, appropriate to porous and non-porous surfaces. Sometimes the design of the primer is unique to the application, having specific 384
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1995 Volume
l
l
A 0.5 mm thick layer of residual sealant resulted in a significant drop in joint performance; priming such a layer did not help. Non-curing and silicone sealant residues, particularly on porous surfaces such as concrete, severely reduced the performance of joints resealed with curing products. A very thin layer, or smear (co.1 mm thick), of a curing sealant residue should provide acceptable adhesion performance. Better adhesion is gained to curing contaminants than to non-curing contaminants when resealing with a curing sealant. The general order of adhesioni,” found from tensile adhesion testing of joints made with the following contaminants during the course of the RESEAL project was: polysulphide > polyurethane > acrylic > butyl rubber > silicone > bitumen > GP mastic. Silicone sealant residues are difficult to remove. However, they represent low energy surfaces and as such are difficult to adhere to other than with identical products (if this represents a suitable option from other considerations). This poses a future problem for
9 Number 6
Sealing
resealing, given the widespread use of silicone materials currently. Clearly tests should be carried out to establish compatibility and adhesion, including simple surface analytical technique#. Additionally, the new sealant must have a lower modulus than the old residue or else any joint movement will impose a disproportionate amount of strain on the existing material, causing rapid failure. Option 3
Oversealing butt-joint seals may sometimes offer the most cost-effective solution. Oversealing may be easy with deeply recessed joints, by creating a new joint in front of the old, or with a bandage joint formed over the face of the original joint. In all cases it is important to separate the new joint seal from the existing material with bond-breaker tape or back-up foam. Various details are shown in Figure 4. Cleaning techniques for resealing
The
applicability
of
various
cleaning
techniques
and resealing
ofjoints:
A. R. Hutchinson
et al.
depends upon joint accessibility and by the nature of the materials surrounding and forming the existing jpint. Most sealants are rubbery or plastic in nature, and if subject to abrasion they tend to spread, clogging abrading tools and spreading with heat. The first stage involves bulk sealant removal with knives and chisels, cutting and scraping along the joint faces. When the contaminating layer has been reduced to a minimum, further removal may be possible by either mechanical means or solvent cleaning. Mechanical removal is possible on surfaces such as stone and concrete, followed by dust removal and solvent wiping. Careful scraping and solvent cleaning is appropriate to more delicate finishes involving metals, painted metals and timber. The use of solvent in conjunction with a mild abrasive, such as nylon pan-scourer, is usually effective on non-porous surfaces. Solvent cleaning itself often represents the final stage of cleaning and preparation but care must be taken in solvent selection for delicate finishes. It should also be noted that traces of original sealant materials may be dissolved and spread, causing staining or further contamination. Finally, most solvents are potentially dangerous and should be used with care. The sealant system and the sealing process The sealant system consists of the sealant itself, primer, and back-up foam or bond breaker tape (if used). The sealing (and resealing) processes are depicted in Figure 5.
F
: ....,’ .
KeY
q q q ISI
vc .I:
-1.:. . ..__..
>,
:::::
. .
IdI
l Adheswe/stiff
Sealant
q
Back-up
Substrate
New
Sealunts
In new sealing the sealant should be selected to have the longest life commensurate with a high probability of successful application to achieve that performance and to suit the expected life of the building. Fundamentally, the sealant must be able to accommodate the expected type, frequency and amplitudes of joint movement whilst also being compatible with the substrates involved, particularly if staining is a possibility. In resealing, sealant choice will probably be a compromise in terms of compatibility, joint cleaning requirements, joint geometries and access difficulties. Identification of the existing sealant materials is a first step based either upon physical characteristics such as elastic recovery, feel, smell and burning odours or by chemical identification in a laboratoryi. In choosing the sealant system the following check procedure should be followed by the specifier or contractor:
sealant
sealant
l Filler
board
-
Bond-breaker
0
Metal
-
Nail
tape
or plastic
l
plate
or screw
l
Figure 4 Oversealing
details
Construction
Identify the type(s) and causes of movement affecting the joint(s). Check the type of movement and select the sealant type appropriate, e.g. elastic, elasto-plastic, etc. Check the joint size and amplitudes of movement to assess the minimum movement accommodation factor (MAF) required. Check surface conditions required for each suitable sealant.
and Building
Materials
1995 Volume 9 Number 6
385
Sealing and resealing of joints: A. R. Hutchinson
et ~1. Back-up materials
Building qssessment Estimation ofjqint movements
I RESEALING
I NEWSEALING
Site survey
f
Potential ne”; joint design
Jointd?gn
1
L
Sealantselection Testing/compatibility trials Draf? specification Application detailshtrwtions Site trials and evaluation of joint preparation and sealant system
Joint depth is controlled by the use of a back-up foam or other joint filler materials. To ensure two-sided adhesion of the sealant to the joint faces only, the back-up or filler should be a material to which the sealant will not adhere. Closed-cell polyethylene foam materials are the preferred choice because they can be compressed in the joint initially, and will expand and contract with the joint providing continuous support for the sealant. As a general rule, the backing foam should be at least 20% wider than the joint and the thickness at least half the width. In resealing joints the back-up material will generally need to be replaced with an appropriate type and size of material to ensure optimum seal performance. Where polystyrene or fibreboard are used as joint fillers, or where oversealing is to take place, bond-breaker tapes should be applied to prevent sealant adhesion.
I
Final specification Access Preparation of joint surfaces Final cleaning Priming Joint width/depth
Masking
Where a flush joint or an especially neat appearance is required, the joint edges may be protected by masking tape before priming, sealing and tooling the joints. After tooling, the masking tape should be removed before the sealant has cured.
Sealantmaterials Applicationmethods Back-upmaterials Tooling Curing
Tooling
This final but essential process compacts the sealant into the joint, fills the corners, eliminates air bubbles, improves wetting and adhesion, and smooths the surface of the sealant. Most joints are tooled to a smooth, slightly concave, surface with a slightly rounded piece of wood kept wet with a suitable wetting agent.
Cleaning up Quality control
Acceptancecriteria Maintenancerequirements Inspectionandre-work Finalinspection
Timing
I Contract
complete
Figure 5 Sealing and resealing processes
l
l
Assess whether the surface conditions can be achieved for each sealant system, i.e. cleaning required, priming, compatibility, etc. Short-list possible sealants and specification; check cost and life expectancy.
Primers
The important performance enhancements promoted by the use of primers on clean surfaces have already been discussed; psychologically, the requirement to prime surfaces also focuses the mind of the sealant applicator on the importance of surface preparation. However it was found during the course of the RESEAL project that primers designed for substrates conferred no performance advantages on sealant residues of about 0.5 mm thick, although they did seem to help on thin layers or smears of contamination. Careful application of primer is essential to ensure that it is confined to the joint area, particularly with highly visible joints. 386
Construction
and Building Materials
Many of the permanent deformation movements such as shrinkage and settlement in buildings occur relatively early in the life of a building. Delaying the sealing of structural, and other, joints until after most of the permanent movements have occurred can reduce the demands on sealed joints and therefore increase life expectancy. Specification Having considered the foregoing points and their implications, together with carrying out laboratory and/or site trials, it is important to document the requirements in the form of a detailed contract specification. This specification should include the following points: construction details, quantities of joints involved and variation in joint dimensions; l minimum and maximum joint sizes, and sealant width/depth ratios to suit the anticipated range of joint sizes; l condition of the joint faces and required surface including methods for the removal preparation, of existing sealants or other contamination if appropriate;
l
1995 Volume 9 Number 6
Sealing
method of final cleaning and priming if required; l type(s) of joint back-up material, including sizes and shapes to suit the range of joint sizes and expected movements; l sealant specification, material, section, width and depth, recessed or flush joint; 0 tooling specification; l procedures to be followed in the event of variance outside the contract specifications; l timescales, including details concerned with unsuitable weather or other limitations which may be experienced during the contract; l planned life of the building, design life for the seal and the maintenance/inspection requirements.
l
A detailed specification represents one of the cornerstones for ensuring adequate quality.
Closing remarks Successful sealing and resealing is achievable with proper joint design, sealant system specification and application. Clearly the application process will be operator-dependent so that the processes of cleaning and preparing the joints, together with preparation and
Construction
and resealing
of joints:
A. R. Hutchinson
et al.
application of the sealant, should be defined to ensure that the applicator is fully aware of the process requirements and the standard required.
Acknowledgements The authors express their appreciation to the EPSRCDOE LINK CMR Management Committee and to all of the sponsors and partners for their support of the RESEAL project.
References Woolman, R. In Resealing of Buildings: A Guide IO Good Practice ed. A.R. Hutchinson, Butterworth-Heinemann, Oxford, 1994 Guide to the Durability of Buildings and Building Elements, Products and Components, BS 7543, British Standards Institution,
1992 Allen, K.W., Hutchinson, A.R. and Paghuca, A. A study of the curing of sealants used in building construction. Int. J. Adhes. Adhesives, 1994, 14, (2) 117-122 Pagliuca, A. and Hutchinson, A.R. Durability of resealed building joints. BRE/RILEM Seminar on Durability of Building Sealants, Building Research Establishment, Watford, UK, 1l-12 October 1994. In Durability of Building Sealants, eds. J.C. Beech and A.T. Wolf, E&FN Spon, 1995 Pagliuca, A. and Hutchinson, A.R. Adhesion properties of sealants in resealed joints. In Science and Technology of Building Seals, Sealants, Glazing and Waterproofing: 6th vol ASTM STP 1286, ed. J C Myers, 1996 (in press)
and Building
Materials
1995 Volume 9 Number 6
387
0950-0618(95)MO50-X
Sealing and resealing of joints in buildings A . R. Hutchinson*‘,
A. Pagliuca*
*Joining Technology ‘Consultant
Research
Received
1994; accepted
11 October
and R. Woolman’
Centre, Oxford
30 March
Brookes
University,
Oxford,
UK
1995
Sealants play a vital role in joints in maintaining the weather-tightness of buildings and engineering structures. This paper outlines the nature of joint movements and the requirements of sealants to fulfil this role. Aspects of economics, joint design, sealant system selection and joint preparation are then discussed, both in the context of new sealing and resealing. Important new guidance is also given in terms of resealing of joints. Finally, the requirements of a detailed contract specification are outlined. Keywords:
buildings; cost-in-service;
curing
Historical perspective The exterior of a building must be weatherproofed to eliminate drafts and to prevent wind-driven rain from entering the structure. Water penetration usually leads to damp and unsightly damage, and may additionally cause structural deterioration. It is the movement of a building or its component parts which often leads to sealed joint failure and resulting leaks sooner or later. Movement occurs in all buildings and structures. In traditional forms of construction involving massive component parts, movement was accommodated by minor cracks and fissures in thick sections or by design features such as overhangs and flashings. Oil-based caulks such as bitumen or putty were sometimes used to fill cavities to prevent leakage. The modern building sealant industry is a post-Second World War phenomenon because modern structural frame buildings are characterised by lighter, thinner and often larger exernal sections such as the elements of a curtain wall. The often frequent cyclic movements at joints must be allowed for and can only be accommodated by modern synthetic sealant materials. Satisfactory performance can no longer be achieved by just filling holes with a sticky mastic. Joints represent discontinuities in a structure, located in positions between either similar or dissimilar materials and they must be designed to enable movements to take place between its component parts. Joints should not be regarded as convenient discontinuities for a contractor to accommodate tolerances, or as unfortunate gaps to be made as small as possible or even invisible. Joints represent vital parts of buildings and need to be
designed, constructed and sealed in a professional manner if they are not to become the weak link in the performance of the building structure. Sealant materials have an onerous role to play in modern building structures in as much as they need to be able to: adhere to a wide variety of substrates; multi-directional movements; resist accommodate degrading environmental factors; retain appropriate aesthetic qualities, and last for as long as possible. Current knowledge, in terms of materials requirements and performance needs, has been accumulated during a time when many buildings have been sealed with varying degrees of success. The building industry is therefore faced with an increasingly large number of buildings which require resealing, some of which may have inherent defects varying from incorrect sealant or poor application to more serious design faults. Even a well-sealed joint has a life expectancy somewhat less than that of a typical building, perhaps 25 years compared to 60 years or more. Thus resealing of joints in building structures needs to take place, perhaps many times, within the lifetime of a building. Currently the repair of sealed joints in the UK accounts for perhaps 75% of all gunnable sealants used, an amount which equates to resealing some 100000 km of joints per year’. The purpose of this paper is to explore the aspects of economics, joint design, sealant system selection and joint preparation, both in the context of new sealing and in resealing. Important guidance is also provided from the RESEAL projectl, particularly in terms of resealing joints. Economics and life expectancy
‘Correspondence to Dr A.R. Hutchinson, Joining Technology Research Centre, School of Engineering, Oxford Brookes University, Gipsy Lane Campus, Headington, Oxford OX3 OBP, UK Construction
One of the major parameters that should be considered in building design is cost-in-service (e.g. see BS 75432). and Building
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Sealing and resealing of joints: A. R. Hutchinson
et al
Maintenance of the weather-tightness of the external envelope is an important part of the cost-in-service, and this includes the repair and resealing of joints. It has already been stated that the life expectancy of joints subject to movement is usually less than that of the building facade. It is important, therefore, that such joints should be designed to allow resealing to be carried out. The cost of sealing or resealing joints in buildings is made up of three major partsi: 0 access; cleaning (more onerous in resealing than in new sealing); sealant and back-up system (primer, 0 sealant material).
etical structural movements of the materials involved, together with experience of the type of construction and anticipated construction tolerances. The movement experienced in different types of joint is depicted in Figure 1. Settlement
Generally slow, urn-directional movement, depending upon the nature of the land, the mass of the structure and its foundations. Less common sources of settlement include subsidence, changes in water table, and clay shrinkage and swelling.
l
The price of the sealant itself is only a minor part of the total and hence financial economies in sealant material have a very small influence on the overall cost but could have a major effect on performance. A large proportion of the cost, perhaps up to 75% of the total, is represented by labour. The largest part of the labour cost is usually joint preparation, particularly in resealing where it is necessary to remove the old sealant material. However, economies in joint preparation may result in early failure, giving rise to a large increase in the effects on cost-in-service. The cost of access is generally less than 20% of the total cost and generally comprises two parts: that of erecting or moving the equipment, and the rental of the equipment. Any extra time taken in proper joint preparation or sealant application has a very modest effect on the cost of access. Joint sealing is often one of the last tasks in new building construction. All too often the time and cost requirements, access needs and weather conditions necessary for proper sealant application are ignored, despite the critical role that sealed joints play in the performance of the structure. However assuming the proper application of sealant material, the life of a sealed joint depends upon the: l l l
l
l l
Moisture
Initial shrinkage of materials such as concrete, blockwork and plaster is inevitable; initial expansion of (dry) brickwork is common. Movement tends to be unidirectional, slow and permanent. Timber and, to a lesser extent, concrete, brick and stone may expand when wet and shrink when dry. Exposure to weathering leads to cyclic changes in moisture content and consequential dimensional changes (Table 1). Absolute movement depends upon section dimensions. Creep
The effects of creep are associated with concrete-framed structures. Unreinforced and reinforced concrete elements creep under load; columns become shorter whilst beams sag. Movement is very stow, load-directional and load-sensitive due to compression.
nature of the sealant; exposure of the joint to weather; amount and frequency of movement affecting the seal; degrading or damaging influences such as UV, ozone, bacteriological attack, vandalism, etc.; geometry of the sealed joint; nature of the joint faces.
Proper joint design and sealant application are primary requirements to ensure suitable joint dimensions, preparation, cleaning, priming, back-up and application of sealant. Sources and nature of joint movement The movement potential of a joint is difficult to determine accurately. Estimates may be based upon theor-
Figure 1 Movement in different types of joint: (a) butt joint sion/compression; (b) butt joint in shear; (c) lap joint in shear
380
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1995 Volume
in ten-
Sealing Table I Reversible
moisture
movement
of building
materials
0.02-0.06 0.03HI.06 0.14.2 0.1-0.25 0.01 0.05W3.08 0.003-0.02 O.OlH).O5 0.15-0.25 0.05-O. I 2.0-8.0
Dense concrete Light aggregate concrete Ultra-lightweight concrete Fibreicement board Limestone (average) Sandstone Brick (clay) Brick (sand-lime) Glass reinforced cement Timber (with grain) Timber (across grain)
of joints:
A. R. Hutchinson
et al.
Colour, surface finish, mass, heat capacity and insulation affect the rate of response of components to temperature change (Figure 2). For example, matt black finish aluminium or plastics react very rapidly to heating from the sun, and heavily insulated panels retain this heat. Sealant materials in butt joints tend to be in compression when it is hot, and in tension when cold. It is a common observation that joint failures predominate on the south-facing elevations of buildings where overall joint movements are greatest due to thermal cycling. Further, the range of joint movement is greatest in spring and in autumn because of large daily temperature changes; cyclic movement due to shading of buildings may also be significant. Stick/slip, rather than gradual, movement is frequently associated with curtain walls whilst ‘pattern-effect’ movements of building facades reflect construction details.
Expansion Dry to saturated Over 3m length % (mm)
Material
and resealing
0.6-1.8 0.9-l .8 3.0-6.0 3.0-7.5 0.3
1.5-2.4 0.09-0.6 0.3-1.5 4.5-1.5
1.5-3.0 Not applicable
Columns may compress 3-4 mm per floor height. The British Standard Code of Practice for Brick and Concrete Cladding requires a 13 mm compression joint at each floor level on concrete-framed buildings to allow for this effect.
Loading
Externally applied loads such as equipment and machinery represent semi-permanent loads resulting in semi-permanent deformations. In contrast, bridge deck joints are subject to rapid transient deformations arising from traffic.
Thermul expansion
Constructional materials expand when heated and contract on cooling. The coefficient of linear thermal expansion (CLTE) represents the amount a material expands for a temperature change of one degree (Tuble 2). The actual movement of a building component depends upon the CLTE, its size and the change in temperature of that component; through-thickness temperature variations can be very high so that surface temperatures are likely to reflect the movement of thin sections rather than thick ones. Table 2 Typical
linear thermal
Material
Clay bricks Concrete (light aggr.) Concrete’(grave1 aggr.) Concrete (limest. aggr.) Aluminium Stainless steel Structural steel Copper Bronze Glass Limestone Granite Sandstone Marble Timber (with grain) Timber (across grain) Glass reinforced cement Acrylic sheet Polyester GRP Vinyl sheet (PVC)
expansion
Coefficient of expansion per “C x 10m6
5.0 8.3 11.7 6.0 23.5 18.0 12.1 18.0 19.8 9.1 2.5-9.0 8.5-11.0 7.0-16.0 13.2 3.8-6.5 50-60 7.0~12.0 70-90 18-50 40-75
of building
Wind
Wind results in non-uniform pressure or suction on building components, giving rise to cyclic movements of Time lag due to thermal mass
materials
Expansion of 3 m length over temperature differential of: 70°C 85°C 100°C (mm) (mm) (mm)
1.05
1.3 1.5 2.1 2.5 3.0 3.5 1.26 1.53 1.8 5.0 6.1 7.15 3.78 4.6 5.4 2.54 3.1 3.63 3.8 4.6 5.4 4.16 5.05 5.94 1.91 2.32 2.13 0.15-2.7 0.5-1.9 0.65-2.3 1.8-2.3 2.1-2.8 2.5-3.3 1.5-3.3 1.84.1 2.148 2.77 3.37 4.0 1.141.95 0.8-1.4 1.0-1.67 15.0-18.0 10.5-12.6 12.7-15.3 1.5-2.5 1.8-3.1 2.1-3.6 14.7-18.9 17.9-23.0 21.0-27.0 3.8-10.5 4.6-12.7 5.4-15.0 8.4-15.8 10.2-19.2 12.0-22.5 1.75 2.45
70°C differential is appropriate for light coloured uninsulated or heavy cladding. 85°C differential is appropriate for light coloured insulated or dark coloured uninsulated cladding. 100°C differential is appropriate for dark coloured insulated cladding. Note: Even greater temperature differentials may occur with high standards of insulation used with high absorption finishes.
Construction
1200
0000
l
Sun dunng normal sunny day (alumlnium)
A
Sun emerges late mornmg, resulting I” rapid temperature nse (alummiumb
l
Sun during normal sunny day (concrete) Note time lag due to thermal mass and total heat capacity
Figure2 Typical movement temperature
2400
of a 3 m length
of material
over a 70°C
change
and Building Materials
1995 Volume
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Sealing and resealing of joints: A. R. Hutchinson
et al.
short periodicity. Tall buildings are subject to the greatest deflections, theoretically giving rise to the largest joint movements at ground level. Vibration Sources of vibration include internal heavy machinery or external traffic. The effect of vibration is frequently to relieve friction between components resulting in large, sudden movements.
Joint design Butt-joints accept movement by deforming in extension and compression; lap-joints can accommodate greater movement than butt-joints by deforming in shear (e.g. see Figure I). However, the latter are often more difficult to incorporate and to seal, and particularly difficult to reseal. The section of sealant in butt- and lap-joints is rectangular, and two-sided adhesion only is essential. Joints rarely fail because they are too large, but they frequently fail because they are too small. A sealant section size less than 6 mm square is out of the question both practically and theoretically. Other sealant shapes such as triangular fillets and Vjoints are widely used but suited only to very low movement joints. This is because movement causes an uneven distribution of strain, leading to overstressing in narrow Table 3 Typical properties
Sealant
type
parts of the joint and consequential adhesion loss.
tearing, splitting or
Design width: butt-joints The maximum and minimum width of joint are determined from the anticipated movement, the sealant performance and the joint tolerances. Different sealant materials possess different movement accommodation factors (MAFs); for example, see Table 3. The MAF is not the same as the extension at failure measured in a short-term tensile adhesion test, but rather a safe indication of relative performance for joint design width calculation purposes. Design joint width, ~ movement x 100 w= + movement MAF Movement refers to the total joint movement between minimum and maximum. Design depth: butt-joints Depth must be sufficient to accommodate an adequate depth of sealant and back-up or bond-breaker material. Sealant section depends on the function of the joint and the elastic properties of the sealant material. For most applications the recommended seal depths are shown in Figure 3.
of sealants Movement accommodation factor (%)
Character
Life expectancy
Joint suitability
(years)
Oil-based Butyl-based
10 10
Plastic Plastic
l-10 15-20
Perimeter pointing Concealed joints (not UV resistant)
Acrylic Water-based
15
Plastic
lo-15
20
Plastic
15-20
Internal joints, plaster cracks, etc. Perimeter pointing, concrete, stone cladding, etc.
Solvent-based
Polysulphide One-part
20-25
Elasto-plastic and elastic
20-25
Two-part
25-30
Elasto-plastic and elastic
20-25
1O-20
Elasto-plastic and elastic
20-25
IO-30
Elastic and elasto-plastic
20-25
20-30
Elastic
20-25
Silicone Low modulus
50-70
Elastic and elasto-plastic
25-30
High modulus
20-30
Elastic
25-30
Flexible epoxy
5-15
Elasto-plastic
lo-20
Two-part high modulus Polyurethane One-part
Two-part
382
Construction
and Building
Materials
1995 Volume 9 Number 6
Perimeter pointing, structural joints, stone cladding, etc. Structural joints, stone and cladding, joints subject to early high movement Paving, traffic, floor joints, etc. Light cladding, curtain walling, structural joints, stone cladding, etc. Light cladding, curtain walling, paving, etc. Perimeter pointing, curtain walling, stone and concrete cladding, structural joints, etc. Glazing, sanitary ware, etc. Floor joints, traffic areas, etc.
Sealing
and resealing
of joints:
A. R. Hutchinson
et al.
Main generic types
Elasto-plasw
Depth = wdth
sealants
Depth = y
Elastic sealants
Depth s y
Figure 3 Joint depths for different sealant types
Elastic sealants perform best in thin sections and are usually applied 6-12mm deep, often in an hour-glass shape. Sealants are frequently used in deeper sections when subjected to pressure, loading or traffic; this reduces the MAF slightly which means that an increase in joint width is required to accommodate the joint movement.
Sealant materials Charucter und general suitubility Elastic sealants, for example polyurethane and silicone materials, are suitable for joints affected by large, frequent, cyclic movement. They are appropriate for lightweight and well-insulated cladding systems and curtain walling. Prolonged distortion increases the risk of adhesive failure, tearing or splitting from minor defects, and degradation. Elastoplastic sealants, such as polysulphides and some acrylics, epoxies and silicones, are best suited to joints affected by slow, cyclic movement and to permanent deformation such as occurs in heavy cladding systems and structural joints. Typical applications include precast concrete cladding, stone cladding and brickwork joints. The stress-relaxing feature reduces the risk of adhesive failure whilst the more elastic versions can accommodate cyclic movements. Plastic sealants, such as non-curing oil-, butyl- and acrylic-based materials, are suitable for joints subject to low movement and permanent compressive deformation. They have been used successfully in joints in traditional construction and in heavy cladding systems. Plastic seals stress-relax under deformation, minimizing the risk of adhesion failure. Large, frequent cycles of movement lead to creasing, folding and eventual splitting. Non-curing sealants also become stiffer with age, reducing their ability to accommodate movement or to deform plastically. Table 3 indicates the typical character of various generic types of sealants and their suitability for different applications. In the absence of loading or other factors (for example, damage- or vandal-resistance), low modulus sealants are always preferred as they impose less strain on adhesion and on the substrate surfaces, reducing the risk of failure. Construction
Sealants are available based on a variety of materials and polymers. A wide range of physical and mechanical properties is available within each generic type, and individual formulations based upon the same generic type may vary markedly between manufacturers. Silicones. These sealants are probably the most durable of all the materials available and tend to be very popular with users. Building sealants are generally based upon one-component systems which cure fairly rapidly on exposure to air. A wide variety of different curing mechanisms leads to characteristic odours and a range of properties from tough, elastic materials to very low modulus materials which can accommodate high movement accommodation. Silicones possess low surface tensions, enabling them to wet out and flow over a wide variety of surfaces. They have a particular affinity for glass, and two-component silicone materials are sometimes used in insulating glass manufacture. Polyurethunes. Polyurethane sealants encompass a huge range of materials with widely varying characteristics and are available as one- or two-component systems. The sealants are generally tough and very elastic, ranging from low modulus to high modulus types. Primers are frequently required to promote adhesion to many substrates. Polysufphides. Polysulphide materials have a long pedigree as sealants and have been used in building and civil engineering applications for many years. The sealants are generally tough and elasto-plastic in nature, and both one- and two-part products are available; slow through-cure is generally associated with the one-part materials. Because of their relatively fast cure, two-part products are particularly suitable for joints subject to large movements soon after sealant application. Acrylics. The two types of sealants in common usage are solvent- and water-based materials. Solvent-based sealants are thermoplastic materials with plasto-elastic properties. They possess good weathering properties and exhibit excellent adhesion to a wide variety of surfaces; for the latter reason they are sometimes used in resealing operations where adequate surface cleaning is difficult. Water-based materials are used primarily for internal sealing. Butyl mastics. These materials, which date back to the 1960s are essentially plastic in nature and are susceptible to UV attack. This type of sealant is seldom used externally today but its residues on joint faces make it very difficult to obtain adequate adhesion with modern sealants. Oil-bused mustics. These are the oldest type of sealants available and are plastic in nature. They are based upon blends of drying- and non-drying oils and plasticisers reinforced with polymers and fillers. The skin of such materials thickens and toughens with age, reducing the flexibility of the seal. Again, the residues of such materials in joints provide difficulties for resealing with modern sealants. and Building
Materials
1995 Volume
9 Number 6
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Sealing and resealing of joints: A. R. Hutchinson
et al.
Curing Development of full cure may take anything between a week and several months depending upon generic type, cure mechanism, sealant mass, joint geometry and atmospheric conditionss. Sealant formulators strike a balance between speed of cure, pot-life, tooling requirements and long-term retention of properties such as elasticity and movement accommodation. The curing of a two-part sealant occurs on mixing two components, the base polymer and a curing agent. Once thoroughly mixed, the sealant will cure in a homogeneous fashion throughout the bulk of the polymer. Two-part products generally achieve a high degree of cure in a shorter time than an equivalent one-part system, assuming proper and complete mixing of the two components. Moisture is required, and the presence of oxygen and elevated temperatures may also accelerate cure development of some materials. One-part sealants rely upon atmospheric moisture and oxygen to initiate cure reactions; moisture often acts as the catalyst for the action of an internal curing system. Cure begins on the surface with the formation of a skin; full cure of the bulk of the polymer then follows from the outside, moving inwards. An increase in temperature, humidity and air flow can accelerate curing.
Joint preparation - new sealing Sealants form a seal by bonding to both sides of a joint and forming a flexible link between the two substrates. Maintaining a seal is dependent on the strength of the bond to the substrates being greater than the forces required to stretch or compress the sealant. Joint performance therefore relies upon the sealant gaining adhesion, and remaining bonded, to the joint faces. Adhesion between sealant and substrate (or primer material) relies upon interfacial forces of attraction, requiring fundamentally: l l
intimate contact between the materials involved; absence of weak layers or contamination at the interfaces such as dirt, dust, grease and existing sealant residues.
affinity to the sealant and to the substrate concerned; substitution of primers can therefore lead to catastrophic failure. Experience has shown that with proper joint design and sealant application, the use of appropriate primer materials results in joints of the greatest durability - particularly under prolonged damp and wet conditions.
Joint preparation - resealing When replacing existing seals, there are essentially three cleaning and preparation options: Option 1: Complete removal of the old seal Option 2: Removal of most of the old sealant Option 3: Oversealing. Option I In most cases the first option of complete removal is intended, particularly if the original seal has failed by loss of adhesion. However, it may be very difficult or even impossible to remove all traces of the existing sealant (and primer, if present) without removing a surface layer of substrate material. Option 2 Where it is impossible to completely remove all traces of existing sealant, and where seal failure was cohesive, it may be acceptable to leave a film of residue, say ~0.5 mm thick, although this will aimost always be at the expense of performance 19. A reduction in joint extension and performance is almost inevitable, in proportion to the type and amount of residue. In the course of the RESEAL project’ a number of important findings were made regarding ‘contaminated’ surfaces: l
l
l
Primers, being relatively low viscosity materials, assist adhesion either by penetrating the pores of a porous surface or by forming a chemical link between the surface and the high viscosity sealant. Primers may also bind and reinforce weak surface layers of certain substrates such as concrete or stone. Where painted surfaces are involved, care must be taken to ensure that the paint will remain securely bonded to the substrate and to ensure that there is no detrimental interaction between the paint and sealant, sealant cleaners or primers. Two main types of primer are associated with the majority of sealant materials, appropriate to porous and non-porous surfaces. Sometimes the design of the primer is unique to the application, having specific 384
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l
l
A 0.5 mm thick layer of residual sealant resulted in a significant drop in joint performance; priming such a layer did not help. Non-curing and silicone sealant residues, particularly on porous surfaces such as concrete, severely reduced the performance of joints resealed with curing products. A very thin layer, or smear (co.1 mm thick), of a curing sealant residue should provide acceptable adhesion performance. Better adhesion is gained to curing contaminants than to non-curing contaminants when resealing with a curing sealant. The general order of adhesioni,” found from tensile adhesion testing of joints made with the following contaminants during the course of the RESEAL project was: polysulphide > polyurethane > acrylic > butyl rubber > silicone > bitumen > GP mastic. Silicone sealant residues are difficult to remove. However, they represent low energy surfaces and as such are difficult to adhere to other than with identical products (if this represents a suitable option from other considerations). This poses a future problem for
9 Number 6
Sealing
resealing, given the widespread use of silicone materials currently. Clearly tests should be carried out to establish compatibility and adhesion, including simple surface analytical technique#. Additionally, the new sealant must have a lower modulus than the old residue or else any joint movement will impose a disproportionate amount of strain on the existing material, causing rapid failure. Option 3
Oversealing butt-joint seals may sometimes offer the most cost-effective solution. Oversealing may be easy with deeply recessed joints, by creating a new joint in front of the old, or with a bandage joint formed over the face of the original joint. In all cases it is important to separate the new joint seal from the existing material with bond-breaker tape or back-up foam. Various details are shown in Figure 4. Cleaning techniques for resealing
The
applicability
of
various
cleaning
techniques
and resealing
ofjoints:
A. R. Hutchinson
et al.
depends upon joint accessibility and by the nature of the materials surrounding and forming the existing jpint. Most sealants are rubbery or plastic in nature, and if subject to abrasion they tend to spread, clogging abrading tools and spreading with heat. The first stage involves bulk sealant removal with knives and chisels, cutting and scraping along the joint faces. When the contaminating layer has been reduced to a minimum, further removal may be possible by either mechanical means or solvent cleaning. Mechanical removal is possible on surfaces such as stone and concrete, followed by dust removal and solvent wiping. Careful scraping and solvent cleaning is appropriate to more delicate finishes involving metals, painted metals and timber. The use of solvent in conjunction with a mild abrasive, such as nylon pan-scourer, is usually effective on non-porous surfaces. Solvent cleaning itself often represents the final stage of cleaning and preparation but care must be taken in solvent selection for delicate finishes. It should also be noted that traces of original sealant materials may be dissolved and spread, causing staining or further contamination. Finally, most solvents are potentially dangerous and should be used with care. The sealant system and the sealing process The sealant system consists of the sealant itself, primer, and back-up foam or bond breaker tape (if used). The sealing (and resealing) processes are depicted in Figure 5.
F
: ....,’ .
KeY
q q q ISI
vc .I:
-1.:. . ..__..
>,
:::::
. .
IdI
l Adheswe/stiff
Sealant
q
Back-up
Substrate
New
Sealunts
In new sealing the sealant should be selected to have the longest life commensurate with a high probability of successful application to achieve that performance and to suit the expected life of the building. Fundamentally, the sealant must be able to accommodate the expected type, frequency and amplitudes of joint movement whilst also being compatible with the substrates involved, particularly if staining is a possibility. In resealing, sealant choice will probably be a compromise in terms of compatibility, joint cleaning requirements, joint geometries and access difficulties. Identification of the existing sealant materials is a first step based either upon physical characteristics such as elastic recovery, feel, smell and burning odours or by chemical identification in a laboratoryi. In choosing the sealant system the following check procedure should be followed by the specifier or contractor:
sealant
sealant
l Filler
board
-
Bond-breaker
0
Metal
-
Nail
tape
or plastic
l
plate
or screw
l
Figure 4 Oversealing
details
Construction
Identify the type(s) and causes of movement affecting the joint(s). Check the type of movement and select the sealant type appropriate, e.g. elastic, elasto-plastic, etc. Check the joint size and amplitudes of movement to assess the minimum movement accommodation factor (MAF) required. Check surface conditions required for each suitable sealant.
and Building
Materials
1995 Volume 9 Number 6
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Sealing and resealing of joints: A. R. Hutchinson
et ~1. Back-up materials
Building qssessment Estimation ofjqint movements
I RESEALING
I NEWSEALING
Site survey
f
Potential ne”; joint design
Jointd?gn
1
L
Sealantselection Testing/compatibility trials Draf? specification Application detailshtrwtions Site trials and evaluation of joint preparation and sealant system
Joint depth is controlled by the use of a back-up foam or other joint filler materials. To ensure two-sided adhesion of the sealant to the joint faces only, the back-up or filler should be a material to which the sealant will not adhere. Closed-cell polyethylene foam materials are the preferred choice because they can be compressed in the joint initially, and will expand and contract with the joint providing continuous support for the sealant. As a general rule, the backing foam should be at least 20% wider than the joint and the thickness at least half the width. In resealing joints the back-up material will generally need to be replaced with an appropriate type and size of material to ensure optimum seal performance. Where polystyrene or fibreboard are used as joint fillers, or where oversealing is to take place, bond-breaker tapes should be applied to prevent sealant adhesion.
I
Final specification Access Preparation of joint surfaces Final cleaning Priming Joint width/depth
Masking
Where a flush joint or an especially neat appearance is required, the joint edges may be protected by masking tape before priming, sealing and tooling the joints. After tooling, the masking tape should be removed before the sealant has cured.
Sealantmaterials Applicationmethods Back-upmaterials Tooling Curing
Tooling
This final but essential process compacts the sealant into the joint, fills the corners, eliminates air bubbles, improves wetting and adhesion, and smooths the surface of the sealant. Most joints are tooled to a smooth, slightly concave, surface with a slightly rounded piece of wood kept wet with a suitable wetting agent.
Cleaning up Quality control
Acceptancecriteria Maintenancerequirements Inspectionandre-work Finalinspection
Timing
I Contract
complete
Figure 5 Sealing and resealing processes
l
l
Assess whether the surface conditions can be achieved for each sealant system, i.e. cleaning required, priming, compatibility, etc. Short-list possible sealants and specification; check cost and life expectancy.
Primers
The important performance enhancements promoted by the use of primers on clean surfaces have already been discussed; psychologically, the requirement to prime surfaces also focuses the mind of the sealant applicator on the importance of surface preparation. However it was found during the course of the RESEAL project that primers designed for substrates conferred no performance advantages on sealant residues of about 0.5 mm thick, although they did seem to help on thin layers or smears of contamination. Careful application of primer is essential to ensure that it is confined to the joint area, particularly with highly visible joints. 386
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Many of the permanent deformation movements such as shrinkage and settlement in buildings occur relatively early in the life of a building. Delaying the sealing of structural, and other, joints until after most of the permanent movements have occurred can reduce the demands on sealed joints and therefore increase life expectancy. Specification Having considered the foregoing points and their implications, together with carrying out laboratory and/or site trials, it is important to document the requirements in the form of a detailed contract specification. This specification should include the following points: construction details, quantities of joints involved and variation in joint dimensions; l minimum and maximum joint sizes, and sealant width/depth ratios to suit the anticipated range of joint sizes; l condition of the joint faces and required surface including methods for the removal preparation, of existing sealants or other contamination if appropriate;
l
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method of final cleaning and priming if required; l type(s) of joint back-up material, including sizes and shapes to suit the range of joint sizes and expected movements; l sealant specification, material, section, width and depth, recessed or flush joint; 0 tooling specification; l procedures to be followed in the event of variance outside the contract specifications; l timescales, including details concerned with unsuitable weather or other limitations which may be experienced during the contract; l planned life of the building, design life for the seal and the maintenance/inspection requirements.
l
A detailed specification represents one of the cornerstones for ensuring adequate quality.
Closing remarks Successful sealing and resealing is achievable with proper joint design, sealant system specification and application. Clearly the application process will be operator-dependent so that the processes of cleaning and preparing the joints, together with preparation and
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application of the sealant, should be defined to ensure that the applicator is fully aware of the process requirements and the standard required.
Acknowledgements The authors express their appreciation to the EPSRCDOE LINK CMR Management Committee and to all of the sponsors and partners for their support of the RESEAL project.
References Woolman, R. In Resealing of Buildings: A Guide IO Good Practice ed. A.R. Hutchinson, Butterworth-Heinemann, Oxford, 1994 Guide to the Durability of Buildings and Building Elements, Products and Components, BS 7543, British Standards Institution,
1992 Allen, K.W., Hutchinson, A.R. and Paghuca, A. A study of the curing of sealants used in building construction. Int. J. Adhes. Adhesives, 1994, 14, (2) 117-122 Pagliuca, A. and Hutchinson, A.R. Durability of resealed building joints. BRE/RILEM Seminar on Durability of Building Sealants, Building Research Establishment, Watford, UK, 1l-12 October 1994. In Durability of Building Sealants, eds. J.C. Beech and A.T. Wolf, E&FN Spon, 1995 Pagliuca, A. and Hutchinson, A.R. Adhesion properties of sealants in resealed joints. In Science and Technology of Building Seals, Sealants, Glazing and Waterproofing: 6th vol ASTM STP 1286, ed. J C Myers, 1996 (in press)
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