Building Envelope Maintenance
The first impression one gets of a well maintained building is the way its exterior envelope looks. Giving the exterior cladding a face lift when it is needed makes a very old building look young and attractive and hence adds to the value of the building and individual units. Before you undertake any repairs, you need to consider the following factors.
Before undertaking repairs to the exterior walls or balconies of a building, a condition assessment to determine existing conditions and the nature and causes of poor performance and visible deterioration, must be conducted. If this information is not obtained, there could be a high risk of the repair failing.The extent of the condition assessment depends largely on the aims and objectives of the repair work. The factors that affect the repair objectives include safety and structural integrity, performance and economic service life. Other factors may include serviceability, appearance and cost.
A condition assessment of exterior walls of a building is sometimes required even if repairs are not contemplated. For example, a condition assessment may be carried out as part of an on-going program of monitoring to establish the performance characteristics and aging characteristics of the walls. This information would be useful in projecting cash flow requirements in a reserve fund study.
A condition assessment is often limited to non-destructive evaluation. Sometimes additional destructive investigation, such as test cuts, is required to confirm or augment the information obtained non-destructively. A thermographic scan of the exterior building envelope may be a useful tool in establishing the condition of the building envelope.
Exterior cladding may consist of brickwork, precast concrete panels, glazing panels, exterior drywall and stucco, exterior insulated finished systems (EIFS), metal siding, wood siding and other materials, and its performance depends on good design, correct construction and regular maintenance.
The two most widely utilized cladding materials in condominiums/strata, are brick masonry and exposed concrete.
While concrete has an impressive track record for durability, as demonstrated by such buildings as the Pantheon, it nonetheless has properties that make the development of cracks inevitable and regular maintenance necessary.
Concrete is a mixture of aggregate and cement paste; as the cement paste sets and hardens, it shrinks by as much as 25mm in 30m. While most of this shrinkage occurs during the construction of a building, some will occur afterwards. This means that concrete always cracks; the number, size and location of the cracks is controlled by the amount of steel reinforcement embedded in the concrete and by the number and location of expansion joints and control joints.
Hardened concrete also deforms. This process continues for a period of time that is estimated to be from 5 to 10 years following construction.
Elastic and creep deformation is the gradual compression of concrete under load. It results in the overall height of a 20 storey building being reduced by up to 30mm and in floor slabs deflecting downwards by as much as 15mm at mid span locations. It will also cause balconies to deflect towards the outside edge.
Floor slab deflection due to creep is a major cause of the cracks in the exposed concrete forming the exterior spandrel walls of many high-rise buildings constructed in the eighties and early nineties.
Shortening of the overall height of a building and deflection of the outside edges of balcony slabs are important considerations to take into account when repairing and maintaining a building envelope.
Both drying shrinkage and creep movement are irreversible processes with the one being effectively completed within the first year and the other within the first five to ten years.
Temperature and Moisture
Concrete expands and contracts with changes in both temperature and moisture content. This movement is mainly accommodated by expansion joints, but unfortunately expansion joints often fail to accommodate all of the seasonal movement that occurs due to temperature changes, movement that may be increased by changes in moisture content.
Change in Width
The movement that does occur at the expansion joint is accommodated by cracks in the walls and slabs created by drying shrinkage and creep deformation. This causes the cracks to change in width between summer and winter.
Concrete prevents embedded steel from rusting due to high pH level of the cement paste, but the presence of salt in the concrete above a certain threshold level, when activated by water, changes this pH level and creates conditions that cause rusting of the reinforcing steel to occur. Since rust occupies a much larger volume than the original steel, tensile stresses gradually develop in the concrete, and eventually the sections of concrete overlying the rusting bars crack and break loose.
Lack of Cover
A similar condition will develop without the presence of salt at locations at which reinforcing bars have very little concrete cover, such as often occurs at shearwalls and floor slab edges, since under these conditions the cement paste component of the concrete no longer has the ability to protect the steel against rusting. Therefore it is important when repairing concrete to properly identify the factors causing the cracks and to design a repair that takes into account the natural properties of the concrete.
For example, the procedure followed in repairing cracks due to creep movement should be different before movement is completed compared to afterwards. Similarly it is important to take into account whether cracks are active or passive, are structural in origin, are due to too little concrete cover over the reinforcing steel, are caused by salt in the concrete, or have developed for other reasons.
Active or Passive Cracks
When repairing cracks, it is important to first establish whether they are “active” or “passive”. Active cracks change in width seasonally while passive cracks do not change in width.
Passive cracks are normally located either inside a building where the concrete remains dry and at a constant temperature or close to expansion joints which accommodate the thermal movement. They also may result from structural distress caused by impact or overloading.
The control of cracks in concrete (and masonry) is achieved by incorporating into a building control (construction) and isolation (expansion) joints at pre- determined locations in floors and walls where cracking or movement is anticipated. These joints must be kept sealed against water penetration.
Construction joints also occur in concrete structures between adjacent pours, such as between floor slabs and shearwalls. These joints, which normally have reinforcing steel passing through them, are prone to water penetration and in floor slabs are weak locations at which drying shrinkage cracks develop. They should be kept watertight and maintained.
Brick cladding is the most common cladding system used in Canada, and for good reason – it is extremely durable and, skillfully applied, one of the best protections possible against the elements. It is also very versatile and can be used to create either solid, cavity or veneer wall design.
Solid masonry consists of an inner skin, or wythe, of concrete masonry and an outer wythe of brickwork. The two wythes are connected by metal wall ties and the joint (collar joint) between them is filled with mortar. Sometimes brickwork itself is used to create the bond between the two wythes. Bricks that occur every sixth course of the veneer brickwork are turned 90° to extend back into the inner wythe.
Clear Space or Cavity
Cavity walls normally consist of an outer wythe of brickwork connected to an inner wythe of concrete masonry with a clear space or cavity in between.
Since a single thickness of brickwork is not watertight against heavy rain, impermeable flashing is a must in the cavity wall. Located at the bottom of each panel, the flashing slopes outward to direct rainwater away from the brick through holes left in the bottom row.
Veneer is an exterior facing of brickwork applied to a structural backing. Again, there is a cavity behind the brick veneer. This makes flashing and weepholes at the bottom of each panel essential. The veneer protects the other wall components from direct exposure to sunlight, wind and moisture.
A widespread problem in brick clad buildings is the localized deterioration of brickwork. This problem rarely occurs in low-rise buildings due to fundamental differences in the design and operation of high-rise and low-rise residential buildings.
There is a potential both for larger expanses of brickwork to be incorporated into high-rise buildings and for greater air and water vapour pressure differentials to exist in high-rise buildings.
Brickwork only deteriorates when it is in a service environment that is too severe for its performance characteristics to accommodate.
Assuming a wall is properly designed, the masonry constructed to a reasonable standard and that the materials satisfy the minimum durability of the Ontario Building Code, then the major factors that may singly, or in combination, cause progressive deterioration of brickwork are:
Differential movement in the brickwork caused by:
- Temperature changes
- Moisture changes
- Movement of the building frame
- Expansion of the brickwork
- Cycles of freezing and thawing with the brickwork in a saturated condition
Experience of the Designer
Unfortunately until recently the Ontario Building Code did not include specific requirements regarding joint location and spacing but rather relied on the experience of the designer. This often resulted in an insufficient number of joints or incorrectly located joints.
Thermal movement can result in the development of both vertical and stepped cracks in the brickwork as well as the gradual deterioration of mortar in the joints. The development of vertical cracks often causes bricks on the line of a crack to break.
Repair the brickwork by the replacement of cracked bricks and tuck pointing of deteriorated and cracked joints, and then to create in the brickwork control joints located as required to accommodate future movement.
A historic footnote to this aspect of masonry deterioration is the widespread incidence among our older brick clad high-rise buildings at locations at which it is supported on shelf angles for it to bulge and spall in horizontal lines coincident with floor slab locations due to absence of horizontal control joints required to accommodate creep shortening of the concrete frame of the building, deflection of the floor slabs at mid spans and expansion of the brickwork.
Freezing and Thawing
Bricks subjected to cycles of freezing and thawing while wet, have a potential to deteriorate due to the development of cracks. This type of failure is normally localized and progressive and is characterized by the bricks separating into thin slices similar to the cross-sectional appearance of an onion. For freeze/thaw failure of brickwork to occur, it is necessary for it to be subjected to cycles of freezing and thawing while in a saturated condition.
Sources of Water
The sources of water available to saturate brickwork are:
- Snow and ice melt water running down over the face of the brickwork from snow and ice accumulations perched on window sills, ledges, balconies and the flat tops of walls
- Water vapour or warm humid air exfiltrating out through the exterior walls during cold weather
Since it does not rain when it is freezing and vice versa, rain is rarely available to saturate the exterior face of brickwork during periods of freezing air temperatures. Rainwater may however be trapped and accumulate either inside the voids that are cast into some bricks as part of the manufacturing process or in the wall cavity and in this way, be present during periods of freezing and thawing.
Melt water from accumulations of snow and ice perched above brickwork will flow down and locally saturate the outside face of the brickwork during periods when ambient air temperature is fluctuating around the freezing point and this creates ideal conditions for deterioration of the brickwork. This is one reason why brickwork deterioration is often associated with the areas below windows and just below the roof.
Another cause of brickwork deterioration below windows is the condensation of water on the inside of windows which ponds on the sills and then finds its way through the joint between the window sill and the window frame and into the brickwork just below the window which could become saturated and vulnerable to freeze/thaw deterioration.
Deterioration of Inside Face
Warm humid air exfiltrating out through the exterior walls of pressurized buildings during periods of cold weather creates a potential for condensation of water vapour to occur on the inside face of the brickwork and this can lead to freeze-thaw deterioration of the inside face. Such deterioration is normally limited in extent and restricted to those locations where excessive exfiltration is occurring, thus brickwork deterioration may occur at the outside face, at the inside face or in the middle of the brickwork with deterioration normally being limited to small areas at typically repetitive locations. Maintenance can change conditions outside or inside the brickwork to eliminate the potential for excessive amounts of water to be present in the brickwork during periods of cold weather.
One of the easiest ways of establishing the extent of deterioration and the locations at which deterioration is occurring is by utilizing a digital camera, sensitive to changes in temperature, known as a thermographic scan.
Essentially the process consists of measuring the surface temperature of the building envelope to record variations in thermal radiation. This process would identify:
- Whether thermal insulation requirements have been met
- Cold spots or leakage paths
- Effectiveness of different retrofit insulation techniques
- Thermal bridging
- Variation in the thermal resistance characteristics of joints in window assemblies
In response to mildew problems on the interior of the building envelope, thermographic scans of the interior surfaces may be carried out to establish the conditions contributing to the development of mold. With older buildings, whose cladding has been allowed to deteriorate, it may be necessary to consider other, more drastic, forms of repair, such as installing a new cladding system. In adopting this type of approach, consideration should be given to the installation of a complete and continuous air and vapour barriers.
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