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BRICKWORK DEFECTS: Façade or Failure?
Brick Masonry is one of the world’s oldest construction materials and dates back to 7000BC where mud bricks were sun dried in warmer climates. Around 3500BC bricks started to be fired in kilns which saw brick manufacturing extending to cooler climates but it wasn’t until the industrial revolution in about 1885 that brick manufacturing adopted machine manufacturing techniques. This saw a boom in both consistency and output of brick manufacturing and lead to an industry boom in the early to mid 19th century.Despite humankinds long history of working with bricks, brickwork defects are still common. Defects can arise in brickwork because of a number of reasons, some of these may include, poor design, incorrect specification, the use of substandard materials, or poor standards of workmanship.
Common material related defects that we regularly encounter can be categorised in three ways:
- Fretting (Salt Attack) – visible in the weathering and disintegration of brick units and brick (mortar) joints.
- Efflorescence (Surface Discolouration) – visible in brickwork surface staining. This is usually seen as the white, powdery scum that can appear on masonry walls after construction but can also be brown green or yellow, depending on the type of salts.
- Thermal Movement and Brick Growth – visible as vertical cracking particularly at the corners of buildings.
Brickwork fretting can also lead to structural defects such as local and overall wall instability by affecting the bricks themselves or compromising the performance of the cavity ties between the inner and outer skins of a cavity brick wall. Let’s have a look at these defects in a bit more detail.
Fretting (or Salt Attack) of Brickwork
Salt attack of brickwork can be characterised by a loosely adhered white precipitate on the surface of the brickwork, and deposition and precipitation of salt, which is highly water-soluble.
The effect of salt attack is variable depending on the surface porosity of the brick and can only occur when three conditions prevail: –
– There is salt present,
– There is water entering the brickwork face, and
– The water must evaporate from the wall
Salt Attack is caused by the penetration of soluble salt (NaCl) into the porous face of the external façade brickwork. In coastal settings the sea spray from the ocean surf penetrates the outer face of the brickwork. The water then evaporates as the brick dries out, leaving the salt behind in the pores of the brick forming salt crystals. These salt crystals expand as they dry and with more saturation and evaporation the salt crystals continue to grow larger. The salt crystals are stronger than the clay brickwork and thus causes the fretting and breakdown of the face of the bricks. This process is shown diagrammatically below:
Saturation of the pores within the brick by dissolved salts. The depth of penetration of the brick is dependent largely on its permeability; the greater the permeability, the greater the penetration and risk of damage.
Migration of the salt solution toward the exposed brick surface by wicking when drying conditions occur.
Removal of the water by evaporation and consequent crystallisation of the salt in the pores to an unknown depth in the brick.
Resulting in damage to the fabric of the brick owing to the internal stress applied to the pores by the crystallised salt.
Fretting continues when the salt attack wetting and drying process is repeated over time.Airborne salt spray or dissolved salts are one source of salt attack. However, salt attack can occur from rising damp and extracting dissolved salts from the ground. This phenomena is limited by the use of Damp-Proof Courses.
A damp-proof course (DPC) is a horizontal barrier in a wall designed to prevent moisture rising through the structure by capillary action. (A damp-proof membrane (DPM) performs a similar function for a solid floor concrete floor cast on the ground.) Unless it has been rendered over, you will normally see it as a little black or silver line between 75mm and 600mm up from the ground level, running all the way around the external walls.A damp-proof course should restrict any damage from rising damp above the damp-proof course but below the damp-proof course may be still be badly affected. It is important to correctly locate the DPC and prevent any material from bridging the damp-proof barrier.
Efflorescence of Brickwork
Efflorescence of Brickwork is the deposit of slightly soluble precipitate on the surface of the brick.
This phenomenon is caused by the presence of water soluble minerals that originate or become impregnated in the material of the bricks and mortar. Efflorescence is often caused by gypsum or lime and in the presence of water commonly results in crystalline salt deposits on the surface of the wall of calcium sulphate, calcium carbonate or sodium carbonate.
The extent of efflorescence may vary throughout the building facades and depends on the extent of firing during brick manufacture, the amount and size of the minerals in the brick or mortar and the volume of water migrating through the facades.
Some efflorescence can be expected as part of the inherent behaviour of the materials but this should typically be minor and be readily cleaned. If efflorescence continues over time this is due to water entering and migrating through the wall which will not only be difficult to clean but may point to more significant and ongoing water ingress issues.
The efflorescence process involves the following stages:
- Saturation of the pores by rainwater to a final depth in the brick that is dependent largely on its permeability; the greater the permeability, the greater the penetration and resultant effect.
- Limited dissolution of the soluble mineral in the water that has penetrated or embedded in the brick or mortar.
- Migration of the dilute mineral solution toward the exposed brick surface by wicking when drying conditions occur.
- Removal of the water by evaporation and consequent crystallisation of the mineral as it reacts to carbon dioxide on the surface pores and to a relatively shallow depth in the brick.
- Discolouration and slight damage to the surface of the brick owing to the stress applied to the surface pores by the crystallised mineral.
Fretting and Efflorescence, how can you tell the difference?
As you can see Efflorescence and Salt Attack are quite similar but can have different consequences. So how to differentiate between Salt Attack and Efflorescence? A basic assessment can actually be done by tasting the precipitates with the tongue! Differentiation of the two can then be based on the following considerations:
1) Salt Attack: Has the taste of the water-soluble sea salt, and can be removed by saliva from the surface of the tongue (water soluble).
2) Efflorescence: Is a low-solubility Calcium Sulphate or Calcium Carbonate which has no taste and reappears quickly after evaporation of saliva from the surface of the tongue.
- BRICKWORK REPLACEMENT
Brickwork that is affected by Salt Attack is likely to be due to inappropriate bricks for the environment and not considered “Exposure Grade” bricks and inadequate for a “Severe Marine Environment.”Removal and Replacement of brickwork and mortar could be considered an effective method of rectification if the bricks and mortar do not have an adequate durability to salt attack.
Adopting Partial Brick Replacement, where only damaged or fretting bricks are removed and replaced, is possible but it is almost impossible to match the colour, texture and appearance of new to existing brick work, which may would need to be “exposure grade” bricks to limit further salt attack damage in the future.
A damp proof course or cavity flashing will need to be installed appropriately if fretting is occurring due to wicking ground moisture.
- BRICKWORK SURFACE RENDERING
Surface rendering a facade will change the appearance and look of the building and may also lead to an increase in property valuation:
2.a) Fretting brickwork
Face brickwork that has entrapped salt due to the penetration of the sea spray and causing the breakdown at the brick face (Salt Attack) could be mitigated by sealing the face of the wall with a surface render provided the following is achieved: –
Removal of the entrapped soluble salts; and
Fully encapsulating the surface to prevent no further water and salt ingress.
Removal of the entrapped soluble salts can be done by application of an absorbing poultice to the external face, such as the Westox Building Products, “Cocoon” system. It may require several applications and need to be tested onsite. The application of a surface render to the external face of the brick work would also need a fully waterproof coating and all junctions of the render, such as around windows, would need to be routed out and a flexible sealant installed. The elastomeric waterproof façade coating would need to be applied and fully maintained or reapplied with wear in the extreme environment.
2.b) Efflorescence on face brickwork
Face brickwork with Efflorescence on the surface could be surface rendered provided there is removal of the surface entrapped minerals and sealing the surface to prevent no further water ingress.
- Removal of the crystalline salts on the brick surface would involve: Use a dry stiff brush to remove a majority of the efflorescence from surface, a damp sponge to remove excess and a weak acid washing to remove the remainder. It may require several applications, then clean water wash. This may etch the surface of the bricks and mortar, and require patch repairs. The difficulty remains that there is still the embodied gypsum and/or lime within the brick. The surface cannot allow external water to penetrate otherwise embodied gypsum within the brick will be drawn out of the brick.
- Applying a surface render and an elastomeric waterproof façade coating to the external face to fully seal the face of the building. Any junctions with elements such as windows will require the installation of a flexible sealant. The waterproof coating would need to be maintained and reapplied as a result of wear in the extreme environment.
3. SURFACE OVERCLADDING
Another means of preventing water and salt penetration into the face of the bricks to mitigate salt attack and efflorescence is to over-clad the building with a watertight external cladding.
The surface cladding would need to be an impervious material that was set off the existing brickwork to allow for a cavity that would drain the existing window sub sills or any water penetration from the cladding. The cladding could be a walling system such as non-combustible cladding panels or a natural stone, or manufactured façade tiling system. These walling systems would be supported on stainless steel shelf angles attached to vertical mullions fixed to the existing façade. Any wall panels would need to be tested and certified for fire compliance.
The surface cladding would create a totally different appearance to the building, and may require Council Approval.