There are many common defects which are found within residential properties, some of these defects are easily preventable with the right knowledge and application when carrying out works to properties.
In this blog we will look at the process known as ‘spalling’, examples of which can be seen in the below photographs. This is a process whereby the face of a brick will ‘blow’ giving an eroded or cracked and flaking appearance, often at lower level within a wall but occurring at any height where the conditions are sufficient for the process to begin.
Some types of bricks are porous and often the older they are, the more porous they are. Mortar (the bedding on which the bricks sit and the pointing between the bricks) is also porous to a greater or lesser extent than the bricks dependant on the mix.
When water freezes it expands by approximately 9% in volume, if there is too high a moisture content within a brick or joint it will shatter/blow the face of the brick/joint. Water will always find the path of least resistance and therefore if the density of the mortar is a lot greater than that of the brick, the water will evaporate via the brick rather than the mortar. Therefore when temperatures drop during the colder periods of the year the water in the face of the brick will expand leading to the ‘blown’ face we can see above.
Two golden rules when selecting mortar are:
-No single mortar is best for all purposes
-Select a mortar type with the lowest possible compressive strength while still meeting project requirements.
Mortar usually accounts for approximately 17.5% of a wall built in a standard stretcher bond (cavity wall), therefore making it essential to have the correct ratio within the mix to ensure best performance of the wall. The primary constituent of mortar is sand, the air gaps in between the grains of sand account for approximately 1/3 of the volume depending on the type of sand used; the binder used to fill these air gaps dictates the properties of the mortar. For example, 1 part cement to 3 parts sand would produce a strong but brittle mortar that was very impervious to water, whereas 1 part cement 1 part lime to 6 parts sand produces a well filled mortar with extra adhesion and flexibility and better permeability to water. The amount of water added will also affect the mortar.
Clearly, mortar used in roofing applications should be impervious to water for obvious reasons but will be brittle meaning that should the building move at all (which is common with old brick buildings) the mortar will crack. Careful consideration must therefore be given to the correct mortar type for the application.
Lime mortar is porous and permeable and therefore allow water to evaporate from the face of a wall via the mortar and pointing, thereby protecting bricks from the process of spalling. Lime mortars are also useful internally to assist with moisture control as the material is breathable.
Spalling therefore most often occurs in older brick built buildings, commonly with softer red bricks which are very porous and where the builder has incorrectly specified a mortar without lime.
To ensure the correct mortar is selected for your requirements good general knowledge should be combined with BS EN 998-2 which is a British Standard document dictating specifications for mortar. There are many considerations which would affect the constituents used such as: weather at the time of laying, required compressive strength, exposure of brickwork to the elements during its lifetime and the type and age of bricks you are using. To ensure that the correct mortar is selected to meet requirements, all these factors should be understood and quantified, so they can be cross referenced with tables provide in BS EN 998-2. This will ensure the correct selection of mortar.
As a rule of thumb, older, softer bricks need to have a softer pointing to protect from spalling. Newer more impervious bricks can have a harder mortar.
If you are concerned feel free to call The Hopps Partnership.