Clay Products Brick
Durability of structural clay products
Materials and processes
Brick clay and fireclay are the materials used to make bricks such as facing and engineering bricks. Both of these materials are essentially sedimentary mudstones. The fireclay is given its name due to its ability to resist heat and therefore making it ideal for facing bricks because of this it has high durability under stress from fire and is therefore a great material used to line furnaces.
Structural clay products are fired in kilns and during the firing process vitrification occurs, which makes the clay in to a hard, solid and a non-absorbent mass which is durable by reducing water absorption (porosity) and in turn increasing the ability to resist frost. Through this process the clay products durability is highly increased.
Structural products can also be glazed and there are two types of glazing. Single and double fired glazing this is where the clay products are sprayed with ceramic glaze before or after drying and then kiln fired, this process makes the clay products impervious to water and water vapour. This therefore increases the durability of the clay product in terms of being able to resist water from infiltrating which is the key ingredient that causes the clay product to break down and decrease the durability to a very low level.Get help with your essay from our expert essay writers...
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Types of damage
Chemical damage to structural clay products is mostly due to the leeching of chemicals from the ground into the brick.
“Efflorescence is the deposit of soluble salts left on bricks when the water in which they were dissolved evaporates”
Efflorescence does not normally cause structural damage and will subside and disappear with time and weathering. The soluble salts that cause efflorescence come from naturally occurring salts in clay, the sand and cement used in mortar or from materials touching the bricks i.e. rain, ground water etc. Calcium hydroxide is bought to the surface by the moisture as it is soluble at low temperatures and on the surface it reacts with carbon dioxide which leaves a whitish deposit that becomes efflorescence.
Due to the types of materials used to make clay products it is virtually impossible to eliminate all the soluble salts that cause the efflorescence. Its not convenient to remove all the salts from the clay products or to prevent them from becoming saturated. Certain steps can be taken to try to minimise the appearance of efflorescence, these steps are limiting water entry, making sure that there is adequate drainage and ensure that the clay product has gone through a proper curing process. Efflorescence can be removed manually but this may cause a unsatisfactory appearance.
Sulphate attacks on clay products affect the mortar and the actual brick work. The sulphate attacks occur when the sulphate solution, from the naturally occurring soluble salts within the clay product reacts with an ingredient of the Portland cement.
“Sulphate attack to brick and hardened mortar joints occurs when sulphates in solution reacts with the tricalcium aluminate constituent, C3A, in Portland cement to form a compound called ettringite or tricalcium sulfoaluminate hydrate, 3CaO·Al2O3·3CaSO4·32H2O. The reaction is accompanied by expansion and when it occurs in brickwork mortars, the effect is an overall expansion of the brickwork”
The brickwork expands causing cracks to propagate and this weakens the brick and the mortar joints causing the durability of the material to be affected.
These sulphate attacks can be avoided under certain conditions. If the brick work is protected from saturation, then the sulphates cannot react with the tricalcium aluminate which therefore stops the expansion of the mortar and brickwork this then reduces the chance of cracks propagating through sulphate based attacks.
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Sulphate attacks can be prevented by using clays with low levels of soluble salts. The mortar used should also contain a high content of cement which will resist the water penetration as water is the key ingredient of sulphate attacks. The cement used should also be sulphate resistant cement.
Frost attack occurs when the clay products become saturated with and the temperature drops below freezing. This type of attack is usually occurs in the winter. In this process the water enters the pore in the clay product and becomes saturated and when the temperature drops below freezing it causes the water to freeze and turns into ice. This water then expands causing the pores in the brick to get bigger and then the process repeats itself until the brick crumbles. Note that freezing temperatures alone do not affect the clay products it is only when they become saturated.
Frost attacks can be prevented by designing the structural clay products to avoid being saturated, and in the places where the freeze thaw process is likely to occur frost resistant clay products should be used. Masonry paints can be used to prevent the clay product from becoming saturated. Acid rain is produced by the chemicals from our cars, factories and the fuels that are burnt everyday. The gasses from these pollutants react with the tiny droplets of water in the clouds to form nitric and sulphuric acids and these come down with the rain as weak acids.
The acid rain leaves deposits of acid particles on the surface of the clay products which cause a reaction with the minerals within the clay products; this then corrodes the clay products causing them to crumble and breakdown therefore weakening the structure in terms of durability.
All the above types of damages that are caused to structural clay products cannot be fully avoided but attempts can me made to protect them from the environmental factors. The clay products can be treated to become water resistant but if a lot of damage has occurred then there is little chance of repairing the damaged members, the best that could be done is to replace the structural member be it a brick, tile or pipe. They can simply be cut out or removed and then replaced with new members to allow the structure to retain its durability by removing the affected members.
Rising damp in buildings can be defined as the vertical flow of water up through a permeable wall structure, the water being derived from ground water. The water rises through the pores (capillaries) in the masonry by a process loosely termed capillarity. This is the spontaneous movement of liquids up or down the capillaries.
Rising damp may not be a problem in most of the buildings as there maybe other source of moisture as was found by the BRE digest.
“The BRE have advised that rising dampness was present in only 10% of the buildings they examined where dampness was assumed to have been a problem”
Before any assessments are carried out external, internal and secondary examinations should be completed to make sure than the proper source of the moisture is identified. Therefore the most economic method of solving the problem can be used.
But in general the assessment of rising damp looks for the following to decide if the problem is rising damp or something else i.e. surface condensation or rain penetration.
First the electrical moisture meter should show that there is a sharp change in moisture at the top of the damp, this point is usually identified by a general salt band at the maximum height of rise. Next there should be little or no sign of mould growth, and there should be no free flowing water on the surface. Because ground water contains small amounts of soluble salts such as chlorides, nitrates and sulphates these should also be present in the wall or the part of the structure where the damp is being identified. Any timber that is in direct contact with the wall should also indicate high moisture content. The most conclusive evidence of rising damp will be to take samples of mortar from a certain depth of the wall and test this for moisture and salt content.
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The remedies for rising damp are damp proof courses; these are usually some type of injections of a chemical solution into the walls which create moisture barriers that stop the moisture rising any further up the wall. The walls have 10mm holes drilled into them, these holes must be 150mm above the ground and 170mm apart, then the solution is injected into these holes
After the solution has been injected into the walls the moisture from the solution evaporates leaving behind a gel which crystallises to form a moisture barrier, in turn stopping the damp from rising.
These cause wetting of the surface of the masonry and therefore when the masonry paints are spread they adhere as a thin continuous layer on the surface, this in turn reduces porosity and increases the durability as it allows little or no water to penetrate into the clay product. The paint also stops the wall from breathing; this traps the moisture that is already in the wall. So when the winter comes the moisture in the wall freezes and expands by 10% and this in turn is causing the freeze thaw affect eventually causing the masonry to crack and breakdown. Different types of masonry paint can be used which allow the wall to breath and stops the freeze thaw affect from occurring as aggressively.