Chloride Attack – Chloride finds its way into and through concrete in a similar process to carbonation, due to its porous nature. Chloride ions which are introduced into concrete from de-icing salts or are within or in close proximity to marine environments can attack concrete aggressively resulting in a faster rate of deterioration compared to carbonation. When chloride passes through the concrete and eventually reaches any reinforcement, corrosion will occur. Salt is a mineral substance which consists primarily of sodium chloride. When the sodium chloride is dissolved in water, which may be present in the pores of concrete, a versatile, highly corrosive and mobile solution is formed of sodium ions (Na+) and chloride ions (Cl-). Once this solution comes into contact with any reinforcement it will attach the passive layer which protects it. The reinforcement will then corrode in the presence of air and water, resulting in corrosion. This will result in cracking and spalling which will appear very similar to the effects of carbonation.
The consequence of chloride attack can be seen on the underside of road bridges and buildings and structures in close proximity to the coast. As discussed above, the impact of chloride attack will appear very similar to carbonation, so it will be necessary to not only consider the environment but also undertake testing to confirm the cause.
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Alkaline silica reaction can usually be identified by random cracking on the surface of the concrete and in advanced cases, a gel like substance may be visible or possible spalling of the concrete. Cracking usually appears in areas with a regular supply of moisture, such as close to the watercourses and ground behind retaining walls etc. In order to confirm the presence of alkaline silica reaction it is necessary for core samples of the concrete to be taken and put under a microscope to establish their mineralogical and chemical characteristics, this is something referred to as petrographic testing.
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Possible sources of sulphates include seawater, oxidation of sulphide minerals in clay (such as copper) adjacent to the concrete (this can produce sulphuric acid which reacts with the concrete), bacterial action in sewers (anaerobic bacteria produces sulphur dioxide which dissolves in water and then oxidizes to form sulphuric acid), In masonry, sulphates are present in bricks and can be released over a long period of time, causing sulphate attack of mortar.
In the UK sulphate attack is particularly common in ‘older’ solid concrete ground floors. The use of fill material became very popular for residential solid concrete ground floor construction from the early 1940s. In the early post war years, waste materials such as burnt colliery shale, blast furnace slag and red ash were promoted by the government as appropriate materials to use for this purpose. However, it was later discovered that these types of fill materials contained high levels of sulphates which often resulted in significant problems and associated expensive remedial works to replace affected floors. Sulphates from these fill materials, with the presence of water, would attack the tri-calcium aluminate (one of the components of Portland Cement), within the concrete, which would result in lifting, expansion and cracking of the concrete floor slab. This problem was significantly reduced with the use of appropriate fill that did not contain these high levels of sulphates, together with the introduction of damp proof membranes (to reduce water penetration) and insulation to improve thermal efficiency. Replacement of a solid ground floor with a new floor is the only practical way of dealing with sulphate attack, which as you can see from the image below can be very disruptive as well as expensive.
|Source: Source: http://www.mybuilder.com/|
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|Source: Source: http://theconstructor.org/|
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