Carbonation of concrete
This is a problem that affects reinforced concrete and has been discovered across a broad range of PRC houses. Carbonation is a natural process that takes place in all concrete but, where insufficient allowance has been made for its effect, it can have disastrous consequences on reinforced concrete, as it can lead to corrosion of the steel and cracking/spalling of the concrete.
Fresh concrete contains calcium hydroxide which is highly alkaline and protects the steel reinforcement, preventing it oxidising. The concrete will, over time, react with carbon dioxide in the air to slowly form calcium carbonate, which is insufficiently alkaline to protect the steel. This process will slowly penetrate the concrete to a depth of 50mm or more. Where the outer layer is also affected by moisture, this can accelerate the process. The effect is to lead to the rusting of any steel within it. The rate of carbonation will depend on a number of factors:
• Quality and density of the concrete - improperly compacted lightweight concretes are particularly prone to the problem
• Exposure of the building - the process requires moisture
• Relative humidity of the atmosphere - carbonation is encouraged where it is between 25 and 75%, in particular, the higher range of 50-75% (this often results in internal components being affected more quickly than wetter external members)
The design of any reinforced concrete component should ensure that the steel is placed at a sufficient depth to prevent carbonation reaching it during the anticipated life of the building. Similarly, the manufacturing process must strictly follow this criterion.
Unfortunately, many prefabricated and cast in situ reinforced concrete components of system houses (and medium and high-rise blocks of flats) were poorly manufactured, with insufficient depth of cover to the steel, as well as poor quality concrete. This inevitably leads to carbonation problems over a period of time. Inadequate thermal insulation was also a common problem with these buildings and, when combined with the former defects, tends to accelerate the carbonation process. Furthermore, the presence of chlorides in the concrete (refer to the next sub-section) increases the depth and rate of carbonation.
In appearance, the affected component will tend to show longitudinal cracking along the line of any steel reinforcement. Its initial appearance will be in the form of hairline cracking, which can occur as early as a few months after construction. Over time, expansion of the rusting steel results in the column cracking along its length, as well as spalling of the surface concrete.
The presence of carbonation can be determined by in situ testing of the concrete. A chemical indicator (manganese hydroxide or phenolphthalein solution) is applied to the surface of the suspected area and will show both the extent and depth of any attack. Determination of the effect upon the steel is then necessary. It is normal to assume that the current rate of carbonation will continue and that, once it reaches the steel, corrosion and longitudinal cracking will commence immediately.
System buildings that have been shown to have instances of carbonation problems include:
• Boot pier and panel cavity (in England, where breeze aggregate was used)
• Cornish Units (in South West England, where high levels of chloride additive were used)
• Easiform cavity wall
• Reema hollow panel
• Orlit (for similar reasons to Cornish Units)
• Parkinson framed
Chloride attack of concrete
Calcium chloride was commonly added to concrete up to the late 1970s to accelerate its curing time, especially during cold weather, and thus speed the construction process. Unfortunately, it can break down the alkaline content of the concrete, especially where it has been introduced as an on-site additive (rather than during cement manufacture). Quality control of on-site additives is always difficult. It often resulted in uneven distribution of the chemical throughout the concrete which tended to exacerbate the problem.
The loss of alkalinity within the surface of the concrete removes its protective capability to stop the encased steel from oxidising. Where carbonation is present, chloride attack can increase the rate of oxidisation of any steel reinforcement. However, chloride attack can lead to the steel suffering corrosion even if carbonation is not present.
The appearance of chloride attack differs from carbonation in that it tends to induce large cracking or bulging within the concrete of a more localised nature. The steel can also suffer sudden failure, especially in the presence of both chloride attack and carbonation, as it can become relatively brittle due to extreme corrosion. Low levels of chloride ions (below 0.4% by weight of cement) are not considered to be of concern, unless carbonation is present. Between 0.4% and 1.0% by weight, cracking is assumed likely to occur within 5 years, even quicker if carbonation is also present. Where high levels of the chloride are present (above 1% by weight) corrosion of steel can occur, even if the concrete is highly alkaline.
System houses experiencing such problems include:
• Cornish Units in South West England
• Reema hollow panel
Very little consideration was given before the early 1970s to the level of thermal comfort enjoyed by the occupants of dwellings. New houses, including system-built housing, had extremely low levels of insulation incorporated into their construction. Generally, system-built houses constructed with a clad steel or concrete frame suffered more severe problems because their structural form and the lightweight materials used were extremely thermally inefficient. This was a situation that was normally exacerbated by a lack of any central heating system.
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