Building Pathology Flashcards
What is regent street disease
Regent street disease / deans gate dilemma occurs in buildings constructed using a steel frame covered in facing masonry or stone. Often found in early twentieth century buildings. The porous materials allow moisture to get into the steel structure which then corrodes.
Corroded metals can expand 7 times in size cause the external masonry to crack.
What is High Alumina Cement ?
HAC is a defect found in building during 1954-1974 (20 years)
HAC gives high early strength and was originally used in maritime applications. It is found mainly in Pre stressed Pre-Cast Concrete Beams. It undergoes a mineralogical change called conversion which increases porosity and reduces and reduces resistance to chemical attack. Therefore where water and chemicals are present it can cause the concrete to become friable and less to a loss of strength.
What is Carbonation?
Carbonation occurs when the alkaline elements of the concrete react with the atmospheric carbon dioxide creating calcium carbonate which lowers the PH levels of the concrete and therefore reduces the passivity of concrete. It will cause gradual neutralization of the alkalinity from the surface inwards. Should there not be adequate coverage of the steel reinforcement within the concrete, carbon dioxide can react with the re-bar and cause corrosion.
Inadequate coverage, mix proportions compaction. Temperature, humidity and CO2 levels can all affect the level of carbonation.
The rate of which carbonation occurs Is inversely proportional to the square root of the age of the building i.e typical portal cement concrete may show a depth of carbonation of 3-5mm after 10 years.
What is Cathodic Protection
Cathodic Protection relies on the passage of a DC current from the environment into the protected metal surface to reverse the direction of electric current associated with the corrosion process. It does not make good previous corrosion but suppressed the continuation of the process
What is Chloride Attack?
Calcium Chloride Additive
Explain Chloride Attack?
Calcium Chloride Additive — Used as an accelerator mainly in the 1950’s and 1960’s (it was banned in 1977). Reduces the passivity of the concrete in damp conditions leading to corrosion in the form of localised pitting of the rebar. The inclusion of calcium chloride ion into concrete increases the electrical conductivity of the concrete and with this, the likely corrosion rates. Sufficient quantities of chloride ion can disrupt the passive layer
around steel reinforcement, and if they exceed a threshold level can prevent the passive layer from being re-established.
However, whilst chlorides are no longer added to concrete as an admixture, they could still be present as a result of poorly washed marine aggregates, or from exposure to deicing salts, salt spray, sea water and the like. Chlorides can be absorbed into the body of the concrete by processes of absorption and diffusion. Potentially, concrete affected by external contamination sources is likely to be at more risk of corrosion
How to Identify Chloride Attack?
Rust staining and delamination of concrete. Examination of the reenforcement bar will show signs of pitting where localized breakdown of the passive oxide layer has occurred. Occasionally the rebar may be completely eaten away leading to
obvious structural concerns
What is the remedial action to Chloride Attack?
Localised concrete repairs can be carried out, however if the original mixture includes chlorides then it is likely that the corrosion will commence in another area. A longer term and more expensive solution is to induce cathodic protection. This is where an external
anode is connected to the metal and the passage of an electric current is used to stop
corrosions.
What would you see when trying to identify Regent Street Disease?
Cracking to masonry around the structural frame.
What is the remedial works for regent street disease?
Today there are two repair options available to address this problem.
1 - sand blasting or needle gunning the exposed steel to remove surface rust and apply a protective coating. (Which is expensive and highly disruptive) which you have to run the risk because you are only treating the exposed steel and can guarantee all the rust is removed.
2 - a structural engineer can asses the steel and if necessary it can be removed and replaced.
What would you see when identifying High Alumina Cement on site?
Identification - the concrete becomes more friable and can often turn a chocolate brown color and it is a particular issue in structural members such as roofs in warm moist environments.
what is the remedial works for High Alumina Cement?
For HAC there is no known cure or treatment. Any patch repair and external protective treatment can only delay the inevitable decay. However any building that has been identified to have HAC should be assessed and monitored.
BRAC (building regulation advisory council) identify 3 stages of investigation identification assessment and durability assessment.
what would you see when identifying High Alumina Cement?
A simple on site test can be carried out by using a simple alkali-sensitive staining technique. Phienophthanlein will turn purple on unaffected concrete where it is still high alkialine. No staining is observed where carbonation has reduced the level of alkalinity. A more robust technique is through petrographic analysis where a sample of the concrete is examined under a microscope.
What is the remedial options for High Alumina Cement?
Typical remedy is to remove the delaminated concrete, clean the re-bar by abrasive blast cleaning and patch in the concrete.
A longer term and more expensive solution is to induce cathodic protection. This is where an external anode is connected to the metal and the passage of an electric current is used to stop corrosion.
What is Alkiali Silica Reaction?
Alkali silica reaction (ASR) is the most common form of alkali-aggregate reaction. Concrete is a highly alkaline material and therefore any pore water within the
concrete will also be alkaline. In certain situations the alkaline water can react with the silica in the aggregate to produce a gel. The gel takes in water, expands and can cause concrete to crack or disrupt.
