Concrete defects Flashcards
When was HAC first developed?
HAC was originally developed by Lafarge and produced in the UK from about 1926.
Why was HAC first developed?
The material developed high early strength and high temperature resistance, as well as resistance to sulphate attack.
What was the typical application of HAC concrete?
Precast elements.
Why was HAC concrete typically used in precasting?
The material was relatively expensive, and so tended to be used in specific circumstances, such as in the precasting industry where early removal from the moulds had economic benefits.
When was the greatest use of HAC concrete?
The greatest use of HAC was in the 1950s to 1960s, predominantly in the manufacture of prestressed ‘X’ and ‘I’ beams for floor or roof decks.
When where the earliest failures of HAC concrete experienced?
During 1973-74 when several school roofs collapsed.
When was HAC concrete banned for structural use?
1976
What is the process which HAC concrete undergoes?
HAC concrete undergoes a process known as conversion, which causes HAC to lose strength with age, increased porosity and vulnerability to chemical attack.
What happens during the conversion process of HAC concrete?
The concrete increases in porosity, which in turn results in a loss of strength and a reduction in resistance to chemical attack.
In the manufacturing process of HAC concrete, what effects how quickly conversion takes place?
The higher the temperature during the casting of the concrete, the more quickly conversion takes place.
How might HAC concrete become vulnerable to acid, alkaline and sulphate attack?
Water as well as the chemicals must have been present persistently over a long period of time at normal temperatures.
How can chemical attack be identified in HAC concrete?
Chemical attack is usually very localised in nature and the concrete typically degenerates to a chocolate brown colour and becomes very friable.
What are the generic stages of HAC concrete investigation?
Stage one: identification.
Stage two: strength assessment.
Stage three: durability assessment.
When does concrete provide good protection to steel reinforcement?
When in highly alkaline concrete (pH 12-13), because a passive layer forms on the steel to protect against corrosion.
How is the passive layer in concrete disrupted?
Once the pH of concrete falls to 9-10 the passive layer is disrupted leading to the risk of corrosion.
