1960s Concrete Portal Frames Flashcards
Can you tell me some typical defects to 1960s concrete portal frames?
Leaks to asbestos cement roof sheets.
Carbonation to concrete frame.
Cavity wall tie failure.
Cracking and delamination to ground bearing slab.
What was the primary type of asbestos used in asbestos cement roof sheets and how long should they last for?
Chrysotile (white) asbestos.
Between 40 – 60 years.
How might you identify if the asbestos cement roof sheets are leaking / defective?
Cracking parallel with the long edge
Cracking across the width of the sheet
Corrosion of the fixings
Moss growth and surface deterioration
How are the cracks to the width of the sheets caused?
Structural movement.
Overloading.
Wind loads coupled with defective fixings.
How does corrosion of the fixings lead to a defective roof sheet?
The expansion of the fixings leads to cracks in the sheets and subsequent water ingress.
How does moss growth cause defects and leaking to the sheets?
Carbonation increases porosity, in damp conditions, the surface is suitable for moss and algae growth.
The algae growth produces acidic run off which damages other components.
What remedial works can you undertake to asbestos cement roof sheets?
Encapsulation
Overlaying
Full replacement
Describe how you would replace an asbestos cement roof.
Consult the HSE guidance note A14 and A12 which relates to the handling of asbestos and its cleaning during removal.
Protect nearby surfaces with sheeting.
Dampen and remove fasteners and place in a waste container.
Double wrap large pieces in bags and seal.
Dispose of waste at a licensed disposal centre.
How would you identify carbonation to a concrete frame?
Cracking along the line of the reinforcement bars.
Spalling and delamination of whole areas of concrete cover.
How is carbonation caused?
Atmospheric carbon dioxide reacts with the water in concrete which forms a carbonic acid which neutralises concretes natural alkalinity.
Alkalinity of concrete is reduced, passivity lost and the steel reinforcement eventually corrodes.
Corrosion makes the steel expand which eventually makes the concrete cover crack and spall off
11How do you identify that carbonation has occurred?
Visual inspection.
Hammer test to discover unsound concrete.
The depth of carbonation is ascertained by Phenolphthalein which is a pink dye which indicates where carbonation has not occurred.
What repair options can be undertaken to address carbonated concrete?
Isolated replacement.
Electrochemical re-alkalisation.
Cathodic protection.
Tell me how you would undertake isolated repairs to carbonated concrete?
Cut out the concrete surrounding the rebar and over the full corroded length
Clean the steel reinforcement bar and apply corrosion inhibitor, i.e. with a Sika/Fosroc product.
If badly corroded, replace the steel bar by welding new piece to existing.
Replace the concrete cover with proprietary product to restore alkalinity.
Apply anti-carbonation product to external surface.
How does cathodic protection work?
In corrosion cells, anodes attract oxides which cause deterioration, whilst cathodes do not.
If an electric current can be applied to metal, it can be made into a cathode.
in an anode system, coating or mesh, is sprayed or fixed to the whole surface of the concrete.
A current is applied which then passes to the reinforcement turning the anode to cathode.
What are the signs of cavity wall tie failure?
Horizontal cracks to mortar joints at regular intervals.
Expansion of the outer leaf of brickwork.
Wall bulges at 900mm horizontally and 400mm vertically.
