Technical - Building Pathology Flashcards
Can you describe what Carbonation is?
Atmospheric carbon dioxide and other pollutants mix with water and concrete to form carbonic acid. This acid increases the PH values and neutralizes the alkalinity in concrete. The concrete becomes more acidic and breaks down the passive (typical when cover is inadequate less 20mm) layer and increases the risk of corrosion to steel reinforcement.
Can you tell me about ways to treat instances of Carbonation?
- Wire brush corroded areas
- Apply a corrosion inhibitor
- Repair cement mortar
- Consider a finishing coat to prevent further carbonation
- Petrograhpic analysis in order to test.
What are the basic components of concrete?
- Water
- Aggregate
- Cement
Do you know of any British Standards relating to the correct level of steel reinforcement cover?
BS 8110 - Reinforcement cover
Reinforcement should have at least 20 - 70m of concrete dependant on its use. Insufficient cover leads to corrosion
n.b. steel can increase x4 its original size from corrosion.
Can you describe what Alkali Silica Reaction is?
Alkalines found in cement react with aggregates which contain silica. This forms a silica gel which expands when moisture is present which causes cracking and blistering.
Can you describe some symptoms of Alkali Silica Reaction?
- Map cracking
- Visible silica gel on surface
- Spalling of concrete
- Loss in strength
Can you describe some treatments for Alkali Silica Reaction?
- Monitor
- Undertake core samples
- Limit exposure to water/moisture
- Strengthen or replace affected areas
- Examine thin section of concrete under a petrographic microscope
Can you describe how you might test concrete?
- Slump Test (Measures workability of fresh concrete)
- Cube Test (Measures the compresses strength of concrete)
- Cover meter (Measures concrete cover over reinforcement)
- Concrete sampling (Laboratory sampling for chloride content(HAC))
- Test hammers (determines surface hardness of concrete e.g. Schmidt hammer
- Battery operated gauge (Detects rebards, wall ties, studs
- Core test (Determines the build-up of concrete e.g. precast and screed
In your Ethics Competency you note cracking to a building, how did you determine the severity of the crack?
On site I took measurements and photographs of the cracks, I then referred to BRE Digest 251 to assess the damage it provides guidance on the size in mm of a crack and the severity
In this example the cracks were between 15-25mm which is noted as extensive damage. Whereby as I observed windows and door frames were distorted and not fully operational.
Explain to me what penetrating damp is?
Penetrating is the movement of moisture laterally, through the walls or fabric of a building.
Explain what rising damp is?
Rising Damp is the movement of moisture upwards through capillary action through porous parts of building fabric.
What can cause penetrating damp?
Typically this would be caused by leaks to pipework, damage to the building fabric allowing water to penetrate, high ground levels, blocked drains, leaky gutters, cracked masonry, broken flashings.
What can cause rising damp?
This is generally accepted to the result of incorrect placing, faults associated with or a lack of a damp proof course. Moisture below the ground level fabric draws moisture up to generally 1m above the ground floor level. Ground levels, floods and leaks below ground can increase the severity.
How would you identify either type of damp?
Firstly visual inspection, as this is the most reliable method to understand the extend and immediate damage and potential causes.
If rising damp, internally you would be looking for 1m high tide marks at ground floor level and you may see salts rising.
Whereas, penetrating damp watermarks and damp patches will occur on inner walls and internal fabric, you may also find damaged, stained and saturated external or moss growth build up on external brick or masonry.
To better understand the severity a surveyor can use a moisture meter, drilled samples via a carbide meter or thermal imaging cameras.
How can you test for damp?
To better understand the severity a surveyor can use:
- A moisture meter (Protimeter)
- Drilled samples via a speedy carbide meter or
- Thermal imaging cameras.
What is carbonation?
It is a defect which occurs to concrete and steel reinforcement. It occurs when alkaline in concrete react with atmospheric carbon dioxide which causes the PH (alkalinity) of the concrete to lower.
This causes gradual neutralisation from the surface inwards. When then is an inadequate concrete coverage around the reinforcement, the carbon dioxide can react with the steel/re-bar can lead to corrosion and expansion can occur causing the concrete cover to spall and blow.
All concrete suffers from carbonation but it is the reaction with rebar which cause the specific defect. The rate of carbonation depends on the age of the building, atmospheric pollutants, aggregate used and the porosity of the concrete.
How do you identify carbonation?
Visual indentation of cracking which follows the lines of reinforcement and in instances rusting of the exposed reinforcement below blown concrete cover.
As simple test concrete for carbonation can be undertaken with a alkaline-sensitive staining technique (phenolphthalein indicator). The solution will turn purple/pink if still alkaline or will show clear if neutralised.
Lab test of concrete samples can be undertake via microscope known at petrographic analysis.
How do you remedy carbonation?
Typically, advice would be to remove the delaminated concrete, clean the re-bar by way of an abrasive blast clean and then apply an anti-carbonation coating before patching in a new concrete cover mix.
Alternatively, a longer term and more expensive method is via cathodic protection. When an external anode is connected to the metal reinforcement and an electrical current is used to prevent further corrosion, but it does not revert.
What other related defect might cathodic protection be used as a remedy and what is this defect?
Regent St. Disease / Deansgate Dilema is similar to carbonation but affects facing masonry or stone rather than concrete.
What is Regent St. Disease/Deansgate Dilemma?
Often found in early 20th C. buildings when large masonry or stone units were built tightly built around the steel frame. Masonry and stone materials used are porous materials allowing moisture to penetrate to the steel structure leading to corrosion and expansion up to 7 times the original size. Causing cracking to the masonry.
Identified with knowledge of the age and construction and cracking to masonry around the steel frame.
How would you remedy Regents Street Disease?
- Cathodic Protection, passage of a DC electrical current into the steel frame to reverse the currents associated with corrosion. It only suppresses corrosion does not make good.
- Removal of masonry and stone and sand blast cleaning the steel corrosion via sand blast of needle gunning and treating the steel with a protective coating.
What is cut edge corrosion?
When the edges of profiled metal cladding panels have been cut in the factory and the factory finish delaminates, leaving the edges of the metal panel exposed to the elements and at risk of rust and corrosion.
How do you identify cut edge corrosion?
Visual inspection. Early signs are loss of colour, and delamination of the plastisol/factory coating. More severe s rust and corrosion of lap and joint ends.
How would you treat cut-edge corrosion?
Apply a silicon based paint such as Giromax to provide edge protection or replace the sheet entirely if corrosion is severe.
What is subsidence and what are some typical causes?
Subsidence is the vertical down movement of part or whole of a building on its foundations due to changes in soil conditions, causing a loss of support in the soil.
Typical when:
- Common in Cohesive (Clay) soils
- Water Table Changes Drought/Flood/Leaking Drains
- Trees changing water table
- Filled or made ground (best practice to asses the depth and composition)
- A lack of adequate foundation design in the context of the soil
What is heave and what causes it?
Otherwise known as shrinkage. This is the upward movement of a building and its foundations.
This is less common, generally associated with the removal of trees in clay soil. Causing swelling of the soil due to increased water.
Cohesive soils expand contract due to moisture. Non cohesive are prone to wash out (erosion of soil due to increases in water)
What cracking would you see from subsidence and heave and what is the difference?
Subsidence – Vertical cracking in the centre of a building or diagonal at a corner. Cracks will be larger at the top and thinner at the bottom.
Heave – Vertical cracking in the centre of a building or diagonal at the corner. Cracking will be larger at the bottom and thinner at the top.
