Building Pathology (L1) Flashcards
- What is wet rot?
- What moisture content does it require?
- A natural decay of timber due to high moisture levels.
- Moisture content of around 30 - 50%
Wet rot is confined to a localised damp area and does not spread, making it less destructive than dry rot. However, wet rot can be just as destructive as dry rot, and, if left untreated, can seriously weaken structural timbers and lead to major structural damage.
What are causes of wet rot?
A source of moisture such as:
- Defective plumbing
- Leaking gutters
- Broken pipes
- Leaking appliance i.e. washing machine
What are the indicators of wet rot?
- Dark brown staining on the timber
- Splitting or longitudinal cracking along the grain
- A damp, musty smell
- Timber that is damp to touch
- Localised fungus growing on the timber
- Affected area is darker in colour than the surround area
- Wood is bleached around the area
- Timber shrinkage
- Mycelium strands on timber
- The fungus does not spread
- Distortion, discolouration, loss of strength
How can you treat wet rot?
- Determine the cause of the wet rot
- Expose and excess the full extent of the infestation
- Remove all timbers that have been destroyed and/or damaged
- Repair any damage to the surrounding timbers and treat adjacent timbers with fungicidal solution
- What is dry rot?
- What moisture content does it require?
- Dry rot is a fungal timber decay known as Serpula Lacrymans.
- Moisture content above 20%
Dry rot is the most serious form of fungal decay. Dry rot fungus is very destructive as it spreads across masonry and destroys all timber in its path including structural timbers, skirting boards, door frames, and flooring. It affects all types of properties, new and old, and can be detrimental to a property if left untreated as the fungus can weaken the structural integrity of a building and cause it to collapse.
What are the causes of dry rot?
Moist, damp and poorly ventilated conditions.
Where is dry rot most likely to occur?
- Under stairs
- In lofts and attics
- In flooring
- Behind plaster
What are the key indicators of dry rot?
- Shrinking and warping of timber
- Cuboidal cracking across the grain of the wood
- A damp fungal smell
- Dry, brittle and fractured wood that will crumble and break by hand
Depending on what the stage of fungus is at in it’s life cycle:
- Spores: red dust, orange, brown-coloured spore dust
- Hyphae: fine grey strands
- Mycelium: white or grey cotton wool-like substance that spreads across masonry in search of wood for food
- Sporophore: Fruiting body. Red dust, orange in colour and looks like a flat fleshy pancake
How can you treat dry rot?
- Eliminate fungal growth by stopping the source of moisture
- Introduce/improve ventilation
- Remove all infected timbers and damaged wood including skirtings, panelling, flooring
- Treat the existing timbers with a fungicidal solution
- Replace damage timbers with pre-treated timber.
What are the common wood-boring insects found in the UK?
-Common Furniture Beetle (Most common in the UK)
- Deathwatch Beetle
- Powder Post Beetle
- House Longhorn Beetle (Found mostly in Surrey, prefer warmer South climate)
What are the indicators of an active insect attack?
- Damp timber, moisture content above 20%. Insects thrive at around 27% MC
- Holes with sharp edges rather than round
- Frass, colour of freshly-cut timber
How would you identify the species of beetle?
- Magnifying glass with focused lens
- The type of timber
- Location within the UK
- Time of year
- Size of holes
- Rub frass between fingers (CFB will feel gritty)
How would you treat dry rot?
- Find the source of moisture and rectify the fault
- Determine the full extent of the dry rot outbreak (Lift floorboards, remove plaster etc)
- Once identified, cut back the timbers in-situ to at least 500mm beyond the last signs of fungal growth or hyphae strands.
- Remove all fruiting bodies and visible spores. Clean all walls to remove surface growth
- Replace any timber with treated timber
- All new and existing timber to be treated with fungicidal solution, including masonry
What is Reagents Street Disease? (RSD)
- Also known as Deansgate Disorder is a structural/material condition affecting early 20th Century Steel-framed, masonry-clad buildings
In the late-Victorian, Edwardian and inter-war period, many buildings were constructed with skeletal structural frames
of steel and/or iron. These buildings were then clad in masonry with the voids around the frame loosely infilled with
grout, rubble and/or concrete. Although there are several advantages to this form of construction [rigidity, fire-protection]
the result is that, in certain circumstances, this environment will lead to expansive corrosion of the metal.
How to identify Reagents Street Disease?
- Property construction early 20th century
- Steel-frame with masonry-cladding
- Cracking to masonry around the structural frame from the corroding steel
How do you repair a building affected by Reagents Street Disease?
There are two options:
Option 1: Removal the external masonry to repairs the steel work; clean to remove surface rust by sand blasting or needle gunning and treating the steel with an appropriate applied protective paint coating. (Process is costly and not all corrosion may be treated)
Option 2: Cathodic Protection; relies up on the passage of a DC current from the environment into the protected metal surface to reverse the direction of electric currents associated with the corrosion process.
NOTE: It does not make good previous corrosion but suppresses continuation of the process.
What is High Alumina Cement? (HAC)
-High Alumina Cement is also known as calcium aluminate cement and it was used in concrete which gives high early strength.
-Originally used in maritime applications.
What applications is High Alumina Cement used?
- Pre-stressed precast concrete beams
Why is High Alumina Cement an issue?
- High Alumina Cement undergoes a mineralogical change called ‘Conversion’ which increase porosity and reduces resistance to chemical attack.
- Where water and chemicals are present, the concrete becomes friable and leads to a loss of strength
What are the indicators that High Alumina Cement was used concrete?
- The concrete becomes friable
- Can turned the colour of the concrete brown
- Found in structural members
How do you remedy concrete containing High Alumina Cement?
- There is no known cure or treatment
- Inevitable decay
- Where HAC is identified, it’s strength should be assessed and monitored
What is carbonation in concrete?
Carbonation of concrete is the chemical reaction between carbon dioxide in the air and calcium hydroxide and hydrated calcium silicate in the concrete to give mainly carbonates.
How does carbonation affect reinforced concrete?
-Carbonates slowly change the pH in concrete, starting at the outer layers, from 13 to between 8-9.
- The pH change can disrupt the corrosion passivation of the steel reinforcement and, if moisture and oxygen are available, the steel reinforcement can start to corrode
- This can lead to spalling of the concrete surface and a loss of mechanical strength.
- Ensure reinforced steel is adequately covered with 40-50mm of concrete.
How do you identify carbonation in concrete?
- An on-site test can be carried out by using a simple alkali sensitive staining technique
- Phenolphthalein will turn purple on unaffected concrete where it is still highly alkaline
- No staining is observed where carbonation has reduced the level of alkalinity.
- A more robust technique is through petrographic analysis (Sample of concrete examined under a microscope).
How is carbonation remediated?
Option 1:
- Assess the extent
- Remove the delaminated concrete
- Clean the steel reinforcement by abrasive blast cleaning
- Treat the exposed steel using corrosion inhibitor.
- Prime existing concrete and apply concrete repair mortar
Option 2:
- A long more extensive 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 suppress any further corrosion.
What is Chloride Attack on concrete?
Chlorides attack the passivity layer around the reinforcing steel.
Chlorides are found in de-icing salts, the ground, salt spray, the sea and even admixtures (banned in 1977).
