General Flashcards
What is the difference between subsidence/heave and settlement?
Subsidence/Heave: Upward/Downward movement of ground level.
Settlement: Downward movement due to increased load i.e a new or altered building.
How would you diagnose subsidence or heave?
Cracks smaller at bottom and winder at top or vice versa.
Vertical cracking if in the centre of building.
Diagonal cracking
What are the causes of subsidence?
Differences in the type of soil leading to differences in the bearing capacity.
A change in the water course or high water course.
A defective drain.
Voids in ground collapsing.
Growth of trees removing water from the soil.
What soils are susceptible to heave?
What causes heave in these?
Cohesive soils such as clay.
Removal of trees.
Change in water table
Leaking drains.
Inadequate foundations that are built within the movement zones.
Can you name defects on asphalt roofs?
Flowing.
Rippling.
Cracking.
Crazing.
Defective joints.
Punctures from occupier installations i.e. AC units.
How would you assess whether to replace or repair a flat roof?
- Age.
- Life expectancy.
- Condition.
- Extent of Defect.
- Type of defect.
- Client intention and budget.
- Building use.
- Cost.
- Use.
- Life Cycle Costing.
- Disruption associated with repair and replacement.
What are the different types of damp in building?
- Rising damp.
- Penetrating damp.
- Damp caused by defective plumbing.
- Damp caused by condensation.
What common defects affect commercial properties? Describe how you would identify them and potential remedial actions?
Asbestos
Regents Street Disease - To early 1900s steel and masonry hybrid buildings.
High Alumina Cement (HAC) - To pre-stressed concrete building elements.
Carbonation - To reinforced concrete.
Chloride Attack - To concrete in damp conditions.
Alkali Silica Reaction (ASR) - To concrete,
Filler Joist Floor - In 1880-1920 buildings with steel joists and concrete infill panels.
Woodwool Slabs - In permanent shuttering to concrete.
Brick Slips and Brick Panels - In 19602 - 1970s concrete frame buildings.
Sulphate attack - To concrete.
Explain regents street disease, how to identify and how to remedy it.
Defects:
Occurs in buildings constructed using a steel frame covered in facing masonry or stone. Often found in early twentieth century buildings where large masonry or stone units were tightly built around the steel frame. These porous materials allow moisture to get to the steel structure which then corrodes. Corroded metals can increase up to 7 times their original size. This puts pressure on the surrounding masonry causing cracking.
Identification:
Cracking to masonry around the structural frame.
Remedies:
- Removal of external stone/masonry to repair steelwork - This process involves cleaning the exposed steelwork to remove the surface rust by either sand blasting or needle gunning and treating the exposed surface with an appropriate site applied protective paint coating. This is clearly expensive and highly disruptive. It normally only covers the exposed elevations of the steel frame so there is a risk that no all corrosion is treated. However, it does allow the structural engineer to assess the condition of the exposed steel and its structural adequacy and if necessary it can be removed and replaced.
- Cathodic Protection - CP relies 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. It does not make good previous corrosion but suppresses the continuation of the process.
Explain carbonation, how to identify and how to remedy it.
Defect:
Carbonation occurs when the alkaline elements of the concrete react with the atmospheric carbon dioxide creating calcium carbonate which lowers the PH level of the concrete and therefore reduces the passivity of the concrete. It will cause gradual neutralisation of the alkalinity from the surface inwards. Should there not be adequate coverage of 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 at which carbonation occurs is inversely proportional to the square root of the age of the building i.e. typical Portland cement concrete may show a depth of carbonation of 3-5mm after 10 years.
Identification:
A simple 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 where a sample of the concrete is examined under a microscope.
Remedy:
- 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 corrosions.
Explain Chloride Attack, how to identify and how to remedy it.
Defect:
When Calcium Chloride additive was 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 de-icing 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
Identification:
Rust staining and delamination of concrete. Examination of the re-enforcement 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.
Remedy:
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.
Explain Alkali Silica Reaction (ASR), how to identify and how to remedy it.
Defect:
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.
Identification:
In unrestrained concrete, the cracks have a characteristic random distribution often referred to as ‘map cracking’. In restrained concrete, the cracking tends to run parallel to the main reinforcing bars. In particularly active situations, the cracks may be bordered by a colourless gel. This leads to surface pop-outs and spalling. Petrographic examination is the only sure way of identifying the attack.
Remedy:
Three conditions are necessary for ASR to occur: certain silica in the aggregate, sufficient moisture, high alkalinity. Removal of moisture by repairing damaged rain water goods is the first measure. Removal of the delaminated concrete and an isolated patch repair may also be an alternative. Secondary structural reinforcement can also be introduced.
Explain Filler Floor Joist, how to identify and how to remedy it.
Defect:
‘Filler joist’ is an early form of floor slab construction, which was commonly used between about the 1880s and 1920s. It was formed from iron or steel joists spaced between 600 and 900mm apart and in-filled with unreinforced concrete. The concrete often had coke breeze in it acting as an aggregate. The cover on the steel joists was often insufficient. The coke releases acids which change the passivity of the concrete and the lack of cover increases the risk of carbonation. Both lead to corrosion of the steel joists, which in turn cause cracking of the concrete and brickwork. The lack of cover also poses the issue of lack of fire protection and strength.
Identification:
Long cracks along the joists at approximately 600/900mm centres.
Remedy:
Expose the steel, remove the rust and treat with protective paint coating or cathodic protection.
Explain Woodwool Slabs, how to identify and how to remedy it.
Defect:
When used as permanent shuttering for concrete it may lead to grout loss, honeycombing or voids. Therefore, can lead to reduced fire resistance, reinforcement corrosion or loss of structural strength.
Identification:
Non-destructive testing techniques can be employed to identify whether any problems are likely to exist, although physical sampling may be more reliable.
Remedy:
Removal of the shuttering and repairs using hand placed mortars or sprayed concrete may be necessary if defects are found.
Explain Brick Slip and Brick Slab Panel defects, how to identify them and how to remedy them.
Defect:
Popular in the 1960, and early 1970s, brick slips were seen as an easy method of concealing the exposed edges of a concrete frame at each floor level. A failure to recognise the propensity of a concrete framed building to shrink created circumstances where the brick slips could be stressed and forced off the building, often with serious risks to health and safety. The provision of movement joints above and below the course of slips is essential, but great care is needed to ensure that even the smallest amount of bridging of the soft joint does not occur. Brick panels within either exposed concrete frames or supported on knibs can similarly be affected by unplanned shrinkage in a concrete frame.
Identification:
Elements coming loose.
Remedy:
Depends on the severity of the defect but in essence sufficient movement joints are required to take account of the different movements of the different elements.
