Materials - Typical Defects Flashcards
What are typical defects associated with concrete?
Defect in the concrete can be broken down into mechanical, chemical and physical.
Mechanical defects include:
- impact
- overload
- movement - settlement
Chemical
- carbonation
- sulphate attack
Physical
- freeze/thaw
- thermal
- shrinkage
- wear
What are common techniques for identifying defects in concrete?
visual survey
hammer test
Cover meter
Breaking out - intrusive
Phenolphthalein spray
Half-cell mapping
Laboratory testing
What is carbonation?
Concrete is a naturally alkaline environment which protects the steel reinforcement from corrosion
Carbonation is a chemical reaction whereby atmospheric gases such as Carbon dioxide enter the concrete through pores, crack and damages areas, which reduces the alkalinity of the passive layer resulting on carbonation which attack the steel reinforcement.
What is the british standard for concrete repair?
British Standard EN 1504 Products and systems for the protection and repair of concrete structures
What action plan should you follow for concrete repair?
Assess condition of the structure - what are the main agencies of damage?
Consider options for action - repair, demolish, make safe, do nothing
Decide upon methodology restore, protections, barrier coating, patch repair?
Select materials based on performance standards.
Undertake repair
Establish and define a maintenance and control strategy for the future.
What methods might you use to test for carbonation?
Breaking out - intrusive
Phenolphthalein spray
Half-cell mapping
Laboratory testing
How can you test for concrete repairs using phenolphthalein spray?
Drill two holes10mm in dimeter 15 mm apart on the effected area. Break out the area between.
Spray the freshly exposed concrete with Phenolphthalein
If carbonation present, remains clear.
If no carbonation, turns pink.
How would you undertaken a concrete patch repair on carbonation?
Determine the primary cause of the failure
Undertake hammer test and remove loose areas of concrete
Hack off loose concrete from around the exposed steel bars.
grit blast to remove corrosion and other deposits from the exposed steel reinforcement including any hidden back face
apply a proprietary bar primer to the surface of the reinforcement such as an alkali based epoxy resin.
Patch repair concrete
secondary measures - re-line gutter.
What are principal methods for repairing carbonation in concrete?
break out and replace
cathodic protection
Migrating corrosion inhibitors - where initial failure has not occurred, MCI spray applied to the structure migrated through the covercrete to the reinforcement forming a chemical barrier.
What is minimum sufficient cover for concrete?
20-30mm
What methods might you use to identify carbonation?
Visual
Hammer testing
Half cell survey
Can you name some defects associated with concrete?
Carbonation
Calcium chloride concrete
High Alumina Cement
Sulphate attack
What is calcium chloride concrete and how would you diagnose it?
Calcium chloride was commonly use as an additive in in-situ cast concrete as an accelerator in the 1950s and 1960s (banned 1977). However, it typically reduces the passivity of concrete in damp conditions and increases the risk of corrosion of the reinforcement.
Diagnosis: Pitting to the concrete surface, chemical testing.
How else can chloride enter concrete?
De-icing salts and car parks and external staircases
Marine environment
Inclusion of unwashed marine aggregates
What are repair options for chloride in concrete?
cathodic protection
sacrificial zink anodes in patch repair.
migrating corrosion inhibitors
What is sulphate attack?
As the concrete absorbs the sulphated water, it expands causing spalling and cracking of the concrete. Can happen to chimneys or foundations. If to ground bearing slab then it is often the case that the concrete will need to be dug up and replaced with a sulphate resistant concrete.
What is high alumina cement and how would you diagnose it?
Mainly used in manufacture of precast beams and lintels to provide concrete resistant to chemical attack between 1954 and 1974. Strength of this concrete can decrease with age, often when high temperatures and/or high humidity is involved resulting in chemical change
Diagnosis: Concrete turns brownish colour and friable. Need chemical testing to confirm ID.
What defects would you associate with Timber?
Rot
Insect attack
What is rot?
Decay caused by the actions of a fungi in the correct environmental conditions in the presence of moisture.
What are the stage of fungal attack?
- Spores react with timber in the right environmental conditions
- Spores grow to produce fine white strands called hyphae
- Hyphae germinate producing mycelium
- If untreated fruiting bodies will develop.
What is the difference between wet rot and dry rot? What are the levels of moisture in which they occur?
the dry rot fungus tends to decay wood at lower moisture contents (i.e. 20–22%) than the wet rot organisms (i.e. 22–50%); and
the wood tends to be left in a dry state after attack by a dry rot fungus.
What factors should you consider when inspecting biodeterioration?
- the material;
- the environment;
- the organism.
What types of wet rot are there?
White rot: Cause the wood to become lighter in colour and lint-like in texture, without cross cracks. All white rots are wet rots.
Brown rot: Cause the wood to become darker in colour and to crack along and across the grain (Fig 2); when dry, very decayed wood will crumble to dust. Many common wet rots are brown rots; dry rot is also in this group.
What are common locations for wet rot?
Cellars
Roof voids
Window and door frames
External joinery
How can you diagnose wet rot?
Fruiting body
Mycelium and strands
Form of decay - all white rots wet rot
Damp meter to check moisture content
How would you treat wet rot?
Primary measures
- remove source of moisture
- promote rapid drying
Secondary
- establish size of attack
- remove rotten wood and apply localised preservative if required
- use preservative treated replacement timbers
*structural engineer if structural timbers.
Which BRE guide deal with wet rot?
BRE Digest 345
What conditions does wet rot manifest itself?
Wood-rotting fungi can develop on susceptible wood if the moisture content of the wood remains above about 22% regularly and for prolonged periods
What conditions does dry rot manifest itself?
God source of moisture – 20-22% moisture content required
What are common locations for dry rot?
Floor voids
Cellars
Unventilated roof voids
Behind timber panelling and plaster
Which BRE guide deal with dry rot?
BRE Digest 299
How can you diagnose dry rot?
wood light weight and crumbles in fingers
dull brown colour
no longer fresh resinous smell
cuboidal cracking
fruiting bodies - shaped like pancakes or brackets with a centre covered in small pores or folds. Margin light but centre rusty red when spore production begins.
What detection methods could you use for wet rot?
visual - cuboidal cracking
smell - musty
touch - spongy/friable
Sniffer dogs
moisture meter
lab sample testing
How would you treat dry rot?
Primary measures
- locate and eliminate source of moisture
- promote rapid drying
Secondary
- determine extent of outbreak
- remove rotten wood 300-450mm beyond last indication of rot
- contain fungus within the wall (typically if remove source of moisture this is not required but otherwise can use fungicidal treatment)
- treat remaining sound timbers with preservative (Several fungicide treatments are available)
- use preservative treated replacement timbers
- introduce support measures.
What support measures would assist in preventing rot?
ventilation pathways between sound timber and wet brickwork
where ventilation is not possible, provide a barrier such as a damp-proof membrane or joist hangers between timber and wet brickwork
What are typical indications of insect attack?
Evidence of flight holes and frass
How would you determine between different types of insect attack?
- size of flight hole
- type of timber
What treatments are available for insect attack?
Brush wood with poison on all sides of timber. If localised, then only need to treat up to 300mm beyond exit holes.
gas fumigation (kills but no residual protection),
heat sterilisation of kiln dried woods.
Remove and burn all badly damages timber and replace by splicing in a new timber
What are common defects in masonry walls?
Wall tie failure
thermal movement due to lack of movement joints
What is wall tie failure and how would you diagnose it?
old or ungalvanized wall ties become corroded wall ties open up horizontal mortar joints and displacement of brickwork (typically 450mm)
How can you treat wall tie failure?
Metal detector used to locate and remove wall ties prior to retrofit of stainless steel mechnicaly fixed ties.
What British Standard cover corrosion of steel in concrete?
BS444 Corrosion of Steel in concrete
