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Condition Survey and Diagnosis of Building Defects ... · Diagnosis of Building Defects/ Remedial...
Transcript of Condition Survey and Diagnosis of Building Defects ... · Diagnosis of Building Defects/ Remedial...
APC Revision Course
1 August 20141
Condition Survey and
Diagnosis of Building Defects/
Remedial Methods
APC Revision Course
1 August 20143
Condition Survey
Surveyors Responsibilities
Duty of care (reasonable care to avoid acts or omissions)
Reasonable level of competence and knowledge associated with a member of the surveying profession
Guidelines as set down by professional bodies are used as a reference
Professional Negligence (Point of Law)
Duty of care exists
Breach of duty of care
Financial/non-financial loss of client
Reasonable test
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Condition Survey
Inspection procedures
Digest client’s instructions. What does he/she want? MBIS??
Establish type and extent of survey
Undertake survey preparations (access & equipment)
Undertake desktop study (third party documentation)
Undertake preliminary survey
Undertake detail survey (external & internal, destructive/non-
destructive)
Assimilate findings and analyse results
Prepare report and conclusions
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Condition Survey
Equipment required
Plans
Torch
Hammer
Camera
Recorder
ipad
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Condition Survey
Testing techniques
Type of tests:
Destructive test
Non-destructive test
Field/ In-situ tests
More accurate and representative of performance
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Condition Survey
Laboratory tests
Removal of sample of material and subsequent testing at test
laboratory
Take sample at various locations
Large amount of samples allow comparison and the result
would be more justifiable
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1 August 2014
3 Main types of Defects
1. Design and Workmanship
- Wrong mix
- Wrong design
- Misplacement of reinforcement
- Inadequate cover to reinforcement
- Poor construction joints
- Not enough compaction—honey comb
- Too much water
- Poor curing
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2. Chemical
- Chlorides
- Carbonation
- Sulphates
- Alkali-aggregate reaction
- Acids
- Electrolysis
- Grease, oil & waste water
3. Physical
- Overloading
- Fire damage
- Mechanical Impact
- Adverse temperature or inclement weather
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Chlorides (Calcium Chloride)
- High concentrations of chloride ion in concrete (above 0.4% by weight) will
have a corrosive effect on steel bars
- Only soluble chlorides are involved in the corrosion process, therefore the
concrete must be porous and moist for this to happen
- Symptoms: Efflorescence on surface or deterioration of paint finishes, rust
stains tend to be very dark, often in patches, and show deep pitting
- Degree of chloride content: Low (0.4% content), Medium (0.4 – 1.0% content),
High (over 1.0% content)
- Sources: admixtures (hardening), salt water, marine sand, course aggregate,
cement, airborne, leaking flusing pipes, toilets
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• Collect samples for selected building at wall, beam, column at different locations
• Obtain drilling powder samples.
Engineering Assessment –
Chemical Composition Analysis (Chloride Content Test)
Field Work
• Cement content determined according to BS1881: Part 124: 1988
• Chloride content determined according to CS1: 1990, section 21
• Chloride content by weight of cement (%) is determined.
• The presence of chloride ions can depassivate the steel and promote corrosion.
• The most widely accepted reinforcement corrosion threshold is concrete that
contains more than 0.4% chloride by weight of cement (i.e. approximately 0.06%
by weight of concrete sample).
Engineering Assessment –Chemical Composition Analysis (Chloride Content Test)
Assessment Criteria
Source: The Concrete Society – Technical Report No. 54, Diagnosis of Deterioration in Concrete Structures
APC Revision Course
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Carbonation
- A natural process starts at the surface and penetrates into the concrete
- The concrete itself is not harmed, in fact there may be a slight increase in
strength
- Caused by carbon dioxide in the atmosphere slowly and steadily transforms
the calcium hydroxide into calcium carbonate (limestone)
- Carbon dioxide forms about 0.03% by volume of the atmosphere although it
can increase to over 0.35 in urban areas, due to industrial activity
- The pH value will then drops thus causing corrosion of the reinforcement bars
- pH value ranges from 1.0 to 14.0. When pH value over 12, reinforcement is
protected from corrosion
- Rate of carbonation depends on: time, cover on re-bars, density of conc.,
cement ratio, cracks, alkalinity of the original concrete
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Carbonation Front
Carbon Dioxide Penetration from Atmosphere
High pH >12protects the reinforcement
Carbonation Front
Reduced pH
H2OCO2
Active Corrosion within carbonated
zone
• Reinforcement steel does not corrode when embedded in highly alkaline concrete
despite high moisture levels.
Source: Currie R.J. , Robery P.C. ; (1994) Repair and Maintenance of Reinforced Concrete; Building Research Establishment, Garston, Watford, WD2 7JR; chapter 2.
Carbonation Process
• Carbonation process: hydrated cement is neutralised, and a carbonation front
progresses from outer concrete surface inward.
• Once concrete cover is carbonated, protection to steel reinforcement is lost.
Building Age > 30 yrs
• Universal indicator (colourless) – phenolphthalein, is used to determine the
carbonation front. Colour change is a direct measure of carbonation depth.
• Colour change from colourless to purple-red indicates alkaline, hence NO
occurrence of carbonation in concrete.
• Colourless reaction indicates carbonated cement.
Engineering Assessment –
Carbonation Depth Test
Assessment Criteria
Carbonation Depth Test
Scoring
SystemCriteria
(Best) 1 0mm to 5mm, < reinforcement depth
2 6mm to 25mm, < reinforcement depth
3 At reinforcement depth
(Worst) 4 Beyond reinforcement depth
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Electrolysis
- There are differences in electrical potential between different parts of the
reinforcement steel due to the differences in soluble salt concentration
- If these anodic (+ve) and cathodic (-ve) areas are connected by an
electrolyte such as salt solutions in the hydrated cement, an electro-chemical
corrosion process is set up and a corrosion cell is formed
- Positively charged metal ions at the anode pass into solution as Fe++ and the
free electrons pass along the steel to the cathode. They are absorbed by the
electrolyte and on combining with oxygen and water form hydroxyl ions.
- These in turn combine with ferrous ions to form ferric hydroxide and are
converted to rust
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Engineering Assessment –
Half-Cell Electrochemical Potential Survey
• Test locations were selected.
• Measures the potential of an embedded reinforcing bar relative to a reference half-cell placed on the concrete surface
Source: ASTM International Standards Worldwide, http://www.astm.org/Standards/C876.htm
Measures
the Potential
Difference
Reference
Electrode on
Concrete
Surface
On ReinforcementBar
Engineering Assessment –
Half-Cell Electrochemical Potential Survey
Assessment Criteria
• Survey conducted according to ASTM C876.
• To investigate the probabilities of occurrence of corrosion activities in reinforcement bars.
• In the vicinity of corrosion within a structure, the value of the free corrosion potential becomes increasing negatve.
• Test locations were selected.
Engineering Assessment –Concrete Resistivity Measurement Field Work
• A four probe device is connected to a high impedance resistivity meter.
• An electrical current is passed through the outer electrodes while the
voltage drop between the inner electrodes is measured.
Engineering Assessment –Concrete Resistivity Measurement
Assessment Criteria• Resistivity measurement is according to BS 1881 – 201 : 1986
• The apparent resistivity of the concrete is calculated from the current,
voltage drop and electrode spacing.• The moisture content primarily affects the electrical resistivity of the
cement paste medium surrounding the steel bar which provides the electrolyte in the electrochemical corrosion process, supporting the transport of ions from the cathode to the anode.
• The higher the resistivity the lower rate of corrosion supported by the concrete, if the reinforcement is corrosively active (note the resistivity does not indicate if the reinforcement is actually corroding).
• 75mm/100mm dia. concrete core samples per
selected building at different locations
• Rebound hammer test at different locations.
Engineering Assessment –
Concrete Core Compression Test, Schmidt Rebound Hammer Test
Field Work
Engineering Assessment –
Concrete Core Compression Test, Schmidt Rebound Hammer Test
Assessment Criteria
• Concrete coring method and compression test according to CS1: 1990
• Rebound hammer test according to BS EN 12504 – 2 : 2001
(superseded BS 1881 – 202 : 1986)
• Expected concrete strength is:
12.5 MPa (Pre-1959 age band) ;
20 MPa (1959-1980 age band)
Source: B.D. Surveys -B.D. Consultancy Agreement CAO C55, Dec 1995; B.D. Consultancy Agreement CAO E25, Sep 1999