APC Revision Course

19 August 20151

Building Diagnosis And

Maintenance

APC Revision Course

19 August 20153

Building Diagnosis—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

APC Revision Course

19 August 20154

Condition Survey

Inspection procedures Digest client’s instructions. Establish type and extent of survey Preparations (access & equipment) Desktop study (third party documentation) Preliminary survey Detail survey (external & internal, destructive/non-destructive) Findings and analysis Prepare report and conclusions

APC Revision Course

19 August 20155

Condition Survey

Equipment required Plans Torch Hammer Camera Recorder Ipad Mirror

Visual Inspection (V.I.)Site Proforma

Visual Inspection (V.I.)Site Proforma

APC Revision Course

19 August 20158

Condition Survey

Necessary TestingType of tests: Destructive test Non-destructive test

Field/ In-situ tests More accurate and representative of performance

APC Revision Course

19 August 20159

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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Defects in Concrete

APC Revision Course

19 August 2015

APC Revision Course

19 August 2015

3 Main types of Concrete 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—mapping crack

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APC Revision Course

19 August 2015

2. Chemical- Chlorides- Carbonation- Sulphates- Acidic- Grease, oil & waste water

3. Physical- Overloading- Fire damage- Mechanical Impact- Adverse temperature or inclement weather- Uneven settlement

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APC Revision Course

19 August 2015

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

19 August 2015

Carbonation

- A natural process starts at the surface and penetrates into the concrete- 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 FrontCarbon 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 TestScoring System Criteria

(Best) 1 0mm to 5mm, < reinforcement depth2 6mm to 25mm, < reinforcement depth3 At reinforcement depth

(Worst) 4 Beyond reinforcement depth

Test for Carbonation- by coring and application of phenolphthalein

APC Revision Course

19 August 2015

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 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.

Concrete Coring- concrete compressive strength test by laboratory testing

• 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

Surface Hardness Test- by rebound hammer (also known as “Schmidt hammer”)

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

Typical Building Condition Change with Short Term Repair (For Typical HK Pre-1980 Buildings)

0 10 20 30 40 50 60 70

Year(s)

Good

Satisfactory

Varied

Poor

Beyond Economic Repair>$200k

$15k

Typical Building Condition Change with Short Term Repair

(For Typical HK Age 30+ Buildings)