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

APC Revision Course

1 August 20144

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

APC Revision Course

1 August 20145

Condition Survey

Equipment required

Plans

Torch

Hammer

Camera

Recorder

ipad

Visual Inspection (V.I.)Site Proforma

Visual Inspection (V.I.)Site Proforma

APC Revision Course

1 August 20148

Condition Survey

Testing techniques

Type of tests:

Destructive test

Non-destructive test

Field/ In-situ tests

More accurate and representative of performance

APC Revision Course

1 August 2014

9

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

10

Defects in Concrete

APC Revision Course

1 August 2014

APC Revision Course

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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APC Revision Course

1 August 2014

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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APC Revision Course

1 August 2014

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

1 August 2014

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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Test for Carbonation- by coring and application of phenolphthalein

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

APC Revision Course

1 August 2014

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

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