Building Condition Survey and Diagnosis

Transcription

1 APC Revision Course 1 8 July 2013 Building Condition Survey and Diagnosis

2 APC Revision Course 2 8 July 2013 Condition Survey Common Practice in Hong Kong Fixed fee for a report and hourly rate for presentation at Court if required Testing fees that require the use of special equipment and participation of specialist personnel will be separately reimbursed

3 APC Revision Course 3 8 July 2013 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

4 APC Revision Course 4 8 July 2013 Condition Survey Inspection Procedures Digest client s instructions. What does he/she want? 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/nondestructive) Assimilate findings and analyse results Prepare report and conclusions

5 APC Revision Course 5 8 July 2013 Condition Survey Tools & Equipment Required Plans Tapping Rod Marble Ball Torch Screw Driver Compass Laser Pointer Electricity Test Driver Brush Pocket Mirror Mallet/Hammer Tape rule Magnifying Glass Stripping Knife/Scrapper Recorder/ ipad Binoculars Spirit Level/Plumb Rule Marker/Pen Camera Filler Gauge/Crack Gauge PPE

6 APC Revision Course 6 8 July 2013 Condition Survey Testing Techniques Type of tests: Destructive test Non-destructive test Field/ In-situ tests More accurate and representative of performance

7 APC Revision Course 8 July 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

8 APC Revision Course 8 8 July 2013 Defects in Concrete

9 APC Revision Course 9 8 July 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

10 APC Revision Course 10 8 July 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

11 APC Revision Course 11 8 July 2013 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 ( % content), High (over 1.0% content) Sources: admixtures (hardening), salt water, marine sand, course aggregate, cement, airborne, leaking flushing pipes, toilets

12 Engineering Assessment Chemical Composition Analysis (Chloride Content) Test) Field Work Collect samples for selected building at wall, beam, column at different locations Obtain drilling powder samples.

13 Engineering Assessment Chemical Composition Analysis (Chloride Content Test) Assessment Criteria 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). Source: The Concrete Society Technical Report No. 54, Diagnosis of Deterioration in Concrete Structures

14 APC Revision Course 14 8 July 2013 Carbonation Normally start at surface and penetrate into concrete; No harm to concrete itself and may slightly increase concrete strength; Caused by carbon dioxide in the atmosphere slowly and steadily transform calcium hydroxide into calcium carbonate (limestone); ph value will then drop thus causing corrosion of reinforcement bars; ph value ranges from 1.0 to When ph value over 12, reinforcement is protected from corrosion (Passivation); Rate of carbonation depends on: time, concrete cover, concrete density, cement ratio, cracks, alkalinity of original concrete.

15 Test for Carbonation - by coring and application of phenolphthalein

16 Carbonation Front Carbon Dioxide Penetration from Atmosphere Carbonation Front High ph >12 protects the reinforcement

17 Carbonation Process Building Age > 30 yrs Reinforcement steel does not corrode when embedded in highly alkaline concrete despite high moisture levels. 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. Source: Currie R.J., Robery P.C. ; (1994) Repair and Maintenance of Reinforced Concrete; Building Research Establishment, Garston, Watford, WD2 7JR; chapter 2.

18 Engineering Assessment Carbonation Depth Test Assessment Criteria 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. Carbonation Depth Test Scoring System Criteria (Best) 1 0mm to 5mm, < reinforcement depth 2 6mm to 25mm, < reinforcement depth 3 At reinforcement depth (Worst) 4 Beyond reinforcement depth

19 APC Revision Course 19 8 July 2013 Electrolysis There are differences in electrical potential between different parts of 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.

20 Engineering Assessment Half-Cell Electrochemical Potential Survey Reference Electrode on Concrete Surface Measures the Potential Difference On Reinforceme ntbar Select test locations Measures the potential of an embedded reinforcing bar relative to a reference half-cell placed on the concrete surface Source: ASTM International Standards Worldwide,

21 Engineering Assessment Half-Cell Electrochemical Potential Survey Assessment Criteria Survey conducted according to ASTM C876. Investigate the probabilities of occurrence of corrosion activities in reinforcement bars. In the vicinity of corrosion within a structure, the value of free corrosion potential becomes increasing negative.

22 Engineering Assessment Concrete Resistivity Measurement Field Work Select test locations 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.

23 Engineering Assessment Concrete Resistivity Measurement Assessment Criteria Resistivity measurement is according to BS : 1986 The apparent resistivity of 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).

24 Engineering Assessment Concrete Core Compression Test, Schmidt Rebound Hammer Test Field Work 75mm/100mm dia. concrete core samples per selected building at different locations Rebound hammer test at different locations.

25 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 : 2001 (superseded BS : 1986) Expected concrete strength is: 12.5 MPa (Pre-1959 age band) ; 20 MPa ( age band) Source: B.D. Surveys - B.D. Consultancy Agreement CAO C55, Dec 1995; B.D. Consultancy Agreement CAO E25, Sep 1999

26 Typical Building Condition Change with Short Term Repair (For Typical HK Pre-1980 Buildings) Good Satisfactory Varied $15k Poor Year(s) >$200 k Beyond Economic Repair