APC Revision Course
19 August 2015
1
Building Diagnosis
And
Maintenance
APC Revision Course
19 August 2015
3
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 2015
4
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 2015
5
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 2015
8
Condition Survey
Necessary Testing
Type of tests:

Destructive test

Non-destructive test
Field/ In-situ tests

More accurate and representative of performance
APC Revision Course
19 August 2015
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
APC Revision Course
19 August 2015
10
Defects in Concrete
APC Revision Course
19 August 2015
11
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
APC Revision Course
19 August 2015
12
2. Chemical
-
Chlorides
-
Carbonation
-
Sulphates
-
Acidic
-
Grease, oil & waste water
3. Physical
-
Overloading
-
Fire damage
-
Mechanical Impact
-
Adverse temperature or inclement weather
-
Uneven settlement
APC Revision Course
19 August 2015
13
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
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.
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
APC Revision Course
19 August 2015
16
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
Carbonation Front
Carbon Dioxide Penetration from Atmosphere
Active Corrosion
within carbonated
zone
CO2
Carbonation Front
H2O
Reduced
pH
High pH >12
protects the reinforcement
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.
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
Test for Carbonation
- by coring and application of phenolphthalein
APC Revision Course
19 August 2015
21
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
Engineering Assessment –
Half-Cell Electrochemical Potential Survey
Reference
Electrode on
Concrete
Surface
Measures
the Potential
Difference
On
Reinforceme
ntBar
• 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
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
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.
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
BuildingCondition
Condition Change
with Short with
Term Repair
TypicalTypical
Building
Change
Short Term
(For Typical HK Pre-1980 Buildings)
Repair
(For Typical HK Age 30+ Buildings)
Good
Satisfactory
Varied
$15k
Poor
Beyond Economic
Repair
>$200k
0
10
20
30
40
Year(s)
50
60
70