Strength assessment techniques

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STRENGTH ASSESSMENT TECHNIQUES
ENGINEERSTALK
Mohammad Alhusein, PhD
Managing Partner
M: +971 552592001
E: malhusein@superarc.net
W: www.superarc.net
STRENGTH ASSESSMENT TECHNIQUES
www.superarc.net
STRUCTURAL ASSESSMENT & REPAIR

WEBINAR OUTLINE
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Introduction
Main objective of condition assessment
Deterioration / Building materials
Condition assessment
Condition Survey Planning
Visual inspection
Destructive testing (DT)
 Cores into the concrete for visual inspection and strength
 Case Study
 Conclusions

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Introduction

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What Are the Different defects involved in the deterioration of concrete?
1- SCALING
What is it?
Scaling is referred to the loss of the surface portion of concrete
(or mortar) as a result of the freezing and thawing. It is a
physical action that usually leaves the aggregates clearly
exposed.
How it happens?
Scaling happens when the hydraulic pressure from water
freezing within concrete exceeds the tensile strength of
concrete. Scaling is more common in non-air-entrained
concrete, but can also occur in air-entrained concrete in the full
saturated condition.
Severity (Ontario Structure Inspection Manual (OSIM)
Light - Loss of surface mortar to a depth of up to 5 mm without exposure of coarse aggregate;
Medium - Loss of surface mortar to a depth of 6 to 10 mm with exposure of some coarse aggregates;
Severe - Loss of surface mortar to a depth of 11 mm to 20 mm with aggregate particles standing out from the concrete
and a few completely lost.
Very Severe - Loss of surface mortar and aggregate particles to a depth greater than 20 mm.

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2- DISINTEGRATION
What is it?
Disintegration is the physical deterioration (such as scaling) or
breaking down of the concrete into small fragments or
particles.
How it happens?
It usually starts in the form of scaling. It may be also caused by
de-icing chemicals, sulphates, chlorides or by frost action.
Light - Loss of section up to 25 mm in depth with some loss of coarse aggregate;
Medium - Loss of section between 25 mm and 50 mm deep with considerable loss of coarse aggregate and exposure of
reinforcement;
Severe - Loss of section between 50 mm and 100 mm deep with substantial loss of coarse aggregate and exposure of
reinforcement over a large area.
Very Severe - Loss of section in excess of 100 mm deep and extending over a large area.

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3- EROSION
What is it?
Erosion is the deterioration of concrete surface as a result of
particles in moving water scrubbing the surface.
How it happens?
When concrete surface is exposed to the water-borne sand and
gravel, the surface gets deteriorated by particles scrubbing
against the surfaces. Flowing ice particles can also cause the
problem. It is an indicator of poor durability of concrete for that
specific exposure.
Light - Loss of section up to 25 mm in depth with some loss of coarse aggregate;
Medium - Loss of section between 25 mm and 50 mm deep with considerable loss of coarse aggregate and exposure of
reinforcement;
Severe - Loss of section between 50 mm and 100 mm deep with substantial loss of coarse aggregate and exposure of
reinforcement over a large area.
Very Severe - Loss of section is in excess of 100 mm deep and extending over a large area.

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4- CORROSION OF REINFORCEMENT
What is it?
Corrosion is the deterioration of steel reinforcement in concrete. Corrosion
can be induced by chloride or carbonation. The corrosion can result in
cracking in the concrete cover, delamination in concrete decks, etc.
How it happens?
When the concentration of chloride ions above the surface of reinforcement
reaches the threshold limit (which is the amount required to break down the
passive film) corrosion begins. The volume of resulting material (rust) is 6-7
times, which increases the stress around the rebar, and causes fracture and
cracking. The cracks extend to the surface of concrete over time; that is when
we can visually see the sign of rust over the surface of concrete.
Light - Light rust stain on the concrete surface;
Medium - Exposed reinforcement with uniform light rust. Loss of reinforcing steel section less than 10%;
Severe - Exposed reinforcement with heavy rusting and localized pitting. Loss of reinforcing steel section between 10%
and 20%;
Very Severe - Exposed reinforcement with very heavy rusting and pitting. Loss of reinforcing steel section between 10%
and 20%;

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5- DELAMINATION
What is it?
“Delamination is defined as a discontinuity of the surface
concrete which is substantially separated but not completely
detached from concrete below or above it.” Delamination is
often identified by the hollow sound by tapping or chain
dragging of concrete surface.
How it happens?
The corrosion of reinforcement and subsequent cracking of the
cover can cause delamination. When the rebar have small
spacing, the cracking extends in the plane of the reinforcement
parallel to the exterior surface of the concrete.
Intercoat Delaminations
Light - Delaminated area measuring less than 150 mm in any direction.
Medium - Delaminated area measuring 150 mm to 300 mm in any direction.
Severe - Delaminated area measuring 300 mm to 600 mm in any direction.
Very Severe - Delaminated area measuring more than 600 mm in any direction.

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6- SPALLING
What is it?
Spalling can be considered an extended delamination. In fact,
when the delamination continues, the concrete fragments
detach from a larger concrete mass.
How it happens?
Very Severe Spalling and Delamination in Concrete Beams
If delamination is not repaired on time, the progress of damages
as a result of external loads, corrosion, and freezing and thawing
can break off the delaminated pieces.
Light - Spalled area measuring less than 150 mm in any direction or less than 25 mm in depth.
Medium - Spalled area measuring between 150 mm to 300 mm in any direction or between 25 mm and 50 mm in depth.
Severe - Spalled area measuring between 300 mm to 600 mm in any direction or between 50 mm and 100 mm in depth.
Very Severe - Spalled area measuring more than 600 mm in any direction or greater than 100 mm in depth.

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7- ALKALI-AGGREGATE REACTIONS
What is it?
It is the internal cracking of concrete mass as a result of a chemical
reaction between alkalis in the cement and silica in the aggregates.
The AAR/ASR (Alkali Silica reaction ) cracking are very famous for
their crack patterns.
How it happens?
The alkalis in the cement can react with the active silica in the
aggregates to form a swelling gel. When this gel absorbs water, it
expands, and applies pressure to surrounding environment which
makes the concrete crack.
Severity
Light - Hairline pattern cracks, widely spaced, with no visible expansion of the concrete mass.
Medium - Narrow pattern cracks, closely spaced, with visible expansion of the concrete mass.
Severe - Medium to wide pattern cracks, closely spaced, with visible expansion and deterioration of concrete.
Very Severe - Wide pattern cracks, closely spaced, with extensive expansion and deterioration of concrete.

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+ Plastic shrinkage
After Hardening
+ Drying shrinkage
+ Thermal contraction
+ Sub-grade settlement
8- CRACKING OF CONCRETE
What is it?
A crack is a linear fracture in concrete which extends partly or
completely through the member.
How it happens?
Some people believe that concrete is born with cracks; that its
ingredients, and how it is produced - from the batching plant
to pouring, setting, and curing - is influenced by so many
factors that cracking of concrete does not come as a big
surprise; and to a great extent, that might be true. Cracking of
concrete can happen in different stages: It can happen before
hardening of concrete, and it can happen in an old concrete
structure:
Before Hardening
+ Settlement within concrete mass
Severity
Hairline cracks - less than 0.1 mm wide.
Narrow cracks - 0.1 mm to 0.3 mm wide.
Medium cracks - 0.3 mm to 1.0 mm wide.
Wide cracks - greater than 1.0 mm wide.

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9-SURFACE DEFECTS
- Stratification;
- Segregation;
- Cold Joints;
- Deposits - efflorescence, exudation, incrustation, stalactite;
- Honeycombing;
- Pop-outs;
- Abrasion and Wear;
- Slippery Surface.
Surface defects are not necessarily serious in themselves; however, they are indicative of a potential weakness in
the concrete, and their presence should be noted but not classified as to severity, except for honeycombing and
pop-outs.

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STRATIFICATION is the separation of the concrete components into horizontal layers in over-wetted or overvibrated
concrete. Water, laitance, mortar and coarse aggregates occupy successively lower positions. A layered
structure in concrete will also result from the placing of successive batches that differ in appearance.
SEGREGATION is the differential concentration of the components of mixed concrete resulting in non uniform
proportions in the mass. Segregation is caused by concrete falling from a height, with the coarse aggregates settling
to the bottom and the fines on top. Another form of segregation occurs where reinforcing bars prevent the uniform
flow of concrete between them.
COLD JOINTS are produced if there is a delay between the placement of successive pours of concrete, and if an
incomplete bond develops at the joint due to the partial setting of the concrete in the first pour.
DEPOSITS are often left behind where water percolates through the concrete and dissolves or leaches chemicals
from it and deposits them on the surface. Deposits may appear as the following:
Efflorescence - a deposit of salts, usually white and powdery.
Exudation - a liquid or gel-like discharge through pores or cracks in the surface.
Incrustation - a hard crust or coating formed on the concrete surface.
Stalactite - a downward pointing formation hanging from the concrete surface, usually shaped like an icicle.

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HONEYCOMBING is produced due to the improper or incomplete
vibration of the concrete which results in voids being left in the
concrete where the mortar failed to completely fill the spaces
between the coarse aggregate particles.
Severity
Light - Honeycombing to a depth less than 25mm and 50mm.
Medium- Honeycombing to a depth between to a depth between
25mm and 50mm
Severe - Honeycombing to a depth between 50mm and 100mm.
Very Severe - Honeycombing to a depth greater than 100mm.
Efflorescence Incrustation
Stalactite
HONEYCOMBING Exudation

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Pop-outs Severity
Light - Pop-outs leaving holes up to 25 mm in depth.
Medium- Pop-outs leaving holes between 25 mm and 50 mm in depth.
Severe - Pop-outs leaving holes between 50 mm and 100 mm in depth.
Very Severe - Pop-outs leaving holes greater than 100 mm in depth.
POP-OUTS are shallow, typically conical depressions, resulting from the breaking away of small portions of the
concrete surface, due to the expansion of some aggregates or due to frost action. The shattered aggregate
particle may be found at the bottom of the depression, with a part of the aggregate still adhering to the pop-out
cone.

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ABRASION is the deterioration of concrete brought about by vehicles or snow-plough blades scraping
against concrete surfaces, such as, decks, curbs, barrier walls or piers.
WEAR is usually the result of dynamic and/or frictional forces generated by vehicular traffic, coupled with
the abrasive influx of sand, dirt and debris. It can also result from the friction of ice or water-borne particles
against partly or completely submerged members. The surface of the concrete appears polished.
SLIPPERY CONCRETE SURFACES may result from the polishing of the concrete deck surface by the
action of repetitive vehicular traffic.
Severity
There are no severity descriptions given for slippery concrete surfaces as this is a serious and potentially
hazardous situation.

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 Main Objective of Condition Assessment

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 Condition assessment planning
YES
Is further
investigation
required?
Preliminary investigation:
1. Review of relevant
documents
2. Visual inspection, with
documentation of defects
3. Field and laboratory testing
4. Preliminary analysis and
evaluation
Detailed investigation:
1. Review of additional
documents and data source
2. Additional field
observations, and field and
laboratory testing
3. Detailed analysis and
evaluation
NO
Is
repairing
required?
Identify and analysis repair
options
YES
Final
report
Identify special conditions
to further considered (e.g.
maintenance, planning
NO

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Review of plans and relevant documents
• To review documents from design
and construction process as well as
inspection and maintenance reports
is in general the easiest way of
gathering data about the structure to
be assessed.
• It has to be assured that the reviewed
documents are correct.
• Loads can be usually determined from
current loading codes and
environmental conditions may be
obtained from inspection reports.
• Resistance properties like material
and structural properties and
dimension can be obtained from:
– Construction specifications -
Codes
– As-built drawings--architectural,
structural, mechanical, and
foundation plans
– Construction documents )e.g.
material delivery documentation(
– Documentation of performance,
defects, maintenance, and
changes (Alterations(
– Reports of earlier inspection and
maintenance.

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VISUAL INSPECTION

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Scope of Visual Inspection
• Prior to the starting of visual inspection,
the structural engineer is to obtain a set
of the building’s structural layout plans
from the building owner.
• The availability of the structural layout
plan will help the structural engineer to:
a understand the structural system and
layout of the building;
b) identify critical areas for inspection;
c. identify the allowable imposed loads,
in order to assess the usage and
possibility of overloading; and
d. verify if unauthorised addition or
alteration works that affect the
structure of the building have been
carried out.

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Visual Inspection Tools and Instruments
• Simple tools and Instruments like:
– Camera
– Magnifying glass
– Binocular
– Gauge for crack width
measurement
– Chisel and hammer are usually
needed.
– Pocket knife, screwdriver
– Occasionally, a ladder or light
platform/scaffold tower can be
used for access to advantage.

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Scope of Visual Inspection
c any addition or alteration works
affecting the structure of the
building
• to identify any addition or alteration
works which can result in
overloading or adverse effects on
the structure
If there are no signs of any structural
deterioration or defects, the visual
inspection should suffice and unless
the structural engineer otherwise
advises, no further action needs to
be taken
A visual inspection is generally carried
out of:
a. the condition of the structure of
the building
• to identify the types of structural
defects
• to identify any signs of structural
distress and deformation
• to identify any signs of material
deterioration
b. the loading on the structure of the
building
• to identify any deviation from
intended use, misuse and abuse
which can result in overloading

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Visual inspection report (example)
1. General Information of the Building
• address, usage of the building,
maintenance history etc.
2. Structural System of the Building
• reinforced concrete, prestressed
concrete, steel, etc
3. Date and Scope of the Inspection
4. Survey of addition or alteration works
to building structure
5. Survey of signs of structural defects,
damages, distress, etc.
6. Survey of exposure to aggressive
environment
7. Conclusions on the structural
condition
8. Sketches, plans and photographs

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Examples of typical defects found by visual inspection
Reference: ACI 201.1R-08 (Guide for Conducting a Visual Inspection of Concrete in Service
Erosion of Brick Face Efflorescence Brick Spalling/Delaminating

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Examples of typical defects found by visual inspection
Crack and Spall of
Concrete Around Steel
Member
Delaminating
Concrete
Over Reinforcement
Concrete Crack

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What is Detailed visual inspection?
A detailed visual inspection is Element-by-element “close-up” visual assessment of:
a) Material defects,
b) Performance deficiencies
c) Maintenance needs
Answer: a or b
Who can perform it?
or c all of them
a) professional engineer
b) or a technician with structures inspection experience working under the direction of a
professional engineer.
Answer: a or b all of them

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DESTRUCTIVE TEST METHODS OF
HARDENED CONCRETE

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Strength of Hardened Concrete
• Two important feature:
-Strength
-Durability
• Strength(kgf/cm2 or Mpa): The ability to resist strain
-Compressive strength
-Tensile strength
-Bending strength

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Strength of Hardened Concrete
• Depends on
-Strength of paste
-Strength of aggregates
-The bond between the cement paste and the aggregates. (no obstacle between cement and
aggregates)
• Also:
-Water/Cement ratio of the concrete mix
-Quality of the mixing water
-Properties of the cement
-Properties of the aggregates
-Mixing, transportation, placing, and compaction operations applied to the concrete
-Curing conditions and age of the concrete.

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Effect of the Water/Cement Ratio
The w/c ratio affects the content of the total
capillary porosity of the cement paste. The
higher the w/c ratio, the higher is the total
capillary porosity of the paste, thus the lower
concrete strength.

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Strength Of Hardened Concrete

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Effect of the Properties of Cement
• Compound composition and fineness of the
cement affect the rate and amount of the gel
production upon hydration of the cement.
• As the gel production increases, the capillary
porosity in he cement paste decreases.

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Effect of the Quality of Mixing Water on Strength
Presence of excessive amounts of impurities in
the mixing water causes not only harmful effects
but also reduce the strength and durability of
the hardened concrete

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Effect of the Properties of Cement
• Compound composition and fineness of the
cement affect the rate and amount of the gel
production upon hydration of the cement.
• As the gel production increases, the capillary
porosity in he cement paste decreases.

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Effect of Aggregates Properties
• Gradation
• Max. aggregate size
• Shape of aggregate particles
• Amounts of deleterious materials in aggregates.
Affect the strength of concrete.
• Improper gradation
• Odd shaped particle
• Presence of excessive amount of clay lumps
• Increase the water requirement, decrease the strength of concrete.
• Presence of very fine materials as a coating on the aggregate particles
reduces the bond between aggregate particles and the cement paste

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Effect of Mixing, Transportation, Placing, Compaction
• Inadequate mixing concrete results in a non-uniform
mixture
• Prolonged mixing not only causes the aggregate
particles to break but also increases the concrete
temperature (increases water requirement)
• Improper transportation and improper placing may
lead to segregation.
• Inadequately compacted concrete may contain a large
number of voids, which lead to reduction of strength.
• Long vibration times cause segregation.

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Effect of Mixing, Transportation, Placing, Compaction
• Inadequate mixing concrete results in a non-uniform mixture
• Prolonged mixing not only causes the aggregate particles to break
but also increases the concrete temperature (increases water
requirement)
• Improper transportation and improper placing may lead to
segregation.
• Inadequately compacted concrete may contain a large number of
voids, which lead to reduction of strength.
• Long vibration times cause segregation.

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Compressive Strength of Concrete
• The maximum resistance of the concrete to axial
compressive loading.
• In structural applications, concrete is employed
primarily to resist compressive stresses
• Calculations for the design of structures are
usually based on the compressive strength of the
concrete.
• Strength of concrete is usually determined by the
standard test methods.

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Compressive Strength of Concrete
Most common used test method.
• Conducting compressive strength test on
standard test specimens
• Drilling cores from the hardened concrete and
determining the compressive strength by testing
the core specimens
• Determination of the compressive strength by
measuring the rebound hardness of the
concrete’s surface.

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Techniques for Strength Assessment
• Core testing represents the most reliable method of
establishing in-situ concrete strength.
• Core-drilling is relatively ‘destructive’ by reason that it left a
hole in the structure.
• Other ‘partially-destructive’ techniques for assessing
strength of surface concrete are generally less reliable than
cores but cause less damage & give instant results :
penetration resistance, pull-out, pull-off, internal fracture &
break-off methods.

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Core Drilling Machine

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Core Drilling & Testing
• Refer Concrete Society TR11 (1987) & BS 1881 Part 120 (1983) –
detailed guidance & recommendations.
• Other uses of cores : visual inspection to assess concrete
uniformity (aggregate distribution & compaction);
samples for petrographic or chemical analysis.

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Purpose of Core Drilling & Testing
• Assessment of Potential Strength
• Assessment of In-situ Cube Strength
• Assessment of Load Factor to carry : actual, designed &
projected loads
• Assessment of degree & extent of deterioration due to loads
or environmental causes

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Planning for Core Drilling
• Establish reasons for core testing : specification
compliance; structural adequacy; etc.
• Obtain information on location of suspect concrete
(NDT survey or records).
• Establish locations, number & size of cores.
• Other logistics planning.

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Coring Locations
• For in-situ cube strength : cores may be drilled from any
location of interest (avoiding reinforcement steel). Zones
of lowest strength are expected to be towards the top
elements.
• For potential strength : should avoid –
- top 20% of lifts
- badly compacted concrete
- reinforcement steel
• Covermeters used to locate position of steel.

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Number of Cores
• ASTM C823/C823M recommends a minimum of five core test specimens be obtained for each
concrete category with a unique condition or specified quality, specified mixture proportion, or specified
material property. ASTM C823/C823M also provides guidance for repeating the sampling sequence for
large structures.
• ACI 214.4R states that it is preferable to obtain specimens with nominal diameters of 4 to 6 in. (100 to
150 mm) and l/d ratios between 1.5 and 2 to minimize error introduced by the strength correction
factors (Neville 2001).
• ASTM: C42/C42M − Correction Factor
• (ACI 228.1R) requires a minimum of six to nine test locations for cores (with two cores that should
be drilled from each location).

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ASTM: C823/C823M (Standard Practice for Examination and Sampling of Hardened Concrete in Constructions
Minimum Depth of Sampling of Concrete for Testing Purposes

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CASE STUDY

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Table 12 — Compressive strength classes for normal-weight and heavy-weight concrete
4.3.1 Compressive strength classes, EN 206-1:2013 ( Concrete — Specification, performance, production and conformity)

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- The 3rd party report is mainly based on the test results of 15 nos. concrete cores.
- Core diameters as per 3rd party test reports are 69 mm.
- L/D ratios are 1.0
- Cores had only been retrieved from external ground floor columns.
- Max. Cores lengths are 69.0mm.
- Average measured concrete cover is 32 mm.

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 Conclusions
 Core test is the most direct method to evaluate the concrete strength in the existing
structure. However, it is obvious that this method suffers from many drawbacks:
a) The number of cores required by the standards is large which leads to high investigation
cost,
b) When planning the core locations in the structures, we are not fully free in selecting these
locations due to: structural considerations because drilling excessive cores in the high
stressed members can weaken the structure and consequently affect its capacity to bear the
applied loads; drilling considerations because the access of large and heavy drilling
machine may be difficult (or impossible) in several situations,
c) Drilling is a complicated process because it includes setting up the machine, checking the
perpendicularity and extracting core and for each step the standards provide several
requirements to be respected. Consequently, a skill operator is mandatory in order to
obtain a core specimen that is undamaged and representative of the in-situconcrete.

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 Conclusions
d) Before the compressive testing, it is required to follow several preparation and storage
conditions and the shortening in any of these conditions leads to misleading core
strength. Therefore this is another reason for the expensive cost of core test.
e) Core test takes a lot of time as compared with the other tests due to drilling, preparation,
testing processes,
f) The obtained strength of core specimen differs from the true in-situ value existing in the
structure at location where the core was extracted.

INTEGRITY PROBLEMS OF CONCRETE PILES
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