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| Cement Content Measuring
the cement content of hardened concrete is useful to assess the
concrete's compliance with specification requirements if a failure is
suspected, or as an aid to predict the likely physical characteristics
of the concrete in question.
The test is generally conducted chemically. Wherever possible
representative samples of the cement and aggregate used in making the
concrete should be obtained and chemical analyses carried out on them
in a similar manner for the concrete analytical sample.
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| Chloride Content The
total chloride content of concrete containing steel reinforcement is
usually designed to be as low as possible to avoid early onset of
corrosion of the reinforcement.
The test can be conducted in a variety of chemical means, from
approximate dip-stick measures through to potentiometric methods. The
usual method is by titration.
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| Chloride Permeability This
test covers the determination of the electrical conductance of concrete
to provide a rapid indication of its resistance to the penetration of
chloride ions. The test method is applicable to types of concrete where
correlations have been established with this procedure and long-term
chloride ponding procedures such as those described in AASHTO T259. |
| Chloride Ponding This
test is used to determine the depth to which chloride ions can ingress
into concrete over a period of time in standard conditions. It can be
used to assess a concrete for its resistance to chloride attack and
thus protection of the reinforcement from corrosion.
Samples cast in a standard manner have one face continually in
contact with a chloride saturated solution for ninety days. After this
time, specimens are extracted in increasing depth from the face and the
chloride content of each specimen is measured.
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| Core Compressive Strength This
test is used to determine the compressive strength of a concrete core,
which has usually been extracted from an existing structure. The value
of compressive strength can then be used in conjunction with other
measured properties to assess the condition of the concrete.
Using a masonry saw, the core is first trimmed to the correct test
length, which varies upon the standard being adopted. Following
trimming, the core will have its ends either ground perfectly flat, or
be capped in a material to produce a smooth bearing surface. After the
prescribed curing has taken place, the specimen is then crushed to
failure noting the maximum load achieved. From the values of load and
dimensions, the compressive strength of the core can be calculated.
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| Coring, 10cm diameter Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability. Extraction of
concrete cores, achieved by rotary drilling using a diamond tipped
hollow barrel, serve as a means of taking a sample of concrete which
can then be used to determine various physical properties, but most
commonly compressive strength.
Cores can generally be extracted from wherever access allows, and
would include floor slabs, walls and columns. 10cm diameter cores,
generally afford the minimum diameter required for a representative
sample and require less reinstatement than larger diameter cores.
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| Coring, 15cm diameter Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability. Extraction of
concrete cores, achieved by rotary drilling using a diamond tipped
hollow barrel, serve as a means of taking a sample of concrete which
can then be used to determine various physical properties, but most
commonly compressive strength.
Cores can generally be extracted from wherever access allows, and
would include floor slabs, walls and columns. 15cm diameter cores will
generally give more representative samples than smaller diameters but
require more reinstatement.
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| Cover meter survey Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability. The cover
meter survey will give a detailed view of the reinforcement closest to
the surface within a concrete structure. Information such as:
reinforcement spacing, diameter, depth from surface and location can be
determined.
The test involves scanning the surface with an electromagnetic meter, which can detect the ferrous elements within the concrete.
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| Cube Compressive Strength This
test is used to determine the compressive strength of a concrete cube,
which has usually been made from fresh concrete cast in a standard test
mould. The value of compressive strength can then be used to assess
whether the batch that the concrete cube represents meets the required
compressive strength.
Following cube manufacture and curing, which should both be closely
controlled, the cube is crushed at a stated constant speed until it can
sustain no further increase in load. The strength is then derived by
calculation using the maximum load and cube dimensions.
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| Cylinder Compressive Strength This
test is used to determine the compressive strength of a concrete
cylinder, which has usually been made from fresh concrete cast in a
standard test mould. The value of compressive strength can then be used
to assess whether the batch that the concrete cylinder represents meets
the required compressive strength.
Following cylinder manufacture and curing, which should both be
closely controlled, the cylinder is crushed at a stated constant speed
until it can sustain no further increase in load. The strength is then
derived by calculation using the maximum load and cylinder dimensions.
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| Depth of carbonation Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability. Over time
concrete surfaces react with the carbon dioxide in air, which then
carbonates the concrete, reducing its passivity and thus protection of
the reinforcement from corrosion.
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| Drying shrinkage/wetting expansion Values
of drying shrinkage and wetting expansion of concrete are often useful
to measure at the time of mix design trials to ensure the concrete will
exhibit values that are within normal ranges for concrete.
The test consists of making a number of prisms of the concrete of
known exact length, then either subjecting them to a drying or wetting
environment. Any change is length over time is then recorded.
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| Dust drilling, 3 increments to 100mm Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability. Dust
drilling, by dry rotary percussion, provides a simple method of
extraction of test samples at increasing increments of depth within the
concrete. These test samples can later be analysed for chloride
content, in order to establish how far chlorides have ingressed into
the concrete. |
| Flexural Strength Although
generally not such an important property of concrete than compressive
strength, tensile strength values are often important to know when the
concrete may be used free of reinforcement and may be subjected to some
tensile force.
The test method essentially involves applying a load at the centre
of a beam of concrete supported at its ends. The load required to break
the specimen is then recorded.
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| Half-cell survey Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability.
Depassivation of steel by carbonation or ingress of chlorides will
promote anodic and cathodic activity at sites throughout the
reinforcement. Metal is dissolved at the anodes where the oxidation of
metal ions takes place, and metal oxide corrosion products are
subsequently formed. Simultaneously, electrons are released and these
are conducted through the metallic reinforcement (flow of electrical
current) to a cathodic site where a reduction process occurs.
Associated with the flow of current is a variation in electrical
potential along the length of the corroding reinforcement, which may be
detected with a suitable half cell electrode. For steel in concrete,
anodic regions normally correspond to more negative potentials, whilst
cathodic regions correspond to less negative potentials.
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| Hammer sounding Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability.
A rapid method of assessing whether there may be any delamination
of the concrete surface is to conduct a hammer sounding. This involves
striking the concrete surface and listening for a noise, distinctive to
the trained operator of delamination.
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| ISAT This
test method provides data for assessing the uniaxial water penetration
characteristics of a concrete surface. The applied pressure, normally
200mm head of water, is worse than the severest weather exposure due to
driving rain in such climates as the UK might experience.
The results may be considered to be related to the quality of the
finish and to the durability of the surface under the effects of
natural weathering. The results are of little relevance to behaviour
under high water pressures and cannot be used to assess the
permeability of a body of concrete.
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| Porosity Porosity
of concrete is an important factor is classifying its durability.
Generally, concrete of a low porosity will afford better protection to
reinforcement within it than concrete of high porosity.
Porosity can be measured by vacuum saturation of a concrete
specimen, measuring its weight gain and expressing this as a percentage
of the mass of the sample.
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| Pull-off test Following
major concrete repairs, it is often useful to check the bond strength
of the repair material to the concrete substrate. This is achieved
using pull off apparatus.
The test involves coring through the repair material and into the
underlying concrete, to produce a regularly shaped test area of known
diameter. A dolly is then fixed to the centre of the test area. A pull
off machine is attached to the dolly, which will then measure the
tensile force taken to either break the concrete substrate, break the
repair material, or to separate the repair material from the concrete.
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| Rebound hammer survey Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability.
Rebound hammer surveys can be conducted to provide a rough estimate
of concrete strength, where it is not possible to determine this by
more accurate means such as core extraction and testing.
The test involves using a special rebound hammer, which measures
the recoil of a piston against the concrete surface when a standard
force is applied. This recoil can then be correlated approximately to
concrete strength.
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| Sulphate Content Measuring
the sulphate content of concrete is often used to check that cast-in
sulphate levels are low enough to avoid any later problems with
deterioration of the concrete. This test can be used at concrete mix
design stage to ensure low sulphate levels have been achieved, or on
older concrete structures to measure the level of ingress of sulphates.
The test can be conducted in a variety of chemical means.
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| Ultrasonic survey Often,
existing concrete structures will need to examined and tested to ensure
the concrete remains of adequate strength and durability.
Measurement of the velocity of ultrasonic pulses of longitudinal
vibrations passing through concrete achieved using special portable
apparatus, may be used in applications such as: the determination of
the uniformity of concrete or the detection and approximate extent of
cracks, voids and other defects.
It can also be used when pulse velocity and strength are correlated to provide a measure of concrete quality.
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| Water Absorption Water
absorption of concrete is an important factor is classifying its
durability. Generally, concrete of a low water absorption will afford
better protection to reinforcement within it, than concrete of high
water absorption.
Water absorption can be measured by soaking concrete specimens,
measuring their weight gain and then expressing this as a percentage of
the mass of the sample.
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| Water Penetration Water
penetration of concrete is an important factor is classifying its
durability. Generally, concrete of a low water penetration will afford
better protection to reinforcement within it, than concrete of high
water penetration.
Water penetration can be measured by applying water at a known
pressure against one side of a cubic specimen. After the prescribed
time, the actual depth to which the water has penetrated the cube face
can be measured by splitting the cube and measuring the ingress with a
rule.
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