Materials and methods
Concrete1- Introduction:Concrete is mixture of cement or lime,
sand, brick or stone ballast and water, which when placed in forms
and allowed to cure, becomes hard like stone. The hardening is
caused by the chemical reaction between cement and water. The
cement and water form a paste which, upon hardening, binds the
aggregate to a permanent mass. cement is called binding material ,
the stone or brick ballast called coarse aggregate as distinguished
from the fine aggregate which is sand . The mortar used in concrete
called (matrix) cement concrete when used by itself known as mass
concrete. Concrete is stronger in compression than tension. Enable
it to resist tensile stressed it is reinforced or strengthened with
steel in the form of steel bars or wire netting etc. the concrete
so obtained is called (reinforced concrete). A- fresh concrete B-
hardened concrete A- Fresh concrete:- The first 48 hours are very
important for the performance of the concrete structure. - Fresh
concrete: from time of mixing to end of time concrete surface
finished in its final location in the structure - Operations:
batching, mixing, transporting, placing, compacting, surface
finishing For fresh concrete to be acceptable, it should: 1. Be
easily mixed and transported. 2. Be uniform throughout a given
batch and between batches. 3. Be of a consistency so that it can
fill completely the forms for which it was designed. 4. Have the
ability to be compacted without excessive loss of energy. 5. Not
segregate during placing and consolidation. 6. Have good finishing
characteristics.
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2- properties of concrete:-
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Workability of Fresh Concrete: Fluidity & compatibility
known as workability Higher workability concretes are easier to
place and handle but obtaining higher workability by increasing
water content decreases strength and durability Measurement of
workability: 1- Slump test 2- Compaction factor test 3- Vebe
consistometer test 4- Flow table test The slump test is the most
well-known and widely used test method to characterize the
workability of fresh concrete. The inexpensive test, which measures
consistency, is used on job sites to determine rapidly whether a
concrete batch should be accepted or rejected. The test method is
widely standardized throughout the world, including in ASTM C143 in
the United States and EN 12350-2 in Europe (In British standard BS
1881 Part 2).
a- Slump test :-
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Advantages:1- The slump test is the most widely used device
worldwide. In fact, the test is so well known that often the terms
workability and slump are used interchangeably, even though they
have different meanings. 2- Specifications are typically written in
terms of slump. 3- The slump test is simple, rugged, and
inexpensive to perform. Results are obtained immediately. 4- The
results of the slump test can be converted to yield stress in
fundamental units based on various analytical treatments and
experimental studies of the slump test. 5- Compared to other
commonly used concrete tests, such as for air content and
compressive strength, the slump test provides acceptable
precision.
Disadvantages:1- The slump test does not give an indication of
plastic viscosity. 2- The slump test is a static, not dynamic,
test; therefore, results are influenced by concrete thixotropy (is
the viscous behavior where the apparent viscosity decreases with
shear stress). The test does not provide an indication of the ease
with which concrete can be moved under dynamic placing conditions,
such as vibration. 3- The slump test is less relevant for newer
advanced concrete mixes than for more conventional mixes.
Limitations: The slump test is not considered applicable for
concretes with a maximum coarse aggregate size greater than 37.5 mm
(1.5 inches). For concrete with aggregate greater than 37.5 mm in
size, such larger particles can be removed by wet sieving.
2-Compacting Factor Test:The compacting factor test (Powers
1968; Neville 1981; Bartos 1992; Bartos, Sonebi, and Tamimi 2002)
measures the degree of compaction resulting from the application of
a standard amount of work. The test was developed in Britain in the
late 1940s and has been standardized as British Standard
1881-103.
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Compacting factor shape The apparatus, which is commercially
available, consist of a rigid frame that supports two conical
hoppers vertically aligned above each other and mounted above a
cylinder, as shown in Figure. The top hopper is slightly larger
than the bottom hopper, while the cylinder is smaller in volume
than both hoppers The compacting factor is defined as the ratio of
the weight of the concrete compacted in the compaction factor
apparatus to the weight of the fully compacted concrete. The
standard test apparatus, described above, is appropriate for
maximum aggregate sizes of up to 20 mm. A larger apparatus is
available for concretes with maximum aggregate sizes of up to 40
mm.
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Degree of workability Very low Low
Slump mm 0 - 25 25 - 50
Compaction factor Small Large apparatus apparatus 0.78 0.85 0.80
0.87
Application Vibrated concrete in road or other Large sections
Mass concrete foundation without vibration , simple reinforced
section with vibration simple reinforced work without vibration
Section with congested reinforcement without vibration
medium High
50 - 100 100 - 180
0.92 0.95
0.935 0.96
3-Vebe consistometer test:1- The Vebe test measures the
remolding ability of concrete under vibration. The test results
reflect the amount of energy required to remold a quantity of
concrete under given vibration conditions. 2- The Vebe test is
applicable to concrete with slumps less than 2 inches.
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Vebe consistometer test1- The apparatus, shown above , consists
of a metal cylindrical container mounted on a vibrating table,
which produces a sinusoidal vibration. 2- In the version of the
test standardized in Europe as EN 12350-3, a slump cone is placed
in the center of the cylinder and filled in the same manner as in
the standard slump test. 3- After the slump cone is removed, a
clear plastic disk is set atop the fresh concrete. 4- The Vebe
table is started and the time for the concrete to remold from the
slump cone shape to the shape of the outer cylindrical container is
recorded as a measure of consistency. 5- The sliding clear plastic
disk facilitates the determination of the end of the test.
B- Hardened concrete :12345Principal Properties of harden
concrete are: Compressive Strength Tensile strength Porosity
Permeability Fatigue and Fracture, etc.
- The properties are determined by: 1Laboratory Testing 2In-Situ
Testing Concrete Testing Approaches 1- Destructive Testing - In
this approach separately cast concrete samples and/or cores/cut
samples are tested to destruct. 2- Non-Destructive Testing - In
this approach tests are conducted without destructing the concrete.
This approach is preferred when assessment the quality of existing
concrete structures is required - Destructive Tests:- The standard
method of evaluating the quality of concrete in buildings or
structures is to test specimens cast simultaneously for
compressive, flexural and tensile strengths. - The main
disadvantages are that results are not obtained immediately; that
concrete in specimens may differ from that in the actual structure
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Materials and methods
different curing and compaction conditions; and that strength
properties of a concrete specimen depend on its size and shape.
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This category of testing covers the following tests:Engineering
& Mechanical properties Compressive strength Flexural strength
Tensile strength Modulus of elasticity Durability Permeability
Chloride migration The most common test preformed on concrete is
for compressive strength. There several reasons for this: It is
assumed that the most important properties of concrete as directly
related to compressive strength; Concrete has little tensile
strength and is used primarily in compression; Structural design
codes are based on compressive strength; The test is relatively
simple and inexpensive to perform.
Compressive Strength: ASTM C39 Cylinder Test The normal
compressive specimen in North America is a cylinder with length to
diameter ratio of 2:1. 1- Molds may be reusable, made of
heavy-gauge metal or single-use, made from sheet metal or waxed
cardboard. 2- Cardboard molds have been found to yield slightly
lower strength (+/-3%) than other types.
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3- Specimen should be cast on a firm level surface, free from
vibration. If the slump is more than 3 inches, concrete is
consolidated by rodding; if the slump is less than 1 inch, the
concrete is consolidated by vibration. 4- Poorly compacted
cylinders will have lower strength. 5- If the specimen is to be
rodded, it should be filled in three equal layers, each rodded 25
times with 5/8 inch diameter steel rod with a rounded end.
Compressive Strength: BS 1881-116/BS EN 12390-3 Cube Test 1- Cube
test, standard in Great Britain and Germany, uses a 150mm cubic
mold, which is filled in three layers, rodded 35 times with a 25 mm
square rod or compacted with a vibrator. 2- The cube is tested at
right angles to the position casted and therefore required no
capping or grinding. The loading rate is 0.03 kg/mm2/s, for 150mm
standard cube it will be about 5kN/s. Destructive Tests: Tensile
strength There is as yet no standard test for directly determining
tensile strength. However, there are two common methods for
estimating tensile strength through indirect tensile tests. Split
Cylinder Test Flexural Strength Tensile strength: Split Cylinder
Test BS 12390-6 BS EN 12390-6 supersedes BS 1881-117, Method for
the determination of tensile splitting strength. BS EN 12390-6
specifies a single method for determining the tensile splitting
strength of concrete cylinders as the reference method. Splitting
test carried out on a standard cylinder specimen by applying a line
load along the vertical diameter. BS 1881-117 placed no essential
dimensional requirements on the tests specimens. For cylinders, BS
EN 12390-6 requires compliance with the dimensional requirements of
Clause 4.3 of BS EN 12390-1. It is not practical to apply the true
line load to the cylinder because the side is not smooth enough and
because it would induce high compressive stresses at the surface.
Therefore, a narrow loading strip made of soft material is used.
Tensile Strength: Flexural Strength Test BS EN 12390-5 BS EN
12390-5 supersedes BS 1881-118, Method for the determination of
flexural strength. BS EN 12390-5 specifies two methods for the
determination of flexural strength of concrete by either centre or
two-point loading of cast or sawn prisms.
1-
2-
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3- The two-point loading test is essentially the same as the
test specified in BS 1881118, except in detail, whilst the
centre-point loading test was not included in BS1881. 4- A specimen
beam 150 x 150 x 500 mm is molds in two equal layers each rodded 60
times. 5- The beam may be vibrated and should be cured in the
standard way. 6- This test tends to overestimate the true tensile
strength by about 50%. 7- This can be explained by the fact that
the simple flexural formula used is based on a linear stress-strain
distribution whereas concrete has a nonlinear distribution. 8- This
is an important test because it model how a concrete beam is
normally loaded.
Non-Destructive Testing:These tests are useful to: Quality
control; Determination of the time for form removal; and Help
assess the soundness of existing concrete structures. NDT
Categories: Rebound Test: The rebound hammer is a surface hardness
tester for which an empirical correlation has been established
between strength and rebound number. The only known instrument to
make use of the rebound principle for concrete testing is the
Schmidt hammer, which weighs about 4 lb (1.8 kg) and is suitable
for both laboratory and field work. It consists of a
spring-controlled hammer mass that slides on a plunger within a
tubular housing. 1- The hammer is forced against the surface of the
concrete by the spring and the distance of rebound is measured on a
scale. 2- The test surface can be horizontal, vertical or at any
angle but the instrument must be calibrated in this position.
REBOUND HAMMER TEST
NDT Categories: Dynamic Test
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At present the ultrasonic pulse velocity (UPV) method is the
only one of this type that shows potential for testing concrete
strength in situ. 1- It measures the time of travel of an
ultrasonic pulse passing through the concrete. 2- The fundamental
design features of all commercially available units are very
similar, consisting of: A - a pulse generator, and B- a pulse
receiver. 3- Pulses are generated by shock-exciting piezo-electric
crystals, with similar crystals used in the receiver. 4- The time
taken for the pulse to pass through the concrete is measured by
electronic measuring circuits.
- NDT Categories: Penetration Test:1- The Windsor probe is
generally considered to be the best means of testing penetration.
2- Equipment consists of a powder-actuated gun or driver, hardened
alloy probes, loaded cartridges, a depth gauge for measuring
penetration of probes and other related equipment. 3- A probe,
diameter 0.25 in. (6.5 mm) and length 3.125 in. (8.0 cm), is driven
into the concrete by means of a precision powder charge. 4- Depth
of penetration provides an indication of the compressive strength
of the concrete. 5- Although calibration charts are provided by the
manufacturer, the instrument should be calibrated for type of
concrete and type and size of aggregate used.
- NDT Categories: Radioactive Methods 1- Radioactive methods of
testing concrete can be used to detect the location of
reinforcement, measure density and perhaps establish whether
honeycombing has occurred in structural concrete units. 2- Gamma
radiography is increasingly accepted in England and Europe. 3- The
equipment is quite simple and running costs are small, although the
initial price can be high. 4- Concrete up to 18 in. (45 cm) thick
can be examined without difficulty.
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Radioactive Methods figure
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