materials of construction lab application - testing of concrete · 2020. 3. 12. · materials of...

MATERIALS of CONSTRUCTION Lab Application - Testing of Concrete

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1. TESTING FRESH CONCRETE

1.1. Slump Test (EN 12350-2)

The fresh concrete is compacted into a mould in the shape of a frustum of a cone. When the cone is withdrawn upwards, the distance the concrete has slumped provides a measure of the consistency of the concrete.

Slump cone The interior of the mould shall be smooth and free from projections, such as protruding rivets and shall be free from dents. The mould shall be in the form of a hollow frustum of a cone having the following internal dimensions:

diameter of base: 200 mm; diameter of top: 100 mm; height: 300 mm.

The base and the top of the mould shall be open and parallel to each other and at right angles to the axis. The mould shall be provided with two handles near the top and fixing clamps or foot pieces near the bottom to hold it steady.

Testing procedure

Dampen the mould and base plate and place the mould on the horizontal base plate/surface. During filling of the mould hold it firmly against the base plate/surface by clamping in place, or by standing on the two foot pieces.

Fill the mould in three layers, each approximately one-third of the height of the mould when compacted. Compact each layer with 25 strokes of the tamping rod. Uniformly distribute the strokes over the cross-section of each layer. For the bottom layer this will necessitate inclining the rod slightly and positioning approximately half the strokes spirally toward the centre. Compact the second layer and the top layer each throughout its depth, so that the strokes just penetrate into the immediately underlying layer. In filling and compacting the top layer, heap the concrete above the mould before tamping is started.

If the tamping operation of the top layer results in subsidence of the concrete below the top edge of the mould, add more concrete to keep an excess above the top of the mould at all times. After the top layer has been compacted, strike off the surface of the concrete by means of a sawing and rolling motion of the compacting rod.

Remove spilled concrete from the base plate/surface. Remove the mould from the concrete by raising it carefully in a vertical direction.

Perform the operation of raising the mould in 5 s to 10 s, by a steady upward lift, with no lateral or torsional motion being imparted to the concrete.

Immediately after removal of the mould, measure and record the slump by determining the difference between the height of the mould and that of the highest point of the slumped test specimen.

The test is only valid if it yields a true slump. If the specimen shears, as shown in Figure 3 (a), another sample shall be taken and the procedure repeated. If two consecutive tests show a portion of the concrete shearing off from the mass of the test specimen, the concrete lacks the necessary plasticity and cohesiveness for the slump test to be suitable.

Figure 1. Slump cone

Figure 3. Types of concrete slump

Figure 2. Slump measurement


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1.2. Density (EN 12350-6) This standard specifies a method for determining the density of compacted fresh concrete both in the laboratory and in the field. Fresh concrete is compacted into a rigid and watertight container of known volume and mass and is then weighed. The volume of the container shall not be less than 5 l.

Means of compacting the concrete Compacting method may be one of the following:

a) Internal (poker) vibrator with a minimum frequency of approximately 120 Hz (7 200 cycles per minute), b) Vibrating table with a minimum frequency of approximately 40 Hz (2 400 cycles per minute), c) Compacting rod of circular cross-section, straight, made of steel, having a diameter of approximately 16

mm, length of approximately 600 mm and with rounded ends; d) Compacting bar, straight, made of steel having a square cross-section of approximately 25mm x 25mm

and length of approximately 380 mm.

Procedure Weigh the container to determine its mass (m1) and record the value indicated. Place the concrete into the container by compacting in a minimum of two layers. Compact each layer by

applying mechanical vibration or hand compaction. If internal vibrator or vibrating table is used, apply the vibration for the minimum duration necessary to

achieve full compaction of the concrete. Full compaction is achieved using mechanical vibration, when there is no further appearance of large air bubbles on the surface of the concrete.

If hand compaction is applied, distribute the strokes of the compacting rod, or bar, in a uniform manner over the cross section of the mould.

Ensure that the compacting rod, or bar, does not forcibly strike the bottom of the container when compacting the first layer, nor penetrate significantly any previous layer. Subject the concrete to at least 25 strokes per layer. In order to remove pockets of entrapped air but not the entrained air, after compaction of each layer, tap the sides of the container smartly with the mallet until large bubbles of air cease to appear on the surface and depressions left by the compacting rod or bar, are removed.

After the top layer has been compacted, smooth it level with the top of the container, using the steel float. Wipe the outside of the container clean.

Weigh the container with its contents to determine its mass (m2) and record the value indicated. The density is calculated from the formula:

where D is the density of the fresh concrete, in kilograms per cubic meter; m1 is the mass of the container, in kilograms; m2 is the mass of the container plus the mass of the concrete specimen in the container, in kilograms; V is the volume of the container, in cubic meters.


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1.3. Air Content (EN 12350-7) There are two test methods, both of which use apparatus which employ the principle of Boyle-Mariotte's law. For the purpose of reference, the two methods are referred to as the water column method and the pressure gauge method and the apparatus as a water column meter and a pressure gauge meter.

Means of compacting the concrete Compacting method may be one of the following:

a) Internal (poker) vibrator with a minimum frequency of approximately 120 Hz (7 200 cycles per minute), b) Vibrating table with a minimum frequency of approximately 40 Hz (2 400 cycles per minute), c) Compacting rod of circular cross-section, straight, made of steel, having a diameter of approximately 16

mm, length of approximately 600 mm and with rounded ends; d) Compacting bar, straight, made of steel having a square cross-section of approximately 25mm x 25mm

and length of approximately 380 mm.

1.3.1 Water column method

Water is introduced to a predetermined height above a sample of compacted concrete of known volume in a sealed container and a predetermined air pressure is applied over the water. The reduction in volume of the air in the concrete sample is measured by observing the amount by which the water level is lowered.

Procedure

Using the scoop, place the concrete in the container having a capacity of at least 5 l. Place the concrete in three layers, approximately equal in depth. Compact the concrete by applying mechanical vibration or hand compaction.

If internal vibrator or vibrating table is used, apply the vibration for the minimum duration necessary to achieve full compaction of the concrete. Full compaction is achieved using mechanical vibration, when there is no further appearance of large air bubbles on the surface of the concrete.

If hand compaction is applied, distribute the strokes of the compacting rod, or bar, in a uniform manner

over the cross section of the mould. Subject the concrete to at least 25 strokes per layer. Thoroughly clean the flanges of the container and cover assembly. Clamp the cover assembly in place. Fill the apparatus with water and tap lightly with the mallet to remove air adhering to the interior surfaces

of the cover. Bring the level of water in the standpipe to zero by bleeding through the small valve with the air vent

open. Close the air vent and apply the operating pressure, P, by means of the air pump. Record the reading on the gauge tube, h1, and release the pressure. Read the gauge tube again and if

the reading, h2, is 0.2 % air content or less record the value (h1 - h2) as the apparent air content, A1. If h2 is greater than 0.2 % air content apply the operating pressure, P, again, giving a gauge tube reading

h3 and a final reading h4 after release of the pressure. If (h4 - h2) is 0.1 % air content or less record the value (h3 - h4) as the apparent air content. If (h4 - h2) is greater than 0.1 % air content, it is probable that leakage is occurring and the test shall be disregarded.

Figure 4. Water column method apparatus


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1.3.2 Pressure gauge method A known volume of air at a known pressure is merged in a sealed container with the unknown volume of air in the concrete sample. Procedure

Using the scoop, place the concrete in the container having a capacity of at least 5 l. Place the concrete in three layers, approximately equal in depth. Compact the concrete by applying mechanical vibration or hand compaction.

If internal vibrator or vibrating table is used, apply the vibration for the minimum duration necessary to achieve full compaction of the concrete. Full compaction is achieved using mechanical vibration, when there is no further appearance of large air bubbles on the surface of the concrete.

If hand compaction is applied, distribute the strokes of the compacting rod, or bar, in a uniform manner over the cross section of the mould. Subject the concrete to at least 25 strokes per layer.

Thoroughly clean the flanges of the container and cover assembly. Clamp the cover assembly in place. Close the main air valve and open valve A and valve B. Using a syringe, inject water through either valve A or B until water emerges from the other valve. Lightly

tap the apparatus with the mallet until all entrapped air is expelled. Close the air bleeder valve on the air chamber and pump air into the air chamber until the hand on the

pressure gauge is on the initial pressure line. After allowing a few seconds for the compressed air to cool to ambient temperature stabilize the hand on

the pressure gauge at the initial pressure line by further pumping in or bleeding off air as necessary. During this process lightly tap the gauge.

Close both valve A and valve B and then open the main air valve. Tap the sides of the container sharply. Whilst lightly tapping the pressure gauge, read the indicated

value, which is the apparent percentage of air, A1.

Figure 5. Pressure gauge method apparatus


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2. TESTING HARDENED CONCRETE

2.1. Compressive Strength Test (EN 12390-3) Specimens are loaded to failure in a compression testing machine. The maximum load sustained by the specimen is recorded and the compressive strength of the concrete is calculated. Procedure

Wipe the excess moisture from the surface of the specimen before placing in the testing machine. Wipe all testing machine bearing surfaces clean and remove any loose grit or other extraneous material

from the surfaces of the specimen that will be in contact with the platens. Position the cube specimens that the load is applied perpendicularly to the direction of casting. If required (particularly in cylinders), the ends of the specimens should be adjusted by grinding, calcium

aluminate cement mortar, sulfur mixture or sandbox. Centre the specimen with respect to the lower platen. If auxiliary platens are used, align them with the top and bottom face of the specimen. Select a constant rate of loading within the range 0.2 MPa/s to 1.0 MPa/s. Apply the load to the specimen

without shock and increase continuously, at the selected constant rate until no greater load can be sustained.

Record the maximum load indicated. Calculate the compressive strength using the equation:

where fc is the compressive strength in MPa F is the maximum load up to failure in N and Ac is the cross-sectional area of the specimen (in mm

2) on which the compressive force acts.

Explosive failure

NOTE: All four exposed faces are cracked approximately equally, generally with little damage to faces in contact with the platens.

Figure 6. Satisfactory failure of cube specimens

Figure 7. Satisfactory failure of cylinder specimens


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2.2. Flexural Strength Test (EN 12390-5) Prismatic specimens are subject to a bending moment by the application of load through upper and lower rollers. The maximum load sustained is recorded and the flexural strength is calculated. Procedure

For specimens stored in water, wipe excess moisture from the surface of the specimen before placing in the testing machine.

Wipe clean all testing machine bearing surfaces and remove any loose grit or other extraneous material from the surfaces of the specimen that will be in contact with the rollers.

Place the test specimen in the machine, correctly centred and with the longitudinal axis of the specimen at right angles to the longitudinal axis of the upper and lower rollers.

Ensure that the reference direction of loading is perpendicular to the direction of casting of the specimen. Select a constant rate of stress within the range 0.04 MPa/s to 0.06 MPa/s. Apply the load without shock

and increase continuously until no greater load can be sustained.

where fcf is the flexural strength in MPa f is the maximum load in N l is the distance between the supporting rollers in mm and d1 and d2 are the lateral dimensions of the cross-section in mm.

Figure 8. Two-point loading Figure 9. Centre-point loading


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2.3. Tensile Splitting Strength Test (EN 12390-6) A cylindrical specimen is subjected to a compressive force applied to a narrow region along its length. The resulting orthogonal tensile force causes the specimen to fail in tension.

Procedure

The test specimens shall be cylindrical, conforming to EN 12390-1 (length/diameter ratio; l/d=2), but l/d ratio as low as 1 shall be acceptable for cores.

For specimens stored in water, wipe any excess moisture from the surface of the specimen before placing in the testing machine.

Wipe the bearing surfaces of the jig, packing strips, loading pieces and platens. Clean and remove any loose grit or other extraneous material from the surface of the specimen that will be in contact with the packing strips.

Place the test specimen centrally in the testing machine, optionally using a jig. Carefully position packing strips and, if required, loading pieces, along the top and bottom of the plane of loading of the specimen.

Ensure that the upper platen is parallel with the lower platen, during loading.

Ensure that the specimen remains centred

when the load is first applied, either by

means of a jig or by temporary supports.

Select a constant rate of loading within the

range 0.04 MPa/s to 0.06 MPa/s. Apply

the load without shock and increase

continuously, at the selected constant

rate, until no greater load can be sustained.

Record the maximum load indicated. The tensile splitting strength is given by the formula:

where fct is the tensile splitting strength in MPa, F is the maximum load in N, L is the length of the line of contact of the specimen in mm and d is the designated cross-sectional dimension in mm.

1: Steel loading piece 2: Hardboard packing strips

Figure 10. Jig for testing cylindrical specimens


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2.4. Determination of Static Modulus of Elasticity and Poisson’s Ratio in Compression (ASTM C469)

This test method covers determination of (1) chord modulus of elasticity (Young’s) and (2) Poisson’s ratio of molded concrete cylinders and diamond-drilled concrete cores when under longitudinal compressive stress. Procedure

Use at least two companion specimens to determine the compressive strength prior to the test for modulus of elasticity.

Place the specimen, with the strain-measuring equipment attached, on the lower platen or bearing block of the testing machine. Carefully align the axis of the specimen with the center of thrust of the spherically-seated upper bearing block.

Load the specimen at least three times. Do not record any data during the first loading. Base calculations on the average of the results of the subsequent loadings.

Apply the load continuously and without shock. Set testing machines of the screw type so that the moving head travels at a rate of about 1 mm/min when the machine is running idle. In hydraulically operated machines, apply the load at a constant rate within the range 250 ± 50 kPa/s. Load the specimen until the applied load is 40 % of the average ultimate load of the companion specimens. This is the maximum load for the modulus of elasticity test.

During the first loading, observe the performance of the gauges. Correct any attachment or alignment defects that may be causing erratic readings prior to the second loading. For the second and subsequent loadings, obtain each set of readings as follows: Record, without interruption of loading, the applied load and longitudinal strain at the point (1) when the longitudinal strain is 50 microstrain and (2) when the applied load is equal to 40 % of the ultimate load of the companion specimens. Longitudinal strain is defined as the measured longitudinal deformation of the specimen divided by the effective gauge length.

If Poisson’s ratio is to be determined, record the transverse strain at the same points. The transverse strain is the measured change in specimen diameter divided by the original diameter.

If a stress-strain curve is to be determined, take readings at two or more intermediate points without interruption of loading; or use an instrument that makes a continuous record of the gauge readings.

Calculate the modulus of elasticity as follows:

where E: chord modulus of elasticity, in MPa S2: stress corresponding to 40% of the ultimate load, in MPa S1: stress corresponding to a longitudinal strain, ε1, of 50 millionths, in MPa ε2: longitudinal strain produced by stress S2.

Calculate Poisson’s ratio as follows:

where μ: Poisson’s ratio, εt2: transverse strain at mid-height of the specimen produced by stress S2, and εt1: transverse strain at mid-height of the specimen produced by stress S1.

Figure 11. Suitable compressometer consisting of two yokes, one of which (see B) is rigidly attached to the specimen and the other (see C) attached at two diametrically opposite points so that it is free to rotate. At one point on the circumference of the rotating yoke, midway between the two support points, a pivot rod (see A) is used to maintain a constant distance between the two yokes.

materials of construction lab application - testing of concrete · 2020. 3. 12. · materials of...

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