Essay: Testing in concrete construction

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  • Testing in concrete construction
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Testing are important in concrete construction. Tests concerned with fresh concrete are to check the workability of concrete. Tests on hardened concrete are to find the strength, creep effects, durability, etc.

Testing of Fresh Concrete

Following tests are commonly employed to measure workability of fresh concrete:

(1)Slump test

(2)Compaction factor test

(3)Flow test

(4)Kelly Ball test

(5)Vee Bee consistometer test

(1)Slump Test

Most commonly used method of measuring the consistency of concrete. It can be conducted in the field or in the laboratory. This test is not suitable for very wet or very dry concrete. Apparatus for conducting the slump test consists of a metallic mould in the form of a frustum of a cone with 20 cm bottom diameter, 10 cm top diameter and 30 cm height. Steel tamping rod of 16 mm dia, 0.6 m long with a bullet end is used for tamping. The internal surface of the mould is thoroughly cleaned and placed on a smooth, non-absorbant horizontal surface. Mould is filled with four layers of equal height. Each layer is compacted by giving 25 blows with the tamping rod uniformly. After filling the mould and roding, the excess concrete is stuck off and levelled.

Table: Slump and nature of concrete

Slump

Nature of concrete mix

No slump

Stiff and extra stiff mix

From 10 to 30 mm

Poorly mobile mix

From 40 to 150 mm

Mobile mix

Over 150

Cast mix

Mould is lifted upwards from the concrete immediately by raising it slowly. This allows the concrete to subside. This subsidence is referred to as slump of concrete. The difference in height of the mould and that of the subsided concrete is measured and reported in mm which is taken as the slump of concrete. Pattern of slump also represents the characteristics of concrete.

If the slump of the concrete is even, it is called a true slump. If one-half of the cone slides down it is called as shear slump. Here, the average value of the slump is considered. Shear slump also indicates that the concrete is not cohesive and reflects segregation (Fig.).

Types of slumps

Slumps recommended for various works of concrete construction are presented in Table .

Sl. No

Nature of concrete construction

Recommended slump

1.

Concrete to be vibrated

10 to 25 mm

2.

Concrete for road construction

20 to 40 mm

3.

Mass concrete

25 to 50 mm

4.

Concrete for horizontal tops of kerbs, parapets, piers, slabs and walls

40 to 50 mm

5.

Concrete for canal lining

70 to 80 mm

6.

Normal R.C.C. Work

80 to 150 mm

7.

Concrete for arch and side walls of tunnels

90 to 100 mm

Slump test can be conducted both in the laboratory and in work site. Slump test results are used to detect the difference in water content of successive batches of the identical mix.

(2)Compacting Factor Test

This is a more refined test than the slump test. Measures the degree of compaction obtained by using a certain energy in overcoming the internal friction of the concrete. This property is a measure of workability. The test apparatus consists of two conical hopers with bottom doors and a separate cylinder kept at the bottom. Concrete is filled in the top hopper fully without compaction and released successively through the two hoppers and into the bottom cylinder (Fig. ).

After striking off the level in the cylinder the weight of the concrete (W1) in the cylinder is determined. Same cylinder is filled with the same batch of concrete and compacted to get the maximum weight (W2). The ratio of the observed weight (W1) to the theoretical weight, (W2), i.e., W1/W2 is the compacting factor. The workability, compacting factor and the corresponding slump are given in Table.

Workbility

Compacting factor

Corresponding slump

Very low

0.80

0 to 25 mm

Low

0.85

25 to 50 mm

Medium

0.92

50 to 100 mm

High

0.95

100 to 180 mm

This test measures the quality of concrete, which relates very close to the workability. This test clearly depicts the workability of concrete. In the figure, the dimensions are plotted in mm.

Compacting factor test apparatus

(3)Flow Test

Gives an indication of the quality of concrete with respect to consistency, cohesiveness and non-segregation. Here, a mass of concrete is subjected to jolting and the flow or spread of the concrete is measured. The flow is related to workability.

The test apparatus consists of a flow table of 76 mm dia on which concentric circles are marked. A mould similar to the one used in the slump test with a base diameter is 25 cm and as upper diameter as 17 cm with a height of 12 cm is used (Fig.).

Mould is kept on the clean table and concrete is filled in two layers with each layer being rodded 25 times with a tamping rod of 1.6 cm diameter and 61 cm long with rounded ends.

Excess concretes on the top of the mould is levelled. The mould is lifted vertically upwards completely. The concrete stands on its own without support.

The table is raised and dropped 12.5 mm with a cam arrangement, 15 times in about 15 seconds. The diameter of the spread-concrete is measured in 6 directions and the average value is taken.

The flow of the concrete is defined as the percentage increase in the average diameter of the spread-concrete to the base diameter of the mould. i.e.,

The value varies from 0 to 150 %. Spread pattern of the concrete also reflects the tendency of the segregation. It is a laboratory test. In the figure, the dimensions are plotted in cm.

Flow table apparatus

(4)Kelly-Ball Test

Consists of a metal hemisphere of 15 cm diameter, weighing 13.6 kg. Concrete base should be 20 cm depth and the minimum distance from the center of the ball to nearest edge of the concrete is 23 cm. Ball is lowered gradually to the surface of the concrete. Depth of penetration is read immediately on the stem to the nearest 5 mm. This test can be done in a shorter periods of about 15 secs. It gives more consistent results than slump tests (Fig). It can be performed in the field and on the concrete placed on the site.

Kelly ball

(5)Vee-Bee Consistometer Test

Consists of a vibrating table, a metal pot, a sheet metal cone and a standard iron rod (Fig). A slump cone with concrete is placed inside the sheet metal cylindrical pot of the conistometer. Glass disc is turned and placed on the top of the concrete in the pot. Vibrator switches on and the stopwatch is started simultaneously. Vibrator is kept on till the concrete in the cone assumes a cylindrical shape. The time is noted, time required in seconds for the concrete to change from the shape of the cone to the shape of a cylinder is known as Vee Bee Degree. It is a good laboratory method and more suitable for very dry concrete. Measures the workability indirectly.

Vee-Bee Consistometer Test

Testing of Hardened Concrete

The following tests are conducted for hardened concrete:

(1)Compressive strength test

(2)Flexural strength test

(3)Split-tension test

(1)Compressive Strength Test

This is an important test. Most of the properties of concrete are qualitatively related to it. It is an easy and most common test. The tests are conducted on cubical or cylindrical specimens. The cube specimen is of size 15 × 15 × 15 cm and the cylinder is about 15 cm diameter and 30 cm long.

The largest nominal size of the aggregates does not exceed 20 mm. The moulds are to be of metal moulds, preferably of steel or cast iron. The moulds are made in such a way that the specimen is taken out without damage. A tamping steel bar of 16 mm diameter 0.6 m long with a bullet end is used for compacting.

The test cube specimens are made as soon as practicable. The concrete is filled into the mould in 5 m deep approximately. Each layer is compacted by tamping rod (25 to 35 strokes depending on 10 or 15 cm depth) or by vibration. After the top layer has been compacted the top of the mould is leveled using a trowel. The top is covered with a glass or metal plate to prevent evaporation.

The specimens are demoulded after 24 hours and submerged in fresh, clean water or saturated lime solution and kept there until taken out just prior to testing. The water should be maintained approximately at 27° C ± 2° C and on no account the specimens are allowed to become dry. The specimen is tested in a compression testing machine at the completion of 7 days and 28 days. Compression on the cube or cylinder undergoes lateral expansion owing to the Poisson’s ratio effect.

Cylindrical specimens are less affected by end restraints caused by plates and hence it is believed to give more uniform results than the cube. Further cylinder simulates the real condition in the field in respect of direction of load. Normally strength of cylindrical specimen is taken as 0.8 times the strength of cubical specimens.

(2)Flexural Strength Test

Concrete is relatively strong in compression and weak in tension. Tensile stresses can develop in concrete due to drying shrinkage, rusting of steel reinforcement, temperature gradient and many other reasons. Hence, the tensile strength of concrete gains importance.

Direct measurement of tensile strength is not feasible. Beam tests are found to be dependable to measure the flexural strength property of concrete. Modulus of rupture is taken as the extreme fibre stress in bending.

The value of modulus of rupture depends on the dimension of the beam and the type of loading. The loading adopted is central or two-point loading.

In the central point loading (Fig a), the maximum fibre stress occurs below the point of loading where the bending moment is maximum.

In the two-point loading (Fig.b), the critical crack may appear in any section, where the bending moment is maximum or the resistance is weak. In general, the two-point loading yields lower value of modulus of rupture than the center point loading.

Loading arrangement in Flexural beam test

Size of specimen is 15 × 15 × 70 cm. In case of concrete with aggregate size less than 20 mm a beam size of 10 × 10 × 50 cm may be used. Mould may be of metal or steel or cast iron. Tamping rod may be of steel of 2 kg weight, 40 cm long and should have a ramming face of 25 mm square.

Testing machine should have the sufficient loading capacity with a specific rate of loading such that the permissible errors on the applied load should not be greater than ±0.50 %. Flexural strength of the specimen is expressed as the modulus of rupture fb as

Where,

P = maximum load in kg applied to the specimen

a = 17 to 20 cm for 15.0 cm specimen or > 13.3 cm for 10.0 cm specimen

b = measured width in cm of the specimen

d = measured depth in cm of the specimen at the point of failure

If a is less than 170 cm for a 15.0 cm specimen or less than 11.0 cm for a 10.0 cm specimen the results of the test be discarded.

(3)Split-Tension Test

This is an indirect tension test, also referred to as Brazilian test. In this test, a cylindrical specimen is placed horizontally between the loading surfaces in a compression testing machine. Load is applied to failure of the cylinder along the vertical diameter. The test specimen is shown in Fig.

Split-tension test

When the load is applied along the genetrix, compressive stresses develop immediately below the two generators to which the load is applied. A larger portion about 5/6th of the depth is subjected to tensile stress.

It is simple to perform and generally gives more uniform results. Tensile strength of split-tension test is almost nearer to true tensile strength than the modulus of rupture. The split-tension test gives 5 to 12 % higher value than the direct tensile strength.

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