OVERVIEW OF CONCRETE TESTS

1. SLUMP TEST

Object

To determine the consistency of concrete mix of given proportions by the Slump test.

Theory

Unsupported fresh concrete, flows to the sides and a sinking in height takes place. This vertical settlement is known as Slump. In this test fresh concrete is filled into a mould of specified shape and dimensions, and the settlement or slump is measured when supporting mould is removed. Slump increases as water content is increased. For different works different slump values have been recommended as shown in the table.

Slump is a measure indicating the consistency or workability of cement concrete. It gives an idea of water content needed for concrete to be used for different works. A concrete is said to be workable if it can be easily mixed and placed, compacted and finished. A workable concrete should not show any segregation or bleeding. Segregation is said to occur when coarse aggregate tries to separate out from the finer material and a concentration of coarse aggregate at one place occurs. This results in large voids, less durability and strength. Bleeding of concrete is said to occur when excess water comes up at the surface of concrete. This causes small pores through the mass of concrete and is undesirable.

Apparatus

Iron pan to mix concrete, trowels, Slump test apparatus and tamping rod.

The slump cone is a hollow frustum made of thin steel sheet, with internal dimensions as: the top diameter 100 mm, the bottom diameter 200 mm and the height 300 mm. It stands on a plane non-porous surface. To facilitate vertical lifting from moulded concrete it is provided with a suitable attachment and suitable foot pieces and handles.

The tamping rod is 16 mm in diameter, 0.6 m long and is bullet pointed at the lower end.

Procedure

1.            Collect the fresh concrete from the Mixer and mix the concrete thoroughly to get a uniform mix.

2.            Place the mixed concrete in the cleaned slump cone mould in the 4 layers, each approximately ¼ of the height of the mould. Tamp each layer 25 times with tamping rod distributing the strokes in a uniform manner over the cross section of the mould. For the second and subsequent layers the tamping rod should penetrate in to the underlying layer.

3.            Strike off the top with a trowel or tamping rod so that the mould is exactly filled.

4.            Remove the cone immediately, raising it slowly and carefully in the vertical direction.

5.            As soon as the concrete settlement comes to a stop, measure the subsidence of concrete in mm which will give the slump.

 The recommended slump values for different placing conditions 
(Ref: DIN EN 206)

References:

1. IS 1199 – 1959             Methods of Sampling and Analysis of Concrete

2.            Strength of Cement Concrete

Object

To determine the Characteristic compressive strength of concrete

Theory and Scope

One of the important properties of concrete is its strength in compression. The strength in compression has a definite relationship with all the other properties of concrete, i.e., these properties are improved with the improvement in compressive strength, hence the importance of the test.

The height of the test specimen in relation to its lateral dimensions greatly influences the results. The more slender the test specimen, lower will be the crushing strength. The ratio of the minimum dimension of the specimen to maximum size of aggregate should be at least 4: 1.

Apparatus

Cube moulds fitted tightly on base plate (size in mm 150x150x150) in required set (One set comprises of 9 moulds, for 3, 7 and 28 days test),

Smooth Tamping rod of size 16 mm dia and length of 600 mm with one rounded edge, Trowels small and large, Collection pan, Flat working platform of adequate size, weighing balance and 200 tones compression testing Machine.

Procedure

A. Sampling

1. Cube moulds shall be verified for dimensions and tight fitting assembly. They shall be lightly oiled and have clean surfaces.

2. Cube moulds shall be arranged in sets on working platform.

3. Collect the fresh concrete from the Mixer and mix the concrete thoroughly to get a uniform mix.

4. Fill the mixed concrete in cube moulds in 3 layers, each approximately 5cm or 1/3rd of the height of the mould. Tamp each layer 35 times with tamping rod distributing the strokes in a uniform manner over the surface of the layer. For the second and subsequent layers the tamping rod should penetrate in to the underlying layer.

5. Care to be taken particularly at corners. Trowels can be used at sides to ensure proper surface of sides and allow entrapped air to escape.

6. Struck off concrete flush with the top of the moulds.

7. Cube concrete surface shall be marked with Cube I.D. , Mix, location of use, date, source of concrete ( additional) when adequately set.

8. Cubes thus cast, shall be covered with wet gunny bags till they are replaced in curing tank.

9. Cubes shall be checked for surface correctness, sharp edges and incorrect filling. Such defective cubes may be rejected there it self.

10.         Details of Cubes thus sampled shall be entered serially in Cube Register maintained for testing compressive strength.

11.         The number of cube samples cast shall also be entered in concrete batch ticket and pour card with proper traceability to cube register.

B. Curing

Specimens are removed from the moulds after 24 hours and placed in curing tank. Cubes shall be arranged in the curing tank sequentially as per ID mark, date wise.

Cubes shall be sorted out for 7 days, 28 days tests. Cubes shall be placed in the tank in layer of three cubes with marked top upside.

Cubes once placed in tank shall not be handled, disturbed frequently so that edges are not broken. Water in tank shall cover cubes by at least 50-75 mm all the time.

Water should be clean at a temperature of 27 ± 30C.

C. Testing

Remove the cubes required for testing from curing tank, wipe clean of water and air dry them to surface dryness.

Stack cubes in sequence as per mix, dates, locations etc. Weigh the cubes and record the weights in the register.

Place the specimen centrally on the location marks of the compression testing machine and load is applied continuously, uniformly and without shock. The rate of loading is continuously adjusted through rate control valve by hand to 14 N/mm2/minute. The load is increased until the specimen fails and record the maximum load carried by each specimen during the test. Also note the type of failure and appearance of cracks.

Format for concrete cube Register

Calculation

Compressive Strength = Failure Load/ Area of Cross section

Area of Cross section (for 150 mm cube mould) = 150 x 150

                                             A = 22500 mm2

Failure Load = L KN

                           = L x 1000 N

Compressive Strength = L x 1000 / A

                                               = L x 1000 / 22500            N/mm2

References:

1.            IS 516 – 1959    Methods of Tests for strength of Concrete

2.            IS 456-2000       Plain and Reinforced concrete-code of practice

Precautions

Both the mould and base plate should be lightly oiled before use to prevent the concrete from sticking to the moulds.

Excess compaction should be avoided, particularly when using vibration with more workable mixes, as this is likely to produce segregation and loss of water from the spaces between moulds and their base plates.

The blows should be evenly distributed over the surface of each layer.

When compaction is completed the moulds should be slightly overfull, the surplus be struck off flush with trowels.

At least three specimens should be used for each test and mean crushing strength of three being taken as crushing strength of concrete. While calculating the average load, if any, individual variation from the average is more than 15 percent the test results are discarded.

Cube should be placed in testing machine centrally on platens.

Frequency of Sampling (Ref: IS 456:2000)

The minimum frequency of sampling of concrete of each grade shall be as below:

Note 

•  At least one sample shall be taken from each shift. Where concrete is produced at continuous production unit such as RMC plant, frequency of sampling may be agreed upon mutually.
• Three specimens shall be made for each sample for testing at 28 days.
• Additional samples may be required for determining strength at 3 days and 7 days or at the time of striking formwork.

Remarks

Sample* means quantity of concrete required to fill one set of cubes (3nos / 6 nos /9 nos) for testing. Hence number of sample is equivalent to number of cube sets (3 / 6/9 nos.). 3 cubes are drawn when only cube testing is done at 28 days. 9 cubes are drawn for tests at 3/7 / 28 days required. Always sampling of cubes should be done as set and these set of cubes (3/6/9 nos.) should be taken from any one of the transit mixer.