FRESH CONCRETE SAMPLING

• The internal surface of the slump cone shall be thoroughly cleaned and freed from superfluous moisture and any set concrete before commencing the test.
• The cone shall be placed on a smooth, horizontal, rigid and non-absorbent surface, such as a carefully levelled metal plate, the cone being firmly held in place while it is being filled.
• The cone shall be filled in four layers, each approximately one-quarter of the height of the cone.
• Each layer shall be tamped with twenty-five strokes of the rounded end of the tamping rod.
• The strokes shall be distributed in a uniform manner over the cross-section of the cone and for the second and subsequent layers shall penetrate into the underlying layer and the bottom layer shall be tamped throughout its depth.

• After the top layer has been rod, the concrete shall be struck-off level with a trowel or the tamping rod, so that the cone is exactly filled.  Any mortar which may have leaked out between the cone and the base plate shall be cleaned away.
• The cone shall be removed from the concrete immediately by raising it slowly and carefully in a vertical direction. This allows the concrete to subside.
• The slump shall be measured immediately by determining the difference between the height of the cone and that of the highest point of the specimen being tested.
• The slump measured shall be recorded in terms of millimeters of subsidence of the specimen during the test.
• Slump to be measured for all transit mixers and the value shall be entered in Concrete Pour Record as per ACT ITP for Civil Work Materials and also on the batch certificate of the particular truck.

8.2 FLOW TEST FOR SELF COMPACTING CONCRETE

• The table and mould shall be clean. The mould shall be cleaned thoroughly from all material and excess water be removed keeping inside of the mould wet.
• The mould shall place on the centre of the table firmly and be filled in two layers each approximately half the volume of the mould.
• Each layer shall be rodded with 25 strokes using a straight metal rod of 61 cm long and 1.6 cm in diameter rounded at the tamping side.
• First layer shall be rodded through out its depth.
• Strokes must be distributed evenly in a uniform manner over the cross section of the mould. Ensure the rod penetrates into the preceding layer.
• After the top layer has been rodded, the surface of the concrete shall be struck off with a trowel so that the mould is exactly filled.

• Excess concrete which has overflowed the mould shall be removed and the area of the table outside the mould again cleaned.
• The mould shall be immediately removed from the concrete by a steady upward pull.
• The table shall then be raised and dropped 12.5 mm, 15 times in about 15 seconds.
• The average of six symmetrically distributed caliper measurements of the diameter of the spread of the concrete shall be taken as the value to the nearest 5 mm.
• The flow of the concrete shall be recorded as the percentage increase in diameter of the spread concrete over the base diameter of the moulded concrete.
• Flow, percent is calculated as  spread diameter in cm – 25 / 2.5 x 100

8.3 MAKING OF COMPRESSION TEST SPECIMENS (CUBES)

• Samples according to the frequency as per technical specification/IS codes are to be collected from the transit mixers.
• The interior surfaces and joints of the assembled mould shall be thinly coated with mould oil to prevent adhesion of the concrete.
• The test specimens shall be made as soon as practicable after collecting the samples, and in such a way as to produce full compaction of the concrete with neither segregation nor excessive laitance.
• The concrete shall be filled into the mould in layers approximately 5 cm deep. In placing each scoopful of concrete, the scoop shall be moved around the top edge of the mould as the concrete slides from it, in order to ensure a symmetrical distribution of the concrete within the mould.

• Each layer shall be compacted manually using the standard tamping bar and the strokes of the bar shall be distributed in a uniform manner over the cross-section of the mould. The number of strokes shall be 35 per layer.
• The strokes shall penetrate the underlying layer and the bottom layer shall be rod throughout its depth. Where voids are left by the tamping bar, the sides of the mould shall be tapped to close the voids.
• Alternatively compacting can be done by vibration, each layer shall be vibrated by means of an electric or pneumatic hammer or vibrator or with the help of a suitable vibrating table until the specified condition is attained.
• After the top layer has been compacted, the surface of the concrete shall be finished level with the top of the mould using a trowel.
• An identification slip (Cube identification number, grade, date…) is to be kept immediately h the cubes till permanent marking can be done.
• All cubes shall be then marked with permanent marker.

8.4 STORING & CURING OF TEST SPECIMENS

• The test specimens shall be stored on the site at a place free from vibration, under damp matting, sacks or other similar material for 24 hours ± ½ hour from sampling.
• After the period of 24 hours, they shall be removed from the moulds and, unless required for testing within 24 hours, stored in curing tank at 27 + 2ºC with clean water till testing day.

8.5 TESTING OF CUBES

• Specimens stored in water shall be tested immediately on removal from the water and while they are still in the wet condition. Surface water and grit shall be wiped-off the specimens and any projecting fins removed.
• The bearing surfaces of the testing machine shall be wiped clean, and any loose sand or other material removed from the surfaces of the specimen which are to be in contact with the compression plates.
• Cubes shall be placed in the machine in such a manner that the load shall be applied to opposite sides of the cubes as cast, that is, not to the top and bottom.

• The axis of the specimen shall be carefully aligned with the centre of thrust of the spherically seated plate. No packing shall be used between the faces of the test specimen and the steel plate of the testing machine.
• The load shall be applied without shock and increased continuously at a rate as per the manufactures instructions until the increasing load breaks down and no greater load can be sustained.
• The maximum load applied to the specimen shall then be recorded and the appearance of the concrete and any unusual features in the type of failure shall be noted.

8.6 CALCULATION OF RESULTS

• The concrete specimen shall be tested for compressive strength at 7 days and 28 days.
• The measured compressive strength of the specimen shall be calculated by dividing the maximum load applied to the specimen during the test by the cross-sectional area, calculated from the mean dimensions of the section and shall be expressed to the nearest kg/cm².
• Average of three values shall be taken as the representative of the batch provided the individual variation is not more than ± 15% of the average. Otherwise, repeat tests shall be made.
• All details shall be entered in the Cube Register and both soft and hardcopies to be maintained.

9.QUALITY CONTROL FOR CONCRETE SAMPLING

• Standard deviation will be used as a means for statistical quality control of concrete delivered to the project.
• SD will be calculated for a minimum of 30 results per grade of concrete separately for different RMC’s and will be entered in the Cube result.
• As the aim of Quality control is to limit the variability as much as practicable it can be assumed that any standard deviation value below less than ‘6’ is acceptable.
• Range from higher value to lower value shows that the degree of control over production, placing and mixing condition of concrete is fair to excellent.
• The cube result will tabulated as per the attached table.

10. ASSOCIATED FORMATS ATTACHED (DOWNLOAD YOUR COPY FOR FREE FOR SAMPLE FORMATS

• Concrete Pour Card for RMC deliveries
• Concrete Compressive Strength Record Register
• Cube Identification System.

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