Guidelines to follow for acceptance criteria for concrete

As per IS 456 : 2000 (C1 15.1), it is known that 28 days compressive strength shall individually be the standard for approval and disapproval of concrete.

But as per IS 456 : 2000 (C1 16), there exist two criterion for acceptance of concrete that range from compressive strength and flexural strength.

Prior to arrange test results for acceptance criteria, it is necessary to examine validation i.e. whether the sample is proper or improper for acceptance criteria.

The test results should be as follow :-

Test results are average of three specimens.

Individual deviation should be under ± 15% of the average.

Suppose M25 grade of concrete is applied in the construction. Let strength of specimen for first one should be 28.5 MPa (Mega Pascal). For the second one, it should be at 26 mega pascal and for third one, it should be 29 mega pascal.

By summing up, we get the average as 27.83 MPa (mega pascal).

If the individual deviation is computed from the average for first specimen, it shall be 2.4%.

For second, it shall be -6.57%

For third, it shall be 4.20%

From the test result, it can be said that the specimens are legitimate for acceptance criteria as the deviation of individual sample does not surpass 15%.

To get more details, go through the following video tutorial.

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Published By
Rajib Dey
www.constructioncost.co

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Slipformed Concrete and its advantages

Slip forming alias continuous poured, continuously formed, or slipform construction stand for a construction process in which concrete is poured into a constantly moving form. Slip forming is employed for high structures (like bridges, towers, buildings, and dams), as well as horizontal structures, like roadways.

Under the slipforming method, the formwork is shifted ceaselessly in sync with the concreting method in a 24-hour operation. The formwork along with the working platform and the hanging scaffold lifted inwardly or on both sides, is attached to the jacking rods in the center of the wall. The lifting jack that runs with hydraulic oil increases the formwork by 15 to 30 cm per hour on the basis of the temperature. The jacking rods are set in pipe sleeves at the top and are supported with the concrete that is already solidified. The rods and sleeves are also expanded endlessly. Mainly, the specialist contractors perform these work completely.

Slipforming is a rapid and well-organized method. The method is specifically applicable for simple, stable ground plans and following high structures:

 High bay warehouses, silos§
 Tower and chimney structures§
 Shaft structures§

As the height of the formwork is normally around 1.20 m and per hour production rate is 20 to 30 cm, the concrete beneath is 4–6 hours old and should be sufficiently rigid to carry its own weight (green strength). Though, it should not have arranged adequately for some of it to fix to the rising formwork (“plucking”). The main purpose of slipforming devoid of any issue is to start concreting all areas at the same level all together, and then perform the setting of these layers all at once. So, the temperature plays an important role together with the need for the constantly optimum w/c ratio.

Composition:
 Aggregate 0–32 mm, or 0–16 mm for close reinforcement§
 Despite the fact that slipformed concrete is primarily crane handled concrete, the fines content should be as for pumped concrete§
 Cement Min 300 kg/m³§
 CEM I 42.5 for close reinforcement and large dimensions, CEM I 52.5 for smaller dimensions (towers, chimneys)§

Workability: The best workability is achieved if a stiff plastic concrete contains a flow diameter of 35–40 cm and a low water content. Note particularly that a wall thickness below 14 cm can create a issue (plucking, anchorage of jacking rods etc.). The newly struck surfaces should be safeguarded as much as possible against wind, sunlight etc.

Article Source civilengineerblog.com

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Published By

Rajib Dey

www.constructioncost.co

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