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28-Days Strength of Concrete in 15 Minutes

By
KAUSHAL KISHORE
Materials Engineer, Roorkee

Determination of compressive strength of concrete, either accelerated or normal 28-days, takes such a long time that remedial action for defective concrete cannot be under-taken at an early stage. By the time cube strength results indicate low strength, it is too late to do any remedy for the defective concrete which has already set in the form, Further in whole day of concreting work, cubes are filled from only a few batches of concrete which do not actually represent the strength of the entire concrete mass being used in the construction. This shows the limitations of cube strength test for the quality control of concrete.

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Dusting of Concrete Slab Surface

By
KAUSHAL KISHORE
Materials Engineer, Roorkee

It is supposed that concrete should give satisfactory service to its entire life. However problems arises, if care is not taken during construction blemish appears on the surface of a concrete slab, it will likely to be one of these: bilisters, cracking, crazing, curling, delamination, discoloration. DUSTING, efflorescence, low spots, popouts, scaling or spelling. This paper will give the details about dusting.

DUSTING
Formation of loose powder resulting from disintegration of surface of hardened concrete is called dusting or chalking and this is composed of water, cement and fine particles. The concrete surface powder under any kind of traffic, and also surface can be easily scratched with nail or even by sweeping.
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Capping Concrete Specimens For Compression Testing

By
KAUSHAL KISHORE
Materials Engineer, Roorkee

CAPPING THE CYLINDERS
It is required that the cylinders ends must be plane within 0.050 mm. The most common way of achieving this planeness requirement is to cap the ends of the cylinder as per ASTM C6176 with suitable materials. Three different capping materials are permitted (a) A thin layer of stiff Portland cement paste may be used on freshly molded specimens. (b) on hardened cylinders, either high-strength gypsum plaster or sulfur mortar may be used (c) A third method is, an elastomeric pad is placed within a metal retaining ring, and the assembly is then placed over the specimen end. The pad conforms to the shape of the cylinder end, but is prevented from spreading laterally by the metal retaining ring. This provides a uniform load across the specimen ends.

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ULTRASONIC TESTING OF CONCRETE

By
KAUSHAL KISHORE
Materials Engineer, Roorkee

Fundamental principle
A pulse of longitudinal vibrations is produced by an electro-acoustical transducer, which is held in contact with one surface of the concrete under test. When the pulse generated is transmitted into the concrete from the transducer using a liquid coupling material such as grease or cellulose paste, it undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves develops, which include both longitudinal and shear waves, and propagates through the concrete. The first waves to reach the receiving transducer are the longitudinal waves, which are converted into an electrical signal by a second transducer. Electronic timing circuits enable the transit time T of the pulse to be measured.

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Testing Concrete Cores

By
KAUSHAL KISHORE
Materials Engineer, Roorkee

The examination and compression testing of cores cut from hardened concrete is a well – established method, enabling visual inspection of the interior regions of a member to be coupled with strength estimation. Other properties which can be measured is also given in this paper.

IS: 456-2000 specified that the points from which cores are to be taken and the number of cores required shall be at the discretin of the engineer-in-charge and shall be representative of the whole of concrete concerned in no case, however, shall fewer than three cores be tested. Core shall be prepared and tested as described in IS: 516.

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