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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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Non-destructive Testing Of Concrete By Rebound Hammer

By
Kaushal Kishore
Materials Engineer, Roorkee

INTRODUCTION
The standard method of determining strength of hardened concrete consists of testing concrete cubes in compression. The quality of entire concrete of a structure cannot be fully assessed by testing a few concrete cubes. The results obtained in testing cubes do not always reflect the actual strength of concrete in construction. In a whole day, concreting work cubes are cast in a few batches, the differences (unintentional and intentional) in the composition are not uncommon, their compaction and their hardening conditions always differ more or less from those of the structure. In addition, the number of test cubes is generally so small that they can only be considered as random tests. Some times, in case of failure of cubes, doubtful concrete, cracks, deterioration of concrete, etc. it becomes necessary to assess the quality and strength of concrete of the structure. As far back as early thirties, the necessity was felt to develop instruments by which in-situ strength of concrete may be obtained. Various non-destructive methods of testing concrete have been developed, which include, Firing method, Skramtayev’s method, Polakov’s method, Magnitostroy method, Fizdel ball hammer, Einbeck pendulum hammer, Ball indentation hammer, Rebound hammer, Pull out techniques, Windsor probe, Ultrasonic pulse velocity methods, Radioactive and nuclear methods, Magnetic and electrical methods. In all these methods of tests, due to simplicity, rebound hammer test based on surface hardness becomes most popular in the world for non-destructive testing of in-situ concrete.

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What are the limitations in multi-stage test of triaxial tests?

The maximum load acted on the soil specimen is limited because a highly deformed soil sample is not suitable for further testings. For instance, soft samples like clay display large failure strain and hence it may be not considered acceptable for multi-stage trial axial tests.

Moreover, multi-stage trial axial tests may not be suitable for residual coils whose cohesion is established based on the remaining rock strength mass. At the stage of shearing, part of cohesion may be destroyed and it is irrecoverable in other stages of triaxial tests.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Multistage triaxial test may not be preferable for consolidated drained test. Why?

In multistage consolidated drained test, the soil sample is consolidated under all round pressure and then loaded by applying an axial stress. Prior to failure, loading is stopped and the specimen is consolidated under a higher confining pressure. The above steps are repeated for 3 stages to
obtain the failure envelope.

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The main problem associated with multistage consolidated drained test lies in the practical difficulty in determining the failure state of the soil sample. Judgment has to be made regarding the condition of “immediately prior to failure” on stress strain curves. It is not uncommon that wrong estimation of the failure state occurs when interpreting the stress strain curves. When there is an underestimation of deviator stress at failure, it would result in overestimation of friction angle and underestimation of cohesion. In case actual failure of soil samples occurs before visual recognition, the sample undergoes overstressing so that the deviator stress at failure in later stages is reduced. As such, this leads to overestimation of cohesion and underestimation of friction angle.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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