Workability Tests

There is no acceptable test which will measure directly the workability as defined. The following tests give a measure of workability which is applicable only with reference to the particular method. However, these tests have found universal acceptance.

Slump Test:

Most popular throughout the world for its simplicity to carry out. Measures subsidence of concrete which can be related to concrete’s suitability to place for various applications. Slump Test Results:

Oxide Composition of Portlant Cement

Very low 0-25 mm
Low 25-50mm little reinforcement use vibration
Medium 50-100mm columns, beams, retaining walls
High 100-175mm heavily reinforced section, flowable concrete Slump test is not suitable for concrete’s of low (< 25mm) and very high (>200mm) workability.

Compaction Test:

This test is not generally accepted method of directly measuring workability, but probably the best test available in the laboratory. Uses the inverse approach: the degree of compaction achived by a standard amount of work is determined.
The degree of compaction, called the compacting factor, is measured by the ratio of the density achieved in the test to the density of the same concrete fully compacted. The higher value of the calculation the more workable concrete.
Very low 0.78
Low 0.85 little reinforcement use vibration
Medium 0.92 columns, beams, retaining walls
High 0.95 heavily reinforced section, flowable concrete Compaction test is not suitable for high workable concrete’s as compaction factor tend to 1. More suitable for field laboratory quality assurance purposes where concrete’s used are of medium workable.

Vebe Test:

A standard slump cone is placed in a cylinder 24cm in diameter and 20cm high. The slump cone is filled in the standard manner, removed, and a disc-shaped rider of standard weight is placed on top of the concrete. Compaction is achieved using a vibrating table with an eccentric weight rotating at 50 Hz. It is assumed that the input of energy required for full compaction is a measure of workability of the mix, and this is expressed in Vebe seconds. Compaction is assumed to be complete when the transparent rider is totally covered with concrete and all cavities in the surface of the concrete have disappeared. This is judged visually. This test is more realistic to field conditions where vibration is used. However, concretes of very low workability and very high workability can not be measured as very stiff concretes result in long time more than 20-30 seconds and many a times may not get compacted. Similarly, highly flowing concretes will not give enough time to record as it will flow in a split second.

Vebe test results:

There are two sets of criteria that we must consider when making concrete;

Low 6-12 seconds little reinforcement use vibration
Medium 2.5-6 seconds columns, beams, retaining walls
High 0-2.5 seconds heavily reinforced section, flowable concrete Other workability test measurements include, Flow table test for high workable concretes, slump flow test (horizontal spread), L-boxtest, J-ring test for Self Compacting Concrete.

STRENGTH, ELASTISITY AND SHRINKAGE: The “strength” of hardened concrete is its ability to resist strain or rupture induced by external forces. The resistance of concrete to compressive, tensile and bending stresses is known as compressive strength, tensile strength, and bending (or flexural) strength, respectively. The resistance of concrete to repeated stresses is called its fatigue strength. Strength is expressed in terms of kgf/cm2 or MPa. The compressive strength of concrete is usually determined at an age of 28 days of the specimen. The 28- day compressive strength is the strength value used in concrete designs. Sometimes, the compressive strength at 7 days is also determined. The 7-day compressive strength is approximately 65-70% of its 28-day strength. At least three specimens should be tested; the average of their compressive strengths is found for determining the compressive strength of a concrete sample on a particular testing day. The compressive strength values obtained for cylinder specimens and cube specimens prepared from the same concrete sample are not the same:.

Compressive Strength Test (Drilling Core Method)

This test is conducted on cylindrical concrete core specimens removed from the hardened concrete by a drilling operation. A core drilling machine is used for cutting and removing the concrete samples. This machine is equipped with diamond cutters located on the end of a cylindrical (tube-like) cutting device. As the machine is operated, the cylinder shaped cutter rotates at a high speed.

The diameter of the concrete core specimen removed from the hardened concrete depends on the inner diameter of the cylindrical cutting device. Usually concrete cores having diameters of 10 cm or 15 cm are obtained. The removed core specimens may have different lengths depending on the thickness of the hardened concrete that they are cut from. If the core specimen is too long, it is shortened so that it will have a length/diameter ratio of 2.0. Core specimens which have a length/diameter ratio of less than 2.0 can also be used for compressive strength testing purposes, but a specimen having a length/diameter ratio of less than 1.0 should not be used. Where is the breaking load? and A is the cross-sectional area of the specimen If the ratio of the length to the diameter of the specimen is less than 2.0, allowance is made; the compressive strength found by the test should be multiplied with the correction factors shown in Table.

Determination of the compressive strength of concrete by testing core specimens is useful in finding the strength of concrete that is present in a structure. As is known, the strength of the concrete in the structure may be different from the strength found by the standard test method. The operations applied to the concrete - in the structure such as placing, consolidation, and curing may lead to these differences in the strength. This method provides the possibility of finding the actual quality of the concrete in the structure.

Will be Continued in Issue 7..

References:
REFERENCE MANUAL FOR FIELD ENGINEERS ON BUILDING CONSTRUCTION

Published by:
Task Force for Quality Assurance in Public Constructions
GOVERNMENT OF KARNATAKA ISBN

Important Note:
This reference manual is intended for the use of Government Engineers.

Disclaimer:
Every effort has been made to avoid errors or omissions in this publication. In spite of this, some errors might have crept in. Any mistake, error or discrepancy noted may be brought to our notice which shall be taken care of in the next edition. It is notified that neither the publisher nor the authors will be responsible for any damage or loss of action to any one, or any kind, in any manner, therefrom.