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Concrete technology

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					1Test on properties of cement;

The manufacture of cement requires stringent control, and a number of tests are performed in
the cement plant laboratory to ensure that the cement is of the desired quality and that it
conforms to the requirements of the relevant national standards.

The tests are as follows:

    Consistency of standard paste
     For the determination of the initial and final setting time and for the Le Chatelier
     soundness test, a neat paste of standard consistency has to be used. It is, therefore,
     necessary to determine for any given cement the water content of the paste which will
     produce the desired consistency. The consistency is measured by the Vicat apparatus,
     using a 10mm diameter fitted into the needle holder. A trial paste of cement and water
     is mixed in a prescribed manner and placed in a mould. The plunger is then brought into
     contact with the top surface of the paste and released. Under the action of its weight
     the plunger will penetrate the paste; the depth of the penetration depends on the
     consistency. This is considered to be standard, in the meaning of EN(European
     standards) 196-3:1987, when the plunger penetrates the paste to a point 6 +/- 1mm
     from the bottom of the mould. The water content of the standard paste is expressed as
     a percentage by mass of the dry cement, usually range of values being between 26 and
     33 percent.

    Setting time
      The setting time of cement is measured using the Vicat apparatus with different
     penetrating attachments. The method is prescribed by European standards (EN 196-3:
     1987). For the determination of the initial set, a round needle with diameter 1.13 +/-
     0.05 is used. This needle, acting under the prescribed weight, is used to penetrate a
     paste of standard consistency placed in the special mould. When the paste stiffens
     sufficiently for the needle to penetrate no deeper than to a point 5 +/- 1mm from the
     bottom, the initial set is said to have taken place. The initial set is expressed as the time
     elapsed since the mixing water was added to the cement. A minimum of 60 minutes is
     prescribed for cement for cement with strengths up to 42.5 Mpa, and 45 minutes for
     cement with higher strengths.
     Final set is determined by a similar needle fitted with a metal attachment hollowed out
     so as to leave a circular cutting edge 5mm in diameter and set 0.5mm behind the tip of
     the needle. Final set is said to have taken place when the needle, gently lowered to the
     surface of the paste ,penetrate it to a depth of 0.5mm but the circular cutting edge fails
     to make an impression on the surface of the paste.
 Soundness
  It is essential that the cement paste, once it has set, does not undergo a large volume
  change. In particular, there must be no appreciable expansion which, under conditions
  of restraint, could result in a disruption of the hardened cement paste. Such expansion
  may take place due to the delayed or slow hydration, or other reaction, of some
  compounds present in hardened cement, namely free lime, magnesia, and calcium
  sulfate.

 Strength of cement

   The strength of mortar or concrete depends on the cohesion of cement paste, on its
   adhesion to the aggregate particles, and to a certain extent on the strength of aggregate
   itself. Strength test are not made on a neat cement paste because of difficulties of
   moulding and testing with a consequent large variability of the test results, cement-sand
   mortar and, in some cases, concrete of prescribed proportions and made with specified
   materials under strictly controlled conditions, are used for the purpose of determining
   the strength of cement. There are several forms of strength tests: direct tension, direct
   compression, and flexure. European Standard EN 196-1: 1987 prescribes a compressive
   strength test on mortar specimens. The specimens are tested as 40mm equivalent
   cubes; they are derived from 40mm x 40mm x 160mm prism, which are first tested in
   flexure so as to break into halves, or areas otherwise broken into halves. Thus an
   optional centre-point test over a span of 100mm is possible.

   The test is performed on mortar of fixed composition, made with ‘CEN standard sand’.
   (CEN is the acronym of the French name of the European committee for
   standardization.) The sand is natural, siliceous rounded sand which can be obtained
   from various sources. The sand/cement ratio is 3 and the water/cement ratio is 0.50.The
   mortar is mixed in a cake mixer and compacted on a jolting table with a drop of 15mm;a
   vibrating table can also be used, provided it results in similar compaction. The
   specimens are demoulded after 24 hours in a moist atmosphere and thereafter cured in
   water at 20 degrees Celsius.

 Fineness of cement

   Because hydration starts starts at the surface of the cement particles, it is the total surface area
   of cement that represents the material available for hydration. Thus, the rate of hydration
   depends on the fineness of the cement particles and, for a rapid development of strength, high
   fineness is necessary; the long-term strength is not affected. The fineness test would be carried
   out by sieve test; but, since this test does not give information on the size of grains smaller than
45µm sieve, and it is the finer that play the greatest part in early hydration. For this
reason, modern standards prescribe a test for fineness by determination of the specific
surface of cement expressed as the total surface area in square metres per kilogram. A
direct method is to measure the particle size distribution by sedimentation or
elutriation: these methods are based on the dependence of the rate of free fall of
particles on their diameter. Stroke’s law gives the terminal velocity of fall under gravity
of a spherical particle in a fluid medium; the cement particles are, in fact not spherical.
This medium must of course be chemically inert with respect to cement. It is also
important to achieve a satisfactory dispersion of cement particle as partial flocculation
would produce a decrease in the apparent specific surface.

				
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Description: Tests on cement properties, it shows types of tests done