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Method of Sampling Cement:

Er. Parbhakar Dwivedi
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Tests on various properties of cement, aggregate and concrete are performed on the samples taken from a heap. The results of the tests strictly speaking are applicable to the samples tested. But since the aim of all such tests is to ascertain qualities of cement, aggregate and concrete, it is imperative that the sample collected represent the typical of average properties of material to be tested. Such sample is said to be the representative sample. Much care and systematic procedure is required to be followed. These methods are now standardized by the ISI.

SAMPLING OF CEMENT

  1. Sampling instrument should be clean and dry when used.
  2. Precautions should be taken to protect the sample, the material being sampled, sampling implements and containers for samples form contamination.
  3. Sample containers should be of such a size that they are almost completely, filled by the sample.
  4. Sample container shall be sealed air tight after filing and marked with full particular of the material and the date of sampling.
  5. Samples shall be stored in such a manner that the temperature of the material does not vary unduly from normal temperature.

Quantity of cement offered by the manufacturing unit at one time is called a Lot. This lot is normally divided into sub lots depending upon is total weight as per table 4.1. Quantity of cement taken by a Single operation is called increment. Aggregates of all these increments form the same sub lot is called gross sample. When these gross samples of each sub lot are reduced by suitable procedures for laboratory testing they are called laboratory samples. Equal quantities of cement from each laboratory sample representing sub lots, are mixed to form composite samples.

Table 1.1 Number of sub lots into which a lot is to be divided

Weight of lot (tonnes) Number of sub lots
Up to 100
101 to 200 3
201 to 300 4
301 to 500 5
501 to 800 6
801 to 1300 7
1301 and above 8

Sampling

For sampling of cement from conveyor, bulk-storage and ship’s hold and wagons, the reader is referred to IS: 3535.

  1. Divide the entire lot into sub lots as per table 1.1.sublots should consist of equal number to bags.
  2. From each sample at least two per cent bags should be sampled.
  3. These bags are to be chosen at random. To ensure randomness following procedure should be followed:

    (a) If there are ‘N’ number of bags in the sublots and ‘n’ is the number of bags to be selected,Then calculate r= N/n.

    (b) Starting from any bag, count the bags in one order as 1, 2, 3, up to r every ‘r’ th bag so counted should be Removed.

    (c) For example, if a lot consists of 3000 bags i .e. 150 tones, divide this lot into 3 sublots i.e. 1000 bags each. Thus N = 1000. Since at least, 2 per cent bags are to be sampled, then n = 20 bags. Therefore r= 1000/20 = 50. Thus starting from any one point, every 50thvbag is to be removed.

    Diagrammatic explanation of obtaining the suitable samplers of cement for testing is given in Fig. 1.1.

  4. ) Increments are to be taken by inserting a tube sampler as specified in IS: 3535. The material so collected from each bag is 0 .75 kg.
  5. ) Material so collected should be placed directly into moisture proof, air tight containers to avoid, moisture absorption and aeration of the sample. Cans, plastic bags of moisture-proof multiple wall paper bags may be used. These containers shall be filled immediately as sealed. The containers shall carry suitable identification marks so that they can be referred back to the particular sublots which the gross sample has been taken.

    Gross-samples thus obtained have to be reduced to laboratory samples by quartering or using riffler box. Quartering should be done as shown in fig.1.1.

    Fig.1.1. Diagrammatic sketch for sampling of cement.

    The material should be scooped into cone-shaped pile. Care shall be taken to drop each scoopful exactly over the same spot, as otherwise the central-axis of the cone will be slackened. After the cone is formed, it shall be flattened by pressing the top of the cone with smooth surface of scoop. Then the cone is cut into the quarters by two lines which intersect at right angles at the centre of the cone. The reduction is achieved by rejecting any two diagonally opposite quarters.

    Reduction should be continued till about 7 kg of material required for laboratory sample are obtained.

  6. Equal quantities of material shall be taken from each laboratory sample representing the sublot and mixed together to constitute composite sample representing the lot as a whole. The weight of the composite sample shall be about 7 kg.
  7. ) All laboratory samples and composite samples should be sealed in an air tight moisture proof container.

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