Sunday, November 11, 2018

Method of Sampling of Construction Materials - Based on Indian Standards [Part 2]

Author: Shubham Raj

Steel plays a vital role in the construction of humongous structures nowadays. It may be used as reinforcement in concrete or as a structural member itself. The correct method of sampling is of high importance to ensure the true characteristics of the
material. The method of sampling of reinforcing steel as per the guidelines of IS: 10790 (Part 2) – 1984 has been briefly discussed below.

4.1.   Sampling for process inspection:
Process inspection is not only required for the conformity of products for prescribed specifications but also to ensure the uniformity in the quality of products. This is done by the manufacturer himself in the course of production. If the manufacturer has his own melting arrangement, then routine inspection and tests shall be done on heat basis separately for materials of each diameter from the heat. Table 1 of IS: 10790 (Part 2) – 1984 suggests the number of samples to be tested for the conformity of different physical and mechanical properties of the material. In case, the manufacturer has not his own melting facility, then tests shall be done on a daily average production basis. The number of samples to be tested for each type of tests depending upon average daily production shall be as per Table 2 of IS: 10790 (Part 2) – 1984. The samples shall be taken at regular interval to ensure the uniformity. The manufacturer shall ensure the quality of the product on the basis of analysis and scrutiny of test results statistically as per different Indian Standards. He shall preserve the test results for an adequate period of time (2 to 3 years) to resolve any conflict in future. The products which do not satisfy specification requirements shall be segregated and efforts shall be made to bring it to prescribed standards by rectifying the deficiencies if any. The manufacturer may issue a test certificate to certify the quality of products.

4.2.   Sampling for lot inspection:
The products formed from the fully tested material of uniform grade and nominal size shall be termed as a lot. Each lot shall be inspected separately by randomly drawing representative samples as per Table 3 of IS: 10790 (Part 2) – 1984. The method for selection of samples randomly from the lot shall be as per IS: 4905 – 1968. The table shows the number of samples to be drawn each for inspection of visual characteristics and weight and dimensional characteristics depending upon the lot size. First of all the samples shall be inspected for visual characteristics for each defect and any sample failing to meet any requirement shall be said defective. If the total number of defective samples is less than the acceptance number as given in Table 3, the lot may be accepted or else each item shall be inspected for visual characteristics. As soon as the lot passes visual characteristics test, samples shall be tested for dimensional and weight characteristics. If a number of defective items is less than the acceptance number as shown in Table 3 of IS: 10790 (Part 2) – 1984, the lot may be declared to pass the dimensional and weight requirement test, otherwise not. After the lot passes the visual test and dimensional and weight test, they are tested for physical and chemical requirements. For determining physical and chemical characteristics, samples shall be taken as per Table 4 of IS: 10790 (Part 2) – 1984 depending upon the lot size. Samples conforming visual, dimensional and weight characteristics may also be taken for inspection of physical and chemical characteristics. For tensile strength, proof stress and elongation, when two sample is tested, both must confirm the requirements as per specification. When more than two sample is tested, mean (xˉ) and range (R) shall be calculated for each characteristic and the lot shall be said to conform the requirements only when the value xˉ - 0.4R is greater than or equal to the minimum value specified in the relevant standard for each characteristic. For bend and rebend test, the lot shall be declared pass only when test result on each sample comes out to be satisfactory. For chemical analysis, drilling shall be taken from each sample and mix together to form a composite sample. The lot shall be said conforming to the requirements if the composite sample meets all specifications as per relevant standards.

Concrete is the most widely used construction material nowadays. The method of sampling of fresh and hardened concrete as per IS 1199 – 1959 to ensure its true characteristics has been discussed below.

5.1.   Sampling of fresh concrete:
Representative samples for fresh concrete may be obtained either directly from concrete mixers or from concrete at the time of placing. However, in either case, the minimum sample taken shall not be less than 0.02 cubic meter in volume. When the sample is taken from a concrete mixer, care shall be taken that at least three equal increments shall be obtained from concrete discharge at regular intervals. A metal receptacle of proper shape that can contain the concrete without segregation shall be used for obtaining increments. When the sample is to be taken from concrete at time and place of deposition, then at least five increments from well-distributed positions shall be taken immediately after the concrete has been discharged at the site. All the increments are thoroughly mixed to obtain a composite sample on which tests shall be conducted. Care shall be taken that the composite sample is uniform and protected from changes in the proportion of any ingredient or from external weather conditions. Information like date and time of sampling, a method of sampling, mix proportions, temperature and weather conditions, etc. shall be recorded for each composite sample obtained.

5.2.   Sampling of hardened concrete:
The samples for ensuring the characteristics from hardened concrete shall be drawn generally after 14 days of casting with the help of drills for extracting cylindrical core or saw for extracting beam specimens from flexural members or pavement for flexure test. For securing a core specimen from the straight and the inclined hole, short drill and diamond drill respectively may be used. The core cut shall have a minimum diameter of 100 mm for determination of pavement thickness while for determining the compressive strength, the diameter of core shall be at least three times the maximum size of aggregate used in concrete. The saw used to secure beam specimens shall have diamond or silicon carbide cutting edge to ensure smooth surface along with an arrangement to adjust the cutting size as per specification. The beam specimen shall have a dimension of 700 mm × 150 mm × 150 mm. Generally, for prism cut from pavements, width and length depend upon the size of aggregate and depth of slab respectively. The core shall be drilled preferably from the middle of the structural unit. A sufficiently large specimen shall be cut from the slab and sawing operation for securing beam specimen shall be done so that concrete is not weakened by vibrations or heat. Any specimen that has broken or damaged while extraction or has any kind of defects shall be rejected from testing. The measurements shall be done for the mean diameter, height and position of reinforcement and data shall be recorded. For determining the mean diameter, three pairs of measurements shall be taken, one at the middle of the core and rest two at both the quarters from top and bottom of the core. Each measurement in a pair shall be taken at right angles and mean of these six readings shall be recorded as the mean diameter of the core. The height of the core is recorded for both its maximum and minimum value. The position of reinforcement shall be recorded as the least distance of the centre of reinforcement from top of the core. The spacing between reinforcements and cover on top and bottom shall also be recorded.

The method of sampling of model trees and logs for inspection and testing of characteristics may be done by selecting model trees in the test area and sticks specimen as per IS: 2455 – 1990. The method of sampling of timber scantlings from depots as per IS: 8720 – 1978 has also been discussed in brief below.

6.1.   Sampling of model trees and logs:
First of all test area shall be selected such that it contains at least 100 trees of each generation of species to be studied. The minimum diameter of trees shall be 140 mm and girth at breast height shall be at least 450 mm. Then, model trees in the test area shall be selected by an expert person who can identify the species of trees and know the implications involved in the conversion and testing of trees of that species. If trees are of varying girths, they shall be first categorized into different groups on the basis of girth as per clause 5.4 of IS: 2455 – 1990. At least 5 trees from each girth group shall be selected at random as per IS: 4905 – 1968 and testing authority shall be informed immediately for further instructions. After selection, trees are marked and converted into logs. Further logs are marked and converted into sticks and sent to the testing authority for proper tests in the order described in clause 12.1 of IS: 2455 – 1990. The number of specimens for each test shall be according to Annexure C of IS: 2455 – 1990. All specimens shall be clear and straight grained, free from decay and other defects and shall be protected from moisture and weather.

6.2.   Sampling of timber scantlings from depots:
The basic information like name of species, source, and date of procurement shall be noted. Materials shall be taken out in a lot of minimum 50 and maximum 500 logs and number of logs in a lot shall be recorded. For testing purposes, only one scantling of about 13 cm × 13 cm × 1.5 m shall be taken out from each log such that each scantling is sound, straight and free from decay or other defects which affect strength properties. 10 per cent of total scantlings, subjected to minimum 20 scantlings are selected at random from the lot and taken as one consignment with proper marking and packing. Each scantling shall be sawn into 4 sticks of equal length and cross-section of 6 cm × 6 cm and kept in the conditioning chamber for minimizing the moisture variation. Then specimens for the test shall be selected as per schedule is given in Appendix A of IS 8720 – 1978 and further cut into length as per test requirements in accordance with IS 1708 (Part 1 to 18) – 1969.

1. Indian Standard (1959), “Methods of sampling and analysis of concrete”, IS 1199, New Delhi.
2. Indian Standard (1963), “Methods of Test for Aggregates for Concrete, Part 1 to 8”, IS 2386, New Delhi.
3. Indian Standard (1968), “Methods for random sampling”, IS 4905, New Delhi.
4. Indian Standard (1969), “Methods of testing of small clear specimens of timber, Part 1 to 18”, IS 1708, New Delhi.
5. Indian Standard (1978), “Methods of sampling of clay building bricks”, IS 5454, New Delhi.
6. Indian Standard (1978), “Methods of sampling of timber scantlings from depots and their conversion for testing”, IS 8720, New Delhi.
7. Indian Standard (1984), “Methods of sampling of steel for prestressed and reinforced concrete, Part 2: Reinforcing steel”, IS 10790, New Delhi.
8. Indian Standard (1986), “Methods for Sampling of Aggregates for Concrete”, IS 2430, New Delhi.
9. Indian Standard (1986), “Methods of sampling hydraulic cement”, IS 3535, New Delhi.
10. Indian Standard (1990), “Method of sampling of model trees and logs and their conversion for timber testing”, IS 2455, New Delhi.