Sunday, November 11, 2018

Method of Sampling of Construction Materials - Based on Indian Standards [Part 1]

Author: Shubham Raj

The present article gives an overview of the method of sampling of commonly used construction materials, based on relevant Indian Standards published by the Bureau of Indian Standards (BIS). The article covers the sampling of cement, aggregates, bricks, reinforcing steel, concrete, wood and timber.
Cement is one of the most important ingredients of concrete which binds the aggregates and influences the strength and durability to a great extent.

There are two main stages of sampling and testing of cement to ensure its desired properties, which are as discussed below.
1.1.   Process inspection:
Process inspection is highly important which is carried out by the manufacturer in order to ensure the consistency in specifications that are being provided to the customers and thus minimizing the fluctuations in qualities. Manufacturers shall follow the recommendations as given in Table 1 of IS: 3535 – 1986 for inspecting various characteristics as a measure for routine control. The tests are conducted at mainly three stages of production. First of all, a sample is obtained every hour from the clinker coming out of the cooling chamber. All such hourly samples are thoroughly mixed together to produce a composite sample of the day. A sufficient amount of clinker from the composite sample is taken out for the chemical analysis and the rest is ground with gypsum and additives for further tests as per specifications. Hourly samples are obtained from the grinding mill to test the setting time. Fineness is tested on samples drawn out every alternate hour. The rest samples are intermixed together and thus daily and weekly composite samples are formed. In packing stage, one composite sample of the day is drawn and tested for properties like setting time, fineness, soundness, compressive strength and degree of whiteness and the rest sample is mixed to obtain a weekly composite sample for conducting other tests as per specification.

1.2. Lot inspection:
The conformity of each lot to the required material specification shall be inspected by obtaining samples (preferably while in motion, loading or unloading) by dividing the lot into sub-lots as per Table 2 of IS: 3535 – 1986. Gross sample is formed for each sub-lot by taking sufficient increments using a receptacle or sampling scoop from the stream or the conveyor belt. For every 10 tons of cement discharge, one increment weighing at least 2 kg shall be taken. The increments taken shall be immediately packed and sealed in moisture-proof bag or container and shall be properly marked for further identification. The increments from sub-lots of material contained in bulk storages (silos) shall be taken when the silo is being charged or discharged to obtain the gross sample. In case the silo is being charged or discharged from two opening, equal increments from both the openings shall be taken.

For a sampling of cement from ship’s hold and wagons, the total quantity of cement shall be divided into a number of sub-lots as per Table 2 of IS: 3535 – 1986. The sub-lots shall contain an approximately equal number of wagons. Increments from the sub-lots are obtained with the help of slotted tube sampler of length in between 1.5 to 1.8 m in case the depth of cement to be sampled is less than 2 m, while a sampling pipe activated by an air jet is used for drawing increments when the depth of cement is more than 2 m for the lots obtained on ship’s hold. For the purpose of gross sampling of cement obtained on wagons, slotted tube sampler of length 60 cm is used for taking increments from at least six evenly distributed points in the wagon. The material may be drawn two or three times to ensure that enough sample is obtained to carry out all the tests as per specifications.

In case the sampling is to be done from bags, the total quantity of cement is divided into sub-lots containing an approximately equal number of bags. The bags shall be selected at random as per IS: 4905 – 1968 from the sub-lots and at least 2 percent subjected to a minimum of five shall be sampled to obtain the representative gross sample. Equal increments from each bag are taken using a sampling tube in two or three operations to ensure enough quantity to carry out tests as per specifications.

1.3.   Reduction of gross sample:
Each gross sample shall be separately reduced and then mixed thoroughly and finally sieved on 850 microns IS sieve. The materials retained on the sieve is discarded. The gross sample may be reduced either mechanically by using sample splitter or riffle samplers of appropriate sizes or by coning and quartering method. In this method, the material is scoped into a cone-shaped pile and then cut equally into four parts by two orthogonal lines. Proper care shall be taken in this method that the centre line of the cone is maintained vertically. Any of the two opposite parts may be discarded to reduce the sample. The reduction shall be continued till 11 kg of laboratory sample is obtained. Equal quantity from each laboratory sample shall be mixed together to obtain a composite sample of around 21 kg that depicts the lot. Finally, the samples shall be kept in airtight bags or containers with proper markings for identification.
Sampling of aggregates is highly important to ensure the true nature and properties of the material which they actually represent. Thus, proper care shall be taken in adopting the sampling procedures with respect to their utility. The sampling procedures for different cases as per IS: 2430 – 1986 has been discussed below.

2.1.  Sampling of aggregates for preliminary investigation of source of supply:
For the sampling of coarse aggregates (stone, boulders, etc.), a detailed investigation of the ledge, quarry, or any other field deposits, shall be made and their visible properties like types, color, texture and their conditions shall be recorded. For each distinct type of aggregates as per preliminary investigation, samples shall be taken such that each sample weighs at least 25 kg. If the sample is to be collected from bank or pit deposits, then several test holes are made depending upon the parameters like quantity of materials to be used, the topography of the area, etc. and sample from each test hole is obtained. All samples are thoroughly mixed with samples from open faces and quartered such that gross sample weighing at least 12 kg is obtained in case of sand and at least 35 kg gross sample is obtained in case of all in aggregate.

2.2.  Sampling for inspection of a shipment of materials and of materials on the site of work:
To conclude a reliable result, it has been recommended that the materials shall be sampled while in motion (on conveyer) or during loading and unloading, by forming gross samples from each sub-lots (as per Table 1 of IS: 2430 - 1986) into which a lot of material has been divided. Thus, there will be as many numbers of gross samples as the number of sub-lots, such that each gross sample weighs as per Table 2 of IS: 2430 – 1986. A suitable sampling scoop is used to take at least 10 increments (each weighing at least 1 kg) from the sub-lot and mix thoroughly to form the gross sample. For sampling from transportation units like wagons, trucks, boats, etc., random selection of carries shall be done as per IS: 4905 – 1968 and a minimum of 25 percent of the carriers shall be selected at random from the sub-lots. To form a gross sample, the increments shall be taken evenly from the carrier at the time of loading and unloading. In case, the gross sample has to be obtained from loaded carriers, the sectional sampling method is used for taking increments. For the sampling of aggregates from stacks or stockpiles, provision for power equipment shall be made to ensure the unbiased preparation of gross sample by taking increments from a small sampling pile. In absence of power equipment, an equal number of increments shall be taken from top one-third, at mid-point and at the bottom one-third of the pile volume from each sub-lot and mixed together to form the gross sample. Trench sampling method may be used to form gross samples from a stationary stack of height up to 1.5 m. In this method, a trench is made from top to bottom ground level along a random line at the surface. Increments are taken randomly with the help of a suitable scoop from the exposed faces in the trench. Finally, the increments are thoroughly mixed to obtain the gross sample.

2.3.   Reduction of gross sample:
The gross sample so obtained is repeatedly mixed and reduced separately till the material required for each test as per IS: 2386 (Part 1 to 8) – 1963 is obtained. The reduction of the gross sample may be done by using a Riffle Divider of different sizes according to the size of aggregates or by Coning and Quartering method. In this method, the gross sample is poured so as to form a cone taking care that there is even distribution of materials and vertical centre line is maintained. Then, the cone is divided into four equal parts using two orthogonal lines and finally, the sample is reduced by rejecting two opposite parts. This method is most suitable for the reduction of the fine aggregate sample or all in aggregate samples.

Burnt clay building bricks may be of various types like solid, hollow or perforated. The methods and scale of sampling of burnt clay bricks to be used for construction purposes have been discussed below.
Method of sampling:
The samples for ensuring the true characteristics of bricks may be drawn when bricks are in motion or from the stack or from lorries and trucks. The samples shall be drawn from lot not exceeding 50,000 bricks by random sampling or by stratified sampling method as per IS: 4905 – 1968. For sampling at the time of loading or unloading, the lot shall be divided into sub lots of minimum 10 bricks and an equal number of bricks shall be taken from each sub-lot or portion to obtain a required number of bricks for inspection and testing as per Table 1 and Table 2 of IS: 5454 – 1978. When it is required to take a sample from the stack, the stack shall be divided into a suitable number of portions and an equal number of bricks shall be drawn from each portion to obtain the required number for tests as per specifications. When the sample is to be taken from trucks or lorries, the number of trucks shall be divided into subgroups of at least 10 trucks and an equal number of bricks shall be drawn from each subgroup to constitute sample for tests and inspection.

3.2. Scale of sampling for conformation criteria of visual and dimensional characteristics:
For the inspection of bricks for visual and dimensional characteristics, an equal number of samples from each lot as per Table 1 of IS: 5454 – 1978 shall be obtained depending upon the lot size. In case the lot contains less than 2000 bricks, the scale of sampling shall be as per the agreement between buyer and supplier. For visual inspection, the number of bricks failing tests shall be less than or equal to the permissible number of defectives as given in Table 1 of IS: 5454 – 1978. For dimensional tolerance, the number of bricks in the sample shall be divided into groups of 20 bricks and tested for dimensional tolerance as per specifications. All the groups shall meet the specified requirements for conformity of the lot. To inspect the dimension for individual bricks, the scale of sampling same as visual inspection shall be used.

3.3. Scale of sampling for conformation criteria of physical characteristics:
The lot which has passed inspection for visual and dimensional characteristics shall further be sampled and tested for physical properties like compressive strength, transverse strength, breaking load, water absorption, bulk density, efflorescence and warpage as per relevant test standards. The number of bricks for sample shall be drawn in accordance to Table 2 of IS: 5454 – 1978. The test results for compressive strength, transverse strength, breaking load, bulk density, and water absorption shall be averaged and must satisfy the minimum requirement as per material specification. The number of defective pieces in the sample shall be less than or equal to the specified number in Table 2 of IS: 5454 – 1978 for efflorescence and warpage.

Method of sampling of construction materials - Part 2