Product Sampling

BS 600 defines these as follows:-

'A random sample is a sample selected without bias, in a haphazard manner, i.e. so that every item has an equal chance of inclusion'.

'A representative sample is a sample in the selection of which planned action is necessary to ensure that a specified proportion of its contents is drawn from different sub-portions of the whole aggregation. The sampling from within each sub-portion may, however, be random'.

Testing Structural Ceramic Products

Over the years British standard specifications have been drawn up for several types of structural ceramics including:-

 Sampling

Rates of sampling for various tests are specified in B.S. 402, 1286, 3679 and 3921 amongst others.

It is most important that samples selected for analysis adequately represents the bulk of material or product. To ensure this, the sample may be selected using one of the following methods.

Dimensional tests

Dimensional tests are non-destructive that can be carried out quickly and on a regular basis; they lend themselves readily to control chart techniques as described in B.S. 3921: 1985 or using standard control charts, which are more statistically accurate.

Water absorption

Water absorption of bricks covered by B.S. 3921:1985 enables products to be graded, such as A and B engineering which are intended for damp-proof courses.

Care should be taken when comparing results as several methods are in use. The most popular one is the 5-hour boil method, others include 24-hour soak at room temperature, 7-day soak and some involve vacuum evacuation before water is introduced to the test.

Compressive strength

Compressive strength is not completely independent of the sample specimen dimensions, so any comparative tests should be carried out using specimens of comparative nominal dimensions.

Accuracy of the testing machine is important and it is recommended that a laboratory whose scope of accreditation by the United Kingdom Accreditation Service, covers this test method to ensure that the relevant calibrations have been carried out.

Compressive strength has been used on occasion to assess the strength of laboratory prepared specimens, the accuracy from these results is fairly low and can only be used as a comparison of raw materials when formed by extrusion or mechanical press and fired to the same temperature profile. For accurate test results, which would compare to finished product, full size bricks must be made.

Transverse Strength

Transverse Strength is a test of the load-bearing capacity, since it measures average breaking load and no allowance being made for variation in specimen thickness. This test is mainly carried out on pavers and roof tiles.

Accuracy of the testing machine is important and it is recommended that a laboratory whose scope of accreditation by the United Kingdom Accreditation Service, covers this test method to ensure that the relevant calibrations have been carried out.

Wet Skid Resistance of Pavers

Pavers tend to be made out of raw materials which have differential wearing characteristics, so has slightly softer material is worn away it leaves a rough surface to reduce the pavers ‘slippiness’. A value greater than 60 units is deemed to be acceptable for road transport.

Soluble salts

Soluble salts content both water and acid soluble were fully covered by BS 3921: 1985, an amendment was issued (AMD 8946 / December 1995) in 1995 analysing only water soluble salts and modifying the acceptable limits.

Bricks since 1995 shall be classified into one of the following categories:-

Low (L) The percentage by mass of soluble ions, measured as described in BS 3921, shall not exceed the following:-

 The limit for calcium was removed in 1995.

 Normal (N)  The percentage by mass of soluble ions, measured as described in BS 3921, shall not exceed the following:-

The sum of the contents of sodium, potassium, and magnesium 0.25%, Sulphate 1.6%

Soluble salts by a slightly different method can be derived for raw materials which gives an indication of the clay to develop a surface scum during the drying and firing processes. Some customers find the clay sulphate and chloride tests useful when determining the amount of anti-scumming additives to use in their product.

Several products for anti-scumming are used in the heavy clay industry which include barium carbonate, Active8, various sugar compounds and lignosulphonates combined with Barium products. Barium compounds tend to be poisonous. Active8 and other sugars are non-poisonous (Active8 used in brick making contains a biocide to prevent fermentation).

Efflorescence

Efflorescence test has been removed from BS 3921 (AMD 8946 / December 1995), however some manufactures still find the results useful as a guide to the likeliness of a brick to efflorescence after it is used in a building.

Frost Resistance – Durability

A panel of bricks or pavers or a model roof of tiles is prepared to undergo 100 cycles of freezing and thawing in a refrigerator, water is sprayed on at the start of each cycle. If the products pass 100 cycles then they are classes as frost resistant and can be expected to service at least 10 years of the most severe exposure to British weather, high up on the north face of a high mountain in Scotland. Under normal circumstances bricks have survived for thousands of years.

Bulk Density of Products

Bulk density is useful when calculating the amount of product that a lorry can carry.

Void Volume

By having the maximum permitted volume permitted by BS3921 the production unit can save on raw materials and energy consumption required during the drying and firing stage. Bricks made throughout Europe tend to have high void space than British made products.

Initial Rate of Suction

This gives an indication of how easy the bricks are to lay on a bed of mortar.

Environmental Concerns

During the latter part of the 20^th century emissions to air, land and water have been taken seriously. Kilns tend to expel acidic gasses such as sulphur dioxide, nitrogen dioxide and fluorine. These gases can be removed by expensive scrubbing technology.

Some manufacturers tend to reduce scrubbing costs by monitoring the raw material for sulphur and fluorine, adjusting both the clay body and the firing profile to limit the emissions.

When quarrying raw material the amount of suspended solids needs to be kept within the permitted value stated on the discharge consent associated with the planning permission for the site. This may need monitoring on some production facilities where there is discharge to the watercourse.

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Last modified: October 10, 2000