VII. Brick testing

VII.1 Purpose of testing

The purpose of testing is to check the production process; to remedy faults to ensure a saleable product; and to guarantee the quality and performance of marketed bricks. Given the intended use of bricks in housing construction, they must resist local weather conditions and should not contain materials which will damage them or the applied finishes such as renderings, plaster or paint. They must be strong enough to withstand both the dead load of the building itself, and the live load imposed by occupancy or wind. The thermal or moisture movements should not also be so large as to build up unduly large stresses. Testing should therefore consist of ensuring that bricks have all the required characteristics for efficient use in building.

VII.2 Initial checks on quality

A quick check of quality consists in striking two hand-held bricks. A high-noted ring indicates that they have been thoroughly fired. On the other hand, a dull sound indicates either cracked or soft-fired material. Similarly, bricks which cannot be scratched and rubbed away with the edge of a coin are hard and of good quality. Nevertheless, bricks without a good ring or which can be marked may still be perfectly suitable for many construction purposes. These two tests only serve to identify some of the best materials.

The general appearance of bricks may give a quick indication of quality. Regular shapes and sizes, sharp arrises, unblemished surfaces and freedom from cracks are signs of good bricks. Colour is difficult to interpret. However, within a batch of bricks from any one brickworks, the darker colours are likely to relate to the harder fired, stronger and more durable bricks.

VII.3 Standard specifications

Many countries have published their own standard specifications for bricks, and reference should be made to these where available. The methods of testing described in the British Standard(37) have been devised after much research and many years of experience, and are of wide interest. However, they may not necessarily be appropriate for other countries. Actual numerical limits must be decided locally according to what is required and what can be made.

VII.4 Sampling

The testing of bricks should be made on a representative sample of the latter since variations in preparation of raw materials, drying and firing produce bricks of variable characteristics. The testing sample should preferably contain 40 bricks picked up at random while they are being unloaded from the kiln. It is more difficult to obtain random samples once they have been placed in a large stack.

VII.5 Dimensions

Since bricks are used in fairly long runs, the testing of brick dimensions need not be carried out on individual bricks. Instead, 24 bricks should be chosen at random from the sample 40 bricks and small blisters or bumps knocked off. The 24 bricks should then be placed against an end stop touching end to end on a long bench. If the nominal length of each brick must be, for example, 21.5 cm, the far end of the row of 24 bricks should be 516 cm from the end stop. The permissible variations (for example 508.5 to 523.5 cm as in the British Standard for 21.5 cm bricks) should be clearly marked above the level of the bricks on a board behind the bench. Similarly, the width and height of the 24 bricks should be checked as bricks which comply with the standards for one dimension do not necessarily comply in other dimensions. Table VII.4 gives the dimensions calculated from batches of African bricks and the requirements of a relevant Standard: none complies in all three dimensions.

Table VII.4

Overall dimensions of 24 bricks

*Dimension complies

Source: 51

VII.6 Compressive strength

The compressive strength of harder fired bricks is greater than that of other bricks although the strength of most bricks is likely to be adequate for simple buildings. Even a fairly weak brick with a compressive strength of only 7 MN/m² may support a 300 m column of bricks without crushing. However, loadings in actual buildings are increased in supporting pillars and around wall openings.

As the strength of dry bricks is higher than that of wet bricks, the former should be immersed in water for 24 hours before testing. Although a simple wooden beam, used as a lever, can be sufficient to crush green bricks or adobe, it is unlikely to be successful for higher strength burnt bricks. Instead, a compression testing machine is commonly used in public works departments, universities, and technical and research institutes. To avoid high local loadings due to small irregularities on the bed faces of bricks in contact with the steel plattens of the machine, thin plywood plattens should be placed on both bed faces. Bricks with frogs or large perforations should be bedded in mortar which is allowed to set prior to testing. Ten bricks should be tested, and the mean strength calculated.

Table VII.5 gives test results obtained on batches of bricks from two different works in East Africa. They were tested by the British Standard method. Although a few individual bricks from one works are weaker than required for a particular grading (not the lowest), the mean value complies with the standard requirement. Similarly, a few bricks from the other works would not meet a requirement of the Indian Standard(36), but the mean value is satisfactory.

Table VII.5

Compressive strength of 10 bricks

Source: 61

For calculated load-bearing structures and civil engineering works, the strength of the bricks should be determined and used in the design calculations.

For non-load-bearing partitions, bricks of only 1.4 MN/m² may be used(37).

A low-cost impact testing method, claimed to give useful information on brick strength, consists in dropping a weight several times on the pieces of a broken brick in a containing cylinder(62).

The density of dry bricks is easily determined, and is sometimes quoted instead of strength.

VII.7 Resistance to erosion by water

Although compressive strengths may be more than adequate, dampness and flowing water can cause severe deterioration in buildings. Unfired earth bricks or adobes are eroded or slaked by water. On the other hand, firing yields bricks with excellent resistance to water erosion. The testing of resistance to erosion by water requires the soaking of brick samples in water for 24 hours. Good bricks should show no sign of softening or slaking.

A more severe test consists in spraying water on bricks for several weeks. This test allows the separation of bricks into various quality groups as low-quality bricks get eroded while high-quality ones remain unaffected.

VII.8. Water absorption

Hard fired bricks will absorb less water than other types of bricks. The quantity of water absorbed by a dry sample of bricks, when immersed in water for 24 hours or eight days(7), or boiled for five hours(37) is often estimated in testing laboratories. However, the results of this simple test are not easy to interpret. Generally, absorptions of less than 15 per cent by weight in the cold tests would be indicative of satisfactory brick strength and durability. Exceptionally, higher absorptions may be found with some types of clay although the bricks may still be satisfactory.

VII.9 Rain penetration

In practice, moderately high water absorption is acceptable in a brick wall, since rain water dries out once good weather conditions return. On the other hand, under some circumstances, rain running on walls made of low absorption bricks may enter the wall surface through small cracks between bricks and mortar. Rain penetration tests may be set up by building walls exposed to either rain or a water spray on one side, and observing the other sheltered side of the wall. The quality of not only the bricks but also of the workmanship in laying them, is of significance in determining the performance of the brickwork.

VII.10 Efflorescence, soluble salts and sulphate attack

Appearance of efflorescence on brickwork is considered unsightly and in extreme cases may cause spalling of faces of bricks. It may occur on initial drying or after subsequent wetting of the bricks. One test for determining the presence of efflorescence consists in half immersing bricks in distilled water for two weeks. Soluble salts, if present, will dry out on the top corners of the bricks as water is being soaked up. If only slight efflorescence occurs, it may be assumed that no problems are likely to arise in practice(7). Alternatively, sample bricks can be covered with a polythene sheet to prevent evaporation from the non-visible face in the finished brickwork, while exposing the other face upwards. A bottle of distilled water is then inverted on this latter face and the water allowed to soak in. Subsequently, the water dries out, and salts present in the bricks are carried to the surface where the amount of such salts may be estimated. Generally, if more than half of the exposed area is covered with salts, or if the surface flakes, the bricks would be regarded as efflorescent(37).

The nature of soluble salts can be determined in the laboratory by standard methods of chemical analysis carried out on powder obtained by drilling or fine grinding of brick samples. More than 3 per cent by weight of salts is considered a high concentration. For special quality brickwork, acid soluble sulphates should not exceed 0.5 per cent, calcium 0.3 per cent, magnesium 0.03 per cent, potassium 0.03 per cent and sodium 0.03 per cent(37).

VII.11 Lime blowing

Hydration of quicklime particles derived from limestone in brickmaking clays can cause pitting on brick faces. When, in spite of precautions in manufacture, a problem persists, it may be possible to alleviate it by docking the bricks (i.e. by immersing them in water(50)). Docking apparently slakes the lime to a softer form which may extrude into neighbouring brick pores. Bricks should be soaked for approximately 10 minutes so that water penetrates at least 15 mm. Otherwise, the problem may worsen. Large quantities of water are required, and the increased weight of the bricks may add to transportation costs. Efflorescence may appear as the water dries out.

Testing for lime blowing can be done by immersing brick samples into boiling water for 3 minutes(50), or preferably into a steamy oven(63).

VII.12 Frost

Frost is one of the most destructive natural agents but only when brickwork is frozen while in a very wet condition. In climates where frost occurs, outdoor test walls or laboratory freezing and thawing tests can be used to assess frost resistance (10, 64). Harder fired, less porous bricks are generally more resistant to frost.

VII.13 Moisture and thermal movement

Reversible and irreversible moisture movements are likely to be small and of little consequence for small buildings. Tests can be carried out to determine these movements if bricks are to be used for long runs of brickwork, or in tall structures. Thermal expansion is of similar significance and may be measured: it is likely to be approximately 4 x 10-6 m per °C (i.e. a change in temperature of 20 °C will cause a 12 m run of brickwork to change 1 mm in length).

VII.14 Durability and abrasion resistance

To investigate performance, including resistance to abrasion by wind-blown sand, small test walls should be constructed outdoors from the various types of bricks made at any works. Changes in the brick surface over various periods of time will indicate the degree of resistance of bricks to abrasion.

VII.15 Use of substandard bricks

Bricks that do not meet the required standards may be used for other purposes, and not be entirely wasted.

Underburnt bricks may be returned for refiring, or may be useful in kiln construction. Overburnt bricks may also be used in kiln construction or can be broken up and used as concrete aggregate.

Reject bricks may be broken up and used for road building or soakaways. If finely ground, they may be used as grogs in brickmaking.

Reject, underfired bricks exhibit pozzolanic properties when ground down to powder. The latter may thus be mixed with lime for the production of a cement substitute. Alternatively, the lime released when ordinary Portland cement sets will react with the brick powder. Thus, crushed soft-fired clay could constitute a useful mortar ingredient.