Tests on Bricks

I) Field Tests on Bricks

The field test on bricks gives an idea about its basic quality based on its shape, size and colour at first observation without any big appliances. They are very common and easiest way to check the quality of brick. Field tests of brick are very helpful on the site. Some very common tests of brick that is followed to find if brick is good at first observation are as follows.

1) Shape and Size

The clay bricks should have a uniform rectangular plane surface, as per standard size and sharp straight edges. BIS recommends the standard size of brick is 190 x 90 x 90 mm and constructional size is 200 x 100 x 100 mm.

2) Visual Inspection

In this test, bricks are closely inspected for its shape. The bricks of good quality should be uniform in shape and should have truly rectangular shape with sharp edges.

3) Hardness

The clay bricks should be sufficiently hard when scratched by a finger-nail and no impression should be left on the brick surface.

4) Colour

The clay bricks should have a uniform deep red colour throughout. It indicates the uniformity of chemical composition and the quality of burning of the bricks.

5) Texture and Compactness

The surfaces should not be so smooth to cause skidding of mortar. The clay brick should have a pre-compact, homogeneous and uniform texture. A broken surface should be free form cracks, holes grits or lumps of lime.

6) Soundness

When two clay bricks are stuck together, a metallic ringing sound should come.

7) Structure

A brick is broken and its structure is examined. It should be homogeneous, compact and free from any defects such as holes, lumps etc.

8) Basic Strength

When dropped flat on the hard ground from a height of about one meter, clay bricks should not break.

II) Laboratory Tests on Bricks

Laboratory tests on brick determine the mechanical properties of brick and give a scientific approach to ensure the quality of bricks. It is essential while purchasing the brick and examine the properties for the quality of construction. Followings brick tests are performed in the laboratory to determine the quality of brick.

1) Water Absorption of Brick

The brick is porous by nature and Porosity is the ability to release and absorb moisture. Therefore, it tends to absorb the water or moisture. It’s an important and useful property of brick. But if brick absorbs more water than the recommended result, then it affects the strength of brick as well as durability of the structure and will damage plaster and paint over walls. Dry bricks are put in an oven at a temperature of 105° to 115°C till these attain constant mass. The weight (W1) of the bricks is recorded after cooling them to room temperature. The bricks are then immersed in water at a temperature of 27° ± 2°C for 24 hours. The specimens are then taken out of water and wiped with a damp cloth. Three minutes later it is weighed again and recorded as W2.

Water absorption test is performed to know the percentage of water absorption of bricks. Water absorption of bricks should not more than 20% by its dry weight. If brick fails in the water absorption test, possible reasons are like manufacturing error, insufficient burning, error in clay composition etc. If brick fails in water absorption as well as efflorescence than never use those bricks because it will cause in permanent problems and it will be very difficult to solve them. Water bath, weight balance and oven are required for performing this test.

2. Compressive Strength of Brick

The compressive strength of the brick is the most essential property of the bricks because in the construction, bricks are widely used in masonry and it also plays a significant role as a load bearing component. When bricks are used in any structure, the bottom-most layer of the brick will be subjected to the highest compressive stress. Therefore, it is essential to know that any particular brick will be able to withstand that load or not. This test is performed to know the strength of bricks because it affects the overall structure in the way of quality, durability and serviceability.

For testing bricks for compressive strength from a sample the two bed faces of bricks are ground to provide smooth, even and parallel faces. The bricks are then immersed in water at room temperature for 24 hours. These are then taken out of water and surplus water on the surfaces is wiped off with cotton or a moist cloth. The frog of the brick is flushed level with cement mortar and the brick is stored under damp jute bags for 24 hours followed by its immersion in water at room temperature for three days. The specimen is placed in the compression testing machine with flat faces horizontal and mortar filled face being upwards. Load is applied at a uniform rate of 14 N/m² per minute till failure. The maximum load at failure divided by the average area of bed face gives the compressive strength.

Recommended Result of Compressive Strength Test of Brick

Test result recommendations are as follows.

• For first class bricks, it should not less than 10 N/mm² (102 kg/cm²).
• For second class bricks, it should not less than 7 N/mm² (71 kg/cm²).
• For third class bricks, it should not less than 3.5 N/mm² (36 kg/cm²).

In India, the northern and the eastern region produce bricks having good compressive strength than the western region because the western region has black cotton soil, while the soil is good in Gangetic region. If the test result is not as per recommendation, there are many reasons behind it such as the clay composition, degree of burning like over burning or insufficient burning, error in the testing appliance or testing procedure etc. If bricks fail in strength as well as water absorption test, then do not use it. If bricks are irregular in some minor shape/size than it can be corrected with mortar.

3) Efflorescence Test

This test should be conducted in a well-ventilated room. The brick is placed vertically in a dish 30 x 20 cm approximately in size with 2.5 cm immersed in distilled water. The whole water is allowed to be absorbed by the brick and evaporated through it. After the bricks appear dry, a similar quantity of water is placed in the dish and the water is allowed to evaporate as before. The brick is to be examined after the second evaporation and reported as follows.

• Nil : When there is no perceptible deposit of salt
• Slight : When not more than 10% of the area of brick is covered with salt
• Moderate : When there is heavy deposit covering 50% of the area of the brick but unaccompanied by powdering or flaking of the surface.
• Heavy : When there is heavy deposit covering more than 50% of the area of the brick accompanied by powdering or flaking of the surface.
• Serious : When there is heavy deposit of salts accompanied by powdering and/or flaking of the surface and this deposition tends to increase in the repeated wetting of the specimen.

Bricks for general construction should not have more than slight to moderate efflorescence.

4) Dimension Tolerance

Twenty bricks are selected at random to check measurement of length, width and height. These dimensions are to be measured in one or two lots of ten each as shown in Fig. 1. Variation in dimensions is allowed only within narrow limits, ±3% for class one and ±8% for other classes.

Fig. 1