NTS STUDY: Testing of Bricks: Procedures and Standards

Testing of Bricks: Procedures and Standards

To ensure the structural integrity of a building, bricks must undergo several laboratory and field tests. The primary tests are as follows:

This is the most critical test to determine the load-bearing capacity of the brick.

Procedure: 5 random bricks are soaked in water for 24 hours. Their frogs are filled with 1:3 cement-sand mortar and cured for 3 days.
Testing: The bricks are placed in a Compression Testing Machine (CTM) between plywood sheets. Load is applied at a rate of 140 $kg/cm^2$ per minute until failure.
Standard: The average of 5 bricks is reported. Any value 20% lower than the average is discarded.

This test determines the durability and porosity of the brick.

Procedure: Bricks are dried in an oven at 110°C to 115°C to get the dry weight ( $W_d$ ). They are then immersed in water at 27°C for 24 hours to get the wet weight ( $W_w$ ).
Formula: $Percentage\ Absorption = \frac{W_w - W_d}{W_d} \times 100$
Limit: For a good quality brick, this should generally not exceed 20% of its dry weight.

Efflorescence is the white powdery deposit caused by soluble salts.

Procedure: Bricks are placed vertically in a dish with 2.5 cm of distilled water. Once the water evaporates, the process is repeated.
Reporting:
- Nil: No salt deposits.
- Slight: Less than 10% area covered.
- Moderate: Up to 50% area covered without powdering.
- Heavy/Serious: More than 50% area with surface flaking or powdering.
Requirement: For general construction, it should not exceed 'Moderate'.

[Image showing different levels of efflorescence on bricks]

These tests can be performed directly at the construction site:

Hardness: Scratch the surface with a finger nail; it should leave no impression.
Soundness: Strike two bricks together; they should produce a clear metallic ringing sound and not break.
Structure: Break a brick to inspect its internal structure. It should be homogeneous, compact, and free from defects like voids or lumps.
Dimensional Tolerance: 20 bricks are measured together to check for size variations. Class I bricks allow a $\pm3\%$ variation.

This is a great practical example for civil engineering students to understand how the 20% rule is applied.

Imagine we are testing a batch of Class 10 (100 $kg/cm^2$ ) bricks. We test 5 samples, and the results are as follows:

Calculate Initial Average:
First, find the average of all 5 tested values.
$Average = \frac{100 + 110 + 95 + 60 + 105}{5} = \mathbf{94\ kg/cm^2}$
Apply the 20% Rule:
As per standards, any individual value that is less than 20% of the average must be discarded.
- 20% of 94 = 18.8
- Lower Limit = $94 - 18.8 = \mathbf{75.2\ kg/cm^2}$
Analyze the Results:
- The strength of Brick No. 4 is 60 $kg/cm^2$ .
- Since 60 is less than the limit of 75.2, this brick is considered a "failed sample" and is removed from the calculation.
Final Average Compressive Strength:
Now, calculate the average of the remaining 4 accepted bricks:
$Final\ Average = \frac{100 + 110 + 95 + 105}{4} = \mathbf{102.5\ kg/cm^2}$

Since the final average (102.5 $kg/cm^2$ ) is greater than the specified class value (100 $kg/cm^2$ ), this batch of bricks passes the test.