NTS STUDY: Testing of Concrete Blocks (As per IS 2185)

Testing of Concrete Blocks (As per IS 2185)

To ensure the structural integrity and durability of concrete block masonry, the following standard tests must be conducted:

This is a preliminary check to ensure the blocks are sound.

Sampling: 20 blocks are randomly selected for every 5000 blocks in a consignment.
Criteria: Blocks must be free from cracks, chips, and other defects that would impair their strength or appearance. Not more than 5% of the consignment should have minor defects (like chips or cracks larger than 25 mm) on exposed faces.

This test verifies if the blocks meet the specified sizes within permissible tolerances.

Procedure: 20 full-sized blocks are measured for length, width, height, and web thickness using a steel scale and calibrated calipers.
Permissible Tolerance:
- Length: ±5 mm
- Height & Width: ±3 mm.

Procedure: 3 blocks are randomly selected, oven-dried at 100°C, and then their weight and dimensions are accurately measured to calculate density.

This is the most crucial test to determine the load-bearing capacity of the block.

Sampling: A batch of 8 blocks is selected randomly.
Capping: To ensure uniform load distribution, the bearing surfaces are capped using a sulfur mixture, plaster of paris paste (min strength 25 $N/mm^2$ ), or other specified material.
Testing: Load is applied perpendicular to the capped surface until the block fails. The strength is calculated by dividing the maximum load by the gross area of the block.
Criteria: Individual values of compressive strength should not fall below 20% of the average strength of the grade being tested.

Unlike clay bricks, concrete blocks are generally not wetted before construction. This test checks their absorption capacity.

Sampling: 3 specimens are tested.
Procedure: Specimens are submerged in water for 24 hours to get the wet weight (A). They are then removed, allowed to drain for one minute, wiped dry, and weighed. Finally, they are oven-dried at 100-115°C for at least 24 hours to get the dry weight (B). The volume is calculated using the weight of the suspended, submerged block (C).
Formula:
- Absorption ( $kg/m^3$ ) = $1000 \times (A-B) / (A-C)$
- Absorption % (by mass) = $100 \times (A-B) / B$
Criteria: Water absorption should not exceed 10% by mass.

Concrete blocks are prone to shrinkage upon drying, which can cause cracks.

Procedure: Specimens are prepared (at least 15 cm long with specific cross-sections) and saturated in water at 27°C for 4 days. Reference points are established on both ends. The initial (wet) length is measured. The specimens are then oven-dried and measured again at regular intervals until a constant dry length is achieved.
Criteria: Drying shrinkage (expressed as a percentage of the dry length) should not exceed 0.1%.

This test measures the expansion of the block when it becomes wet again after being fully dried.

Procedure: The specimens used for the drying shrinkage test are re-immersed in water for 4 days. The increase in length is measured using the same apparatus.
Criteria: Moisture movement (difference between dry and wet length expressed as a percentage of dry length) should not exceed 0.09%.