Bricks are generally used to construct load-bearing masonry walls, columns, and footings. These load-bearing stone constructions mostly experience compression loads. Therefore, it is necessary to determine the compressive strength of bricks in order to determine their suitability for construction.
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Importance of high compressive strength in brick masonry
Compressive strength refers to the capacity of a certain material or structural element to endure loads that cause that material or structural element to shrink in size when they are applied. A sample is subjected to a force that is delivered from both the top and the bottom until the sample breaks or becomes distorted.
The following discussion will cover methods of the compressive strength of bricks.
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Measuring compressive strength in brick construction
- Take five random brick samples and submerge them in room-temperature water for 24 hours.
- After 24 hours, remove them, allow the excess water to drain, and then rinse them.
- Next, fill their frogs (and any other voids) with regular 1:1 mortar (1 part cement with 1 part sand).
- 24 hours later, store these bricks beneath wet sacks (to allow the setting of mortar).
- For seven days, soak the bricks in water. (This is done so that the mortar can harden.)
- Remove the bricks from the water, let the excess water drain, and dispose of the remaining water. After surface drying, the compressive strength of each brick is independently evaluated.
- Place the brick between two sheets of plywood with the frog end pointing up.
- The brick is then placed on the bed of the compressive strength of bricks testing equipment, and axial force is applied at a rate of 140 kg/cm2/minute. (This is quite crucial.)
- Take note of the force at which the brick collapses (gets broken). The compressive strength of a brick is calculated by dividing this load (P) by its cross-sectional area (A) (Co).
Co=P/A
- The arithmetic mean of the compressive strength values of all five bricks should be considered (not for all the bricks of a kiln).
- Based on the above-obtained (Co), the classification of the brick shall be determined.
Compressive resistance of bricks
- First-class brick has a compressive strength of 105 kg/cm².
- The compressive strength of a second-rate brick is 70 kg/cm².
- Common building bricks have a compressive strength of 35 kg/cm²
- The compressive strength of sun-dried brick is between 15 and 25 kg/cm².
Test for water absorption
Select five complete bricks at random
- Dry these samples to a consistent weight at 110 °C +/- 5 °C in a ventilated oven. This could take up to 48 hours.
After chilling, each specimen is weighed individually.
- The samples are then submerged in water at room temperature for twenty-four hours.
- After 24 hours, the samples are extracted. Within three minutes of being extracted from the water, each sample is dried and weighed independently.
- Absorption value is computed using a straightforward relationship.
Absorption% = w2-w1/w1 x 100
Where w1 is the dry weight and w2 is the weight after 24 hours of immersion.
- The water absorption of the brick shall be determined by calculating the mean of five values from five samples.
- It must fall within limits defined for the classification of bricks.
Efflorescence test
- Select five bricks at random.
- Place each brick on its end in a shallow dish with a flat bottom and fill it with distilled water.
Note that the minimum immersion depth for bricks should never be less than 2.5 cm.
- Keep the aforementioned dishes (bricks in water) in a warm (18°C to 30°C), well-ventilated area.
(The water from the dishes will evaporate as a result of absorption by the bricks)
- When the bricks appear to have dried, add a fresh supply of distilled water.
- At the conclusion of the second drying, each brick is inspected for efflorescence, which is the emergence of a white salt patch on its surface.
The efflorescence is only described in qualitative terms
Serious
The salt deposition is heavy and uniformly distributed, and it rises with repeated wetting and drying. Salt powdering is noticeable.
Heavy
Over fifty percent of the surface is covered by salt deposits. There is no inclination to powder.
Moderate
Salt deposits cover 10-50 percent surface area. The salt produces thin layers with no propensity to flake off or become powdery.
Slight
Less than 10 percent of the surface area is covered with a very thin, sticky film of salt.
Nil
Even after repeated wetting, there is no salt deposition observed.
Efflorescence in different types of bricks mustn’t exceed the prescribed level.
For example, in heavy-duty bricks, it must be zero, yet in first-class bricks, it must be minimal.
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IS 3495: Standard guidelines for testing brick compressive strength
In India, the compressive strength of burnt clay building bricks is determined following the procedures outlined in IS 3495 (Part 1): 1992.
This standard specifies the method for evaluating the compressive strength, which is crucial for assessing the load-bearing capacity of bricks used in construction.
key procedures from IS 3495 (Part 1): 1992:
Preparation of specimens:
- Select a representative sample of bricks and remove any unevenness on their bed faces to provide smooth and parallel surfaces.
- Immerse the bricks in water at room temperature for 24 hours.
- After soaking, remove the bricks and drain the surplus water at room temperature.
Testing procedure:
- Place the brick specimen with flat faces horizontal and the mortar-filled face facing upwards between two 3-ply plywood sheets, each approximately 3 mm thick.
- Carefully center the specimen between the plates of the compression testing machine.
- Apply load axially at a uniform rate of 14 N/mm² per minute until the brick fails.
Calculation of compressive strength:
- Record the maximum load at failure in Newtons (N).
- Measure the area of the loaded surface in square millimeters (mm²).
- Calculate the compressive strength using the formula:
Compressive strength (N / mm2) = Maximum load (N) / Loaded area (mm2) - Determine the average compressive strength from the tested specimens to assess the quality of the bricks.
Examples of brick usage by strength:
- First-class bricks (105 kg/cm²): Used in multi-story buildings, load-bearing walls, and foundations due to their high strength and durability. Examples include residential towers and industrial structures.
- Second-class bricks (70 kg/cm²): Ideal for non-load-bearing walls, partitions, and plastered masonry in low-rise residential or commercial buildings.
- Sun-dried bricks (15–25 kg/cm²): Commonly used in temporary structures, rural housing, or low-cost construction where strength is less critical.
Where specific types of bricks are typically used?
Understanding where specific types of bricks—first-class, second-class, and sun-dried bricks—are commonly used helps clarify the importance of their compressive strength in different construction scenarios.
First-class bricks
First-class bricks are known for their superior compressive strength (approximately 105 kg/cm²) and uniform shape, making them ideal for high-load and precision applications. These bricks are used in structural elements like load-bearing walls, columns, and foundations in multi-story buildings and industrial constructions where strength and durability are crucial. They are also preferred for visible masonry, such as facades, where aesthetic appeal is important, as their smooth surface and uniformity enhance the visual quality of the structure.
Second-class bricks
Second-class bricks have a compressive strength of about 70 kg/cm², slightly lower than first-class bricks, and their surface may have slight irregularities or a rough texture. These bricks are typically used in non-load-bearing walls, partitions, or plastered masonry work where the surface finish is not critical. They are a cost-effective choice for residential and low-rise buildings, providing adequate strength for moderate structural demands while reducing material expenses.
Sun-dried bricks
Sun-dried bricks, with a compressive strength ranging between 15 and 25 kg/cm², are the weakest among the types of bricks. They are not fired in kilns, making them less durable and unsuitable for permanent structures exposed to weather or heavy loads. These bricks are primarily used for temporary structures, rural housing, or low-cost construction in areas with mild climatic conditions. Their affordability and ease of production make them a practical option where compressive strength is not a critical factor.
Relevance of compressive strength
The compressive strength of bricks directly determines their suitability for specific applications. First-class bricks are necessary for projects requiring high load-bearing capacity and durability, ensuring the structural integrity of large and complex constructions. Second-class bricks strike a balance between strength and cost, making them ideal for smaller-scale projects or applications with moderate strength requirements. Sun-dried bricks, on the other hand, cater to economically constrained or temporary construction needs, sacrificing strength for affordability and simplicity.
Dimensions tolerance test
The test is performed to determine whether or not the bricks meet the required dimensions. It is finished in the following way:
- Select twenty random bricks.
- Remove from the bricks any loose clay particles and protruding blisters, etc.
- Arrange the bricks in a straight line on a smooth, level surface so that the adjacent surfaces are in contact with each other.
The arrangement will be made in accordance with the measured dimensions of the bricks. Therefore, bricks will be put lengthwise for length.
- Bricks are put on their wide faces for width.
- Bricks are put on the side for height.
- In each instance, the overall length of the brick rows is subsequently measured. These must fall within the restrictions set for various types of bricks.
Why high compressive strength is critical for different construction types?
High compressive strength in bricks is essential for ensuring the safety, durability, and longevity of various construction types. It plays a significant role in the structural integrity of buildings and the ability to withstand different types of loads over time.
Multi-story buildings
In multi-story buildings, bricks must support substantial vertical loads from multiple floors. High compressive strength ensures that the bricks can endure these loads without cracking, buckling, or failing. Weak bricks in these structures can lead to uneven settling, structural instability, or even catastrophic collapse. By using bricks with high compressive strength, the risk of such failures is minimized, ensuring long-term stability and safety.
Industrial structures
Industrial structures often face unique challenges, including heavier loads from machinery, equipment, and storage. High compressive strength in bricks allows these structures to bear such dynamic and static loads effectively. Additionally, industrial buildings are often exposed to harsh environmental conditions, such as chemical exposure or high humidity. Bricks with superior compressive strength are more resistant to these factors, reducing maintenance and repair costs over time.
Impact on safety
Safety is a critical aspect of any construction project. Bricks with high compressive strength contribute to the overall structural soundness of a building, reducing the likelihood of accidents or structural failures. For example, in earthquake-prone regions, strong bricks are better equipped to handle seismic forces, preventing widespread damage and loss of life.
Impact on longevity
High compressive strength directly correlates with the durability of a structure. Bricks that can resist significant compressive forces are less likely to deteriorate under regular use and environmental stress. This longevity reduces the need for frequent repairs or replacements, resulting in lower lifetime costs for the building. Moreover, durable bricks maintain the aesthetic and functional value of a structure over time, making them a wise investment for long-term projects.
Housing.com POV
High compressive strength in bricks is essential for ensuring the safety and longevity of various construction types. In multi-story buildings, bricks with high compressive strength can support heavier loads, maintaining structural integrity and preventing failures. In industrial structures, these bricks withstand substantial mechanical stresses and environmental factors, reducing maintenance needs and enhancing durability. Therefore, selecting bricks with appropriate compressive strength is crucial for the safety and longevity of both multi-story and industrial constructions.
FAQs
What is the brick's minimum compressive strength?
The minimum compressive strength for first-class bricks is 10 N/mm². It may be expressed as 105 kg/cm².
What is the brick's compressive strength in megapascals?
The average compressive strength of brick units is 5.7 MPa, ranging from 4.3 to 6.9 MPa.