Approximately fifty bricks are randomly selected from different sections of the stack to undergo various tests. According to standards outlined in Table 1 (IS: 5454), when sampling for testing purposes, a lot should ideally consist of a maximum of 50,000 bricks. The specific number of bricks chosen to form a sample aligns with the guidelines provided in Table 1. Additionally, Table 2 defines the sampling scale for evaluating the physical characteristics of these bricks.
Scale of Sampling and Permissible Number of Defectives for Visual and Dimensional Characteristics
| No of bricks in the lot | No of bricks to be selected | Permissible No. of defectives in the sample | For dimensional characteristics specified for group of 20 bricks - No of bricks to be selected |
|---|---|---|---|
| 2001-10000 | 20 | 1 | 40 |
| 10001-35000 | 32 | 2 | 60 |
| 35001-50000 | 50 | 3 | 80 |
Note: In case the lot contains 2000 or fewer bricks, the sampling shall be subject to agreement between the purchaser and supplier.
Scale of sampling for physical characteristics
| Lot size | Sampling size from compressive strength, breaking load, transverse strength, bulk density, water absorption and efflorescence | Permissible No. of defectives for efflorescence | Sample size | Permissible No. of Defectives |
|---|---|---|---|---|
| 2000-10000 | 5 | 0 | 10 | 0 |
| 10001-35000 | 10 | 0 | 20 | 1 |
| 35001-50000 | 15 | 1 | 30 | 2 |
Note: In case the lot contains 2000 or fewer bricks, the sampling shall be subject to agreement between the purchaser and supplier
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Dimension Test (IS: 1077)
In accordance with IS: 1077 standards, a dimension test involves the selection of 20 bricks from a set (Referenced in the Table 1) and laying them flat, in three different orientations: first, laid flat with its width facing up; second, positioned with its length facing up; and third, placed upright with its height facing up.
Tolerances for brick sizes are established based on maximum and minimum dimensions given below. These limits apply not to individual bricks but rather to batches of 20 bricks chosen randomly. This approach aims to regulate and maintain consistency in brick dimensions.
However, due to this batch-oriented measurement method, there is a likelihood that some batches may contain bricks exceeding the specified tolerance limits. It’s recommended to reject lots exhibiting such deviations to mitigate concerns regarding perpend variations. This ensures the adherence to quality standards and minimizes potential complaints related to variation in brick dimensions.
| Subclass-A | Subclass-B | ||||
|---|---|---|---|---|---|
| Dimension(cm) | Tolerance(mm) | Dimension(cm) | Tolerance(mm) | ||
| Length | 380 | ± 12 | Length | 380 | ± 30 |
| Width | 180 | ± 6 | Width | 180 | ± 15 |
| Height (i) 9 cm | 180 | ± 6 | Height (i) 9 cm | 180 | ± 15 |
| Height (ii) 4 cm | 80 | ± 3 | Height (ii) 4 cm | 80 | ± 6 |
Water Absorption Test (IS: 3495 - Part II)
The presence of minute pores grants significant capillary properties to brick ceramics, resulting in water absorption by capillary action. The percentage of water absorption serves as a crucial indicator of the degree of burning. True vitrification corresponds to a level of compactness where the brick’s absorption remains below 3 percent after 48 hours of immersion. Studies suggest that when absorption remains under 5 percent, the risk of frost damage becomes negligible.
However, water absorption alone does not wholly predict a brick’s behavior during weathering. While low absorption (below 7%) generally signifies high resistance to freezing damage, certain bricks with higher absorption may also exhibit frost resistance. The ratio of absorption after 24 hours of submersion to that after boiling for 5 hours (C24/B5) emerges as a more effective criterion for freezing resistance, as it depends on the proportion of pore space occupied by water during freezing.
Assessing a brick’s durability involves testing it against frost action, employing alternate wetting and drying. Despite its historical consideration as a measure of durability, the absorption test’s direct correlation to durability is debatable. The rate at which a brick absorbs water, known as its suction rate, significantly impacts the mortar bond during laying. Rapid water withdrawal from mortar by the brick can lead to a weak bond. Measuring a brick’s suction rate typically involves immersing one face of the brick in water and recording the one-minute water uptake (initial rate of absorption). For prolonged immersion periods, the total water absorbed per unit area is calculated using the formula:
w = A *√t
where A represents the water absorption coefficient
t signifies the time elapsed in the test.
Standard methods for determining absorption values of bricks are outlined below. For volume-based absorption, the weight percentage can be multiplied by the apparent specific gravity. The process involves two standard immersion tests:
24 Hours Immersion Cold Water Test: Dry bricks are heated in an oven at 105°C to 115°C until reaching a constant mass (W1). After cooling to room temperature, they are immersed in water at 27°C ± 2°C for 24 hours. Subsequently, the bricks are removed, wiped with a damp cloth, and weighed after three minutes as W2. The water absorption percentage is calculated using the formula:
Absorption (%) = (W2 – W1) / W1 * 100
The average water absorption should not exceed 20% by weight for Class 12.5 and 15% by weight for higher classes. (Table IS-1077 Link)
24 Hours Immersion Cold Water Test Results
| Type of Brick | Percentage by Weight |
|---|---|
| First Class Bricks | <15% |
| Second Class Bricks | <20% |
| Third Class Bricks | <25% |
Five Hours Boiling Water Test: Similar to the previous process, dried bricks’ weight (W1) is recorded. The specimens are then boiled for five hours, cooled to 27°C ± 2°C naturally within 16-19 hours, wiped, and weighed as W3. The water absorption percentage is calculated as:
Water Absorption (%) = (W3 – W1) / W1 * 100
These standardized tests provide essential insights into a brick’s water absorption characteristics, aiding in assessing its suitability for various applications and environmental conditions.
Compressive Strength Test (IS: 3495 (Part I))
Crushing provides a benchmark for comparing brick quality but holds limited significance in determining the strength of a masonry wall, as wall strength primarily hinges on mortar robustness. Although an individual brick’s strength might deviate by 20% or more from the average, permissible stresses allowed for load-bearing walls consider this variability. The basis for permissible stresses rests on an average strength derived from testing six bricks. Consequently, it’s both unnecessary and uneconomical to demand that every brick exceeds a certain strength threshold.
As a structural strength criterion for bricks, transverse failure in a wall or pavement often occurs due to improper bedding. To test bricks for compressive strength from a sample, the two bed faces of the bricks are ground to ensure smooth, even, and parallel surfaces. The prepared bricks are then submerged in room temperature water for 24 hours. After removal from water, excess moisture on the surfaces is wiped off with cotton or a moist cloth. The brick’s frog is leveled with cement mortar, and the brick is stored under damp jute bags for 24 hours, followed by immersion in room temperature water for three days.
During the compression test, the specimen is placed in a compression testing machine with flat faces positioned horizontally, and the mortar-filled face is upward. Load is applied uniformly at a rate of 14 N/m² per minute until failure. The compressive strength is calculated by dividing the maximum load at failure by the average area of the bed faces:
Compressive Strength (N/mm²) = Maximum load at failure (N) / Average area of bed faces (mm²)
The average of these test results should be reported. Any individual brick’s compressive strength tested in the sample should not fall below the specified minimum average compressive strength for the corresponding class of brick by more than 20 percent.
Warpage Test (IS: 3495 (Part IV))
Measuring the warpage of a brick involves using a flat steel or glass surface alongside a measuring ruler graduated in 0.5 mm divisions or a steel wedge sized at 60 × 15 × 15 mm. For this test, a sample comprises 10 bricks from a lot.
Concave Warpage: The flat surface of the brick is positioned along the surface to be measured, selecting the location that exhibits the greatest deviation from straightness. Using a steel ruler or wedge, the maximum distance of the brick surface from the edge of straightness is measured.
Convex Warpage: The brick is placed on a plane surface with the convex surface in contact with the flat surface. The distances of the four corners of the brick from the flat surface are measured. The largest distance among these measurements is reported as the brick’s warpage.
The greater distance obtained from either the concave or convex warpage tests is reported as the brick’s overall warpage.
Efflorescence test (IS: 3495 (Part III))
To conduct the efflorescence test, the ends of the brick are immersed in a porcelain or glass dish with a 150 mm diameter, containing water at room temperature (20°C–30°C) at a depth of 25 mm. The brick remains in the water until it absorbs or evaporates the entire amount. The process is repeated by refilling the water to a 25 mm depth in the dish and allowing it to be absorbed or evaporated by the brick.
Efflorescence presence is classified as follows:
- Nil: Efflorescence is imperceptible.
- Slight: Efflorescence deposit covers less than 10% of the exposed area of the brick.
- Moderate: Efflorescence deposit ranges from more than 10% but less than 50% of the exposed area of the brick.
- Heavy: Efflorescence deposit is more than 50%, but it does not powder or flake away the brick surface.
- Serious: Deposits are heavy and cause powdering or flaking away of the brick surface.
Specifications limit efflorescence to be not more than moderate (10%–50%) for up to class 12.5 and not more than slight (<10%) for higher classes.
