Fly Ash Bricks – Quality Requirements As per IS 16720
Fly ash bricks, technically referred to in IS 16720:2018 as Pulverized Fuel Ash-Cement Bricks, are masonry units manufactured primarily using pulverized fuel ash (fly ash), cement, aggregates and water.
The use of fly ash in bricks provides an effective way of utilizing a thermal power plant by-product while producing a masonry unit suitable for construction. IS 16720 specifies requirements related to materials, dimensions, manufacturing, physical properties, sampling, testing and acceptance criteria.
Applicable standard: IS 16720:2018 – Pulverized Fuel Ash-Cement Bricks — Specification, including Amendment No. 1 (2022) and Amendment No. 2 (2023).
1. Materials Used in Fly Ash Bricks
As per IS 16720, fly ash bricks are manufactured using:
- Pulverized fuel ash conforming to IS 3812 (Part 1) or IS 3812 (Part 2)
- Fine and coarse aggregates conforming to IS 383
- Cement conforming to the applicable Indian Standard
- Water conforming to IS 456
- Chemical admixtures, where used, conforming to IS 9103
- Suitable additives such as gypsum or lime may also be used.
The brick shall contain not less than 35% pulverized fuel ash by mass. The manufacturer may be required to provide a certificate indicating the percentage of fly ash used.
Important amendments
The 2022 amendment revised the coarse aggregate grading requirement. The aggregate shall:
- Pass 100% through 10 mm sieve
- Have 85–100% passing 6.3 mm sieve
- Have 0–10% passing 2.36 mm sieve
The 2023 amendment additionally permits bottom ash as a replacement for sand, provided its loss on ignition is not more than 10% when tested as per IS 1727.
2. Standard Sizes and Tolerances
The standard modular sizes specified in IS 16720 are:
| Type | Length | Width | Height |
| Modular | 190 mm | 90 mm | 90 mm |
| Modular | 190 mm | 90 mm | 40 mm |
| Non-modular | 230 mm | 110 mm | 70 mm |
| Non-modular | 230 mm | 110 mm | 30 mm |
Other sizes can also be manufactured by agreement between the purchaser and manufacturer, but the specified tolerances continue to apply.
The tolerances are:
- Length: ±20 mm for 10 bricks
- Width: ±20 mm for 10 bricks
- Height: ±3 mm for individual bricks
For measurement of length and width, 10 bricks are placed together in a straight line and the overall dimension is measured.
3. Classification of Fly Ash Bricks
Fly ash bricks are classified according to their average 28-day wet compressive strength:
| Class | Minimum average compressive strength |
| 15 | 15 N/mm² |
| 12.5 | 12.5 N/mm² |
| 10 | 10 N/mm² |
| 7.5 | 7.5 N/mm² |
| 5 | 5 N/mm² |
The classification is therefore important when approving bricks for a particular construction application.
4. Physical Requirements
The important requirements specified by IS 16720 are:
Appearance
Bricks should be:
- Sound and free from visible cracks
- Uniform in shape and colour
- Rectangular with smooth faces
- Provided with sharp corners
Minor chipping resulting from normal handling during delivery is not, by itself, a reason for rejection.
Density
Density shall be between 1100 and 2000 kg/m³ when determined in accordance with the specified procedure.
Compressive Strength
The average 28-day wet compressive strength shall meet the specified class requirement.
Note 1: In addition, the strength of any individual brick shall not be more than 20% below the specified minimum average strength.
Example – Class 10 Fly Ash Bricks:
- Required average = ≥10 N/mm²
- Minimum individual result = ≥8 N/mm²
- consider another case:
| Brick | Test result |
| 1 | 12.0 N/mm² |
| 2 | 11.0 N/mm² |
| 3 | 7.9 N/mm² |
- Average = 10.3 N/mm² → Pass based on average
- However, Brick 3 has a strength of 7.9 N/mm², which is below the permitted individual minimum of 8.0 N/mm².
- Therefore, the lot fails even though the average strength is above 10 N/mm².
Note 2: The standard also specifies a special treatment for test results exceeding 125% of the specified strength when calculating the average.
Example — Class 10 Fly Ash Bricks
For Class 10, the specified minimum average compressive strength is:
10 N/mm²
Therefore, 125% of the specified strength is:
1.25 × 10 = 12.5 N/mm²
Suppose the three compressive-strength test results are:
| Brick | Actual test result | Value used for average |
| 1 | 10.5 N/mm² | 10.5 N/mm² |
| 2 | 11.0 N/mm² | 11.0 N/mm² |
| 3 | 14.0 N/mm² | 12.5 N/mm² |
The third brick actually achieved 14 N/mm², but because this is more than 125% of the specified strength (12.5 N/mm²), only 12.5 N/mm² is considered for calculating the average.
Therefore:
Average strength = (10.5 + 11.0 + 12.5) / 3
= 11.33 N/mm²
So, for conformity purposes, the average is 11.33 N/mm², and
not:(10.5 + 11.0 + 14.0) / 3 = 11.83 N/mm²
Want to instantly check whether your fly ash brick sample passes the compressive-strength requirements of IS 16720:2018? Enter the strength results of just 3 bricks in the tool below and get an instant PASS/FAIL assessment.
Water Absorption
After 24-hour immersion in cold water:
- Up to Class 10: maximum average absorption = 20%
- Higher classes: maximum average absorption = 15%
Drying Shrinkage
Average drying shrinkage, based on three bricks, shall not exceed:
0.05%.
Ex: Sampling and Testing of a 30,000-Brick Lot
Consider the following site example:
Quantity received = 30,000 fly ash bricks
Assume all bricks are:
- Same class
- Same size
- Manufactured under relatively similar production conditions
According to IS 16720, a lot can contain a maximum of 50,000 bricks. Therefore, the entire 30,000 bricks can be treated as one lot.
How many bricks should be sampled?
For each lot of up to 50,000 bricks, 10 bricks are randomly selected.
Sampling should preferably be carried out at regular intervals during loading/unloading. If that is not practical, samples may be taken randomly from different portions of the stack.
Test allocation for the 10 sampled bricks
| Test | Number of bricks |
| Visual inspection & dimensions | All 10 |
| Compressive strength | 3 |
| Density | 3 |
| Water absorption | 3 |
| Drying shrinkage | Same 3 used for water absorption |
| Reserve | 1 |
This allocation is directly specified in Clause 9.5 of IS 16720.
Site Example of Lot Acceptance
Assume the 30,000 bricks received at site are Class 10, 190 × 90 × 90 mm bricks.
The following hypothetical test results are obtained.
A. Visual Inspection
Out of 10 sampled bricks:
Defective bricks = 1
Acceptance requirement: Maximum 2 bricks with visual defects
Result: PASS
B. Dimensional Check
For 10 bricks:
- Total measured length = 1,912 mm
- Required range = 1,880–1,920 mm
- Total measured width = 894 mm
- Required range = 880–920 mm
- Maximum individual height deviation = 2.5 mm
- Permitted deviation = ±3 mm
Result: PASS
C. Compressive Strength
Three test results:
| Brick | Strength |
| 1 | 11.2 N/mm² |
| 2 | 10.4 N/mm² |
| 3 | 9.8 N/mm² |
| Average | 10.47 N/mm² |
For Class 10:
- Required average ≥ 10 N/mm²
- Minimum individual allowed = 8 N/mm²
Therefore:
Average = 10.47 N/mm² → PASS
Minimum individual = 9.8 N/mm² → PASS
D. Density
Suppose the three density results are:
1,450 kg/m³, 1,480 kg/m³ and 1,470 kg/m³
Average density:
= 1,467 kg/m³
Specified range = 1,100–2,000 kg/m³
Result: PASS
E. Water Absorption
Suppose the three results are:
18.2%, 17.5% and 19.0%.
Average:
= 18.23%
For Class 10, maximum average absorption = 20%
Result: PASS
F. Drying Shrinkage
Suppose the average result from the three specimens is:
0.038%
Specified maximum = 0.05%
Result: PASS
7. Final Lot Acceptance
The lot is considered conforming when the applicable requirements of Clause 10 are satisfied.
For the above example:
| Parameter | Requirement | Result | Status |
| Visual defects | ≤2 defective bricks | 1 | PASS |
| Dimensions | Within specified tolerance | Within limits | PASS |
| Compressive strength – average | ≥10 N/mm² | 10.47 | PASS |
| Compressive strength – individual | ≥8 N/mm² | 9.8 | PASS |
| Density | 1100–2000 kg/m³ | 1467 | PASS |
| Water absorption | ≤20% | 18.23% | PASS |
| Drying shrinkage | ≤0.05% | 0.038% | PASS |
Conclusion: The 30,000-brick lot can be accepted as conforming, based on the above test results and the requirements of IS 16720.
The standard specifically states that dimensional conformity requires none of the inspected groups to fail, visual defects shall not exceed two, the mean density and compressive-strength requirements shall be satisfied, and average water absorption shall remain within the specified limit.
Reference: IS 16720:2018 – Pulverized Fuel Ash-Cement Bricks — Specification, including Amendment No. 1 (2022) and Amendment No. 2 (2023).
