5 to 400 kNTJ-125
Direct Compression Fixture
Flat platens with the specimen bedded so the load spreads over the whole face. A brick is neither flat nor parallel as fired, and the bedding is what makes it behave as though it were.
SpecificationsTesting standard
IS 3495 (Part 1)
Burnt Clay Building Bricks — Methods of Tests — Part 1: Determination of Compressive Strength
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
IS 3495 (Part 1) : 2019 determines the compressive strength of burnt clay building bricks. It is the fourth revision, and the parts that were once a single 1992 document covering four tests are now issued separately.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- BIS
- Edition
- 2019
- Material
- Wood, concrete & ceramics
- Runs on
- Series 7200 and Series 9000
What the test does
Bricks are selected as a representative sample, conditioned or immersed as the method requires, and their frogs filled. The bearing face is measured to establish the actual area, and the brick is bedded between compression platens so that the load spreads across the whole face rather than onto a high spot. It is then loaded at the specified rate to failure. The maximum load divided by the measured bearing area gives the compressive strength for that brick, and every individual value is reported alongside the average.
What it measures, and why it matters
How much load a brick carries before it crushes — the property masonry design is built on, and one specified statistically rather than as a single figure because bricks vary widely by nature. That variability is not a defect of the test; it is a property of a fired clay product, and it is why a sample is tested and why the spread matters as much as the mean. A wall is only as strong as its weakest courses, so a mean that conceals one very low unit gives the wrong picture of a batch.
A structure of parts
What was one document in 1992 is now a series, and Part 1 is only the compressive strength.
- This part
- Determination of compressive strength
- Other parts
- Water absorption, efflorescence, warpage, initial rate of absorption, modulus of ruptureParts 2, 3 and 4 came with the original series; Part 5 (2021) and Part 6 (2022) were added later.
- Predecessor
- IS 3495 (Parts 1 to 4) : 1992, reaffirmed 2016One document covering four tests. Citing 'IS 3495' without a part is ambiguous for anything after 2019.
- Specimen
- The brick as supplied
- Frog
- Filled as the method requiresAn unfilled frog is a void under the platen, and the brick crushes into it rather than resisting across its bed.
- Bed the brick, do not just place it
- DakA brick is neither flat nor parallel as fired. Bedding is what converts an uneven face into a bearing surface.
Masonry strength is specified statistically. A single brick tells you very little; the sample and its variability are the result, which is why a set is tested rather than one specimen.
Test speed
- Rate
- As the standard specifies
- Reported
- Compressive strength for each brick and the average
- Immersion
- As the method requires before testing
- Record the individual values, not only the mean
- DakBricks vary widely, and a mean that hides one very low unit is the wrong picture of a batch.
Calculations
Compressive strength—
Maximum load divided by the bearing area
- bearing area
- measured, on the face the load is applied to
Measured, not the nominal brick size. Fired clay shrinks unevenly and a brick is rarely its nominal dimension.
Why bedding matters—
Load must spread over the bearing face rather than a high spot
An unbedded brick contacts the platen at three or four points. The stress there is many times the average and the brick spalls long before its section is loaded.
How the test runs
- 01Select a representative sample of bricks rather than the best of a stack.
- 02Immerse or condition the bricks as the method requires.
- 03Fill the frog as specified.
- 04Measure the bearing face to get the actual area.
- 05Bed the brick between the platens so the load spreads across the face.
- 06Centre it on the lower platen.
- 07Load at the specified rate to failure.
- 08Record the maximum load for each brick.
- 09Calculate strength on the measured bearing area.
- 10Report every individual value alongside the average.
- 11Note any brick that spalled at a corner rather than crushing.
The fixture this method needs
What the report has to contain
- Reference to IS 3495 (Part 1) and the edition
- Brick class, source and dimensions
- Sample size and how the sample was selected
- Conditioning or immersion applied
- How the frog was filled
- Measured bearing area
- Rate of loading
- Individual compressive strengths and the average
- Any unusual failure such as corner spalling
- Whether other parts of IS 3495 were also run
What the machine must be capable of
Substantial compressive force — a standard brick at 10 N/mm² needs roughly 250 kN, and higher classes proportionally more — through flat platens that stay parallel. This is a dedicated compression frame rather than a general-purpose one. Beyond capacity the requirements are modest: a controlled rate of loading and enough stiffness to absorb an abrupt failure, since a brick fails suddenly and throws fragments.
What goes wrong in practice
Placing the brick rather than bedding it, so contact happens at three or four high points where the stress is many times the average and the brick spalls at a corner. Leaving the frog unfilled, which puts a void under the platen. Calculating on nominal dimensions when a fired brick rarely matches them. Reporting only a mean, which hides the variability that masonry specification exists to control. And citing IS 3495 without a part number, which after the 2019 restructure no longer identifies a single document, since Parts 5 and 6 arrived in 2021 and 2022 and the 1992 edition bundled four methods together.
IS 3495 (Part 1) or IS 516
| IS 3495 (Part 1) — bricks | IS 516 — concrete | |
|---|---|---|
| Specimen | A fired brick as supplied | A cast cube |
| Variability | High and inherent | Lower, and controllable |
| Preparation | Bedding and frog filling | Casting and curing |
| Specified as | A class, statistically | A grade, statistically |
Both are masonry compression tests judged statistically, but a brick arrives as it was fired while a cube is made to be tested. The preparation burden falls in completely different places.
Questions we are asked about this test
What is IS 3495 (Part 1)?
It is the Indian Standard method for the compressive strength of burnt clay building bricks. The current edition is IS 3495 (Part 1) : 2019, the fourth revision. It is one part of a series whose other parts cover water absorption, efflorescence, warpage, initial rate of absorption and modulus of rupture.
Why does citing 'IS 3495' alone cause confusion?
Because the structure changed. Until 2019 the tests were published as IS 3495 (Parts 1 to 4) : 1992 — one document containing four methods, reaffirmed in 2016. From the 2019 revision each part is a separate publication, and Parts 5 and 6 were added afterwards in 2021 and 2022. An unqualified reference is therefore ambiguous about which document and which edition is meant.
Why does the frog have to be filled?
Because an unfilled frog is a void directly under the platen. The load arrives on a face with a hole in the middle of it, so the brick crushes down into the void instead of resisting across its full bed, and the recorded strength is lower than the brick's own. Filling it restores a continuous bearing surface, which is what the arithmetic assumes.
Why does bedding matter so much?
Because a fired brick is neither flat nor parallel. Placed straight onto a platen it makes contact at three or four high points, and the stress at those points is many times the average, so it spalls at a corner long before its section is properly loaded. Bedding converts an uneven, warped face into a bearing surface — it is the single preparation step that decides whether the number describes the brick.
Why report individual values rather than just the mean?
Because bricks vary widely and the variability is the point. Masonry is specified statistically for that reason. A mean that conceals one very low unit gives a misleading picture of a batch, and a builder needs to know the spread as much as the average — a wall is only as good as its weak courses.
Why measure the bearing area rather than use the nominal size?
Because fired clay shrinks unevenly and a brick is rarely its nominal dimension. The strength is load over area, so using a nominal figure introduces an error directly into the result, and on a product with the dimensional tolerances of a brick that error is not small.
What does a corner spall mean?
Usually that the brick was not bedded properly, or that the platens were not parallel. The load concentrated on one corner, which failed locally at a load that says nothing about the brick's capacity across its bed. It is worth recording separately rather than averaging in, because it points at the test set-up rather than at the material.
Running IS 3495 (Part 1) on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | High — a standard brick at 10 N/mm² needs roughly 250 kN, and higher classes more | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | Class 1 over the working range | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Gripping | Flat compression platens, with the brick bedded and its frog filled as the method requires | Our compression anvils, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.