5 to 400 kNTJ-125
Direct Compression Fixture
Flat parallel platens of a hardness and flatness suited to concrete, with a spherically seated upper platen so the load finds the specimen rather than the specimen finding the platen.
SpecificationsTesting standard
IS 516 (Part 1/Sec 1)
Hardened Concrete — Methods of Test — Part 1: Testing of Strength of Hardened Concrete — Section 1: Compressive, Flexural and Split Tensile Strength
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
IS 516 (Part 1, Section 1) : 2021 is the Indian Standard method for the strength of hardened concrete, covering compressive strength on cubes, flexural strength as modulus of rupture, and split tensile strength. It is the first revision of provisions that previously sat in IS 516 : 1959.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- BIS
- Edition
- 2021
- Material
- Wood, concrete & ceramics
- Runs on
- Series 7200 and Series 9000
What the test does
Three strengths in one section, each with its own specimen and loading rate. Compressive strength is measured on 150 mm cubes, three to a sample, cured in water at 27 ± 2 °C and loaded at 14 N/mm² per minute. Flexural strength is measured as modulus of rupture on a beam in third-point loading at 0,7 N/mm² per minute, with the fracture position recorded because it selects which expression applies. Split tensile strength is measured by laying a cylinder on its side between packing strips and loading it until it splits along its vertical diameter.
What it measures, and why it matters
Whether the concrete that was placed is the concrete that was specified. The point most worth carrying away is that the cube is not a neutral choice. Platen friction restrains the specimen's ends and reaches a fixed distance into it, so it dominates a squat 150 mm cube while largely sparing the mid-height of a 300 mm-tall cylinder — and the cube consequently reads higher for identical concrete. Indian M-grades are cube strengths. Reading one against a cylinder-based specification, or the reverse, misstates the material by a wide margin.
Three strengths, one section
Compression, flexure and splitting are all in this section, and each has its own specimen and its own loading rate.
- Compressive specimen
- 150 mm cube, with 100 mm permitted for accelerated testingCubes, not cylinders — the difference from ASTM C39 and the reason the numbers differ.
- Sample size
- Three cubes per sample
- Curing
- Water at 27 ± 2 °CTested at 7 days and at 28 days, the latter for design verification.
- Compression rate
- 14 N/mm² per minute
- Flexure rate
- 0,7 N/mm² per minuteTwenty times slower, because a beam reaches its peak stress at a far lower load and a fast rate would overshoot it.
- Cast the cubes properly and record the compaction
- DakA poorly compacted cube tests the compaction, not the mix, and it is the commonest cause of a low result on site.
A 150 mm cube reads higher than a 150 x 300 mm cylinder of the same concrete, because platen restraint reaches proportionally further into the squatter specimen. Indian grades are cube strengths; quoting one against a cylinder specification overstates the concrete.
Test speed
- Compression
- 14 N/mm² per minute
- Flexure
- 0,7 N/mm² per minute
- Reported
- Compressive strength, modulus of rupture, split tensile strength
- Record the age at test, always
- DakConcrete gains strength for months. A figure without an age is not a result.
Calculations
Compressive strength—
Maximum load divided by the cross-sectional area of the cube
On the measured area. A worn mould does not produce a 150 mm cube.
Modulus of rupturefb
From the maximum load and the beam geometry, by the section's formula
- —
- the expression depends on where the fracture falls relative to the middle third
The standard gives different expressions according to the fracture position, which is why that position is recorded rather than just the load.
Split tensile strength—
2P / (π l d)
- P
- maximum load
- l
- specimen length
- d
- specimen diameter
A cylinder loaded on its side splits along its vertical diameter. It is an indirect tensile measure, and it is not the same as a direct tensile strength.
How the test runs
- 01Cast the specimens and record how they were compacted.
- 02Cure in water at 27 ± 2 °C to the required age.
- 03Measure the specimen dimensions rather than assuming nominal ones.
- 04For compression, centre the cube on the platen with a cast face against it, not a trowelled one.
- 05Load at 14 N/mm² per minute to failure and record the maximum load.
- 06For flexure, set the third-point rig and load at 0,7 N/mm² per minute.
- 07Record where the beam fractured relative to the middle third.
- 08Select the modulus of rupture expression that matches that position.
- 09For split tensile, lay the cylinder on packing strips and load along its length.
- 10Calculate each strength on the measured dimensions.
- 11Report the age at test with every value.
The fixture this method needs
What the report has to contain
- Reference to IS 516 (Part 1/Sec 1) and the edition
- Mix identification and grade
- Specimen type and measured dimensions
- How the specimens were compacted and cured
- Age at test
- Rate of loading for the test performed
- Maximum load and the calculated strength
- Fracture position for flexure specimens
- Number of specimens in the sample and the individual results
- Whether cubes or cylinders were used
What the machine must be capable of
Very high compressive force — a 150 mm cube at M40 needs roughly nine hundred kilonewtons and higher grades more — with bearing platens suited to concrete and enough stiffness to survive an abrupt failure. Two loading rates have to be held accurately and they differ by a factor of twenty, so the control has to work well at both. The flexural test needs a third-point rig with the specified span, and the split tensile test packing strips of the specified material and width.
What goes wrong in practice
Comparing a cube result against a cylinder specification, which is the error that travels furthest and always in the flattering direction. Bearing on the trowelled face. Reporting a strength without the age at test, which makes it uninterpretable given how long concrete keeps gaining. Loading a flexural beam at the compressive rate, which races past the failure point. And ignoring the fracture position on a beam, when the standard's expression for modulus of rupture depends on whether it broke inside the middle third, and a fracture too far outside it voids the specimen altogether.
IS 516 or ASTM C39
| IS 516 (Part 1/Sec 1) | ASTM C39/C39M | |
|---|---|---|
| Compressive specimen | 150 mm cube | 150 x 300 mm cylinder or a core |
| Reads | Higher for the same concrete | Lower for the same concrete |
| Also covers | Flexure and split tensile | Compression only |
| Grades quoted against it | Indian M-grades are cube strengths | Specified cylinder strengths |
Indian concrete grades — M25, M40 and the rest — are cube strengths. Reading one against a cylinder-based specification, or the reverse, overstates or understates the concrete by a wide margin.
Questions we are asked about this test
What is IS 516 (Part 1/Section 1)?
It is the Indian Standard method for the strength of hardened concrete, covering compressive strength on cubes, flexural strength as modulus of rupture, and split tensile strength. The current edition is the 2021 first revision, which took these provisions out of the long-standing IS 516 : 1959 and reorganised them into parts and sections.
Why does India use cubes when ASTM uses cylinders?
It is a convention rather than a technical necessity, inherited from British practice, and it has a real consequence. Friction between the platens and the specimen restrains the ends and reaches a fixed distance into the concrete, so it dominates a squat 150 mm cube while largely sparing the mid-height of a 300 mm-tall cylinder. The cube therefore reads higher for identical concrete, and Indian M-grades are cube strengths.
Why is the flexural rate twenty times slower than the compressive one?
Because a beam reaches its peak stress at a far lower load than a cube does. The rates are specified as stress per minute rather than load per minute, and the geometry converts them very differently: a modest load on a beam in third-point bending produces a large extreme-fibre stress. Loading a beam at the compressive rate would race past the failure point and overstate the modulus of rupture.
Why does the fracture position matter in the flexural test?
Because the standard gives different expressions for modulus of rupture depending on whether the beam broke inside the middle third or outside it. Between the two inner load points the moment is constant, so a fracture there is at the maximum stress; outside it the moment is lower and the arithmetic has to account for that. Recording the position is what allows the right expression to be used, and a fracture too far out invalidates the specimen.
Which face of a cube goes against the platen?
A cast face, not the trowelled one. The top surface of a cube as cast is finished by hand and is neither as flat nor as dense as the moulded faces, so bearing on it concentrates load unevenly and reads low. Turning the cube on its side so two moulded faces take the load is standard practice and removes a whole class of spurious results.
What is split tensile strength and why not test tension directly?
A cylinder laid on its side and loaded along its length splits down its vertical diameter, and the load at which it does gives an indirect measure of tensile strength. Concrete is tested this way because gripping it in direct tension is very difficult — the grips crush it before it pulls apart. The split figure is an accepted indirect measure and is not numerically the same as a direct tensile strength.
Why must the age at test be reported?
Because concrete keeps gaining strength for months, and a value without an age is not a result. The method's reference points are 7 days as an early indicator and 28 days for design verification, and comparing a 7-day figure with a 28-day requirement — in either direction — is one of the more common misreadings on site.
Running IS 516 (Part 1/Sec 1) on the Series 7200
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 | Very high — a 150 mm cube at M40 needs about 900 kN, and higher grades 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 | A compression frame with bearing platens for cubes, and a third-point flexure rig for the beam test | 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.