Testing standard
ASTM C78/C78M, Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading); ASTM C293/C293M, Standard Test Method for Flexural Strength of Concrete (Using Simple Beam With Center-Point Loading)
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM C78 breaks a concrete beam under third-point loading and reports the modulus of rupture, the design property for concrete paving. Its centre-point companion, ASTM C293, was WITHDRAWN in 2025 with no replacement — and it was never an alternative to C78, because centre-point loading returns a higher figure on the same concrete. The current edition is C78/C78M-22.
A moulded or sawn concrete beam is supported near its ends and loaded in bending until it breaks. The result is reported as the modulus of rupture: the extreme fibre tensile stress in the beam at the failure load.
The two methods differ in where the load is applied. C78 uses third-point loading — two load points, each one third of the span from a support — which produces a constant bending moment and zero shear over the middle third of the beam. C293 applied a single load at mid-span, which puts the maximum moment at one point.
That difference is not academic. Under third-point loading the beam is asked to fail wherever the middle third is weakest, so the result reflects the weakest section in a region. Under centre-point loading it can only fail at one place, whether or not that is where the concrete is weakest. Centre-point loading therefore returns a higher modulus of rupture on the same concrete, and C293 itself stated that it is not an alternative to C78.
Modulus of rupture is the design property for concrete loaded in flexure without reinforcement carrying the tension: pavement slabs above all, and also airfield paving and some precast elements. Highway agencies specify concrete paving on flexural strength rather than compressive strength for that reason.
It is an indirect measure of tensile capacity, computed from beam theory on the assumption that concrete behaves elastically to failure. It does not, so the number is higher than the true tensile strength and higher than a splitting tensile result on the same mix. It is a design convention, applied consistently, rather than a material constant.
The rate of loading is controlled so that the extreme fibre stress increases at 0.9 to 1.2 MPa/min, equivalently 125 to 175 psi/min. On a standard 150 mm beam that corresponds to a load increase of roughly 4.0 to 5.3 kN/min.
Most of what decides a flexural result happens before the beam reaches the machine, and the most damaging of it is invisible.
P L / (b d²)
Applies where the fracture falls within the middle third. Width and depth are measured at the fracture, not nominally.
Constant moment and zero shear over the middle third
The beam fails wherever that middle third is weakest. Under centre-point loading it can only fail at one place, whether or not that is the weakest section.
Beam theory assumes concrete stays elastic to failure
It does not. Modulus of rupture is a design convention applied consistently, not a material constant.
Third-point loading is a four-point bend arrangement: two supports and two loading blocks at the third points. Every block has to be able to rotate and to maintain even contact across the full beam width, so a beam whose faces are not perfectly plane still bears evenly.
SpecificationsA standard 150 mm square beam fails in the tens of kilonewtons, far below the capacity of the frame usually available. A cell sized for the beam is what makes the slow specified ramp controllable.
SpecificationsFailure loads for a standard 150 mm square beam sit in the tens of kilonewtons, so a frame of 100 kN upwards covers ordinary work. Force accuracy to ASTM E4 is the requirement, and a controllable slow load rate matters as much as capacity.
The fixture carries the requirement. Support and loading blocks must be capable of rotating and of maintaining even contact across the full beam width, so that a beam whose faces are not perfectly plane still bears evenly. Spans are set accurately because span enters the calculation directly. No extensometer is required.
Beams allowed to dry before testing are the commonest and most invisible error. Loading faster than the specified rate raises the apparent strength. Bearing blocks that cannot rotate concentrate load on one corner of a slightly warped beam. And quoting a centre-point result against a third-point specification overstates the strength of the concrete, which is why C293 carried the warning it did.
The reason one of these two documents survives and the other does not.
| Third point (C78) | Centre point (C293) | |
|---|---|---|
| Status | Current | Withdrawn 2025, no replacement |
| Load points | Two, at the third points | One, at mid-span |
| Middle third | Constant moment, zero shear | Moment peaks at a single section |
| Failure can occur | Anywhere in the middle third | Only at the load point |
| Result on the same concrete | Lower | Higher |
| An alternative to the other | No | No — C293 said so itself |
Quoting a centre-point modulus of rupture against a third-point specification overstates the concrete. C293 carried an explicit statement that it is not an alternative to C78, and its withdrawal has not made that any less true of historical data.
ASTM C78, published as C78/C78M, determines the flexural strength of concrete using a simple beam under third-point loading. The beam is supported near its ends and loaded at two points, each a third of the span from a support, until it breaks, and the result is reported as the modulus of rupture. The current edition is C78/C78M-22, with revision work registered under WK93728.
C293/C293M-16, the centre-point loading method, was withdrawn in 2025 and the ASTM catalogue names no replacement. It was never an alternative to C78 — the standard said so itself — because centre-point loading returns a higher modulus of rupture on the same concrete. Historical C293 data remains what it was, and it should not be compared directly with C78 results.
Because it lets the beam choose where to break. Third-point loading produces a constant bending moment and zero shear over the middle third of the span, so failure occurs wherever that region is weakest. Centre-point loading concentrates the maximum moment at a single section, and the beam can only fail there whether or not that is the weakest concrete in it. Testing a larger volume at peak stress finds more flaws, which is why the third-point figure is lower and why it is the one specifications use.
Concrete paving, principally. Highway and airfield agencies specify paving concrete on flexural strength rather than compressive strength, because an unreinforced slab fails in bending under wheel loads. It is also used for some precast elements and for fibre-reinforced concrete work, though there the post-crack behaviour measured by ASTM C1609/C1609M usually matters more than the peak.
Because drying puts the outer fibres of the beam into tension before the machine does. Surface moisture leaves faster than moisture in the core, the surface tries to shrink, and the restraint from the interior loads exactly the fibres this test is about to load further. A beam left in a warm laboratory for an hour can read materially low, and nothing about its appearance gives that away.
No. It is computed from beam theory, which assumes the material stays elastic right up to failure — and concrete does not. The stress distribution across the beam at failure is not linear, so the calculated extreme fibre stress overstates the real tensile capacity. Modulus of rupture is a design convention that works because it is applied consistently, and it is higher than both splitting tensile strength and direct tensile strength on the same mix.
A frame of 100 kN upwards covers ordinary beam sizes, with force accuracy to ASTM E4. The controlling requirements are elsewhere: a slow, steady load rate at 0.9 to 1.2 MPa/min of extreme fibre stress, an accurately set span because span enters the calculation directly, and loading and support blocks that can rotate to bear evenly across the full width of a beam that is never perfectly plane.
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 | Tens of kN for a standard 150 mm square beam | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Third-point bend rig with support and loading blocks free to rotate and bear evenly across the full beam width | Our bend fixtures, built to the specimen |
| Environment | Beams kept wet until the moment of testing; drying shrinkage loads the same outer fibres the test loads | 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.
