ASTM C78/C78M Flexural Strength of Concrete: Workflow, Formula, and How Aldoa Simplifies Testing

Flexural strength is critical for understanding how concrete will perform under bending. For pavements, slabs, bridge decks, and many structural elements, flexural performance often matters as much as or more than compressive strength.

ASTM C78/C78M: Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading) is the reference test that geotechnical and CMT firms use to measure this property.

The test procedure is well defined. The friction usually shows up in data collection, calculations, traceability, and reporting, especially when teams still use paper forms or rigid legacy software.

Aldoa gives labs and engineers a modern way to run ASTM C78. Test data is captured once, modulus of rupture is calculated automatically, and reports are generated in minutes.

This guide explains what ASTM C78 is, how the test works, the key parameters you need to track, and how Aldoa streamlines the workflow from sample to final report.

What is ASTM C78/C78M?

ASTM C78/C78M defines a procedure for determining the flexural strength of concrete using a simple beam with third point loading.

• Test objective: Determine the modulus of rupture (MOR), which is an indicator of the tensile strength of concrete in bending.
• Typical specimens: Concrete beams such as 6 in x 6 in x 21 in, or other dimensions allowed by the standard and project requirements.
• Typical applications:
  • Concrete pavements and slabs on grade
  • Bridge decks
  • Structural beams and girders

Because many pavement and slab designs are controlled by flexural performance, this test is a key part of DOT and infrastructure quality control.

Key Parameters in ASTM C78/C78M Testing

An accurate C78 test requires precise measurements and calculations. Common parameters include:

Traditionally, technicians record data on paper or in spreadsheets, then perform calculations manually or with ad hoc formulas. This is slow and introduces opportunities for error.

ASTM C78 Modulus of Rupture Formula

For a simple beam with third point loading and fracture at the middle third, ASTM C78 uses:

R=P×L / BxD^2​

Where:

• R = modulus of rupture (flexural strength)
• P = maximum applied load
• L = span length
• b = average width of the specimen at the fracture
• d = average depth of the specimen at the fracture

Correct unit handling is essential. For example, if P is in pounds and dimensions are in inches, R is typically given in psi. If P is in Newtons and dimensions are in millimeters, the result is in MPa.

In Aldoa, this equation is built into the digital form so technicians do not have to perform any manual calculations.

ASTM C78 Flexural Strength Test: Step by Step

Here is a high level summary of the procedure used in most CMT labs.

1. Prepare and cure specimens

• Mold beams in accordance with the applicable concrete standard.
• Cure specimens under specified conditions until the required test age.
• Mark identification and casting dates clearly so age can be calculated.

2. Measure specimen dimensions

• Measure the width and depth of each beam at three locations around the anticipated fracture area.
• Record measurements and compute average b and d.

3. Set up the testing machine

• Place the beam on the supports with the correct span length L.
• Ensure that third point loading heads are positioned accurately at one third points.

4. Apply load to failure

• Load the specimen at a controlled rate.
• Record the maximum load P at the moment of fracture.
• Note the location of the fracture relative to the supports.

5. Calculate modulus of rupture

• Use the appropriate equation from ASTM C78 based on fracture position.
• Document the calculated modulus of rupture along with specimen age and conditions.

6. Report results

A complete report typically includes:

• Project and specimen IDs
• Dimensions, span, and loading configuration
• Age at test
• Maximum load and calculated MOR
• Fracture description and whether results are acceptable per specification

Challenges When Running ASTM C78 Without Modern Software

Many labs still run C78 tests using clipboards, Excel, or rigid legacy platforms. Common pain points include:

Manual data entry and transcription

• Technicians jot values in notebooks, then re-type them into spreadsheets.
• Simple entry errors can change MOR results and force rework.

Time consuming calculations

• Engineering staff or senior technicians check formulas and unit conversions.
• Multiple beams per test set multiplies calculation workload.

Slow, inconsistent reporting

• Reports are assembled manually in Word or PDF templates.
• Formatting differences between technicians can frustrate clients or DOT reviewers.

Limited traceability

• It is hard to connect each C78 result back to mix design, batch tickets, or earlier field tests.
• Audit or claim investigations become time consuming.

How Aldoa Streamlines ASTM C78/C78M Testing

Aldoa’s Field and Lab Data Management Platform replaces disconnected tools with a single, connected workflow for flexural strength testing.

1. Digital ASTM C78 test forms

• Technicians enter specimen dimensions, span length, age, and test loads directly into Aldoa.
• Required fields ensure that no critical data is missed.
• Dropdowns standardize mix IDs, test locations, and specifications.

2. Automatic modulus of rupture calculations

• The C78 equation is embedded in the form.
• As soon as technicians enter P, L, b, and d, Aldoa calculates MOR.
• The platform enforces consistent units and flags out of range values.

3. Real time field and lab data collection

• Tests can be recorded on any connected device.
• Results sync instantly and are available to project managers, engineers, and QC staff without emailing spreadsheets.

4. One click ASTM C78 reporting

• Aldoa generates clean, professional reports that include:
  • All specimen details and dimensions
  • Calculated modulus of rupture
  • Technician and reviewer information
  • Any notes or project specific requirements

Reports are ready to send to clients or DOTs without additional formatting.

Benefits for Geotechnical and CMT Firms