ASTM C78 Flexural Strength of Concrete Beams

ASTM C78 addresses the flexural testing of concrete used in the construction of slabs and pavements with a "third-point" style flexure apparatus. To conduct the testing, we used our SATEC™ Series 300DX testing system, a flex fixture, and Partner™ Materials Testing Software. Specimen geometry is in the form of a beam that is 6 x 6 inches with a minimum length of 21 inches. Sampling and specimen preparation for fresh concrete taken in the field is covered in ASTM C31; sawed specimens taken from cured material are discussed in ASTM C42; and for specimens made in the lab are referenced in ASTM C192. We suggest referencing the instructions found in the ASTM C78 for the loading rates, proper specimen orientation, and application of preload to assure "gap-free" contact between the specimen and fixture.

The 300DX is a servo-controlled system, which allows the test to perform automatically within a closed loop. We find that operators prefer this style of system over manually controlled systems because of the more consistent control of the loading rate and the less risk of shock to the specimen. The desired results are that modulus of rupture is calculated and the specimen fails within the middle third of the span length in the tension area or underside of the specimen. Partner Software automatically calculates the modulus of rupture based upon the maximum applied load and average dimensional values of the test specimen. If failure occurs outside of the middle third of the specimen, but not more than 5% of the span length, a different formula for calculating the modulus of rupture result is used. If failure occurs outside of the middle third of the specimen's tension area by more than 5%, then the result cannot be used.

We manufacture a variety of testing systems suitable for meeting this specification, including the common construction testing industry standard "compression-only" style systems. We suggest reviewing the standard to fully understand the test set up, procedures, and result requirements.