University of New Mexico
Civil Engineering Department
Civil Engineering Materials Laboratory, CE 305L
FLEXURAL STRENGTH OF CONCRETE
ASTM C78
Scope
This ASTM test method covers the determination of the flexural strength of concrete using a simple beam with third-point loading. The results are calculated and reported as the modulus of rupture.
References
ASTM C78 Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading)
ASTM C496 Splitting Tensile Strength of Cylindrical Concrete Specimens
ASTM C39 Compressive Strength of Cylindrical Concrete Specimens
ASTM C192 Making and Curing Concrete Test Specimens in the Laboratory
Definitions
Modulus of Rupture (MOR) - the tensile strength of a material determined using a flexural specimen.
Apparatus
Compression Testing Machine
Loading Apparatus - Third-point loading mechanism
Materials
6” x 6” x 21” Moist-cured concrete beams
Procedure
Download the procedure for Flexural Strength of Concrete here
Download the Excel form for Flexural Strength of Concrete here
1) Position the specimen in the testing machine. Center the loading apparatus in relation to the applied axial force.
2) Bring the load-applying block in contact with the upper surface of the specimen at the third points between the lower supports. The span distance between the lower supports is 18 in.
3) Apply the load continuously at a rate that increases the extreme fiber stress 125 to 275 psi/minute until rupture occurs (the load rate is 1500 to 3300 lb/minute for 6” by 6” beams with a span of 18”). Note the peak load at failure.
4) If the fracture (rupture) occurs in the tension surface (the bottom surface) outside the middle third of the span length by more than 5% of the span length (about 1 in.), discard the result of the test.
5) Calculate the modulus of rupture (MOR), neglecting the beam weight, as follows:
a) When fracture initiates in the tension surface (i.e., the bottom surface) within the middle third of the beam,
where P is the maximum load at failure in pounds, l is the span length, d is the depth of the beam, and b is the width of the beam. All dimensions are in inches.
b) If fracture initiates in the tension surface (i.e., the bottom surface) outside the middle third of the beam by not more than 5% of the span length,
where a is the average distance, in inches, between the line of fracture and the nearest support measured on the tension surface of the beam
Three point loading set up
Fractured sample
Surface of fractured sample
Report
1) Report the type of concrete, unit weight, and unconfined compressive strength.
2) Report the modulus of rupture to the nearest 5 psi.
Questions
1. What is the ratio of the modulus of rupture strength to the unconfined compressive strength?
2. How does the modulus of rupture flexural tensile strength compare with the strength obtained from the splitting tensile test ( ASTM C496, Splitting Tensile Strength of Cylindrical Concrete Specimens )? Express as a ratio of the split tensile strength to the MOR.
3. An empirical relation for the MOR, according to your textbook, is
How well does this relation predict your measured MOR? What is the percent error?