Flexural strength is the ability to resist an applied bending force such as encountered by concrete pavements or other slabs on ground. A determination of the flexural strength is frequently necessary as part of the design of concrete mixtures to check compliance with established specifications or to provide informa-tion necessary to the design of an engineering structure. In the flexural-strength test, a test load is applied to the sides of a test beam. Although the test can be performed upon beams sawed from existing concrete structures, it is more commonly performed upon beams

Figure 13-25.Alternate testing assembly using CBR test equipment.

When performing the flexural-strength test, you use a concrete beam tester with third-point loading. An example of that equipment is illustrated in figure 13-24. An alternate testing assembly can be made from the loading frame and certain attachments provided with the California bearing ratio (CBR) test set, the breaker (third-point loading), and the 10,000-pound capacity proving ring. That alternate assembly is shown in figure 13-25.

1. Assemble the loading device, as shown in figure 13-24. Turn the test beam so that the finished surface is to the side and centered in the loading assembly. Operate the testing apparatus until the loading blocks are brought into contact with the upper surface of the beam. Be sure to secure full contact between the loading (and supporting) surfaces and the beam. If the surface of the specimen is so rough that full contact is not secured, grind the specimen to secure full contact.

2. Apply the test load at a rate such that the increase in extreme fiber stress in the beam is between 125 and 175 pounds per square inch per minute. The extreme fiber stress corresponding to any load maybe estimated from the equation given in Step 4a below. Obtain readings on the proving-ring dial, and convert them to corresponding total loads in pounds by applying the proving-ring constant. Aside from the reading used to control the rate of application of the load, the only reading necessary is the one that corresponds to the maximum load applied to the beam.

4. The flexural strength, expressed in terms of modulus of rupture, is given in psi, and can be calculated as follows:

a. If the specimen broke within the middle third of the span length, use the following equation:

R = modulus of rupture (in psi)

P = maximum applied load (in pounds)

L = span length (in inches)

b = average width of specimen (in inches)

d= average depth of specimen (in inches)

b. If the specimen broke outside the middle third of the span length by not more than 5 percent of the span length, calculate the modulus of rupture as follows:

If the specimen broke outside the middle third of the span length by more than 5 percent of the span length, discard the results of the test.

5. The report of the test for flexural strength should include the following information:

Values of the modulus of rupture vary widely, depending on the concrete tested. Specification relative to concrete pavements frequently require modulus of rupture in excess of 600 to 650 psi (28-day curing, third-point loading). The flexural strength (modulus of rupture) generally may be expected to be approximately 15 percent of the compressive strength for comparable conditions of age and curing.

An approximate relationship between modulus of rupture and compressive strength can be calculated from the following formula:

fc = compressive strength (in psi)

R= modulus of rupture (in psi).

Figure 13-26.Simplified flow chart showing recovery and refining of petroleum asphaltic materials.