The procedures for conducting the flexural-strengthtest are as follows:1. Assemble the loading device, as shown in figure13-24. Turn the test beam so that the finished surface isto the side and centered in the loading assembly. Operatethe testing apparatus until the loading blocks are broughtinto contact with the upper surface of the beam. Be sureto secure full contact between the loading (andsupporting) surfaces and the beam. If the surface of thespecimen 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 increasein extreme fiber stress in the beam is between 125 and175 pounds per square inch per minute. The extremefiber stress corresponding to any load maybe estimatedfrom the equation given in Step 4a below. Obtainreadings on the proving-ring dial, and convert them tocorresponding total loads in pounds by applying theproving-ring constant. Aside from the reading used tocontrol the rate of application of the load, the onlyreading necessary is the one that corresponds to themaximum load applied to the beam.3. After the specimen has broken, obtain dimen-sions of the cross section at which failure occurred tothe nearest 0.1 inch. These dimensions represent theaverage width and average depth of the section infailure.4. The flexural strength, expressed in terms ofmodulus of rupture, is given in psi, and can becalculated as follows:a. If the specimen broke within the middle thirdof the span length, use the following equation:Where: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 middlethird of the span length by not more than 5 percent ofthe span length, calculate the modulus of rupture asfollows:Where:R=P=a=b=d=c.modulus of rupture (in psi)maximum applied load (in pounds)distance (in inches) between the line offracture and the nearest support, measuredalong the center line of the bottom surfaceof the beamaverage width of specimen (in inches)average depth of specimen (in inches)If the specimen broke outside the middlethird of the span length by more than 5 percent of thespan length, discard the results of the test.5. The report of the test for flexural strength shouldinclude the following information:a.b.c.d.e.f.g.h.Identification numberAverage width to the nearest 0.1 inchAverage depth to the nearest 0.1 inchSpan lengthMaximum applied loadModulus of rupture to the nearest 5 psiDefects in specimenAge of specimenValues of the modulus of rupture vary widely,depending on the concrete tested. Specification relativeto concrete pavements frequently require modulus ofrupture in excess of 600 to 650 psi (28-day curing,third-point loading). The flexural strength (modulus ofrupture) generally may be expected to be approximately15 percent of the compressive strength for comparableconditions of age and curing.An approximate relationship between modulus ofrupture and compressive strength can be calculated fromthe following formula:Where:f ´_{c}= compressive strength (in psi)R = modulus of rupture (in psi).13-33

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