ACI Concrete Strength Testing Technician Practice Test

The formula, R = PL/bd², is used to measure the modulus of rupture at which location?

1. In the middle of the specimen

2. On the compression surface

3. On the tension surface within the middle third of the span length

4. Along the edges of the specimen

The correct answer is: On the tension surface within the middle third of the span length

The formula R = PL/bd² is utilized in the context of determining the modulus of rupture for concrete beams under bending stress. Specifically, this formula calculates the modulus of rupture at the location of maximum tensile stress, which occurs on the tension surface of the beam. When a beam is subjected to bending, the bottom portion experiences tension while the top experiences compression. The critical area for evaluating the tensile strength, and thus the modulus of rupture, is located within the middle third of the span length. This is where the tensile stresses are maximized during loading, and therefore, it's the most relevant location for applying the formula. The tensile forces are particularly concentrated in this area due to the bending moment distribution, and testing at this location provides a clear measurement of the material's ability to resist cracking and failure. Other locations, such as the compression surface or along the edges of the specimen, do not accurately represent the tensile forces acting within the beam, making them less appropriate for this calculation.