ACI Concrete Field Testing Technician - Grade I Practice Exam

How is the top of the concrete specimen identified for slump measurement?

1. By the top edge of the mold

2. By the displaced original center of the top surface

3. By the lowest point of the specimen

4. By the color change of the concrete

The correct answer is: By the displaced original center of the top surface

The correct method to identify the top of the concrete specimen for slump measurement is by the displaced original center of the top surface. This approach ensures that the slump is measured accurately and consistently. When a concrete sample is formed in a cone mold, the top surface becomes displaced during the removal of the mold. The true top of the sample is assessed based on this displacement, which represents the highest point of the concrete after the mold has been removed. This is critical because the slump test measures the vertical deformation of the concrete when the mold is removed, reflecting its workability. By identifying the top surface correctly, any potential inaccuracies that could arise from using the mold's edge or other references are minimized, ensuring that the slump value obtained indicates the actual flow characteristics of the concrete.