Answer:

Introduction:
In the field of civil engineering, tensioning is a method used to apply a pre-load or initial tension to reinforce concrete structures. This pre-load helps to counteract the tensile stresses that the structure may experience during its service life. Anchorage is an essential component of the tensioning system, as it is responsible for transferring the applied force to the concrete member. However, it is crucial to ensure that the maximum tensile stress immediately behind the anchorage does not exceed a certain limit to prevent any structural failure.

Maximum Tensile Stress:
The maximum tensile stress refers to the highest level of stress experienced by the material when subjected to an applied force. In the case of initial tensioning, the maximum tensile stress occurs at the point where the force is applied, which is immediately behind the anchorage.

Ultimate Tensile Stress:
Ultimate tensile stress is the maximum stress that a material can withstand before it fails or breaks. It is an important parameter used to determine the design strength of structural elements. The ultimate tensile stress varies depending on the type of material being used.

Limiting the Maximum Tensile Stress:
To ensure the safe and reliable performance of tensioned concrete structures, it is necessary to limit the maximum tensile stress immediately behind the anchorage. This is because exceeding the material's ultimate tensile stress can lead to structural failure, such as the concrete cracking or the anchorage pulling out.

Correct Option:
According to the given options, the maximum tensile stress immediately behind the anchorage should not exceed 0.80 times the ultimate tensile stress. This means that the maximum tensile stress should be limited to 80% of the ultimate tensile stress. By following this criterion, the risk of structural failure can be minimized, providing a safety factor for the design.

Conclusion:
In conclusion, during the initial tensioning of concrete structures, it is important to limit the maximum tensile stress immediately behind the anchorage to prevent structural failure. The correct option states that this maximum tensile stress should not exceed 0.80 times the ultimate tensile stress. By adhering to this criterion, the safety and reliability of the tensioned concrete structures can be ensured.