M30 grade concrete is often used for post-tensioned members, whereas M40 grade concrete is typically used for pretensioned members. The losses in prestress members owing to various causes range from 250 N/mm2 to 400 N/mm2. Thus, a post-tensioning cable with 50,000 psi (354 MPa) breaking strain will require a minimum diameter of 1/8 inch (3.2 mm). For a pretensioned member, the required cable diameter would be 1/4 inch (6.4 mm). Concrete has high tensile strength in its early stages of development and increases in length before it begins to stretch out. This is called "creep" and occurs because of two factors: thermal expansion and chemical hardening. As the concrete heats up during daylight hours, it expands slightly; this can be as much as 8 percent of its original size. Concrete also hardens over time when water evaporates or chemical agents are added; this process also causes the material to expand. If unconfined creep is not taken into account, then concrete posts will likely fail after only 20 years instead of the expected 100. To prevent this, steel bars are placed within the concrete at regular intervals to provide support and hold it in place while the material creeps away from the first barrow beam.