Bar Post-Tensioning System
Modulus of Elasticity
The modulus of elasticity E is an intrinsic property of steel whose magnitude remains basically constant and is little affected by normal variations in mill processes. For THREADBARS®, this value has been determined to be 29,700 ksi (205 MPa).
A typical stress-strain curve for a stretched and stress relieved bar is substantially different from a typical curve produced for a cold drawn, cold threaded bar. Samples of each are illustrated below.The most notable feature is the lack of a definite yield point characteristic of cold drawn bars.
Relaxation is defined as the loss of prestress load in a post-tensioning steel subjected to a specified initial stress while maintaining the length and the temperature constant. Relaxation tests are usually conducted at an initial load equal to 70% of the strand's actual ultimate strength (see chart below). The tension loss after 1,000 hrs for a THREADBAR® initially stressed to 70% of guaranteed ultimate strength can be assumed between 1.5 and 2%.Tests indicate that the relaxation losses in cold drawn, cold threaded bars are significantly higher.
Under normal circumstances, fatigue is not a primary design consideration for prestressing steels. However, all DYWIDAG bars and accessories have been tested and proven to exceed the fatigue requirements specified by the Post-Tensioning Institute.
Tests have demonstrated that no significant loss of strength occurs when bars are subjected to elevated temperatures up to 1,100 degrees F (593° C). Only the yield strength is reduced when temperatures exceed approximately 750 degrees F (399° C). Bar ductility is not significantly affected by temperatures down to -60 degrees F (-51° C).
Susceptibility to stress corrosion cracking and hydrogen embrittlement
All prestressing steel is susceptible to stress corrosion cracking and hydrogen embrittlement in aggressive environments and therefore must be properly protected. However, accelerated tests have demonstrated that while A 416 strand failed after 5 to 7 hours, bars still held their load when testing was discontinued at 200 hours.
The deformations on the DYWIDAG THREADBAR® exceed the deformation requirements of A 615. Consequently, bond strength is at least equivalent to A 615 reinforcing bars.
High strength bars are not usually used to resist transverse shear loads. However, their untensioned shear strength is similar to that of any other steel.