ESTIMATING THE RELAXATION OF PRESTRESSING TENDONS

The pure relaxation of stress relieved and stabilized (R1 and R2 class) wires was measuredat ambient temperature and at different usual stress levels f= aDjUTS for 5 to abt. 20 thousandhours. In some cases the effect of extreme f levels (0.55 and 0.95) was also studied. The longerdurations were considered to be sufficient to find the constants of three-parameters extrapolat-ing functions. Other test supported the idea that the "final" (ie. practically for 10Gh, abt 114years) relaxation percentages r%) (compared to the initial prestress aD) fit fairly well to straightsof the shape r114 = Af + B, where A and B depend from the type of tendon (R1 or R2, class ofnormal or low relaxation, resp). For standardization purposes an exponential function of twoparameters was chosen to render the well-known S-shaped curve in log t scale. This function wasto cross two points and tends to r114 in infinite time. The two points are: the specified max.relaxation at 1000 h (standards) and the arbitrarily but realistic chosen relaxation of 105 h (abt57 years) r57 ~ 0.95 . r114The values r57 and r114 were compared with the prescriptions of German (FRG) socalled suitabilitv certifications. Hence one can conclude that the r = Af + B straight may bea simplification for some types of tendon in class R2. The two-parameter function seems to beadequate if the time-thickening behaviour of the steel (ie. strainhardening due to segregations)is taken into account by a time variable tc rather than t where the exponent c < 1 makes thefunction to follow the retarded relaxation phenomenon, typical for some low stress levels andsteel products. The functions and the statements concerning pure relaxation and the "final"remaining effective stresses are of course valid only within the limits of maximum stressesallowed by the CEB/FIP Model Code.