ESTIMATING THE RELAXATION OF PRESTRESSING TENDONS

Abstract

The pure relaxation of stress relieved and stabilized (R1 and R2 class) wires was measured at ambient temperature and at different usual stress levels f= aDjUTS for 5 to abt. 20 thousand hours. In some cases the effect of extreme f levels (0.55 and 0.95) was also studied. The longer durations 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 114 years) relaxation percentages r%) (compared to the initial prestress aD) fit fairly well to straights of the shape r114 = Af + B, where A and B depend from the type of tendon (R1 or R2, class of normal or low relaxation, resp). For standardization purposes an exponential function of two parameters was chosen to render the well-known S-shaped curve in log t scale. This function was to 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 (abt 57 years) r57 ~ 0.95 . r114 The values r57 and r114 were compared with the prescriptions of German (FRG) so called suitabilitv certifications. Hence one can conclude that the r = Af + B straight may be a simplification for some types of tendon in class R2. The two-parameter function seems to be adequate 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 the function to follow the retarded relaxation phenomenon, typical for some low stress levels and steel 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 stresses allowed by the CEB/FIP Model Code.