CONCRETE CREEP IN THE CASE OF VARIABLE STRESS, PRACTICAL APPROXIMATION METHODS

FOR AN INCREASING NUMBER OF CONCRETE STRUCTURES IT IS IMPORTANT TO MAKE A FORECAST OF CREEP AND SHRINKAGE EFFECTS. IN THE CASE OF PRESTRESSED CONCRETE STRUCTURES, MORE ESPECIALLY CANTILEVERED BRIDGES, A TIME-RELATED BEHAVIOUR CALCULATION IS INDISPENSABLE. STANDARDS SPECIFY THE INFLUENCE COEFFICIENT DEFINED FOR A CONSTANT STRESS. HOWEVER STRESS IN CONCRETE OFTEN VARIES WITH TIME; THE RESULTING DEFORMATION IS SUPERIMPOSED ON THE CREEP DEFORMATIONS CORRESPONDING TO THE STRESSES IMPOSED AT DIFFERENT MOMENTS. IN ORDER TO APPLY THE SUPERIMPOSITION PRINCIPLE IT IS NECESSARY TO USE NUMERICAL TECHNIQUES. THERE EXIST SEVERAL METHODS THAT SEEK TO AVOID NUMERICAL TECHNIQUES BY USING APPROXIMATIONS. THE LATTER ARE DISCUSSED AND COMPARED. PRACTICAL TABLES ARE APPENDED WHICH ILLUSTRATES THE AGE-ADJUSTED EFFECTIVE MODULUS METHOD THAT COMBINES SIMPLICITY AND ACCURACY. EXAMPLES ILLUSTRATE THE APPLICATION OF APPROXIMATE METHODS AND COMPARE THEIR RESULTS WITH THOSE OBTAINED WITH THE EXACT NUMERICAL METHOD.