3. Assume the behavior of a creeping material to be represented by the Burger's body given below. The material parameters are: E = 20 GPa, n = 10,000 GPa days, E = 50 GPa, 12=5,000 GPa-days. E E2 (a) Find a mathematical expression for the Creep Compliance J in terms of time in days (Note: the unit should be /MPa.) (b) The material is placed under the following stress history (stress unit is MPa): Stress 20 10 Time (day) 0 50 100 Derive expressions for the strain for: (i) 0 < time < 50 days (ii) 50time < 100 days (iii) time 100 days Specifically, what are the strain values? (i) at time = 0 (ii) at 50 days, before and after loading increase (iii) at 100 days, before and after unloading Draw, schematically, the variation of strain with time up to 150 days. 4. A specimen of concrete column (200 x 200 mm) of 1.5 m long has 4 steel rebars (20 mm in diameter) inside with same length. The specimen was loaded with a constant load of 600 kN under 100% relative humidity. Calculate the initial stress in steel and concrete. If the basic creep for plain concrete at 100 days is 600 10-6, and the measured stress in steel at 100 days for this column is 160 MPa, estimate the percentage of creep restrained by steel. (Young's modulus of steel is 210 GPa, the modulus of elasticity of concrete is 25 GPa.) Also, calculate the length change of the specimen.