For the analysis of concrete structures the effect of creep and shrinkage must be considered for checking their instability and serviceability.
There are two different methods of analyzing a section for time-dependent effects using the aging coefficient. One uses the equilibrium equation and compatibility equation. Meanwhile, the other uses the creep transformed section. The first method was used for analyzing a prestressed concrete member with reinforcing steel and a composite steel-concrete member. In the prestressed concrete member, bottom non-prestressed steel restrains the deformation of concrete and results in reduction of prestress loss and camber (or increase in downward deflection) due to creep and shrinkage of concrete. In the composite steel-concrete member, the web reinforcements do not affect greatly time-dependent effects. The second method is particularly convienient in solving the complex problem of the time-dependent stresses and deformations. Thus, as an example, a composite precast-cast-in-situ structure was analyzed by the second method.
The effective modulus approach is used to calculated the time dependent deflections resulting from the creep of cracked reinforced concrete beams. As an example, doubly reinforced concrete beam is analyzed. The change in time-dependent deflection and the change in stress at tension steel decrease as the compression reinforcement ratio increases. But, the change in the stress in concrete at its top ifbre increases as the compression reinforcement ratio increases.
Both ACI model and BP model were used for analyzing the time-dependent behavior of structures, and ACI model gives more time-dependent deflection than BP bodel does.
