Instantaneous Stiffness of Cracked Reinforced Concrete Including Steel-Concrete Interface Damage and Long-Term Effects

In reinforced concrete construction, deflection control is an important performance criterion for its serviceability and sustainability. In this paper, an experimental program aimed at modeling the effects of both short-term and sustained loading on the instantaneous stiffness of reinforced concrete flexural members is described. In the first part of the paper, experimental results are discussed showing that both steel-concrete interface damage (i.e., cover-controlled cracking) attributable to excessive live loading and time-dependent effects such as creep and shrinkage have a major influence on the instantaneous stiffness of beams. In the second part of the paper, a finite-element model that accounts for the influence of cover-controlled cracking on the instantaneous stiffness is proposed. Only the short-term response to load is modeled in this paper. The effect of cover-controlled cracking is taken into account by implementing a damage variable to reduce the bond at the steel-concrete interface as loading is increased or during a period of sustained loading and shrinkage. In addition, a criterion for the initiation of cover-control cracks based on a peak value of the steel stress at the crack location is also defined.