Two popular models that come to mind are known as the Bazant model and Hillerborg's model.
The Bazant model is analogous to Hillerborg's model however the strains
across the crack are analysed instead of the crack opening. The stress-strain relations describe the micro-cracking which occurs in the crack.
For details, check out:
[1] Bazant Z.P., Oh B.H., Crack Band Theory for Fracture of Concrete, Material and Structures, V.16 pages 155-177, 1983
and
[2] Hillerborg A., Modeer R.M., Petersson P.E., Analysis of Crack Formation and Crack Growth in Concrete by Means of Fracture Mechanics and Finite Elements, Cement and Concrete Research, V. 6 pages 773-782, Division of Building Materials Lund Institute of Technology, Sweden, 1976