Brittle and quasi-brittle materials are characterized by the presence of defects that can nucleate, grow, and coalesce, causing catastrophic failures in full-sized structural components. In this context, extensive research studies can be found in the current scientific literature due to the increasing interest in microcracks interaction and in its influence on the mechanical performance of cementitious materials. In the framework of Linear Elastic Fracture Mechanics, this Thesis aims to investigate the damage evolution in quasi-brittle solids characterized by a dominant macrocrack and a distribution of microcracks. Dual Boundary Element Method (DBEM) has been used to describe the propagation of a main crack and its interaction with surrounding microcracks in concrete beams subjected to tensile and flexural loadings.