One of the underlying principles of fracture mechanicsis that unstable fracture occurs when the stress intensityfactor at the crack tip reaches a critical value. This valuefor Mode I deformation and small crack--tip plastic deformationis designated K . K represents the inherent ability of alc lcmaterial to withstand a given stress-field intensity at thecrack tip and to resist progressive tensile crack extensionunder plane strain conditions. An experimental and predictioninvestigation of Mode I crack propagation in concrete areperformed. The object is to study the fracture behavior ofplain and reinforced concrete of beam type member under three-point bending. It is found that, plain concrete fractureoccurs when the applied stress intensity factor reaches acritical value, and this value does not represent a materialproperty of concrete. The reinforced concrete beams fail whenthe stress intensity factor at the crack tip reaches theeffective plane strain fracture toughness of plain concrete.The presence of steel reinforcement creates a closing effectto the crack, and the closing stress intensity factor, K , duesto reinforcing by one bar represents the minimum value ofclosing across the cross-section of the beam. The value of Ksincreases with increasing area of steel, crack length and bondstrength. It is also found that the stability of fractureprocess is greatly affected by area of steel and bond strengthbetween concrete and steel.Good agreement between fracture mechanicsprediction and the experimental results indicatessuitability of fracture mechanics solution forinitiation in lightly reinforced concrete sections.modelthecrack