Detection of internal defects in concrete structures is a difficult task as these anomalies are not always observable at the surface, yet have the potential to expand and damage the structure. The focus of this work is to locate and characterize inclined cracks inside a concrete mass which is essential in monitoring the integrity of many civil structures. For this purpose, three concrete slabs were constructed each having a different sub-horizontal crack. To obtain high resolution images of the concrete interior, an extension of the ultrasonic technique known as SAFT is used. SAFT has shown great potential to produce detailed 3D images of tendon ducts, holes and flaws inside concrete structures. The results of this study show that cracks with angles varying from 5 to 15 degrees can be accurately located inside a concrete slab having a thickness of up to 200 mm.