Among them, the most tightly related with durability are
capillaries and macropores. Regardless of their size, not all
pores contribute in the same degree to the penetration of
aggressive agents into the concrete. An isolated hollow
might have in¯uence in the material resistance as well as
in its fracture, but not in the penetration of aggressive agents
since it has no contact with the exterior.
Due to the complexity of concrete, it was decided to start
analysing a simpler substance like mortar. Two basic
elements are found in the composition of mortar: sand and
cement. Besides, considering the possible apparition of
pores into the cement, we can model mortar as a mixed
material composed by three phases: sand, cement and
pores (Fig. 1). Nevertheless, for a ®rst approach, two-
phase models have been chosen because of their major
simplicity. Ultrasonic signals propagate through an environ-
ment constituted by solid material (one of the phases
composed by sand plus cement mass) and a number of
scatters formed by pores.
On the other hand, the research was undertaken from two
different approaches: