Lame's constants [4]. In addition, some works reach an
analytic relation between acoustic velocity and porosity in
composite materials [5]. These later methods will be used in
our case for mortar.
To apply a micro-mechanic model to the mortar, we
consider the material to be formed by a solid phase, which
occupy a volume V m plus the pores, which occupy a volume
V i
V m 1 V i 1. We also consider that the capillary pores
(of a size inferior to the micron) have a cylindrical and
extended shape, and that its distribution is random in the
solid phase. Starting from these hypotheses, an expression
can be achieved that relates the velocity of propagation in
the medium with porosity [6] as follows:
mv f
Ci;j ; Tklmn ; r; X
1
where v indicates the propagation velocity of the acoustic
mwaves. Ci;j determine the constants of elasticity of the solid
phase, Tklmn indicates the tensor terms which contain the
size, distribution and orientation of pores in the material,
r is the density and X, the porosity percentage.
The average velocity has been measured on the six probes
of each group, by means of two transducers operating in
transmission, in dry, and using a contact of rubber. Three
points per probe have been measured. Fig. 4a shows, for a
1 MHz transducer, the distribution of measures per groups
of probes, and the standard deviation of measured values in
each group. As can be seen, the velocity of propagation is a
parameter, capable of discerning the groups of probes with
different water/cement percentages. Fig. 4b shows the
curves that relate porosity and velocity regarding the
micro-mechanic model. Each curve has been obtained by
®nding the parameters of the model (1) for each water±
cement percentage group of probes. For ®nding the model
parameters, we need the average porosity measure obtained
by NDE ultrasonic methods in the same group. Note in Fig.
4b that each group produces a different model ®tting,
although the curves are close enough to give a promising
method for a precise measure of porosity. On them have also
been drawn the points obtained from the measure of poros-
ity by destructive methods (circles) and measure of velocity
by means of ultrasonic (£). The destructive technique for
measuring the porosity is the one described in the standard
ASTM C 642-90 of the American Society for Testing and
Materials.
4. Characterisation based on attenuation of material
Acoustic attenuation has been used to characterise ma-
terials by ultrasonic means. In particular, this method has
been used to estimate pore density and its size in ceramic
materials [7]. For the present case, we consider mortar to be
composed of a solid phase and of pores whose average
diameter Dm is remarkably inferior to the micron. As a
whole, the mortar will act as a propagative material,
whose scatters are found in Rayleigh zone, characterised