void ratios of 20% and 15%, dripping of paste at the bottom
surface was observed as shown in the circles in
Fig. 6. In these cases, voids at bottom portion are much less
than the other two portions as a result of high content and
high flow of paste. This is not desirable and need to be prevented
as porous nature of concrete at bottom part is
destroyed.
4.2.3. Void at bottom surface
The results of compressive strengths and state of the
voids at the bottom surface are shown in Fig. 7. The compressive
strengths of concrete decrease as void ratios
increase. The strengths of porous concretes with designed
void ratios of 15%, 20% and 25% are between 38–44, 29–
35 and 15–22 MPa, respectively. At full compaction, void
patterns at the bottom surface of concrete cylinders are different
depending on the designed void ratios and flow values
of paste. For the designed low void ratio of 15%, the
P15F3 mix with paste of high flow value of 230 mm shows
low void at the bottom surface of concrete cylinder. Examination
of the surface reveals that concrete is filled with
paste and little void is left. Mix P15F2 with paste of moderate
flow value of 190 mm also contains relatively low
voids at the bottom surface. The low voids are caused by
relatively high flow of paste and high paste content. Under
full vibration, paste with tendency to flow downward starts
to drip and fills the voids resulting in non-porous concrete
at the bottom part of concrete. Although the strengths of
mixes P15F3 and P15F2 are high at 38 and 44 MPa, they
are not good porous concretes as the porous characteristics
are not maintained at the bottom of concretes. Mix P15F1
with paste of low flow value of 150 mm, however, shows
appropriate void distributed across the bottom surface. It
is a good porous concrete with relatively high strength of
39 MPa.
For the designed moderate void ratio of 20%, the P20F3
mix with paste of high flow value shows low void at the
bottom surface. The strength of this mix is high at