FIGURE 4 Stress analysis of pores under external force
As a brittle material with many pores, AC crushing is mainly caused by
the stress concentration formed near the pores under the external
pressure.[19] The sharp pore tip of AC provides
good conditions for stress concentration. In order to clarify the
mechanism of the effects of pore opening and closing states on AC
compressive strength, the stress of the open pore and closed pore are
analyzed. As shown in Figure 4, when the external pressure is put on,
the pore tip of the open pore will generate a strong stress
concentration, so that the tensile stress at the tip is several times
the average tensile stress on AC force surface. When the tensile stress
at the tip is above the AC critical tensile stress, the open pore crack
extend along the tip, and then the AC break. However, it is difficult to
generate strong stress concentration at the tip of round closed pore.
The stress at the tip of slit-type pore is shared by both tips. The
stress on tips of closed pores will be significantly lower than that of
open pores. So, when large number of open pores exist, the closed pore
is not easy to become the weak region of AC compressive strength.
Therefore, the AC is easily broken at the open pores, and the open pores
themselves are the destructive factors of the compressive strength, not
by destroying its skeleton.
2.1.2 Effect of pore size distribution on compressive strength
The linear fitting correlations between AC compressive strength and the
pore volume of 0-2 nm, 2-500 nm and above 500 nm are shown in Figure 5 -
Figure 7. The 0-2 nm and 2-500 nm pore volume are similar, but the
former fitting R2 significantly greater, and the slope
of the fitted line is larger, so the 2-500 nm pores have a more
significant deterioration on the compressive strength. The above 500 nm
pore volume is significantly larger than the pore volume of 0-2 nm and
2-500 nm, but the correlation between above 500 nm pore volume and the
compressive strength is weakest, so above 500 nm pore has a relatively
small damage to compressive strength.
[CHART]
FIGURE 5 Correlation between 0-2 nm pore volume and compressive
strength
[CHART]
FIGURE 6 Correlation between 2-500 nm pore volume and
compressive strength
[CHART]
FIGURE 7 Correlation between above 500 nm pore volume and
compressive strength
It can be known from above analysis that the stress concentration plays
an important role in the AC crushing. The stronger the stress
concentration at the pore, and the more pore where the strong stress
concentration can takes place, the smaller the compressive strength of
AC. As an index to characterize the stress concentration degree, the
stress concentration coefficient can be calculated numerically by
equation (3). In addition to the shape of pore, the K is also
positively correlated with the pore diameter and ratio of length to
diameter [20]. For a single pore, the larger the
pore diameter and the ratio of length to diameter are, the greater theK will be. However, when the pore volume is constant, the larger
the pore diameter is, the smaller the pores number will be. Although the
pores of above 500 nm have a strong damage on compressive strength, the
negative influence on the compressive strength is small because the
pores number is small. Under the same pore volume, the 0-2 nm pores
number is thousands of times of 2-500 nm pores number. But as shown in
Figure 8, the micropore diameter and the ratio of length to width is
relatively small, so the K is smaller, thereby the impact on the
compressive strength is less.