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.
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FIGURE 5 Correlation between 0-2 nm pore volume and compressive strength
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FIGURE 6 Correlation between 2-500 nm pore volume and compressive strength
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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.