Читать книгу Monument Future - Siegfried Siegesmund - Страница 69

Thermal dilatation was tested in two heating cycles each in both a dry and wet environment. The samples CVO, CVE and CAE show pronounced differences between the first and second heating cycle (Fig. 4). All samples expand during heating and then shrink again during cooling. The samples from Oaxaca and Etla have negative residual strain, especially after the second heating cycle. At about 70–80 °C the samples shrink significantly. The samples from Mitla reach their initial state after cooling and the first and second heating cycle are similar. CAE has the highest thermal expansion with 0.78–0.87 mm/m. The tuff varieties from Oaxaca and Etla have the lowest thermal expansion (0.17–0.22 mm/m).

After two dry heating cycles the samples were saturated with water and heated in two wet heating cycles. After the first hour in a wet environment the samples already showed intense hydric expansion with a maximum of more than 2 mm/m. After that first intense expansion the samples 57underwent a thermohydric dilatation. For thermohydric dilatation CRO and CRE show much higher anisotropies than for just thermal dilatation. The highest thermohydric expansion was measured for CVO (2.90 mm/m) and CVE (1.98–2.25 mm/m). The samples from Mitla have the lowest thermohydric expansion.

Figure 4: Thermal and thermohydric expansion.

The second dry cycle shows a significant decrease of residual strain, with negative values representing thermal contraction. After the first wet heating cycle the samples show a decrease in residual strain. Especially CVO, CVE and CAE show this feature. CAO and CAE do not recover from contraction after the second drying cycle. The Mitla samples are rather unaffected by both thermal and thermohydric dilatation.

Figure 5: Maximum hydric expansion values.

Figure 5 shows the hydric expansion of all tested samples. CVO expands the most of all samples, when in contact with water. The maximal hydric expansion is reached by CVO in the Z-direction with 2.36 mm/m. The smallest hydric expansion was measured for MR with 0.04 mm/m in the X-direction. The sample with the highest anisotropy is MR with 73 %. Most samples show higher hydric expansion in the Z-direction, except for CAO, CAE and MG. All samples reach maximal hydric expansion already during the first hours.

The effect of salt weathering on all samples is presented in Figure 6. Most of the samples show a weight increase at the beginning of the salt bursting test. The most resistant sample is MR, while CRE, CRO and CVO show low resistance towards salt weathering. Table 1 shows the number of salt cycles until at least 30 % weight loss is reached.