Siegfried Siegesmund - Monument Future

In contrast to the basaltic ashlar (BF and BW), the natural rock material shows low values of surface hardness ranging between 275 and 300 HLD under dry and 261 HLD under wet conditions. A small 260reduction of 5 % at the lowest point. The clastic material (RC) also shows various values ranging from 130 to 450 HLD under dry conditions also depending on the rock clasts. The clasts show values between 250 and 450 HLD. The matrix reaches an average value of around 250 HLD under dry and 227 HLD under wet conditions, which is a reduction of 9 %. Sanding and back-weathering of the matrix is the main weathering form observed on the rock cut architecture (Figure 3d).

Figure 3: a) The floor plan of the monastery. Areas of investigation indicated. b) Architectonic drawing of the investigated church façade. c) Damage mapping. d) Crack formation or scaling of the rock material. d) Typical back-weathering of a porous basalt ashlar.

In some parts of the upper wall active water infiltration takes place. Electrical conductivity and capacity reaches critical values at various parts of the inner walls of the rock cut structure. The exposed rock also shows crust formation and cracks (Figure 3d). These cracks are partly closed by a restoration mortar to prevent water infiltration.

Water uptake by Karsten test pipes show high values for the rock. This also corresponds to the high porosity of the rock material investigated in the laboratory ( Table 1).

The porosity of the basaltic building stones range between 17.4–23.5 % ( Tab. 1). The rock material shows a very high porosity of 30 % (RC) to even 38.2 % for the RF variety ( Tab. 1). The fine variety (RF) contains a microporosity (0.001–0.1 µm) of 10.2 %, whereas the RC variety contains 23.2 % microporosity (Fig. 2 below).

The saturation degree S of all investigated samples is high. The S-value of the RF variety is 0.94 and for the RC variety 0.97. The two basalt varieties show similar values: 0.97 for the basalt sample of the foundation and 0.95 for the sample from the wall.

Directional water uptake of the rock material, however, shows different values. Z means perpendicular to the bedding, X parallel to the bedding and Y parallel to the bedding and parallel to the lamination of the stone material. Water uptake for the clastic rock (RC) sample attains a value of around 11.8 kg/m 2/√h in all directions. Much lower values 261were determined for the fine-grained variety (RF), which show values of 4.6 and 4.5 kg/m 2/√h for the XY direction and 5.2 kg/m 2/√h for the Z direction, thus attaining an anisotropy (A) of 13 %. This tendency could also be shown by the measurements of ultrasonic velocity, expecially in the case of the fine-grained rock variety (RF), ( Tab. 2).

Table 1: Porosity and density of the investigated stone samples.

The rock material shows a different hydric dilatation. Both varieties show remarkable swelling, while the clastic material (RC) with up to 1.04 mm/m reaches high and critical values (Fig. 4). The fine-grained rock material (RF) only shows half of that dilatation, but a higher anisotropy of more than 50 % (Fig. 4).

Ultrasound velocity decreases in both rock varieties under water-saturated condition ( Tab. 2). This particularly affects the RC variety with an average of 38 % and an average of only 7.5 % for the RF variety ( Tab. 2).

Table 2: Ultrasonic velocity of the rock samples.

Based on its ultrasonic velocity ( Tab. 2) and due to the low values of surface hardness, the natural stone used for the Geghard Monastery can be classified as a low bound stone material with an expected compressive strengh of around 10–20 N/ mm 2(Wedekind et al. 2016). Due to its pore size distribution and the high amount of micropores (Fig. 2 g and h), as well as due to the high S-value, the rock material seems to be sensitive to ice- and salt crystallization (Hirschfeld 1912).

The high hydric dilatation as well as the micoporosity of both rock varieties speak for a certain proportion of swellable clay minerals (Wedekind et al. 2013). Both values correlate with each other (compare Fig. 2 below and Fig. 4). Also, the significant decrease in ultrasound velocity under water-saturated conditions suggests the presence of swellable clay minerals and a softening of the rock structure.

Because of the onsite observed weathering forms and the measured values of hydric dilatation, the two rock varieties were treated with a swelling inhibitor and then their hydric dilatation was measured again. After the treatment, the clastic material shows a plain reduction of the hydric dilalation of around 40 %. In the case of the RC variety, a reduction of the anisotropy of the hydric dilatation to nearly zero is remarkable (Fig. 5). Furthermore, the fine-grained material (RF) shows a reduction of more than 50 % (from 0.53 mm/m to 1.8 mm/m) in the Z direction (perpendicular to bedding), whereas the XY-direction shows a lower reduction reaching 0.093 mm/m.

During the consolidation test with the silica sol, gel formation sometimes occurs on the sample surface. This can also be attributed to the pretreatment with the swelling inhibitor. Silica sols are ion-sensitive and can therefore only be used to a very limited extent on saline substrates. And, a swelling inhibitor is actually a saline solution. Consolidation tests with the silica acid esther showed a lower consolidation effect, but also a low darkening of the sample material.

It is striking that there is only a very slight increase in strength, in the case of both consolidants to the two directions parallel to the stratification (XY) ( Tab. 2). The increase is around 8 % for the RF by using KSE and 7.5 % using the silica sol. The 262RC variety reaches less than 1 % in the case of KSE and around 3 % for the silica sol. The strengthening effect perpendicular to the bedding (Z) is much higher for both rock types and consolidants ( Tab. 2). This attains 16 % for the RF and 30 % for the RC variety by using KSE. By using the silica sol, a consolidation effect can be established in the Z-direction for RF with 19 % and for RC at 30 %.

Figure 4: Hydric dilatation of the two rock varieties.