The power of a bomb explosion to destroy architectural heritage is staggering [34, 35]. Architectural heritage that survives a war and bomb damage continues to be exposed to the sun, frost, damp and other environmental stresses. But a single bomb attack can do as much to weather a building as thousands of years of sun and freezing combined. Bomb explosions cause craters and fractures in a building’s walls. On the wall facing James Street West, there are about 80 visible craters of different sizes and depths, visible area of crater / wall area = 7.76%; on the wall facing Kingsmead North, there are about 60 visible craters of different sizes and depths, visible area of crater / wall area = 11.49%. Cracks will inevitably develop over time, leading to an increase of the permeability coefficient and permeability generally. The RSH and permeametry will differ radically based on the proximity of impacts as well as deterioration associated with ground water rise up to the lead line. In the case of the Bath Labour Exchange, the permeability of the wall stones can be compared from the data in Table 4 and Fig. 4D, which show that the minimum permeability value is 0.0024 mD and the maximum permeability is 13,800 mD. The RSH measurements can be used to determine relative deterioration of the surfaces. As Table 2 and Fig. 4D show that the minimum value of the walls is 200 L and maximum value of is 592 L. It is possible that fractures resulting in a discernible reduction of hardness in the stone have wider micro-fracture networks associated with them [7, 9].
After the bomb exploded at ground level, some of the shrapnel hit the wall, on the one hand, causing a strong impact, forming many craters; on the other hand, the bomb carried burning material, causing the wall to be affected by fire [36, 37]. As shown in Table 3, Table 5 and Fig. 6A, bomb explosions and combustion are the main factors affecting the loss of RSH and enlargement of pore-size of the craters. According to the field investigations and measurements, some of the wall craters are caused by shrapnel directly impacting the wall, and some are caused by other objects hitting the wall of the building due to the formation of fragments caused by the explosion. The high RSH-value areas of two walls are distributed on the upper wall facing Kingsmead N, and under the window near west corner of the wall facing James St W. The low RSH-value areas were distributed at the openings of the door and windows of the wall facing James St W, and near the corner of the wall facing Kingsmead N. The high permeametry-value areas of two walls were distributed on the lower wall facing Kingsmead N, and near the corner of the wall facing Kingsmead N. The low permeametry-value areas were both distributed at east corner of the wall facing James St W.
Architectural heritage after a bomb explosion is more prone to weathering problems (Fig. 8A). These are mainly the results of bomb explosion and combustion, as well as later pollution and weathering. As shown in Figs. 3 and 8, the common forms of weathering are cracks, spalling, staining and biological growth. This is because the explosion has a great impact force, resulting in loose texture, reduced hardness, increased permeability of the Bath stone, and a greater susceptibility to weathering and disintegration in water. Under the action of the external environment for a long time, the internal physical stress of the stone is unevenly distributed. Small cracks form first in the stone, and then these extend to create longer cracks [38, 39]. With a larger area of cracks, the surface stone is subjected to spalling. At the same time, when the humidity reaches a certain level, liverworts and even more substantial plants may grow in cracks.
At the same time, combustion after an explosion is another main reason for the formation of a loose, porous texture, reduced hardness and increased permeability of the Bath stone. Burning at high temperatures can also cause thermal cracking of rocks [40]. As shown in Fig. 7B, according to the microscope study, the main component of the Bath stone is calcite, and its honey colour is related to the presence of iron oxides-hydroxides (goethite-limonite), derived from the oxidation of pyrite [2, 8]. Calcite (calcium carbonate) is insoluble in neutral pH water but is affected by more acidic water, such as rainwater, which causes etching of the limestone. On being heated, as in a fire, calcite can decompose: CaCO3 = CaO + CO2↑ [41, 42], making the limestone readily affected by further weathering by rain and frost. The staining of buildings is also related to explosions and post-explosion combustion (Fig. 3). Bomb explosions can generate temperatures up 5,000 oC; the presence of presence of hydrated iron oxides in Bath stone are altered to ferric oxide (Fe2O3), giving the building walls a reddish-pinkish stain. A temperature higher than 1300°C favours the oxidation of hydrated iron oxides to ferric oxide (Fe2O3)[41, 42].
It is also interesting to note that there are many craters in which the hardness of the inner centre is higher than that of the crater edge, and the permeability of the inner part is lower than that of the crater edge (Fig. 6B). This phenomenon is referred to as Reverse Weathering [43, 44]. A (black) crust can occur on lime containing materials in the presence of SOX (e.g. from pollution) leading to a transformation of calcite into gypsum. The crust can acquire a blackish colour, due to encapsulation of dirt and soot into gypsum. The chemical equation for the formation of the crust is CaCO3 + H2O + H2SO4 = CaSO4 · 2H2O (gypsum) + CO2↑ [41, 42]. According to Stephenson's theory, a thin film of water formed by condensation can hold more dust particles with polluting properties, such as sulphur dioxide, together [45, 46]. The speed of dust concentration depends on the temperature difference between the wall and the air in contact with it. Thus, we find that the most frequent crusts are found in cold, wet craters where water is easily condensed. The black crust continues to feed more gypsum into the porous rock further inside the crater, and the gypsum continues to invade deeper into the rock. With the increase in the thickness of the gypsum layer, the crater has a layer of reverse weathering crust inside, and it is more difficult to be weathered (Fig. 8B). Encrustation also takes place locally on Bath stone in damp areas, as a result of microbial activity and the presence of a biofilm; these are mostly grey crusts of thin, crinkly calcite. Both gypsum and calcite crusts and efflorescent, as well as green microbial-algae growths, can develop near the ground of Bath stone buildings as a result of rising damp.