Structural elements

Natural stone for use in structural stonework should have no trace of decay, splitting of layers or veins of clay. Sandstone and limestone can only be used above ground level; other types of stone can be used both above and below ground level.

Field stones or stones from quarries can be used. Quarry stone can be divided into the following categories:

• Normal quarry stone has been lightly worked.

• Squared stone is produced in rectangular form and has rough surfaces.

• Dressed stone is also rectangular, but the surfaces are smoothly cut.

13.10

Protective dry-stone wall at the Norwegian coast.

The last two types are often called rough and fine-squared stone. If the dimensions of the stone are greater than 20 x 20 x 40 cm, it becomes too heavy to be lifted manually and must be placed by crane. Stone should dry for some time before being used.

Cutting granite, gneiss, sandstone and most slates releases quartz dust, which can cause serious lung damage.

13.11

Dry-stone walling techniques. (a) Cavity wall in field stones is filled with small stones in mortar, clay, lightweight clay aggregate or fossil meal. On the outer leaf the stones should slope outwards so that water runs off. The top of the wall can be protected with slates or a hydraulic mortar. The wall has moderate insulating and moderate to good wind proofing properties as a sheltering wall. (b) Solid wall in field stones can be rendered for stability whilst (c) Solid wall in cut stone is very stable. Both walls (b-c) require additional insulation as house wall. They are best used in foundation walls or foundations to plinths.

Masonry

When building with stone particular care needs to be taken at the wall corners. Normally larger squared stones are placed here, whilst the rest of the wall can consist of smaller lightly worked quarry stones or rubble.

Dry-stone walling demands great accuracy and contact between the stones; it is a real craft (Figures 13.11 and 13.12). The stones have to be placed tightly against each other vertically and through the depth of the wall. Small flat angular stones are put into the joints to fix the stones. Here fully one quarter of the area should have bonders in the form of stones that go through the whole thickness of the wall between the inner and outer leaf.

Dry-stone walling is particularly appropriate for foundation walls as it will prevent any capillary action from occurring - no water can move upwards in such a construction. This form of wall is not windproof. One solution is to have two leaves or parallel walls, with earth or other appropriate fill between them.

Walls bonded with mortar. Many different mortars can be used for masonry (see Table 17.1 in Chapter 17). Generally, lime mortar and cement-lime mortar are the most suitable. The important properties are elasticity and low resistance to moisture penetration, because the stone itself is resistant to moisture penetration. This is especially important for igneous and metamorphic rock species, which can cause condensation problems on the external walls of a normal warm room, no matter which mortar is used. With the exception of marble, sedimentary rocks are best suited for this purpose.

For heated buildings stone is best used for foundations. The exceptions are limestone and sandstone that can well be used for wall construction in milder climates. Both are, however, sensitive to aggressive atmospheres, in the same way as concrete. They are also susceptible to frequent freeze-thaw cycles. These as well as increased precipitation are expected to increase in regions such as Northern Europe and Russia as a result of global climate changes. In other parts of Europe these risks are likely to decrease.

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