Stone from fields and beaches lay freely scattered in nature. Throughout the centuries these stones have been used and carefully stored. In Denmark, as recently as the twentieth century, round beach stone was so highly valued that several parts of the coast have been totally emptied. This round stone is particularly suitable for building in or near water, especially for piers. But the possibilities are still relatively limited, since cement has difficulty bonding to smooth stone surfaces (Figure 7.1). For larger buildings these loose stones have usually been cut into rectangular blocks for ease of handling.
Quarry stone has been extracted since the early Middle Ages. The work has been by pure muscle power, chisels, sledgehammers and pickaxes. This method was used well into the twentieth century. The stone quarryman's work is one of the least modernized, despite the introduction of explosives and saws, flame cutting tools and other cutting machinery. Traditionally, stone quarrying has always been based on simple and labour-intensive technology, which couldn't compete with growing industrialization. However, in many countries with low and medium industrialization, stone can cost as little as a quarter of the price of concrete. In highly industrialized countries there are signs of improved competition as part of an aesthetic and qualitative trend. A significant factor that will strengthen the case for using local stone is that, in conventional concrete production the amount of energy used comprises 25-70% of the price of the product, and is likely to increase.
Extraction methods for various types of stone vary slightly, but the main principles are as follows.
Reconnaissance. The rock is inspected and samples are taken and tested for damp absorption, strength, etc. It is important to split the rock without cracking it or causing it to crumble or disintegrate. Layered and slate-like rock is the least problematic, but the distance between splits should not be too small. Rock of the same structure is often evaluated by the sound it makes when hit with a hammer, and by the splinters or angular forms that split off.
Stone need to go through two further tests: for water absorption and heat resistance. The water test involves leaving the stone in water for several days and checking that it does not increase in weight. To test heat resistance the rock is placed in glowing coals and must retain its form and structure when raked out afterwards. A good roof slate passes both tests. Another condition is that a white surface film should not form when exposed to air and moisture.
Quarrying. The surface of a rock is cleared of trees, loose stones, earth and all other organic matter. Holes are drilled for charges. Placement of these is determined by the thickness of the block and the layer formation. The depth of the hole is also important. A 'rimmer' is knocked into the hole. This makes ruts in the wall of the hole along which the block will crack. The hole is then filled with gunpowder, rather than dynamite. Gunpowder has a lower rate of burning and gives a more muted explosion. Dynamite causes microscopic hairline cracks in the blocks that decrease their strength, although for crushed stone this is of no consequence.
Soft stones such as marble, limestone and soapstone can, in many cases, be removed with a wire saw. This consists of a long line of diamonds that cut 20-40 cm per hour. For rock rich in quartz, such as granite, a jet flame can be used. The equipment for this is a nozzle mounted on a pipe with paraffin or diesel under pressure. The temperature of the flame is about 2400 °C, and the speed is high. A jet flame can cut out about 1-1.5 m3 of stone blocks per hour.
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