Weathering forms

Table 2 Simple classification of weathering forms on buildings...

The focal areas of this book have relatively minor relationships to the primary formation of the Earth s crust which has caused a certain distribution of the chemical elements. They mainly deal with products of the alteration of the crust in geologic processes. We can presently still observe the weathering of solid rocks, the erosion of mountain ridges, and the transport of eroded materials as suspended and dissolved constituents in river and rain water, in ice and wind. In-situ weathering forms soils, and soils are the basis of food production for human nutrition. Therefore, soUs need special protection against the impact of toxic substances (see Part I, Chapters 4 and 5). 

Weathering of the silicate minerals Is primarily a process of hydrolysis. Much of the silica that Is released by weathering forms silicic acid but, when liberated in large quantities, some of it may form colloidal or amorphous silica. Mafic silicates usually decay more rapidly than felsic silicates and, in the process, they release magnesium, iron and lesser amounts of calcium and alkalies. Olivine is particularly unstable, decomposing to form serpentine, which forms talc and carbonates on further weathering. Chlorite is the commonest alteration product of augite (the principal pyroxene) and of hornblende (the principal amphibole). 

The exterior form of MDF is used in special appHcations requiring durabiHty and resistance to water or weather exposure. Highway signs would be an example of this use of exterior MDF. It is an extremely expensive product and thus is used only for special appHcations requiring its special properties. Another example of use would be where a customer would be willing to pay the additional cost to use a composite which has the exceptional quaHties of MDF, but also has virtually no formaldehyde emissions. 

Polymer compositions containing pentaerythritol are also used as secondary heat-, light-, and weather-resistant stabilizers with calcium, zinc, or barium salts, usually as the stearate, as the prime stabilizer. The polymers may be in plastic or fiber form (87—89). 

Sources. Iron ore deposits were formed by many different processes, eg, weathering, sedimentation, hydrothermal, and chemical. Iron ores occur in igneous, metamorphic, and sedimentary deposits. Normally, as-mined iron ore contains 25 to 68% iron. 

In contrast to the sulfide ores, the lateritic ores were formed over long periods of time as a result of weathering of exposed nickel-containing rocks. The lateritic weathering process resulted in nickel solutions that were redeposited elsewhere in the form of oxides or siUcates. One type of laterite is nickeliferous limonitic iron laterite (Ee, Ni)O(OH) which consists primarily of hydrated iron oxide in which the nickel is dispersed in soHd solution. 

Emerson Gumming, Inc. eventuaUy bought the rights to the Sohio process and produced a variety of microspheres. Union Carbide was Hcensed to produce the phenoHc microspheres offered under the name PhenoHc MicrobaUoons (Table 16). When PhenoHc MicrobaUoons are introduced into a cmde-oU storage tank, they form a fluid seal that rises and faUs with the level of the oU. A continuous vapor-barrier seal is formed, which reduces evaporational losses up to 90%. Tests have been conducted under various mechanical and weather conditions and with cmde oUs of varying vapor pressure. 

Aluminum and aluminum alloys are employed in many appHcations because of the abiHty to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. This film is 5 —10 nm thick when formed in air if dismpted it begins to form immediately in most environments. The weathering characteristics of several common aluminum alloy sheet products used for architectural appHcations are shown in Eigure 30. The loss in strength as a result of atmospheric weathering and corrosion is small, and the rate decreases with time. The amount of... 

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

Roofs are a basic element of shelter from inclement weather. Natural or hewn caves, including those of snow or ice, ate early evidence of human endeavors for protection from the cold, wind, rain, and sun. Nomadic people, before the benefits of agriculture had been discovered and housing schemes developed, depended on the availabiUty of natural materials to constmct shelters. Portable shelters, eg, tents, probably appeared early in history. Later, more permanent stmctures were developed from stone and brick. SaUent features depended strongly on the avadabihty of natural materials. The Babylonians used mud to form bricks and tiles that could be bonded with mortars or natural bitumen. Ancient buildings in Egypt were characterized by massive walls of stone and closely spaced columns that carried stone lintels to support a flat roof, often made of stone slabs. 

Sheet form, in widths as agreed upon by purchaser and seUer, composed of asphalt-saturated organic felt coated on both sides with asphalt and surfaced on weather side with mineral granules, except for selvage. Classified, in minimum net mass of granule-surfaced portion, as Type I, 3610 gm/m (74.0 lb/100 fC) and Type II, 3490 gm/m (71.5 lb/100 fC). 

Sheet form, 914 mm (36 in.) in width, or widths agreed upon by purchaser and suppHer, composed of asphalt-saturated organic felt with approximately half the width of weather side coated with asphalt and surfaced with mineral granules, for use as cap sheet in constmction of BUR. Materials covered by this specification, in minimum mass per unit area, are Type I, 1806 g/m (37.0 lb/100 fU) Type II, 2260 g/m (46.3 lb/100 fU) Type III, 1733 g/m (35.5 lb/100 fU) and Type IV, 2090 g/m (42.8 lb/100 fU). 

Cloud Seeding. In 1947, it was demonstrated that silver iodide could initiate ice crystal formation because, in the [ -crystalline form, it is isomorphic with ice crystals. As a result, cloud seeding with silver iodide has been used in weather modifications attempts such as increases and decreases in precipitation (rain or snow) and the dissipation of fog. Optimum conditions for cloud seeding are present when precipitation is possible but the nuclei for the crystalliza tion of water are lacking. 

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