Die deutsche Hausindustrie

The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1003351123

URN:: urn:nbn:de:zbw-retromon-16006

Document type:: Monograph

Author:: Koch, Heinrich

Title:: Die deutsche Hausindustrie

Edition:: Zweite, bedeutend erweiterte Auflage

Place of publication:: M. Gladbach

Publisher:: Volksvereins-Verlag GmbH.

Year of publication:: 1913

Scope:: 1 Online-Ressource (294 Seiten)

Digitisation:: 2017

Collection:: Economics Books

Usage license:: Get license information via the feedback formular.

Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Achtes Kapitel. Hilfe von aussenstehenden Kreisen

Collection:: Economics Books

BUILDING STONES AND ROAD METALS 183 The stones used in macadam must be tough so as to resist compression and sudden blows. The best road metals are igneous rocks in which the constituents are intergrown, and especially those containing a fibrous or prismatic mineral such as hornblende. The pyroxenes are less useful as their grains are torn apart, and they yield more readily along the cleavage. Large felspars also break along the cleavages. Among the best of the acid rocks are the granophyres in which the base consists of a fine-grained micropegmatitic intergrowth of quartz and felspar. The basic rocks have the advantage that being heavy, pieces are less easily dis- placed, though a given weight of stone covers a smaller area ; those with an ophitic structure, such as dolerite, are the best. The sedimentary rocks are usually less satisfactory as their rounded grains are easily torn out of the cement, and powdered quartz has a low cementing value. Limestone, though soft, has the advantage that its powder acts as a natural cement. Coarse gritty sandstones, such as the gray- wacke of the Southern Uplands of Scotland and the harder seams in the Old Red Sandstone of the North of Scotland serve as fair road metal; they are known as whin—a term given to any rock that was difficult to quarry or resisted decay into soil. The term is now often used as if applicable only to igneous rocks. An important factor in road metal is its adherence to tar and bitumen; hence granite is unsuitable for macadam, as if overheated the cleavages open on cooling and the rock becomes friable ; if inadequately heated the tar peels off and does not bind the material properly under the vibration of traffic. Road metal is tested by two methods—abrasion against a revolving iron plate, and the attrition test, by loss of weight when road metal is rotated on a cylinder. Love- grove’s test (Attrition Tests of Road-making Stones, with Petrological Descriptions, by J. S. Flett and J. A. Howe, 1905), often adopted in the London district, determines the per- centage of material that will pass through a sieve with spaces one-sixteenth of an inch in diameter after 16 pieces of the stone which together weigh 4 Ib. have undergone 5 hours rotation. at the rate of 20 revolutions per minute, in an

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