City roads and pavements suited to cities of moderate size. . or slide under passing loads,and thus loosen all the fragments which touch them. Further, these rounded cobble-stones which werestrewed broadcast over parts of the country during theglacial period, came from the most widely differentlocalities in the northern part of the continent andinclude all varieties and degrees of hardness. Granite, syenite, quartz, limestone, flint and slatewere found to make up one-tenth of a mass of them, ofwhich the remaining nine-tenths were sandstone, ofwhich at least one-half were so disintegrated or we
City roads and pavements suited to cities of moderate size. . or slide under passing loads,and thus loosen all the fragments which touch them. Further, these rounded cobble-stones which werestrewed broadcast over parts of the country during theglacial period, came from the most widely differentlocalities in the northern part of the continent andinclude all varieties and degrees of hardness. Granite, syenite, quartz, limestone, flint and slatewere found to make up one-tenth of a mass of them, ofwhich the remaining nine-tenths were sandstone, ofwhich at least one-half were so disintegrated or weather-worn—so rotten, as the workmen call them—as tobe worthless for any purpose: for road-surface metalthey are worse than worthless, as their only effect is todestroy the good material with which they chance to bemixed. Crushed cobble-stones may be selected to form thelower or base course, if nothing better is available, byrejecting all which are inferior and by selecting, to becrushed and screened, only the hardest and best. 144 TESTS OF ROAD 145 TESTS OF ROCK FOR ROAD-MAKING. The various rocks available for road-making arecompared as to their relative endurance, by subjectingsimilar sets of samples of each kind to similar abrasionin machines like that here shown, which was devisedby Deval in 1878. Each set of samples consists ofeleven pounds, or five kilograms, of roughly cubicalselected fragments, none smaller in any way than oneand one-quarter inches, nor larger than two and one-half inches. These are cleaned, washed, dried andaccurately weighed, and enclosed in one of the cylindersand tightly sealed. Similar sets of samples are put ineach cylinder and the whole machine is then slowlyrevolved at the rate of 2,000 revolutions per hour forfive hours, or until a cyclometer registers 10,000revolutions. The fine dust worn from each set of samples is thensaved for cementation tests, and the fragments arewashed, dried and again weighed: comparison of thepercentage of
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