SECTION XIII 



REENFORCED CONCRETE 



94. Physical properties. The use of concrete dates from the 

 time of the Romans, who obtained a good artificial stone from a 

 mixture of slaked lime, volcanic dust, sand, and broken stone. The 

 modern use of concrete, however, is of comparatively recent develop- 

 ment, its universal use being a matter of only the last quarter of a 

 century, while reenforced concrete is of still more recent origin. 



Concrete is made by mixing broken stone, varying in size from 

 a walnut to a hen's egg, with clean, coarse sand and Portland cement, 

 using enough water to make a mixture of the consistency of heavy 

 cream. The proportion of these three materials depends on their 

 relative size ; in general, enough sand being needed to fill the voids 

 in the broken stone and enough cement to fill the voids in the sand. 

 The cement and water cause the mass to begin to stiffen in about 

 half an hour, and in from ten to twenty-four hours it becomes hard 

 enough to resist pressure with the thumb. In a month the mixture 

 becomes thoroughly hard, although the hardness continues gradually 

 to increase for some time. 



Portland cement was invented by Joseph Aspdin of Leeds, Eng- 

 land, who took out a patent for its manufacture in 1824, the name 

 Portland being due to its resemblance to a popular limestone 

 quarried in the Isle of Portland. Its manufacture was begun in 

 1825, but its use did not become general until 1850, when the 

 French and the Germans became active in its scientific production 

 and succeeded in greatly improving both the method of manu- 

 facture and the quality of the finished product. Portland cement 

 was first brought to the United States in 1865, but not until 1896 

 did its annual domestic production reach a million barrels. 



When ordinary limestone (calcium carbonate) is heated to about 

 800 F., carbon dioxide is driven off, leaving an oxide of calcium 

 called quicklime. This has a great affinity for water, and when 



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