MATERIALS FOE CONCRETE CONSTRUCTION IN MANILA. 471 



The variation of the efficiency in the Sangley, Pasig and Quingua sands, 

 all of which are pebble sands, is probably largely due to the difference in 

 the character of the rock fragments which compose them. Andesitic and 

 basaltic materials are conspicuous in the Pasig sands and basaltic grains 

 constitute nearly all of the rock material in the Sangley sands, while the 

 Quingua contains harder materials which do not so readily decompose and 

 become spongy or porous, but under the action of water are gradually 

 worn down to smaller sizes by attrition and abrasion. The efficiency of 

 the Sangley sand, which is the lowest, may be further accounted for by the 

 presence of many minute shells Arhich do not fill with cement. 



The fact that the Quingua sand is more efficient than the Grant and 

 Tarlac, is probably due to the shape of the grains, which being rounded 

 decreases the voids. Moreover, the vitreous surfaces of the crystal frag- 

 ments in the Oraniand Tarlac sands may also cause them to wet less 

 readily than the grains of the Quingua, which have minutely pitted 

 surfaces and capillary openings. 



A comj)arison of the efficiency of the Pasig and Orani sands was also 

 made by crushing concrete blocks which were made in a uniform way, 

 using the same cement and Pasig- gravel in each, but different sands. 

 In order to follow working conditions the sands were both screened 

 through a -1-mesh sieve. The blocks were stored twenty-five days in 

 moist air and afterwards exposed to the sun and rain. They were crushed 

 at the end of three months and gave the following results : Pasig sand, 

 1:3:5: mixture, average of three breaks; first crack, 912 pounds per 

 square inch; ultimate, 1,543 pounds per square inch. Orani sand, 1:3:5 

 mixture, average of three breaks; first crack, 1,568 pounds per square 

 inch; ultimate, 1,951 pounds per square inch. 



Specific gravity, percentage of voids and weights per cubic meter. — 

 Samples of the various sands were screened through a 10-mesh sieve and 

 their specific gravities, percentage of voids and weights per cubic meter 

 determined as shown in the following table : 



Sand. 



Specific 

 gravity. 



Percentage 

 of voids. 



Weight per 

 cubic 

 meter in I 

 kilograms, j 



Pasig 



Quingua _. 

 Sangley — 



Orani 



Corregidor 



2.67 

 2.71 

 2.58 

 2.62 

 2.77 



37.5 

 35.2 

 42.0 

 42.9 

 41.7 



1,671 

 1,757 

 1,500 

 1,500 

 1,615 



Photographs of these samples are shown in Plates I and II. 



