CONCRETE CONSTRUCTION IN THE PHILIPPINE ISLANDS. 133 



No. 1, very few stones broken. No. 2, a considerable number of broken stones. 

 No. 3, soft mortar and broken stones. No. 4, some stones broken, mortar fair. 

 No. 5, very soft mortar. No. 6, very soft concrete. No. 2 sand gives the highest 

 efficiency, as is shown by No. 5 gravel cubes. 



At another time laboratory and field tests did not agree. 1 :2 :4 

 mixtures when tested after seven days in moist air and twenty-one days 

 in air withstood an average pressure of 1,491 pounds per square inch. 

 Over 50 per cent of the gravel was cracked. That which cracked was 

 andesite containing glassy feldspars in a soft matrix. The 1 :2 :4, 5, and 

 6 mixtures again gave practically the same strength. Later, we received 

 from the contractors concrete cubes which gave higher results. In these 

 cubes all the andesite gravel with glassy feldspars present, cracked, but 

 whereas the original gravel sent to us contained about 75 per cent of this 

 friable andesite, that in the contractors' cubes contained only about 33 

 per cent of this material and some large stones over 24 inches in di- 

 ameter. The cubes had been submerged in water for six days and then 

 exposed to the air, thus producing a very strong mortar. The cone 

 showed that the surrounding mortar was sound in many places where 

 the stone had crushed. 



The facts above mentioned show the necessity of proper representative 

 sampling and a uniform method for the treatment of concrete samples 

 after they have been gauged. The same concrete preserved under 

 different conditions will give variable results. Cubes are sometimes 

 received which have apparently not been wetted since gauging and con- 

 sequently show poor strength when tested, even though the sand, gravel 

 and cement are good and have been mixed properly. A previous inspec- 

 tion of material often enables the engineer to improve the quality by 

 simple methods. A few examples will serve to illustrate this. 



Sometimes is its advisable thoroughly to wash the material. At other times 

 this expense is not necessary. Thus Novaliches sand gave much higher tests 

 washed than unwashed, while that used for the Bocaue Bridge contained less than 

 3 per cent of dirt and fine material and its removal was not advantageous. 



Novaliches Sand (dirt and fine material 5.0 per cent by weight). 



Sand. 



Strength- 1: 2 mortar, 7 days. 



Tensile. 



Compres- 

 sion. 



Per cent 



efficiency 



tensile 



strength. 



Novaliches (washed) 



Novaliches (unwashed) 



Standard quartz 



288 

 224 

 308 



2,485 

 2,087 



93.5 



72.7 







