122 REIBLING. 



upon small variations during manufacture and the subsequent exposure. 

 Whenever possible, samples of concrete should be taken at the construc- 

 tion work, preserved and tested according to specific instructions given 

 to the engineers and compared with results obtained at the laboratory. 

 Also, it would be advisable, with no great outlay of funds as compared 

 with the results to be obtained, to have each district engineer examine 

 the sand and gravel beds of his territory and send a few barrels of the 

 most promising material to the laboratory for investigation. A record 

 of short and long time tests, together with other information as to the 

 location, quantity, shipping facilities, labor conditions, etc., would cer- 

 tainly be of much value for future use. Construction could be most 

 economically specified, and contractors could bid more accurately. The 

 demand for this information and its economic value will subsequently 

 be illustrated briefly by a few references to work already accomplished 

 in this laboratory with the apparatus at our disposal. 



THE GRAVEL AND STONE USED IN CONCRETE CONSTRUCTION. 



The main bulk of concrete is composed of gravel or crushed stone. 

 The more of the latter materials which can be used per unit volume, 

 the cheaper will be tbe concrete. The voids in this aggregate are 

 filled, and the individual stones are bound together by the mortar. The 

 strength of the concrete depends primarily upon : First, the strength 

 of the mortar; second, the efficiency of the gravel or stone; and third, 

 the density of the concrete. 



In order properly to specify the relative volumes of cement, sand, and 

 gravel for concrete it is necessary to know enough about the raw materials 

 to "formulate the best and most economical mixture suitable for the 

 purpose. The use of more than enough mortar to fill the voids and 

 cover the gravel adds to the cost and subtracts from the strength. 

 Even in a well-proportioned concrete the ultimate strength is often 

 limited by the texture or strength of the coarse aggregate. 



EXPERIMENTAL WORK ON GRAVEL AND SAND. 



Pasig Eiver gravel, so much used in Manila, has often proved to be 

 of inferior quality. Much of it is so weathered and decomposed that 

 it cracks under the force of a very light blow. Its low efficiency' is shown 

 in the following results obtained on 6 inch cubes constructed according 

 to the method described in Bulletin No. 329 of the United States Geo- 

 logical Survey: 



