136 



REIBLING. 



mixture is already specified and the ratio of mortar to gravel already 

 efficient, a further reduction of the voids in the gravel by means of sand 

 can only result in lowering the quality of the concrete. 



For example, in one instance a 1:2:5 concrete was specified, but the gravel 

 being used contained 22.1 per cent sand (fine material passing 0.51 centimeter 

 rectangular openings ) . Therefore, the actual mixture employed was far inferior 

 to that intended by the specifications. This is shown by the following figurfes : 



Gravel. 



Sand. 



Voids. 



Relative 

 volumes 

 2/5 sand 



+ 1 

 gravel 



Actual pro- 

 portions, by 

 volume, of a 

 1:2:5 mix- 

 ture. 



Cement- 

 sand 

 ratio. 



Mortar- 

 gravel 

 ratio. 



Volume 

 of excess 

 mortar. 



As received. _ 



Per cent. 

 22.1 

 



Per cent. 

 29.1 

 33.9 



1.25 



1. 215 



1:3.1:4.8 

 1:2 : 5 



1:3.1 

 1:2 



2.86 : 4.8 

 2.16:5 



1.41 

 0.44 



Screened on 4-mesh sieve. 



The ratio of cement to sand was decreased and the mortar strength was 

 weakened accordingly. The volume of mortar became so excessive that it 

 separated the gravel or flushed to the surface. The volume of concrete per unit of 

 cement was increased and therefore the strength of the concrete was decreased. 



Unfortunately, it is impossible to specify a fixed mixture under the 

 assumption that the fine material is uniformly distributed in such a way 

 as to make adequate allowance for the sand present in gravel or broken 

 stone, since subsequent rehandling and weathering tend to concentrate 

 the fine material from the coarse. 



Carefully selected, representative samples taken at different times from one 

 large gravel pile showed variations in sand contents of 4.3, 8.0, 13.0, 20, and 

 24.8 per cent. The gravel taken from the bottom of this pile contained the 

 most sand, ana concrete taken from the mixer when it was being used had less 

 strength at the end of eighty days than a like mixture of the same cement, sand 

 and gravel (except that the sand was removed from the gravel) at only twenty- 

 nine days, as the following results will show: 



Pasig sand and gravel concrete. 



Dimensions, in 

 inches. 



Propor- 

 tions, by 

 volume. 



Age. 



Total strength, 

 in pounds. 



Strength in 

 pounds, per 

 square inch. 



Mark on cubes. 



In 



moist 



air. 



In air. 



First 

 crack. 



Ulti- 

 mate. 



First 

 crack. 



Ulti- 

 mate. 



6 y 6 6 



1:2:5 



Days. 



7 



7 



7 



Days. 

 22 



73 



73 



46, 770 

 27, 720 

 16, 390 



52, 450 

 32, 705 

 44, 125 



1,300 



770 

 455 



1,457 



908 



1,225- 



No sand in the 



gravel. 

 Consi d e rable 



sand in gravel. 1 

 Do. 



* The 80-day cubes showed few broken stones, but the mortar was weak and porous. 



