PORTLAND CEMENT CONCRETE PAVEMENTS. 

 Table I. — Granulometric analysis of quartz sand. 1 



Size of grains. 



Grams. 



Per cent. 



Retained on — 



0.0 



74.0 



124.5 



266.0 



460.0 



624.0 



930.5 



1,139.5 



1, 159. 5 



1, 198. 5 



51.5 



0.0 





5.9 





10.0 





21.2 





36.7 





49.8 





74.3 





91.1 





92.7 





96.5 





3.5 











.100.0 



i Total weight of sample, 1,250 grams; weight of sample after washing, 1,208 grams. 



Experience has shown that when first-class sand is used very good 

 results are obtained by using a proportion of 1 part of cement to 1^ or 

 If parts of sand and making the proportion of coarse aggregate such 

 that the resulting concrete will contain slightly more mortar than is 

 sufficient to fill all voids. If a well-graded gravel is used as coarse 

 aggregate, the proportion should be about 1 : H : 3, while in most cases 

 where broken stone is used as coarse aggregate it will be found desir- 

 able to make the proportion about 1 : If : 3, and in some cases, where 

 the particles of stone are of uniform size, even a still greater propor- 

 tion of mortar will be required, but this should be effected by de- 

 creasing the amount of coarse aggregate and not by further increas- 

 ing the amount of sand. 



Since the bottom course of a two-course pavement is not subjected 

 to the wearing action of traffic, it would appear that the rules for 

 proportioning the materials for this course might be considerably 

 modified. On the other hand, using different proportions in the top 

 and bottom courses undoubtedly results in the concrete of the two 

 courses having different coefficients of expansion and different moduli 

 of elasticity, and these differences might tend to cause a separation 

 of the two courses. The fact that such separations sometimes occur 

 strengthens this theoretical objection. 



CONSTRUCTION. 



There are two general types of concrete pavement, known as the 

 one course and the two course. These designations are due to 

 the fact that the former consists of one course of concrete, all of 

 which is mixed in the same proportion and composed of the same 

 kind of materials, while the latter consists of two courses of concrete, 

 usually mixed in different proportions and containing different kinds 

 of aggregate. Plate X, figure 1, shows a typical cross section for a 



