82 HIGHWAY CONSTRUCTION 



sand or other incoherent material; and when the concrete is replaced 

 and set, the covering may be reset at its proper level, without the un- 

 certain allowance for settlement which is necessary in other cases. 



Thickness of Concrete. The thickness of the concrete bed must 

 be proportioned by the engineer; it should be sufficient to provide 

 against breaking under transverse strain caused by the settlement of 

 the subsoil. On a well-drained soil, six inches will be found sufficient; 

 but in moist and clayey soils, twelve inches will not be excessive. On 

 such soils a layer of sand or gravel, spread and compacted before pla- 

 cing the concrete, will be found very beneficial. 



The proportions of the ingredients for concrete used for pavement 

 foundations are usually: 



1 part Portland cement 

 3 parts Sand 

 7 parts Broken Stone. 

 Or, 



1 part Natural Hydraulic Cement 



2 parts Sand 



5 parts Broken Stone. 



The question is sometimes raised as to whether Natural or Port- 

 land cement should be used. Natural cement is more extensively 

 employed on account of its being cheaper in price than Portland. 

 There is no advantage gained in using Portland cement. Concrete 

 should not be laid when the temperature falls below 32 F. 



The concrete foundation, after completion, should be allowed to 

 remain several days before the pavement is placed upon it, in order 

 that the mortar may become entirely set. During setting, the con- 

 crete should be protected from the drying action of the sun and 

 wind, and should be kept damp to prevent the formation of drying 



STONE BLOCK PAVEMENTS 



Stone blocks are commonly employed for pavements where traffic 

 is heavy. The material of which the blocks are made should possess 

 sufficient hardness to resist the abrasive action of traffic, and sufficient 

 toughness to prevent them from being broken by the impact of loaded 

 wheels. The hardest stones will not necessarily give the best re- 

 sults in the pavement, since a very hard stone usually wears smooth 

 and becomes slippery. The edges of the block chip off, and the 



