PORTLAND CEMENT CONCRETE PAVEMENTS. 5 



sections have been given identical treatment, a proportionally greater 

 number of cracks have usually formed in the gravel concrete. It has 

 been observed, however, that when some varieties of stone are used as 

 coarse aggregate the resulting concrete shows very little, if any, 

 superiority over gravel concrete as regards the formation of cracks. 

 It therefore seems possible that the quality of stone, rather than the 

 angular shape of the particles, may be responsible for the apparent 

 advantage of crushed stone over gravel. 



There are not sufficient data available to warrant making a definite 

 comparison of the advantages possessed by the different varieties of 

 stone when used as coarse aggregate. But so far as cracks are con- 

 cerned, limestone appears to have made a better record than any 

 other variety of stone which has been used to any considerable extent. 



The coarse aggregate, whether of crushed stone or gravel, should 

 possess at least as great resistance to wear as the mortar which fills 

 the voids between the particles of stone. Any sound stone or gravel, 

 moderately hard and tough, will meet this requirement, but in general 

 the harder and tougher the coarse aggregate, the greater the re- 

 sistance to wear of the concrete. The best available stone should 

 therefore always be used. 



The difficulties experienced in securing a satisfactory quality of 

 coarse aggregate are frequently caused by a lack of proper facilities 

 for preparing the natural materials locally available. There are 

 very few gravel pits which furnish a gravel suitable for use in con- 

 crete pavement construction without washing, and properly equipped 

 washing plants are both difficult and expensive to construct. On the 

 other hand, a great many stone quarries contain pockets of clay or 

 inferior stone which should not be contained in the aggregate, and 

 it is sometimes very difficult to remove these objectionable materials 

 while the stone is being crushed and screened. It is also frequently 

 difficult to screen out the dust of fracture. formed in crushing some 

 varieties of stone. 



It is very desirable that the particles composing the coarse aggre- 

 gate be well graded in size between proper limits in order that the 

 percentage of voids may be as small as practicable. It is convenient 

 to fix the limit of variation by specifying a certain screen upon which 

 coarse aggregate shall all be retained, and another screen which it 

 shall all pass. A J-inch mesh screen for the lower limit and a screen 

 having 1^-inch circular openings for the upper limit have been most 

 frequently specified for coarse aggregate used in concrete pavements. 

 The upper limit of 1| inches seems to be entirely satisfactory in 

 nearly all cases, but the lower limit of |- inch frequently results 

 in a failure to remove as much fine material from the aggregate 

 as is desirable. For example, when the coarse aggregate is se- 



