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 l^-inch circular openings for the upper limit have been most 
frequently specified for coarse aggregate used in concrete pavements. 
The upper limit of \\ inches seems to be entirely satisfactory in 
nearly all cases, but the lower limit of J 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- 
