28 BULLETIN 249, TJ. S. DEPABTMENT OF AGBICULTUEE. 
of sticks, lumps of clay, particles of unsound stone, or other objec- 
tionable material in the aggregates. When cup holes first appear 
they are usually from 1 to 2 inches in diameter and from J to 1 inch 
in depth, but they become gradually enlarged by the action of traffic 
in loosening and abrading the concrete around their edges, and unless 
promptly repaired they may soon have an area of several square feet 
and a considerable depth. The action of traffic also gradually breaks 
away the concrete at the edges of cracks and joints, and if proper 
maintenance is not provided a considerable area of the surface of the 
pavement will be destroyed. The maintenance of cup holes, cracks, 
and joints usually consists of filling them with tar and covering the 
tar with coarse sand, pea gravel, or stone chips. Satisfactory results 
can be secured by this method only when a crew with proper equip- 
ment and materials goes over the road making necessary repairs at 
least two or three times a year. 
Where defects of any considerable size are to be repaired the edges 
should be chiseled down until they are approximately vertical and not 
less than about 1 inch deep. The hole should be thoroughly cleaned 
and painted with tar, after which it should be filled with clean, coarse 
stone chips thoroughly grouted with tar. The surface of the patch 
should then be covered with coarse sand, pea gravel, or fine stone 
chips. 
Either refined water-gas or coal-gas tar may be used for making 
such repairs, and the Office of Public Roads has obtained satisfactory 
results with both kinds. There is some difference of opinion among 
engineers as to just what consistency the tar should possess in order 
to give the best results, but the most general requirement in this 
particular seems to be that the tar when subjected to the float test in 
water at 50° C. will permit the float to sink in about 100 seconds. In 
order to apply a tar of this kind satisfactorily it is necessary that it 
be heated to about 225° F. 
The repair equipment ma}^ consist of a small portable tar kettle, a 
horse and cart, pouring pots, wire brooms, hammers, and stone 
chisels. 
When it becomes necessary to renew any poition of the pavement 
with Portland cement concrete that portion should be entirely closed 
to traffic, and the concrete should be mixed, placed, and cured in the 
manner described in the discussion of construction. The edges of the 
old concrete should be thoroughly cleaned and coated with neat 
cement mortar before the new concrete is placed. 
A properly constructed concrete pavement ought to wear down 
uniformly and develop few defects. Poorly constructed and poorly 
maintained contraction joints are probably responsible for more 
defects of the kind described than can be attributed to any other one 
