28 BULLETIN" 240, U. S. DEPARTMENT OF AGRICULTURE. 



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 ^ 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 Trill 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 may consist of a small portable tar kettle, a 

 horse and cart, pouring pots, wire brooms, hammers, and stone 

 chisels. 



WTien it becomes necessary to renew any portion 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 



