PORTLAND CEMENT CONCRETE ROADS. 59 
u, about 100 seconds. In order to apply a tar of this kind satisf ac- 
>rily, it is necessary that it be heated to about 225° F. 
I The repair equipment may consist of a small portable tar kettle, 
light truck, pouring pots, wire brooms, hammers, and stone chisels. 
The tar kettle is usually hauled by attaching it to the rear of the 
truck. 
AY hen it becomes necessary to renew any portion of the pavement 
with concrete, that portion should be entirely closed to traffic, and the 
concrete should be mixed, placed, and cured in the same way as a 
new pavement. 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 uni- 
formly and develop few defects. Poorly constructed and poorly 
maintained joints are probably responsible for more defects of the 
kind described than can be attributed to any other one cause. For 
this reason the joints should receive very careful attention at the time 
of construction. 
RESURFACING OLD CONCRETE PAVEMENTS. 
Under certain traffic conditions it may be necessary at tmes to 
resurface old concrete pavements so as to provide an additional thick - 
less of pavement for the increased traffic. The thickness of the 
resurfacing layer should be not less than 3 or 4 inches at any point, 
and the concrete should be mixed in the proportions of 1 : 1J : 3, using 
a coarse aggregate graded from J inch to 1^ inches in size. Steel re- 
inforcement weighing at least 25 pounds per 100 square feet, placed 
in the middle of the resurfacing layer, should preferably be used 
where the resurfacing is to be 3 inches thick. If the resurfacing 
layer is to be 4 inches thick, it is not believed that any reinforcement 
is necessary. Where an old concrete pavement is to be resurfaced, 
it should be thoroughly cleaned and all the bituminous filling used 
to cover cracks and small holes removed. The new concrete is placed 
and finished in the manner previously described for concrete pave- 
ments. Joints should be provided in the resurfacing layer directly 
over those in pavement below. The service records of a number of 
resurfaced concrete pavements indicate that it is immaterial whether 
or not a bond is secured between the two layers of the concrete. 
