PORTLAND CEMENT CONCRETE PAVEMENTS. 29 
cause. For this reason the contraction joints should be given very 
careful attention at the time of construction. 
It has been claimed that the difficulty involved in properly main- 
taining defects in joints and cracks and the inconvenience attending 
periodic renewals of the pavement may be largely eliminated by 
maintaining a bituminous wearing surface over the concrete. Until 
further improvements are made in this method of treating concrete 
pavements, however, no specific recommendations can be made. 
CONCLUSION. 
In concluding this discussion of concrete roads the principal points 
may be summarized as follows : 
(1) The economic efficiency of concrete roads is undetermined at 
present, but the indications are that this type of construction will 
prove to be well suited for certain conditions. 
(2) The one-course type of concrete pavement is greatly to be pre- 
ferred to the two-course type, but there are conditions under which 
the adoption of the two-course type of construction may be justified. 
(3) The proportion of cement to the sand and coarse aggregate 
combined should not be less than about 1 to 5, and the proportion of 
sand to coarse aggregate should not be less than 1J to 3, nor greater 
than 2 to 3. Ordinarily, when gravel is used as coarse aggregate, the 
proportions may be made 1 part of cement to 1J parts of sand to 
3 parts of gravel, and when crushed stone is used as coarse aggregate, 
1 part of cement to If parts of sand to 3 parts of crushed stone. 
(4) All types of contraction joints which have yet been devised 
require careful and frequent attention in order to prevent rapid 
deterioration of the pavement in their vicinity. It appears that bet- 
ter results are obtained by spacing the joints at an angle of about 75° 
to the center line of the road than when they are placed at an angle 
of 90°. 
(5) Thin bituminous wearing surfaces for concrete pavements can 
not be economically justified at present. It is possible that through 
experimental investigations some method of constructing such surfaces 
to give uniformly satisfactory results may yet be devised. If this is 
done, the maintenance of concrete pavements and the contraction- 
joint problem will be greatly simplified. 
(6) Intelligent engineering supervision is absolutely essential in 
concrete pavement construction, because defective materials or work- 
manship can not be readily repaired after the pavement is completed, 
and they are not usually apparent until the pavement has been in use 
for some time. 
(T) It is believed that the following specifications typify the best 
practice which has been developed in concrete pavement construction. 
