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 1^ 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 1|~ 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. 



(7) It is believed that the following specifications typify the best 

 practice which has been developed in concrete pavement construction. 



