PORTLAND CEMENT CONCRETE ROADS. 21 



vertical as little as 6°, this movement was likely to occur and 

 it was found difficult in construction work to avoid even greater 

 variations. 



These findings led to the experiment of building pavements with- 

 out expansion joints, and it was found in pavements so built that the 

 transverse cracks did not occur more frequently than in those built 

 with expansion joints and that the shrinkage due to the setting and 

 initial drying out of the concrete provided sufficient room for such 

 expansion as occurred later from changes in temperature and mois- 

 ture content, except in pavements laid in cold weather. In pave- 

 ments laid in cold weather it appeared that the shrinkage due to set- 

 ting and initial drying out did not provide sufficient space for subse- 

 quent expansion caused by changes in temperature and moisture, and 

 local failures of the pavement were not infrequent.'' Experience, 

 therefore, indicates that transverse joints are unnecessaiy in pave- 

 ments laid when the air temperature is generally above 50° F., but 

 are necessary in pavements laid in cold weather. The tnajority of 

 plain concrete pavements are now constructed without joints. Trans- 

 verse cracks will occur in pavements so constructed at more or less 

 regular intervals, averaging 30 to 50 feet apart. These cracks in 

 general are less objectionable than joints. They do not adversely 

 affect the riding qualities of the pavement, slipping of the slabs rarely 

 occurs, the cost of maintaining them is no greater, and, if properly 

 maintained, they do not materially injure the pavement. 



It is customary to construct transverse joints in reinforced pave- 

 ments. They are generally spaced from. 40 to 80 feet apart. The 

 method most often used in constructing transverse joints is to sep- 

 arate the sections of the pavement by means of specially prepared 

 bituminous felt boards. These are usually held in place by means 

 of properly shaped steel templates until the concrete is deposited 

 against them, after which the templates are removed and the con- 

 crete flows around the boards. The thickness of this joint has varied 

 in common practice from one thicknessi of two-ply tar paper up to 

 about one-half inch. A thickness of one-quarter inch seems to give 

 very satisfactory results when the joints are spaced not more than 

 40 feet apart. Joints of this kind are sometimes provided with metal 

 armor, which is intended to keep the adjacent edges of the concrete 

 from spalling off. It is claimed that armored joints require less 

 maintenance than other types, but they are more expensive to con- 

 struct. As the amount of abrasive traffic on country pavements is 

 steadily decreasing, there does not appear to be any necessity for this 

 type of joint except under unusual conditions. 



« A full discussion of the expansion and contraction of concrete roads may be found in 

 U. S. Department of Agriculture Bulletin 532. 



