22 BULLETIN 1077, U, S. DEPAKTMENT OF AGRICULTURE. 



The use of longitudinal joints along the central axis of the road 

 has generally been confined to pavements exceeding 20 feet in width. 

 Where such joints have been used, it has been customary to construct 

 one-half of the pavement width at one operation. After this portion 

 of the pavement has been completed, the remaining half portion is 

 constructed. The edges of the longitudinal joint are rounded with 

 an edging tool, and after curing the joint is filled with bituminous 

 material. The method of constructing a pavement in two half sec- 

 tions is particularly advantageous on some heavily traveled roads 

 where it is nof possible to divert the traffic. The construction of a 

 pavement of this type can be carried on without diverting the traffic, 

 although the operations of the contractor are hampered somewhat, 

 resulting in slightly increased costs. 



It has not been general practice to use a longitudinal joint in 

 the construction of pavements 16 to 20 feet wide, when the full 

 width of pavement has been constructed at one operation, but there 

 are several, arguments in favor of this form of construction. From 

 observation of a large mileage of concrete pavements, it is found 

 that longitudinal cracks rarely occur in pavements 9 or 10 feet wide, 

 but frequently occur in pavements exceeding 16 feet in width. It is 

 reasonable to assume, therefore, that a longitudinal joint along the 

 central axis of the pavement would practically eliminate cracking. 



Longitudinal cracks are more objectionable than transverse cracks 

 because they have a tendency to gradually increase in width. Wlien 

 they occur along the line of wheel traffic the edges of the cracks 

 deteriorate rapidly unless carefully maintained. Another important 

 advantage of a longitudinal joint along the central axis of the road 

 is that it serves to define sharply the limits of travel in each direc- 

 tion, thus providing a desirable factor of safety for road travel. 



A longitudinal joint for full-width pavement construction should 

 be of the submerged type. A joint of this type usually extends from 

 the bottom of the pavement to within approximately three-fourth 

 inch of the surface. The purpose of the submerged joint is to facili- 

 tate and simplify the operations of striking, tamping, and finishing 

 the surface of the pavement, which would otherwise be rather difficult 

 with the joint extending through the pavement. A strip of 18 or 

 20 gauge metal, held rigidly in place by pins driven into the sub- 

 grade, will usually prove satisfactory. The metal should prefer- 

 ably be corrugated or deformed sheets so as to key the two sections 

 together. Reinforcing steel should be used to tie the two sec- 

 tions of the pavement together and prevent any lateral movement. 

 The reinforcing steel should be placed halfway between the top 

 and bottom of the slab. The practice of the Illinois highway de- 

 partment is to use five-eighths-inch deformed bars, 5 feet long, spaced 



