22 BULLETIN 1077, U. S. DEPARTMENT 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 not 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. When 
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 
