14 BULLETIN" 249, U. S. DEPARTMENT OF AGRICULTURE. 



condition of the subgrade at the time the concrete is placed upon it, 

 and the method employed in curing the concrete. It is common prac- 

 tice to space the transverse joints from 25 to 50 feet. 



If there were no curves in the alignment, or summits in the grade 

 of a road, it is doubtful if any provision for expansion would be 

 necessary in constructing the joints, because the elasticity of the 

 concrete should be sufficient to take care of the expansion caused by 

 changes in temperature and moisture content. In nearly all cases, 

 however, there are curves in alignment and changes in grade which 

 might permit a displacement of the pavement before a very high 

 compressive stress was developed. For this reason it is advisable 

 that joints be constructed to provide for a slight amount of expan- 

 sion as well as for contraction. 



There are a number of different methods of constructing joints, 

 but none of them appear to be entirely satisfactory from every 

 standpoint. Probably the simplest type of joint is that made by 

 introducing into the pavement a board about five-eighths inch 

 thick and shaped to conform with the cross section. This board is 

 held in place by means of stakes until the concrete is placed against 

 it on both sides. The stakes are then removed and the board is left 

 in place with its upper edge even with the surface of the pavement 

 and its lower edge resting upon the subgrade. The principal objec- 

 tions to this joint are that the board wears rather rapidly and does 

 not protect the adjacent edges of the concrete. 



A second method is to form a plane of weakness by placing a board 

 so that its top edge is about 3 inches below the surface of the pave- 

 ment. Then, when the contraction of the concrete has caused a crack 

 to form immediately over the board, the crack is filled with bitumi- 

 nous material. This joint is said to have proved very satisfactory for 

 dense concrete where the distance between joints is comparatively 

 small, but it is subject to the objection that compressive stresses de- 

 veloped by expansion of the concrete are likely to be concentrated 

 in the upper part of the pavement and to cause spalling at the joints. 



Another method is to use a board, such as that first described, which 

 is removed before the concrete has taken its final set. The opening 

 thus left is later filled with bituminous material. The principal diffi- 

 culty with this method is that when the board is withdrawn the adja- 

 cent edges of the concrete are usually disturbed and a rough joint is 

 produced. 



Probably the method most often used in constructing joints is to 

 separate the successive sections of the pavement by means of spe- 

 cially 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 concrete flows around the boards. The thickness of this 



