20 BULLETIN 373, U. S, DEPARTMENT OF AGRICULTURE. 



cushions may be advantageously omitted if proper longitudinal 

 cushions are provided. The principal objection to the use of trans- 

 verse expansion cushions is based on the fact that the material com- 

 posing the cushions frequently softens during warm weather and 

 runs out toward the curb, thus leaving the edges of the adjoining 

 brick exposed to destructive impact from the wheels of passing 

 vehicles. Even if the cushion consists of a material which does not 

 run in warm weather, it is necessarily softer than the brick, and the 

 natural result is still the development of unevenness in its immediate 

 vicinity. No such objection can exist concerning longitudinal ex- 

 pansion cushions if they are placed adjacent to the curbs and con- 

 structed of proper material. They not only furnish a means for the 

 pavement to expand and contract with changes in temperature, but 

 they also eliminate to a large extent the disagreeable rumbling which 

 has been so frequently associated with grout-filled brick pavements. 



The bituminous material of which the expansion cushions are made 

 should be such as to remain firm in summer and not to become brittle 

 in winter. It should also possess the quality of durability. In order 

 to insure that any given material is suited for such a purpose, it is 

 usually considered necessary to prescribe certain laboratory require- 

 ments to which it must conform, and examples of these, which have 

 been found to give good results, are contained in the section entitled 

 "Typical specifications." (Gf. p. 26 et seq.) 



Expansion cushions should be provided for at the time the brick 

 are laid. This may be done by placing a board of the required thick- 

 ness on edge adjacent to each curb, as shown in figure 3. Small iron 

 wedges, such as are shown in this figure, may be inserted between the 

 curb and the board at the time the board is set. These wedges may 

 be readily loosened and removed after the brick have been laid and 

 grouted, and may consequently be made to facilitate the removal of 

 the board which provides space for the bituminous filler. If pre- 

 ferred, a bituminous felt board may be satisfactorily substituted for 

 the poured cushion just described. 



The proper thickness for expansion cushions is a matter concerning 

 which much difference of opinion exists among highway engineers. 

 Some engineers advocate a minimum thickness of 1 inch, while others 

 claim to have secured their best results by using expansion cushions 

 having a minimum thickness as low as three-eighths inch for very 

 narrow pavements. It is generally agreed that the thickness of the 

 cushion should vary with the width of the pavement. The following 

 suggestions for proportioning the cushion are offered as being fairly 

 representative of the best practice. 



