20 BULLETIN 3*73, 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. Xo 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." (Cf. 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 . 
