22 BULLETIN 23. U. S. DEPARTMENT OP AGRICULTURE. 
Stone curbing. — All stone curbing shall have the shape and size shown 
on the plans, and shall be hauled and set before the subgrade is com- 
pleted. The stone used shall be hard, tough, and of a homogeneous 
texture, and no section of curbing shall have a length less than 4 
feet. Stone curbing shall be set true to line and grade, and shall be 
securely bedded in either broken stone or gravel. 
Concrete curbing. — All concrete curbing shall be constructed before 
the subgrade is completed. It shall have the cross section shown on 
the plans, and shall be composed of Portland cement concrete, mixed 
in the proportion 1 part of cement, 2 parts of sand, 4 parts of broken 
stone or washed gravel, and sufficient water to make a quaky mixture. 
The specifications as to quality of the materials, mixing and placing 
the concrete, etc.. hereinafter given under "Concrete foundation," 
shall apply also to concrete curbs. 
Forms for concrete curbs shall be constructed of dressed lumber, 
and the curb shall be constructed in sections not less than 4 feet nor 
exceeding 12 feet in length. When complete the curb shall present 
a smooth, neat, uniform appearance, and shall be true to fine and 
grade. 
Marginal curb.— Marginal curbs shall be constructed at the ends 
of the pavement and at all intersections with other roads and drive- 
ways. The marginal curbs shall conform to the specifications given 
above for the kind of curbing employed, except that they shall be 
shaped to conform to the cross section of the pavement. 
Broken stone or gravel foundation. — If a broken stone or gravel 
foundation is called for. it shall be constructed of sound, durable 
material, and to the compacted depth shown on the plans. The 
material shall be well graded in size between that which will just be 
retained on a screen having |-inch circular openings and that which 
will just pass a screen having 1^-inch circular openings. After the 
broken stone, or gravel has been spread, sufficient clean sand or stone 
screenings to fill the voids shall be spread over it and flushed in by 
means of sprinklhig and rolling. When completed the foundation 
shall be well compacted, free from depressions, and uniform in grade 
and cross section. 
Concrete foundation. — If a concrete foundation is called for. it shall 
be constructed to the depth shown on the plans in the following 
manner : 
The subgrade shall be completed for a distance of at least 50 feet 
in advance of the foundation work. The foundation shall be con- 
structed of Portland cement concrete mixed in the proportions 1 part 
of Portland cement, 3 parts of sand, 6 parts of broken stone, and 
sufficient water to bring the mass to a condition coinnionly described 
as quaky. The concrete shall be thoroughly mixed to the satisfac- 
tion of the engineer, either by hand or in a mechanical mixer approved 
