Page 400 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.62 THE WORK

FIGURE 8.49 Cutting a joint. (Courtesy of Portland Cement
Association.)

Fully hardened soil-cement is shaved back to solid material with a grader blade or disc, with hand-
tool assistance as necessary, cleaned of loose material, and moistened, after which the new mix is placed
against it with some excess.
Curing. The freshly finished soil-cement surface must be protected from drying. The standard
method is to cover it with a thin bituminous coating, as soon as possible after completion.
The surface should be damp, and free of any loose material. Cutbacks RC-250, MC-250, and
emulsion RS-2 are suitable curing materials.
Sanding is required if traffic is to use the surface before the asphalt has set. The soil-cement
can then safely carry loads that are not greater than the compaction equipment used, but low speeds
are advisable for a few days.
Asphalt is the most convenient curing material, but anything that will hold moisture will do.
Plastic or waterproof paper sheets, wet cloth, straw, or sand, or membrane spray can be used.

Surface. A soil-cement surface is ordinarily too friable and absorbent to stand up under exposure
to traffic. It is therefore customary to apply a regular bituminous surface on top of the curing coat.
This may be placed immediately, or delayed a month or more to allow shrinkage cracks to develop.
For light traffic, a single-seal coat may be sufficient. A double-seal coat with a total thickness of
3 1
⁄ 4 inch is suitable for general use, except in snow areas. There, 1 ⁄ 2 inches of plant mix blacktop is
recommended, for assurance against peeling by snow plows.
A single-seal coat may be applied immediately, and a second coat or blacktop put on several
months later, preferably in cool weather, to bridge over shrinkage cracks.
These cracks are inevitable in soil-cement, as the hydration process causes shrinkage. However,
breaks are so irregular that the edges support each other, so structural weakening is slight. The
principal damage is to the appearance of the surface, which can be minimized by delaying the top
coat until after they have formed.

SOIL-CEMENT BASE

Soil-cement base for concrete pavement is constructed in the manner just described, except that
curing is likely to be done with some nonbituminous material.
Getting correct surface grade is critical, and may be somewhat difficult. If a gravel or other base
is found to be off-grade just before concrete is laid, correction is made rather easily by removing
material or bringing it in. With hardened soil-cement this is not practical, so great care must be
taken to get it right.