A good road or pavement requires a good foundation. The pavement base provides the thickness and stiffness necessary to support traffic and provide long-term performance.
Stabilized pavement bases, such as soil-cement and cement-treated base, have provided economical, long-lasting pavement foundations for more than 70 years. Soil cement's improved strength and durability, combined with its low first cost and ease of construction, make it an outstanding value for use as a base and subbase material.
Soil-cement is a simple, highly compacted mixture of soil, portland cement, and water. As the cement reacts, or hydrates, the mixture gains strength and improves the engineering properties of the raw soil. The major variables that control the properties and characteristics of soil-cement mixtures are the type of soil or aggregate material, the proportion of cement in the mix, the moisture conditions, and the degree of compaction. It is possible, simply by varying the cement content, to produce soil-cement that ranges from a basic modification of the compacted soil (or cement-modified soil) to fully hardened soil-cement that is strong, durable, and frost-resistant.
Although soil-cement is known by various local and regional names, the three most commonly used terms are described below.
Cement-modified soil (CMS)—This describes a soil that has been treated with a relatively small amount of cement in order to improve its engineering properties so that it is suitable for construction. CMS, for example, may be used to decrease a clay or silty clay soil's cohesiveness (plasticity), decrease its volume change characteristics, increase its bearing strength, or transform a wet, soft subgrade into a surface that will support construction equipment.
Cement-treated base (CTB)—This refers to all hardened soil-cement that meets the project specified minimum durability and strength requirements for a base. Using more cement than CMS, CTB becomes a strong, durable, frost-resistant layer for the pavement structure.
Full-depth reclamation (FDR)—This describes a special case of CTB in which aggregate for the cement-treated base is obtained by pulverizing and recycling the old asphalt surface and base material into a new fully hardened, durable, frost resistant base.
WHY USE SOIL-CEMENT?
The use of soil-cement can be of great benefit to agencies responsible for building and maintaining roads. Its cost compares favorably with that of granular-base pavement. When built for equal load-carrying capacity, soil-cement is almost always less expensive than other low-cost site treatment or pavement methods.
The use or reuse of in-place or nearby borrow materials eliminates the need for hauling of expensive, granular-base materials; thus both energy and materials are conserved.
This low cost has made soil-cement an attractive alternative to designers of roads and pavements. In addition, soil-cement has considerably more load-carrying capacity than flexible pavements, requiring less thickness to carry a given load. Pavement engineers praise soil-cement's performance, its low first cost, long life, and high strength. Soil-cement is constructed quickly and easily—a fact appreciated by local government agencies and the traveling public.
ECONOMIC, ENVIRONMENTAL, AND ENGINEERING BENEFITS
Soil-cement has many benefits, including:
Low first cost—Soil-cement is often more economical to construct than granular bases because the soil material is found on or near the paving site. Generally, any in-place nonorganic, low-plasticity soils can be used. Also, nearby granular borrow soil can provide an excellent material source, requiring lower cement contents than clay and silt soils. Borrow soils do not have to be expensive base-course material; almost any granular material is suitable.
Fast construction—Modern methods and equipment make soil-cement processing simple and efficient. In-place soils are processed at the paving site. When borrow soil is used, it is usually mixed in a central plant at the borrow source, then hauled to the paving site to be compacted, finished to grade, and cured. There is no mellowing period or other delays in the construction process. In addition, soil-cement is stable immediately after construction and gains strength rapidly.
Recycling of existing materials— Making good soil-cement out of old flexible pavement is nothing new; it has been done for years. Failed flexible pavements contain materials that can be salvaged economically by recycling—breaking them up, pulverizing them, and stabilizing them with a minimum quantity of portland cement to make a new soil-cement base. There is no disposal problem as is commonly found when old pavements are dug out. Since approximately 90% of the material used is already in place, handling and hauling costs are cut to a minimum. Many granular and waste materials from quarries and gravel pits also can be used to make soil-cement, thus conserving high-grade materials for other purposes.
Stiffness—Soil-cement is a low-cost pavement base offering the feature most essential for long-lasting parking areas and roads: stiffness. Large paved areas must maintain their original grade and must not develop depressions or potholes if they are to drain freely during rains, thereby preventing puddles and damage from water that seeps through and weakens the underlying soil. The stiffness of a cement-stabilized base acts to distribute loads over a wider area, reducing subgrade stresses and allowing the base to maintain its original grade for many years without costly resurfacing or repairs.
Soil-cement does not rut or consolidate. As a cemented material, it does not soften when exposed to water. When rutting occurs in an unstabilized base material or the underlying subgrade soil, a simple overlay of the pavement surface is insufficient to correct the cause of the rutting. With a stabilized base, rutting is confined to the asphalt surface layer and is relatively simple and less expensive to correct.
Great strength—Cores taken from soil-cement pavements furnish proof of its strength. Samples taken after 15 to 20 years show considerably greater strength than samples taken when the pavement was initially built. Because the cement in soil-cement continues to hydrate for many years, soil-cement has “reserve” strength and actually grows stronger. Soil-cement thickness requirements are less than those for granular bases carrying the same traffic over the same subgrade. This is because soil-cement distributes loads over broad areas. Its slab-like characteristics and beam strength are unmatched by granular bases. Strong, stiff soil-cement resists cyclic cold, rain, and spring-thaw damage.
Superior performance—More than 70 years of collective experience have demonstrated that different kinds of soil-cement mixtures can be tailored to specific pavement applications, all achieving superior performance as a result of soil-cement's strength and durability. Thousands of miles of soil-cement pavements across the United States and in all the Canadian provinces are still providing good service at low maintenance costs.
Cement-treated bases are designed to be virtually impermeable, so that even under frost conditions, no ice lenses can form in the base layer. Poor drainage or rising groundwater can affect a granular or unbound material by saturating the base, causing significant strength losses. The cement-stabilized layer, on the other hand, will maintain significant strength even in the unlikely event it becomes saturated.
The higher stiffness of cement-treated bases leads to lower pavement deflections and lower asphalt strains, resulting in longer fatigue life for the asphalt surface. The use of soil-cement actually reduces the occurrence of fatigue cracking, a common pavement failure.
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