Soil-cement (cement-modified soil) is a highly compacted mixture of soil/aggregate, cement, and water. It is widely used as a low-cost pavement base for roads, residential streets, parking areas, airports, shoulders, and materials-handling and storage areas. Its advantages of great strength and durability combine with low first cost to make it the outstanding value in its field. A thin bituminous surface is usually placed on the soil-cement to complete the pavement.
Soil-cement is sometimes called cement-stabilized base, or cement-treated aggregate base. Regardless of the name, the principles governing its composition and construction are the same.
What Type of Soil is Used?
The soil material in soil-cement can be almost any combination of sand, silt, clay, gravel, or crushed stone. Local granular materials, such as slag, caliche, limerock, and scoria, plus a wide variety of waste materials including cinders, fly ash, foundry sands, and screenings from quarries and gravel pits, can all be utilized as soil material. Old granular-base roads, with or without bituminous surfaces, can also be reclaimed to make great soil-cement.
How is Soil-Cement Built?
Before construction begins, simple laboratory tests establish the cement content, compaction, and water requirements of the soil material to be used. During construction, tests are made to see that the requirements are being met. Testing ensures that the mixture will have strength and long-term durability. No guesswork is involved.
Soil-cement can be mixed in place or in a central mixing plant. Central mixing plants can be used where borrow material is involved. Friable granular materials are selected for their low cement requirements and ease of handling and mixing. Normally pugmill-type mixers are used. The mixed soil-cement is then hauled to the jobsite and spread on the prepared subgrade.
Compaction and curing procedures are the same for central-plant and mixed-in-place procedures.
There are four steps in mixed-in-place soil-cement construction; spreading cement, mixing, compaction, and curing. The proper quantity of cement is spread on the in-place soil material. Then the cement, the soil material, and the necessary amount of water are mixed thoroughly by any of several types of mixing machines. Next, the mixture is tightly compacted to obtain maximum benefit form the cement. No special compaction equipment is needed; rollers of various kinds, depending on soil type, can be used. The mixture is cemented permanently at a high density and the hardened soil-cement will not deform or consolidate further under traffic.
Curing, the final step, prevents evaporation of water to ensure maximum strength development through cement hydration. A light coat of bituminous material is commonly used to prevent moisture loss; it also forms part of the bituminous surface. A common type of wearing surface for light traffic is a surface treatment of bituminous material and chips .5- to .75-inch thick. For heavy-duty use and in severe climates a 1.5-inch asphalt mat is used.
Contractors bidding on soil-cement jobs know that construction will be relatively easy and problem-free; weather delays rare; and reworking of completed sections unnecessary.
Why Use Soil-Cement?
Failing granular-base pavements, with or without their old bituminous mats, can be salvaged, strengthened, and reclaimed as soil-cement pavements. This is an efficient, economical way of rebuilding pavements. Since approximately 90 percent percent 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 can also be used to make soil-cement; thus high-grade materials are conserved for other purposes.
Benefits of Soil-Cement
- Reduces Costs – Improves soil without replacement and hauling expenses
- Eliminates waste – No removal or replacement of inferior soils is necessary
- Reduces plasticity/cohesiveness – Improved constructability of marginal on-site soils
- Improves strength – Treated soils are stronger than native soils
- Improves performance – Better support and bearing capacity
- Resists leaching – Chemical and physical improvements of the soil are permanent
- Decreases volume change characteristics
- Adaptable to any type of soil – Improves strength and durability from poorly graded sands to highly plastic clays
Highway and city 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 owners and users alike.
How Does Soil-Cement Perform?
Soil-cement thicknesses are less than those required for granular bases carrying the same traffic over the same subgrade. This is because soil-cement is a cemented, rigid material that distributes loads over broad areas. Its slab-like characteristics and beam strength are unmatched by granular bases. Hard, rigid soil-cement resists cyclic cold, rain, and spring-thaw damage.
Old soil-cement pavements in all parts of the continent are still giving good service at low maintenance costs. Soil-cement has been used in every state in the United States and in all Canadian provinces. Specimens taken from roads show that the strength of soil-cement actually increases with age; some specimens were four times as strong as test specimens made when the roads were first opened to traffic. This reserve strength accounts in part for soil-cement’s good long-term performance.
Is Soil-Cement Economical?
The cost of soil-cement 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 pavements. Economy is achieved through the use or reuse of in-place or nearby borrow materials. No costly hauling of expensive, granular-base materials is required; thus, both energy and materials are conserved.
