Denver’s E-470 is the first major highway in Colorado to use cement-treated soil (CTS) for subgrade construction. E-470 is a new toll road being built along Denver’s eastern suburbs to service growth and provide access to the Denver International Airport.
The current project encompasses more than 30 miles of new highway construction. The first 10-mile section was opened in 1996 and the remainder of the project was to open to traffic in April 1998.
First highway using cement-treated soil
Initially, the design called for full-depth asphalt on a gravel base. However, engineers found that an 8- to 10-in. stabilized subgrade with 7- to 10-in. asphalt pavement was less expensive. This was principally because by using CTS, the stabilized subgrade adds to the structural support of the pavement and allows reduced asphalt thickness and total cost savings.
Several stabilizers were considered including cement, lime, fly ash and cement kiln dust. Besides having a cost advantage on a per-ton basis, cement offered several advantages, including:
Soils varied considerably on the project from sandy soils with low plasticity, to clay soils with plasticity indexes of 20 or more. Cement can stabilize a wide range of soils from low- to high-plasticity. Lime would be limited to the higher plasticity clays, thus necessitating using cement for sandy soils, and requiring frequent changeovers of stabilizer, with commensurate added time and cost;
CTS reduces swell potential of subgrade soils;
CTS produces a weather-resistant working platform for asphalt paving operations;
CTS gains strength with time to increase structural capacity should traffic exceed expectations;
Cement stabilization has established durability criteria for freeze-thaw environments, thus allowing it to be considered as a structural element of the pavement; and
Cement has a track record of more than 60 years for use in successful subgrade stabilization, as well as extensive laboratory and field testing by the Portland Cement Association (PCA) and other organizations.
Combating reflective cracks
One major concern of the toll road authority was the possibility of reflection cracking through the asphalt surface. Cement stabilized materials, as a by-product of gaining strength and losing moisture, exhibit certain shrinkage characteristics, which generally result in fine cracks at intervals ranging from 10 to 60 ft, depending on the soil, the cement content and the moisture/curing conditions. If constructed properly, the hairline cracks are not a problem.
In consideration of controlling this phenomenon, the specifications included provisions to allow no more than a combined total of 6 in. of cracks in 500 ft of paving, with no crack greater than 3/8 in.
Actual cracking has been considerably less than this. One study found only 1.66 in. of cracks in 500 ft, with all cracks less than 1/16 in.
One effective method used to minimize reflection through the asphalt pavement is to employ curing procedures, which include:
Spraying the surface of the cement-treated material with periodic water applications until a prime coat is applied;
Applying a prime coat of asphalt emulsion at 0.20 gal per sq yd as soon as possible after finish grading; and
Allowing the subgrade to cure with the prime coat for at least 28 days prior to asphalt surfacing. Studies have found that up to 80%of shrinkage will occur in the first 28-day period. If asphalt surfacing can be delayed, reflection cracking is minimized.
Both the contractor and the toll road authority have quality control/assurance labs set up on site to monitor the cement treated subgrade. These labs ensure compliance with material specifications of 250 psi in seven days, with a minimum cement content of 55 although cement contents were measured to about 8 for some parts of the project.
When some low strengths were encountered, Dick Suedkamp, president of Ground Engineering, recommended performing falling weight deflectometer testing to determine if the modulus of the pavement was greater than the 26,000 psi required. The tests showed that the CTS had a resilient modulus of about 150,000 psi, and had uniform structural characteristics throughout, regardless of strength variations.
The project used approximately 40,000 tons of cement, supplied by Dacotah Cement.
As with many other projects, CTS has proven to be an effective method of saving costs in pavement design and that construction can proceed fast-even with a variety of soil conditions.
The history of soil cement
In 1935, engineers constructed the first experimental soil-cement pavement. The 1.5-mile stretch of road outside of Johnsonville, S.C., represented a significant development because it proved to be a long-sought means to stabilize local soils and provide good economic road base. More than 60 years later, soil-cement pavements are still giving good service at low maintenance costs and more than 100,000 miles of highway have been built using soil-cement.
Soil-cement, also referred to as cement-modified soil and cement-treated aggregate base, is a dense, highly compacted mixture of soil or roadway material, portland cement and water. Soil material can be almost any combination of sand, silt, clay, gravel or crushed stone. Granular soils are preferred, however, because they pulverize more easily and require less cement to achieve the required strength and durability.
Laboratory tests are performed to determine the proper cement content, compaction and water requirements of the soil material to be used. The soil-cement can be mixed in a central plant or mixed-in-place. Central plant mixed soil-cement requires a non-cohesive, usually granular material. For mixed-in-place operations, clay or granular soils can be mixed.
For mixed-in-place, construction contractors follow four basic steps of soil-cement paving: spreading, mixing, compacting and curing. When the roadway has been shaped to grade and the soil loosened, the proper quantity of cement is spread on the in-place soil. Mixing machines then thoroughly mix the soil. The mixture is next tightly compacted with rollers, shaped to the proper contour and rolled again to achieve a smooth finish. Finally, the soil-cement is cured by spraying water and sealing with a bituminous mixture to supply and maintain the moisture needed for hydration.
Soil-cement’s advantages of high strength and durability combine with low first cost make it an economical material. About 90 of all the material needed for soil-cement is already in place, keeping handling and hauling costs to a minimum. Like concrete, soil-cement continues to gain strength with age. Because soil-cement is compacted into a tight matrix during construction, the pavement does not deform under traffic or develop potholes as unbound aggregate bases.
Soil-cement is capable of bridging over weak subgrade areas and is highly resistant to deterioration caused by seasonal moisture changes and freeze/thaw cycles.
The use of soil-cement has expanded since its initial development in 1935. Soil-cement has been used primarily as a base course for roads, streets, highways, airports and parking areas. Soil-cement also is used as slope protection, ditch lining and foundation stabilization. Soil-cement is used in every state in the U.S. as well as in all the Canadian provinces.