Among the many factors involved in placement of a quality concrete pavement, the subgrade is an important component often overlooked. The subgrade has a substantial impact on base and subsurface drainage requirements and on long-term pavement ride quality and overall performance.
Although we've long understood that a stable “platform” is necessary to construct a smooth, uniform pavement, subgrade stability and strength are often misunderstood as part of design and construction requirements.
Also, the uniformity of subgrade soils affects both design and construction. Concrete pavements can be designed for relatively poor soil conditions, as long as the level of support is consistent and accounted for in the design. The design procedure established by the American Association of State Highway and Transportation Officials (AASHTO) is only moderately sensitive to subgrade support values (in terms of “k” or modulus of subgrade reaction).
Several options are available to produce a uniform subgrade, including cross hauling, soil modifying or stabilizing, and removing and replacing. The most desirable option is one that minimizes cost while providing improved constructability and performance.
To allow construction to proceed, soil stabilization is sometimes accomplished by drying a subgrade soil and stabilizing the working platform for the subbase or pavement. Depending on the soil type, portland cement, hydrated lime, quicklime, fly ash, kiln dust (cement or lime), or other agents may be used. Thorough laboratory testing is almost always necessary to determine the effectiveness of the soil stabilizer in reducing the plasticity of the soil (plasticity index value), increasing the strength, determining the optimum addition, and establishing density requirements. If a pavement design is based on a stabilized subgrade strength value, the long-term durability of the material must be considered (that is, consider if the stabilized soil will retain its strength for the life of the pavement).
Concrete pavements can be designed and constructed for all soil types. The key is to provide a uniform, stable subgrade platform for construction equipment and the rest of the pavement structure. This will make for a constructible pavement that provides excellent long-term performance.
THE SUBBASE COMPONENT
The importance of a subbase (or base) often is overlooked during the pavement design phase. Specifying the right subbase will enhance concrete pavement performance. A number of factors must be considered while specifying a subbase, including traffic, environmental conditions, and in-place soil conditions.
A subbase imparts many benefits for moderate- to heavy-traffic concrete pavements. For example, subbases will accomplish the following functions:
Support layers should be considered a design element; a construction platform; and a stable, uniform support during the life of the pavement. In spite of their importance, they are not emphasized, nor are they addressed in great detail, in the commonly used 1993 AASHTO Pavement Design Guide. One reason is that certain combinations of subbases have relatively little influence on the required slab thickness.
For example, a heavily trafficked, rural interstate highway built over a relatively weak subgrade might have a design thickness of 11.1 inches if a 6-inch low-quality, unbound granular subbase were used. A high-quality, unbound granular subbase could reduce the thickness to 11.0 inches.
On the other hand, a 6-inch layer of high-quality, cement-treated subbase could reduce the thickness requirement to 10.7 inches. (Note that a pavement built directly on the subgrade with no subbase would still require the 11.1-inch thickness.) Clearly, the addition of a subbase cannot be justified solely on the basis of reducing the required thickness of pavement.
When incorporating subbases in low-volume roads, cost effectiveness should be considered. Generally, low-volume roads do not require subbases. If the use of a subbase is justified, the type of subbase chosen will be dependent upon availability of materials, subgrade type, anticipated construction operations, budget, and numerous other factors. The use of drainable subbases is somewhat controversial because they are not believed to have a material effect on pavement performance. Therefore, drainage should be analyzed thoroughly on a project-by-project basis. It's also important to consider that a drainage system requires a long-term commitment to maintaining that system.
Subbase material stability is another important consideration. Densely graded granular materials and materials stabilized with cement or asphalt create firm support for construction equipment. Unstabilized permeable layers, which became popular in the 1990s, have caused some construction and pavement performance problems—in particular, cases of early cracking.
An important balance must be met between the degree of drainage and the stability of the unstabilized subbase layer. Subbase stability should not be sacrificed for the sake of drainage. A target permeability of 200 to 300 feet per day has been found to produce a stable, drainage layer that will support the paving equipment, construction vehicles, and the pavement in the long term.
— Reprinted by permission of the American Concrete Pavement Association. Copyright 2005, American Concrete Pavement Association. No portion of this article may be reproduced or redistributed either electronically or mechanically without the expressed written permission of the American Concrete Pavement Association,www.pavement.com.
Terms of the art
Cross haul—Excavating soil from one location to another and blending it in place.
Soil modification—Modifying characteristics of soil through additions of stabilization agents, granular materials, or soils with improved properties.
Soil stabilization—The addition of chemicals such as cement, cement kiln dust, lime, lime kiln dust, fly ash, or others to improve the quality and support characteristics of the soil.
Addressing cracks prior to whitetopping
A common question during whitetopping preparation is whether it is necessary to repair cracks in distressed asphalt prior to overlayment. The short answer is, it depends on the severity of the cracking.
Cracks in asphalt do not reflect through concrete overlays. The reason the asphalt cracks usually do not cause a crack in a concrete surface is a result of the different modulus of elasticity between the two materials. Concrete has a significantly higher modulus of elasticity, which means it can withstand more elastic pressure than asphalt. When bonded, the concrete is more likely to crack the asphalt.
As a result, it is usually not necessary to repair cracks in the asphalt. Also, it is usually not necessary to use a synthetic fabric or stress-absorbent interlayer to prevent reflective cracks in a concrete overlay of asphalt. However, serious asphalt distress in advanced stages—such as severe rutting, shoving, or potholes—must be repaired. Areas showing subgrade failure, which, in turn, will not provide uniform support of the overlay, should be removed and replaced.
After repair, there are a few options for addressing a distorted surface before placing the overlay:
Setting fixed forms before concrete placement
Properly setting forms (alignment and elevation) is key to producing a smooth pavement. Fixed-form concrete paving is often used for streets and local roads, parking lots, short paving segments, and irregularly shaped pours. Here are some tips to keep in mind while setting fixed-forms.
Good form—First, forms should be straight, clean, and in acceptable condition. Ten-foot steel forms are most common, particularly for straight sections, but wooden forms also can be used on small jobs if they are not reused too many times. Plywood forms also are used frequently for short radius turns, where they can be bent to the radius of the curve.
Setting the form—Next, the quality of the support beneath the forms should be assessed. Settlement of the forms under paving equipment can be a source of built-in roughness. The base of the form should bear against the subbase or subgrade surface completely and not lie on any clumps of dirt or rocks.
Forms should be set in place according to the stringline and grade, and fastened to the sub-base with three pins or stakes. The forms should not be shimmed up more than ¼ inch, to reduce the deflection of the form caused by paving equipment.
Before concrete placement—Next, the wedges and form locks should be driven tight, and the horizontal and vertical alignment of the forms checked either with a straightedge or by eye. The forms should be given a light application of a form release agent to permit easy removal after the concrete has hardened. The last step, before concrete placement, usually involves final preparation and shaping of the subbase or subgrade.