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NEW PAVEMENTS

Pavements, especially flexible pavements, are constantly under changing conditions, thus they are inherently unstable. Water infiltration weakens the underlying soil condition and variable loading moves those conditions throughout the pavement structure. Asphaltic concrete pavements are constantly under the debilitating effects of oxidation and the actions of water stripping the asphaltic binder from the aggregate structure.

The use of chemical stabilization in roadway design speaks directly to these issues of long-term life-cycle stability. 

Pavement Design Factors

The design of pavements is based on the premise that a minimum specified structural quality will be achieved for each layer of material in a pavement system. Each layer is required to resist shearing, avoid excessive deflections that cause fatigue cracking within the layer or in overlying layers, and prevent excessive permanent deformation through densification. As the quality of the soil layer is increased, the ability of that layer to distribute the load over a greater area is increased so that a reduction in the required thickness of the upper and surface layers may be permitted.

Pavement Types

Hard surfaced pavements are typically categorized as flexible or rigid:

  • Flexible pavements. Those which are surfaced with bituminous (or asphalt) materials. These types of pavements are called "flexible" since the total pavement structure "bends" or "deflects" due to traffic loads. A flexible pavement structure is generally composed of several layers of materials, which can accommodate this "flexing".
  • Rigid pavements. Those which are surfaced with Portland cement, concrete (PCC). These types of pavements are called "rigid" because they are substantially stiffer than flexible pavements due to PCC's high stiffness.

Each of these pavement types distributes load over the subgrade in different ways. A rigid pavement, because of PCC's high stiffness, tends to distribute the load over a relatively wide area of subgrade. The concrete slab itself supplies most of a rigid pavement's structural capacity. Flexible pavements use more flexible surface course and distribute loads over a smaller area. It relies on a combination of layers for transmitting load to the subgrade.

Engineers are frequently required to incorporate poor quality soil and aggregate into pavement designs. These poor quality materials typically have the potential to demonstrate undesirable engineering behavior such as low bearing capacity, high shrink/swell potential, and poor wet-dry durability. Thus, engineers frequently seek to improve the engineering properties of poor quality soils and aggregates through chemical stabilization.

When applying a chemically stabilized section within a traditional pavement design the goals center around improving the bearing capacity, durability, and permeability of the native soil or unbound aggregate base layers. This ability to improve bearing capacity, as measured by unconfined strength, R-Value, CBR, or other strength measurements, leads to replacement or reduction of traditional design sections. These engineering perimeters are met while reducing the environmental impact and cost of constructing new pavements or reconstructing existing pavements.

Designing with Stabilized Sections

Chemical stabilization of various pavement sections can be in the form of in-situ subgrade improvements, stabilization of lesser quality subbase materials, or treatment of aggregate base material in the form of cement treated base.

The most common improvements of chemical treatments include better soil gradation, reduction of plasticity index or swelling potential, and increases in durability and uncured strength. In wet weather, stabilization may also be used to provide a working platform for construction operations. These types of soil quality improvements are referred to as Chemical Modification.

The process known as Chemical Stabilization is required when considering a pavements long-term performance. Chemical stabilization requires a greater degree of engineering design, performance testing, and construction quality control when considering as a permanent pavement foundation. When chemical stabilization is achieved then reduction in traditional design thickness can be realized.

This level of performance requires a design approach that optimizes all phases of the chemical stabilization process. Through various test methods, a minimum strength is determined to meet design performance, while long-term strength can be established to assure that strength and stiffness do not exceed flexural requirements.

A critical element for pavement designers and project managers is performance verification and reduction of post treatment concerns of shrinkage cracking of cementious binders. HSI follows an optimal approach to these concerns through the concept of Lab/Field Synchronization and Data Point Testing.

When considering chemical stabilization for flexible pavement design the most advantageous approach is to consider stabilization of the native subgrade soils. Since empirical pavement designs require section thicknesses to increase as subgrade soils become weaker, any improvement in these soils will inherently reduce the need for additional section thicknesses, thus reducing cost and environmental impacts of the constructed roadway.

The criteria for establishing the engineering properties of soils used for pavement base courses, subbase courses, and subgrades by the use of chemical additives are applicable to all pavement types, including parking lots, low and high volume roads, and airfields.