Subgrade Support and Stabilization

Faculty Investigator(s): Erol Tutumluer and Marshall Thompson

Overview:

Subgrade performance is a key factor in the overall performance of a pavement system. This project provides testing and analysis to establish subgrade support stabilization requirements in support of the design of O’Hare Airport pavements, which will be constructed as part of the O’Hare Modernization Program (OMP). The new 7500-ft North Runway (9L-27R) paving is programmed as the first runway to be constructed starting in 2007. The 9L-27R runway subgrade soils are primarily “fill” although there are some cut areas as indicated by the runway plans. A considerable quantity of excavated soil from the nearby “Deep Pond,” which was previously stockpiled was used as fill material. Such issues as the adequate subgrade support requirements, modification and moisture treatment of the stockpiled soils and the “Stabilized” Subgrade Zone have been recently considered. Inputs to OMP for their evaluation and adoption to draft construction specifications and design decisions have been provided. More recently, activities related to subgrade soils for the South Airfield of the O’Hare Airport have also started with particular emphasis on the evaluation of local stockpiled soils.

Objectives:

The project objectives are:

• Establish pavement design input(s) for subgrade support such as the modulus of subgrade reaction “k”
• Establish subgrade soil support requirements with respect to:
  • Need for subgrade modification/moisture treatment
  • Need for pavement subgrade stabilization
  • Selection of lime and other suitable admixture stabilization considerations
  • Stabilization depth
• Estimate “Subgrade Support” for various combinations of subgrade modification/stabilization treatments and prepared subgrade conditions.

Potassium Acetate and ASR

Faculty Investigator(s): Leslie J. Struble

Overview and Objectives:

The objective of the work on alkali-silica reaction (ASR) in concrete proposed for 2007 is to understand the interaction between reacting mortar and potassium acetate deicing solution. This work will provide a technical basis for recommendations concerning concrete materials. The work is divided into three parts.

Part 1: Chemistry of Reaction The first part is to study the chemistry of the reaction of mortar in deicing solution. The potassium acetate solution is very concentrated. The alkali concentration is about 5 times the concentration of the test solution in ASTM C 1260, the standard test for alkali-silica reactivity of aggregate. However, what drives aggregate reaction is not the alkali concentration, but rather the hydroxide ion concentration (commonly reported as pH). The deicing salt solution has a pH of about 10.5, not high enough to initiate reaction with aggregate. However, we observe a major increase in pH when mortar is placed in the solution. Even a small amount of calcium hydroxide causes the pH to increase to a level above 14.0, a very high value and sufficient to initiate reaction with aggregate. We find that portland cement and hydrated cement causes a similar increase in pH. It is important to understand what chemical changes produce this increase in pH. Calcium hydroxide is present in hydrated cement and is known to react so as to increase the pH in the presence of other neutral salts. Precipitation of calcium acetate, although not expected according to its solubility values, would explain the increase in pH. We have examined the solid phases in mixtures of calcium hydroxide and potassium acetate and found no calcium acetate. Therefore we must look more deeply to understand the chemical reaction between potassium acetate and calcium hydroxide responsible for the increase in pH.

Part II: Reactivity of Chert The second part is to verify that chert is resistant to deleterious effects of deicing solution. Experiments during 2006 showed quite clearly that mortars made using sands containing chert, the typical reactive constituent in sands in the central US, do not expand deleteriously in the deicing salt solution. This is a very unexpected result. Silica glass, on the other hand, expands much more in the deicing salt solution than in the standard test solution. In order for OMP to be confident that sands containing chert will not deteriorate on exposure to the deicing salt solution, it is important to carry out additional experiments to verify these results. Mortar and concrete experiments are proposed, with measurement of length changes, combined with petrographic examination of test specimens to check for evidence of alkali-silica reaction.

Part III: Assess Mitigation Materials The third part is to explore whether mitigating materials (fly ash, slag) are effective in preventing ASR reaction in the deicing salt solution in concrete containing reactive aggregate. The use of materials such as fly ash and slag are well known to mitigate ASR expansion by reducing the pH of the pore solution. We have tested several fly ash and slag materials in our market area and seen that some are effective in mitigating expansion produced by sands in our area. However, it is also important to explore whether these materials can mitigate in the presence of the deicing salt solution, which contributes such an overwhelming amount of alkali. Mortar and concrete experiments are proposed in which reactive aggregates are used in combination with known mitigating materials and immersed in deicing salt solution, with measurement of length changes. In addition, chemical measurements in model solutions will be carried out to determine whether pH values are reduced by the addition of mitigating materials.

Effect of Climate and Concrete Constituents on Airfield Rigid Pavement Performance

Faculty Investigator(s): Jeffery Roesler and David Lange

Overview

The performance of concrete pavements is a complex interaction between the concrete constituents, pavement geometry, climatic conditions, and construction techniques. The early-age properties are a good indicator of the long-term performance potential of a concrete pavement. The early-age behavior of slabs is controlled by the temperature and moisture states in the concrete and how these stresses relate to the concrete’s strength and fracture properties. The concrete materials selected coupled with the ambient conditions and construction techniques must overcome the initial environmentally-induced concrete stresses as well as the mechanical loading that will be applied over its service life.

Objectives

The objective of this research proposal is to investigate concrete material properties required to achieve early and long-term performance at the Chicago O’Hare International Airport (ORD), develop material constituents and proportions to assure the desired material properties, and predict the integrated effects of mix constituents, climatic conditions, and pavement layer geometry on early-age behavior.

Airport Wildlife Safety Management

Faculty Investigator(s): Edwin Herricks and Bruce Branham

Overview:

Avian wildlife in proximity to airports present significant aircraft safety concerns. Birds and in particular geese, are attracted to large expanses of green space, open water, and patches of habitat that provide food, cover and roosting areas. In the open areas, airports use turfgrasses to cover the land that is not paved with concrete to prevent soil erosion from wind and rainfall, reduce dust and particulate matter, and absorb sound. A healthy turf contains soil insects such as grubs, earthworms, cutworms, etc. that are valuable food sources for birds. In addition, geese will feed upon the foliage of the turf. Thus a healthy turf is a food supply for many avian species and actually presents a hazard to aircraft safety because of its attractiveness to birds. In addition to safety issues in open areas, surface waters also present a safety hazard. Open waters are common around airports and occur naturally dut to the setting or are created as a part of storm water management where both delivery channels and detention basins may hold water. These waters also provide food and resting areas that attract a wide range of wildlife. Although open areas of turf grass dominate airports, topographic conditions, the actual land use, and other factors give rise to patches of vegetation, trees, wet and wetland areas, and places where uncontrolled vegetation growth occurs. All of these landscape features should be subject to management and control to assure safe conditions for aircraft movement.

Objectives:

The objective of the proposed research project is to develop a system that comprehensively manages airport properties to discourage wildlife use. In the open areas, developing soil/vegetation conditions provides the benefits of a well-managed turf while minimizing the attractiveness to avian species. In water resources management, the design and placement of control practices, and the management and maintenance of those practices can also minimize the attractiveness to wildlife. Finally, the management of the entire airport as a system, considering all potential habitat from buildings and bridges to patches of vegetation and water are also needed to minimize wildlife use at airports.

Non-Destructive Evaluation and Preparation for Field Validation of Constructed Pavement Layers at O’Hare

Faculty Investigator(s): John Popovics (PI), Jeffery Roesler, Marshall Thompson, David Lange

Overview:

A portion of the O’Hare north runway (9-27) paving is programmed for spring/summer 2008 construction. At that time, the pavement support layers will be completed and the construction of the paving layers will commence. The nominal pavement design for the O’Hare Modernization Program is:

• 15-17 inches of Portland cement concrete pavement (PCCP) Surface
• 6-inch Hot Mix Asphalt (HMA) Base
• 6-inch Asphalt Treated Permeable Base
• “Stabilized” Subgrade Zone (SSZ)
• Prepared Subgrade

Information about the in situ response of the slabs to environmental loading conditions and as built conditions, such as layer thickness, are of interest.

Objectives:

Building upon previous efforts carried out on a replica test slab cast at the ATREL research facility of the University of Illinois, a field testing and instrumentation testing setup to monitor the effects of environmental loads (temperature and moisture) on the behavior and response of the constructed pavement at O’Hare will be proposed. In the mean time, long-term pavement behavior data from the existing instrumented test slab will continue to be collected and disseminated to other OMP researchers. Finally, non-destructive evaluation methods that can effectively and accurately characterize the in place pavement structure will be investigated and evaluated through trials on the Illinois test slab.

Reflective Cracking and Improved Performance of Grooved Asphalt

Faculty Investigator(s): William G. Buttlar and Imad L. Al-Qadi

Overview:

This study will focus on two main topics: reflective cracking and its reduction/mitigation and the performance of grooved flexible pavements. This project was initiated in January of 2006. The proposed work plan describes the research tasks to be conducted during the period January 2007 until December 2007.

Reflective Cracking Hot-mix asphalt (HMA) overlays are typically applied to existing flexible and rigid pavements when the structural or functional conditions of the pavement system have reached an unacceptable level of service. However, adequately designed overlays may still show cracking similar to the ones which existed in the old pavement or the joint patterns after a short period of time. This distress is known as ‘reflection cracking,’ and can begin to occur sometimes within a year of the overlay construction. In addition to decreasing the serviceability of the overlay, reflection cracking may accelerate other pavement distresses such as the weakening of subgrade and aggregate base layer through water infiltration, stripping of hot-mix asphalt (HMA) layer, and loss of existing concrete support.

The most popular techniques to enhance pavement resistance against reflection cracking include increasing overlay thickness, use of polymer-modified HMA, rubblization of existing concrete, and use of interlayer systems. Recently, studies conducted at UIUC have led to the development of new laboratory tests and modeling tools that can be used to study the mechanisms of reflective cracking and to optimize overlay systems to reduce the development of reflection cracking.

Pavement Grooving A major consideration in safe aircraft operations is to ensure that adequate friction is present at the tire-pavement interface on runways. Due to the high speed of aircraft during takeoff and landing, friction requirements for airports are more stringent than those for highways and more frequently measured. In order to enhance friction at airport pavements, grooves are often sawn into concrete and HMA surfaces in areas where low friction is experienced or where high friction is needed, particularly on runway takeoff and touchdown areas. This becomes more critical in areas exposed to frequent rainfall and snowfall. Although grooves would improve surface friction and reduce hydroplaning, grooves in HMA may accelerate several forms of deterioration, including the following: permanent deformation (rutting), thermal cracking, reflective cracking (if applicable), and raveling. In the case of permanent deformation, the presence of laterally unsupported HMA at the surface of the pavement can make the pavement more susceptible to rutting and could reduce the effective thickness of the overlay. The factors which may influence the rate of rutting in grooved pavements are related to the mix design and its characteristics as well as that of its constituents. This includes aggregate type, shape, size, gradation, and, polishing susceptibility; asphalt binder shear and stiffness properties; and mixture volumetrics. For potential crack development, grooves can be viewed as notches in the pavement surface, which create points of stress concentration in the surface of the flexible pavement. As a result, HMA that can reduce the potential for crack development is desired. The grooves may also accelerate the rate and depth of penetration of asphalt binder aging, which would also tend to accelerate pavement cracking and possibly raveling.

Objectives:

The overall objectives of this study are the following:

• Model reflective cracking of HMA overlays
• Evaluate binder properties to design materials that can reduce the potential for cracking, including the evaluation of environmental effects that impact distress mechanisms.
• Evaluate stability of grooves in HMA surfaces.
• Make recommendations to maximize overlay life and/or minimize life cycle costs based upon considerations of thermal cracking and groove stability