Download the complete report at fhwa.dot.gov/pavement/concrete/pubs/hif09009/hif09009.pdf

Impact of New Design Guide

Part of the new complexity of subbase design, and ultimately construction, derives from the ongoing adoption of new highway pavement design procedures set forth in the Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, Final Report (NCHRP, 2004), now referred to as the Mechanistic-Empirical Pavement Design Guide (MEPDG), and in the process of adoption by DOTs from coast-to-coast.

Mechanistic-empirical are big words that describe a very simple concept. Mechanistic refers to the interaction between the materials and structure of a pavement, and how it stresses and strains under load deflection. The MEPDG paradigm relates these pavement mechanics to empirical or experimental performance data obtained in field or lab.

The guide uses mathematical models to describe this relationship, and the primary basis for all mechanistic-based pavement performance predictions methods is cumulative axle load applications.

“The benefit of a mechanistic-empirical approach is its ability to accurately characterize in situ material (including subgrade and existing pavement structures),” says the Washington State DOT in its online tutorial. “This is typically done by using a portable device to make actual field deflection measurements on a pavement structure to be overlaid. These measurements can then be input into equations to determine existing pavement structural support (often called backcalculation) and the approximate remaining pavement life. This allows for a more realistic design for the given conditions.”

The existing 1993 edition of the AASHTO Guide for Design of Pavement Structures is based on empirical equations derived from the well-known, but outdated, AASHTO Road Test. This program conducted performance testing between 1958 and 1960 of a limited number of structural sections at one location, Ottawa, Ill., and based on much-reduced traffic levels compared those of the 21st century.

Under the new design guide, a designer of any pavement must first consider site conditions such as traffic, climate, subgrade, existing pavement condition for rehabilitation and construction conditions in proposing a trial design for a new pavement or rehab. Then, using the software, the trial design will be evaluated through prediction of key distresses and smoothness. If the trial does not meet the demanded performance criteria, the pavement design must be revised until it does.

The new guide also incorporates procedures for performing traffic analyses, includes options for calibrating to local conditions, and incorporates measures for design reliability. Engineers can use the guide to analyze common causes of pavement distress, including fatigue, rutting and thermal cracking in asphalt pavements, and cracking and faulting in concrete pavements.

Reclaimed Materials in Bases

There is no question that recycled concrete aggregate (RCA) also may be used in road subbases and bases, so long as it is treated as an engineered material — that is, crushed, screened, processed and tested as though it were a virgin aggregate. (See Better Roads, Two for the Price of One, April 2010, pp. 16-29.)

In that Road Science Tutorial, we reported that TxDOT has researched and used RCA with good success for about 17 years. In the years 2006-2008, TxDOT saved approximately 1.8 million tons of virgin aggregates by incorporating RCA in cement treated base, flexible base, continuously reinforced concrete pavement (CRCP), filter dams, gabion walls, concrete traffic barriers, flowable fill and select backfill for mechanically-stabilized earth walls. “This equates to an estimated savings of $12.6 million from reduced or eliminated landfill and virgin aggregate associated costs,” TxDOT reports. “Savings from using RCA has the potential to increase tenfold based on current availability of RCA.”

But recycled aggregate from structures may perform just as well as RCA from demolished highways, say Dana V. Martin and Gregory W. Halsey, undergraduate research assistants, and Jeffrey S. Melton, research assistant professor, Department of Civil Engineering, University of New Hampshire-Durham, in their 2011 Transportation Research Board paper, Comparison of Building Derived Aggregate in Comparison to Crushed Stone.

Use of recycled concrete aggregate for road construction has become a widely accepted practice throughout the United States, and has proven to be an excellent substitute for crushed stone in road base applications. More than 45 states allow its use in highway construction, they write, adding the most common source of RCA is from the demolition of highway infrastructure. “While the use of RCA has become commonplace,” they say, “the use of building-derived aggregate (BDA) in roadway construction has not.”