IMPROVING UNBOUND AGGREGATE ROADS THROUGH A COMPARISON OF THE PERFORMANCE OF MATERIALS

iii

ABSTRACT

Unbound aggregate roads, or unpaved roads, generally do not hold up to the expectations of the general public.  This forces agencies to often pave roads unnecessarily. Wearing course materials used for unpaved roads and the subgrade they are placed on need to be improved in order to see an improvement in these roads. Research was conducted to prove if there is a benefit to using Driving Surface Aggregate (DSA), a material developed specifically for use as a wearing course, over another commonly used aggregate gradation, PennDOT 2A, designed for use as a base course. The performance of DSA and PennDOT 2A under scaled loading with the one-third Model Mobile Load Simulator (MMLS3) was compared in terms of rutting.  The materials were evaluated in the laboratory at a lightweight traffic setting (simulating passenger vehicles, pick-up trucks and SUVS) and at a heavy traffic setting (simulating tri-axial and semi-trailers). The MMLS3 was also used in the field on the lightweight traffic setting to evaluate the materials in an as-placed setting.  DSA consistently performed better than PennDOT 2A with lower average rut depths, although the predictability of the rut depth formation should be studied in more detail.

In considering subgrade weakness, a rehabilitation practice called full-depth reclamation (FDR) has been used to increase roadbed stiffness in paved roads.  The impact of using this method for unpaved roads has been evaluated. In situ stiffness testing using the lightweight deflectometer (LWD) and condition assessments of four roads in Erie County, Pennsylvania were compared to evaluate the potential of FDR as a practice to improve the subgrade for unpaved roads. A road consisting of FDR with a tar and chip surface treatment had the highest overall stiffness when compared to a FDR road without a surface treatment, an aggregate-surfaced road and an unsurfaced road.  Both FDR roads showed higher stiffness over the spring-thaw period proving potential benefit to adopting this method as an unpaved road practice; however, future research is needed to evaluate the performance of a FDR road with an aggregate surface. All four

 

roads showed decreasing condition ratings over the spring-thaw period, but the FDR road with a tar and chip surface treatment required maintenance over the summer months, highlighting one of the major financial downfalls of paving a road.

 

TABLE OF CONTENTS

LIST OF FIGURES ………………………………………………………………………………………………….. vi

LIST OF TABLES ……………………………………………………………………………………………………. vii

ACKNOWLEDGEMENTS ……………………………………………………………………………………….. viii

Chapter 1  Introduction ……………………………………………………………………………………………… 1

Problem Statement …………………………………………………………………………………………….. 2

Chapter 2  Literature Review ……………………………………………………………………………………… 4

Pavement Design ……………………………………………………………………………………………….. 4

Traffic Loading ………………………………………………………………………………………….. 8

Aggregate Wearing (Surface) Courses …………………………………………………………… 9

Subgrade Pavement Layer ……………………………………………………………………………. 10

Material Properties …………………………………………………………………………………………….. 12 Angularity …………………………………………………………………………………………………. 13

Moisture and Fines Contents ………………………………………………………………………… 13

Density ……………………………………………………………………………………………………… 15

Climate ……………………………………………………………………………………………………… 15

Improving Material Properties …………………………………………………………………………….. 16 Driving Surface Aggregate …………………………………………………………………………… 16

Full-Depth Reclamation ………………………………………………………………………………. 17  The Construction Process ……………………………………………………………………… 18  Addition of Stabilizer …………………………………………………………………………… 19  Cross Section ………………………………………………………………………………………. 19

Cement Stabilization ……………………………………………………………………………. 20  Cement-Stabilized Soils …………………………………………………………………. 21

Cement-Stabilized Aggregate …………………………………………………………. 23 Pavement Analysis ……………………………………………………………………………………… 23

Accelerated Pavement Testing ………………………………………………………………. 24

Chapter 3  Experimental Methods ………………………………………………………………………………. 26

Tire Considerations ……………………………………………………………………………………………. 27 Laboratory Testing …………………………………………………………………………………………….. 29

Surface Aggregates ……………………………………………………………………………………….. 33

Driving Surface Aggregates ……………………………………………………………………………. 34

PennDOT 2A ……………………………………………………………………………………………….. 38

Field Testing …………………………………………………………………………………………………….. 41 Surface Aggregates ……………………………………………………………………………………….. 42 Condition Assesments and Stiffness Testing …………………………………………………….. 45 Chapter 4  Results …………………………………………………………………………………………………….. 48

MMLS3 – Aggregate Testing ……………………………………………………………………………… 48

Condition Assessments and Stiffness Testing ………………………………………………………… 61 Reservoir Road …………………………………………………………………………………………… 63

Himrod Road ……………………………………………………………………………………………… 65

Church Road ………………………………………………………………………………………………. 67

Benson Road ……………………………………………………………………………………………… 69

Chapter 5  Discussion ……………………………………………………………………………………………….. 72

MMLS3 Discussion …………………………………………………………………………………………… 72

Condition Assessment and LWD Discussion ………………………………………………………… 76

Chapter 6   Conclusions …………………………………………………………………………………………….. 80

References ……………………………………………………………………………………………………………….. 82

Appendix A  LWD Results ………………………………………………………………………………………… 85

Appendix B  MMLS3 Results…………………………………………………………………………………….. 100

Appendix C  Weather History …………………………………………………………………………………….. 133

Chapter 1

 

Introduction

Improving the integrity of an unpaved road would require evaluation of both the subgrade and the wearing course, since both have an effect on the performance and overall rideability of a road (Huang, 2004).  The Center for Dirt and Gravel Road Studies at The Pennsylvania State University developed a densely-graded aggregate called Driving Surface Aggregate (DSA) as an attempt to improve the wearing course on unbound aggregate roads (Bloser, 2007).  To improve subgrade performance, full-depth reclamation has previously been used to rehabilitate paved roads.  In Erie County Pennsylvania, an area with subgrade soils subjected to moisture and harsh winters, FDR with cement stabilization was used to improve the subgrade performance on several roads in the area. Most of these roads remained unpaved.  The performance of DSA compared to PennDOT 2A as a wearing course and FDR as a means for improving subgrade soils for unpaved roads was evaluated.

Problem Statement

Unbound aggregate roads, commonly referred to as unpaved roads, do not hold up to the expectations of the general public.  With this, many agencies are forced to spend money to pave roads unnecessarily.  Many of the specifications for aggregate-surfaced roads were developed over time according to an empirical approach of “what has worked” and “what hasn’t worked.”  They were also not developed for driving surface purposes.  With little money available to maintain roads that do not hold up to expectations, a definitive solution is needed to determine the best practice and designs to use when surfacing unpaved roads with aggregate.

 

2 Driving Surface Aggregate (DSA) is an aggregate gradation that was developed relatively recently specifically for use as a surface for unpaved roads. This material is not as well-known and has more strict construction specifications than another commonly-used aggregate gradation, PennDOT 2A, possibly making it less desirable to use.  Further research is needed to prove if there is a benefit to using DSA over PennDOT 2A.   A rehabilitation practice, full-depth reclamation (FDR), has previously been used to increase roadbed stiffness in paved roads.  The impact of using this method for the design of unpaved and aggregate surfaced roads needs to be evaluated.

Objectives

The objectives of the research were to determine the following:

1. How does Driving Surface Aggregate (DSA) perform compared to PennDOT 2A as a result of accelerated pavement testing?

 

2. How much do the benefits of a cement stabilized base provide against frost and moisture? Is it enough to decrease seasonal parameters in unpaved road design?

 

3. Does lack of a surface overlay on a cement-stabilized full-depth reclamation base affect the performance of the road?

Scope of Research

The research focused on collecting three types of data consisting of rut depths, stiffness measurements, and condition assessments.  Rut depths as a result of accelerated pavement testing with the one-third scale Model Mobile Load Simulator (MMLS3) were measured in order to compare PennDOT 2A aggregate with DSA.  Both stiffness and condition of full-depth reclamation roads were measured and assessed six times in a one-year span; once a month from February to May, once in the summer (August) and once in the fall (October).  This was collected to assess how well the cement-stabilized FDR base held up over the spring-thaw period and in order to determine if an overlay is needed on an FDR road.

IMPROVING UNBOUND AGGREGATE ROADS THROUGH A COMPARISON OF THE PERFORMANCE OF MATERIALS

Sharing is caring!