Research In Progress
Pavements
Home > Soil-Cement > Cement-Treated Bases > CTB Research Evaluation
of Soil Cement Bases in the New AASHTO 2002 Pavement Design Guide
Principal Investigator Tom Scullion, Texas Transportation
Institute
Objectives
AASHTO is in the final stages of developing a new flexible pavement
design guide to be used by all State Highway Agencies (SHA’s)
for layer thickness design. For the first time this procedure will
be mechanistically based with the design life for the Soil Cement
(S-C) Base computed from flexural fatigue considerations. The material
property to be used in design will be the resilient modulus value
of the S-C base which can be obtained either from dynamic laboratory
testing or backcalculated from deflection data collected with Falling
Weight Deflectometers. Frequently with S-C bases arbitrary factors
are also included in the design process to reduce their modulus
value to account for shrinkage cracking. In the analysis procedure
the tensile strains induced at the bottom of the S-C layer by the
design wheel load are computed using a layered elastic program.
These are then used to calculate the pavement life in terms of the
number of repetitions to cause load associated slab cracking.
It is important to realize that this is a major national development
effort but to date the focus of the AASHTO team has been on asphalt
stabilized and granular base materials and little consideration
has been given to Soil Cement bases or the benefits of Cement Modified
soils (CMS). This new design approach presents both opportunities
and challenges to the cement industry. S-C bases typically have
high moduli values often 10 times that of unstabilized granular
materials. However it is also important to safeguard against under
design where thin layers are placed which may fatigue rapidly under
heavy truck loadings.
The new procedure is scheduled for preliminary release in later
2000 and Texas is a lead state. The role of the lead states will
be to pilot test the proposed procedure and make recommendations
to AASHTO before general release. The goal of the proposed research
project is to prepare PCA for this new design procedure, to document
existing practices with mechanistic design, to develop recommendations
on how to measure resilient modulus in the lab and field and to
develop realistic values for a range of both S-C bases and CMS subgrades.
Market Significance of Project
All SHA’s in the US are required to use the approved AASHTO
design procedure for all federally aided projects. It is important
that realistic design moduli values be used for both S-C bases and
CMS subgrades. It is important for PCA to gear up for both the opportunities
and challenges that this potential major change will provide.
Utilization of Results
The results generated by this study will be used to review the
reasonableness of the recommendations of the AASHTO research team
and to provide documented evidence of in-situ material properties
and pavement performance. As part of this study laboratory test
procedures for determining the moduli values for S-C bases will
be studied and recommendations developed for possible consideration
by ASTM.
Project Description
While complete details of the new procedure are not known it is
anticipated that it will provide pavement engineers with 3 design
options with different levels of sophistication. At the highest
level (1) designs will involve extensive laboratory testing of all
the materials in each layer of the pavement. The primary design
values will be Resilient Modulus and Poisson Ratio of all pavement
layers. In level 2 designs the material properties will be obtained
from standard test results such as unconfined compressive strength.
Level 3 designs will involve table look ups. It is anticipated that
the vast majority of designs will be either levels 2 or 3 with level
1 restricted to research or a few major projects. In each case the
PCA should have recommended test procedures and moduli values for
all of the materials of interest. The work plan below includes tasks
to a) summarize what we already know in this area (Task 1), b) evaluate
the design properties in the lab and field (Tasks 2 and 3) and,
c) conduct a sensitivity analysis of the AASHTO recommended procedure
and provide comparisons with currently available procedures (Task
4).
Work Program Tasks (Including Deliverables):
Task 1.0 Literature Search
As the first phase of this project it will be necessary to document
the basics of mechanistic pavement design for Soil-Cement (S-C)
bases and Cement Modified Soils (CMS). This will include reviewing
the design practices in DOT’s around the US and agencies around
the World.
It is known that other countries particularly in Europe, Australia
and South Africa already have implemented mechanistic design procedures.
This knowledge should be captured and summarized. In conducting
this literature search the following areas will be investigated:
- Methods of measuring layer moduli in laboratory
Most methods in current use were developed for either asphalt
or unstabilized soils and base materials. The current laboratory
procedures use expensive test equipment and may not be adequate
for stiff soil cement materials. New procedures are under evaluation
such as seismic methods which are simpler and less expensive, these
will be evaluated in Task 2. It will also be useful to review the
correlations that exist between resilient modulus and more common
parameters such as unconfined compressive strength.
- Methods of measuring moduli in the field
Several DOT’s prefer to develop moduli values for field
testing preferably analysis of deflection data. The benefit of
field testing is that long term studies can be conducted and the
impact of environment and shrinkage cracking can be determined.
For CMS the dynamic cone penetrometer (DCP) is increasing used
to evaluate the permanency of soil stabilization and to develop
field moduli values. The existing methods will be summarized.
This will be studied further in Task 3.
- Typical moduli values used by SHA’s for S-C bases and CMS
layers
The values in common use by agencies should be documented.
- Methods of accounting for shrinkage cracking
One popular design approach is to assume that the S-C base will
crack and that this cracking will reduce the effective modulus
of the base and increase the tensile strain at the bottom of the
layer. The goal of the thickness design is then to provide sufficient
thickness to prevent secondary load associated cracking which
initiates at the shrinkage cracking. This is done by either Finite
Element Analysis of by field test results over cracks. Substantial
work has already been performed in this area and it should be
summarized.
- Existing mechanistic design methods for stabilized layers.
Several agencies and countries have developed and implemented
mechanistic procedures. These experiences should be capture for
use later in this project.
An important part of this review will be to compare the recommendations
from AASHTO with the current practices in the US and around the
World. It is anticipated that the draft AASHTO procedure will
be available in late 2000 or early 2001. This comparison will
also be included in this task.
Deliverables: Literature review will be complete 9 months after
project initiation
Task 2.0 Laboratory Determination of Layer moduli for
Cement modified materials
Given that the design property of interest (resilient modulus
and poisson’s ratio) are known the PCA should be in a position
to recommend realistic values and to provide information on how
to obtain values from either field or laboratory test. In this
task it is proposed to review the different methods of determining
moduli values and to compare the tradition triaxial test method
with the new seismic methods.
The traditional method (which will be recommended by AASHTO)
involves fairly sophisticated and expensive test equipment which
is not widely available. The resilient modulus test has not been
optimized for C-S samples with regard to sample size, stress conditions,
sensor placement and bias and precision. These will be studied
as the first priority in this task.
Once a test procedure is finalized it is proposed to measure
the resilient modulus properties on a variety of C-S and CMS samples.
The array of materials and test conditions to be studied will
include,
| C-S type |
Open Graded CTB, ACP Reclaimed with Cement, Soil-Cement |
| CMS |
Two types (one low one high PI) |
| Percentage Cement |
4 different levels will be used |
| Age |
7 days, 28 days, 6 months 1 year |
| Wet/Dry |
Influence of moisture |
As part of these studies the resilient modulus will be measured
with both a triaxial procedure and the low cost automated seismic
method developed by the Texas DOT. In this method the shear wave
velocity is measured using an accelerometer and instrumented hammer.
As both tests are nondestructive measurements can be made on the
same samples.
Deliverables: Recommendations on Resilient Modulus values for typical
C-S and CMS materials and a recommended lab testing procedure
Task 3.0 Comparing Laboratory and Field Moduli values
Techniques for obtaining field moduli values for deflection equipment
are well established. However limited work has been done at comparing
these field moduli values to lab values for C-S and CMS materials.
In is proposed to collect Falling Weight Deflectometer (FWD) data
on an array of pavements with either S-C bases or CMS soils. Samples
of the S-C and CMS will them the extracted from the pavement and
returned to the laboratory for testing. A minimum of three bases
and three subgrades will be tested in this task
Deliverables: Recommendations on how to determine field moduli
values from field deflection data and how these correlate with laboratory
determine values
Task 4.0 Sensitivity Analysis of new AASHTO procedure
Once the proposed AASHTO system is available it is proposed to
use the system to perform a sensitivity analysis to compare thickness
predictions with those from existing procedures. The design values
obtained from the laboratory test conducted in task 2 will be used
as part of this analysis.
Deliverables: Recommendations to PCA within 2 months of the release
of the AASHTO recommendations
Task 5.0 Implementation Recommendations and Final Report
Deliverables: A final report documenting all research findings
will be submitted to PCA at the end of the study
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