Concrete Pavement Construction: Subgrades and Subbases

Michael E. Ayers
Global Pavement Consultants, Inc.
Introduction and terminology.
Subgrades and subbases as a design element.
Subgrade characteristics, construction
requirements and quality control/quality
assurance (QC/QA) measures.
Subbase types, materials, construction
requirements and QC/QA procedures.
Subbase (stabilized
in this case)
Subgrade (also
used as a haul
road on this
Subgrade – Usually the naturally occurring soil
on which the pavement is constructed. The
subgrade is compacted to specification and
trimmed to the elevation or grade shown on
the plans.
Subbase – The material directly under the
pavement slab. The subbase is also compacted
to specification and trimmed to the proper
elevation before the concrete is placed.
Concrete pavements require a uniform level of
support for good long-term performance.
Abrupt changes in support can lead to
cracking, poor ride quality and other pavement
Concrete pavements do not benefit
significantly from very strong support layers.
Even repairs require uniform
There are a number of concrete pavement
design methods in common use.
Although each method looks at the support
layers in a slightly different way, none of them
are very sensitive to small changes in support.
In reality, the importance of a good foundation
under a concrete pavement cannot be
Support for concrete pavements is expressed in
terms of “k” or the modulus of subgrade
This value used to be determined by a plate
load test but now is generally correlated with
other strength parameters.
The “k” used in design varies by the design
method, so be sure to check that you use the
correct value.
The easiest way to estimate the effect of “k” is
by looking at required slab thicknesses for a
fixed set of design conditions.
Poor support requires an 8
inch slab
Very good support
requires a 6.4 inch slab
Support layers, particularly subgrade soils may
be prone to the following:
Frost heave.
Designing for these conditions is not realistic.
Therefore, remedial measures must be
employed to resolve these issues prior to
Material properties.
Moisture content.
Equipment type.
Equipment operation.
Subgrade soils are characterized in terms of their
engineering properties.
Gradation (sieve analysis).
 Liquid limit.
 Plasticity index.
 Mineralogy is also important although not a part of the
classification scheme.
The most common characterization for highway
use is the AASHTO Soil Classification System.
Soils classed as A-1 through A-3 are generally
good subgrade soils.
Soils classified as A-4 through A-7 are marginal
and may require remediation.
Subgrades must provide uniform support over
the life of the pavement.
The subgrade should ideally be erosion
resistant (effectively limiting the amount of fine
particles (minus #200 sieve).
The soil should have very limited frost heave
and shrink/swell potential.
It is neither economical or good practice to
compensate for poor soils with increased slab
and subbase thickness.
Soil compaction is strongly related to the
uniformity of the subgrade and
strength/deformation characteristics.
Example of a typical moisture-density curve
Depending on the soil characteristics, different
compaction equipment and methods may be
Photos courtesy of PCA
Depending on the soil characteristics, proper
compaction can alleviate many problems.
For instance, compaction at 1 or 2 points above
optimum moisture content significantly reduces the
expansion characteristics of most soils.
If compaction alone is not sufficient, a number
of options are available to remediate poor
subgrade soils:
Remove and replace.
Chemical stabilization (Portland cement, lime and
fly ash are the most common).
Cross hauling and blending.
Effective stabilization requires the right type of
stabilizer placed in the correct amount,
thoroughly blended and compacted.
Photos courtesy of Joe O’Grady (PCA)
The subgrade soil must be graded to the proper
elevation following compaction.
The tolerance for trimming varies by project
but has a direct bearing on the thickness of the
subbase and potentially the concrete pavement
Trimming operations can utilize survey hubs,
stringlines or electronic guidance controls.
Photos courtesy of Gomaco
and ACPA
The most common QC/QA measure for
subgrade soils is relative density.
The density is usually expressed as a
percentage of the maximum dry density as
determined by a Standard Proctor test
(AASHTO T-99).
Field testing typically relies on nuclear density
gage results although other devices may also
be used.
Typical Testing Requirements for
Requirements for subgrade
depths can vary from as
little as 6 inches (150 mm)
up to 2 feet (0.6 m)
depending on governing
Measures in-place
engineering values
using structural layer
stiffness, klbf/in
(MN/m) and Young’s
Modulus of a material,
kpsi (MPa).
Modified subgrade MUST be
stable before next pavement
course is constructed!
Proof-rolling is the most
commonly accepted practice.
There are 2 general types of subbases for
concrete pavements.
Unbound granular materials.
Stabilized granular materials.
The primary purpose of the subbase is to
provide uniform support and minimize erosion
and faulting under traffic.
Subbases are not required for pavements with
low traffic volumes but may be advantageous
from a constructability standpoint.
A variety of materials may be used including,
but not limited to, the following:
Crushed stone.
Crushed gravel.
Recycled concrete or asphalt.
Others depending on locally available materials.
Important material parameters include:
Gradation (particularly the maximum aggregate size
and the amount passing the #200 sieve.
Durability and abrasion resistance.
Availability and cost.
Unbound subbases are generally 4 to 6 inches
Thicker layers are rarely justified due to
performance issues and higher material and
placement costs.
Conventional thicknesses are placed in one lift
and compacted to either AASHTO T-99
specifications or the more stringent Modified
Proctor specification (T-180)
Stabilized granular subbases can generally
make use of the same materials as the unbound
The most widely used type of stabilized
subbases include the following:
Portland cement (commonly termed CTB or cement
treated base).
Asphalt cement (commonly termed ATB or asphalt
treated base).
Stabilized subbases dramatically increase the
“k” value and may reduce slab thickness.
Construction methods
Road mixed.
 Place cement with spreader.
 Mix with single-pass or multiple-pass mixers.
Central-plant mixed.
 Blend granular material, cement, and water
 Spread to the proper depth and width.
Compact, finish to grade, cure.
A common gradation
requirement is for
100% to pass the
1.5-inch (38 mm)
sieve and a minimum
of 60% to pass the
No. 4 (4.75 mm)
sieve (ASTM C136).
A common moisture
requirement is to be
within 2% of the
laboratory established
optimum moisture
content (ASTM D558).
The density
requirement is to be a
minimum of 95% of
the established
laboratory standard
Proctor density (ASTM
Research and experience have shown that
properly constructed subgrades and subbases
can add substantially to concrete pavement
longevity and performance.
It is neither cost effective or practical to
compensate for support deficiencies with
increased slab thickness.
Proper materials characterization and/or
selection are required.
A stable subgrade is necessary for adequate
compaction of the subbase layer.
The pavement support layers provide a
“working platform” during construction.
Stabilized subgrades and subbases can provide
enhanced pavement performance.
For additional information, please contact
Michael Ayers, Ph.D.
 Global Pavement Consultants, Inc.
 Cell 2127-621-3438