AASHTO Method for Flexible Pavement Desi

GRID-DE-6
1999 TENAX Corporation
4800 East Monument Street
Baltimore, Maryland 21205
tel: (410) 522 - 7000
fax: (410) 522 - 7015
TENAX
Technical Reference GRID-DE-6
GEOGRID REINFORCEMENT OF FLEXIBLE PAVEMENTS:
A PRACTICAL PERSPECTIVE

GRID-DE-6
Geogrid Reinforcement of Flexible Pavements: A Practical Perspective
By Aigen Zhao and Paul T. Foxworthy
Recent efforts by the AASHTO Subcommittee on Materials, Technical Section 4E, to develop a
geogrid/geotextile specification for pavement reinforcement have initiated very positive
discussions. The Geosynthetic Materials Association has participated in the discussion and made
recommendations to AASHTO with the presentation of a draft “White Paper” addressing
installation survivability and specifications. The overwhelming comments back from the
reviewers of the “White Paper” clearly show the need to 1) demonstrate the performance and
cost benefits of geogrid reinforcement, and 2) develop a design procedure incorporating geogrid
with value-added benefits, in addition to the installation survivability aspects already well
documented.
Geogrid reinforcement has been used in the design and construction of pavements for over a
decade, yet there exists no design method incorporating geogrid mechanical properties as direct
design parameters. Due to the complexity of layered pavement systems and loading conditions,
there may never be a simple design method identifying the properties of a geogrid as direct
design parameters for reinforced pavement systems. Rather, a series of performance based tests
should be conducted to evaluate the structural contribution of geogrid reinforcement to pavement
systems, from which design parameters could be derived and incorporated into a design
methodology.
This paper presents a practical perspective to address: 1) a modified AASHTO design method for
reinforced pavements, 2) performance tests to support and verify the design parameters, and 3)
cost benefit and constructability analyses. Performance data and analyses presented here are
limited to multilayered polypropylene biaxial geogrids.
Modified AASHTO Design Method for Geogrid Reinforced Flexible Pavements
Existing design methods for flexible pavements include: empirical methods, limiting shear
failure methods, limiting deflection methods, regression methods, and mechanistic-empirical
methods. The current AASHTO method is a regression method based on the results of road tests.
The AASHTO method utilizes an index termed the “structural number” (SN) to indicate the
required combined structural capacity of all pavement layers overlying the subgrade. The
required SN is a function of reliability, serviceability, subgrade resilient modulus, and expected
traffic intensities. The actual SN must be greater than the required SN to ensure long term
pavement performance.
The actual SN value for a unreinforced pavement section is calculated as follows:
22211 mdadaSN ∗∗+∗= Eq. (1)
where a1 a
2 are the layer coefficients characterizing the structural quality of the asphaltic
concrete (AC) layer and the aggregate base course (BC) in a pavement system. A subbase layer

GRID-DE-6
can be included in Eq. (1) if desired. d1, d2 are their thicknesses; and m2 is the drainage
coefficient for the granular base.
A modification to equation (1) is introduced to account for the structural contribution of a
geogrid reinforcement to flexible pavements.
Eq. (2)
where LCR is the layer coefficient ratio. Equation (2) can be used to calculate the base course
thickness for geogrid reinforced pavements by rearranging its terms:
Eq. (3)
When the layer coefficient ratio, LCR, is greater than 1, the thickness of the geogrid reinforced
base course is reduced compared to unreinforced sections; similarly, if the base course thickness is
held constant, the structural number of the reinforced section increases. An increased structural
number implies an extended service life of the pavement for the same traffic level.
The concept of layer coefficient ratio was introduced over a decade ago (Carroll, Walls and Haas
1987, Montanelli, Zhao, and Rimoldi, 1997) to quantify the structural contribution of a geogrid
in a flexible pavement. This concept was established based on the reinforcing mechanism that
geogrid provides lateral confinement to the base course material and improves the layer coefficient
of the reinforced base. The next section addresses the controlled laboratory pavement tests
performed to develop this design parameter for multilayered polypropylene biaxial geogrids. The
following sections provide field verification through nondestructive tests and full-scale in-ground
tests.
Controlled Laboratory Pavement Testing
Laboratory tests were performed to study flexible pavement systems under cyclic loading
conditions, and to quantify the structural contribution of a geogrid reinforcement. The test setup
is shown in Figure 1. Cyclic loading was applied through a rigid circular plate with a diameter of
300 mm. The peak load was 40 kN with an equivalent maximum stress of 570 kPa. Asphaltic
concrete, aggregate base course and subgrade soil layers were included in the pavement sections.
The asphalt thickness was 75 mm, and the base thickness was 300 mm. A multilayered
polypropylene geogrid manufactured by continuous extrusion and orientation processing was
used in the test, its properties are listed in Table 1. The details of the laboratory tests are
presented by Cancelli et al. (1996).
SN a d LCR a d m
=∗+ ∗
11 2 2 2
**
22
11
2*maLCR daSN
d∗∗
−
=

GRID-DE-6
Figure 1. Controlled laboratory pavement tests
Table 1. Properties of the Multilayered Geogrid Used in the Tests
Machine Direction Cross Machine Direction
Unit weight g/m2 240
Open Area % 75
Peak tensile strength kN/m 13.5 20.5
Tensile modulus @2% strain kN/m 220 325
Tensile modulus @5% strain kN/m 180 260
Junction strength kN/m 12.2 19.2
Figure 2 shows pavement surface rutting for both control and geogrid reinforced sections. The
number of loading cycles versus subgrade CBR is presented in Figure 3 for rut depths of 12.5mm
and 25 mm respectively.

GRID-DE-6
100 1000 10000 100000
CYCLE, [-]
0
50
100
150 VERTICAL SETTLEMENT, [mm]
300 mm GRAVEL
Unreinforced CBR 1%
Reinforced CBR 1%
Unreinforced CBR 3%
Reinforced CBR 3%
Unreinforced CBR 8%
Reinforced CBR 8%
Unreinforced CBR 18%
Reinforced CBR 18%
Figure 2. Pavement surface ruts for control and reinforced sections.
10
100
1000
10000
100000
1000000
0 3 6 9 12 15 18
CBR, [%]
CYCLE, [-]
Unreinforced 25mm RUT
Reinforced, 25mm RUT
Unreinforced 12.5mm RUT
Reinforced, 12.5mm RUT
Figure 3. Loading cycle number for control and reinforced at two rut depth.
Figure 4 depicts the relationship between the calculated layer coefficient ratio and subgrade CBR
based on pavement testing data from both control and reinforced sections. The layer coefficient