17 Comparative Analysis of Flexible Pavement Design Methods …
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pavement design charts were developed based on the maximum strain at the bottom
of the surface layer and the highest vertical compressive strains at the top of the
subgrade layer. The basic principle in the asphalt institute method is determining
the least thickness of the pavement surface to sufficiently resist the compressive
stresses at the surface and subgrade layer, and the tensile strain below the asphalt
layer developed. Design charts for different range of traffic loads have been prepared
(Nicholas and Lester 2002). The four different criteria required to determine proper
thickness of the pavement layer in Asphalt Institute method are traffic loading of the
pavement in ESALs; material properties, especially the resilient modulus of the soil
at the foundation layer; the mean annual air temperature of the pavement location,
the desired base materials: granular or hot mix asphalt (Lavin 2003). The detail
procedure for determining pavement thickness using the asphalt institute method
can be found in (Nicholas and Lester 2002).
17.2.2 AASHTO 1993 Method
This is an empirical method for pavement design developed by AASHTO. The result
of the field performance of roads conducted in Ottawa and Illinois in 1958–60 forms
the basis of this method. The approach of this method is to design for specific loss in
serviceability at the end of the pavement intended life (AASHTO 2001). The stresses
due to variations in temperature and moisture, traffic loading, time constraints and
other design variables are used to design the pavement layers as to make the pavement maintain the required serviceability throughout the pavement design life (Khan
et al. 2012). Inputs for the design of flexible pavement in AASHTO procedure were
classified into (i) design variables (time constraints, traffic, reliability, environmental
impacts) (ii) performance criteria (serviceability, allowable rutting, aggregate loss)
(iii) material properties for structural design (effective roadbed soil resilient modulus,
effective modulus of subgrade reaction, pavement layer material characterization,
PCC modulus of rupture, layer coefficient) and (iv) the structural characteristics (
drainage, load transfer, loss of support).
17.2.3 California Bearing Ratio CBR Method
In 1928, the highway division in California developed the first empirical method to
design flexible pavement which is known as the CBR method. United States Army
Corps adopted the method in 1945. The thickness of the pavement constituents is
assessed by the strength of the subgrade expressed as CBR index value (Pereira and
Pais 2017). The ratio of test load to the standard load for specified plunger penetration
expressed in percentage is defined as CBR (Gill and Maharaj 2015). Thickness of
the surface, base and subbase layers are obtained using results of the CBR test
conducted on the subgrade, base and subbase materials combined with the empirical
charts. Pavement thickness depends on the CBR of value of the subgrade and the
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pavement design charts were developed based on the maximum strain at the bottom
of the surface layer and the highest vertical compressive strains at the top of the
subgrade layer. The basic principle in the asphalt institute method is determining
the least thickness of the pavement surface to sufficiently resist the compressive
stresses at the surface and subgrade layer, and the tensile strain below the asphalt
layer developed. Design charts for different range of traffic loads have been prepared
(Nicholas and Lester 2002). The four different criteria required to determine proper
thickness of the pavement layer in Asphalt Institute method are traffic loading of the
pavement in ESALs; material properties, especially the resilient modulus of the soil
at the foundation layer; the mean annual air temperature of the pavement location,
the desired base materials: granular or hot mix asphalt (Lavin 2003). The detail
procedure for determining pavement thickness using the asphalt institute method
can be found in (Nicholas and Lester 2002).
17.2.2 AASHTO 1993 Method
This is an empirical method for pavement design developed by AASHTO. The result
of the field performance of roads conducted in Ottawa and Illinois in 1958–60 forms
the basis of this method. The approach of this method is to design for specific loss in
serviceability at the end of the pavement intended life (AASHTO 2001). The stresses
due to variations in temperature and moisture, traffic loading, time constraints and
other design variables are used to design the pavement layers as to make the pavement maintain the required serviceability throughout the pavement design life (Khan
et al. 2012). Inputs for the design of flexible pavement in AASHTO procedure were
classified into (i) design variables (time constraints, traffic, reliability, environmental
impacts) (ii) performance criteria (serviceability, allowable rutting, aggregate loss)
(iii) material properties for structural design (effective roadbed soil resilient modulus,
effective modulus of subgrade reaction, pavement layer material characterization,
PCC modulus of rupture, layer coefficient) and (iv) the structural characteristics (
drainage, load transfer, loss of support).
17.2.3 California Bearing Ratio CBR Method
In 1928, the highway division in California developed the first empirical method to
design flexible pavement which is known as the CBR method. United States Army
Corps adopted the method in 1945. The thickness of the pavement constituents is
assessed by the strength of the subgrade expressed as CBR index value (Pereira and
Pais 2017). The ratio of test load to the standard load for specified plunger penetration
expressed in percentage is defined as CBR (Gill and Maharaj 2015). Thickness of
the surface, base and subbase layers are obtained using results of the CBR test
conducted on the subgrade, base and subbase materials combined with the empirical
charts. Pavement thickness depends on the CBR of value of the subgrade and the