Use of FWD deflection basin parameters (SCI, BDI, BCI) for pavement condition assessment/Deflektometru nustatyto ilinkio dubens parametru (PII, PPI, PGII) naudojimas dangu buklei vertinti/Kritosa svara delektometra (FWD) izlieces baseina parametru (SCI, BDI, BCI) izmantosana segas stavokla novertejuma/FWD vajumikausi parameetrite (SCI, BDI, BCI) kasutamine katendi seisukorra hindamisel.
Talvik, Ott ; Aavik, Andrus
1. Introduction
Arraigada et al. (2009) have been studied the use of accelerometers
to measure pavement deflections due to traffic loads. The finite element
models (FEMs) revealed the inability of the accelerometers to measure
very slow or quasistatic motion. Chea and Martinez (2008) were carried
out three-dimensional finite element (3D FE) simulations of the
deflection under a standard axle load in order to detect interface flaws
between bituminous and hydraulic layers of composite pavements
Most backcalculation programs used to evaluate the pavement layer
properties assume static deflections even though dynamic deflections are
generated from the Falling Weight Deflectometers (FWD). Losa et al.
(2008) proposed a statistical model for the straight evaluation of
critical strains in pavements by using the deflections measured by the
FWD and the layer thicknesses without backcalculating layer moduli. The
model was calibrated on the basis of experimental data and it is useful
to evaluate statistical parameters of the homogeneous sub-sections with
the aim to evaluate the residual pavement life taking into account the
reliability concepts. A pseudo-static backcalculation procedure Dynamic
BALMAT (DYN-BAL) was developed by Seo et al. (2009) to calculate the
layer moduli after converting dynamic deflections into static
deflections. From the test results, it was found that DYN-BAL gives the
most reliable results when compared with several other computer codes in
use. The results of Bayrak and Ceylan (2008) study demonstrated that the
ANN-based models, which were trained to predict the layer moduli by
using the FWD deflection basin data and the thickness of the concrete
pavement structure, are capable of successfully predicting the rigid
pavement layer moduli with high accuracy.
A dynamic analysis based on the spectral element method Grenier et
al. (2009) described for the interpretation of FWD tests on flexible
pavements. While the deflection basin currently used in static methods
gives some details of the pavement response under transient loading, the
simulations of FWD tests using the dynamic model suggest that the time
histories should be included as well for the interpretation of FWD
deflection measurements. In fact, important dynamic phenomena due to
inertial effects and viscous effects are only revealed by deflection
histories. Grenier and Konrad (2009) presented a robust backcalculation
methodology that uses the Levenberg-Marquardt iterative minimization
technique to identify the value of unknown layer parameters from FWD
tests using a dynamic approach based on the spectral element method,
too. The efficiency of the proposed methodology is demonstrated by
interpreting FWD tests on three flexible pavements that cover a variety
of structures, soil, and bedrock conditions. Results indicate that the
dynamic approach is capable of simulating quite well the measured
deflection histories using effective backcalculated moduli. In addition,
comparison of critical strains between static and dynamic interpretation
of FWD tests indicates that both approaches predict similar traction
strains at the bottom of the asphalt concrete layer. However, the
prediction of the compression strain in the subgrade with the static
approach is erratic compared with the dynamic method. Donovan and
Tutumluer (2009) presented a methodology based on analyzing FWD test
data between trafficked and non-trafficked lanes to determine the
degradation and rutting potential of flexible pavement unbound aggregate
layers in comparison to the subgrade damage.
According Dawson et al. (2009) several procedures can be used for
the determination of the resilient modulus: laboratory testing,
backcalculation with Non-Destructive Testing (NDT) data, and
correlations to other soil parameters (California bearing ratio,
density, and water content). Backcalculation with NDT data procedure is
relatively inexpensive and fast and can be designed to cover
representative soils under the pavement network. NDT devices are used to
determine pavement structural capacity and for pavement condition
assessment. FWD are mostly used NDT devices all over the world because
of the testing accuracy, repetitiveness and similarity to the real
loading magnitude and duration.
Since using the NDT devices, many different parameters have
developed describing their deflection basins. The main purpose of the
parameters is to evaluate whole pavement or single layer condition.
Widely used FWD deflection basin parameters (DBPs) are presented in
Table 1. Different researches (Kim et al. 2000; Park 2001; Tiehallinnon
2006) have shown their utility possibilities as calculating pavement
layers modulus of elasticity (E modulus) or assessing pavement
structural condition. Current research focuses on the three basic DBPs
(SCI, BDI, BCI) and is trying to find relationship between FWD
deflections and pavement condition:
--Surface Curvature Index (SCI)--difference of deflections measured
with load cells in the center of the loading plate (d0) and 300 mm from
the center (d300): (d0 - d300), which is characterizing condition of the
pavement layers;
--Base Damage Index (BDI)--difference of deflections measured with
load cells in the distance 300 mm ([d.sub.300]) and 600 mm
([d.sub.600]): ([d.sub.300]-[d.sub.600]), which is characterizing
condition of the base layers;
--Base Curvature Index (BCI)--difference of deflections measured
with load cells in the distance 1200 mm (d1200) and 1500 mm
([d.sub.1500]): ([d.sub.1200]-[d.sub.1500]), which is characterizing
condition of the subgrade.
2. Initial database of the research and analysis of data
The aim of the current research was to study employing of FWD
deflection basin parameters for pavement condition assessment in
Estonia. As the analysis had to rely on larger data than 26 control FWD
measurement points (measured 1999-2006 every year) used in the
researches until now, it was decided to construct database based on
paved state road network data. The data of defects (29790 100 m
sections), rut depths (24333 100 m sections) and FWD measurements (37936
points) was derived from the Estonian Road Data Bank to the initial
database. Additionally, different pavement types and traffic loadings
were taken into account.
2.1. Analyze groups of the research ans analysis of data
As pavement design depends on forecasted traffic loading in the end
of service life, it was purposeful to divide analyze groups according to
traffic loadings. It has to be mentioned that most Estonian roads have
reached the end of their service life, but the traffic loading for the
current analysis was determined according to the actual traffic volumes
based on the counting data of 2006. Estonian Standard for Road Design
(Metsvahi et al. 2005) is determining required min equivalent E modulus
([E.sub.req]) and related forecasted traffic loadings and those were
used for dividing data into analyze groups (Table 2).
2.2. Transformation of measured deflections to the standard load
level and standard temperature
During the standard FWD measurement the dropped weight and dropping
height are always the same, but in reality, the load applied to the
pavement depends on site conditions. Applied load ([p.sub.measured]) is
affected by the pavement stiffness, its surface profile and the
properties of the FWD device.
To have comparable deflection values, they have to be normalized to
the standard load by multiplying by the factor
([p.sub.target]/[p.sub.measured]). In our case the target load is 50 kN
as standard axle load used in Estonia for pavement design is 100 kN. The
contact pressure equivalent on a 300 mm diameter plate for 50 kN load is
707 kPa according COST 336:1999 Falling Weight Deflectometer.
E modulus of the bituminous-bounded layers is dependant on the
temperature. Therefore, measured deflections of the same structure at
different temperatures are different and depending on the stiffness of
the bituminous-bounded layers. During the FWD measurements the
temperature of the bituminous pavement can vary in the range +5 ...
+35[degrees]C. As result of this the measured deflection values have to
be corrected to the standard temperature. According to the Estonian
guidelines for flexible pavement design Procedure 2001-52, in the case
of calculation of the pavement structure to the elastic deformation,
standard temperature is +10[degrees]C. For correction of FWD measured
deflection values to the standard temperature (+10[degrees]C) can be
used temperature correction factors (Kt), calculated using Eqs in Table
3 (Aavik 2003), depending on the bituminous pavement type and the
average temperature of the bituminous layer during the FWD measurement
(T).
FWD measured deflections are transformed to the standard load level
(50 kN) and standard temperature (+10[degrees]C) using following Eq:
[d.sub.r50kNT] = [d.sub.r] x ([p.sub.target]/[p.sub.measured]) x
[K.sub.t], (1)
where: [d.sub.r50kNT]--deformation at the load 50 kN and
temperature +10[degrees]C at the distance r (mm) from the center of the
loading plate, [micro]m; [d.sub.r]--FWD measured deflection at contact
pressure [p.sub.measured] (kPa) at the distance r (mm) from the center
of the loading plate, [micro]m; [p.sub.target]--contact pressure,
corresponding to the 50 kN load ([p.sub.target] = 707 kPa);
[K.sub.t]--temperature correction factor (Table 3).
2.3. Relationship between Deflection Basin Parameters (DBPs) and
pavement defects
In the current research only longitudinal cracking and alligator
cracking (fatigue cracking) were examined. Those types of defects are
forming if whole pavement or single layers have insufficient structural
capacity and therefore bituminous layers submit fatiguing easier.
As data of these surface deflections has gathered separately, the
Partial Defect Sum (PDS) parameter was taken into use to give better
comparison. PDS is describing extent of cracks in % of road surface area
on the section of 100 m (Eq 2). The Eq 2 is found on the basis of Defect
Sum Eq (used in Estonian Pavement Management System), where are
presented all types of defects with their weight coefficients.
PDS = (0.5 X LCRACK + 1.0 X ALLIG) X 100/RWIDTH X 100, % (2)
where: LCRACK--length of longitudinal cracks, m; ALLIG--extent of
alligator cracking, [m.sup.2]; RWIDTH--width of road, m.
Analysis of data showed that there is no definite relationship
between DBPs and investigated road surface defects. It was clearly
stated that presenting any of DBPs and PDS on the graph, the dispersion
of data is extensive. Values of determination coefficients (R2) were
less than 0.1 ([R.sup.2] < 0.1), which is indicating the absences of
relationship. For example the SCI vs PDS of dense asphalt concrete
surface of analyze Group 2 is presented in the Figure 1.
[FIGURE 1 OMITTED]
Main reason for absence of the relationship can be the difference
between data collection principles: FWD measurements are performed only
once per every 100 m and the measurement represents only the condition
of the pavement at this exact point, but other condition indicators are
collected from all length of the 100 m sections. In addition to that the
determinations of defects and FWD measurements have been done in
different times.
2.4. Relationship between DBPs and rut depth
Measurements of rutting in Estonian roads are performed two times a
year: in the spring and in the autumn. The depth of rut in the spring is
usually smaller than in the autumn, because of different driving
trajectory with studded tires in the winter. Reducing the affect of
studded tires to the data of rut depths and to survey better permanent
deformations, rut depths measured in the autumn were only taken into
account.
As the FWD measurements are carried out on the spot of the right
wheel lane, the rut depth is taken also from the right wheel lane.
[FIGURE 2 OMITTED]
It is clearly perceivable in the Fig. 2 that similar rut depths
have appeared in the case of different values of SCI. The same situation
appeared with other parameters and groups. It is complicated to
determine relationships because the rut depths are presented as mean
values of the sections, but DBPs represent the pavement condition at the
exact point.
2.5. Relationship between Deflection Basin Parameters (DBPs) and
pavement equivalent E modulus ([E.sub.eq])
The Eq for back-calculation of pavement equivalent E modulus
([E.sub.eq]), which is expressed in the BCH 46-83 [TEXT NOT REPRODUCIBLE
IN ASCII] muna" [Guidelines for Flexible Pavement Design VSN 46-83]
(the previous Soviet Union flexible pavement design procedure), which
derivation the Procedure 2001-52 is as follows:
[E.sub.eq] = 0.25[pi] x (1 - [v.sup.2]) x F x S/[d.sub.0] (3)
where: [E.sub.eq]--pavement equivalent E modulus at the center of
the loading plate, MPa; v--Poisson's ratio (in Procedure 2001-52 v
= 0.3); F--contact pressure under the loading plate, kPa; S--diameter of
the loading plate, mm; [d.sub.0]--deflection at the center of the
loading plate, [micro]m.
The Eq for the calculation of the [E.sub.eq] comparable with the
Procedure 2001-52, taking into account possible different known
influencing variables, can be written in the form (Aavik 2003):
[E.sub.eq2001-52] = C x [E.sup.e.sub.eq] x [T.sup.t] x [R.sup.r] x
[M.sub.i] x [H.sub.j], (4)
where [E.sub.eq]--pavement equivalent E modulus at the center of
the loading plate, MPa, calculated using Eq (3); T--mean temperature of
the bituminous pavement surface at the moment of FWD measurement,
[degrees]C; R--summarized amount of rainfall in 30 days before FWD
measurement, mm; [M.sub.i]--factor taking into account the month when
FWD measurement is performed (i = 4, 10, April-October);
[H.sub.j]--factor taking into account the height of embankment at the
FWD measurement site (j = < 0.5 m; 0.5-1.0 m; > 1.0 m); C, e, t, r
- empirical constants.
As during the FWD measurements carried out in the network level the
height of the embankment or amount of the rainfall is not known at every
measurement site, the Eq (4) can be transformed as follows:
[E.sub.e9200l-52] = C x ]E.sup.e.sub.eq] x [T.sup.t] x [M.sub.i]
(5)
where e = 0.793; t = 0.098; C = 2.039 (Aavik 2003) and factors
taking into account the month M; according to the Table 4.
In the analysis onwards used [E.sub.eq] are calculated using Eq
(5).
Relationships between DBPs (SCI, BDI, BCI) and back-calculated
[E.sub.eq] (Eq (5)) were analyzed. There were examined separately the
pavements with and without surface defects, to determine, if there are
differences between distressed pavements and undamaged [E.sub.eq] and
DBPs. In all analyze groups different pavement types were studied
separately.
Getting visual survey from characters relationships, they were
presented in the dispersion graphs and regression curves were added. It
can to be seen from the Figs 3 and 4, that most suitable regression line
is power function in the form: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE
IN ASCII].
It was found, that relationship between DBPs and [E.sub.eq2001-52]
are quite strong. Relations SCI-[E.sub.eq2001-52] and
BDI-[E.sub.eq2001-52] are described by mathematical functions, which
give values of [R.sup.2] between 0.5-0.9. In the case of relations
BCI-[E.sub.eq] values of [R.sup.2] were smaller than 0.5. Also was
found, that among the roads, forming the database, appeared to be
different values of BCI (characterizing condition of subgrade) in the
case of similar E modulus. In the case of different values of SCI and
BDI (characterizing upper layers) the divergence of Eeq was smaller.
This confirms that pavement structural capacity in Estonian roads is
assured if there are strong upper layers on the weak subgrades. While
the upper layers are also weak, then the whole bearing capacity of the
road structure is insufficient. It is illustrated on the Figs 3 and 4,
how the value of the DBP was determined according to the min required E
modulus ([E.sub.req2001-52]).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
During the analyses was found, that there exists no tendency, like
pavements with surface defects have bigger values of DBPs. Nevertheless,
it was recognized, that values of DBPs are decreasing if the traffic
loading (or [E.sub.req]) is increasing. This confirms that pavements
with higher structural capacity have smaller deflections under the
loading.
As usually the [R.sup.2] of mathematical models describing
BCI-[E.sub.eq2001-52] were lower than 0.5 and mostly lower than 0.3,
then pavements that are stabilized with complex binders (bitumen +
cement) were found to have higher [R.sup.2] values (0.55-0.88). This
shows that subbases with good structural capacity affect strongly on
BCI-[E.sub.eq2001-52] relationship.
2.6. Determination of limit values for Deflection Basin Parameters
(DBPs)
On the assumptions of preliminary analysis, based on the min
[E.sub.req] of particular pavement, the Eqs were developed for
calculation the max limit values of deflection basin parameters for
different types of pavements. Graphs, where [E.sub.req] and DBPs are
presented, were composed to as many pavement types as possible. Used
mathematical models are power functions, because of the former research
which showed non-linearity between parameters and E modulus:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (6)
where: x - min [E.sub.req], MPa; y - deflection basin parameter
(SCI, BDI, BCI); [a.sub.0], [a.sub.1] - constants according to Table 5.
For example relationship between SCI-[E.sub.req] of dense asphalt
concrete on top of existing pavement (Fig. 5) can be described as
follows:
SCI = [1795660E.sup.-1.70.sub.req], [R.sup.2] = 0.83. (7)
[FIGURE 5 OMITTED]
Based on similar Eqs it is possible to calculate max limit values
for min [E.sub.req] (Table 6).
The Eq for calculation min [E.sub.req] according Procedure 2001-52
is following:
[E.sub.req] = (alog(Q) + b)[K.sub.tt], (8)
where Q - (forecasted) traffic load, standard axle load vpd
([E.sub.req] [greater than or equal to] 2); a, b--constants (Table 7);
[K.sub.tt] - pavement strength factor (Table 8).
3. Conclusions
Even though it was not succeeded to identify relationships between
deflection basin parameters (SCI, BDI, BCI) and pavement surface
deflections, the strong relationship with back-calculated [E.sub.eq]
proved the practical utility possibilities of DBPs. Stronger
relationships were found between upper layers indicators (SCI and BDI)
and [E.sub.eq] ([E.sub.eq2001-52]), as relationship between subgrade
indicator BCI and [E.sub.eq2001-52], found in the research, was not very
strong.
Analyses confirm that poor condition of Estonian road pavements is
due to weak subbases and subgrades. Pavements that are stabilized with
mixed binders (bitumen + cement) were found to be with higher [R.sup.2]
of mathematical models representing the relationship between BCI and
[E.sub.eq2001-52].
As the statistical analyses of such extensive database have been
done for the first time in Estonia, the determined limit values have to
be evaluated in practical use and if needed corrected. Initially,
deflection basin parameters limit values, developed in this research,
can be used for pavement condition assessment in network level. It is
possible to determine road sections with insufficient pavement
structural capacity using FWD measurements and proposed method for
determine max limit values of deflection basin parameters.
DOI: 10.3846/1822-427X.2009.4.196-202
Received 22 August 2008; accepted 11 November 2009
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parameetrite SCI, BDI ja BCI kasutamine teekatendi seisukorra hindamisel
[Use of FWD Deflection Basin Parameters (SCI, BDI, BCI) for Pavement
Condition Assessment]. Magistritoo [Master thesis]. Tallinn: TTU
Teedeinstituut [Tallinn University of Technology, Dept of
Transportation], 163 p.
Ott Talvik (1), Andrus Aavik (2)
Dept of Transportation, Tallinn University of Technology, Ehitajate
tee 5, 19086 Tallinn, Estonia
E-mails: (1) ott.talvik@ttu.ee; (2) andrus.aavik@ttu.ee
Table 1. Widely used FWD deflection basin parameters
(Kim et al. 2000; Talvik 2007)
Deflection
basin
parameter Equation Unit
Surface SCI = [d.sub.0]--[d.sub.300], [micro]m,
Curvature SCI = [d.sub.0] - [d.sub.r] mm
Index (used also r [member of]
[450, 600])
Base Damage BDI = [d.sub.300]- [micro]m,
Index [d.sub.600] mm
Base BCI = [d.sub.600]--[d.sub.900] [micro]m,
Curvature (used in USA)
Index BCI = [d.sub.900]--[d.sub.1200] mm
(used in Finland)
BCI = [d.sub.1200]--[d.sub.1500]
(used in Estonia)
Area AREA = 6 ([D.sub.0] + mm
2[D.sub.1] + 2[D.sub.2] +
2[D.sub.3]/[D.sub.0]
AREA = 150 ([d.sub.0] +
[2d.sub.300] + [2d.sub.600]
+ [d.sub.900])/[d.sub.0]
Area under AUPP = 5[d.sub.0] + mm
pavement [2d.sub.300] + [2d.sub.600]
profile + [d.sub.900])/[d.sub.0]
Shape [F.sub.1] = ([d.sub.0] - --
factors [d.sub.600])/[d.sub.300],
[F.sub.2] = ([d.sub.300] -
[d.sub.900])/[d.sub.600]
Deflection DR = [d.sub.600]/[d.sub.0] --
ratio
Deflection
basin Parameter's
parameter objective
Surface Characterizing
Curvature condition of
Index bound layers
Base Damage Characterizing
Index condition of
base layers
Base Characterizing
Curvature condition of
Index subbase or
subgrade
Area Characterizing shape of the
deflection basin close to
the load by the normalized
area on the top of the
deflection basin
Area under Characterizing condition
pavement of the pavement upper layers
profile
Shape Determination of condition
factors of the layer at the
equivalent depth
Deflection Determination of condition
ratio of the layer at the
equivalent depth
Note: [d.sub.0], [d.sub.300], [d.sub.600], [d.sub.900],
[d.sub.1200], [d.sub.1500]--measured deformations at
the distance of 0, 300, 600, 900, 1200, 1500 mm from
the center of the loading plate; [D.sub.0], [D.sub.1],
[D.sub.2], [D.sub.3]--measured deformations at the
distance of 0 ft, 1 ft (305 mm), 2 ft (610 mm), 3 ft
(914 mm) from the center of the loading plate.
Table 2. Analyze groups, traffic loadings
and required [E.sub.eqmin]
(Metsvahi et al. 2005)
Analyze Traffic loading, Required
group standard axle load [E.sup.eqmin],
(100 kN), vpd MPa
1 < 30 140
2 30-59 160
3 60-114 180
4 115-224 200
5 225-439 220
6 440-869 240
7 > 870 260
Table 3. Bituminous pavement layer temperature
correction factors [K.sup.t] (Aavik 2003)
Pavement Temperature correction
layer type factor [K.sub.t]
Asphalt [K.sup.t] = [0.000203T.sup.2] -
concrete 0.014841T + 1.127603
Cold [K.sup.t] = 0.000205T2 -
bituminous 0.015198T + 1.135192
mix
Table 4. Values of factor M;, taking into
account the month when measurement is
performed (Aavik 2003)
[m.sup.4] [m.sup.5] [m.sup.6] [m.sup.7]
1.000 0.911 0.830 0.816
[m.sup.4] [m.sup.8] [m.sup.9] [m.sup.10]
1.000 0.831 0.825 0.817
Table 5. Values of constants a0 and a1 in Eq (6) for
calculation of the max allowable deflection basin
parameter (SCI, BDI, BCI) values
(Talvik 2007)
Type of Deflection Value of constants
pavement (based basin
on Estonian parameter [a.sub.0] [a.sub.1] [R.sup.2]
Road Data Bank) y
AC pavement on SCI 1 795 660 -1.70 0.83
top of existing BDI 1 265 966 -1.74 0.78
pavement BCI 51 220 -1.36 0.68
AC pavement on SCI 655 780 050 -2.76 0.87
top of leveling BDI 15 319 713 999 -3.47 0.93
milling BCI 11 182 -1.13 0.09
AC pavement on SCI 169 150 407 -2.54 0.92
top of leveling BDI 104 111 -1.27 0.38
layer BCI -- -- --
AC pavement on SCI 88 410 -1.113 0.54
bitumen- BDI 62 337 -1.161 0.51
stabilized base BCI 985 977 -1.909 0.35
AC pavement on SCI 1 225 980 -1.63 0.53
complex- BDI 137 949 -1.307 0.85
stabilized base BCI 497 43 -0.492 0.14
AC on crushed SCI 498 577 -1.45 0.87
stone base BDI 10 645 -0.84 0.21
BCI 51 984 -1.31 0.61
Cold bituminous SCI 834 463 -1.55 0.97
mix BDI 2 055 457 -1.84 0.94
BCI 983 446 -1.99 0.84
Oil shale ash SCI 2 491 -0.44 0.14
stabilized BDI 1 325 498 -1.80 0.75
pavement BCI 12 473 680 -2.43 0.44
Surface-dressed SCI 13 705 -0.72 0.66
gravel pavement BDI 258 341 445 -2.83 0.96
BCI 255 760 -1.87 0.95
Table 6. DBPs limit values for [E.sup.req],
AC pavement on top of existing pavement
[E.sub. SCI BDI BCI
req], MPa
140 403 233 62
160 322 185 51
180 263 151 44
200 220 125 38
220 187 106 33
240 161 91 30
260 141 80 27
Table 7. Values of constants a and b
according Procedure 2001-52
Load a b
group
Lorry A 67.60 61.3
B 73.37 -7.7
Bus A 77.00 62.0
B 84.70 0
Table 8. Pavement strength factors according
Procedure 2001-52
Road class Pavement [K.sub.tt]
Motorway, I, II permanent pavement 1.00
III permanent pavement 0.94
III, IV, V light pavement 0.90
IV, V transient pavement 0.63
V, out of class roads primitive pavement 0.63
