FALLING WEIGHT DEFLECTOMETER BOWL PARAMETERS AS ANALYSIS TOOL FOR PAVEMENT STRUCTURAL EVALUATIONS Professor Emile Horak, Head of Department of Civil Engineering, University of Pretoria, Pretoria, South Africa and Director of Kubu Consultancy Pty Ltd, Pretoria Professor (Extraordinary) Stephen Emery, Department of Civil Engineering, University of the Witwatersrand, Johannesburg, South Africa and Director of Kubu Consultancy International Pty Ltd, Perth, Australia ABSTRACT The falling weight deflectometer (FWD) is used world wide as a well established and valuable non-destructive road testing device for pavement structural analyses. The FWD is used mostly for rehabilitation design investigations and for pavement management system (PMS) monitoring on a network basis. On the project level investigations a mechanistic or theoretically correct approach of using multi-layered linear elastic theory in back-calculation procedures is often used to provide elastic moduli for the pavement structural layers for detailed structural evaluations. As an alternative to this an semi-mechanistic semi-empirical analysis technique has been developed in South Africa where deflection bowl parameters measured with the FWD are correlated with individual pavement layer structural strength. This paper briefly describes the current practice and basis of this use of deflection bowl parameters and illustrate the usefulness in project investigations as applied to a current pavement rehabilitation project underway in SA. HORAK, E and EMERY, S J (2006) Falling Weight Deflectometer Bowl Parameters as Analysis Tool for Pavement Structural Evaluations. 22nd ARRB Conference, 29 Oct-2 Nov 2006, Canberra
Transcript
FALLING WEIGHT DEFLECTOMETER BOWL PARAMETERS AS ANALYSIS TOOL FOR PAVEMENT
STRUCTURAL EVALUATIONS
Professor Emile Horak, Head of Department of Civil Engineering,
University of Pretoria, Pretoria, South Africa and Director of Kubu
Consultancy Pty Ltd, Pretoria
Professor (Extraordinary) Stephen Emery, Department of Civil
Engineering, University of the Witwatersrand, Johannesburg,
South Africa and Director of Kubu Consultancy International Pty
Ltd, Perth, Australia
ABSTRACT
The falling weight deflectometer (FWD) is used world wide as a well established and valuable
non-destructive road testing device for pavement structural analyses. The FWD is used
mostly for rehabilitation design investigations and for pavement management system (PMS)
monitoring on a network basis. On the project level investigations a mechanistic or
theoretically correct approach of using multi-layered linear elastic theory in back-calculation
procedures is often used to provide elastic moduli for the pavement structural layers for
detailed structural evaluations. As an alternative to this an semi-mechanistic semi-empirical
analysis technique has been developed in South Africa where deflection bowl parameters
measured with the FWD are correlated with individual pavement layer structural strength. This
paper briefly describes the current practice and basis of this use of deflection bowl
parameters and illustrate the usefulness in project investigations as applied to a current
pavement rehabilitation project underway in SA.
HORAK, E and EMERY, S J (2006) Falling Weight Deflectometer Bowl Parameters as Analysis Tool for Pavement Structural Evaluations. 22nd ARRB Conference, 29 Oct-2 Nov 2006, Canberra
2
INTRODUCTION
Deflection measurements of pavement structures are used to do structural analyses for the
purpose of rehabilitation design as well as for network monitoring of pavement networks. The
older equipment like the Benkelman beam and La Croix deflectograph were used extensively
in the past and various empirical relations were developed for analysis and overlay design by
organisations like Shell, the Asphalt Institute, and TRRL (Jordaan, 1988). In most cases only
the maximum deflection were utilised and the shape of the deflection bowl and the
significance of its relationship with the pavement structural response were basically ignored
and wasted. Since the 1980s significant improvement of non-destructive deflection measuring
devices resulted in the ability to measure the whole deflection bowl accurately. It also enabled
an appreciation of the value of the whole deflection bowl in structural analysis of roads and
pavements (Horak, 1988).
The extensive use of the modified Benkelman beam, the road surface deflectometer (RSD),
with accelerated pavement testing (APT) devices, like the heavy vehicle simulator (HVS) in
South Africa (SA), coupled with the use of the in depth deflection measurements with the
multi-depth deflectometer (MDD), helped to give credibility to the back-calculation of elastic
moduli with various multi-layered linear elastic computer models. The extensive test
programmes of the HVS in SA helped to correlate such back-calculated elastic moduli with
pavement performance and deterioration modelling and helped to increase the credibility and
use of back-calculated elastic moduli derived from surface deflection measurements. (Horak,
et al, 1992).
A brief overview of the evolutionary use of the full deflection bowl is given to describe the
rationale behind the development of the use of a number of deflection bowl parameters in a
well established semi-mechanistics-empirical analysis procedure in SA. A well documented
current rehabilitation project is used to demonstrate the value of these parameters in
structural analysis and rehabilitation design .
APPRECIATION OF THE FULL SURFACE DEFLECTION BOWL
When a pavement deflects under a load, the influence of the load can extend over an area
measurable 1 to 2meters away from the point of loading in three dimensions. This is
illustrated in Figure 1 for a uniform circular and truck dual axle loading situation. This
deflected area tends to form a circular deflected indentation called a deflection bowl. The size
and shape of the deflection bowl vary and depend on different factors such as pavement
composition and structural strength, size of load contact area, load magnitude and duration of
loading, the measuring device used, temperature, etc. (Horak, 1987 and 1988 and Lacante,
1992).
3
Prior to the arrival of electronic measuring equipment the deflection bowl was measured
mostly with the Benkelman beam which measured maximum deflection and resulting in
various empirical design and analysis procedures based solely on this single point on the
deflection bowl (Jordaan,1988 and Horak, 1988). These measuring techniques had a number
of short comings. The Benkelman beam required a standard axle loaded truck to position over
the point of the beam between the dual tyres and pull away to register the “re-bound”
deflection measurement. This rebound measurement included plastic deformation
components due to the static loading situation before the truck moved. One of the side-effects
was the “pinching” effect which occurred between the dual wheels as illustrated in Figure 1.
This is very pronounced on soft bases and warm asphalt surfacings (Horak, 1988 and
Dehlen, 1961).
The wealth of information in the rest of the deflection bowl went virtually wasted in analysis
methods developed in the early 1950s and 1960s. However, Dehlen (1961) used the
Benkelman beam to record the deflection at 75mm intervals to plot the whole deflection bowl.
Particular attention was given to the detail of the inner 600mm close to the point of maximum
deflection. The radius of curvature at the point of maximum deflection was obtained by
determining the circle which best fit to the curve over the central 150 to 250mm. Dehlen
(1961) noted that a circle fitting the deflected surface in the field is an approximation of either
an ellipse or sinusoidal or parabolic form, but the error by means of this approximation with a
fitted circle created a an error of less than 5%. The Dehlen curvature meter was subsequently
developed by Dehlen (1962) which enabled the measurement of the curvature directly as
illustrated in Figure 2. The relation between curvature and differential deflection may be
deduced by simple geometry by fitting an appropriate curve to the three points on the road
surface defined by the instrument.
In the late 1980s the Falling Weight Deflectometer (FWD) became the new electronic
deflection measuring tool of choice which could simulate a moving wheel load, measure
elastic response and the critical points on the whole deflection bowl up to a distance of 1.8m
to 2m away from the point of maximum deflection or loading (Coetzee et al, 1989). This
measurement of the whole deflection bowl led to the definition of various deflection bowl
parameters which described various aspects of the measured deflection bowl. In Table 1
various deflection bowl parameters and their formula are summarized and their association
with pavement structure and structural elements. Of these nine deflection bowl parameters
listed in Table 1, Horak (1988) found that only the first five gave good correlations with the
relevant pavement structural condition and individual pavement layer associations. The use of
these first five measured deflection bowl parameters in the evaluation of the structural
capacity of a pavement has subsequently been suggested and used by several researchers
(Horak et al. 1989, Maree and Bellekens, 1991 and Rohde and van Wijk, 1996).
4
Table 1: Deflection Bowl Parameters (Horak et al, 1989)
Parameter Formula Structural indicator
1 .Maximum deflection D0 as measured Gives an indication of all structural layers with about 70% contribution by the subgrade
2. Radius of Curvature (RoC) RoC= (L)2
2D0(1-D0/D200)
Where L=127mm in the Dehlen curvature meter and 200mm for the FWD
Gives an indication of the structural condition of the surfacing and base condition
3.Base Layer Index (BLI)
BLI=D0-D300
Gives an indication of primarily the base layer structural condition
4.Middle Layer Index (MLI)
MLI=D300-D600
Gives an indication of the subbase and probably selected layer structural condition
5. Lower Layer Index (LLI)
LLI=D600-D900
Gives an indication of the lower structural layers like the selected and the subgrade layers
6.Spreadability, S S={[(D0 +D1 +D2+D3)/5]100}/D0,
Where D1, D2, D3 spaced at 300mm
Supposed to reflect the structural response of the whole pavement structure, but with weak correlations
7. Area, A A=6[1+2(D1/D0) +2(D2/D0) + D3/D0] The same as above
8.Shape factors F1=(D0-D2)/D1
F2=(D1-D3)/D2
The F2 shape factor seemed to give better correlations with subgrade moduli while F1 gave weak correlations
9. Slope of Deflection SD= tan-1(D0-D600)/600 Weak correlations observed
Maree and Bellekens (1991) analysed various pavement structures (granular, bituminous
and cemented base pavements) as measured with the FWD. Pavement structures were
analysed mechanistically, remaining lives determined and correlated with measured
deflection basin parameters. The remaining life is expressed in terms of standard or
5
equivalent 80kN axle repetitions (E80s). These relationships and correlations derived from
this research and development work are shown in Figure 3 for three distinctively different
pavement types found in SA, namely; granular, bituminous and cemented base pavements.
These deflection bowl parameters have been refined and promoted with success as semi-
empirical-mechanistic indicators of the structural strength and condition of the pavement.
(Horak et al, 1989; Rohde and van Wijk, 1996 and Joubert, 1995). These curves and
associated criteria have subsequently been included in the TRH12 guideline for rehabilitation
design and analysis in SA (CSRA, 1996). This guideline is currently under revision with a
stronger utilization of deflection bowl parameters in the proposed new procedures.
DEFLECTION BOWL PARAMETERS APPLICATION
The rehabilitation of the M2 Motorway in Johannesburg
In Figure 4 the motorway system of Johannesburg is shown. A section of this multi-lane road
is currently under rehabilitation as indicated on the plan shown in Figure 4. This 10km
motorway section carries in excess of 70 000 vehicles per day per direction and is mostly
running at capacity on four lanes dropping to three lanes in the eastwards direction. Some
sections are elevated bridge structures with a number of busy interchanges in between. The
pavement structure have been rehabilitated before after about 15 years service (Read and
Maree, 1984 and Papendorf et al, 1985) and nearly ten years later again with various
innovative construction technologies involved (Horak et al, 1994a&b). In 2005 this section of
the M2 Motorway, under the jurisdiction of the Johannesburg Roads Agency (JRA), had to be
rehabilitated again. A very good record existed of the pavement structures and history of
maintenance and rehabilitation. This makes this section of motorway ideal to demonstrate the
value of the use of the deflection bowl parameters as part of the detailed condition
assessment for the rehabilitation design of this complex high traffic volume road. FWD
surveys were done on the slow and middle lanes of this multi-lane motorway at 100m
intervals in both directions. For the purposes of demonstration only the FWD results for the
slow lane in the eastwards direction is used and shown here.
Setting tolerances for structural condition indications
Structural indicators describing sound, warning and severe were developed for the previously
mentioned five deflection bowl parameters shown in Table 1 (Horak et al, 1989). These
structural condition descriptions were developed to link in with the standard visual survey
methodology used in rehabilitation methodology in SA (CSRA, 1996). These criteria are
normally used in association with other survey techniques such as visual surveys, Dynamic
Cone Penetrometers (DCP) soundings and field material sampling, etc. The combination of
these techniques enhances the confidence in the rehabilitation investigation and analysis
6
(CSRA, 1996). Ranges for such structural indicators can be set for specific pavement base
types and traffic classes by using the correlations (Maree and Bellekens, 1991) as previously
shown in Figure 3. As demonstration of the derived structural indicators criteria or tolerances
can be derived for a granular base pavement for specific 80kN (E80) standard axle repetition
situation and is summarised in Table 2. The slow lane of the M2 eastwards direction has a
granular based pavement structure. These criteria may obviously vary for different pavement
types (e.g. also bituminous and cement treated bases) and for differing traffic situations.
TABLE 2: Condition rating criteria for deflection bowl parameters for granular
pavements
D0 (mm) RoC (m) BLI (mm) MLI (mm) LLI (mm)
Sound <0,4 >120 <0,15 <0,1 <0,06
Warning 0,4-0,75 40-120 0,15-0,5 0,1-0,2 0,06-0,1
Severe >0,75 <40 >0,5 >0,2 >0,1
In Figure 5 the maximum deflection is used to calculate and plot the cumulative differences
over the length of the section. This approach is used to distinguish between different uniform
section over the length of the road as the change in gradient of the plot indicate distinct
differences in pavement structural response. The position of the bridge structures are
indicated and at least 9 different uniform sections can be discerned in this way as a first
indication of variable structural capacity over the length of road. Common practice defines
that about 70% of the maximum deflection measured originates from the subgrade, whilst the
remaining deflection originates from the other pavement structural layers. However the
maximum deflection alone is a blunt instrument as all the other pavement layers may filter this
effect and will not allow the precise location of the structural deficiency in the pavement
structure . For that reason other deflection bowl parameters can be used to get a more
detailed indication of the structural capacity of individual layers.
Subgrade and selected layer structural condition
The lower layer index (LLI) values correlate well with the structural condition of the selected
layer and to that of the subgrade layer. The results of the LLI values are shown in Figure 6 .
The sections where the LLI falls in the sever condition range are circled in Figure 6 and
coincides with two of the uniform sections identified in Figure 5. These are sections which
clearly has subgrade and selected layer weaknesses. The visual condition surveys done (not
shown here) confirmed these sections have longer undulations and surface deformations
7
which are characteristic of subgrade failure. Other sections in the warning condition also show
the same early signs of failure in the visual condition surveys, but are not highlighted here.
Subbase layer structural condition
The MLI corresponds well with the structural condition of the subbase layer. In Figure 7 it can
clearly be seen that the same sections of concern, linked to subgrade and selected layers
failure, are also failing in the subbase layers due to the lack of support from the layers below
this subbase.
Base layer and surfacing
The BLI values shown in Figure 8 correlate fairly well with the indications of the other
structural layer deficiencies being reflected from below. However in this case the base layer
does not reach into the severe condition. Therefore it is clear that even though the granular
base layer has at least four sections in the warning condition, it is obviously in the process of
deterioration due to the lack of support from below.
The radius of curvature (ROC) and base layer indices (BLI) generally give good indications of
the base and surface layer structural condition. However the radius of curvature (RoC)
normally can discern better what the structural condition is closer to the surface. In Figure 9
the RoC values are shown. The ROC values show that the majority of the road length is in a
sound condition, but the same sections showing structural failure in the subgrade, selected
and subbase layers correspond here with severe conditions.
CONCLUSIONS
Modern non-destructive survey equipment like the FWD can accurately measure the elastic
response of the whole deflection bowl. This enables the use of the whole deflection bowl in
either empirical or theoretically based (mechanistic) analysis procedures of pavement
structures. Correlations between a number of deflection bowl parameters and mechanistically
determined structural evaluations of a number of pavement types offers the possibility to use
these parameters in a semi-empirical-mechanist fashion to analyse pavements. Such
parameters can be used in a complementary fashion with visual surveys and other
assessment methodologies to describe pavement structural layers as sound, warning and
severe regarding their structural capacity. This technique can be used in a “sieving” action to
identify structural failure and pin point it to specific layers for further detailed investigations
with other assessment methodologies. The example shown on a high traffic volume road
demonstrated the approach and value of this fuller use of the deflection bowl and associated
parameters in the structural evaluation and assessment of pavements in rehabilitation
analyses
8
REFERENCES
Coetzee NF, van Wijk AJ and Maree JH (1989) Impact Deflection Measurements.
Proceedings of the Fifth Conference on Aspahlt Pavements in Southern Africa. Swaziland,
1989
Committee of State Road Authorities (CSRA) (1996) Guidelines for rehabilitation design of flexible pavements. Technical Recommendations for Highways 12 (TRH 12), Department of Transport (DoT), Pretoria.
Dehlen GL (1961) The use of the Benkelman beam for the measurement of deflections
and curvatures of a road surface between dual wheels CSIR, Special report, R.2 NITRR,
RS/11/61, CSIR, Pretoria, South Africa
Dehlen GL (1962) A simple instrument for measuring the curvature included in a road
surfacing by a wheel load. Civil Engineer in South Africa. Vol. 4,No9, September 1962,
South Africa
Horak E (1987) The use of surface deflection basin measurements in the mechanistic
analysis of flexible pavements. Proceedings of the Fifth International Confenernce on the
Structural design of Asphalt Pavements. Ann Arbor, Michigan, USA, 1987.
Horak E (1988). Aspects of Deflection Basin Parameters used in a Mechanistic
Rehabilitation Design Procedure for Flexible Pavements in South Africa. PhD thesis,
Department of Civil Engineering at the University of Pretoria, Pretoria, South Africa.
Horak E, Maree JH and van Wijk AJ (1989) Procedures for using Impulse Deflectometer
(IDM) measurements in the structural evaluation of pavements. Proceedings of the
Annual Transportation Convention Vol 5A, Pretoria, South Africa.
Horak E, Kleyn EG, du Plessis JA, de Villiers EM and Thomson AJ (1992). The impact and
management of the Heavy Vehicle Simulator (HVS) fleet in South Africa. Proceedings of
the 7th International Conference on Asphalt Pavements, Nottingham, England. August 1992.
Horak E, Hagemann D, Rust FC and van Heerden C (1994a). Technology transfer of large aggregate mix bases (LAMB) on Johannesburg roads Proceedings of the 6th Conference on Asphalt Pavements for Southern Africa, 9 to 13 October, Cape Town. Horak E, Verhaeghe MMJA, Rust FC and Van Heerden C (1994b) The use of porous
asphalt on major roads in Johannesburg. E Horak, BMJA Verhaeghe, FC Rust and C van
Heerden (1994). Proceedings of the 6th Conference on Asphalt Pavements for Southern
Africa, 9 to 13 October 1994, Cape Town.
9
Jordaan GJ (1988) Analysis and development of some pavement rehabilitation design
methods. PhD Thesis, Department of Civil engineering, University of Pretoria, Pretoria, South
Africa.
Joubert PB (1995) Structural Classification of pavements through the use of FWD
deflection basin parameters. Proceedings of the Annual Transportation Convention,
Pretoria, South Africa.
Lacante, S.C. 1992. Comparative Study of Deflection Basins Measured on Road
Structures With Various Non-Destructive Measuring Devices. Thesis for MTech.
Technikon of Pretoria, Pretoria, South Africa.
Maree JH and Bellekens RJL (1991) The effect of asphalt overlays on the resilient
deflection bowl response of typical pavement structures. Research report RP 90/102. for
the Department of Transport. Chief Directorate National Roads, Pretoria , South Africa.
Papendorf, W Holtshousen, C Stephanou, D John, R and Jeoffreys, M.(1985) Rehabilitation of the M2 motorway in Johannesburg,. The civil engineer in South Africa. February 1985.
Read, DS and Maree, JH (1984) Rehabilitation Recommendations for the Johannesburg Francois Oberholzer Motorway (M2 Urban Freeway). Fourth Conference on Asphalt Pavements for Southern Africa. Volume 1 Proceedings. March 1984.
Rohde, G.T. and Van Wijk, A.J. 1996. A Mechanistic Procedure To Determine Basin
Parameter Criteria. Southern African Transportation Conference, Pretoria, South Africa.
AUTHOR BIOGRAPHIES
Emile Horak and Steve Emery are directors of a specialist consultancy, Kubu Consultancy
based in Pretoria, South Africa and Perth, Australia. Emile is professor and head of the
department of civil and biosystems engineering department of the school of engineering of the
University of Pretoria. Steve is extraordinary professor with the department of civil engineering
of the University of the Witwatersrand, Johannesburg. Through Kubu Consultancy they are
involved in a number of specialist consultancy projects involving roads, airports and materials
expertise in SA as well as in Australia. They both have a history of research and technology
transfer which dates back to several years of working together at the CSIR, in Pretoria, SA..
Their linked association also involves working for various subsidiaries of a major contractor
firm in SA in various specialist capacities. Emile was city engineer, roads and stormwater of
the City Council of Johannesburg and head of Service Delivery of the Greater Johannesburg
Transitional Metropolitan Council before moving into education. Steve was professor of the
SABITA Chair in Asphalt Technology at the University of Stellenbosch, SA before moving to
the abovementioned contractor association.
10
Figure 1. Illustration of deflection bowl shapes under various forms of loading
Figure 2. Illustration of the principles of the Dehlen curvature meter (Source: Horak,
1988)
11
Maximum
Deflection
Figure 3. Correlation between deflection bowl parameters and remaining life (Source: Maree and Bellekens, 1991)
