PREDICTION OF REMAINING STRUCTURAL

World Conference on Pavement and Asset Management, WCPAM2017 Milan, Italy - June 12/16, 2017

PREDICTION OF REMAINING STRUCTURAL SERVICE LIFE USING TRAFFIC SPEED DEFLECTOMETER Sittampalam Manoharan1, Gary Chai2, Sanaul Chowdhury3 and Andrew Golding4

1

PhD Candidate, School of Engineering, Griffith University, Queensland, QLD 4222, Australia. E-mail: [email protected] 2

Senior Research Fellow, School of Engineering, Griffith University, Queensland, QLD 4222, Australia. E-mail: [email protected] 3

Senior Lecturer, School of Engineering, Griffith University, Queensland, QLD 4222, Australia. E-mail: [email protected] 4

Director, Portfolio Investment and Programming Branch, Queensland Department of Transport and Main Road, Brisbane, QLD 4001, Australia. E-mail: [email protected]

The Traffic Speed Deflectometer (TSD) is a vehicle mounted Doppler laser system that is capable of continuously measuring the structural bearing capacity of a pavement whilst moving at traffic speeds. The device’s high accuracy, high speed and continuous deflection profiles are useful for network level applications such as predicting road rehabilitations needs and remaining structural service life. The objective of this paper is to develop a simplified methodology for predicting structural service life using maximum deflection measured by TSD. This methodology was developed by using tolerable deflection concept which is customarily used to design pavement overlay thickness for granular and asphalt pavement layers. The existing tolerable deflection equations are based on Falling Weight Deflectometer (FWD) maximum deflection. This study successfully established a detail procedure for predicting remaining structural life without complex calculations using TSD deflection data. The outcomes of this study is established a methodology for predicting structural service life at network level utilising TSD deflection data. Keywords: Traffic Speed Deflectometer (TSD), Remaining Structural Service Life (RSSL), Falling Weight Deflectometer (FWD), Tolerable Deflection (Dtol) and Maximum Deflection (D0)

Introduction Australia consists over 823,000km road network which includes 356,000km paved and 466,000km unpaved roads (1). Queensland (QLD) is Australia’s second largest state, covering approximately 1.8 million square kilometers. QLD has approximately 174,000 km of road network managed by state and local governments (2). Predicting pavement performance is vital for maintaining the road network and to address pavement defects in a prompt manner which can reduce needs for large scale rehabilitation and minimize the whole-of-life cost of the road network. The ability to predict future pavement deterioration is highly advantageous for road asset managers as a means of developing investment plans.

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Falling Weight Deflectometer (FWD) technology has been used by many road agencies globally since 1980 for evaluating pavement structural bearing capacity and is widely recognized as one of the most suitable non-destructive testing devices that accurately predicts bearing capacity. Over the last 30 years extensive research has been undertaken into FWD usage in overlay design and determining remaining structural life.

Figure 1: Traffic Speed Deflectometer (TSD) (3)

The Traffic Speed Deflectometer (TSD) is a new state-of-the-art technology vehicle which was developed by Greenwood Engineering in the early 2000’s using Doppler Laser-based laser technology. This technology is capable of continuously measuring the bearing capacity of a pavement whilst moving at traffic speeds. The device’s high accuracy and continuous deflection profiles are useful for network level applications such as predicting road rehabilitation needs and remaining pavement life (4). There are eight TSD vehicles in use around the world including USA, China, UK, South Africa, Poland, Italy, Denmark and a shared vehicle New Zealand & Australia. The TSD measures vertical deflection velocity using a Doppler laser and the slope is calculated by dividing vertical velocity by vehicle driving velocity. Therefore, the accuracy of the deflection bowl is dependent on the manner in which the Doppler laser is mounted on the vehicle.

2


Tolerable Deflections Tolerable deflection is a maximum acceptable deflection under a specified load which a given pavement may exhibit if the pavement is to achieve a specified number of load repetitions satisfactorily (5). Tolerable deflection concept is used to design overlay as known as “deflection reduction” method. The overlay depth requirement is the amount of overlay material required to reduce pavement deflection to a tolerable deflection. Tolerable deflection concept is a direct method to use maximum deflection for evaluate pavement structural capacity. This concept is used by several road authorities in Australia. The tolerable deflection is the limit between the sound and critical pavement condition. The Figure 2 illustrates the tolerable deflection curves for unbound granular pavements (6)

Figure 2: Tolerable Deflection Criteria for unbound Pavement. (Lister and Kennedy 1997) (6)

Design of pavement rehabilitation treatments by deflection reduction method. The overlay design methodology using deflection reduction (tolerable deflection) methodology is outlined in Austroads Guide to Pavement Technology (AGTPT Part 5) (7). Also,the details of design procedure and limitations of this methodology are clearly detailed in the Department of Transport and Main Roads Pavement Rehabilitation Manual Part5 (8). It should be noted as granular overlay and asphalt overlay design charts are different, the appropriate chart is to be used for each particular treatment type. In this analysis the granular overlay is only considered as most of Australia’s road network is granular overlay with spray seal. Figure 3 depicts the design chart and Table 1 shows the equations for that charts and these equations are developed based on FWD deflection with 40 KN loading. Therefore, these

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tolerable deflections equations are to be modified to use TSD deflection or TSD deflection is to be converted to FWD equivalent and use the current FWD based equations.

Tolerable Deflection - Normal Design Standard-FWD 2.000 CBR3 - 1 CBR3 - 3

1.800

CBR3 - 2 Tolerable Deflection (FWD-mm)

1.600

CBR5 - 1 CBR5 - 2

1.400

CBR5 - 3 CBR7 - 1

1.200

CBR7 - 2 CBR7 - 3

1.000

CBR10 - 1 CBR10 - 2

0.800

CBR10 - 3 0.600

CBR15 - 1 CBR15 - 2

0.400

CBR15 - 3 CBR20 - 1

0.200 1E+05

1E+06

1E+07

1E+08

CBR20 - 2 CBR20 - 3

Traffic (ESA)

Figure 3: Tolerable Deflection (FWD-D0) – Normal Design Standard Curves (8)

Table 1; Tolerable deflection (FWD-D0) curves equations

Tolerable Deflection Curves for FWD and 40 KN loading Normal Design Standard CBR (%)

D900 (mm)

Traffic (ESA) From

3

5

0.330

0.230

Equation

To

1E+05

2E+06

Dtolerable = 7.5364x(DESA)-0.137

2E+06

7E+06


7E+06

1E+08


1E+05

1E+06


4


7

10

15

20

0.180

0.145

0.110

0.090

1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


Methodology Development tolerable deflection equations for TSD deflection. It is very useful to have TSD deflection based tolerable deflection charts instead of using FWD based tolerable deflection charts. This will enable TSD data to be used directly rather than converting it to a FWD equivalent. To develop TSD based tolerable deflection charts, it is essential that FWD deflection to be converted to TSD equivalent. As such the following equation which was developed by Manoharan et al (9) is used.

D0(TSD) = 0.9845* D0( FWD) − 40.129

(1)

Where:

D0(TSD ) = maximum deflection measured by TSD in microns.

D0( FWD) = maximum deflection measured by FWD in microns.

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The excel spreadsheet program is used to regenerate the eighteen separate curves for TSD deflections by using the Equation 1. Temperature corrections were adopted in accordance with methodology provided in Pavement Rehabilitation Manual (8). The manual stipulates that flexible pavements with less than 50mm thick asphalt surfaced no temperature correction is necessary. The eighteen curves were made-up for six different CBR types and each CBR type consist of three curves. Once the curves were plotted, the equations were derived from the best fit line of each curve. The newly developed equations is shown in Table 2 and the chart is depicted in Figure 4

Tolerable Deflection - Normal Design Standard -TSD 2.000 CBR3 - 1 1.800

CBR3 - 3 CBR3 - 2

Tolerable Deflection (TSD-mm)

1.600 CBR5 - 1 CBR5 - 2

1.400

CBR5 - 3 1.200

CBR7 - 1 CBR7 - 2

1.000

CBR7 - 3 0.800

CBR 10 CBR 10

0.600

CBR 10 CBR 15

0.400

CBR 15 0.200 1E+05

CBR 15 1E+06

1E+07

1E+08 CBR 20

Traffic (ESA)

Figure 4: Tolerable Deflection (TSD-D0) – Normal Design Standard Curves

Table 2: Tolerable Deflection (TSD -D0) – Normal Design Standard Curves

Tolerable Deflection Curves for TSD (D0) Normal Design Standard CBR (%)

D900 (mm)

Traffic (ESA) From

Equation To

6


3

5

7

10

15

20

0.330

0.230

0.180

0.145

0.110

0.090

1E+05

2E+06

Dtolerable = 7.6202X(DESA)-0.142

2E+06

7E+06


7E+06

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


1E+05

1E+06


1E+06

1E+07


1E+07

1E+08


Predicting Remaining Structural Service Life (RSSL) using tolerable deflections concepts Several methods have been used by various roads jurisdictions to estimate the structural service remaining life of pavement for them to maintain the road network with minimum whole –of-life cost. In this study a simple model was proposed by using the newly developed TSD based tolerable deflection equations shown in Table 2 for predicting remaining life for network level analysis. The following flow chart outlines the main steps involved in predicting structural remaining life using newly developed TSD based tolerable deflections equations.

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Methodology for to calculate remaining structural service Life (RSSL) using TSD based tolerable deflection chart

Select a road section where TSD measured maximum deflections were measured

Calculate design ESA from selected curves

Check whether if any ESA's a left (design ESA minus consumed ESA)

Determine Subgrade CBR using D900 deflection data

Calculate the initial annual average daily traffic (AADT) using pavement age and current AADT

If yes , Calculate the remaining ESA

Select appropriate equation based on determined CBR from tolerable deflection carvers

Calculate consumed ESA using initial AADT and pavement age

Calculate remaining life from remaining ESA

The detail description of the main stages are out lined below: •

Determine Subgrade CBR from D900 deflection data

The subgrade strength details not available in inventory database. Therefore, the strength of subgrade is derived from measured deflection at 900mm or 450 mm from the centre of the impulse load. Chai et al developed a methodology to predict subgrade CBR from D450 reading (10). In this analysis the TMR method is used to calculate subgrade CBR from D900 CBR subgrade =0.5996(D900)-1.4543

•

(2)

Select appropriate tolerable deflection equation based on determined CBR and calculated

There are there equations for each CBR values for different DESA ranges, based on calculated design equivalent standard axle (DESA) the appropriate equation is to be selected.

8


•

Calculate Consumed Equivalent Standard Axle

First calculate initial AADT by using current AADT, annual traffic growth and pavement age. Then consumed ESA can be estimated utilising calculated initial AADT and other required parameters in accordance with Austroads pavement design guidelines. (11).

•

Calculate remaining ESA

By subtracting consumed ESA from design ESA •

Calculate remaining structural service life (RSSL)

RSSL can be predicted from calculated remaining ESA using Austroads pavement design guidelines. (11) The predicted remaining life can be grouped into five proposed bands as shown in the Table 3 which is useful for screening network based on the remaining life. This will allow to choose appropriate rehabilitation treatments by considering remaining structural life.

Table 3: Proposed Remaining Structural Service Life Bands Bands

Remaining Life is Remaining Life is >= years < years

Very Good

30

Good

20

29

Fair

10

19

Poor

4

9

Very Poor

0

3

Discussion of results The aim of this study was to develop a methodology for predicting remaining structural service life using TSD deflection data. RSSL is a good indicator of pavement that could provide its intended level of service before fails. As pavement age is comely used an indicator for predicting remaining life of pavement, however this was used due to availability of structural data at network level. The established methodology to predicting RSSL is very useful for asset manager for choose appropriate treatment based on their RSSL utilizing TSD deflection data. Additionally, in the last 30 years FWD deflection data was used by many road agencies as this proposed a methodology will enable to directly use TSD deflection for RSSL.

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CONCLUSIONS A real benefit can be achieved if asset managers use RSSL appropriately in investment decisions which will sustainability reduce whole of cost of an asset and maximize the benefit. Structural data collection at the network level is generally considered cost prohibitive, however when such data is used in a network needs analysis; the benefit can be much higher than the data collection cost. Hence the RSSL estimation using TSD deflection data methodology is very useful for asset managers for their decision making process at network level for improving the timely identification of road sections requiring rehabilitation treatments. This study draws some remarkable findings in particular that deflection data collected by TSD can be utilized in forecasting remaining structural life. Therefore, these newly developed TSD based tolerable deflection curves can be used in the interim basis for use of TSD data until comprehensive studies have been conducted by various research organizations. This study focused exclusively on the existing FWD based tolerable deflection relationship could be modify to TSD based relationships as existing tolerable deflection relationships. Therefore, further research is recommended to review the existing tolerable deflection relationships which was developed in late nineteen nineties. Furthermore, the existing relationships were developed only for sprayed and thin (