A Study on FWD Deflection Bowl Parameter

A Study on FWD Deflection Bowl Parameter

To Assess Structural Condition of Flexible Pavement

Ujjval J. Solanki1, Prof.(Dr.) P.J. Gundaliya2, Prof. M.D. Barasara3

1 Research Scholar, School of Engg. and Technology, R.K. University, Rajkot, INDIA,

2Professor, Department of Civil Engg.,L.D. College of Engineering, Ahmedabad, INDIA

3Prof. M.D. Barasara, Head, Civil Engg Deptt, Darshan Institute of Engg. and Technology-Rajkot, INDIA

Abstract

The Falling weight deflectometer (FWD) is used for structural evaluation of flexible pavement layers. The magnitude of the load, duration and area of loading is so adjusted that it corresponds to the effect of loading due to standard axle on in-service pavement. The FWD study outcomes is deflections at radial distance about 0,300,600,900,1200,1500 and 1800.

The objectives of the study are condition assessment of in service pavement using FWD deflections without backcalculation for modulus value of different layer. Varying age and different location within country the flexible pavement is evaluated using FWD. Based on deflections different deflection bowl parameters are calculated, three significant divisions for preliminary structural maintenance is classified as need overlay, need to observe and no need of overlay.

Key words: Structural Evaluation, Backcalculation, Falling Weight Deflectometer FWD

1.0 Introduction

Use of Falling Weight Deflectometer (FWD) for the evaluation of pavements is gaining popularity in many countries, as it is possible to simulate the magnitude and duration of load applied by a fast moving vehicle on highways using this equipment. In falling weight deflectometer (FWD) test an impulsive load is applied on the road surface. The magnitude of the load, duration and area of loading is so adjusted that it corresponds to the effect of loading due to standard axle on in-service pavement. The instantaneous deflections of the road surface is measured at a number of points at different distances radially outward from the center of the falling weight. Thus, the shape of deflection bowl is obtained. Information on structural health condition can be extracted from analysis by back calculation of the FWD data. The backcalculation process need expertise, complete understanding of pavement layer performance and field experience to assess the pavement condition. The backcalculation process need estimation of moduli range for all the three layers of pavement again it needs correct judgment. The process of backcalculation require field experience for the input of range of modulus of elasticity of bituminous, granular and subgrade layer, and its required number of trial to find such optimal moduli with the observed FWD deflection on the field. The use of these deflection bowl parameters were in effect promoted as a first order structural analysis of pavements or as de facto benchmark method to structurally evaluate flexible pavements and associated pavement layers Benchmarking is also a valuable screening option when analyses are done with a pavement management system (PMS) at network level.

2.0 Objectives

(i) Condition assessment of the existing in service pavement includes distresses, age, treatment done up to date, layer thicknesses.

(ii) FWD deflection study on different age and location in India.

(iii) To calculate different deflection bowl parameter.

(iv) To assign preliminary benchmarking value to know structural condition of in service flexible pavement.

3.0 Literature review

HORAK, E and EMERY, S J (2006)

A semi-mechanistic, semi-empirical analysis technique has been developed in South Africa in terms of which deflection bowl parameters, measured with the FWD, are used in a relative benchmarking methodology in conjunction with standardized visual survey methodology to give guidance on individual layer strengths and pinpoint rehabilitation needs. This benchmark methodology enables the determination of the relative structural condition of the pavement over length and in depth without the requirement for detailed as-built data. A further correlation study with calculated surface moduli and deflection bowl parameters was presented.

Richard N. Stubstad Consulpav LTPP ( Long term pavement performance) Data Analysis(2002)

This study used only very simple, straightforward methods of analyzing the FWD load deflection data. These methods included the calculation of the "composite" modulus, or apparent stiffness, of the materials under each layer interface for unbound material tests, and an approximate calculation of the stiffness of a single bound layer for the bound layer tests. The stiffness of any bound layer was derived through a simple formula, using the composite modulus, E and the "AREA" of the deflection basin. AREA is similar to a basin shape factor or curvature index of the FWD’s deflection basin out to a distance of 3 feet (~0.9 m). The research confirms that these values represent reasonably well the material properties and their variations, in terms of modulus or stiffness, along the pavement sections studied.

EMILE HORAK (2008)

A semi-mechanistic, semi-empirical analysis technique has been developed in South Africa in terms of which deflection bowl parameters, measured with the FWD, are used in a relative benchmarking methodology in conjunction with standardized visual survey methodology to give guidance on individual layer strengths and pinpoint rehabilitation needs. This benchmark methodology enables the determination of the relative structural condition of the pavement over length and in depth without the requirement. Based on study the pavement condition is identified as sound, warning and severe for different benchmarking value for different deflection bowl parameter and also incorporate the traffic range in terms of msa.for detailed as-built data.

Emile Horak, Arno Hefer, James Maina and Steve Emery (2010)

The origin of the empirical structural number (SN) method is from the American Association of State Highway Officials (AASHO) road tests in the late 1950’s. The SN method is described as an index methodology and has found use and application world-wide through the AASHTO design guide. In the study adjusted Structural number (SNP) can be derived from FWD deflection bowl information as an approximation of the SNP values normally derived from detailed material and layer thickness information. SNP is calculated from deflection value and Structural condition index (SCI) is calculated from deflection bowl parameter. A large database of South African flexible pavements was used to correlate successfully the SNPNZ with SNPeff not only for granular pavements, but also for asphalt base and cemented base pavements. The good correlation is testimony to the value of better utilization of the full deflection bowl with inherent structural response information.

The above study carried out at South Africa, America and New Zealand they have some preliminary benchmarking values based on deflection bowl parameter, and used for maintenance treatment decision. The same should have in our country. There is need to make such data base of structural evaluation of flexible pavement using FWD deflection in the country so some unified deflection based parameter can be fixed for decision of maintenance treatment.

4.0 Deflection bowl parameter

Deflection bowl parameters are calculated from the measured FWD deflections, Dr, at geophones positioned at various offsets, r in mm from center of loading. In the study typical FWD geophone set up is spaced at zero (D0) (under the center of the FWD loading plate, which itself has a diameter of 300mm), 200mm (D200), 300mm (D300), 600mm (D600), 900mm (D900), 1200mm (D1200), 1500mm (D1500),1800mm (D1800) resulting from dropped weights such as 40kN (0.56 MPa contact stress) are measured at these offsets.

In the deflection bowl parameters following inputs are considered besides deflection at r offsets.

μ = Poisson’s ratio considered 0.4, a = Radius of loading plate 150 mm, σ0= Contact pressure 0.56 MPa.

The twelve deflection bowl parameter is calculated as shown in Table -1. All the parameter is indicates the structural condition of pavement layer and its significance is given in Table-1. These parameters are suggested in south Africa and New Zealand.

Table-1 Deflection bowl parameter, its significance and formula

Based on observed deflection value, deflection bowl parameter is calculated as shown in Table-3. These parameters are average in kilometer observation.

5.0 Study area

The FWD study is carried out in three different stretch of highway. First and second study stretch is located at Punjab and third is located near Hyderabad.

The first study area is located in the State Punjab, District: Sangrur, Mansa & Bathinda. The work contract Type- Output & Performance Based Road Contract- (OPRC) which is first in India and supported by the World Bank. The study area as showed in Figure-1 and 2. It is located in the three districts bathinda, mansa and sangrur. The study stretch is ODR-9, the section is S4, length is 20.00 Km Mansa to Talwadi sabo. In Punjab study the FWD application is carried out two times on same 20 km road at 200 mt interval, first study on heavily distressed road and second study on same road but with newly prepared road as BC+DBM+WMM+ GSB after six month of construction. The subgrade soil is fine silty sand.

Figure 1 Punjab study area-S4 Road Figure 2 Pavement Evaluation using FWD

The second study area is located in the state of Punjab The project road comprises section of NH-15 starts from Existing Km 131.00 of NH-15 and terminates at existing Km. 157.00 of NH-15 having total length of 26.00 km. It is is situated in Amritsar and Tarn Taran Districts in the state of Punjab. Pavement condition is fair to good. No major undulations or depressions was observed, minor localized cracking observed. The FWD deflection is observed at 125 mt interval on right hand and left hand side considering visually good condition of pavement. The subgrade soil type is silty clay- ML

The third study stretch is located near hyderabad at adone to mahav. FWD study road length is 30 km and visually good condition very small fine cracks no rut observed. The FWD deflection is observed at 500 mt interval considering visually good condition of pavement. The subgrade soil type is silty sand, SM and SM-SC.

The details of road name, month of evaluation, length, pavement composition, condition and purpose of evaluation is tabulated in Table-2.

Table-2 Details of study area

6.0 Data collection and analysis

6.1 Condition assessment

All the three study stretch condition survey has been carried out as per IRC: 115 at an interval of 100 m. It revealed the extent of potholes, cracking, patching, rut depth which have been classified based on the percentage area of distress the condition of the study stretch is as shown in Table-3.

Table 3 Condition assessment

6.2 FWD Study and calculation of deflection bowl parameter

FWD deflection study is carried out using Dynatest 8000 model FWD. The deflection value observed at 0,200,300,450,600,900,1200,1500 and 1800. All the deflections are normalized at 40 kN. In old and New Punjab road the observation is taken at 200 mt interval. In Adone-mahav road the FWD observation is taken at 500 mt interval as per visual condition of pavement and consultant instruction. The deflection observation is taken at the pavement temperature range of 25° C to 45°C in all the three study stretch. The deflection bowl parameters are calculated at each point of observation as per formula given in Table 2. For simplification the calculated deflection bowl parameter are averaged for each kilometer and all are averaged as a whole. The calculated average deflection bowl parameters are tabulated in Table 4.

Table 4 Average Deflection Bowl Parameter