Performance of Deflection Measurement Equipment and Data Interpretation in France Jean-Michel SIMONIN1, Jean-Luc GEFFARD1, Pierre HORNYCH1 1
LUNAM Université, IFSTTAR, Route de Bouaye, CS4, F-44344 Bouguenais Cedex France Phone: +33 2 40 84 58 29, Fax: +332 40 84 59 94 email:
[email protected],
[email protected],
[email protected]
Abstract In France several systems are used to measure pavement deflections: the curviameter, the Falling Weight deflectometer (FWD), and several types of deflectographs. These devices use different measurement principles and sensors (geophones or rotary coder) and have different operating speeds (static to 18 km/h). They also differ by their level of performance, and domain of use. The paper presents a comparison of the metrological performance of these different devices. Then, a sensitivity analysis is performed, to evaluate the influence of measurement frequency and temperature on deflection measurements, and correction methods to take into account these parameters, for determination of bituminous layer moduli are proposed. . Finally recommendations are proposed concerning deflection measurement in two different contexts: network monitoring and pavement reinforcement studies Keywords: Pavement monitoring, Deflection, Temperature correction, Non Destructive Technique, Metrological Performance
1. Introduction Pavement deflection, which represents the vertical displacement of the pavement under a known load, is one of the main parameters used for structural evaluation of pavements, and back-calculation of pavement layer moduli. In France, the deflectograph has been the first apparatus developed by the French road laboratories in the years 1960 for the measurement of pavement deflections. Successively, several improved versions have been developed. The FWD was developed at the same period in other countries. More recently, the research center of public works (CEBTP: Centre d’Expérimentation pour le Bâtiment et les Travaux Publics) has developed the idea of the Curviameter. Although deflection measurements are relatively standard for pavement monitoring, improving the quality of the measurements, and the interpretation of the test results is still an important issue. After describing the different devices, and discussing their accuracy, the paper proposes different approaches for improving deflection data analysis: Corrections for taking into account temperature and loading frequency variations Examples of analysis of deflection measurement results to detect different pavement deterioration mechanisms such as layer debonding or vertical cracks.
2. Presentation of deflection measurement methods A load applied on a pavement, induces a vertical displacement of the pavement surface. The shape of the deformed pavement surface is called the deflection bowl. The main parameters determined form the deflection bowl are the maximum deflection (maximum vertical displacement) under the center of the load, and the radius of curvature, defined as the radius of the circle that fits best the deflection bowl, at the point of maximum deflection.
The deflection bowl can be measured by a fixed transducer placed on the pavement, under a moving load (principle of the deflectograph and of the curviameter), or by several transducers, placed at different distances from a fixed load (principle of the FWD). Three main types of equipment are used for pavement deflection measurements: The deflectographs, developed in France since the years1960; The curviameter, developed first in France by the CEBTP then in Spain by EUROCONSULT; The Falling Weight Deflectometer (FWD). 2.1 The deflectograph A deflectograph [1] uses a beam, equipped with a rotary coder, laid down on the pavement to measure the vertical displacement of the rear axle of a truck (figure 1). It comprises: a truck which moves at a constant speed and applies the wheel load; a measurement beam, which makes two vertical displacement measurements in the 2 wheelpaths. This beam is brought forward by an automatic system before being laid down again, to perform the next measurement. Several types of deflectographs (n°2 to n°4) have been successively developed, to improve the accuracy of the deflection measurement. This has been achieved mainly by increasing the length of the truck and the length of the measurement beam, to increase: the initial distance between the displacement transducer and the rear axle of the truck; the distance between the support points of the beam and the front axle of the truck. Front axle
Reference beam
Movement of the truck
Rear axle
Rotary coder
Figure 1. Measurement principle of the deflectograph
Deflectographs generally perform one measurement every 4 to 5 meters, at a speed of 3 to 4 km/h. The last version, (Flash deflectograph [2]), can reach a speed of 7 km/h, with a measurement spacing of 10 m. To measure the radius of curvature, an inclinometer, measuring the variation of the slope of the deflection basin, is added. The radius of curvature is defined as the derivative of this parameter. Only the Flash deflectograph and some type 4 deflectographs are equipped with an inclinometer. 2.2 The curviameter The principle of the curviameter, developed by the CEBTP, consists in using a geophone, which measures the velocity of vertical displacement of the pavement, under the passage of the rear axle of a truck (see figure 2). The deflection is obtained by integration of the geophone
measurement. A chain system ensures the placement of the geophone on the pavement surface, in front of the rear axle, and its retrieval, after the axle has moved. Several geophones are mounted on the chain, and measure the deflection bowl every 5 meters, at a speed of 18 km /h. The measurement starts 1 meter before the wheel, and stops 3 meters behind. The main limitation of the curviameter is related with the integration process which requires an accurate calibration of the geophones, the need to respect a constant speed and to assume that the deflection is zero at a distance of 3 meters from the load. This assumption is acceptable, except for very stiff, heavy traffic pavements. Another limitation is the impossibility to make measurements in sharp turns (radius lower than 40 m).
Figure 2. View of the curviameter (source BRRC)
2.3 The Falling Weight Deflectometer (FWD) The FWD applies a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. The load is produced by dropping a large weight, and transmitted to the pavement through a circular load plate - typically 300mm in diameter. A load cell measures the applied load. The loading frequency is chosen to reproduce a vehicle speed of 70 km/h. A set of 9 to 15 geophones, mounted radially at different distances from the center of the plate, measure the deformation of the pavement. The FWD offers the possibility to change the load level, and to make measurements at precise locations (for example close to a crack). It also offers a better accuracy than the deflectograph or the curviameter. However, it has two important limitations: A slow measurement rate ( a maximum of about 60 measurements per hour, representing 300 m per hour with a spacing of 5m), while the deflectograph or curviameter can cover respectively 4 km and 18 km per hour. The impossibility to measure the deflection under the loading plate, which leads to a loss of information, and makes impossible the estimation of the radius of curvature. Due to its slow measurement rate, the FWD is mainly suitable for detailed investigations on short road sections (for pavement reinforcement studies), and not for surveys at network level.
3. Metrological evaluation of the different measurement methods Studies have been performed in France, to evaluate the performance of the different types of deflectographs (type 02, 03, 04 and Flash) [3], and also of the curviameter [4] and FWD [5]. These evaluations consisted in comparing the measurements of each apparatus with a reference vertical displacement transducer, anchored at a depth of 6 m, measuring the displacement of the pavement surface. The main results of these evaluations are summarized on figure 3. This figure compares the deflection values measured by the different devices, in comparison with the reference displacement transducer, for a deflection range of 0 to 500 µm. For each device, the repeatability of the measurements is indicated, by an error bar (corresponding to a level of confidence of 99 % or 3 times the standard deviation). Clearly, the FWD and the curviameter present a better measurement accuracy than the different deflectographs. For intermediate deflections, (200 to 500 µm), the maximum measurement error is about 150 µm for the deflectographs, and 50 µm for the curviameter and FWD. The repeatability is also better for the FWD (±43 µm), than for the deflectographs and curviameter (±70 µm). For small deflections, lower than 200 µm, the accuracy of the deflectographs is clearly insufficient, while the curviameter and the FWD still give reasonable results (measurement error lower than 30 µm). The FWD also presents the best repeatability (±20 µm). For better comparison, table 1 presents the estimated measurement range, for different deflection levels, for each apparatus. The results clearly indicate that: The deflectographs (04 and Flash) give imprecise results for deflection levels lower than 50 µm. Their measurements cannot be used for backcalculation of layer moduli. They can only be used qualitatively, for comparing relative deflection levels of different sections, or for evaluating the evolution of deflections with time. The curviameter delivers more accurate results, and is well suited for network level evaluation (definition of homogeneous sections, analysis of the evolution of deflections with time). However, it tends to underestimate slightly the real deflection level, and this needs to be taken into account when performing backcalculations. The FWD is the most accurate apparatus. Due to its slow testing rate, it is particularly suitable for detailed analysis of small sections, ad for backcalculation of layer moduli.
Table 1. Estimated measurement intervals, for different deflectometers Apparatus Reference displacement level Deflectograph 04 Flash Curviamètre FWD
Estimated measurement level in µm (confidence level 99 %). 100 µm 200 µm 300 µm
500 µm
[120 - 220] [10 - 90] [50 - 110] [80 - 120]
[430 - 570] [420 - 560] [410 - 550] [450 - 530]
[200 - 300] [50 - 140] [160 - 220] [180 - 220]
[290 - 430] [170 - 310] [230 - 370] [250 - 330]
50 FWD 45
Curviameter Flash
Deflection measured (mm/100)
40
Deflectograph 04
35
Y=X 30 25
20 15 10 5 0 0
5
10
15
20 25 30 Reference deflection (mm/100)
35
40
45
50
Figure 3. Comparison of deflection values measured by different device with reference values obtained with a displacement transducer, for deflection levels below 500 µm
4. Influence of temperature and frequency on deflection measurements on bituminous pavements The mechanical response of bituminous pavements is very sensitive temperature and frequency. This section presents the results of a sensitivity analysis, performed with the pavement design software ALIZE [7] and VISCOROUTE [8], to assess the influence of these parameters on deflection. The calculations are made assuming a homogeneous temperature in the whole pavement structure, which is a simplification, because temperatures in a pavement structure generally present a positive or negative vertical gradient. Calculations have been performed for a thick bituminous pavement (30 cm of bituminous materials), for the temperature range [5 – 35] ° C, and for 3 loading frequencies, 0.5, 2 and 10 Hz, corresponding approximately to loading speeds of 3.5, 18 and 70 km/h. The calculated deflections are presented on figure 4. For the linear elastic calculations made with the software ALIZE, and for a temperature close to 20 °C, the results show that: For a temperature variation of 5 °C, the variation of the deflection is between 17 µm and 30 µm, depending on the loading frequency; The influence of frequency is also significant, with a variation of deflection of 40 µm, when the frequency increases from 0.5 to 10 Hz. Above 30 °C, the influence of temperature becomes very important (about twice larger than at 20 °C). For this reason, in France, deflection measurements are not performed above 30 °C. Calculations performed with the software Viscoroute, (considering visco-elastic behavior of bituminous materials) show a higher sensitivity to temperature than with the linear-elastic model (figure 4). Due to the large influence of temperature and frequency on pavement behavior, it is necessary to correct the measured deflections, to convert them to the same reference values of frequency and temperature (in France generally 15 °C and 10 Hz).
900
800
Linear elastic model; f=0.5Hz
Visco-elastic model; f=0.5Hz 700
Linear elastic model; f=2Hz Visco-elastic model; f=2Hz
Linear elastic model; f=10Hz
600 Déflection (µm)
Visco-elastic model; f=10 Hz 500
400
300
200
100
0 0
5
10
15
20 Temperature (°C)
25
30
35
40
Figure 4. Example of influence of temperature on the deflection of a thick bituminous pavement
5. Temperature corrections for deflection measurements Different approaches can be used for correcting temperature effects on deflection: A modelling approach: a pavement model is used to fit the deflection measurements at in situ temperature, and then to extrapolate the deflection at 15 °C, using the temperature dependence of the material (determined by laboratory complex modulus tests). A simplified approach, which consists in using empirical correction formulas. The simplified approach is generally preferred, because it does not require any material testing and avoids the fitting process. For analysis of deflectograph or curviameter results, 3 main correction formulas are used: The formula proposed in the French guide for pavement reinforcement [9] (called here “French method”). This formula is based on a reference temperature of 15 °C, and takes into account the type of pavement structure. The formulas proposed in Spain and in Belgium for the curviameter (“Spanish method” and “Belgian method”), both based on a reference temperature of 20 °C. Table 2. Temperature correction formulas for deflection measurements
French method
Spanish method 200𝑑 𝑇 𝑑20 = 3𝑇 + 140
Belgian method dT 𝑑20 = 𝑘(𝑇)𝑑 𝑇 d15 T 15 𝑘(𝑇) = 0.00026008T² − 0.02589279T 1 K . 15 + 1.4111835 dT = deflection at temperature T d15 = deflection at 15 °C d20 = deflection at 20 °C T surface temperature (or temperature at mid depth of the pavement layers in the French method) K coefficient depending on the pavement structure – K = 0.2 for a thick bituminous pavement
The three correction formulas are presented in table 2. In the French method, the temperature T is defined, in theory, as the temperature at mid-depth of the pavement layers. In practice, as measuring the internal temperature is difficult, the surface temperature is used in all methods. Figure 5 shows the effect of the different correction methods, in comparison with the initial deflection values. For simplicity, only results obtained at 2 Hz are presented. It can be seen that: The Belgian and Spanish correction methods give practically identical results (≠
