FLEXIBLE PAVEMENTS STRUCTURAL EVALUATION BASED ON DEFLECTION BASIN PARAMETERS

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1 FLEXIBLE PAVEMENTS STRUCTURAL EVALUATION BASED ON DEFLECTION BASIN PARAMETERS Flaviane Melo Lopes * Master of Civil Engineering, Universidade Estadual de Campinas, Campinas, Brasil, flavianeml@globo.com Rita Moura Fortes Professor of Civil Engineering, Universidade Presbiteriana Mackenzie, São Paulo, Brasil Carlos Yukio Suzuki Professor of Civil Engineering, Universidade de São Paulo, São Paulo, Brasil ABSTRACT: In Brazil it is significant the increase in the use of an equipment type FWD for deflection analysis and structural characterization of existing pavements. This characterization is still held only through a maximum deflection measure. However, the isolated association of maximum deflection ( ) may not be conclusive enough to diagnose the structural condition of the pavement. Thus, the current study aims, through parametric study, to propose a simplified structural assessment methodology of asphalt pavement condition with granular base using measurements of deflection basin. Through the known correlations simplified models will be proposed to estimate the remaining life of a pavement according to deflection parameters. The proposed methodology has proved to be promising. The use of the information on deflection basin can help out in troubleshooting pavement rehabilitation, avoiding premature failures in diagnosing faults. KEY WORDS: structural evaluation, deflection basin parameters, Falling Weight Deflectometer, Elsym-5 computer program, flexible pavements, remaining pavement life 1. INTRODUCTION Since the mid 1980s, in Brazil, the Falling Weight Deflectometer (FWD) is used as a well-established nondestructive testing device for evaluating the structural capacity of pavements in service for rehabilitation designs. The growth of the use of FWD it can be attributed to the advantages of this equipment. These advantages are the speed and security of execution in the field work, reliability of results as the repeatability and the ability to accurately obtain the whole shape of deflection basin. The advantages showed in relation to the traditional Benkelman Beam, which is an easy operated device and its cost less than FWD device. The Benkelman Beam is a device that works in a slow time and demands a hard field work. Currently, the Brazilian restoration standards are used as reference measurements with equipment Benkelman Beam, these standards are known in Brazilian technical group of pavement. In order to increase the interpretation of the deflection basin based on a parametric study ever conducted, this work propose a methodology for assessing the structural condition of asphalt pavements using deflection basin parameters. The parameters of curvature of the basin deflection used in this study are internationally known and readily obtained by surveying deflection measurements with the equipment FWD. These deflection basin parameters (DBP) are used to develop correlations between them and the critical strain generated in the pavement structure when requested. The elastic and geometric characteristics of the layers are used in correlations to verify its representation in the form of the basin and the values of DBP. Using the correlations, we intend to establish a methodology that can provide simplified models to estimate the remaining life of a pavement in operation through the deflection parameters. Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 1

2 The Brazilian methods for calculating the pavement overlay, as DNER-PRO-11/79 [2] and DNER-PRO-269/94 [3], recommend the determination of deflection basin through deflection surveys, however only the DNER-PRO- 269/94 [3] method makes use of additional measures to calculate the radius of curvature (R). The importance of the knowledge of the basin deformation are discussed in Moreira [9] in accordance with Figure 1, where two deflections curvature basin represent the deformed obtained at two different places of the surface on the same type of pavement, caused by application of load wheel. APPLIED LOAD D r DEFLECTION BASIN - A DEFLECTION BASIN - B = MAXIMUM DEFLECTION D r = DEFLECTION DIFFERENCE AT OFFSET r Figure 1. Maximum Deflection ( ) for two different basins deformation (adapted from [4]) The maximum deflection ( ) values in both cases (A and B) of figure 1 are the same, so this values indicates a same structural capacity. However, it is known when analyze the deflection basin the structural conditions are different, even if expressed the maximum deflection ( ). In the literature, it can be seen that many researchers tried to develop methods for pavement analyzes using different devices and curvature parameters to obtain the values of deflection. 2. PREDICTION OF PAVEMENT REMAINING LIFE In the technical literature there are some works of the deflection basin parameters that have deflections prediction model as a function of the traffic number of loading repetitions, as shown by Maree and Jooste [8] and Horak [5] on Figure 2, where different levels of traffic and different parameters are estimated the pavement remaining life with granular, asphalt or cemented bases. Kilareski et al. [6] reported that many highway departments estimate the remaining life of the pavement by using only the measurements of deflections. Thus, the authors attempted to relate the maximum tensile strength with the parameters of curvature and to make more practical. They produced the graph showed in Figure 3, which introduced the concept of the structural number (SN) related to remaining life and deflection basin parameters. Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 2

3 Maximum Deflection (D máx ) SCI BDI BCI Figure 2. Correlation between DBP and prediction of the traffic number of loading repetition (adapted from [8]) Figure 3. Remaining pavement life based on SCI and SN [6] 3. PARAMETRIC STUDY The purpose of the parametric study ever conducted in Lopes [7] was to determine relationships between deflection basin parameters and indicators that reflect the structural capacity of pavements, to assist in the evaluation of the estimated remaining useful life of the pavement from surveys conducted with FWD device. To calculate the stresses, strains and displacements in the simulated structures of pavement, made use of the ELSYM-5 - Elastic Layered System computer program. This program admits up to five layers, including the Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 3

4 subgrade, and up to ten loads of wheel contact area with tire-pavement loop. ELSYM-5 program is a well-known and widely used because of its low processing time and ease of use. The structures of flexible asphalt pavement simulated consist of three layers: surface, base and subgrade. The first layer is composed of asphalt concrete (AC) the base composed of granular material overlying the subgrade with resilient modulus ranging from 250 to kgf/cm². The summary characteristics of the layers are shown in Table 1. Table 1. Characteristics of pavement structures Layer Thickness (cm) Modulus (kgf/cm²) Poisson's Ratio H AC ,35 H BASE ,40 H SG ,45 According to the correlation equations of Lopes [7], the most representative DBP relative to structural indicators are SCI, BDI and BCI, that forming a set of measurements to provide adequate information representing all layers of the pavement from the subgrade to the surface. Besides the selection of the parameters SCI, BDI and BCI, for subsequent studies, were also analyzed and R, parameters already established in the transportation standards. Table 2 shows the formulas of DBP and units mentioned above. Table 2. Deflection basin parameters used in parametric study Deflection Parameter Formula Unit - Maximum Deflection R - Radius of Curvature SCI - Surface Curvature Index BDI - Base Damage Index BCI - Base Curvature Index 4. PERFORMANCE OF PREDICTION MODELS [10-2 mm] Depending on performance, models can be found to make structural evaluation of pavements, estimating its remaining useful life, from the values of DBP in a direct and relatively simple form. With the aim of determining deflection parameters models based on traffic to flexible pavements, fatigue models recommended in the literature were used, in which the number N correlate with the maximum deflection ( ), the horizontal tensile strain at bottom of asphalt (ε t ) and the vertical compressive deformation at top of subgrade (ε v ). It is noteworthy for the adoption of a fatigue mathematical equation that evaluates the quality and performance of a pavement, it is necessary to understand how and under what conditions this equation was determined. as field trials or laboratory, magnitude, frequency and duration of cyclic loading, etc. R = OBS:, D 25, D 30, D 60, D 90 [10-2 mm] Deflection distance from point of maximum deflection *( -D 25 ) SCI = - D 30 BDI = D 30 - D 60 BCI = D 60 - D 90 R [m] SCI [10-2 mm] BDI [10-2 mm] BCI [10-2 mm] Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 4

5 DEFLEFECTION VALUES [10-2 mm] AND RAIUS OF CURVATURE [m] For each parameter deflection were prepared correlations between ε t, ε v alone or together, and then selecting the final one to produce better representation. The proposed method allows obtaining of the abacus shown in Figure 4, which shows the fatigue models proposed to estimate the remaining life of the pavement and assist the structural evaluation of flexible pavements as a function of, R, SCI, BDI and BCI and models fatigue already consolidated in Brazil. For the development of the abacus of Figure 4 were used separately or together with the model of Asphalt Institute with ε t and the model of the Shell (50% reliability) with ε v REMAINING PAVEMENT LIFE PRO 11/79 R 100 PRO 269/94 10 BCI BDI SCI 1 1,00E+05 1,00E+06 1,00E+07 1,00E+08 NUMBER N PRO 11/79 PRO 269/94 Do R SCI BDI BCI Figure 4. Abacus fatigue models proposed In Figure 5 presented two sets of equations that represent the abacus model proposed here. The difference between the boxes are the type of mathematical formulation adopted. For any of the three structures, the set of responses is the same. = 2.605,3602 N -0,2465 N = 7,2024 x ,0568 x SCI = 2.077,7335 N -0,3037 N = 8,3928 x x SCI -3,2927 BDI = 632,4255 N -0,2603 N = 5,7636 x x BDI -3,8417 OU BCI = 142,4329 N -0,1959 N = 9,8496 x x BCI -5,1046 R = 1,5727 N 0,3099 N = 0,2320 x R 3,2268 Figure 5. Equations of fatigue models proposed to estimate the pavement remaining life There were developed ranges of values for the study and fatigue models proposed. These ranges showed in Table 3 for parameters, R, SCI, BDI and BCI, and the lanes of traffic in the methods usually adopted in Brazil. As can be seen in the manual DNIT [1]. Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 5

6 Table 3. Range of recommended values for the proposed model DBP N 1x10 7 1x10 7 > N 1x10 6 1x10 6 > N 1x10 5 N 1x10 5 [10-2 mm] R [m] SCI [10-2 mm] BDI [10-2 mm] BCI [10-2 mm] < > 150 > < 60 < > 60 < > 30 < > 15 To illustrate the fatigue models proposed a theoretical example was designed to estimate the remaining life of the structures and was analyzed the structures of selected critical points and compared with geometric and elastic characteristics. 4. THEORETICAL EXAMPLE For this example we used three theoretical structures designated by the letter D and they have the deflection value equal to 62,9x10-2 mm and they are presented in Figure 6. Structure D1 Structure D2 Structure D3 62,9x10-2 mm 62,9x10-2 mm 62,9x10-2 mm HAC = 5 cm EAC = kgf/cm² ε t 2,47x10-4 cm/cm HAC = 10 cm EAC = kgf/cm² ε t 2,81x10-4 cm/cm HAC = 10 cm EAC = kgf/cm² ε t 4,95x10-4 cm/cm HBASE = 45 cm HBASE = 15 cm EBASE = kgf/cm² HBASE = 30 cm EBASE = kgf/cm² ESG = 750 kgf/cm² ε V 4,65x10-4 cm/cm ESG = 750 kgf/cm² ε V 8,79x10-4 cm/cm EBASE = kgf/cm² ESG = kgf/cm² ε V 5,67x10-4 cm/cm SCI = 30,4 x 10-2 mm E 1 /E 2 = 7,5 SCI = 19,7 x 10-2 mm E 1 /E 2 = 20 SCI = 32,2 x 10-2 mm E 1 /E 2 = 10 BDI = 10,6 x 10-2 mm E 2 /E 3 = 4 BDI = 17,2 x 10-2 mm E 2 /E 3 = 3 BDI = 15,8 x 10-2 mm E 2 /E 3 = 1,2 BCI = 5,6 x 10-2 mm E 1 /E 3 = 30 BCI = 9,1 x 10-2 mm E 1 /E 3 = 60 BCI = 5,3 x 10-2 mm E 1 /E 3 = 12 R = 116,7 m R = 198,3 m R = 115,3 m R/ = 1,85 m /10-2 mm R/ = 3,15 m /10-2 mm R/ = 1,83 m /10-2 mm Figure 6. Deflection basin of structures with = 62,9 x10-2 mm For each structure was estimated the pavement remaining life, through the value of DBP calculated. The structure D1 presented the value =62,9 x10-2 mm, Nremaining=3,63 x10 6, however critical number N is equal to 1,08 x10 6 for the parameter R, which is very close to the result for the parameter SCI (Nremaining= 1,10x10 6 ). Both results reflect the concrete asphalt with 5,0 cm of thickness and modulus (E CA ) equal to kgf /cm ². The structure D2 also showed = 62,9 x10-2 mm and Nremaining = 3,63x10 6, however the critical number N is equal to 1,04 x10 6 for BDI. Unlike the D1 structure with disabilities in the concrete asphalt, the structure D2 has a thinner base (HBase = 15,0 cm) with lower modulus of resilience, therefore, reflected in the lower number N. As its concrete asphalt has a thickness of 10,0 cm and modulus of kgf/cm², their number N for the parameters SCI and R are more than 4,63 x10 6. The estimated remaining life for the structure D3 is shown in Figure 7, and its critical number N is equal to 9,13x10 5 for SCI. Although the structure has 10,0 cm of concrete asphalt thickness and the modulus of resilience is equal to kgf/cm ², this value contributed to the result of the number N critical. This structure has a Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 6

7 DEFLECTION [10-2 mm] AND RADIUS OF CURVATURE [m] DEFLECTION [10-2 mm] AND RADIUS OF CURVATURE [m] DEFLECTION [10-2 mm] AND RADIUS OF CURVATURE [m] value of subgrade modulus equal to kgf/cm 2, characterized by a number N, for parameter BCI is equal to 1,92 x REMAINING PAVEMENT LIFE ,63E+06 1,52E+07 1,08E+06 1,10E+06 6,73E ,00E+05 1,00E+06 1,00E+07 1,00E+08 NUMBER N STRUCTURE D PRO 11/79 Do SCI BDI BCI R PRO 269/94 Do - Calculado SCI - Calculado BDI - Calculado BCI - Calculado R - Calculado ,63E+06 6,00E+06 1,26E+06 4,63E+06 1,04E ,00E+05 1,00E+06 1,00E+07 1,00E+08 NUMBER N STRUCTURE D PRO 11/79 Do SCI BDI BCI R PRO 269/94 Do - Calculado SCI - Calculado BDI - Calculado BCI - Calculado R - Calculado ,04E+06 3,63E+06 1,92E+07 1,44E+06 9,13E ,00E+05 1,00E+06 1,00E+07 1,00E+08 NUMBER N STRUCTURE D3 PRO 11/79 Do SCI BDI BCI R PRO 269/94 Do - Calculated SCI - Calculated BDI - Calculated BCI - Calculated R - Calculated Figure 7. Estimation of remaining life for the structures D1, D2 and D3 Although the three structures have the same value of deflection, deficiencies in each one occurs in different layers. By means of the methodologies available of restoration, for example, the procedure DNER PRO-11/79 [2] to restore solution for three structures would be exactly the same, since values of maximum deflection are identical. Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 7

8 The use of DBP assists in the identification of the problems in each layers of the pavement and can obtain a better solution for restoration an existing pavement. CONCLUSIONS The use of DBP (Deflection Basin Parameters) enables an expansive flexible pavement structural evaluation of existing pavement, identifying what are the critical layers through the deflection values that are usually obtained in the field, using the FWD equipment. It was observed that although some surfaces have the same value of maximum deflection ( ), they may have different deflection of other parameters, demonstrating that the maximum deflection ( ) alone is not sufficient to characterize the condition of a pavement, that claim the studies found in the literature, as Paiva [11], Suzuki [13], Souza [12], Nagao [10] and others. With these models can be verified in a simplified form with the results of FWD, the evolution of structural parameters as a function of traffic and period of time, or to estimate the pavement remaining life according to the deflection basin. REFERENCES: [1] DEPARTAMENTO NACIONAL DE INFRAESTRUTURA DE TRANSPORTES DNIT. Manual de Pavimentação. Rio de Janeiro, [2] DEPARTAMENTO NACIONAL DE ESTRADAS DE RODAGEM - DNER. DNER-PRO 011/79. Avaliação estrutural dos pavimentos flexíveis: Procedimento B. Rio de Janeiro, [3] DEPARTAMENTO NACIONAL DE ESTRADAS DE RODAGEM - DNER. DNER-PRO 269/94. Projeto de restauração de pavimentos flexíveis. Rio de Janeiro, [4] HORAK, E. Aspects of deflection basin parameters used in a mechanistic rehabilitation design procedure for flexible pavements in South Africa Thesis (PhD of Civil Engineering) - Department of Civil Engineering, University of Pretoria, Pretoria - South Pretoria, [5] HORAK, E.; EMERY, S. Falling Weight Deflectometer bowl parameters as analysis tool for pavement structural evaluations. In: ARRB CONFERENCE, 22., 2006, Canberra. Proceedings... Australian Road Research Board, [6] KILARESKI, W. P.; ANANI, B. A. Evaluation of in-situ moduli and pavement life from deflection basins. In: INTERNATIONAL CONFERENCE ON THE STRUCTURAL DESIGN OF ASPHALT PAVEMENTS, 5., 1982, The Netherlands. Proceedings... Ann Arbor: University of Michigan, [7] LOPES, F. M. Pavimentos flexíveis com revestimento asfáltico avaliação estrutural a partir dos parâmetros de curvatura da bacia de deformação Dissertação (Mestrado em Engenharia Civil) - Programa de Pós-Graduação da Faculdade de Engenharia Civil, Arquitetura e Urbanismo, Universidade Estadual de Campinas, Campinas, [8] MAREE, J. H.; JOOSTE, F. J. Structural classification of pavements through the use of IDM deflection basin parameters. PR-91/325, South African Roads Board, Sandton, [9] MOREIRA, M. R. P. Um método de avaliação de pavimento flexíveis com base no raio de curvatura da linha elástica das deflexões. In: REUNIÃO DE PAVIMENTAÇÃO URBANA, 13., 1977, Rio de Janeiro. Anais... Rio de Janeiro: ABPv, [10] NAGAO, E. M. Considerações de parâmetros de curvatura da bacia de deformação e do número estrutural no dimensionamento de reforço de pavimentos flexíveis Dissertação (Mestrado em Engenharia Civil) - Escola Politécnica, Universidade de São Paulo, São Paulo, [11] PAIVA, C. E. L. Contribuição para análise e interpretação de bacias de deformação Tese (Doutorado em Engenharia Civil) - Escola Politécnica, Universidade de São Paulo, São Paulo, [12] SOUZA, P. R. B. Considerações sobre a utilização de parâmetros de curvatura da bacia de deformação para avaliação estrutural de pavimentos Dissertação (Mestrado em Engenharia Civil) - Escola Politécnica, Universidade de São Paulo, São Paulo, Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 8

9 [13] SUZUKI, C. Y. Contribuição estudo de pavimentos rodoviários com estrutura invertida (sub-base cimentada Tese (Doutorado em Engenharia Civil) - Escola Politécnica, Universidade de São Paulo, São Paulo, Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 9