LONGITUDINAL SURFACE CRACKING IN ASPHALT PAVEMENTS ON STEEL BRIDGE DECKS

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1 LONGITUDINAL SURFACE CRACKING IN ASPHALT PAVEMENTS ON STEEL BRIDGE DECKS Tatsuo Nishizawa 1, Kenji Himeno 2, Kitaro Uchida 3, Kenichiro Nomura 4 and Sigeo Higashi 5 (1) Ishikawa National College of Technology, Japan (2) Chuo University, Japan (3) Sato Road Corporation, Japan (4) Taisei Rotech Corporation, Japan (5) Kajima Road Corporation, Japan Abstract Longitudinal surface cracks (LSC) are often observed in asphalt pavements on steel bridge deck. The cracks appear longitudinally from pavement surface in wheel paths and their spacing coincides with a half of spacing of longitudinal ribs. The objective of this study is to find the causes and mechanism of the LSC. A condition survey was conducted on more than 5 steel deck bridges to investigate actual conditions of pavements on the bridges. The survey revealed that LSC actually starts from pavement surface but does not reach its bottom, and that they appear not only above the webs of main girder or longitudinal ribs but also between the webs. A computer simulation was performed using a newly developed FE model, in which a steel deck structure is modeled with a strip element and the pavement on it is modelled with a prism element. To take into account viscosity of asphalt materials, the Burger model was incorporated. In the simulation, axle loads traveled over a pavement and variations of stresses and strains in the pavement with time were calculated. The dissipated energy in the pavement was estimated. The simulation results suggested that high dissipated energy at the pavement surface between webs might cause the LSC there. 1 Introduction In Japan, since a steel plate deck was first utilized in a road bridge in 1954, this technology has been developed and allows us to construct huge steel bridges, such as Akashi Strait Bridge, which is the longest suspension bridge in the world. Along with the development of this technology, pavements on steel bridge decks have also been improved in their structures and materials.

2 The most serious problem in the pavements on steel bridge decks is longitudinal surface cracks (LSC) that appear relatively early after construction. The cracks occur longitudinally from the pavement surface in wheel paths and their spacing coincides with the spacing of longitudinal ribs. Therefore, it is expected that the positions of the cracks are related not only to wheel load positions but also to the locations of the longitudinal ribs. Since research on the LSC in pavements on steel bridge decks is very limited 1,2,3,4), its causes and mechanisms are not clear at the present time. The objective of this study is to investigate the causes and mechanisms of the cracks and find the prevention measures. First, a condition survey was conducted for more than 5 pavements on steel bridge decks to investigate the actual pavement conditions. The finite element analysis for a pavement on a steel bridge deck was performed to examine the effects of viscous behavior of asphalt concretes on the occurrence of LSC, using a new FE model with the prism and strip elements. 2 Condition Survey In order to grasp the actual condition of pavements on steel bridge decks, a condition survey was conducted on 5 steel bridges in the Tokyo metropolitan area, where traffic load condition is very heavy. We observed pavement surfaces and recorded types and positions of distresses as well as types of steel deck plates and longitudinal ribs. The bridges investigated included 29 box girder bridges, 1 plate girder bridges, 5 suspension bridges and 6 other types. 2.1 Distress of Pavement Fig. 1 summarizes a result of subjective evaluation of pavement distresses observed in the survey. The most serious distress of the pavements is LSC and 1% of them are judged to need rehabilitation as soon as possible. It was found that there is no clear relationship between the pavement distress and the bridge type Figures in the graph indicate the number of occurrnce per lane Pot Hole Rutting Longitudinal Crack None Lane No.(Top) Number of lane per direction(bottom) Fig. 1 Distress of Pavement on Steel Plate Deck.

3 2.2 Type of Longitudinal Rib Types of the longitudinal ribs investigated included open section ribs in 25 bridges, close section ribs in 18 bridges and unknown types in 7 bridges. In this survey, it is observed that LSC appeared more frequently on the pavements on close section ribs than those on the open section ribs. 2.3 Longitudinal Surface Crack Most of LSCs occur in the wheel paths and the cracks run in parallel with a spacing of about 15 mm. Situations of the cracking at various stages are shown in Fig. 2, where positions of LSCs as well as structures of plate decks are indicated. It can be seen that LSCs appeared on the pavement surface not only above webs of longitudinal ribs but also between the webs. Fig. 3 shows a typical LSC on a pavement on a bridge. An excavation survey of a cracked pavement confirmed that the cracks started from the surface and propagated downward as shown in Fig. 4. Cross section Cross section Plan Plan Cracks Cracks Surveyed section (a) Early Stage (b) Final Stage Fig. 2 Positions of Longitudinal Surface Cracks. Fig. 3 Situation of Longitudinal Cracking. Fig. 4 Excavation of Cracked Section.

4 Pavement Deck Plate w z y v x Longitudinal Rib u Prism Element Link Element Strip Element Fig. 5 SLPE Model for Pavement and Deck. 3 Simulation Method 3.1 FEM Model The structural model employed in this study is SLPE model based on the finite element method 2). The model uses a strip element and a prism element that are bonded by a link element, as shown in Fig. 5. The strip element represents the bending behavior of the elastic plate, modelling complicated deck structures. The prism element is an eight node isoparametric element that is able to describe the local deformation of asphalt material with relatively low stiffness. The role of the link element is to bond the strip element to the prism element, modelling the insulation layer between the steel deck and the pavement. Pavements on steel bridge decks are subjected to dynamic loading of traveling vehicles. The dynamic behavior of these pavements is more important than normal pavements on grade, because the whole bridge structure is more flexible. The structural model has an ability to take into account the dynamic effect of the pavement and the viscosity of the asphalt materials. 3.2 Structural model A computer simulation was performed for a pavement on a steel deck bridge as shown in Fig 6. A 6 mm thick asphalt pavement is constructed on a 12 mm thick steel deck with longitudinal closed ribs. The asphalt pavement and the steel deck were assumed perfectly bonded with each other at the interface. A span between lateral girders was taken as a region for the structural model. The length of the span was assumed 5 m in the simulation. Stresses and strains at midspan generated by an axle load travelling longitudinally on the pavement were calculated. Two travelling speeds were assumed in the simulation: 4 km/h and 2 km/h. The mesh used in the simulation is shown in Fig. 7(a). The pavement was divided into two layers and areas under the tires were meshed with small elements as shown in Fig. 7(b). Two cases of loading was considered: Loading Position A where right tires are

5 located between the webs of the longitudinal ribs and Loading Position B where the right tires are located just above the webs. A tire load was modelled as a combination of a uniformly distributed load in the vertical direction z and triangular distributed loads in the horizontal direction x as shown in Fig.7(c). The total amount of the axle load was 98 kn. z y x Span: L=5 m Steel Deck: t=12 mm Pavement: t=6 mm Longitudinal Rib: t=6 mm Box Girder: t=1mm Fig.6 Structural Model Considered in the Simulation Fig. 7 Mesh and Load Model Used in the Simulation 3.3 Material Model In the simulation, the steels of the deck and ribs are assumed to be an elastic material and the asphalt concrete is modeled with the Burger model as shown in Fig 8. The values of the parameters are summarized in Table 1. Two types of asphalt concrete were considered. C G,C K Table 1 Material Parameters 1/C G 1/C G1 η G = η G1 η K = η K1 MPa MPa GPa*sec GPa*sec Steel Asphalt Concrete Asphalt Concrete η G,η K C G1,C K1 η G1,η K1 Fig. 8 Burger Model

6 4 Simulation Results Simulation results are presented in terms of transverse tensile strain and dissipated energy in the pavement, both of which are supposed to be strongly related to the fatigue cracking of asphalt concrete 5,6). In this study, the dissipated energy is defined as follows : w σ dε (1) f = ij ij σ, ε : stress and where, w f : the dissipated energy per unit during one loading cycle, ij ij strain tensors, respectively. The integral is performed for one loading cycle. Himeno, et. el. presented a unique relationship between w f and fatigue life that can be applied to various asphalt concretes 5). Therefore, w f is a good indicator of the possibility of fatigue cracking. Compressive Strain Tensile Strain (a) Unit Dissipated Energy (b) Transverse Strain Fig 9. Contours of Dissipated Energy and Transverse Strain under Tires Fig. 9 shows contours of the transverse strains and the dissipated energy in the pavement under the right tires. From this figure, it can be seen that the large horizontal strains are generated at the pavement surface just above the webs of the rib. The dissipated energy is high at the surface between the webs. However, both of them rapidly decrease in the lower part of the pavement. These contours suggest that the fatigue cracking most likely starts from the surface in the pavement, which agrees with the results of the condition survey mentioned earlier. Deflection(mm) 5 Axle Load Deflection Horizontal Strain Dissipated Energy Strain(*1 6 ) 1 Energy(J/m 3 *1 1 ) Deflection(mm) 5 Axle Load Deflection Horizontal Strain Dissipated Energy 1 Strain(*1 6 ) Energy(J/m 3 *1 1 ) Distance x (m) Distance x (m) (a) Load Position A (b) Load Position B Fig. 1 Distributions of the Tensile Strain and Dissipated Energy at Pavement Surface

7 Deflection(mm) 5 Axle Load Deflection Horizontal Strain Dissipated Energy Strain(*1 6 ) 1 Energy(J/m 3 *1 1 ) Deflection(mm) 5 Axle Load Deflection Horizontal Strain Dissipated Energy 1 Strain(*1 6 ) Energy(J/m 3 *1 1 ) Distance x (m) Distance x (m) (a) Asphalt Concrete 2 (b) Axle Speed : 2km/h Fig. 11 Distributions of the Tensile Strain and Dissipated Energy with Other Conditions (Loading Position A) Fig. 1 shows the distributions of the surface transverse strain, which is obtained when the axle load is located at the midspan, and the surface dissipated energy in the case of Asphalt Concrete 1 and the axle speed of 4 km/h. In Load Position A (Fig. 1(a)), large tensile strain and dissipated energy are observed just above the web of the box girder, which corresponds to a tire edge of the left wheel. On the other hand, under the right wheel, the tensile strain is not so high. The dissipated energy, however, is relatively high between the webs of the ribs, suggesting the LSC there. In Load Position B (Fig. 1(b)), the tensile stress and dissipated energy are totally much lower than those of Load Position A. Therefore, the mechanical behavior of pavement on steel bridge deck is very sensitive to tire position and geometry of the deck and ribs. Accordingly, LSC in the pavement on steel deck can not be explained with only tensile stress at the pavement, because the strain state between the web, where LSC is sometimes observed, is generally compressive. The dissipated energy could explain the cause of LSC not only above the webs but also between the webs. Fig.11 shows similar distributions under other conditions. When the viscosity of the asphalt concrete is low and the axle speed is the same as in Fig. 1, the dissipated energy becomes much larger than the case of high viscosity, as shown in Fig. 11(a). Lowering the axle speed from 4 km/h to 2 km/h has the similar effect on the dissipated energy, as shown in Fig. 11(b). 5 Conclusions In this study, to investigate the causes and mechanism of LSC in pavements on steel bridge decks, a condition survey on actual bridge pavements was conducted. Its results revealed that LSC is a major distress in the pavements. Also, it was found that the LSC is observed not only above the webs but also between the webs. A computer simulations using SLPE model was performed for a typical pavement on a box girder. Its results

8 showed that the large tensile strain is generated at the position where a tire edge coincides with the position of a web, and that the LSC between the webs could be explained by the large dissipated energy under the tire. Lowering the viscosity of asphalt concrete and the axle load speed increase the dissipated energy and thus increase the possibility of occurrence of LSC between webs. 6 Acknowledgements This paper was supported financially by a Grand-in-Aid for Scientific Research, Japan Society for the Promotion of Science, and a 22 Research Project for Promotion of Advanced Research at Graduate School, Chuo University. 7 References 1. Nishizawa, T., Himeno, K., Sato, S. and Sato, I., Study on Analytical Method for Asphalt Pavement on Steel Plate Deck, Journal of Materials, Concrete Structures and Pavements, JSCE, No.627, V-44, (1999) , (in Japanese). 2. T. Nishizawa, T., Himeno, K., Nomura, K. and Uchida, K., Development of a New Structural Model with Prism and Strip Elements for Pavements on Steel Bridge Decks, The International Journal of Geomechanics, Vol.3, (21) Huurman, M., Kasbergen, C., Liu, X., Scarpas, A., Molenaar A.A.A. and Medani, T.O., 3D-FEM for the Estimation of the Behavior of Asphaltic Surfacings on Orthotropic Steel Deck Bridges, Proceedings of 3 rd International Symposium on 3D Finite Element for Pavement Analysis, Design and Research, Amsterdam, the Netherlands, (22) Kolstein, M.H., Medani, T.O., Molenaar, A.A.A. and Scarpas, A., Design Aspects for Wearing Courses on Orthotropic Steel Bridge Decks, Proceedings of 9 th International Conference on Asphalt Pavements, Copenhagen, Denmark, (22) 1:6-3,. 5. Himeno, K., Watanabe, T. and Maruyama, T., Estimation of the Fatigue Life of Asphalt Pavement, Proceedings of 6 th International Conference on Structural Design of Asphalt Pavements, Ann Arbor, USA, (1987) Rowe, G.M., and Brown, S.F., Fatigue Life Prediction Using Visco-Elastic analysis, Proceedings of 8 th International Conference on Asphalt Pavement, Seattle, Washington, (1997)