Evaluation of Optimum. Rubblized Depth to Prevent Reflection Cracks

Transcription

1 Lee, Bae, Han and Stoffels 1 Evaluation of Optimum Rubblized Depth to Prevent Reflection Cracks Seung Woo Lee, Ph.D. Assistant Professor, Department of Civil Engineering, Kangnung National University 123 Jibyeon-Dong, Gangneung, Gangwon-Do, Korea Tel: ; Fax: ; swl@kangnung.ac.kr Jae Min Bae Graduate Student, Department of Civil Engineering, Kangnung National University 123 Jibyeon-Dong, Gangneung, Gangwon-Do, Korea Tel: ; Fax: ; boy2nice@kangnung.ac.kr Seung Hwan Han, Ph.D. Chief Research Engineer, Korean Highway Cooperation San 50-5, Sanchuk-ri Dongtan-myeon Hwaseong-si Kyunggi-do, Korea Tel: ; Fax: ; hansu@freeway.co.kr Shelley M. Stoffels, D.E., P.E. (*Corresponding Author) Associate Professor of Civil Engineering 201 Transportation Research Building The Pennsylvania Transportation Institute at The Pennsylvania State University University Park, PA Tel: ; Fax: ; sms26@engr.psu.edu August 1, 2005 Submitted to the Transportation Research Board for presentation at the 85 th Annual Meeting and for publication in the Transportation Research Record Words: 3250 Figures: 12 Tables: 1 Total : 6500

2 Lee, Bae, Han and Stoffels 2 Evaluation of Optimum Rubblized Depth to Prevent Reflection Cracks ABSTRACT Reflection cracking is a common distress in asphalt overlays on cracked concrete pavements. Rubblization, which can minimize reflection cracks, has been used as an efficient rehabilitation method for aged concrete pavements in the United States. When applying the rubblization method, it is typical for the upper layer of aged concrete pavement to be rubblized to 40mm- 70mm in size, while the lower layer of the aged concrete pavement remains at a larger size, often over 300mm. When much larger sizes remain, reflection cracks in the asphalt overlay may result. For thick concrete slabs, it is difficult to rubblize the entire depth of thick concrete slabs in terms of both construction and cost. The high impact energy necessary to rubblize the entire depth may cause degradation of the structural capacity of the pavement substructure. Korean highways are typically paved with thick concrete pavement slab (30-33cm concrete slab with 15cm lean concrete base on granular subbase). This study conducted in Korea determined the minimum depth of effective mm size rubblization to deter a reflection crack. An indoor simulation experiment was designed to assess the effects of the depth of the mm rubblized layer on the prevention of reflection cracks. The experiment simulated two modes of reflection crack initiation: Mode I (bending failure) and Mode II (shear failure). Mode I and Mode II tests were carried out for rubblization depths of 0cm, 10cm, and 20cm. Reflection cracking did not occur with the simulated rubblization depths of 10 cm or 20 cm. INTRODUCTION In Korea, asphalt overlays on asphalt pavements have been used as a common method to extend the life of aged pavement. However, large-scale rehabilitation of concrete pavement has not been in practice, as the oldest concrete pavement was not more than 25 years old as of In 2010, however, more than 3000 km of concrete pavements will be more than 25 years old, since most of the Korean concrete pavements were constructed in the late 1980s in Korea (1). Therefore, efficient methods to rehabilitate concrete pavements need to be adopted, and rubblization is being considered as a potential method. A typical pattern of distress in asphalt overlays on cracked concrete pavements is reflection cracking. The mechanism of reflection cracks is illustrated in figure 1. Mode I reflection cracking (bending) is caused by a tensile failure of the overlay layer due to the horizontal movement of the concrete slab's contraction and expansion. In many cases, this reflection crack is shown at the discontinuous section (cracked joint) of the concrete at the first year of the overlay and is accelerated by repeated traffic loading. Generally, a thin joint crack

3 Lee, Bae, Han and Stoffels 3 starts during the first winter of the overlay pavement, and the damage to the overlay layer due to crack is observed within a few years of the overlay. Mode II reflection cracking (shearing) is caused by the traffic load. Vertical traffic loading at various parts of concrete slabs induces relative vertical movements and cause shear failure. Figure 1. Progress of a reflection crack (2). To prevent reflection cracking, many methods have been suggested. These methods include sawing and sealing at the joint, or installation of a transition layer made of wire mesh, textile grid, cushion courses, or other materials to reduce strain or stress on the overlay. Overlaying using special materials such as rubber asphalt, fiber-reinforced asphalt, or polymer

4 Lee, Bae, Han and Stoffels 4 asphalt, or increasing the thickness of the overlay have also been adopted to protect against reflection cracking. These methods showed partial success, slowing down the initiation and progression of a reflection crack, but the problem of a reflection crack still reappeared a few years later (3, 4). Rubblization has already been widely used in the United States (5). To prevent reflection cracking while maintaining structural capacity, specialized expertise such as retaining adequate size of the rubblized aggregate is required. There are currently approximately twentyone states that are using rubblization; most of them have reported excellent performance of the rubblized sections (5). Common types of rubblization equipment include the Resonant Frequency Breaker or a combination of Badger Breaker (MHB) and Vibratory Grid Roller. The Resonant Frequency Breaker uses a 5,400 Kg hammer of 15cm in width to rubblize the pavement by applying an amplified frequency of 45 per second at 13mm (135kg - 2,250 kg). The Resonant Frequency Breaker does not require special safety devices and is relatively fast (between 3km and 6km per hour). A method of using the combined equipment of the Badger Breaker (MHB) and the Vibratory Grid Roller rubblizes the concrete slab by dropping a 545kg- 680kg hammer free-fall; a Vibratory Grid Roller is used as a safety device after rubblizing. Rubblization has the advantage of being environmently friendly (in the sense that the rubblized pavement is recycled at its original place of problem) and relatively fast in its implementation. Additionally, it lets the drivers use a part of the road while the road is being repaired by the rubblization technique (6). It is difficult to rubblize the entire depth of the thick concrete slab, in terms of both the construction implementation and the cost. Also, the high impact energy to required to rubblize the entire depth may cause degradation of the structural capacity of the pavement substructure. Because Korean concrete pavements use a lean-concrete subbase and thicker concrete slabs than typical of the pavements currently being rehabilitated in the United States, a research on finding the optimal depth of rubblization is required. This study examined resistance characteristics of rubblized concrete pavement to a reflection crack through an indoor simulation experiment. The experiment varied the rubblization depth to 0cm, 10cm and 20cm for the rubblized aggregate size of 40mm-70mm as generally accepted by transportation agencies in the United States. Then, the suggested effective rubblization depth was determined. RUBBLIZED AGGREGATE SIZE AND DEPTH Adequate size and depth of rubblization is important to prevent reflection cracking and to provide structural capacity. Guides for the rubblized-aggregate gradation in United States are shown in Table 1 (5). Most of the states restrict the average particle size to 25.4mm-76.2mm at the top of the rubblized concrete layer. The maximum allowable rubblized particle size is

5 Lee, Bae, Han and Stoffels 5 specified to be 127mm-203mm and 228mm-381mm in 11 and 10 states, respectively (6). As the rubblized particle size differs widely, the relative layer coefficient and the thickness of the required asphalt overlay layer differ accordingly. TABLE 1. Design Standards for Rubblization Layer Coefficient No. of States Thickness of Asphalt Overlay No. of States Maximum Particle Size No. of States mm mm mm mm mm Figure 2 shows the excavation of a pavement after the application of the rubblization. As shown in figures 2 and 3, the broken PCC slab can be differentiated into two materials, designated as Rubblized PCC (RC) and Fractured Concrete (FC). The size of RC ranges from 40 to 70mm, and the maximum allowable particle size of RC is 70mm. FC maintains a high structural unity with crack spacing more than 300 mm. Since the structural unity of FC is very tight, the movement of FC may exhibit behavior much like that of a continuous slab. The continuous slab movement due to the structural integrity of FC is very positive in terms of bearing capacity; however, it may not be effective to prevent reflection cracking. As the RC layer gets thicker, its ability to prevent reflection crack may improve. No specific recommendation has been suggested for the minimum depth of RC. Thus, it is desirable to identify the optimum depth of RC to prevent reflection cracks. EXPERIMENTAL SIMULATION OF A REFLECTION CRACK This experiment was designed to investigate the effect of rubblized depth on the resistance against reflection cracks. Reflection cracks are caused by the tensile strains induced by the horizontal movement of the concrete slab and by the shear strains induced by the vertical traffic loading. Horizontal strain was simulated with a given vertical load on the asphalt layer on a cracked PCC and rubblized PCC as shown in figure 4. Repeated horizontal strain was developed with the repetition of vertical loading, and the initiation and propagation of crack was

6 Lee, Bae, Han and Stoffels 6 monitored. The effects of rubblized depth on reflection cracks was examined based on the comparison of crack growth rates with 0, 10 and 20 cm rubblization depth. Figure 2. Rubblized PCC slab. Figure 3. Typical cross-section of a rubblized PCC pavement.

7 Lee, Bae, Han and Stoffels 7 (a) HMA on the cracked PCC slab (b) HMA on the rubblized PCC Figure 4. Simulation of tensile strain in HMA overlay on (a) cracked PCC and (b) rubblized PCC: Mode I. To simulate the shear strain due to traffic loading, a vertical moving load was applied on the surface of the asphalt overlays on cracked PCC and rubblized PCC as shown in figure 5.

8 Lee, Bae, Han and Stoffels 8 (a) HMA on the cracked PCC slab (b) HMA on the rubblized PCC Figure 5. Simulation of shear strain in HMA overlay on (a) cracked PCC and (b) rubblized PCC: Mode II. Repeated shear strain was developed with the repetition of vertical moving load, and the initiation and propagation of crack is monitored. The effects of rubblized depth on reflection cracks was examined based on the comparison of the crack growth rates with 0, 10 and 20 cm rubblization depth.

9 Lee, Bae, Han and Stoffels 9 Specimen and Test Set-Up HMA and Rubblized Layer The asphalt material chosen for this study was of 19mm in granularity. AP-5 binder, a common PG binder that is produced by S company in Korea, was used. A coarse aggregate of maximum size 19mm that is produced by a crushing process by D company in Choongjoo city of Choongnam province was used. This aggregate is of good quality and has been used on the test road of Korea Highway Corporation in Choongbu Expressway. For each additive, the Marshall Mix Design procedure was followed. The optimal asphalt content was set at 4.5%. The Marshall s stability index, percentage of air voids, flow value, and indirect tensile strength for each additive were measured by the standard testing methods. The rubblized concrete was simulated by crushed and recycled aggregate of existing concrete pavement, and the aggregate was mixed at the ratio of 15% between 70mm-40mm and 85% below 40mm. The maximum particle size of the rubblized aggregate chosen for this study was 100mm. An engineered synthetic rubber pad was used to allow adequate deflections for the Mode I and Mode II tests. Test Set-Up for Mode I: Simulation of a Reflection Crack Induced by Repeated Tensile Strain Three specimens of 300mm x 200mm x 50mm asphalt slabs were fabricated for the bendingmode (Mode I) simulation experiment. For the simulation of asphalt overlay directly on cracked concrete, the specimen was attached to a cracked concrete slab. The width of crack in the concrete was set at 10mm, which would be considered a severe crack. To simulate a partial depth of rubblized concrete, a jig of 300mm x 200mm x 500mm was constructed in order to lodge the rubblized aggregate (10cm and 20cm) between the cracked concrete block and the asphalt experimental specimen. To allow adequate deflection for a given vertical load, an engineered synthetic rubber pad (3cm thick) was installed beneath the cracked concrete as shown in figure 6. The aforementioned recycled aggregate, with 15% between 70mm and 40mm and 85% below 40mm composition, was laid out for 10 or 20cm on a concrete block. A wheel tracking device was used to compact it with 800Kg load by 20 repetitions (back and forth). After compaction, asphalt was affixed by a tack-coat pattern on the compacted and rubblized aggregate. The contact between the wheel and pavement surface was also simulated by a thin rubber pad placed between the loading device and the test piece. A periodic loading was simulated by an oil-pressure instrument (SHIMADZU MODEL 4800), exerting its load through a round loading board of 100mm in diameter at a periodic speed of 10HZ. The maximum load was set at 550Kg to simulate the tire pressure of 7.03 Kg/cm 2 (100 psi). The horizontal displacement of the asphalt test piece by Mode I was measured by a crack gauge. The test

10 Lee, Bae, Han and Stoffels 10 piece was painted with a white water-based paint in order to observe the progress of the reflection crack visually. The experiment was carried out until the vertical crack reached the entire depth of the test piece. Figure 6. Set-up for Mode I Test Test Set-Up for Mode II: Simulation of a Reflection Crack induced by Repeated Shear Strain Asphalt test-slabs of 600mm X 300mm X 50mm in size were fabricated for a simulation experiment of the reflection crack due to shearing (Mode II). To simulate an asphalt overlay on cracked concrete, the test-slabs were affixed by overlaying on a cracked concrete slab of 60 mm

11 Lee, Bae, Han and Stoffels 11 x 150mm x 100mm, with a gap of 10mm. For the case of simulating an asphalt overlay on partially rubblized concrete, the rubblized aggregates were inserted between the asphalt overlay slab and the cracked concrete slab, by using a jig to provide confined stress condition during the simulation experiment. The elastic support of the subbase was simulated by placing a 30mm rubber pad on the bottom of the test piece. The aforementioned recycled aggregate, with 15% between 70mm and 40mm and 85% below 40mm composition, was laid out for 10 or 20cm between a concrete block and the asphalt test-slab. A back and forth moving loading was repeated by a wheel with a contact pressure of 7.03 Kg/cm 2 (100 psi), as shown in figure 7. Figure 7. Set-Up for Mode II Test The experiment was carried out until the crack extended the entire depth of the asphalt slab.

12 Lee, Bae, Han and Stoffels 12 The test slab was painted on one side with a white water-based paint in order to observe the progress of the reflection crack visually. The progress of the reflection crack was measured and monitored at the interval of every 500th loading. Test Results and Analysis In the simulated experiment of Mode I reflection cracking, when the asphalt test specimen was overlaid directly on the cracked concrete slab, a crack was exhibited at the 16,000th loading. The crack propagated through the entire height of the asphalt test slab at the 300,000th loading; the horizontal deformation as the crack progressed is illustrated in figure 8. As shown in figure 9, when the rubblized aggregate of 10cm and 20cm in depth was present below the overlay, no crack exhibited at any time through the end of test at 300,000 loadings. A photograph of the cracking pattern, without the rubblized layer, is provided in figure Horizontal Deformation (mm Rubblized aggregate of 0cm in depth Rubblized aggregate of 10cm and 20cm in depth Cycles Figure 8. Horizontal deformation at mid-depth of AC slab in Mode I test.

13 Lee, Bae, Han and Stoffels 13 Progress of a Vertical Crack (mm) Rubblized aggregate of 0cm in depth Rubblized aggregate of 10cm and 20cm in depth Cycles Figure 9. Vertical crack propagation in Mode I tests. Figure 10. Reflection cracks in a Mode I test. In the case of Mode II (shear failure) simulation tests, the cracking began at the 2,000th

14 Lee, Bae, Han and Stoffels 14 loading, and penetrated the entire depth of the asphalt test specimen by the 12,000th loading, as illustrated in figure 11. A photograph of the resulting reflection cracks is provided in figure 12. However, when the rubblized aggregate was used as the middle layer, a reflection crack did not take place before testing was stopped after 20,000 loadings. 50 Progress of a Vertical Crack (mm) Rubblized aggregate of 0cm in depth Rubblized aggregate of 10cm and 20 cm in depth Cycles Figure 11. Vertical crack propagation in Mode II tests.

15 Lee, Bae, Han and Stoffels 15 Figure 12. Reflection cracks in a Mode II test. For both Mode I and Mode 2 tests, the asphalt overlay over 10cm of rubblized concrete did not show the initiation of reflection cracks after the number of loading cycles that showed initiation of reflection cracks for the asphalt overlay on the cracked concrete slab. Although these tests do not directly provide the reflection-crack life, they can indicate a relative comparison of the resistance against reflection cracking with the variation of effective rubblized depth. After testing up to 20 times the number of loading cycles required for the initiation of reflection cracks for the initiation of reflection cracking. Generally, reflection cracks occur in asphalt overlays on cracked concrete pavement after one year, after passing the maximum temperature variation of a year (7). This may imply reflection cracking in the asphalt overlay on rubblized concrete should be prevented up to 20 times longer than that of asphalt overlay on cracked concrete to meet a pavement design life of 20 years. Thus, the minimum depth of rubblized aggregate is suggested as 10cm when applying the rubblization technique to deter reflection cracking. CONCLUSIONS This study employed laboratory simulation to attempt the determination of an optimal depth of rubblization to deter a reflection crack in aged concrete pavements. The research employed the standard of rubblized gradation within the typical policies of state transportation agencies in the

16 Lee, Bae, Han and Stoffels 16 United States (40mm-70mm rubblized aggregate) as the middle layer. Indoor simulation experiments of Mode I (bending failure) and Mode II (shear failure) reflection cracking were carried out for each depth of the rubblized aggregate of 0cm, 10cm and 20cm. The results showed that a reflection crack started at 16,000 loadings and 2,000 loadings for Mode I and Mode II, respectively, when there was no rubblized aggregate layer beneath the overlay. However, when a rubblized aggregate of 10cm or 20cm in depth was between the overlay and the underlying concrete, a reflection crack did not take place within the testing cycles. Thus, the effectiveness of the rubblization technique was affirmed. Moreover, when applying the rubblization technique, the optimum (minimum) depth of the rubblized aggregate layer was found to be 10cm in consideration of the characteristics of domestic (Korean) concrete pavements. ACKNOWLEDGEMENTS This research is a partial result of a focus project to develop a rubblization method for recycling aged concrete pavements, sponsored by the Korea Institute of Construction and Transportation Evaluation and Planning. REFERENCES 1. Eom, J. Y., Yang, S. C., Kim, S. H., Lee, S. G., Kim, G. W., Lee, G. M., and Park, T. S., Investigation of Methods for Deterring Pavement Crack and Reflection Crack on the Road Surface," Research Institute of Korea Highway Corporation 00-21, National Asphalt Pavement Association, Guidelines for Use of HMA Overlays to Rehabilitate PCC Pavement, NAPA Information Series 117 (1999). 3. Caltabiano, M. A. and Brunton, J.M., "Reflection Cracking in Asphalt Overlays," Proceedings of the Association of Asphalt Paving Technologists, Vol. 60, Huffman, J. E., "Reflection Cracking and Control Methods," Proceedings of the Canadian Technical Asphalt Association, Vol. 23, Ksaibati, Khaled, Miley, William and Armaghani, Jamshid, Rubblization of Concrete Pavements," Transportation Research Record 1684, Transportation Research Board, Niederquell, Michael G., Baladi, Gilbert Y. and Chatti, Karim, Rubblization of Concrete Pavements Field Investigation, Transportation Research Record 1730, De Bont, A., Anti-reflective Cracking Design of (Reinforced) Asphalt Overlay," Ph.D. Thesis, Delft University of Technology, 1999.