ADHESION OF REINFORCEMENT GRIDS IN ASPHALT OVERLAYS

Transcription

1 P.O. Box 1 Phone ZG Avenhorn Fax The Netherlands Research & Development Conference Papers C.P. Plug, A.H. de Bondt ADHESION OF REINFORCEMENT GRIDS IN ASPHALT OVERLAYS prepared for 5th World congress on emulsions Lyon, France October 2010

2 ADHESION OF REINFORCEMENT GRIDS IN ASPHALT OVERLAYS C.P. Plug, Ooms Nederland Holding bv, P.O. box 1, 1633 ZG Avenhorn, The Netherlands, tel: , fax: , A.H. de Bondt, Ooms Avenhorn Groep bv, P.O. box 1, 1633 ZG Avenhorn, The Netherlands, tel: , fax: , An old road construction can be rehabilitated by overlaying with a new surface course. However, cracks in the old pavement will eventually come through from beneath in the new overlay. This phenomenon called reflective cracking is due to the inability of the overlay to withstand the shear and tensile stresses caused by traffic loading or thermal movements. Reinforcement grids are used to prevent reflective cracking. To do so, a levelling course is applied on the cracked old pavement and reinforcement grids are laid at the location of the underlying cracks or joints. On top of this, a tack or bond coat and a new surface course are constructed. An important factor for the effectiveness of the grid is the adhesion with the surrounding asphalt layers. Trial sections were constructed with two different types of a standard tack coat (unmodified bitumen emulsion) on top of a reinforcement grid. Furthermore, sections were constructed using different spray rates of a standard tack coat and a polymer modified bond coat emulsion. On the latter sections, the adhesion of the grid itself was tested on the levelling course just after installing the grid and after spraying the bitumen emulsion by using a spring balance. After paving the surface course, cores were drilled regularly in time and examined by using the Leutner shear test device. The tests indicate that the shear strength increases in time for the applied tack and bond coats. Furthermore, applying a polymer modified bond coat will result in a better and more durable bonding of the pavement layers. Keywords: reinforcement grids, polymer modified bitumen, emulsion bond coat, interface adhesion

3 1. Introduction A road structure usually consists of several different layers. Rehabilitation of an old road can be done by overlaying. However, cracks in the old pavement will eventually propagate through the new overlay. This reflective cracking is due to the inability of the overlay to withstand shear and tensile actions induced by the old pavement. To prevent the new surface from this type of cracking, reinforcement grids are used [1]. An important factor for the effectiveness of the grid is the adhesion with the surrounding asphalt layers. Testing of the layer interface properties can be done by means of direct shear tests, such as the Leutner shear test. The Leutner shear test procedure was developed in Germany in the 1970s [2] as a simple shear test on adhesion between two asphalt layers. The principle of the test is to apply a shear displacement at a constant rate at the interface of Ø 150 mm samples consisting of two layers. For this study, trial sections were constructed consisting of a binder course and a surface course with in between a reinforcement grid and 3 types and different amounts of emulsion tack coats. The objective was to determine the influence of the type of used tack coat and the effect of curing (in time) on the shear strength. 2. Reinforcement grid and emulsions In this study a commercially available reinforcement grid was used with a high-strength polymer coated fibreglass mesh and self-adhesive backing. The specified strength of the grid was > 100 kn/m with an elongation of break of less than 3% (ASTM D6637) [3]. To provide an adequate adhesive bond, a tack coat (or bond coat) has to be applied. In this study, the adhesive of the grid itself and the adhesion of both asphalt layers have been tested. For this, two types of conventional commercially available bitumen emulsion tack coats and an experimental emulsion bond coat consisting of an SBS polymer modified bitumen were used. Details of the emulsions are given in table 1. Table 1: Properties of used bitumen emulsions. Emulsion Tack coat Tack coat Bond coat Test method Type A Type B Type D Binder content [%] EN Viscosity 40 C [mpa.s] EN Residual binder Penetration [dmm] EN R&B [ C] EN > 70 Ductility 5 C [J/cm ] EN (Energy till break) > 16.5 Elastic recovery 25 C [%] EN > 85 Viscosity 135 C [mpa.s] EN

4 3. Trial sections The first trial section using tack coat type A and B was constructed on an existing asphalt pavement in First, a leveling course was laid down consisting of a standard AC 11 binder course asphalt layer. On top of this the reinforcing grid (GlasGrid 8501) was installed. Then half of the covered section was sprayed with 0.3 kg/m 2 tack coat type B (Emutop B) and the other half with 0.4 kg/m 2 tack coat type A (Emutop A), so that the residual binder content was approximately equal for both sections. After breaking of the emulsions, a standard AC 16 surface course was laid down. Repeatedly, Ø 150 mm cores were drilled from this (untrafficked) section during 2 years after construction. From these cores the interface shear strength was determined in the laboratory using the Leutner shear test device. The second trial section using tack coat type A and a slow setting polymer modified bond coat type D (Emuflex D) was constructed on an existing asphalt pavement in First, a leveling course was laid down consisting of a standard AC 8 surface course asphalt layer. On top of this, the reinforcement grid was laid down. Then the section was divided into 5 parts, of which 4 parts were covered with respectively 0.5 and 1.0 kg/m 2 tack coat type A and with respectively 1.0 and 1.5 kg/m 2 bond coat type D. The remaining part was sprayed with water only, so no tack coat was used (to check bond strength under wet conditions). After breaking of the emulsions and 2 days waiting, a standard AC 16 surface course was laid down on top of the construction. This waiting period was done on purpose to test the adhesion of the grid. Figure 1: Experimental spraying of tack coats on reinforcement grid. Before, 10 minutes after spraying of the emulsion and just before laying of the surface course, the adhesion of the reinforcement grid itself was tested by a spring balance. Furthermore, Ø 150 mm cores were drilled 1 month and 1 year after completion of the trial section for shear strength measurements.

5 4. Leutner shear test device The used test frame for the Leutner test (see figure 2) is a modified version of the original frame and was the same as used and described by Choi et all [4]. It is specified in the UK in MCHW_1 clause 954 [5]. The tests were performed at the standard temperature of 20 C using a stiff Marshall -press with a standard shear displacement rate of 0.85 mm/s. Figure 2. Test frame Leutner device. The shear strength at the interface and the displacement (slip) of both layers was measured. From these collected data the shear stiffness (k) could be determined, which is defined as the peak shear stress ( max ) divided by the corresponding displacement ( max ) as shown in figure 3. Figure 3. Example of shear stress vs displacement [5].

6 5. Measurement results 5.1. Adhesion reinforcement grid The adhesion of the tack coat reinforcement grid combinations of the second trial section were tested with a spring balance as shown in figure 4. The test results are given in table 2. Figure 4. Testing adhesion using a spring balance. Sub section Emulsion Table 2. Adhesion of reinforcement grid. Before spraying emulsion* 10 minutes after spraying emulsion* 2 days after spraying emulsion* 1 Type A; 0.5 kg/m 2 7 kg 7-8 kg kg 2 Type A; 1.0 kg/m 2 7 kg 7-8 kg kg 3 Type D; 1.0 kg/m 2 8 kg 5 kg kg 4 Type D; 1.5 kg/m 2 7 kg 5 kg kg 5 Water 7 kg 4 kg No adhesion *] Force in kg needed to pull reinforcement grid from the surface. After placing the reinforcement grid and before spraying a tack coat, the initial adhesion of the grid is higher than the minimum required adhesion of 5 kg [3]. After spraying the type A tack coat the adhesion improved a little after 10 minutes and doubled almost in 2 days. The type D bond coat decreases the adhesion in the first 10 minutes to the minimum acceptable value, but after 2 days the grid adhesion is the same as with type A. Only water, gives as expected no adhesion at all after 2 days. The reason for the decreasing adhesion of the type D bond coat (after 10 minutes) could be the slow setting character of this emulsion.

7 5.2. Laboratory shear tests From the trial sections repeatedly cores were drilled and tested using the Leutner shear test. After testing of the samples, the two halves of the core were forced apart for visual inspection of the interface. Examples of the spliced cores are given in figure 5 and 6. Remarkably was that the interface looked damp and brownish (especially the cores drilled in the first months). This indicates that the emulsion was not fully broken. Furthermore, the reinforcement grid was found to be fixed (in almost all the tested cores) to the underlying course. When the grid is forced to come loose, the remains of the yellow glue from the self-adhesive at the bottom of the strands become visible (figure 6). Figure 5. Brownish color at the interface. Figure 6. De-adhesion of the bottom of the grid. The measured interface shear strength of the cores (average of at least 4 cores) from the first trial section are displayed in figure 7. It can be seen from this graph, that the strength for both tack coats is almost constant during the first month and then increases rapidly during the first 150 days. After this the increase is more gradually and asymptotically for both tack coats Interface shear strength [MPa] type B type A Curing period [days] Figure 7. Average interface shear strength against curing period.

8 Furthermore, the interface shear stiffness from the same cores are displayed in figure 8. As can be seen, the development of the stiffness follows the shear strength development. 0.6 Interface shear stiffness [MPa/mm] type B type A Curing period [days] Figure 8. Average interface shear stiffness against curing period. The sprayed amount of residual binder of both variants was approximately equal. There was only a difference in emulsion viscosity (type A: low viscosity; type B: high viscosity). This finding suggests that a high viscosity emulsion will give a higher interface shear strength and interface shear stiffness. From the second (also untrafficked) trial section cores were drilled 1 month and 1 year after construction. The average interface shear strength from these cores is displayed in figure 9. It can be seen that the interface shear strength for both tack and bond coats increases in time. As expected, the 0.5 kg/m 2 type A variant is vulnerable to early age loadings. 0.5 kg/m² type A 1.0 kg/m² type A 1.0 kg/m² type D 1.5 kg/m² type D 0.6 Interface shear strength [MPa] month 1 year Curing period Figure 9. Average interface shear strength against curing period.

9 The average interface shear energy at maximum displacement (8 mm) from the same cores are displayed in figure 10. As expected, an increase in spray rate or using a polymer modified bond coat will result in a better and more durable bonding. After 1 year the interface shear energy increases due to curing of the construction. 0.5 kg/m² type A 1.0 kg/m² type A 1.0 kg/m² type D 1.5 kg/m² type D 4.0 Interface shear energy [MPa.mm] month 1 year Curing period Figure 10. Average interface shear energy against curing period. 6. Conclusions The shear strength of a tack coat emulsion interface increases in time after construction. Even up to a factor 3 in the long run. A polymer modified bond coat will generate an improved (initial) shear strength. It will also have a higher shear energy than a standard tack coat during service life of the pavement. The results show that there is room for optimization and improvement of tack and bond coats. 7. References [1] De Bondt, A.H.; Anti-Reflective Cracking Design Of (Reinforced) Asphaltic Overlays; Ph.D-thesis, Delft University of Technology; [2] Leutner R. (1979); Untersuchung des Schichtenverbundes beim bituminösen Oberbau (in German), Bitumen 41-3, pp [3] Saint-Gobain; GlasGrid Technical Manual ; [4] Choi, Y.K., Collop, A.C., Airey, G.D. and Elliott, R.C 2005; A Comparison between Interface Properties Measured using the Leutner Test and the Torque Test, Journal of the Association of Asphalt Paving Technologists, vol.74e (CD). [5] The Highways Agency 2008; Specifications for highway works, Manual of Contract documents for Highway Works, Volume 1 (MCHW_1), August 2008, London, UK.