FIELD STUDY OF REPAIR METHODS FOR TRANSVERSAL CRACKS

Transcription

1 FIELD STUDY OF REPAIR METHODS FOR TRANSVERSAL CRACKS Jarkko Valtonen and Ilmo Hyyppä Helsinki University of Technology, FINLAND Abstract Transversal thermal cracks are a difficult problem on Finnish roads. Those cracks occur especially on minor roads but also the heavily used roads suffer from transversal thermal cracks. Traditionally hot bitumen has served as a repair method. Unfortunately it has happened very often, that transversal cracks occur exactly on the same place during the following spring. Even if a whole new asphalt layer has been laid on the road, there have been as many cracks after maybe just two years. In July 2000 a field test was performed. On the main road 1 altogether 49 transversal cracks were repaired by eight different methods and the location of every crack was determined carefully. Four of the methods were based on fibre grids and the other four were more traditional methods. After the repair work a new SMA layer was laid on the road. The following spring 2001 the test area was examined. The main result: not even one of the cracks repaired with grids had occurred again. Traditional methods had not been as successful; some cracks were already visible. After two winters in spring 2002 the difference with grids and traditional methods was even bigger. Then it seemed as if there is a difference also whether the grid has been laid straight on heated remixed surface or on a milled box. The spring 2003 showed the final order of the tested repair methods. It seems that the best method to prevent transversal cracks is to lay a grid on a milled box. 1. Introduction Transversal thermal cracks are a difficult problem on Finnish roads. Those cracks occur especially on minority roads but also the heavily used roads suffer for transversal thermal cracks. Asphalt paving companies have traditionally repaired transversal cracks with hot bitumen. If a new asphalt layer is needed because of rutting, no treatment was

2 applied to the underlying cracks. In recent last years milling the old pavement has become more common and that has had a slight influence on the deterioration. Unfortunately it has happened very often, that transversal thermal cracks occur exactly on the same place next spring. Even a new asphalt layer has not helped much. The Finnish Road Enterprise got a contract to repave a 10 km section of the main road 1 during the summer There were a large number of transversal thermal cracks on the road; the average was seven cracks on every 100 m. The cracks were very annoying especially for heavy vehicles with heavy loads. To produce a high quality pavement, it was obvious, that something had to be done to prevent cracking. To find out the best method of repairing cracks a field test was carried out. 2. Field test The Laboratory of Highway Engineering at Helsinki University of Technology (HUT) investigated 800 m of the road and chose 49 best cracks to be repaired by different methods. The cracks were chosen for the study, if they were straight or at least nearly straight and the length of them was equal to the width of the road. The location of every test crack was determined carefully so that the locations were comparable to the locations of cracks occurring following years. HUT planned the test programme with eight different methods. Four of them were based on fibre grids and the other four were more traditional methods. After repairing the crack a new SMA 16 layer was laid on the road. The used grid was Glasgrid Geogrid This is a fibre glass reinforcement with modified polymer coating and pressure sensitive adhesive backing. The mash size is 25 x 25 mm and the tensile strength across the width is 100 KN/m. This kind of grid was used because it is the most familiar one. By using glass fibre grids every kind of repaving method is possible in the future. Figure 1: Glasgrid Geogrid 8511 preventing transversal cracks

3 The grids were installed on the bottom of a milled box. This is a transversal channel where the first layer of the asphalt got milled off. The box is approx. 4-5 cm deep and either 1.75 or 1.00 m wide (method 3 and 4). Or the grids were installed on a hot milled levelled surface (method 6 and 7). Hot milled levelled surface means that the asphalt surface was taken away with a hot mill and got levelled with levelling mass in the same time. This surface was then 2 cm deeper than the road surface. In all cases the grid was either 1.5 or 0.75 m wide. The repair methods were as follows. 1. levelling the old surface, hot milling and asphalt surfacing + SMA 16 (reference section) (seven cracks) 2. milling the form of a box, 1.5 m wide grid, levelling the old surface, hot milling and asphalt surfacing + SMA 16 (six cracks) 3. milling the form of a box, 0.75 m wide grid, levelling the old surface, hot milling and asphalt surfacing + SMA 16 (six cracks) 4. sawing a crack, filling the crack with hot modified bitumen, levelling the old surface, hot milling and asphalt surfacing + SMA 16 (six cracks) 5. sawing a crack, filling the crack with a band of modified bitumen, levelling the old surface, hot milling and asphalt surfacing + SMA 16 (six cracks) 6. hot milling and asphalt surfacing, 1.5 m wide grid + SMA 16 (six cracks) 7. hot milling and asphalt surfacing, 0.75 m wide grid + SMA 16 (six cracks) 8. hot milling and asphalt surfacing + SMA 16 (reference section) (six cracks) At first the methods from 1 to 5 were performed at the stage after levelling the surface. The mixture used to level the surface, called levelling mass, was open graded AC 12. It was necessary to do the work lane by lane, because the road is heavily used (10000 vehicles/day) and there was no possibility to reroute the traffic. Figure 2: Milled box ready for the grid

4 The sawed cracks were 20 mm wide and 40 mm deep. It was quite difficult to saw exactly all the small curves of some cracks, so the sawed cracks were straighter than the original ones. Altogether 12 cracks were sawed. Six of them were filled with hot modified bitumen, which was poured from a can. This work had to be done twice, because the bitumen was flowing slowly inside the crack. The other six cracks were filled with a band made of modified bitumen. The bands were then melted with hot gas. Figure 3: Sawing the cracks Figure 4: Filling the crack with hot bitumen Figure 5: Filling the crack with a band made of modified bitumen After these stages the surface was glued by bitumen emulsion and the asphalt mass for levelling was laid. The following day was the turn of methods from 6 to 8. At first the old surface with the levelling mass was hot milled. The grids were laid on the milled surface and then a layer of SMA 16 with a thickness of 4 cm was laid on the whole road.

5 Figure 6: Milling a box Figure 7: Compacting the asphalt on grid After the repair work the locations of the test cracks were marked on the shoulder of the road to find the exact places during the following years. Figure 8: State of the test side before the final SMA layer

6 3. Situation in the spring 2001 HUT was investigating the cracks the first time in April At that time there were four methods, which succeeded in preventing the reoccurrence of the cracks. Every one of those four methods was a grid method. Not even one single crack, which was repaired with a grid, had occurred again. So there was no difference between the methods 2 and 3 compared to the methods 6 and 7. And there was no difference either between the diverse widths of the grid; the narrow grid succeeded as well as the broad one. Methods 4 and 5, the saw methods, were not as successful. One crack repaired by method 4 had occurred again and also one crack repaired by method 5. Additionally two cracks repaired with method 5 were just to start occurring. Already after one winter it became clear, that method 1 is not able to prevent the transversal cracks to occur very soon again. Only one of the cracks was as long as the road is wide, but beside that four other cracks were starting to occur again. Astonishing was that method 8 gave significantly better results than method 1. Only one of the cracks could slightly be seen, the others were not visible at all. To saw the cracks does not seem to be better than just hot milling and repaving the road without paying any attention to the cracks. In addition to that, sawing is very laborious and causes delays to the traffic. That is why it is not a very recommendable method Amount of Cracks Method Cracks starting to occur Cracks as long as the road is wide Figure 9: Situation in spring 2001

7 4. Situation in the spring 2002 The second investigation was done in April Still there were three methods, which were totally successful: method 2, 3 and 7. The cracks which were repaired with these methods had not occurred again; not even after two winters. Quite astonishing was that five cracks, repaired with method 6, were visible. Three of those cracks were as long as the road is wide and two of them were half as long. So the broader grid did not prevent the cracks from occurring anymore. The narrower one seemed to act much better. The grids laid on the bottoms of milled boxes were functioning perfectly all the time. Method 5 gave as bad results as method 6. Three of the cracks were as long as the road is wide and only one crack had not occurred again. The pouring method 4 had a little better success. Only one crack was as long as before the study and two cracks started to occur. So it seemed that pouring hot bitumen in a sawed crack is more efficient than putting a bitumen band in a crack. The reference methods gave also bad results. Six of the cracks repaired by method 1 were clearly to be seen, and the seventh one was only 2 m long. Method 8 seemed somehow to be a little better one; four of the cracks were to be seen, but two of them not Amount of Cracks Method Cracks starting to occur Cracks as long as the road is wide Figure 10: Situation in spring 2002

8 5. Situation in the spring 2003 The third investigation was made in April Now the superiority of the box milling became clear. Not a single one of the six cracks repaired by method 2 and five repaired by method 3 were to be seen. So only one crack repaired by method 3 had occurred again. Instead of that every crack repaired by method 6 had occurred again and also four of the cracks repaired by method 7. Five cracks repaired by method 5 had occurred again and all the six cracks repaired by method 4 were to be seen, although three of them were quite short. All the cracks repaired by method 1 and 8 were as long as the road is wide, so the situation is totally the same as before the study Amount of Cracks Method Cracks starting to occur Cracks as long as the road is wide Figure 11: Situation in spring Conclusions It seems that the grids are very helpful in preventing the reoccurrence of the transversal thermal cracks after repaving. But just laying the grids on hot milled surface seemed not to be enough. If a road is so heavily used that the traffic should not be influenced negatively the best method to avoid the nearly annual repair work is to lay the grids on the bottom of a milled box. Not a clear answer was given on how wide the box and the grid should be. Both, the grid with 1.5 m width as well as the 0.75 m wide grid, were acting well. The main thing to know when solving this question is the width of the grid package. When the package width is more than 1.5 m, it definitely could be sawed in two parts.