ASPHALT REINFORCEMENT FOR THE PREVENTION OF CRACKING IN VARIOUS TYPES OF PAVEMENTS: LONG TERM PERFORMANCE AND OVERLAY DESIGN PROCEDURE C.G.J. Jenner 1 and B.G.J. Uijting 2 (1) Tensar International Ltd, Blackburn, United Kingdom (2) Tensar International SARL, Bordeaux, France Abstract Asphalt reinforcement using stiff polymer grids has reached a worldwide experience of 20 years now. Successful applications have been achieved in various applications as roads, highways and airports in all weather conditions from Arctic to Middle East. Examples of evaluations in various countries and conditions are given. Recent developments in computer modelling based on OLCRACK developments have made it possible to improve design procedures of geogrid reinforcement in asphalt layers. A step-by-step approach is presented in which all relevant requirements are considered for a successful design. Several design examples will be presented based on actual cases. 1. Introduction Life cycle costs of asphalt pavements are high due to two basic flaws in asphalt: 1. Low tensile stress fatigue performance of bitumen against cracking 2. Viscous-elastic deformation of asphalt These basic flaws results in a number of typical problems asphalt pavements, Fatigue, Reflective cracking in overlays, Rutting, Road widening, cracking, Bearing capacity failure of road foundation The development of a stiff polymer grid solution to reduce these problems is based on 2 principles [1,2]: 1. the principle of interlocking the stones in the asphalt mixtures within the apertures of a stiff grid, taking over the tensile stresses after initial cracking of the bitumen. 2. the use of polymers with a high fatigue resistance thus improving design life enormously. 2. Laboratory research Between 1981 and 1985 an extensive programme of research was carried out at the University of Nottingham under the supervision of Professor S F Brown. Laboratory testing work investigated the benefits of the grid with respect to the control of permanent
deformation, the control of reflective cracking and improvement of the fatigue life of the pavement. The research work was carried out over approximately 4 years and consisted of simulative testing leaving installation techniques to be developed in the field Details of the laboratory work have been published by Brown [3,4]. 2.1 Permanent deformation Tests on reinforced and unreinforced specimens showed that for all asphalt types, the reinforced specimens were more resistant to permanent deformation. It was also shown that the lateral flow of the material away from the loaded area was well restrained by the grid [3]. 2.2 Reflective cracking This first part of the test involved asphalt beams subjected to vertical cyclic loading. A 10 mm wide joint in the base was used to simulate a joint in a rigid pavement. The set up represented a rigid pavement being overlaid for the first time and also a previously overlaid rigid pavement having another overlay applied. These small scale tests used to examine pavement deformation and reflective cracking performance were repeated in the full scale Pavement Test Facility at the University of Nottingham [4] and the significant increase in performance was confirmed at full scale. 2.3 Fatique These tests examined similar beams to those used in the reflective cracking tests. Only vertical cyclic loading was used and strains were measured at various points over the depth of the slab. Results showed that in general the life of a pavement was extended by a factor of 10 to a given level of traffic induced strain within the asphalt (micro strain) [3]. To gain this benefit the grid should be positioned at the base of the asphalt layer. 2.4 Temperature trials Extensive research has been done on the effect of hot rolled asphalt on the polymer grids [9, 11] and no effect was measured on the properties or performance of the grids. Worldwide applications in various climatic conditions confirm this as well. 2.5 Removal and recycling tests Although asphalt reinforcement increases the life of the pavement considerably there will be situations were a new overlay or removal of the asphalt is required. Comparative testing between different types of asphalt reinforcement in Holland proved that stiff polymer grids create no problems in grinding old asphalt layers [5]. Polymer fibres and steel nettings proved to have considerable problems in removing them. Further testing was done in the recycling of the grinded grid in new asphalt [6]. These tests proved that recycling brings no problems in the quality for new asphalt layers.
3. LONG TERM PERFORMANCE Many full scale trials were carried out in different geographical and climatic regions which led to a number of installation techniques being developed to suit local conditions.. In 2000 several evaluations were made to monitor the long term performance and to evaluate the economic and technical performance during the life of the reinforced pavement. 3.1 Long term performance of B80044 road, UK A trial section of road was used to determine the benefits of using a stiff polymer grid reinforced overlay. The initial overlay thickness was 100mm. The road was initially regulated with a thin layer of 6mm medium dense base course. The grid was installed using the standard fixing and tensioning method. A 60mm thick layer of base coarse was placed and compacted over the grid by hand, followed by machine laid 40mm wearing course. A control section was constructed to the same overlay thickness. Immediate improvements were noticed with regard to the vertical deformation that the pavement compared with deformations that had exhibited under a single lorry axle pass. This was reduced from between 10-20 mm to less than 5 mm. Fatigue cracking reappears in the control section within 3 months and within 6 months the control section had failed. The reinforced section showed no deterioration at all. In the later projects, the reinforced overall thickness was reduced to 70 mm. [i.e. 30mm base course and 40mm wearing course]. Over the next 14 years the road was regularly monitored by Engineers from the road authorities. All the reinforced sections have performed satisfactory leading to the network manager stating that he was extremely satisfied and that the reinforced sections had saved a number of additional overlays and that these sections continue to perform by supporting the traffic load without the need for structural maintenance. 3.2 Long term performance of 7 roads in the Netherlands Between 1985 and 1989 many roads in the Netherlands were renovated with a new asphalt overlay using stiff polymer grid reinforcement. In recent years, many road authorities have requested information on the long-term performance of asphalt reinforcement in the evaluation of the application of the correct type of reinforcement. AIl roads are located in area's with low bearing capacity, which resulted in heavily cracked roads in 1985-1989. In order to prevent reflective cracking, these roads have been renovated by overlaying the existing asphalt road, using a stiff polymer grid reinforcement. The independent survey by consultant KOAC.WMD has investigated 7 of these roads in the Netherlands: Dronrijp, Menaldum, Uithuizermeeden, Purmerend, Monnickendam, Graft-De Rijp en Alkmaar [7]. The conclusion of the survey is that the reinforcement has clearly extended the lifetime of the asphalt overlay. Boreholes have been made to determine the effect of the reinforcement in the pavement and to investigate causes of the few cracks that appear in the existing raad. The boreholes clearly show the performance of the reinforcement and the very good bonding between the old asphalt and 10-15 year old overlays.
The analysis of the few appearing cracks shows a clear connection with installation errors. It was concluded that the installation guidelines need to be followed strictly in order to prevent cracking due to installation mistakes. 4. DESIGN PROCEDURE The design procedure for the proper design of a reinforced asphalt pavement or overlay has to define a range of design parameters. Table 1: criteria for the selection of asphalt reinforcement. Design life Overlay thickness Rutting Location of reinforcement layer Asphalt mixture temperature Chemical resistance Installation Removal and recycling Costs Each type of asphalt reinforcement has a specific design life; only through full-scale testing a life expectancy can be determined; comparative testing between different types of reinforcement give clear indications of differences in life expectancy. Overlay thickness is related to the specific purpose for the design; renewal of the wearing course requires another overlay thickness as a fit-and-forget design for a rural road. With some types of reinforcement overlay thickness can be reduced. It is important not to specify a reinforcement with a higher minimum thickness than required The prevention of rutting requires specific types of reinforcement which are not based on adhesion with bitumen, but on interlocking with the granular particles in the asphalt layer. Comparison tests have indicated that some types are not reducing rutting. In case of prevention of rutting, it is important to have determined the cause for the rutting, this determines the locations of the reinforcement. Generally all reinforcement are suitable for common asphalt mixtures and temperatures. For mixtures with a temperature exceeding 180 degrees specific types of asphalt reinforcement are required. The chemical resistance of the reinforcement should be sufficient in accordance with design life; resistance against corrosion, weathering, oil is required. Each type of asphalt reinforcement requires a strict and specific method of installation. The method of installation also determine the cost of the solutions; some types of reinforcement require 5 steps where other only require 3 [8] Extensive testing on removal techniques and problems and recycling of grinded reinforcement have been done [5,6]. The cost evaluation of asphalt reinforcement can only be done by evaluating life-cycle costs of the design. It is important to consider the cost of the product installed as installation costs are a major part of the total costs.
The following step-by-step approach is developed to guide the designer: 1. determine primary loads on the pavement construction 2. determine (potential) causes of failure 3. determine the location of the reinforcement 4. determine installation conditions 5. determine removal and recycling conditions 6. select the reinforcement 4.1 Step 1: determine primary loads on the pavement construction The primary loads determine primarily the type of reinforcement. The main distinction between types of loadings are [1,2]: dynamic or (quasi) static; dynamic forces are caused by traffic are short and passing. Static loads can be caused by temperature influences, by differential settlements or static loads on the pavement. temperature: day/night and seasonal differences in temperature can cause major loadings on the asphalt pavement for instance by frost heave or deformation of concrete slabs. differential settlements; the bearing capacity is an important parameter, for instance a road widening with insufficient bearing capacity can cause differential settlements and thus cracking in the asphalt. 4.2 Step 2: determine (potential) causes of failure In new pavements, the potential problems can be evaluated and prevented by using an proper asphalt reinforcement. In existing pavements, actual problems need to be resolved. Typical problems in asphalt pavements are fatigue, reflective cracking, rutting, insufficient bearing capacity. Fatigue in asphalt pavements is caused by the failing of the bitumen. After a given number of loadings bitumen starts to crack due to fatigue behaviour. The solution is to take over the role of the bitumen at the moment it starts cracking and thus extending the life of the pavement. Reflective cracking is also caused by the limited fatigue performance of bitumen in combination with a crack or discontinuity beneath the pavement. It is generally accepted that overlaying a cracked pavement with reinforcement can reduce this problem considerably. The fatigue behaviour of the reinforcement determines the design life of the overlay. Rutting is caused by the viscous-elastic properties of asphalt; static loads can be the cause (such as parked airplanes) or repeated loading in an restricted area. Rutting can be related to the asphalt type (rounded stones/high bitumen content) or the bearing capacity of the sub-base. It is important to determine what is the cause as it determines the location of the reinforcement. Insufficient bearing capacity of the sub-base is important cause for failures. Deformations caused by the traffic result in differential settlements and cracking in the asphalt. It particular important in road widening and rural roads in area's with soft soils. 4.4 Step 3: determine location of the reinforcement The location of the reinforcement is crucial in solving or preventing problems in asphalt pavements [7,8]. Prevention of rutting is on example; an incorrect placed reinforcement will not solve the problem. Other considerations are design life of the reinforced
pavement; a fit-and-forget solution for rural roads often utilise a 70 mm overlay in combination with a stiff polymer grid asphalt reinforcement. On the other hand a fixed location requires often a specific type of reinforcement. For instance, regular renovations of the asphalt wearing courses in heavily trafficked roads often requires a 4cm overlay renewal. It makes no sense in selecting a reinforcement with a design life much longer than the maintenance interval. In some cases where an insufficient bearing capacity is the main cause, a solutions with sub-base reinforcement might be utilised, in which with a similar construction depth the bearing capacity can be improved up to 5 times [2]. 4.5 Step 4: determine installation conditions The conditions under which the reinforcement should be installed are vital for the selection of the type of geogrid. Extensive research on installation methods determine clearly for each type of reinforcement what method should be used [7,8]. Nowadays, more and more the time for installation is restricted due to limitations in disruptions of traffic. Modern composites have been introduced to reduce time and cost of the installation considerably. 4.6 Step 5: determine conditions for removal and recycling Asphalt reinforcement can increase the design life of a pavement considerably, but at a given moment it should be removed and replaced with a new pavement. Further, often other works require often the (temporary) removal of the pavement. Modern machines to remove asphalt layers are more and more used nowadays. These machine grinds the asphalt layers simply away and produce a recycling product that can be used in the production of new asphalt or sub-bases. Therefore it is important to assess the conditions regarding removal and recycling as some types of reinforcement are impossible to grind or at least very difficult and time consuming to remove [9]. The wrong selection could impose high costs at the end of the design life of the reinforcement. 4.7 Step 6: selection of the reinforcement After all conditions are determined the selection of the reinforcement can be done and the pavement design finalised. Installation and maintenance instructions can be prepared. For the selection of the reinforcement, it is necessary to start to determine the relevant parameters on which the reinforcement can be selected. In table 1 these parameters are given as a checklist. Extensive work has been done to determine design life of asphalt reinforcement [1, 3, 4, 7, 8, 12, 13]. The design life is related to the specific design of the reinforcement and the specific conditions. Certain types of reinforcements are very suitable in dynamic conditions and other types are suitable for high static loadings. The incorrect application of a reinforcement type can result in a non-effective application with a strongly reduced life expectancy.
5. Design programs The development of design programs for reinforced asphalt overlays started in the early 1990's based on the application of Finite Element Modelling. These models are not easy to apply for standard design purposes [10, 12], but can be tuned for specific reinforcements for the development of design charts. Recent developments in design programs specifically tune for specific products [12, 13]. The OLCRACK model [13] has been further tuned for stiff polymer grids and is currently used for specific designs purposes, but continues to be refined by verifying with empirical testing and long term site evaluations. 6. Conclusions Utilizing the design procedures can effectively solve a whole range of problems in asphalt pavements using reinforcement. The design procedure can be made easier following a step-by-step approach in which all relevant elements are investigated and evaluated. Design parameters are identified which can be used for the identification or verification of the type of reinforcement. Design life of reinforcements is directly related to the reinforcement mechanism of each type of reinforcement. Long term site evaluations confirm the estimated design life based on extensive full-scale laboratory testing. Design programs are not yet commonly available for standard design of reinforced asphalt overlays, but for some products design charts are derived and tuned using finite element modelling. A specific program has been developed and is further tuned and verified for stiff polymer grid applications. 7. References 1. Hughes, D A B et al: Tensar reinforcement of asphalt. Proc Symp Polymer Grid Reinforcement in Civil Eng, SERC/Netlon Ltd, London, March 1984. 2. Haas, R: Structural behaviour of Tensar reinforced pavements and some field applications. Proc Symp Polymer Grid Reinforcement in Civil Eng, SERC/Netlon Ltd, London, March 1984. 3. Brown, S F et al: Polymer grid reinforcement of asphalt. AAPT, San Antonio, Texas, February 1985. 4. Brown, S F: Polymer reinforced grid to limit cracking and rutting in pavements. 3rd IRF Middle East Regional Meeting, Riyadh, Saudi Arabia, 1988. 5. Louis: Investigations into the temperature behaviour of Tensar RG grids in asphalt overlays (in German). IFTA, Essen, June 1992 6. CROW, "geen vrees voor de frees", CROW, Ede, Holland, 1994 7. Van Drent, Onderzoek betreffende toepassing van Tensar-wapening in de wegenbouw, KOAC.WMD, Apeldoorn, Holland, 1999 8. Vanelstraete, A et al, On site behaviour of interface systems, 4 th international RILEM conference, Ottawa, 2000.
9. Gilchrist, A J T, et al The development of a grid/geotextile composite for bituminous pavement reinforcement, Reflective Cracking in Pavements,m RILEM 1996. 10. Caltabiano, M A et al: Reflection cracking in asphalt overlays. AAPT, March 1991. 11. Belgian Road Research Centre, High Temperature Installation Test on Tensar AR-G, Brussels, 1998 12. Thom, N.H. a simplified computer model for grid reinforced asphalt overlays, 4 th international RILEM conference, Ottawa, 2000. 13. Thom, N.H. Geogrid overlays, prediction the unpredictable. Mairepave 03, Guimaraes, Portugal, 2003.