REFLECTIVE CRACKING INCLUDED INTO ROUTINE DESIGN OF NEW ASPHALTIC PAVEMENTS

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1 P.O. Box 1 Phone ZG Avenhorn Fax The Netherlands Research & Development Conference Papers J.G.F. Schrader A.H. de Bondt REFLECTIVE CRACKING INCLUDED INTO ROUTINE DESIGN OF NEW ASPHALTIC PAVEMENTS prepared for 8th RILEM International Conference on Mechanisms of Cracking and Debonding in Pavements Nantes, France June 7 9, 2016

2 Reflective Cracking Included into Routine Design of new Asphaltic Pavements J.G.F. Schrader, A.H. de Bondt Ooms Civiel bv, Scharwoude, the Netherlands, Abstract During the past decades, it has been experienced that reflective cracking is a very complex phenomenon. Not only which one of the possible mechanisms behind the reappearance of cracks in new pavement surfaces (traffic, temperature variations in time or uneven settlements) is dominant, depends on the typical circumstances of a specific project, but also a variety of maintenance solutions often seems to be applicable. Examples are: (combinations of) thick overlays, use of modified asphaltic mixtures, application of stress-relieving systems or the incorporation of reinforcement. At motorway and airfield (maintenance and rehabilitation) projects there usually is time, budget and information. At those large projects it pays tribute to include reflective cracking into the routine design, because a cement treated base or concrete slabs are quite often present. From these specific pavement layers, cracks or joints propagating into and through the asphaltic overlay is the dominant mechanism. Ooms Civiel has developed an analysis method to include the crack driving mechanisms temperature variation and traffic into the routine design of new asphaltic pavements. The temperature influence is analysed by means of ARCDESO ; the traffic loading requires finite elements simulations. This paper explains the design method, which meets the challenge mentioned above. The method has been calibrated with long term field experience (crack mapping data) and is being validated continually. Keywords Pavement design, FEM-analysis, Reflective cracking, Polymermodified asphalt. 1 Introduction During the maintenance or rehabilitation planning of motorways and airfields a cement treated base (CTB) or concrete slabs (PCC) often imply challenges. In the

3 2 CTB cracks will be present, as well as joints in between the PCC-slabs. The PCC can also contain cracks. Lastly, also new pavements are designed with a (pre-) cracked CTB. In general, in (current) design software a CTB or an old PCC is simulated as a uniform base layer with a specific Young s modulus (E). This material modulus can be reduced to an equivalent layer stiffness to incorporate the effect of cracks and joints. However, reflective cracking from the joints into the asphalt layers has not been implemented into standard design software yet. For many clients and contractors this is the reason to exclude the use of a CTB or an old PCC into their pavement design. According to Ooms Civiel it is very interesting to use these pavement materials as they are cheap, widely available and in general they possess a higher Young s modulus than unbound granular base materials. This results into thinner asphalt layers on top of the base layer and/or a thinner base layer itself. Ooms Civiel has recognized the risk of reflective cracking by adding two additional pavement design analyses to the traditional multi-layer thickness design approach. First of all the pavement structure has to be designed by using the traditional design software. Afterwards the structure has to be analysed by means of the software programme ARCDESO for the risk of reflective cracking caused by temperature induced (horizontal) slab movements. These analyses are described in chapter 2. The vertical slab movements caused by traffic loadings are analysed by means of the finite elements programme CAPA-3D and will be described in chapter 3. In chapter 4 the results are superimposed. 2 Temperature Analyses During the hardening of a CTB, (restraint) shrinkage can initiate cracking. These cracks can propagate into the asphalt layers on top of the CTB because of horizontal movements of the CTB. The horizontal movements are caused by the (daily) temperature drops and these movements can initiate cracking in the CTB as well. In the end a random crack pattern shall develop with an average crack distance of more than 6 m. By pre-cracking over a shorter distance, the asphalt overlay is relaxed and a regular crack pattern shall arise. The width of the pre-cracks will be less than the width of the natural cracks and the seasonal/daily widening of the pre-cracks will also be less as the temperature induced movements of shorter CTB-slabs will be smaller. Ooms Civiel has developed the (web-based) software programme ARCDESO to design pavement structures with respect to the failure mechanism thermal induced reflective cracking ( ARCDESO stands for Anti- Reflective Cracking DEsign SOftware and the project was with the (partial) financial support of ADFORS Saint-Gobain. ARCDESO is a mechanistic empirical program that is based on a) the PhD. research by Arian de Bondt at Delft University of Technology from 1989 to 1996/1999, entitled Anti-Reflective Cracking De-

4 3 sign of (Reinforced) Asphaltic Overlays (de Bondt 1999) and b) later work by the R&D-department of Ooms Civiel (de Bondt 2012). The software development took a decade (from 2001 to 2011) and afterwards Ooms has updated the programme, by validating and calibrating it on numerous long-term field and laboratory data. ARCDESO is (one of) the first type(s) of design software in the world that deals with reflective cracking in asphalt layers. The viscoelastic behaviour of the asphalt layers is indirectly taken into account, and the amount of fatigue in the structure is controlled by Miner s law and presented as the Cumulative Damage Factor (CDF TEMPERATURE ). Variability in material parameters and thicknesses are taken into account automatically in the analysis. Please remind that according to EU COST action 348 (de Bondt 2006) crack initiation is dominant versus crack propagation in case of temperature cycles. In ARCDESO the pavement details have to be inputted, including the distance between a) the (pre-)cracks of the CTB-base layer, b) the cracks in an old asphalt layer or c) the joints in the PCC-slabs. Also the monthly temperature profile has to be inputted. ARCDESO transforms this input into mean daily temperature variation cycles, combined with one extreme temperature cycle per month, and one seasonal temperature cycle (summer winter drop). ARCDESO calculates the reflective cracking behaviour from the cracks for the given (hundreds of) maintenance options. The maintenance exists of an asphalt concrete overlay of a certain thickness and an optional addition of (glass fibre) asphalt reinforcement that could be combined with a tack coat, SAMI or a PMB bond coat, via a spring model. There is also the possibility to change the conventional asphalt into polymermodified asphalt. In ARCDESO the optimum slab size can be determined given one maintenance option, but also the maintenance option can be optimized for a certain slab length. In common, the maintenance option to start with corresponds with the result of the general (thickness) design software. In Figure 1 an example of these optimizations is shown. Fig. 1 ARCDESO optimization overlaying a new 250 mm thick CTB-base layer From Figure 1 it becomes clear that a shorter notch distance results into less reflective cracking. Also the positive effects of applying PMB-asphalt and glass fibre reinforcement are obvious.

5 4 An important parameter in this part of the pavement design is the shear stiffness of the interface between the asphalt concrete and the underlying CTB. As there are relatively few temperature loadings (only one cycle per day) and the temperature loading (shear rate) is very slow, it is not likely that fatigue of the shear stiffness at the interface will occur. Of course, the value of the actual shear stiffness depends on the temperature, the loading time and the pavement structure characteristics (which materials are e.g. used as tack coat or bond coat?). ARCDESO contains a wide data base with (tested) materials and interfaces. 3 Traffic load Analyses The traffic loads can also initiate reflective cracking from the notches into the asphalt layers. In general the effect of the traffic loads reduces as the thickness of the asphalt concrete layers increases (better: the flexural deformation decreases). However, there is no rule of thumb at which thickness the effect of traffic loading cannot be ignored. This means that case dependent, the effect of traffic loading has to be analysed. For this Finite Element Modelling (FEM) software is necessary. Ooms Civiel performs this FEM-analyses by using the CAPA-3D software, developed at the section Structural Mechanics of Delft University of Technology (Scarpas and Karsbergen 1999). Figure 2 shows a section of a created element model that is loaded by a wheel load located exactly on top of the notch. The deformations in Figure 2 have been enlarged 800 times. Fig. 2 Vertical deformations in FEM-model caused by wheel load on top of the notch In Figure 2 the upper part of the CTB is shown, including the opened notch in the middle. On top of the CTB the asphalt concrete is visible. During the analyses the wheel load has been moved over the asphalt concrete pavement and the corresponding stresses and strains in all individual elements have been analysed. The first step in the analysis is to determine the critical wheel load position(s) and therefore the maximum strain at the bottom of the asphalt concrete has to be determined. Figure 3 shows that the maximum principal strain

6 5 is caused by wheel load position 21. This position equals to the wheel load exactly on top of the notch, which means that bending of the asphalt concrete is the normative failure mechanism for this specific case (project). Fig. 3 Principal strain at the bottom of the asphalt concrete above the notch caused by wheel load Figure 3 shows that the shear stiffness (D_tt) of the interface between the asphalt concrete and the CTB (modelled as a 1 mm thick, separate horizontal layer in between the cubes) is indeed an important input parameter for the traffic load analysis. In general this shear stiffness varies from 1 to 10 (N/mm)/mm², but it might reduce to only 0.1 (N/mm)/mm² in case of fatigue in between these layers (the shear resistance is generated by a combination of dry friction which is constant and adhesion which shall reduce in time). The calculations without a notch have been introduced to calibrate this design method to the traditional pavement design methods (when layers are homogenous). When the normative failure mechanism (bending or shear) has been determined, the corresponding CDF TRAFFIC can be calculated for several interface shear stiffness values. In this calculation amongst others the temperature, the fatigue performance of the asphalt concrete (determined via CE type-testing), the lateral wander and the time required for a crack to propagate from the bottom of the asphalt concrete to the pavement surface, are taken into account. The crack propagation is based on the time delaying factor determined in the PhD. research by M. Jacobs at Delft University of Technology (Jacobs 1995). According to EU COST action 348 crack propagation is as dominant as crack initiation for traffic loadings. 4 Design life Determination The determined CDF-values in chapter 2 and 3 should be superimposed to determine the design life of the pavement structure. In principle, this combined CDF represents only the expected design life of the asphalt concrete on top of the notches in the CTB. So a 100% value of the combined CDF means that cracks

7 6 from all notches have propagated to the pavement surface. Given for example a notch distance of 3.5 m and an average cracked area of 0.1 m width, this results in an overall damage of 3% of the pavement area. However, in CTB layers with a low fatigue strength, the combination of traffic and temperature loads could result into additional cracking of the CTB-layer itself and additional crack propagation. There is then a chance that cracking at the asphalt bottom might originate also at other locations than above the notch. For these two reasons Ooms Civiel has decided to limit the combined CDF to a maximum of 0.5. Given this criterion it implies that structures with a small interface shear stiffness (near 0.1 (N/mm)/mm²) often do not survive. In other words, an adequate interface shear resistance (but not too high) is required. 5 Conclusions Ooms Civiel has performed thermal and traffic load analyses to determine the expected lifetime of a pavement structure including a (CTB-)layer, by amongst others verifying the resistance of the structure against reflective cracking from a notch or (pre-)crack. The new or existing pavement design review on reflective cracking is possible to complete within a period of three weeks, for a given project. The results of the FEM-analysis without a notch correspond to the results of the traditional pavement design programmes and therefore this analysis method is adequate. It is possible to eliminate the chance of reflective cracking by pre-cracking the CTB and applying enough asphalt concrete (with the right quality) on top. Reflective cracking can be included into the routine design of (new) asphaltic pavements by adding a thermal ARCDESO analysis and an FEM-analysis. References de Bondt A (1999) Anti-Reflective Cracking Design of (Reinforced) Asphaltic Overlays. PhD- Thesis, Delft University of Technology, the Netherlands de Bondt A (2006) EU COST ACTION REINFORCEMENT OF PAVEMENTS WITH STEEL MESHES AND GEOSYNTHETICS Work Package 4: Selection of Design Models and Design Procedures, 3 January 2006 de Bondt A (2012) 20 years of research on asphalt reinforcement Achievements and future needs. Paper presented at the 7th RILEM Conference on Cracking in Pavements, Delft, the Netherlands, June 2012 Jacobs M (1995) Cracking in Asphaltic Mixes. PhD-Thesis, Delft University of Technology, the Netherlands Scarpas A, Karsbergen C (1999) CAPA-3D User s Manual, Delft University of Technology, the Netherlands