PAVEMENT DAMAGE BY CRACKING UNDER SEVERE FROST CONDITIONS

Transcription

1 PAVEMENT DAMAGE BY CRACKING UNDER SEVERE FROST CONDITIONS M. Boutonnet 1, Y. Savard 2, N. Pouliot 2 et P. Hornych 3 (1) Consultant for LCPC, Besançon, France (2) Ministère des Transports du Québec, Canada (3) Laboratoire Central des Ponts et Chaussées, Nantes, France Abstract A France-Québec cooperation project between the Laboratoire Central des Ponts et Chaussées (LCPC) and the Ministère des Transports du Québec (MTQ) led to the design, construction and rigorous monitoring of behaviour over time of four experimental road sections established in Québec. The project s main objective is the validation and improvement of French and Québec pavement design methods under severe frost conditions, and more specifically the calibration of damage models to the deterioration observed on the experimental sections. The two types of pavement most used in France (road foundations treated with hydraulic binders and bituminous pavements) were selected. Two test beds of each type were constructed, one of them thermally insulated with panels of extruded polystyrene. After four years of monitoring, no cracks have appeared in the bituminous pavements, while the cement-treated base pavements exhibit fatigue and thermal shrinkage cracks. These observations confirm the fatigue damage forecasts of the dimensioning methods and the effectiveness of the cement-treated base precracking technique to control the reflection and opening of thermal shrinkage cracks in the surface bituminous mix. 1. Introduction The thermal behaviour of pavements in winter and their mechanical behaviour in thaw periods have a major influence on pavement design ( 1 ) ( 2 ). To optimize pavement design while taking into account for the risks of frost and thaw damage, the Laboratoire Central des Ponts et Chaussées (LCPC) and the Ministère des Transports du Québec (MTQ) have models to forecast the propagation of frost and thaw phenomena depending on the thermal characteristics of the projects and depending on the climate they will support ( 3 ). They also have frost damage models (heaving forecast) and thaw damage models. Given the progress of road technology and the absence of exceptional winters in France since these models were perfected, the LCPC and the MTQ initiated a cooperation project in 1998, with the objective of optimizing their frost-thaw pavement design method ( 4 ). The two types of pavement most used in France (cement-treated base and bituminous pavements) were selected. This project is also the opportunity for the MTQ to observe the behaviour, under winter weather and operating conditions specific to Québec, of pavements containing a precracked cement-treated base course.

2 One of the project s objectives is the calibration of the damage models to the observed on-site distress. The pavement structures of the uninsulated test beds were designed for a three-year life cycle so that the calibration could be completed in a short time. This aspect will be presented in this paper, which will also present observations on the thermal shrinkage of cement-treated bases, shrinkage crack sealing and the ability of this sealant to withstand stresses over time. 2. Pavement behaviour under severe frost conditions Under severe frost conditions, a pavement structure should be designed to withstand the stresses transmitted to it under the effect of traffic and satisfy the criteria of protection against the effects of frost and thaw. The effects of frost can be considerable: thermal shrinkage cracking of the bound materials, frost heaving and loss of load-bearing capacity during thaw. When a bound material (asphalt or cement) is cooled, it contracts, which induces tensile stresses at the surface. If the stress exceeds the material s tensile strength, a transverse crack appears on the surface of the course and quickly propagates downward. The influence of transverse cracks on a pavement s structural behaviour depends on the opening of the cracks. If the crack remains narrow, the overlapping aggregates assure an adequate load transfer between the walls of the crack. If the crack is wide, however, the load transfer is no longer possible and the vertical stress may be increased at this location ( 5 ). Pavement frost heaving is mainly caused by the formation of ice lenses in the subgrade soil ( 6 ). These soils are classified as frost heave susceptible because they have a fine gradation, which favours migration of the interstitial free water by capillarity to the frost front and ice lens formation. Since the nature of the soils and the frost depth are very rarely uniform, this additional ice causes unequal differential heaving on the road surface and affects riding comfort. During the thaw, the melting of ice lenses translates into an increase in the water content in the vicinity of the frost front and a reduction in the pavement s load-bearing capacity. It is recognized that this weakening during thaw is the cause of accelerated fatigue deterioration and structural pavement rutting ( 7 ), ( 8 ), ( 9 ). 3. Description of the experimental test site 3.1 Local characteristics The experimental site was constructed on national Route 155 at St-Célestin, Québec, Canada. This is a road with two contiguous lanes. The Site is located in the plain of the St. Lawrence River about 100 km Northeast of Montréal. The road runs North- Northwest while the prevailing winds blow from the West. The site is located in a rural sector bounded by fields and the land is flat. Route 155 was chosen because of the site s homogeneity, the presence of frostsusceptible soils (silty sand resting on clay) and signs of pavement fatigue damage. It is subject to a harsh climate (freezing index around 1150 C days/year) and supports substantial truck traffic (750 trucks/day/direction). Preliminary studies made it possible to characterize the bearing capacity of the subgrade soil, its frost susceptibility level, the drainage conditions, the site s climatic conditions, the annual traffic growth and the

3 properties and composition of the asphalt mixes and the cement-treated base to be produced. 3.2 Test Beds The test beds were constructed by half-roadway, maintaining one lane for traffic. The previous pavement was excavated to the desired depth to install the layers constituting the Test Beds (Figure 1). The wearing course of all test beds is made of hot-mix asphalt with a gradation of 0/10 mm (EB10S). Test Bed 1 has a base course of hot-mix asphalt with a gradation of 0/20 mm (EB 20). It is of Québec design and serves as a control section. Test Bed 2 has a structure similar to the control test bed, with thermal insulation added. The other two test beds, of French design, have a cement-treated base (CTB) course. One is thermally insulated (Test Bed 3) and the other is not (Test Bed 4). The thermal insulation has been incorporated to isolate the effects of traffic from those related to the frost-thaw cycle of the subgrade soil. The cement-treated base course is made in a plant batch by blending its components (0-5 mm and 5-20 mm crushed aggregates, 3.5% by weight of Type 10 Portland cement, 1.9% of hydratation stabilizer by weight of cement and 1.5% by weight of added water). The cement-treated base was precracked transversely with a spacing interval of 2.5 m over 50% to 60% of its thickness to control thermal shrinkage cracking. The precracks were produced after 10 passes of a roller compactor (2/3 of the compaction energy) by means of a saw cut and the slot was filled with a cationic asphalt emulsion to prevent setting. The thermal insulation used is composed of a double layer of extruded polystyrene in plates 25 millimeters thick. The base course of all test beds is made of 0/20 mm granular materials (MG20). The bottom of the cut consists of subbase sand left in place on a subgrade of natural clayey soil. TEST BED 1 TEST BED 2 TEST BED 3 TEST BED m 100 m 100 m 100 m EB 10S 6 cm EB cm Cement treated-base: 25 cm MG20 : 30 cm MG20 : 35 cm MG20 : 30 cm Sand : 45 cm Sand : 28 cm Sand : 20 cm Polystyrene : 5 cm Sand : 15 cm Sand : 45 cm Silty sand: 30 cm (nonexistent for Test Beds 3 and 4 in direction 1) Clay: indeterminate thickness Figure 1 - Composition of the Test Beds The constituents of the pavement materials, their manufacturing and their implementation were controlled very rigorously. The controls related to the pavements thermal behavior pertained to:

4 - the mineralogical nature of the aggregates and the measurement of their optimum Proctor density, - composition of the EB 10S and EB 20 mixtures and particularly their asphalt binder content, - composition of the cement-treated bases and particularly their water, cement and hydration stabilizer admixture content, - measurement of the density in place and the water contents of each pavement layer (EB 10S, EB20, cement-treated base, MG 20, subbase sand) and the bottom of the cut, - thickness measurement of the pavement courses and the underlying courses next to the thermal instrumentation. A pavement condition zero point was obtained one month after the pavements were commissioned. It involved taking core samples of the bound pavement courses. The measurements performed on the core samples made it possible to validate the thickness and composition of the courses. Destructive boring was also done to validate the thickness of the unbound courses and the position of the extruded polystyrene layers. 4. Field data collection During construction, each test bed was equipped with a set of sensors to monitor the behaviour of the pavement and its different courses periodically. The test beds are equipped with frost depth gauges (progression of the frost-thaw front), piezometers (position of the water table), thermistors (temperature measurement) TDR (Time Domain Reflectometer) sensors (volumetric water content) and heaving sensors. On the uninsulated bituminous pavement and cement-treated base test beds, multi-level deflectometers are added to measure the deflection of each course. The uninsulated bituminous pavement test bed has also been equipped with longitudinal and transverse deformation gauges at the base of the bound course. Sensors linked to a continuous data acquisition system have also been added under Test Bed 1 to measure the volumetric water content of the different courses (Theta-Probe) and temperatures to a great depth (thermistors down to 10.5 m). A weather station and a weigh-in-motion station complete the site instrumentation. A detailed annual program of readings and observations of the behaviour of the test beds has been in progress since commissioning. It includes non-destructive deformability measurements of the pavement structure in summer, fall, thaw and recovery periods with the following apparatus: FWD (Falling Weight Deflectometer), inclinometer and ovalization (French devices), multi-level deflectometers and deformation gauges (Laval University apparatus). Detailed distress survey mapping and measurement of rut depth, surface roughness (IRI index), frost depth and heaving are also performed periodically. 5. Behaviour of the experimental test sections 5.1 Thermal cracking Transverse cracks appeared only in the cement-treated base test beds (Test Beds 3 and 4). These cracks correspond to the thermal shrinkage of the cement-treated base because they appear in the winter period (Figure 2). They are located above the precracks. After 4 years of monitoring, 75% of the precracks (137 m) have reflected to the surface of Test

5 Bed 3 in whole or in part, compared to 95% (157 m) in Test Bed 4. The total length of reflective cracking represents about 50% of the 240 m of cement-treated base precracks. The precracks generally open 1 to 2 mm, thus assuring load transfer. They appear on part of the pavement and generally propagate over the entire width Direction 1 Direction Direction 1 Direction 2 Meters (m) Meters (m) oct-98 oct-99 oct-00 Date oct-01 oct oct-98 oct-99 oct-00 Date oct-01 oct-02 a) Test Bed 3 (insulated CTB) b) Test Bed 4 (uninsulated CTB) Figure 2 Evolution of transverse cracking 5.2 Fatigue cracking Fatigue cracks have appeared only in the cement-treated base test beds (Figure 3). The cracks developed first in Test Bed 4 (uninsulated) and propagated more quickly in direction 2, which corresponds to the heaviest loaded lane (Figure 3b). Fatigue cracks total 224 m on Test Bed 4, compared to only 26 m on Test Bed 3. The cracks first appeared at locations where deflection was greatest when the test beds were commissioned. Meters (m) oct-98 Direction 1 Direction 2 oct-99 oct-00 Date oct-01 oct-02 Meters ( m ) oct-98 Direction 1 Direction 2 oct-99 oct-00 Date oct-01 oct-02 a) Test Bed 3 (insulated CTB) b) Test Bed 4 (uninsulated CTB) Figure 3 Evolution of fatigue cracking 6. Maintenance Thermal shrinkage cracks, even in temperate climates, are inherent in the cement-treated base technique. In the absence of sealing, these cracks evolve quickly under the action of water infiltration and traffic, which particularly result in separation of the surface course/base course interface and wear by attrition of the lips of the cracks, phenomena accompanied by crack ramification, alligator cracking and potholes. It was therefore

6 decided to seal the cracks when the first cracks appeared in This sealing operation was renewed in 2000 and 2001 to treat the cracks that appeared in the second and third years. In France, the cement-treated base precracking technique was developed to delay the appearance of surface thermal shrinkage cracks and control their opening. On Route 155, despite the implementation of this technique, and due to the harsh winter, 25% of the precrack length had already appeared on the pavement surface at the end of the first winter after the pavement was commissioned. Thermal shrinkage and fatigue cracks were sealed according to the MTQ method, both concerning the nature of the product (polymer-modified asphalt base) and its conditions of application (cleaning and drying of the cracks with compressed air and then with a hot compressed-air lance less than 5 minutes before application of the product, maintained at a temperature of 185 to 190 C). The product is applied with a nozzle equipped with a scraper (Figure 4a), making it possible both to fill the cracks and control the quantity applied by limiting the width of the ribbon to 4 cm and its thickness to 3 mm. To prevent it from sticking to vehicles tires, the sealing product is protected with a biodegradable absorbent paper (single-layer toilet paper) and left out of traffic for 30 minutes (Figure 4b). a) Applying the product b) Biodegradable absorbent paper Figure 4 Crack sealing 7. Analysis of the results 7.1 Precracking In France, the usually retained step of precracking is 3 meters. To take into account the more important annual thermal gradient in the region of the experimental site (-30 C to + 35 C) while it is of -20 C to +35 C in the East of France, it was decided to reduce the step of precracking. The 2,5 meters value was retained and it corresponds to a relative reduction of the same order of height as the relative difference between the annual thermal gradients. It was also retained to avoid to lead slabs of too small dimension as well as to allow to realize the precracking in a compatible delay with the speed of advance of the construction site. Observation of thermal shrinkage cracks on cementtreated base test beds shows that the proportion of precracks that have partially or totally reflected through the wearing course is 75% on Test Bed 3 and 95% on Test Bed 4. The precracking technique therefore worked correctly by generating closer cracks, and thus short slabs (2.5 m long) with low dimensional variations under the effect of annual thermal gradients. Crack opening remains low and interlocking of the edges of the cracks

7 is good. This successful precracking translates into correct behaviour of pavements under load to at the vicinity of the precracks (no sensitive movement at the edge of the slab) and an absence of evolution of transverse cracks except for the normal growth of their length (no ramifications or spalls). The fact that the linear length of precracks appearing on the surface only represents 50% of the precracks created confirms the effectiveness of the technique. 7.2 Fatigue Design of the cement-treated base structures had been forecast, with the French methodology, for a three-year period with a 25% calculation risk. The purpose of this choice was to calibrate the LCPC design method over a short period with appearance of fatigue cracks within three years on pavements without thermal insulation. A recalibrate of this design calculation during construction of the pavements showed that the damage to the uninsulated cement-treated base pavement should appear in the second year of its life cycle (cumulative damage = 1.7 after three years of traffic). In fact, the first longitudinal fatigue cracks appeared on Test Bed 4 at the end of the first year of service and fatigue cracking affected the entire length of Test Bed 4 in direction 2 after two years of service. As for thermally insulated Test Bed 3, the damage calculations showed that the cumulative damage, with a 25% calculation risk, should reach 0.86 after three years of traffic, while cumulative damage of 1 should therefore be reached in the fourth year of service. In fact, while the first fatigue cracks appeared after one year of service in direction 1 and after three years of service in direction 2, the linear pattern of fatigue cracks remains limited after four years of service (21 meters in direction 1 and 6 meters in direction 2). The bituminous pavement test beds were designed by the Québec method for a 15-year life cycle. Calibration of this dimensioning calculation with the French method during construction showed that uninsulated bituminous pavement damage calculated with a 25% risk should reach the value of 0.66 at the end of the third year of service. Cumulative damage of 1 should be reached in the fifth year of service. The fifth year of service of the test site is presently being monitored. 7.3 Crack sealing With the crack sealing technique described in Chapter 6, the sealant withstood traffic and weathered stresses for 4 years, without ripping or tearing, with an annual thermal gradient of 35 C (-10 to + 25 C) at the base of the cement-treated base. The condition of the cracks did not worsen and they remained single, clean and fine, indicating an absence of deterioration of the surface course/base course interface and interlocking of the sides of the cracks. 8. Conclusion The results presented in this paper represent part of the four years of cooperation between the MTQ and the LCPC, which led to the production of 4 test beds. Attentive monitoring of the construction of these test beds, their instrumentation and all of the measurements and readings taken constitute an essential database for validation of the methods used by the two organizations for frost-thaw pavement design. No cracks have appeared in the bituminous pavements after 4 years of observation, while the cement-treated base pavements exhibit fatigue and thermal shrinkage cracks. The

8 results indicate that the cement-treated base precracking technique is effective to control the reflection of thermal shrinkage cracks and thus assure adequate behaviour of pavements under load. At this stage of experimentation, the observations of the Route 155 test beds confirm the fatigue damage forecasts of the design methods, both regarding the effect of traffic and the effect of thaw. This conclusion still has to be confirmed when the complete analysis of the traffic withstood by these pavements and their behaviour under load are completed. The crack sealing technique applied by the MTQ is effective in maintaining the integrity of cracks and corresponds to a 5-year life cycle, taking into account the product s wear by traffic. 9. References ( 1 ) Corté, J-F., Odéon, H. et Boutonnet, M., Vérification au gel des structures de chaussées, Bulletin de Liaison des Ponts et Chaussées, vol. 198, 1995, p ( 2 ) Corté, J-F., Boutonnet, M., Savard, Y. et St-Laurent, Denis., Validation de la méthode française de dimensionnement au gel/dégel des chaussées, Recueil des communications du Sommet Mondial de la Nordicité, Québec, 2-5 février 1999, p ( 3 ) Boutonnet, M., Savard, Y., Lerat, P., St-Laurent, D. and Pouliot, N., Thermal aspect of frost-thaw pavement dimensionning : in situ measurement and numerical modeling, to be published in Journal of Tranportation Research Record, TRB, National Research Council, Washington D.C., ( 4 ) Rioux, N., Savard, Y., Corté, J-F. et Boutonnet, M., La collaboration scientifique franco-québécoise sur le dimensionnement des chaussées au gel-dégel, Revue Générale des Routes et Aérodromes, Hors série 2, 1999, p ( 5 ) Pittman, David W., Development of a Design Procedure for Roller-Compacted Concrete (RCC) Pavements, US Army Corps of Engineers, Waterways Experiment Station, Technical Report GL-94-6, March ( 6 ) Konrad, J. M. and Morgenstern N.R., The Segregation Potential of Freezing Soil, Canadian Geotechnical Journal, No 18, ( 7 ) White, T.D. and Coree, B.J., Threshold Pavement Thickness to Survive Spring Thaw, Proceedings of the Third International Conference on Bearing Capacity of Roads and Airfields, Trondheim, Norway, pp ( 8 ) Janoo, V.C. and Berg, R.L., Thaw Weakening of Pavement Structures in Seasonal Frost Areas, Transportation Research Record, No 1286, TRB, National Research Council, Washington D.C., 1990, pp ( 9 ) Doré, G. and Savard, Y., Analysis of Seasonal Pavement Deterioration, Proceedings of the TRB 77 th Annual Meeting, Washington D.C., 1990.