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Paper No. 535 RUTTING IN FLEXIBLE PAVEMENTS – A CASE STUDY † V.K. SINHA*, H.N. SINGH** & SAURAV SHEKHAR*** ABSTRACT Flexible pavements are generaly adopted for construction of roads in India. Bitumen as a binder is known to be highly sensitive to high temperatures. Dis tresses in the form of ruts, cracking, ageing etc. are common on Flexible pavements . These are still observed on pavements constructed presently with thick layers of binder courses at high cost. Rutting one of the commonly observed permanent nature distress is the subject matter of this case study. The effect of high pavement temperature on the stability of mix in conjunction with lower softening point of bitumen has been studied in the context of prevailing high temperature in top pavement layers. Study brings out the inadequacies in existing specifications and suggests some follow up actions to improve the existing specifications. Use of modifiers in the top binder courses like DBM to enhance the thermal dependent characteristics of the bituminous mixes is one of the recommendations. Adoption of catalogue type performance based specifications covering different climatic regions of the country are also suggested. 1. INTRODUCTION Flexible pavements have been traditionally provided on most of the important highways of the country. Thick bituminous pavement layer broadly comprising of a DBM layer of 160 to 180 mm topped with 50 mm bituminous concrete are being provided presently by way of strengthening. The bitumen used in the design of mixes for SDBC, PC, DBM and BM is typically of 60/70 grade. However, in few cases wearing course, having bituminous concrete 50 mm thick is also being pr ovided with modified bitumen. Modifiers in such cases are either CRMB or PMB. Use of modifiers, however, is not common. The design of mixes are being done as per Marshall method with normal 75 blows for all locations without considering the effects of climatic, traffic variations etc. Despite the construction of thicker pavement, such bituminous pavements suffer from rutting frequently in quite early age. Such def ormations in the form of rutting are more pronounced at locations of intersections, curves and in stretches where heavy traffic operates with low speed and is subject to frequent stop/start condition . Such early rutting of the flexible pavements should concern all highway engineers. This is particularly so, when constructing long performing pavements is the moto of all highway agencies in view of huge investment being made on the construction of such highways. The Paper is based on a case study representing a typical rutted stretch of a four-lane road which has been * Secretary General, IRC } E-mail: secretarygen@irc.org.in ** Executive Engineer (Retd.) PWD Bihar, Material Engineer, Quest Consultants Pvt. Ltd. *** Director, SA Infrastructure Consultants Pvt. Ltd. † Written comments on this Paper are invited and will be received upto 31 st December, 2007 widened and strengthened recently with thick bituminous pavement layers. The effect of high temperature of pavement layers on in-service behaviour of compacted bituminous mixes is the key obje ctive of this case study. 2. STUDY STRETCH The stretch considered is about 250 m long, suffering substantial rutting to a maximum depth upto 35 mm. This stretch is near an intersection. Heavy trucks with high axle loads in large number (about of 4500 trucks per day) are operating on this stretch, at a relatively low speed with frequent stop/start condition. The crust composition of this stretch is given in Table 1. S. No. T ype of layer Thickness (mm) 1 . Bituminous Concrete (BC with CRMB 60) 5 0 2 . Dense Bitumen Macadam (DBM Layer II) 8 0 3 . Dense Bitumen Macadam (DBM Layer I) 8 0 4. Wet Mix Macadam (WMM) 2 5 0 5. Granular Sub-Base (GSB) 260 The study stretch comprises both types of surfaces (i) Exposed DBM surface without BC and (ii) DBM layer covered with BC surface in adjoining length. Same traffic is operating on both these surfaces. Time lag between laying of DBM and laying of BC on the DBM is on average about six months plus. TABLE 1. CRUST COMPOSITION AT THE STUDY STRETCH
