Sabit KUTLUHAN, Emine AĞAR

Bitümlü sıcak karışımlarda tekerlek izi oluşumunu etkileyen faktörler ve azaltmaya yönelik öneriler

Tekerlek izi oluşumu (oluklanma), esnek yol üstyapılarında görülen önemli bozulma türlerinden biridir. Taşıt tekerleklerinin yola değdiği bölümlerde, yol boyunca oluşan düşey kalıcı deformasyon-lar olarak tanımlanır. Tekerlek izi oluşumu yolun enine düzgünlüğünün bozulmasına neden olur. Enine düzgünlüğü kabul edilebilir bir sınırın üzerinde bozulan bir yol, konfor ve güvenlik yönünden büyük sorunlar ortaya çıkarır. Ağır taşıt sayılarındaki artış, bunların taşıma sistemlerinin değişmesi, dingil ağırlıklarının ve lastik iç basınçlarının artması gibi değişmelere bağlı nedenlerle, yollardaki bozulmalar hızla artmıştır. Yollarda görülen bozulma türleri; kalıcı deformasyonlar (tekerlek izi, çökme, kabarma, yığılma, ondülasyon), çatlamalar (yorulma ve termal) ve ayrılmalar (sökülme, soyulma) 'dır. Bu bozulmalar arasında tekerlek izi oluşumu, son yıllarda dikkate alınması gereken en önemli sorun haline gelmiştir. Tekerlek izi oluşumuna neden olan başlıca faktörler, yukarıdaki gelişmelere ek olarak, ticari taşıtların yasal sınırın üzerinde yüklenmesi, bunların düşük hızla hareket etmesi, uzun süreli veya durağan yükler, aşırı yük tekrarı, trafik yüklerine ve iklim koşullarına uygun olmayan malzeme kullanımı, tasarım ve yapım hataları olarak sıralanabilir. Bu çalışmada, bitümlü sıcak karışımlardaki tekerlek izi oluşumunda etkili faktörler, dış faktörler ve iç faktörler olmak üzere iki grupta incelenmiştir. Dış faktörler; araç faktörleri, iklim koşulları ve yol geometrik parametreleridir. İç faktörler ise; agrega, bitümlü bağlayıcı, bitümlü sıcak karışım bileşimi ile tasarım ve yapım hatalarıdır. Çalışmanın sonunda tekerlek izi oluşumunu azaltmaya yönelik öneriler sunulmuştur.

Factors effecting rutting in bituminous hot mixtures and suggestions to reduce rutting

Rutting is one of the important distress types that occur in flexible pavements. It is described as vertical permanent deformation in contact area between wheel and road surface along the way, longitudinally. Rutting causes transversely unevenness in road surface. A road of which transversely uneven-ness is over an acceptable limit reveals many problems in point of comfort and safety. The passengers in a vehicle may be disturbed and control of vehicle gets difficult during changing line, water accumulates in wheel path in rainy days, water accumulated causes icing in cold weather and aquaplaning, consequently, breaking distance gets longer. In this study, rutting mechanism, rutting types and effectual factors have been explained in detail. Densification (decrease in volume, compaction) and shear deformation play important roles in rutting mechanism of bituminous hot mixtures. Rutting mechanism consist of two stage. In the first stage, permanent deformation below the tires is greater than the one at the edge of wheel path. In this stage, compaction effect (densification) of traffic is more effectual than shear deformation. And then, decreasing volume below the tires gets approximately equal to increasing volume at the edge of wheel path. This case that is the initial of second stage indicates to complete the compaction of pavement under traffic. And so, the reason of further rutting is shear deformation without volume change. It is difficult to determine that how much part of rutting is resulting from densification or shear deformation. However, shear deformation plays more important role in the part near the surface than the lower part of pavement. In addition, many scientific researches have indicated that shear deformation is more influential parameter than compaction in deformation behavior in the greater part of life of pavement. There are four types of rutting formation based on different reasons. These are structural rutting, flow (instability) rutting, wear rutting and consolidation (compaction) rutting. Structural rutting is resulted from deformations in one or more layer beneath the bituminous pavement layer. Because of this type of rutting formation is that stresses occurred in pavement layers exceed the materials resistance. The material rising is not appeared at the edge of the wheel path in this type of the rutting formation. Flow rutting is resulted from deformations in the bituminous pavement layer or layers. Because of this type of rutting formation is that stresses occurred in bituminous pavement layer(s) due to loading exceed the bituminous materials resistance. The material rising is appeared at the edge of the wheel path in this type of the rutting formation. Shear deformation plays an important role rather than densification (compaction) in this type of rutting. Wear rutting, that is named surface rutting type, is resulted from wearing the aggregates on the surface of pavement by the using of studded tires in winter. This type of rutting formation appears especially in the north Europe countries using studded tires. The parameter to be considered in wear rutting formation is aggregate toughness. Fourth type of rutting formation is consolidation (compaction) rutting, that is named surface rutting type too. Consolidation rutting has a same cross -section with that of wear rutting. This type of rutting formation occurs because of laying the bituminous mixture in cold weather and (or) insufficient compaction of pavement. Factors effecting rutting formation in bituminous hot mixtures are into two groups named external factors and internal factors. External factors consist of vehicle factors (axle load, tires inflation pressure, loading time etc.), climatic conditions (temperature, raining, freezing and thawing etc.) and geometrical standards of road (gradient, curb radius etc.). Internal factors consist of aggregate (shape, roughness, size etc.), bituminous binder (penetration, softening point, etc), bituminous hot mixtures composition (air void, bitumen content, void in mineral aggregate, void filled with asphalt, etc.), and design and construction wrong (inappropriate pavement thickness, insufficient compaction, laying pavement in cold weather etc.) An agregate gradation of which nominal maximum agregate size and coarse agregate percent are high should be preferred for resistance to rutting in bituminous hot mixtures. For example, stone mastic asphalt (SMA) mixture of which nominal maximum agregate size is bigger than 12.7 mm, coarse agregate percent is high and binder is modified bitumen provides high performance for rutting resistance. In addition to this, SMA mixtures can be used in two-layer system that is preferred in France. In this system, while the upper thin layer provides durability, impermeability and skid resistance, the lower thick layer provides resistance to rutting and fatigue cracking. SMA mixture should be the lower layer of this system

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