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  Fa c tor s ff e cti ng Pa ve me nt D esig n 3 Surface course __ i ~~~ ~ (bi tumin ous) ~~ '......, ______   _ _ Base S ur face co ur se - (~ §~1 ~~ Sur face co ur se (granular) il ifj lil}jjii i   r nular ) ' ' ·' (bi tumin ous) Sub-base ( gr anular) Sub -g rade a) Buum inous pavement Suh-base  . ( gr avel soil) Bi tum inous - base b) Gra nu la r pa ve ment c) Full - de pth bi tuminous pa veme nt Figure 1 2 Cros s- sectio nal de tails of ty pi cal flexi bl e pa veme nt s tru c tur es The surface course of flexible pavements acts as a flex ible layer. On wheel loading, the surface lay er bends with elasticity into a c   ncave shape. This bowl-like surface moves in the longitudinal direction under the moving wheel (on the load path). Every application of wheel load on the pavement surface causes elastic deflection (or elastic strain). Repetitive application of wheel loads in different climatic conditions causes unrecoverable deformations in pavement structure which are termed as plastic deformations (or uneven surface). Plastic deformations will occur in the vertical and horizontal directions due to vertical compressive stresses and horizontal tensile stresses respectively. Occurrence of such plastic deformations in the vertical direction, viz. rutting, consolidation, shear failure, upheaval or any kind of undulation in any one of the layers, causes a similar trend of deformation in the rest of the layers. These deformations may propagate in any vertical direction ranging from the sub-grade to the surface course and vice versa (Figures 1.3 and 1.4). The plastic deformations caused by horizontal tensile stresses are generally observed as cracking in bound layers and displacement of materials in case of unbound layers. Propagated un   ulations on pavement surface Surface course Base course Sub-base course Sub-grade Figure 1 3 Pavement surface undulations caused due to plastic deformation of the sub grade Propagated undulations at different layers Surface course Base course Sub-base course -- - =-----------....::..;:,--- Sub-grade Figure 1 4 Undulations caused due to rutt i ng of s urface c ourse  Fa c tors ec ting Pavement De sign Sub-base layer is provided for the following reasons/conditions (Figure 1.5 (b )) (Collins and Sharp 195 8; Childs et al. 15 59). ã To decrease deflections of slab with an accompanying reduced tendency for pumping mud . ã Sub-base layer is required if the plastic sub-grade soil is susceptible to pumping and has a tendency to weaken severely in the presence of melting frost. ã Sub -base layer should be provided if the sub-grade tends to change its volume signifi cantly with varying moisture content . Use of sub-base which is thicker than normal to prevent pumping of mud does not greatly enhance the load - carrying capacity of the pavement. Moreover, thicker sub-bases may not be structurally economical. The structural functions of the remaining layers below the surface course are similar to the layers of a flexible pavement. 1 2 3 Composite Pavements In this category, the wearing course of the pavement is constructed by spreading bricks or pre-cast rectangular cement concrete blocks, or interlocking concrete blocks (Figure 1.6 (a)). The joints between them are filled with fine sand of standard gradation. These pieces of blocks act as small rigid plates. Hence, the wearing course constructed of these blocks is neither truly flexible nor is it a truly rigid layer. It possesses dual properties and is suitable for low ~ol~m~ village roads or foot paths. A composite pavement consists of cement concrete or cement-treated granular and bitu minous layers. These kinds of pavements are observed when an old concrete pavement is overlaid with a bituminous layer or any granular layer below a bituminous wearing course is treated with cement or lime (Figure 1.6 (c) and (d)). While designing composite pavement structures, the flexural strength of the base slab or treated granular material should be taken into consideration for resisting tensile stresses. ™ _ Su~face ~ourse ·,, \;{',~ (with bncks or ã ã Base (granular) . c. c._c._c._c ~c ãc ãc ãc ãc Sub-base _ ~c ãc ãc ãc ãc (granular) ~ ~ã~ã~ã~ã~ ~,...... ~ Sub-grade_.~ a) With sub-base course P re-cast cement concrete blocks) ã~ã ã ã ã ã - ãããããã Base course ---------~ ã ã ã ã ã . Granular cement or hme treated or ~-~- ~--~-- :  ~~ dry lean concrete) ~~ ;;. .%.~·-0 ,-  m~~ Sub- grad~ 6*'~ b) Without sub-base course Figure 1 6 Cross   sections of typical composite or semi-flexible or semi   rigid pavement structure s  Lane distribution factor LDF) Lane distribution factor is a decimal value which indicates the concentration of placement of wheel load repetitions along a road stretch. As a general tendency, on a multi-lane undivided one-way pavement, a driver places his vehicle towards the centre or away from the edge . Therefore, lower concentration of wheel load repetitions is observed near the edges . Due to this reason, as an economic consideration, based on the number of lanes, only a part of the commercial traffic volume is taken as the design traffic volume. The lane distribution factor LDF) should be determined from realistic and appropriate field surveys based on special repeatability of wheel paths of different vehicles in the transverse direction of a pavement surface. In the absence of such data, the following LDF may be used to design flexible pavements IRC : 37-2001). able 1 1 Lane distribution factors /RC: 37-2001 SI No. Type of carriageway LDF Total number of commercial vehicles considered in 1 Single-lane 1.00 2 TwQ ·-lane single 0.75 3 Four-lane single 0.40 Both directions 4 Dual two-lane 0.75 5 Dual three-lane 0.60 Each direction 6 Dual four-lane 0.45 Q Determine the traffic volume just after construction of a road project using the following data: Present traffic volume = 1090 commercial vehicles per day Growth rate of raffic = 7 1 Duration of road construction = four and half years A Using equation (1.3), A = P l ,y e = 1090 1 0.071) 4·5 = 1484 commercial vehicles per day CVPD) . 1.3.3 Factors Related to Axle and Wheel Loads Axle Load Wheel Load and Configuration of Wheels The total weight of a vehicle is carried by its axles. The load on the axles is transferred to the wheels and this load is ultimately transferred on to the surface of the pavement in contact with the tyres. To keep wheel load induced stresses on pavements within allowable limits , the total vehicle load is distributed onto wider areas of pavement by using more axles and wheels. This is the reason why more number of axles and wheels are fitted to heavy load-carrying trucks . Figure 1 7 shows the different types of configurations of axles and wheels used in the manufacture of road veh ides . '  Fa c tors ffecting Pavement D esig n Table 1 2 omparison o load weighing me thods SI.No. Static weighing method Weight in motion WIM) 1 Vehicles are stopped and weighed Vehicles are weighed automatically, while in motion, without disturbing the driver. 2 Accurate weight measurement Weight measurements may be influ-enced by parameters related to vehicle speed, suspension system of the vehicle, tyre pressure, acceleration and deceler-ation of the vehicle and dynamic forces produced due to pavement roughness, wind velocity etc. 3 Takes more time and interrupts free flow of There is no such interruption to flow traffic; may pose problems related to safety. of traffic; can weigh high volumes of traffic. 4 Other information such s body type of vehicles, Collection of other information is not loading type etc can be physically ascertained. possible by automated WIM equipment . 5 Less number of vehicles can be measured; se-More number of vehicles can be mea -lected vehicles are weighed; need more person- sured; better coverage of all vehicles nel, time and space to weigh all vehicles. since it is automatic. 6 Less installation and maintenance cost. High installation and maintenance cost. 7 The weigh pads can be installed at any location. The WIM equipment can be installed at fixed locations only. In the static method, the axle load of a vehicle is weighed using portable weights or load-pads Figure 1.9) or a weighing platform. Only axle loads having more than or equal to 3 tonnes are taken into account for analysis since the damage caused by axles weighing less than 3 tonnes is negligible. For traffic analysis, vehicles having axle weight = 3 tonnes are referred to s commercial vehicles. Single axle with dual wheel assembly Load pads Battery Electronic circut Load cells onnecting wire Figure 1 9 xle load me asur eme nt Axle load display unit
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