Design of Pavement - Lab Report Example

Design of pavement Based on the structural behaviour the pavements are ified into two categories flexible pavements and rigid pavements. Theflexible pavements are those which as a whole have low or negligible flexural strength and are rather flexible in their structural action under the loads. The flexible pavement layers reflect the deformation of the lower layers on to the surface of the layer. Thus, if the lower layer of the pavement or the subgrade is undulated, the flexible pavement surface also get undulated. The important components of the flexible pavements are (i) Soil subgrade (ii) sub-base course (iii) base course (iv) surface course (Mathew and Rao, 2007). Rigid pavements on the other hand, posses significant flexural rigidity or flexural strength. The stresses are not transferred from grain to grain to the lower layers as in the case of flexible pavements. The load carrying capacity of rigid pavements is decided mainly due to the rigidity and the high modulus of elasticity of the slab. The pioneering effort in the design of rigid pavements has been made by H.M.Westergard. Westergard considered the rigid pavement as thin elastic plate resting on soil subgrade, which is assumed as a dense liquid. The upward reaction is assumed to be proportional to the the deflection, p = KΔ , where K is the modulus of subgrade reaction. The unit of K is kg/cm2 per unit deflection thus expressed as kg/cm3 . A certain degree of resistance to slab deflection is offered by the subgrade. This is dependent upon the stiffness or pressure deformation properties of the subgrade material. The tendency of the slab to deflect is dependent upon its properties of flexure strength (Mathew and Rao, 2007). Factors to be considered in the design of pavements (Mathew and Rao, 2007) Design wheel load Subgrade soil Climatic factors Pavement component materials Environmental factors Special factors to be considered specific type of pavements. Flexible pavement design procedure The design process of flexible pavements include determination of required thickness for the layer and its composition and ensures that under the application of load none of the layers is over stressed. Thus, no section of the pavement section would be subjected to excessive deformation to form a localised depression or settlement. The maximum intensity of stress is experienced in the top layers of the pavement and the magnitude of the stress reduces to the lower layers. As the stresses experienced by the top most layer of the pavement is the maximum and least on the top of the sub grade (Mathew and Rao, 2007). Hence, they are arranged in the order of descending load bearing capacity with maximum capacity material at the top and least load bearing material at the bottom. Thus superior materials are provided at the top. The most advanced design approaches are using the elastic layered system analysis based on the stress deformation characteristic of the pavement. The important parameters to be considered in this context is the load due to traffic and the temperature variation being experienced by the road surface. Two methods of the design of pavements are widely practiced - Empirical design and mechanistic design (Mathew and Rao, 2007). The typical cross section of a flexible pavement is given in the figure 1. Empirical design : The methodologies in the empirical design is based on the information and results obtained from the experiment or experience. It could be undertaken either with or without support of soil investigation and strength assessment. California bearing ratio (CBR) test is the most widely used soil strength test used for highway pavement design (Mathew and Rao, 2007). Mechanistic design : The design is based on the stress - strain behaviour of the pavement materials subjected to the input values like wheel load. These relationships are established with the help of mathematical models which captures the total behaviour of the system. These methods are also able to incorporate the design reliability and the life cycle costing (Seeds, n.d.). Figure 1 The layers in flexible pavement (Source : http://www.dur.ac.uk/~des0www4/cal/roads/pavdes/thicknes.html#next-step) Materials for flexible pavements. The important materials used for the construction of flexible pavement are as follows (Pavement design, n.d.) Dense Bitumen Macadam (DBM) : Contains very low binder content , continuously graded and offers less resistance to the cracking by fatigue. Hot Rolled Asphalt (HRA) : Contains relatively higher binder concentration, gap graded and offers good resistance to fatigue. High Density Macadam (HDM) contained relatively higher proprotion of finer material than dense bitumen macadam . It has highest resistance and resistance to cracking. DBM50 is DBM with a 50 penetration binder. It is marginally inferior to HDM but is superior to ordinary DBM.. Dense Tar Macadam uses tar and is identical to DBM . Though the performance of dense tar macadam is better, it is more sensitive to temperature fluctuation. Descriptions of the design problem (Pavement design, n.d.) Flexible pavement design for two way two lane road for following descriptions Design parameters Design period= 20 years Project reliability= 80% Present year two-way AADT (3500) Directional distribution= 50/50 Percentage of heavy vehicles (10 %) Compound heavy vehicle growth rate= 3% Sub-grade CBR : 3 % The materials chosen for the pavement design have following charecteristics Asphalt : The tensile strain along the horizontal at the bottom of the layer. Asphalt concrete layer: This layer is of 7.5 to 9 cm thick decided on the expected traffic. It consist of mineral aggregates and asphalt (Pavement design, n.d.) Granular materials (Unbound) : Not taken into account in this analysis Cemented : The tensile strain along the horizontal at the bottom of the layer. Subgrade (Selected subgrade) : Compressive strain in vertical direction. Granular base : This layer is prepared using crushed stone, crushed or uncrushed slag, crushed or uncrushed slag, gravel or sand. Sub grade: This layer serves as the foundation of the pavement structure. The layer is well compacted to reduce the voids. Sub base: This layer is immediately located just above the sub grade. The material used for this layer is of a superior quality than the sub grade and shall be stabilized based on the requirement (Pavement design, n.d.) Assumptions in design (Pavement design, n.d.) The pavement system considered is multilayered and elastic All the layers comprising the pavement like base course and Asphalt concrete layer is infinite in horizontal direction. Design of pavement The design ESA : 106 Where, fd is the design lane factor, AADT is the annual average daily traffic, Gjt is the growth factor, Ni number of axles on each vehicle. And EAS is computed as 106 . Subgrade CBR : 3 % Sub grade modulus (MPa) = 10 CBR = 30 MPa From Chart Thickness of Asphalt layer is 120 mm and the unbound granular layer is 200 mm. Trial 1 : Asphalt layer over cemented material Asphalt modulus = 3000 MPa Cement Material modulus = 5000 MPa. Subgrade Modulus = 30MPA Trial 2 : Sub grade modulus varies Asphalt modulus 1000 MPa Trial 3 : Sub grade modulus varies Asphalt modulus 5000 MPa Trial 4 : Sub grade modulus varies Asphalt modulus 3000 MPa References Anonymous (n.d.), flexible pavement design, [Online] Available at [Accessed on 8 June 2009] Gaurav, G., Wojtkiewicz, S., F. and Khazanovich, L., Optimal design of flexible pavements using a framework of DAKOTA and MEPDG, Transporation Research Board [Online] Available at < http://pubsindex.trb.org/document/view/default.asp?lbid=881618 > [Accessed on 8 May 2009] Mathew, T.V. and Rao, K.V.K. ( 2007), Introduction to transportation engineering [Online] Available at [Accessed on 8 June 2009] Seeds, S.B. (n.d.), Flexible pavement design - summary of the state of art [Online] Available at [Accessed on 8 June 2009] Ullidtz, P, Analytical tools for the design of flexible pavements [Online] Available at < http://www.civil.uwaterloo.ca/isap/GRAPHICS/Keynote2002.pdf > [Accessed on 8 June 2009] Pavement design -thickness design (n.d.), [Online] Available at [Accessed on 8 May 2009] Read More