structural design of pavement thickness

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Highway Engineering

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STRUCTURAL DESIGN OF PAVEMENT THICKNESSIntroduction - One of the basic requirements for a pavement is that it should be of sufficient thickness to spread the surface loading to a pressure intensity that the underlying sub-grade is able to withstand, with the pavement itself sufficiently strong to deal with the stresses incident on it. Thickness is thus a central factor in the pavement design process. (In order to reflect European harmonization, the names of the various pavement layers have been altered within the context of thickness design, as seen in the figure).

Flexible pavements The pavement should be neither too thick nor too thin. If it is too thick, the cost will become excessive. If it is too thin, it will fail to protect the underlying unbound layers, causing rutting at formation level. A flexible pavement is defined as one where the surface course, binder course and base materials are bitumen bound. Permitted materials include hot rolled asphalt (HRA), high density macadam (HDM), dense bitumen macadam (DBM) and dense bitumen macadam with 50-penetration bitumen (DBM50). Flexible composite pavements involve surface course and upper base materials bound with macadam built on a lower base of cement bound material (CBM). Wearing courses are either 45mm or 50mm of hot rolled asphalt or 50mm of porous asphalt (PA). (If PA is used, it is assumed to contribute only 20mm to the overall thickness of the pavement for design purposes.) The bitumen within dense bitumen macadam road bases and base courses must be at least 100 penetration grade, with hot-rolled asphalt containing 50 pen binder. Road Note 29 Pavement thickness design methods have historically been empirically based, with the performance of pavements being analyzed and design charts being compiled based on the information obtained from the on-site observations of researchers. This approach led to the publication of Road Note 29 (Department of the Environment, 1973). This document was based on scrutiny during the 1960s of the behaviour of sections of highway pavement in UK trafficked by up to 10 million standard axles. It formed the basis for pavement design philosophy in the UK from then until the mid 1980s. Road Note 29 (RN29) takes account of increasing axle loads and vehicle numbers while also differentiating between the performance characteristics of different road base materials. (The road base is assumed to satisfy the entire strength requirements for the entire pavement, with the surfacing considered to make no significant contribution to the strength of the pavement. The primary function of the surface material is to provide surface texture and regularity.) The RN29 procedure is best explained as a series of design steps.

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Step 1 - Determine the cumulative number of commercial vehicles expected to use the highway from its first day of use to the end of its design life, taken as 20 years. Step 2 - Determine the cumulative number of commercial vehicles expected to use the design lane over its design lifetime. (The design lane is the most heavily trafficked lane in any given direction.) Step 3 - Determine the equivalent number of standard axles incident on the road over its design life, based on the commercial vehicle usage. Based on a standard axle of 80kN, the required value is obtained from the product of the cumulative number of commercial vehicles and a term called the damage factor which varies for different road types. The maximum value of this conversion factor is 1.08, used for motorways and trunk roads designed to cater for over 1000 commercial vehicles per day in each direction. For a motorway: Equivalent No. of standard axles = No. of commercial vehicles x 1.08 Step 4 - Determine the sub base thickness. This is dependent on both the CBR of the sub grade and the cumulative number of standard axles over the design life of the highway. For a cumulative number of standard axles of 1 million (1 msa), a minimum sub base thickness of 150 mm is required where the CBR is greater than 6%, rising to 440mm where the CBR is 2%. Where the CBR is less than 2%, an additional 150mm of sub base should be added to that required for a CBR of 2%. The CBR of the sub base should be at least 30%. If the CBR of the sub grade is in excess of this value, no sub base need be used. Step 5 - Determine the road base and surfacing thickness. This parameter depends purely on the cumulative number of standard axles over the pavements design life. For cumulative standard axles in excess of 10 million, the surfacing should be 100 mm thick (60 mm base course plus 40mm wearing course). If dense bitumen macadam is used, a road base thickness of just under 150 mm is required to cater for 10 msa, giving a total bound thickness of 250mm. LR1132 Road Note 29 was the sole officially recognized pavement design methodology throughout the 1970s and early 1980s. While it was considered to be generally effective, it had certain inherent deficiencies: It was seen as unresponsive both to improvements in the quality of available raw materials and to changes in construction processes. The RN29 method is valid for designs up to 40 msa. Many highways were, by the mid 1980s, well in excess of 50 msa, with some approaching 150msa over their 20-year design life. The 20-year design life implied that, after this period, a surface rut of 20mm or more, or severe cracking or crazing had developed. The pavement was then considered to be in a failed state and in need of major strengthening or partial reconstruction. It has been shown that attempting to strengthen a pavement damaged to such an extent did not automatically result in satisfactory performance afterwards.

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LR1132 revised RN29 by redefining pavement failure, thereby delivering a thicker but longer lasting highway likely to be in a less deteriorated state after 20 years. The design criteria adopted by LR1132 were: (1) The sub grade must be able to sustain traffic loading without excessively deforming. This is achieved by limiting the vertical stress at formation level. (2) Bituminous or cement bound materials used in the flexible pavement must not be subject to fatigue cracking. This is achieved by limiting the horizontal tensile stresses at the bottom of the bituminous/cement bound road base. (3) The load spreading capability of granular sub bases should be enough to provide an acceptable construction platform. (4) When a pavement is composed of a considerable depth of bituminous material, its creep must be restricted in order to stop the rutting which arises from internal deformation. Some of the stresses referred to above are illustrated in the figure..

In contrast with RN29, where the failure condition was presented as a 20mm rut with severe cracking / crazing, LR1132 defined the end of a pavements design life as indicated by a 10 mm rut depth or the beginning of cracking in the wheel paths. These less severe indications were chosen on the basis that they are the precursors of significant structural deterioration. They mark the latest time when the application of an overlay will have maximum effect and will be expected to make best use of the original structural quality of the pavement. In other words, the design life as thus defined is the latest time at which the application of an overlay will deliver another few years of high quality motoring. This is termed pre-emptive overlaying, a process carried out at the onset of critical structural conditions within the pavement. If application is postponed to a point later in the pavements life, it may well have deteriorated to a stage where extensive pavement reconstruction will be required. 3

Since the LR1132 approach maximizes the use of the existing pavements strength, a pavement of more uniform strength will result. In addition, as deterioration can be predicted without too much difficulty, ultimate reconstruction can be more easily planned. This definition of design life results in LR1132 designing a pavement having an additional period of serviceable life before major reconstruction, a period that would not be available if Road Note 29 were used. A design life of 20 years is normally employed. Given the adoption of the design life concept as detailed within LR1132, the cumulative number of equivalent 80 kN standard axles to be carried during the design life of the highway must now be estimated. Observed or estimated 24 hour commercial vehicle flows must be converted to annual flows. If there is more than one lane in each direction, an allowance must be made for the proportion of this traffic travelling in the nearside lane, assumed to be the lane carrying the majority of commercial vehicles (RN29 makes this same assumption). The annual traffic is then multiplied by the vehicle damage factor an estimator of the damage effect of an average commercial vehicle. The design procedure can be summarized as follows. Step 1 - Calculate Tn, the total number of commercial vehicles using the slow lane over the n years design life, as follows:

where F0 = initial daily flow (base year), r = commercial vehicle growth rate n = design life

P = proportion of commercial vehicles using the slow (nearside) lane P = 1 if it is assumed that all vehicles use the nearside lane. Step 2 - Calculate the damage factor, D. In order to convert Tn into equivalent standard axles, it must be multiplied by the vehicle damage factor, D, calculated for the mid year of the design life, Fm. The damage factor is calculated as follows:

where Fm = number of commercial vehicles per day in one direction (mid-term year) t = mid-term year minus 1945.

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Step 3 - Calculation of N, the cumulative number of standard axles N = Tn x D Sub grade strength The CBR test is taken as a direct measure of the strength of the in-situ sub grade material. Despite concerns regarding the limited accuracy of this test, it is utilized on the basis

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