- QUALITATIVE COMPARISION - ANALYSES OF SEMI-RIGID PAVEMENT DESIGN METHODS AND A PROPOSAL FOR VIETNAM’S. - Abstract: Semi-rigid pavements (mặt đường nửa cứng) will have shown the potential for Vietnam’s nationwide application, especially the country’s new construction of freeways and expressways (đường cao tốc) having got high traffic volumes and heavy and large trucks and currently Portland cement concrete (PCC) highways’. - Therefore, the author of this article has been using literature review method and engineering knowledge and experience to identify the nation’s definition of semi-rigid pavement, and qualitatively summarize, analyze and compare semi-rigid pavement design methods employed in the country and some developed ones in the world. - Resulted from this preliminary study, the author might have been able to propose adjustable and more accurate semi-rigid pavement design method in Vietnam’s circumstances.. - Keywords: semi-rigid pavement, design, method 1. - shows that semi-rigid pavements, which encompass AC surface layer(s) on rigid base layer being possibly cement-treated base layer, PCC one, lean PCC one, or steel- reinforced PCC one, are the most worldwide used. - Therefore, composite pavements are hereafter studied within this article when they include rigid base layer(s) which is the main load-bearing one under flexible layer(s) which is friction one as shown through typical examples in Figure 1.. - Figure 1: Typical cross-sections of semi-rigid pavements [3].. - Semi-rigid pavements had been widely applied into highways serving high traffic volumes (greater than 50 million design axles) including a great number of heavy vehicles or having their cycle lifespan which needs increasing but their overlay replacement is minimized for rehabilitation (Nunn, Jofre and Fernandez, 2004).. - Especially, simi-rigid pavement construction costs will be the most reasonable when the structures are applied into highways with high traffic volumes (Nunn et al., 1997 and Nunn, 2004). - In accordance with [1], there had been approximately 155 thousand kilometers of semi-rigid pavements with asphalt surface layer on PCC base layer constructed in America until 2001. - Semi-rigid pavements were used in early 1980s in Europe, which are accounted for 30% to 50% pavement structures of European highway system (Thogersen et al., 2004). - However, [1] reveals that semi-rigid pavements had been majorly applied into PCC pavement maintenance and rehabilitation and rarely used in new construction. - In Vietnam, semi-rigid pavements were used on Northern Thang Long - Noi Bai Road Segment, at Can Tho Airport, at Cat Bi Airport in Hai Phong, within bidding packet No. - Pursuant to the researches of Donald in 2003, Jofre and Fernandez in 2004 and Nunn in 2004, semi-rigid pavements have got several merits as the follows:. - The base layer(s) is stiff and strong, which make stresses in base layer(s) and the subgrade decreased because of cement;. - Not only structural but also functional Level of Performance Index of semi- rigid pavements is higher than the one of conventional pavements including flexible pavements and rigid ones;. - Semi-rigid pavements provide drivers with comfort, smooth and quiet driving surface;. - Flexible surface layer(s) comprises adequate friction properties;. - Flexible surface layer(s) may prevent surface water from penetrating into rigid layer(s). - Temperature gradient in rigid base layer(s) may go down due to the upper flexible surface layer(s).. - The greatest merit of semi-rigid pavements in accordance with Donald’s research in 2003 is that critical tensile strain which cause fatigue crackings, specifically bending fatigue, moves from the bottom of lower asphalt concrete layer of flexible pavements to the bottom of lower rigid base layer of semi-rigid pavements because of the presence of cement in rigid base layer(s). - However, [3] summarized through real application that semi-rigid pavements may be primarily deteriorated because of fatigue, wheel-tracking, and reflective cracking. - Among them, reflective crackings are the most popular failure, which are crackings appearing in asphalt surface layer(s) due to horizontal movements and vertical ones at the joints, periodical temperature gradient cracking development from rigid base, and/or vehicular loading impacts. - reduces the strengths of base and subgrade materials and makes material of asphalt surface layer(s) surrounding the crakings separated from each other. - Reflective crackings with remarkable width will become pavement joints, which cause stress concentration phenomenon in semi-rigid pavements [3]. - Reflective cracking failure can be eliminated by increasing the thickness of flexible surface layer, or installing aggregate base layer or geo-fabric material on rigid subbase layer but under flexible surface one [1].. - SEMI-RIGID PAVEMENT DESIGN METHODS. - Until now, a number of methods have been used by different institutions to design semi-rigid pavements. - Before 1960, almost all the institutions determined the thickness of flexible surface layer when basing on professional experience and assessment. - Since 1960, employing non-demolishing tests to measure pavements’ deflection has been being acceptable and more and more semi-rigid pavement design methods (SRPDM) based on pavement deflection measurements have been developed. - [12] can be used to design semi-rigid pavements in two situations: (I) flexible surface layer on cement-treated base layer and (II) AC surface layer on PCC base layer being possibly jointed PCC or continuously steel-reinforced PCC layer.. - The key performance of this SRPDM when flexible surface layer placed on cement-treated base layer is to choose layer coefficient a 2 which should be fit for determination of structural factor SN:. - where a 1 , a 2 and a 3 are layer coefficients, m 2 and m 3 are drainage coefficients, and D 1 , D 2 and D 3 are the thicknesses of pavement layers from the top to the bottom of pavement structure including flexible surface, rigid base and the subgrade, respectively. - In the second alternative when AC surface layer is on PCC base layer, [12] guide designers to apply the formulas (4) to (6):. - When semi-rigid pavement structures are newly constructed, these factors are all valued at 1 because both rigid base and flexible surface layers have not been deteriorated. - In 2004, US Department of Defence had developed a Pavement Design Manual for Roads, Streets, and Open Storage Areas when basing on asphalt’s deformation standard:. - AC = allowable strain at the bottom of asphalt layer. - The Highways Agency in the UK published two output parameters to design semi- rigid pavements. - The first one which is rigid base thickness is obtained by using a nomograph when resulted from minimum compressive strength at 7 th -old-day of base material and the foundation stiffness (modulus of resilience). - The Institute’s research is focused on fatigue failure of the rigid base, in which a parameter showing the failure is tensile strain at the bottom of rigid base. - The design standard resulted from formula (12) should be used to prevent semi-rigid pavement structures from fatigue failure at 75 percent reliability:. - In pursuant to [2], the minimum lifespan of semi-rigid pavements is 50 years under conditions when the overlay must be renewed periodically during their operation process to secure pavement friction level and driving qualities. - Thus, flexible surface layer, rigid base layer and the subgrade are computed by using formular (13) to (15) so that horizontal tensile strains within flexible overlay and rigid base could be always less than 33. - 1.1 = 12.0 and = 1.6.. - Pursuing this method, semi-rigid pavement structures must be design to make sure that fatigue cracking phenomenon will not happen within rigid base layer by controlling maximum tensile strain at this layer not greater than 36. - Among input data of rigid base layer design in accordance with US mechanistic- empirical SRPDM, the maximum dry unit weight is obtained through standard Proctor test, the optimum moisture content is determined according to ASTM D698, the resilient modulus in compression is attained by implementing AASHTO T307 test, and. - M rt = resilient modulus in tension of rigid base layer;. - Output data of US mechanistic-empirical SRPDM are maximum tensile strain at the bottom of rigid base (19) and fatigue life of AC (20). - The fatigue life of asphalt surface layer which expresses overall pavement performance efficiency is evaluated by using a fatigue cracking model.. - t = tensile strain at bottom of rigid base layer;. - Thickness calculation of PCC base layer below AC overlay mentioned in [6] is the first guidance in semi-rigid pavement design of Vietnam. - Among methods presented in this article, 2.1(I) is empirical method using linear multi-variable regression function applied into the situations when flexible surface layer is on cement-treated base one and E c and a 2 are repetitively computed via linear univariable regression function. - 2.1(II) is an effective thickness method when output data of pavement structures including AC overlay on PCC base layer are repetitively computed via linear multi-variable regression function. - 2.2 had been developed in order to secure fatigue resistance of flexible surface layer by establishing non-linear multi-variable functions and its calculations must be repetitive. - 2.4 is an empirical approach when rigid base layer is determined in order to secure fatigue resistance of flexible overlay. - Danish Road Institute (2.5) did the research on semi-rigid pavement behavior and developed a mechanistic semi-rigid pavement design method by conducting a full-scale test on six real highway segments (every two segments had a different pavement structure) and choosing a generalized incremental-recursive model based on tensile strain at the bottom of the cement-treated base layer in order to verify the pavement deterioration model (Thogersen et al., 2004). - Moreover, comparison between the research results and operational conditions of constructed semi-rigid pavements that had been in service for more than 20 years show that the proposed deterioration model was accurate. - According 2.6 method, semi-rigid pavements are proposed through mechanistic analyses to make sure that fatigue crackings will not occur in flexible overlay (13), rigid base layer will always work in pre-cracking stage during design span (14), and permanent deformation will not happen within the subgrade (15). - In 2.6 contents, wheel-tracking which is one of primary deterioration modes of semi-rigid pavements was mentioned but not modelled through mechanistic analyses. - In accordance with 2.7 guidance, semi-rigid pavement behavior being demonstrated via strain and stress. - The basis of empirical semi-rigid pavement design methods is tests measuring factual working strengths of pavements and monitoring data base during the pavements’ operational process. - Mechanistic-empirical semi-rigid pavement design approach also includes behavioral calculation of the pavements under vehicular impacts causing pavements’ failure and accumulated fatigue deterioration, specifically calculation of strains and stresses causing structural working capacity decline with respect to time. - On mechanistic-empirical approach (2.8), elastic multi-layer theory was employed to attain structural behavior of semi-rigid pavements expressing via stresses, strains and deflection. - According to 2.8, semi-rigid pavements are considered as elastic multi-layer system with assumptions that all their layers are linear elastics below circular surface loading distribution area and vehicular loading is analyzed in axis-symmetric space causing stresses, strains and vertical deflections at specified locations within pavement structures. - Within this design approach, working properties of semi-rigid pavements are assumed to be taken as default values corresponding to a chosen design level of flexible pavements.. - Being compared with empirical method, specifically method of [12], mechanistic- empirical allows using actual load distributions so semi-rigid pavement behavior is assessed under wheel load of 40 kN on the surface layer. - ratio, Atterberg limits, maximum unit dry weight, optimal moisture content, failure modulus and unit weight must be also determined as input data of rigid base calculation. - The current SRPDM of Vietnam (2.9) is effective thickness method in which rigid base layer thickness is calculated with respect to non-linear relation with elastic modulus and flexible surface layer thickness is determined through professional experience of designers. - Pursuant to 2.9, however, generalized elastic modulus on the surface of rigid base layer is determined by referring to the nomograph established for dual-layer system of flexible pavements regulated in [7] and this nomograph is kept being used in [8]. - This regulation is not conformable because factual working characteristics of semi-rigid pavements are different from flexible pavements’ ones. - In 2001, semi-rigid pavement design was mentioned in Vietnam’s [11] but design proposals within these specifications belong to American Association of State Highway and Transportation Officials while empirical data of semi-rigid pavements in Vietnam are still restricted. - In 2008, [3] carried out semi-rigid pavement calculations and designs by using methods in 2.1 to 2.5 with the same input data set to obtain an output data set as shown in Figure 2. - Via statistical evidence in Figure 2, it is visual to reveal that methods 2.3 and 2.4 have got similar results, methods 2.2 and 2.5 give lowest thickness of AC layer, and only method 2.5 propose an aggregate base layer.. - Figure 2: Calculation and design results of semi-rigid pavements pursuant to [3]. - Semi-rigid pavements include flexible overlay on rigid base layer(s) have been being used worldwide but their mechanistic analyses are the most difficult because they involves in two different material types. - However, it has been very difficult to model the lower layer because of this rigid base layer’s presence of crackings. - [1] supposes that it is likely to apply slab theory into stress determination within bending slabs under an assumption that AC surface layer adhered to rigid base layer which is the mainly loading resistance component. - On the other hand, semi-rigid pavements will be able to be analyzed by applying elastic layerd theory if stress adjustment factors are determined when loading position is at the slab edge and corner. - However, semi-rigid pavements will be suitable for new freeway, expressway, runway and taxiway construction and widely appropriate for existing PCC highways in Vietnam owing to the combination of different merits of their flexible overlay and rigid base layer. - But semi-rigid pavement should be researched when pursuing mechanistic-empirical approach to apply effectively into Vietnam because of various climate, material, economic and technological conditions of different countries and. - del Val (2016): Guidelines for the design of semi-rigid long-life pavements, International Journal of Pavement Research and Technology 9.. - Musharraf Zaman (2017): Design of semi-rigid type of flexible pavement, International Journal of Pavement Research and Technology 10.. - Vietnam’s Ministry of Transport (1995): 22TCN 223-95 Rigid highway pavement design standards.. - Vietnam’s Ministry of Transport (1993): 22TCN 211-93 Specifications for flexible pavement design.. - Vietnam’s Ministry of Transport (2006): 22TCN 211-06 Standards and Specifications for flexible pavement design.. - Trần Thị Thuý (2017): Some noticeable issues of rigid and semi-rigid base utilization in highway and airport pavement structures in Vietnam, Journal of Transportation 07/2017.. - Lưu Ngọc Lâm (2013): Semi-rigid pavement structures and their applicable orientation in Vietnam, Journal of Transportation 09/2013.. - Vietnam’s Ministry of Transport (2001): 22TCN 274-01 Specifications for flexible pavement design.
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