Benefit analysis of subgrade and surface improvements in flexible pavements 2

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Benefit analysis of subgrade and surface improvements in flexible pavements 2

  1. 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME385BENEFIT ANALYSIS OF SUBGRADE AND SURFACEIMPROVEMENTS IN FLEXIBLE PAVEMENTSDr. K.V.Krishna ReddyProfessor & Principal, Chilkur Balaji Institute of Technology, Hyderabad-75, AP, IndiaABSTRACTIn the present study, an attempt is made to highlight the benefit of improvementsmade in the subgrade and surface layers individually and in combination when compared tothe conventional flexible pavement. FPAVE program has been used to evaluate the designthickness of various combination of improvements made at the subgrade and surface layers.The results obtained represent that the strain levels reflected on the top surface and subgradeare low in case of pavement with improvements made at both the subgrade and surface layerswith same pavement thickness and the design thickness required decrease resulting in a costbenefit of 50%Key Words: FPAVE, Subgrade stabilization, Surface course improvements, Cost benefitanalysis, Flexible pavements1. INTRODUCTIONFlexible pavements have the advantage of easy and simple construction, adaptabilityfor stage construction and are best suitable for sustainable development. However, they havethe disadvantages of having shorter life, and require constant maintenance. Among thevarious types of distress in flexible pavements, block / edge / longitudinal / transversecracking, pothole formation, water bleeding, pumping and swelling are load, moisture anddrainage dependent. Fatigue / alligator / edge cracking, shoving, pothole formation,corrugations are load induced apart from the climatic effect. Rutting, bleeding, polishedaggregates, lane to shoulder drop off, corrugation and depression depend essentially on thematerials, load and moisture.Most of the pavement failures initiate in the subgrade and some are attributed to thedeficiencies in the surface course. The other layers of the conventional flexible pavementsbeing aggregate and granular material in the base and subbase course, both of which haveINTERNATIONAL JOURNAL OF CIVIL ENGINEERING ANDTECHNOLOGY (IJCIET)ISSN 0976 – 6308 (Print)ISSN 0976 – 6316(Online)Volume 4, Issue 2, March - April (2013), pp. 385-392© IAEME: www.iaeme.com/ijciet.aspJournal Impact Factor (2013): 5.3277 (Calculated by GISI)www.jifactor.comIJCIET© IAEME
  2. 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME386good material properties and sufficient strength to transfer the loads coming from the toplayers and need no replacement or additives except for strict quality control duringconstruction, thus leaving the subgrade and surface course for modification to obtain thedesired performance.This study involves modification of the surface course material with waste tire rubberand clayey subgrade with pond ash and lime. Evaluate pavement thickness required usingFPAVE program for various combinations of the improvements to understand the benefit ofimprovements.2. RESEARCH METHODOLOGY2.1 Laboratory analysisLaboratory experimentation is done to determine the optimum additives content forstabilizing the clayey subgrade with pond ash and lime and optimum crumb rubber content tomodify the surface course.2.2 Pavement Analysis and DesignAnalysis and design of pavements for full-scale loading has been considered todetermine the pavement thickness. FPAVE, a software program for analysis and design offlexible pavements is used to evaluate the same. FPAVE program analyses for the stressesand strain values at various depths of the pavement section based on the input elastic modulusand Poisson ratio values. For the purpose of design, the program has the input option for theconventional (80/100-b8, 60/70-b6 and 30/40-b3) penetration grade bituminous mixes. In thepresent study the input elastic modulus values have been calculated based on the guidelinesgiven in IRC37 –2001 and b8 is considered for evaluation. The thickness of the pavementshas been evaluated for 10 million standard axel repetitions. The average pavementtemperature has been considered to be 400C.3. DATA ANALYSIS3.1 Subgrade stabilizationInitially it was determined to check for the improvement in the properties of the claysoil by addition of pond ash alone. Pond ash was added at the rate of 15, 20, 25 30 and 35%by weight of soil. IS heavy compaction test has been conducted on three samples of blackcotton soil and that of each modified mix to determine the optimum moisture content (OMC)and maximum dry density (MDD). CBR test was conducted on three samples of each mix atOMC, after curing for 7 days by covering with wet sand followed by 96 hours of soakingunder a surcharge weight of 7.5 kg. The CBR test was carried out up to penetration value of7.5mm, and corrections for initial concavity are made based on the same. CBR of the curedand soaked samples increased with addition of pond ash content up to 25% to a CBR of 10and then decreased on further addition. This may be due to the replacement of clay fines bysilty natured pond ash particles. It also indicated that the calcium oxide present in the pondash was not sufficient to initiate any reaction in the pond ash soil mix.The Atterberg limits and swelling characteristics were determined for the 25% pondash soil mix and the results depicted that the volume stability needs to be attended. It wasproposed to use lime to cater for the volume stability and additional strength needs. HydratedLime at 3, 4 and 5% was added to the 25% pond ash soil mix. Lime was added in terms of
  3. 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME387percentage by weight of clay. The optimum lime content for volume stability was determinedto be 5%. The optimal mix from strength and stability considerations was determined toconstitute 25% of the pond ash and 5% hydrated lime by weight of clay.Table 1 indicates the Atterberg limits, free swell index, swell potential, CBR and 7-dayunconfined compressive strength (UCC) of the optimal mix. The optimal mix resulted in aCBR of 16.34 and a 7day unconfined compressive strength of 310kN/m2.Table 1 Mix, Atterberg limits, swelling and strength characteristics of pond ash / limestabilized Clay soilS.NoPropertyType of MixClayClay +25%PAClay +25%PA +5%lime1Atterberg limitsLiquid Limit (%)Plastic Limit (%)Plasticity IndexShrinkage Limit (%)79.3031.4647.8412.2060.8329.2831.5519.2356.5044.3012.2039.802 Free swell index (%) 110 52.50 303 Swell potential (%) 27.07 11.13 1.204 Soaked CBR (%) 2.65 10 16.605 UCC kN/m261 - 310 (7D)3.2. Bitumen modificationThe conventional bitumen is modified using locally available waste tire rubberpowder (>0.6mm &<1.18mm). 80/100-penetration grade bitumen is considered forexperimentation. Wet process of mixing waste tire rubber was adopted, where in the wastetire is added to the conventional bituminous binder before incorporating the same into thefinal mix. Waste tire rubber is added after heating the bitumen to a temperature of 1630C. Itwas added in various percentages varying from 8 to 14. Laboratory tests, namely,penetration, softening point, ductility and loss on heating test are conducted to determine theproperties of the waste tire rubber modified bitumen before and after loss on heating test.Aggregates with grade II specifications as per MORTH code have been collectedfrom local quarry. Marshall mix design methodology has been considered for mix design ofthe bituminous concrete. Marshall stability value has been considered as the value for judgingthe optimum bitumen / additive content for the mix and the other properties like air voids andflow value are checked for to be within limits. It was found that an optimum bitumen andwaste tire rubber content of 4.3 and 11.8% respectively for conventional and modifiedbituminous concrete yielded the best results. The properties of the conventional and modifiedbituminous concrete are tabulated in Table 2
  4. 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME388Table 2 Properties of conventional and waste tire modified bituminous concrete mixS.No Mix / Property ConventionalWaste tiremodified mix1 Optimum Bitumen / Waste tirecontent4.3%11.8%2 MSV (Kg) 1300 24053 Air voids (%) 3.875 3.4254 Flow value (mm) 2.375 3.7505 Bulk density (g/cc) 2.520 2.4703.2 Pavement Analysis and DesignDifferent trails have been considered, varying the thickness of the bituminous layer(h1) and the other component layers (h2) through trials 1 to 4 for each combination ofimprovements. The combinations being conventional subgrade with conventional surface,stabilized subgrade with conventional surface, conventional subgrade with modified surfaceand stabilized subgrade with modified surface, the layer thickness of the pavements is arrivedat for the various combinations. The tensile strain under the bituminous layer and thecompressive strain on the top of the subgrade are arrived for the optimum design layerthickness. The input parameters along with the corresponding output are tabulated in Table 3to 6Table 3 Summary of input and output parameters for conventional pavement structureInputOutput(mm)Design thickness (mm)Strain values basedon outputTrailnoCBR(%)E3(Mpa)E1(Mpa)H1(mm)H2(mm)E2(Mpa)h1h2BCDBMLBMGr.BaseGr.SubbaseepZ epT1 2.65 26.5 797 50 650 357 285 650 40 110 160 250 400 5.52E-04 4.27E-042 2.65 26.5 797 200 750 391 280 750 40 100 160 250 450 4.78E-04 4.21E-043 2.65 26.5 797 230 800 409 275 800 40 100 155 250 550 4.48E-04 4.22E-044 2.65 26.5 797 175 900 441 265 900 40 100 145 250 650 3.95E-04 4.28E-04h1: Thickness of bituminous layer h2: Thickness of granular base and subbaseE1: E of Bituminous material E2: E of granular base and subbaseE3: Elastic modulus(E) of Subgrade BC: Bituminous concreteDBM: Dense Bituminous Macadam LBM: Lean Bituminous MacadamGr. Granular epZ: Vertical strain on the top of the subgradeepT: Tangential strain at the bottom of the bituminous layer
  5. 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME389Table 4 Summary of input and output parameters for pavement with Pond ash andLime modified subgrade with Conventional surfaceInputOutput(mm)Design thickness(mm)Strain values basedon output designthicknessSnoCBR(%)E3(Mpa)E1(Mpa)H1(mm)H2(mm)E2(Mpa)h1h2BCDBMGr.BaseGr.SubbaseepZ epT1 16 104 797 50 300 271 195 300 50 165 250 150 5.94E-03 3.81E-042 16 104 797 100 400 308 110 400 50 80 250 150 5.96E-04 4.24E-043 16 104 797 60 450 325 65 450 40 40 250 200 5.94E-04 3.25E-044 16 104 797 50 450 325 65 450 40 40 250 200 5.94E-04 3.25E-04Table5 Summary of input and output parameters for pavement with conventionalsubgrade and modified surfaceInputOutput(mm)Design thickness(mm)Strain values basedon output designthicknessSnoCBR(%)E3(Mpa)E1(Mpa)H1(mm)H2(mm)E2(Mpa)h1h2BCDBMGr.BaseGr.SubbaseepZ epT1 2.65 26.5 1200 50 850 110 247 850 40 100 400 550 4.44E-04 3.72E-042 2.56 26.5 1200 80 750 104 254 750 40 100 350 550 5.03E-04 3.71E-043 2.65 26.5 1200 40 900 113 30 900 40 100 250 550 7.52E-04 4.43E-044 2.65 26.5 1200 60 800 107 250 800 40 100 360 550 4.73E-04 3.72E-04
  6. 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME390Table6 Summary of input and output parameters for pavement with stabilizedsubgrade and modified surfaceInputOutput(mm)Design thickness(mm)Strain values basedon output designthicknessSnoCBR(%)E3(Mpa)E1(Mpa)H1(mm)H2(mm)E2(Mpa)h1h2BCDBMGr.BaseGr.SubbaseepZ epT1 16 104 1200 40 360 294 41 360 40 0 250 200 8.77E-04 1.98E-042 16 104 1200 50 350 290 50 350 60 0 250 200 8.73E-04 2.89E-043 16 104 1200 60 340 286 61 340 60 0 250 200 8.69E-04 3.52E-044 16 104 1200 65 340 286 60 340 40 20 250 200 8.50E-04 3.72E-044. RESULTS4.1 Strain analysisThe vertical strain on the top of the subgrade is evaluated for a constant thickness ofthe pavement irrespective of the improvements made at subgrade and surface course. Thepavement thickness provided for the conventional test track section is considered for allcombinations to evaluate at the benefit in life based on the vertical strain at the top of thesubgrade and shear strain at the bottom of the bituminous layer. Table 7 depicts the strainvalues at the top of the subgrade and the number of standard load repetitions the pavementcan take. It is found that the strain is least in case of test track section improved at bothsubgrade and surface level compared to the other sections resulting in high life benefit.Table 7 Summary of Strains and load repetitions for a standard thickness despiteimprovements madeSectionType of pavementStrain on top ofsubgrade (epZ)TangentialStrain at thebottom of thebituminoussurface (epT)No. of Loadrepetitionsbased onfatiguecriteriaNo. of Loadrepetitionsbased onruttingcriteria1 Conventional 5.48E-04 5.80E-04 3E6 25E62 Subgrade stabilised 1.40E-04 1.19E-04 144E6 12359E63 Surface modified 5.39E-04 6.10E-04 1.7E6 27E64Subgrade and surfaceimproved0.80E-04 1.29E-04745E6 156260E6
  7. 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME3914.2 Cost AnalysisThe cost of the pavement for two lanes has been worked out based on the ratesobtained from Roads and Buildings Department. The layer thickness of the pavements forvarious combinations considered along with the cost is tabulated in Table 8. The resultsindicate that, for the same design life of the pavements, there is substantial reduction in thepavement thickness requirement and the cost of pavement construction due to modifications.Table 8 Pavement layer thickness and Cost of two lane Pavement for Km lengthS.NoPavement typeBC /CRMDBMLBMGranularBaseGr.SubbaseCost(Lakhs)1 Conventional 40 100 155 250 550 117.02 Surface modified 40 100 135 250 500 162.03 Subgrade Stabilised 65 40 0 250 200 65.04Subgrade and SurfaceImproved40 20 0 250 200 58.05. ACKNOWLEDGEMENTAt the outset the author would thank the Head, CED and TE division, and otherprofessors at NIT Warangal for their valuable guidance and encouragement duringexperimentation.6. CONCLUSION1) Clayey soils in particular can be effectively stabilized adding 25% Pond ash + 5%lime by weight of soil to have a CBR of 15.2) The strain levels of the subgrade stabilized soils indicate that the number of loadrepetition the pavement can take with the same thickness as that of conventionalpavements is 48 times.3) Analysis and design using FPAVE indicated that stabilization of subgrade aloneresults in a pavement structure that can be formed with a cost of 55% of that requiredfor construction of conventional pavement4) Use of bitumen modified with 12% of tire rubber in formation of bituminous concretealone without stabilizing subgrade has not resulted in much improvement both interms of cost and number of load repetitions when compared to conventionalpavement structure.5) Life benefit in pavements is pronounced when the poor subgrades are improved alongwith the surface course. Life benefit by modifying both subgrade and surface coursesis 200 times in terms of the number of load repetitions for the same pavementthickness as that of conventional pavement.
  8. 8. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME3926) Improving both subgrade and surface course results in a pavement structure that canbe formed with a mere 50% amount of that required to form conventional pavementsection.7) The key to effective performance of flexible pavements is to understand the causes offailures and the action needed for correction. Sound judgment should be used todetermine which part of the existing pavement structure is weak and if the subgrade isaffected, total reconstruction of the pavement or full depth reclamation should beconsidered for best performance rather than improving the surface alone.REFERENCES1. Albritton, G.E., and Gatlin, G.R. (1996), “Construction and Testing of crumb RubberModified Hot Mix Asphalt Pavement”, Rep. No. FHWA/MS-DOT-RD-96-115,Washington D.C.2. Bhasin N.K., Dhawan, P.K., and Mehta, H.S. (1978), “Lime requirement in soilstabilisation”, Road Research Papers, Rep. no.149, CRRI, India.3. Chaturvedi, A.C. (1977), “Expansive Soil in India with special reference to U.P.”, Proc. ofFirst National Symposium on Expansive Soils, HBTI- Kanpur, India, pp 2-1 to 2-5.4. Chopra, S.K., Reshi, S.S., and Garg, S.K. (1964), “ Use of Fly Ash as a Pozzolana”, Proc.Symposium on Pozzolan, their survey, Manufacture and Utilizat5ion, CRRI, India, p.18.5. Chu, S. C., and Kao, H. S. (1993), “A Study of Engineering Properties of a Clay Modifiedby Flyash and Slag”, Flyash for Soil Improvement Geotechnical Special Publication, Vol.36, pp 89-99.6. Chu, T.Y. (1955), “Soil Stabilisation with Lime Fly Ash mixture, Preliminary studies withSilty and Clayey Soils”, HRB, No.108, p.102.7. Collins, R. J., and Ciesielski, S. K. (1992), “Highway Construction use of wastes and By-products” Utilization of Waste Materials in Civil Engineering Construction, Published byASCE, New York, pp.140-1528. Daly, W.H., and Negulescu (1997), “Characterization of Asphalt Cements Modified withcrumb Rubber from Discarded Tires” TRR-1583, TRB, pp 37-44.9. Durga Prasad, K. (2002), “A study of lime and flyash on the performance of bituminousconcrete mix”, Mtech thesis, NITW.10. Eades, J.L., and Grim, R.E. (1960), “Reaction of Hydrated Lime with Pure Clay Minerals inSoil Stabilisation”, HRB, No.262, pp.51-63.11. FPAVE, “Software program for Analysis and Design of Flexible Pavements”,Transportation Engineering Section, Civil Engineering Department, IIT, Kharagpur.12. IRC 37 - 2001: Guidelines for Design of Flexible Pavements.13. IRC : SP : 53 - 2002: Guidelines on Use of Polymer and Rubber Modified Bitumen in RoadConstruction.14. IS 2720 (part 16) - 1979, “Methods of Test for Soils; Laboratory Determination of CBR”,Bureau of Indian Standards, New Delhi.15. Wason, O.P., and Bhattnagar, O.P. (1980), “Economizing Rural road Construction cost”,Indian roads congress journal.16. Yoder, EJ and Witczac, M.W (1975), “Principles of Pavement Design”, 2ndEdition, JohnWiley & Sons.17. Ravin M. Tailor, Prof. M. D. Desai and Prof. N. C. Shah, “Performance observations forGeotextile Reinforced Flexible Pavement on Swelling Subgrade: A Case of Surat, India”,International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2,2012, pp. 347 - 352, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

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