• Croney (1977) • Design is the process of developing the most economical combination of pavement layers in relation to both thickness and type of wheel load to suit to the soil foundation and the cumulative traffic to be carried during the design life.

Design of Highway Pavements • • • •

Types of pavements 1) Flexible pavements. 2) Rigid pavements. 3) Semi rigid pavements.

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Design factors for pavements. 1) Design wheel load. 2) Subgrade soil. 3) Climatic factor. 4) Pavement component material. 5) Environmental factors.

Flexible pavement design methods • 1) Group Index (G.I) method. • 2) CBR method. • 3) California R-value or Stabilometer method. • 4) Mc-Leod method. • 5) Tri axial test method. • 6) Burmister method.

IRC–37-2001

1.INTRODUCTION • THE DESIGN OF FLEXIBLE PAVEMENT INVOLVES THE INTERPLAY OF SEVERAL VARIABLES,SUCH AS,THE WHEEL LOADS,TRAFFIC CLIMATE,TERRAIN AND SUBGRADE CONDITIONS. • THE IRC FIRST BROUGHT GUIDELINES IN 1970,THESE WERE BASED ON CALIFORNIA BEARING RATIO METHOD. • TO HANDLE LARGE SPECTRUM OF AXLE LOAD,THESE GUIDELINES WERE REVISED IN 1984.

• THE EQUIVALENT AXLE LOAD CONCEPT WAS USED IN THIS APPROACH,PAVEMENT THICKNESS WAS RELATED TO THE CUMULATIVE NUMBER OF STANDARD AXLES TO BE CARRIED OUT FOR DIFFERENT SUBGRADE STRENGTHS. • DESIGN CURVES WERE DEVELOPED TO CATER UPTO 30 MILLION STANDARD AXLES. • WITH THE RAPID GROWTH OF TRAFFIC,THE PAVEMENTS ARE REQUIRED TO BE DESIGNED FOR HEAVY VOLUME OF TRAFFICOF THE ORDER OF 150 MILLION STANDARD AXLES.

• ON THE BASIS OF MATHEMATICAL MODELLING OF THE PAVEMENT STRUCTURE USING MULTIPLE LAYER ELASTIC THEORY AND ON THE PERFORMANCE OF THE EXISTING DESIGNS,’GUIDELINES FOR THE DESIGN OF FLEXIBLE PAVEMENT’ WAS FINALLY REVISED IN 2001.

2.SCOPE

• THESE GUIDELINES ARE APPLICABLE TO EXPRESSWAYS,NATIONAL HIGHWAYS,STATE HIGHWAYS,MAJOR DISTRICT ROADS AND ROADS CARRYING MOTOTISED VEHICLES. • FLEXIBLE PAVEMENTS ARE THOSE WHICH HAVE BITUMINOUS SURFACING AND GRANULAR BASE AND SUBBASE COURSES CONFIRMING TO MORTH SPECIFICATIONS SECTION 400 .(SUB BASES,BASES,NON BITUMINOUS AND SHOULDERS.) • SECTION 500 BASES AND SURFACE COURSES BITUMINOUS.

• THESE GUIDELINES APPLY TO NEW PAVEMENTS. • FOR DESIGN OF STRENGTHENING MEASURES OR OVERLAY FOR EXISTING PAVEMENTS IRC 81 “TENTATIVE GUIDELINES FOR STRENGTHENING OF FLEXIBLE ROAD PAVEMENTS USING BENKELMEN BEAM DEFLECTION TCCHINIQUE” • FOR TIME TO TIME REVISION OF THIS CODE IT IS SUGGESTED THAT ALL THE ORGANISATIONS INTENDING TO USE THE GUIDELINES,SHOULD KEEP A DETAIL RECORD OF THE YEAR OF CONCTRUCTION ,SUBGRADE CBR,SOIL CHARACTERISTICS,TRAFFIC,PAVEMENT PERFORMANCE,OVERLAY HISTORY,CLIMATIC CONDITIONS,ETC.

3.RECOMMENDED METHOD OF DESIGN. • PAVEMENT DESIGN GIVEN IN IRC 37 1984 WERE APPLICABLE TO DESIGN TRAFFIC UPTO 30 MSA. • NEW SET OF DESIGN ARE TO CATER TRAFFIC UPTO 150 MSA. • DESIGN APPROACH AND CRITERIA. • FLEXIBLE PAVEMENT HAS BEEN MODELLED AS A THREE LAYER STRUCTURE STRESSES AND STRAINS AT CRITICAL LOCATIONS (ANNEXURE 1) HAVE BEEN COMPUTED USING THE LINEAR ELASTIC MODEL FPAVE DEVELOPED UNDER THE MORT&H RESEARCH SCHEME R-56 “ANALYTICAL DESIGN OF FLEXIBLE PAVEMENT”.

ANNEXURE-1 CRITICAL LOCATION IN PAVEMENT.

• TO GIVE PROPER CONSIDERATION TO THE ASPECT OF PERFORMANCE,THE FOLLOWING THREE TYPES OF PAVEMENT DISTRESS RESULTING FROM REPEATED APPLICATIONS OF TRAFFIC LAODS ARE CONSIDERED : • 1) VERTICAL COMPRESSIVE STRAIN AT THE TOP OF THE SUBGRADE.IF THE STRAIN IS EXCESSIVE,THE SUBGRADE WILL DEFORM RESULTING IN PERMANENT DEFORMATION AT THE PAVEMENT SURFACE DURING THE DESIGN LIFE.

• 2) HORIZONTAL TENSILE STRAIN AT THE BOTTOM OF THE BITUMINOUS LAYER .LARGE TENSILE STRAIN S CAUSE FRACTURE OF THE BITUMINOUS LAYER DURING THE DESIGN LIFE. • 3) PAVEMENT DEFORMATION WITHIN THE BITUMINOUS LAYER.

• THE PERMANENT DEFORMATION WITHIN THE BITUMINOUS LAYER CAN BE CONTROLLED BY MEETING THE MIX DESIGN REQUIREMENTS AS PER THE MORT&H SPECIFICATIONS,THICKNESS OF THE GRANULAR AND BITUMINOUS LAYER ARE SELECTED USING THE ANALYTICAL DESIGN APPROACH SO THAT STRAINS AT CRITICAL POINTS ARE WITHIN THE ALLOWABLE LIMITS.

• FOR CALCULATING THE TENSILE STRAINS AT THE BOTTOM OF BITUMINOUS LAYER AND THE RELATIONSHIP FOR ASSESSING THE ELASTIC MODULI OF SUBGRADE,GRANULAR SUB-BASE AND BASE LAYERS ARE GIVEN IN ANNEXURE-1.

• THE PAVEMENT DESIGNS ARE GIVEN FOR SUBGRADE CBR VALUES RANGING FROM 2% TO 10% AND DESIGN TRAFFIC RANGING FROM 1 MSA TO 150 MSA for an annual pavement temperature of 35c.

• For design purposes • 1) design traffic in of cumulative number of standard axles(8160kg);and. • 2) CBR value of subgrade. • Is required. • For estimating design traffic following information is needed: • 1) initial traffic after construction in of CVPD. • 2) traffic growth rate during the design life. • 3) design life in number of years.

• 4) vehicle damage factor (VDF). • 5) distribution of commercial traffic over the carriageway. • For the purpose of structural design ,only the number of commercial vehicles of gross weight of 3 tonnes or more and their axle loading is considered. • To obtain a realistic estimate of design traffic,due consideration should be given to to the existing traffic or that anticipated based on possible changes in the road network and land use of the area served,the probable growth of traffic and design life.

• Estimate of the initial daily average traffic flow for any road should normally based on atleast 7 days 24 hour classified traffic counts. • Traffic estimate can be made on the basis of potential land use and the traffic on the existing routes in the area. • Traffic growth rate may be estimated by • 1) by studying the past trends of traffic growth. • 2) IRC 108 “GUIDELINES FOR TRAFFIC PREDICTION ON RURAL HIGHWAYS”.

• If adequate data is not available it is recommended that an average annual growth rate of 7.5 % may be adopted. • Design life. • Design life is defined in of the cumulative number of standard axles that can be carried before strengthening of the pavement is necessary. • It is recommended that pavements for national highways and state highways should be designed for a life of 15 years.Expressways and urban roads may be designed for a longer life of 20 years. For other categories of roads, a design life of 10 to 15 years may be adopted.

Vehicle damage factor (VDF)

• It is a multiplier to convert the no. of commercial vehicles of different axle loads and axle configurations to the number of standard axle load repetitions. • It is defined as equivalent number of standard axles per commercial vehicle. • The VDF varies with the vehicle axle configuration,axle loading,terrain,type of road and from region to region.the VDF is arrived at from axle load surveys on typical road sections so as to cover various influencing factors,such as traffic mix,mode of transportation,commodities carried,time of the year,terrain,road conditions and degree of enforcement.

• The axle load equivalency factors recommended in the AASHTO guide are given in Annexure-2.

• These are used to convert different axle load repetitions into equivalent axle load repetitions. • It is recommended that designer should take the realistic values of VDF after conducting the axle load survey. • Where sufficient information on axle loads is not available and the project size does not warrant conducting an axle load survey, the indicative value of VDF are as :

Distribution of commercial traffic over the carriageway • A realistic assessment of distribution of commercial traffic by direction and by lane is necessary as it directly affects the total equivalent standard axle load applications used in design.In the absence of adequate and conclusive data following distribution may be assumed . • 1) Single lane roads • traffic tends to be more channelised on single lane roads than two lane roads and to allow for this concentration of wheel load repetitions,the design should be based on total number of commercial vehicles in both directions.

• Two lane single carriageway roads: • 75% of the total number of commercial vehicles in both directions. • Four lane single carriageway roads: • 40% of commercial vehicles in both directions. • Dual carriageway roads: • 75% of the commercial vehicles in each direction,for dual three lane carriageway & dual four lane carriageway distribution factor will be 60% and 45% respectively.

• Where significant difference between the two streams can occur,condition in the more heavily trafficked lane should be considered for design.

• Computation of design traffic. • The design traffic is considered in of the cumulative number of standard axles (in the lane carrying maximum traffic) to be carried during the design life.

Subgrade • Subgrade in cut or fill should be well compacted. • Expressways, NH, SH, MDR 97% of dry density achieved by IS 2720 part 8. • Other roads - 97% dry density by IS 2720 part 7. • IRC:36 “Recommended practice for the construction of earth embankment for road works”. • Material used as subgrade in expressways,national highways,state highways should have the dry density not less than 1.75gm/cc.

• CBR gives the strength of subgrade soil in cut and fill at most critical moisture condition likely to occur in situ. • CBR can be determined as per IS 2720 part 16 in lab.static/dynamic compaction can be adopted. • In-situ CBR is not recommended for design purposes.

• Soaking for 4 days may be unrealistically severe moisture condition,in certain cases were the climate is arid throughout the year i’e the annual rainfall is less than 500mm is less and the water table is too deep,CBR value may be prepared at the natural moisture content of soil at subgrade depth immediately after recession of the monsoon. • Use of expansive soil is not allowed for subgrade construction particularly for heavily trafficked roads. •

• When expansive soils are unavoidable compaction requirement as per Annexure-4 of this code should be followed. • Expansive soils swells very little when compacted at low densities and high moisture but swells greatly when compacted at high densities and low moisture. • It is recommended that moisture content should be 1-2 % wet of optimum moisture content.

• The design should be based on the CBR value of weakest soil type. •

4.Pavement Thickness And Composition.

• For the design of pavement to carry traffic in the range of 1-10 msa and traffic in the range of 10-150 msa the pavement thickness design chart is given in fig.1 and fig.2. • The design curves relate pavement thickness to the cumulative number of standard axels to be carried over the design life for CBR values of subgrade ranging from 2-10%.

• Based on the recommended designs minimum thickness and compositions of pavement layers for new constructions are given in the pavement design catalogue, plates 1 and 2.

4.2 Pavement composition • Sub-base material comprise natural sand, moorum, gravel, laterite, kankar, brick metal, crushed stone, crushed slag, crushed concrete or combinations thereof meeting the requirements as per clause 401 of MORT&H specifications for road and bridge works. • The material ing 425 micron sieve when tested in accordance with IS 2720 pt 5 should have liquid limit and plasticity index of not more than 25 and 6 respectively.

• The sub base material should have minimum CBR of 20% for traffic upto 2msa and 30% for traffic exceeding 2msa. • From drainage consideration the granular sub base should be extended over the entire formation width in case the subgrade soil is of relatively low permeability. • The thickness of sub base should not be less than 150 mm for design traffic less than 10msa and 200 mm for design traffic of 10msa and above.

• Where the CBR value of the subgrade is less than 2% the design should be based on subgrade CBR value of 2% and a capping layer of 150 mm thickness of material with a minimum CBR of 10% shall be provided in addition to the sub base.

4.2.2 Base Course • WBM, WMM or other equivalent granular construction conforming to IRC/MORT&H specification shall be adopted. • The recommended minimum thickness of granular base is 225 mm for traffic upto 2msa and 250 mm for traffic exceeding 2msa. • For WBM construction carrying traffic more than 10msa the thickness of WBM base shall be increased from 250 mm to 300 mm (I’e 4 layers of WBM grades II and III each of 75 mm compacted thickness) keeping the overall thickness unchanged.

4.2.3 Bituminous surfacing • The most commonly used wearing courses are surface dressing, open graded premix carpet, mix seal surfacing, semi dense bituminous concrete and bituminous concrete. • Use BM for traffic upto 5msa and DBM for traffic more than this. • In case the granular material is manually laid or if recommended by engineer, the DBM may be preceded by a 75 mm thick BM layer. • 10 mm BM can be taken as equivalent to 7 mm DBM.

• Choice of appropriate type of bituminous wearing course will depend upon several factors like design traffic, sub grade conditions, rainfall etc. For wearing course ref. annexure 5 and for selecting the grade of bitumen annexure 6. • Where the wearing surface adopted is open graded premix carpet of thickness upto 25 mm, the thickness of the surfacing should not be counted towards the total thickness.

4.3 Pavement Design catalogue. • Refere plate 1 and 2. • In some cases the total pavement thickness given in the recommended designs is slightly more than the thickness obtained from the design chart this is in order to :• A) provide the minimum thickness of sub base. • B) adapt the design to stage construction which need some adjustment. • DBM shall be constructed in 2 layers when prescribed thickness is more than 100 mm.

• For intermediate traffic ranges, the pavement layer thickness will be interpolated linearly. • For traffic exceeding 150msa, the pavement design appropriate to 150msa may be chosen further strengthening to extend the life based on pavement deflection measurements as per IRC:81.

5.Drainage Measures • The performance of a pavement can be seriously affected if adequate drainage measures to prevent accumulation of moisture in the pavement structure are not taken. • Some of the measures to guard against poor drainage conditions are:• 1) Maintenance of transverse section in good shape. • 2) Provision of appropriate surface and subsurface drains.

• 3) The difference between the bottom of subgrade level and the level of water table/high flood level should not be less than 0.6-1m. • 4) In water logged areas installation of suitable capillary cut-off as per IRC:34. • 5) On a relatively low permeability subgrade, the granular base should be extended over the entire formation width.

• Drainage of the pavement structural section can be greatly improved by providing a high permeability drainage layer satisfying the following criteria:-

• D85 means the size of sieve that allows 85% by weight of the material to through it. • 6) The permeable sub-base when placed on soft erodible soils should be underlain by a layer of filter material to prevent the intrusion of soil fines into the drainage layer. Non woven geosynthetic can be provided to act as filter. • 7) Where large inflows are to be taken care of, an adequately designed sub-surface drainage system consisting of an open graded drainage layer with collection and outlet pipes should be provided. • 8) The shoulders should be well shaped and if possible, constructed of impermeable material.

Design of Rigid pavement as per IRC:58-2002.

Tension and Compression • The intensity of normal force per unit area is expressed as normal stress, and is expressed in units of force per unit area.

• If the forces applied to the ends of the bar are such that the bar is in tension, then tensile stresses are set up in the bar. If the bar is in compression we have compressive stresses.

• Normal Strain :- The elongation per unit length or can be defined as change in length by original length. • For any material having a stress strain curve it is evident that the relation between stress and strain is linear for comparatively small volumes of strain. This linear relation between elongation and the axial force causing it was first noticed by Sir Robert Hook in 1678 and is called Hook’s law.

• Modulus of Elasticity/Young’s Modulus E is the ratio of unit stress to the unit strain. • • Poisson’s ratio :- When a bar is subjected to a simple tensile loading there is an increase in the length of the bar in the direction of load, but a decrease in the lateral dimensions perpendicular to load. The ratio of the strain in the lateral direction to that in axial direction is defined as Poisson’s ratio. • For most metals it lies between 0.25 to 0.35.

Rigid pavement – where and why? • Vehicles, like, agricultural tractor trailers, light goods vehicles, buses, animal drawn vehicles, motorized 2 wheelers and cycles. • Light and medium trucks carrying sugarcane timber, quarry material etc. • Neglected maintenance. • Concrete pavements offer an alternative to flexible pavement where • the soil strength is poor, • the aggregates are costly and • drainage conditions are bad, waterlogged area

• 3. FACTORS GOVERNING DESIGN – Wheel Load • Pavement may be designed for a wheel load of 102 kN. – Tyre Pressure • 0.7 MPa

– Design Period • The design methodology is based on wheel load stresses. •

Design life of not less than 20 years.

– Characteristics of the subgrade •

Modulus of subgrade reaction, k, which is determined by carrying out a plate bearing test, using 750 mm dia. Plate according to IS:9214-1974.

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Subgrade strength is desirable to determine during or soon after the rainy season.

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An idea of the k value of a homogeneous soil subgrade may be obtained from its soaked CBR value using table 1.

– Sub-Base • • • • •

The Provision of a sub-base below the concrete pavements has many advantages such as: It Provides a uniform and reasonably firm It Prevents mud-pumping on subgrade of clays and silts It acts as a leveling course on distorted, non-uniform and undulation sub-grade It acts as a capillary cut-off

– Concrete Strength •

Since concrete pavements fail due to bending stresses, it is necessary that their design is based on the flexural strength of concrete.

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It is suggested that the 90-day strength be used for design instead of the 28-day strength as the traffic develops only after the lapse of a period of time. The 90 day flexural strength may be taken as 1.20 times the 28-day flexural strength or as determined from laboratory tests.

• For rural roads, it is recommended that the characteristic 28-day compressive strength should be at least 30 MPa. The characteristic 28-day flexural strength shall be at least 3.8 MPa.

4. DESIGN OF SLAB THICKNESS • Critical Stress Condition •

Concrete pavements in service are subjected to stresses due to a variety of factors.

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The factors commonly considered for design of pavement thickness are traffic loads and temperature variation, as the two are additive.

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The effects of moisture changes and shrinkage are not normally considered critical to thickness design.

• Three different regions slab-corner, edge and interior-which react differently from one another to the effect of temperature differentials, as well as load application.

• The concrete pavements undergo a daily cyclic change of temperature differentials, the top being hotter than the bottom during day and cooler during night.

• The consequent tendency of the pavement slabs to warp upwards (top convex) during the day and downwards (top concave) during the night, and restraint offered to this warping tendency by selfweight of the pavement induces stresses in the pavement, referred to commonly as temperature stresses.

• These are flexural in nature, being tensile at bottom during the day and at top during night. • As the restraint offered to warping at any section of the slab would be a function of weight of the slab upto the section, it is obvious that corners have very little of such restraint. • The restraint is maximum in the slab interior, and somewhat less at the edge. Consequently, the temperature stresses induced in the pavement are negligible in the corner region, and maximum at the interior.

• Under the action of load application, maximum stress is induced in the corner region, as the corner is discontinuous in two directions especially when load transfer steel dowels are not provided in rural roads.

• The edge being discontinuous in one direction only has lower stress, while the least stress is induced in the interior where the slab is continuous in all directions. • The maximum combined tensile stresses in the three regions of the slab will thus be caused when effects of temperature differentials are additive to the load effects. • This would occur during the day in case of interior and edge regions, at the time of maximum temperature differential in the slab. • In the corner region, the temperature stress in negligible, but the load stress is maximum at night when the slab corners have a tendency to lift up due to warping and lose partly the foundation .

• Considering the total combined stress for the three regions, viz., corner, edge and interior, for which the load stress decreases in that order while the temperature stress increases. • The critical stress condition is reached in the edge region where neither of the load and temperature stresses are the minimum. • It is, therefore, felt that both the corner and the edge regions should be checked for total stresses and design of slab thickness should be based on the more critical condition of the two.

Temperature differential: between the top and bottom of concrete pavements causes the concrete slab to warp, giving rise to stresses. The temperature differential is a function of solar radiation received by the pavement surface at the location, losses due to wind velocity, etc., and thermal diffusivity of concrete, and is thus affected by geographical features of the pavement location. As far as possible, values of actually anticipated temperature differentials at the location of the pavement should be adopted for pavement design. For this purpose guidance may be had from Table 4.

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Corner stresses: The load stress in the corner region may be obtained as per westergaard’s analysis as modified by Kelley, from the following correlation :

Design Charts For calculations of load stress in the edge and corner regions of rigid pavement slabs.

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5. TS

Types of ts • • • • •

Rural Roads are generally of single-lane, and the full lane width (3.0 m-3.75m) is concreted in one operation. Thus, there is no need for a longitudinal t for single-lane rural roads. Transverse ts, they are of three types: Contraction ts Construction ts Expansion ts

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Spacing of ts Transverse contraction and construction ts: The spacing of transverse contraction ts or construction ts in alternate bay construction may be kept 2.50 m-3.75m. The length of the (in the direction of traffic) shall not be less than the width of the . The details of the t are shown in fig. Expansion ts: Expansion ts are necessary where concrete slabs abut with bridges and culverts. The details of the ts are shown in Fig.

Longitudinal ts: Where the width of concrete slab exceeds 4.5 m as in the case of causeways, etc., it is necessary to provide a longitudinal t as per the details given in fig. in the mid-width of the slab.

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• • •

• •

– Load Transfer at Transverse ts Since rural roads have low traffic low traffic with small wheel loads, the slab thickness being 150-250 mm, the aggregate interlock at the sawn ts is itself adequate for load transfer and no dowel bars are necessary. If slabs are cast in alternate s, keyed ts can be formed as in fig.. Day’s work should normally be terminated at a contraction t. At expansion ts, where the ts width may be 20 mm, dowel bars are required as shown in, fig. 6 (c). Dowel bars shall be 25 mm diameter, 500 mm long and spaced at 250 mm centre to centre. In the case of Roller Compacted Concrete pavements, the contraction ts may be formed by cutting ts with concrete saw at the spacing suggested. If aesthetics of the road is not an important consideration, the sawing of ts may be omitted and the cracks allowed to form on their own.

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Recommended Design Procedure

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Select design wheel load, concrete flexural strength, modulus of subgrade reaction, modulus of elasticity of concrete, Poisson’s ratio, coefficient of thermal expansion of concrete.

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Decide t spacing and lane width.

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Select tentative design thickness slab.

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Ascertain maximum temperature stress for the critical edge region from Equation (6).

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Calculate the residual available strength of concrete for ing traffic loads.

• Ascertain edge load stress from Equation (3) or Fig. 2 or Fig. 4 as relevant and calculate the factor of safety. • In case the available factor of safety is less than or far in excess of 1, adjust the tentative slab thickness and repeat steps 3 to 6 till the factor of safety is 1 or slightly more. • Check the adequacy of thickness in the corner region by ascertaining corner load stress from equation (7) or fig. 3 or fig. 5 as relevant and readjust the thickness if inadequate.