Flexible Pavement

ByD.K.Nanda

Geometric Design

Geometric design of Highways

“Geometric design” deals with visible elements of a highway. Sound geometric design results in economical operation of vehicles and ensures safety.

The Geometric design essentially consists of the design of :

Horizontal Alignment. Vertical Alignment Cross profile.

Horizontal Alignment

The selection of road alignment is broadly based on the following:

a)Maximum comfort to road user.b)Least social and environmental adverse impact.c)Least displacement and loss of public property , monuments , religious structuresd)Location of required drainage structures.e)Availability of landf)Topography

Horizontal Alignment Contd….As road alignment changes in direction are often necessary owing to restrictions imposed by considerations discussed earlier, it is accomplished by introducing Horizontal and transition curves in the alignment.

The general principles to be followed while designing these curves is to coordinate both the horizontal and vertical alignments. Some of the important considerations followed are:

Horizontal Alignment Contd…Sharp horizontal curves should be avoided at or near the apex of pronounced summit/sag vertical curves from safety considerations.

Vertical and horizontal curves should coincide as far as possible and their length should be more or less equal. If this is difficult for any reason, the horizontal curve should be somewhat longer than the vertical curve

The degree of curvature should be in proper balance with the gradients.

Horizontal Alignment Contd…

Short curves gives appearance of kinks, particularly for small deflection angles, and should be avoided. The curves should be sufficiently long and should have suitable transitions to provide pleasing appearance.

As a general rule, curve lengths should be at least 150 meters for a deflection angle of 5 degrees and should be increased by 30 meters for each one degree decrease in the deflection angle . For deflection angle less than one degree, no curve is required to be designed.

Horizontal Alignment Contd…Reverse curves may be needed in difficult terrain. It should be ensured that there is sufficient length between the two curves for introduction of requisite transition curves.

Curves in the same direction separated by short tangents should be avoided as far as possible in the interest of aesthetics and safety and replaced by a single curve. If this is not feasible, a tangent length corresponding to 10 seconds travel time must at least be ensured between the two curves.

Horizontal Alignment Contd….Compound curves may be used in difficult topography only when it is impossible to fit in a single circular curve . To ensure a smooth transition from one curve to the other, the radius of the flatter curve should not be disproportional to the radius of the sharper curve with a ratio of 1.5:1 as the limiting value.

To avoid distortion in appearance , the horizontal alignment should be coordinated carefully with the longitudinal profile, keeping in mind that the road is a three dimensional entity.

Horizontal CurvesIn general horizontal curves consists of a circular portion flanked by spiral transitions at both ends.

Design speed, super elevation and coefficient of side friction affect the design of circular curves.

Length of transition curves is determined on the basis of rate of change of centrifugal acceleration or the rate of change of super elevation.

Super elevationSuper elevation required on horizontal curves is calculated using the following formula, which assumes that centrifugal force corresponding to three-fourth the design speed is balanced by super elevation and the rest counteracted by side friction.

e=V²/225R where, e= super elevation, V=design speed in Km/hr. R=radius in meters The super elevation obtained from the above equation should however be kept limited to the following values.a)In plain & rolling terrain 7%b)In snow bound areas 7%c)In hilly areas not bound by snow 10%

Super elevation contd…When the value of super elevation obtained by using the formula is less than the road camber , then the normal cambered section is continued on the curved portion without providing any super elevation.The normal cambered section of the road is changed into super elevated section in two stages.First stage is the removal of adverse camber in the outer half of the pavement.In the second stage, super elevation is gradually built up over the full width of the carriageway so that required super elevation is available at the beginning of the circular curve.

Circular curvesOn a horizontal curve, the centrifugal force is balanced by the combined effects of super elevation and side friction . From the basic equation of equilibrium it can be found that R = V²/127(e+f) where, V=speed in Km/hour, e = Super elevation , f = coefficient of side friction between tyre and pavement( taken as 0.15) ,R= Radius of circular curve in meter Based on this equation, and the maximum permissible values of super elevation, the radius of circular curve corresponding to design speed is worked out.

Transition curves

Transition curves are necessary for a vehicle to have smooth entry from a straight section into a circular curve. The transition curves also permit gradual application of super elevation at the horizontal curves. Spiral curves are generally used as transition curves.

The minimum length of transition curves required is determined from the following two considerations and the larger of the two values is adopted for design.

Transition curves contd…The rate of change of centrifugal acceleration should not cause discomfort to drivers. From this consideration, the length of transition curve is given by

Ls = 0.0215 V²/CR, where Ls = Length of transition curve in meter. V = Speed in Km/hour , R = Radius of circular curve in meter. C = 80/(75+V) subject to maximum of 0.8 and minimum of 0.5

Transition curve contd….The rate of change of super elevation should be such as not to cause discomfort to travellers or to make the road appear unsightly . The rate of change should not be steeper than 1 in 150 in plain and rolling terrain, and 1 in 60 in mountainous/steep terrain.The formula for minimum length of transition curve on this basis are For plain and rolling terrain, Ls = 2.7 V²/RFor Mountainous / steep terrain Ls = 1.0 V²/R whereR = Radius of circular curve in meters, V = Speed in Km/hour, Ls = Length of transition curve in meters.

Vertical AlignmentThe vertical alignment should provide for a smooth longitudinal profile consistent with the category of the road and lay of the terrain.

Grade changes should not be too frequent as to cause kinks and visual discontinuities in the profile

Desirably there should be no change in grade within a distance of 150 meters.

A short valley curve within an otherwise continuous profile is not desirable and can be hazardous.

Two vertical curves in the same direction separated by a short tangent should be avoided and preferably replaced by a single long curve.

The longitudinal profile should be coordinated suitably with the horizontal alignment

Vertical alignment Contd…

GradientsGrades should be carefully selected keeping in view the design speed, terrain conditions and nature of traffic. The recommended values of gradients for different classes of terrain are as below. Terrain Ruling Limiting Exceptional Gradient Gradient. Gradient.Plain/ Rolling 3.3% 5% 6.7%

MountainousElevation>3000m 5% 6% 7%

MountainousElevation <3000m 6% 7% 8%

Gradient Contd…On un kerbed pavement s in embankment, near-level grades are not objectionable when the pavement has sufficient camber to drain the storm water laterally.

In cut sections or where the pavement is provided with kerbs, it is necessary that the road should have some gradient for efficient drainage.

Desirable minimum gradient for this purpose is 0.5 percent if the side drains are lined and 1 percent if these are unlined.

Vertical Curves Vertical curves are introduced for smooth transition at grade changes. The vertical curves are designed as square parabolas.The length of vertical curve is controlled by sight distance requirements, but curves with greater lengths are aesthetically better. Design speed Grade change in % not Minimum length of requiring vertical curve Vertical curve in mts. 50 1.0 30.00 80 0.6 50.00 100 0.5 60.00

Summit CurvesThe length of summit curves is governed by the choice of sight distance. The length is calculated on the basis of the following formula.

For safe stopping sight distance:

a)When length of curve exceeds the required sight distance i.e. L>S L = NS²/4.4 where N= Algebraic difference between the two grades, L = Length of curve in meters , S = Sight distance in meters.

b) When L is less than S

L = 2S – 4.4/NFor Intermediate or overtaking sight distance

a)When L is greater than S, L = NS²/9.6

a) When L is less than S, L = 2S – 9.6/N

Summit Curves

Valley CurvesThe length of valley curves should be such that for night travel, the headlight beam distance is equal to the stopping sight distance . The length of the valley curve is calculated as under. When L is greater than Sa)L = NS²/(1.5+0.035S)When L is less than Sb)L = 2S – (1.5+0.035S)/N where.N = Algebraic difference between the two grades, L = Length of curve in meters.S = Stopping sight distance in meters.

Design of Pavement

Pavement

• Pavement is part of the road crust above the sub grade/formation .

Granular Base

Bituminous Layer Granular Base

 Granular Sub base

Sub grade 

The function of the pavement is to carry heavy wheel loads of vehicles and to transfer the same over a wide area of the sub grade soil permitting the deformations within elastic or allowable range.

Function of Pavement

Pavement ClassificationPavements are broadly classified into two

categories namely:i. Flexible Pavement.ii.Rigid Pavement.The choice of Pavement to be adopted for

construction for any particular section of road is dependent on factors such as rainfall in the area, availability of budget etc.

Components of Flexible Pavement The components of flexible pavement are as

under:a. Granular sub baseb. Granular Basec. Wearing course. The thickness of above components are

dependent mainly on the design life of pavement,traffic volume, and axle loads etc.

Traffic

For the purpose of design of flexible pavements only the number of commercial vehicles of laden weight of 3.0Mt or more and their axle load is considered.

The initial daily traffic for any road is normally based on 7days 24 hour traffic count.

An estimate of likely traffic growth rate is obtained by studying the past trends. In the absence of any past trends an average value of 7.50 percent is adopted for design.

Design of Flexible Pavement based on IRC-37

The thickness of Pavement is dependent on the following factors:a)Traffic volume defined in terms of cumulative standard axles(8160kg) to be carried during the design life.b)The CBR value of the Sub grade

Design Life

It is considered appropriate that roads in rural areas to be designed for a design life of 10-15 years.

Initially due to financial reasons if it is not possible to construct the full depth of pavement at the same time, stage construction techniques is resorted to in such cases as the traffic increases.

Computation of trafficThe design traffic is considered in terms of cumulative number of standard axles to be carried during the design life of the pavement.The following equation is used for the computation:

N = 365xA{(1+r) - 1}ª x F xDxL/r Where, N=Cumulative no of standard axles to be catered for design A=Initial traffic, in the year of construction,in terms of number of commercial vehicles per day duly modified to account for lane distribution.

r = Annual growth rate of commercial traffic.a = Design life in yearsF = Vehicle damage factor( Number of standard axles per commercial axle)D = Direction distribution factorL = Lane distribution factor

Computation of traffic

Distribution of traffic

Single lane road(3.75m width): Traffic tends to be more channelised on single lane roads than on two lane roads and to allow for this concentration of wheel load repetitions the design should be based on the total number of commercial vehicles per day in both directions multiplied by 2.Intermediate lane roads( 5.5m width): The design should be based on total number of commercial vehicles per day in both directions multiplied by 1.5

Two lane single carriageway roads : The design should be based on 75 percent of the total number of commercial vehicles in both directions.Four lane single carriageway roads : The design should be based on 40 percent of the total number of commercial vehicles in both directions.Dual carriageway roads : The design of dual two lane carriageway roads should be based on 75 percent of the number of commercial vehicles in each direction. The distribution factor shall be reduced by 20 percent for each additional lane.

Distribution of traffic

Vehicle Damage Factor

The damage factor is a multiplier for converting a commercial vehicle into equivalent standard axle load repetitions. The factors are arrived at from axle load surveys of commercial vehicles and the AASHO axle load equivalence factors . The VDF varies for different class of roads . This may be computed from axle load measurements. Where sufficient information is not available, values as given in next slide may be adopted.

Indicative VDF Values

Initial No of  Terrain VDF  Values VDF  Values VDF  Values

Commercial vehicle per day

  Unsurfaced Thin bituminous surfacing

Thick bituminous surfacing

Less than 150 Hilly 0.50 0.75   Rolling 1.50 1.75   Plain 2.00 2.25 150-1500 Hilly   1.00 1.25  Rolling   2.00 2.25  Plain   2.50 2.75More than 1500 Hilly   1.25 1.50  Rolling   2.25 2.50  Plain   2.75 3.00

Pavement Thickness calculation Once the cumulative standard axles are found out, the same is converted to million standard axles.From the pavement thickness design chart given in IRC-37 for different CBR values of sub grade , the total pavement thickness is calculated.

From the total thickness of pavement, the thickness of different layers are calculated using the thickness combination block given in IRC-37 (Fig-2)

Construction

The construction of a highway essentially involves the construction of the following layers of pavement and under laying embankment.

Bituminous wearing courseBituminous base course Granular base course Granular sub baseSelected sub gradeEmbankment

Resources

The resources required for the construction of a highway are broadly classified under the following three heads.

Construction Plants & Equipments

Construction Material

Manpower

Major Plants & Equipments For Embankment/Sub grade:a)Excavatorb)Dumperc)Dozerd)Motor Gradere)Vibratory Rollerf)Tractor fitted with rotavatorg) Related Laboratory Equipments

Major Plants & Equipments

For Granular Sub Base:a)Wheel Loaderb)Dumperc)Motor Graderd)Water Tankere)Vibratory Rollerf)Related Laboratory Equipments

For Wet Mix Macadama)Wheel Loaderb)Pugmil c)Dumperd)Sensor Pavere)Vibratory Rollerf)Water Tankerg)Related Laboratory equipments

Plants & Equipments

Plants & Equipments For Bituminous works:a)Hot Mix Plantb)Wheel Loaderc)Dumperd)Sensor Pavere)Tandem Vibratory Rollerf)Pneumatic Tyre Rollerg)Bitumen Sprayerh)Water Tankeri)Asphalt cutterj)Related Laboratory Equipments

Plants & Equipments

For Concrete Worksa)Batch mix plantb)Wheel Loaderc)Transit Mixersd)Concrete Pumpe)Needle Vibratorsf)Water Tankerg)Related Laboratory Equipments

a) Stone Crusherb) DG Setsc) Articulated Trailord) Light Mastse) Mobile Service Unitf) Bar shearing & Cutting Machineg) Latheh) Circular sawi) Welding Equipmentj) Conveyance Vehicle & Ambulancek) Magazine

General Plants & Equipments

Construction Material The major construction material involved in construction of Highway are

Soil/Fly ash Stone aggregatesCement & LimeSteel and HT strandsBitumen & Bitumen EmulsionBearings & Expansion JointsRoad furniture such as Thermoplastic road marking, Metal beam crash barrier, Road signage, Kilometer stones, Roadway indicators, Road studs, etc.

Manpower The typical organization of Manpower at a project site are as follows:

Employer representative: Project Director & supporting staff.

Engineer & his representative appointed by the Employer.

Contractor’s representative & his supporting staff.Contractors skilled & Unskilled labor.

Contractual pre requisites for Construction Following the award of contract and prior to commencement of construction at site , the following are the pre requisites to be fulfilled:

Notification of Engineer by the Employer.Mobilization of Engineer’s representative.Mobilization of Contractor’s representative & supporting staff. Handing over of site by the Engineer to Contractor.Submission of Construction program by the ContractorApproval of Construction Program by the Engineer.Mobilization of resources by the contractorEngineer’s notification for commencement.Issue of drawings by the Engineer.

Sequence of Construction

Setting outClearing & GrubbingEarthworksCross Drainage structuresGranular sub baseGranular baseAsphalt worksProtective worksRoad furniture.

Setting out

This essentially includes establishment of: Control Points.Working bench marks.Road center line on the ground.Existing ground levels & Cross sections. Method statements for different items of work.Field laboratory.Borrow areas and stone Quarry.Establishment of plants & their successful trial.Arrangement for traffic safety & Control.

Clearing & Grubbing This includes cutting of trees , removal of stumps , bushes ,shrubs , roots , grass , weeds , & top organic soil not exceeding 150mm of thickness which in the opinion of the Engineer are unsuitable for incorporation in the works , from the area of road land containing road embankment , drains, cross drainage structures and such other areas as may be specified in the drawings or approved by the Engineer.Clearing & Grubbing is performed in advance of earthwork operations.The equipments deployed for clearing and grubbing is a crawler mounted dozer of adequate capacity having ripper attachment/ or a hydraulic excavator with adequate dumpers for disposal of unsuitable materials.

Method Statement First the toe line of the embankment after allowing for drains is marked on the ground using pegs/lime powder by taking measurements from the road centre line.

Once this is done, the trees/ structures falling within this limit is identified & marked for their removal.

Pre measurements of the tree girths/ dimension of structures to be removed are then recorded prior to their removal.

A suitable equipment such as an excavator/Dozer is deployed for removal of the trees/structures and the materials are then disposed off at designated location using dumpers/tractors as approved by the Engineer.

Embankment & Sub grade-Materials Materials: The material to be used in embankment/sub grade shall be soil , moorum , gravel , or a mixture of these and shall be free from logs, stumps , roots rubbish or any other material likely to deteriorate or affect the stability of the embankment/Sub grade . The material to be used in embankment/sub grade shall satisfy the following general requirements: Liquid limit < 70.Plastic limit < 45. Free swelling Index < 50% for embankment and 0% for sub grade.Maximum size < 75mm for embankment & 50mm for sub grade.MDD > 1.55 gm/cc for embankment & 1.78 gm/cc for sub grade.CBR > Design CBR as specified in the contract for sub grade.

Embankment & Sub grade – General requirements

The materials to be used shall be from approved sources with preference given to materials becoming available from nearby roadway excavation if any. Arrangement of borrow areas complying requirements of Ministry of environment and forest in respect of its excavation.Collection of representative sample from the identified borrow areas and its testing in the site laboratory for its suitability.Submission of test results in respect of MDD & CBR to the Engineer for his approval.Approval of the borrow area by the Engineer for use.Testing of representative samples of the existing ground within embankment limits ascertain its suitability for reuse and MDD.

Construction Operation After the site has been cleared, the limit of embankment/Sub grade is marked with pegs at suitable intervals on the ground after allowing extra 500mm for trimming of embankment slope at a latter date to ensure that the remaining material is to the desired density and conforms to specified side slope. The original ground is then levelled , watered and then compacted by rolling so as to achieve minimum dry density of 95%.If the ground forms a part of the sub grade and the materials within the subgrade limit is suitable but does not have 97% compaction ,then the ground is loosened upto subgrade bottom, watered and then compacted in layers not exceeding 200mm thickness to 97% of maximum dry density . If the materials within the subgrade limit is not suitable, then the same is removed and replaced by suitable materials and compacted in layers of 200mm thickness to 97% MDD.

Construction Operation contd.. Once the ground supporting the embankment/ sub grade is tested and cleared, then approved materials from borrow areas are excavated and transported by dumpers to the construction site. Materials are then dumped in pre determined locations based on the volume requirement of the layer under construction taking into account the compaction factor, loose volume carried by each dumper so as not to exceed 200mm compacted thickness of the layer.The materials so dumped are then roughly spread to a more or less uniform thickness covering the entire width of the layer under construction by the help of a crawler mounted dozer of adequate capacity.

Construction operation Contd…

A motor grader is then used to give a finish grading of the material to allow for easy movement of water tanker on top of the layer for watering if the natural moisture of the material is found to be below the OMC requirement.Depending on the natural moisture content determined using a rapid moisture meter & the optimum moisture content of the material determined earlier in the laboratory and approved by the Engineer, either the material is allowed to dry or water is added using a water tanker fitted with a distributor for controlled application of water in required quantity so as to achieve OMC. The layer is then left undisturbed for some time for the applied surface water to percolate through the layer.

Grading of Embankment Layer

Grading of Sub grade Layer

Construction Operation Contd… Then the layer is scarified and mixed thoroughly using tractor fitted with a rotavator so as to ensure uniform mixing of water.A motor grader is then used to finish the layer surface to desired level and cross fall with the help of level stakes fixed on each side of the layer at regular intervals of 20m/10m . Rolling is then started using a vibrating roller of adequate capacity from the lower side to the upper side of the layer covering the entire width of the layer without vibration to ensure material are packed in their place.Then required number of vibratory passes as determined during the trial stretch are given and the rolling is suspended with a final plain pass.

Compaction of Embankment Layer

Construction Operation Contd…

Density measurements are then taken using either a Nuclear density gauge or sand replacement method as approved by the Engineer. If the observed densities are found to be above the required Percentages of MDD, then subsequent layer construction is continued.In the event of density measurements falling below the required % compaction, then the layer is scarified to full depth using a motor grader fitted with a ripper or a Dozer fitted with a ripper attachment.The entire process starting from grading, watering and rolling is repeated till the densities are achieved.The sub grade is then checked for levels and compared with the design level with a tolerance of +20mm to -25mm.

Granular Sub base Material: The material to be used for the work shall be natural sand , moorum ,gravel ,crushed stone or combination thereof depending upon the required grading. The grading to be adopted can be any one of the grading specified in Table 400-1 or 400-2 and is contract specific.The material shall have 10 percent fines value of 50KN or more. Water absorption <2% Liquid limit < 25%Plasticity Index < 6%CBR>30 for grading-I, 25 for grading-II and 20 for grading-IIISand equivalent value > 50 Minimum % compaction: 98% of MDD

Spreading & Compaction Prior to laying of sub base material, the sub grade surface shall be prepared by lightly sprinkling with water and rolled with two passes of a vibratory roller.Once the sub grade surface is ready, the sub base material of required quantity conforming to specified grading shall be dumped uniformly over the entire width of the sub grade within the toe line marked with lime powder to prevent spillage of material beyond the toe line. The material is then roughly spread over the entire bed using a motor grader. Required quantity of water is then applied uniformly over the surface of sub base by a water tanker having facility for controlled application of water.

Grading of Granular sub base layer

Compaction of GSB

Spreading & Compaction The sub base is then scarified with the help of a motor grader for uniform mixing of water and then checked of water content using a rapid moisture meter. Once the moisture content is found to be within +1% to -2% of OMC , the material is then finished to desired level and cross fall by the help of a motor grader. Immediately thereafter the rolling shall start with a vibratory roller of 80 to 100KN from the lower edge and proceed towards the upper edge .Each pass of the roller shall uniformly overlap not less than one third of the track made in the preceding pass.During rolling the grade and the cross fall shall be checked and any high spots or depression corrected by removing or adding fresh material. The speed of the roller shall not exceed 5 Km/hour.

Rolling shall be continued till the density achieved is atleast 98% of the maximum dry density for the material determined in the laboratory.The surface of the material on completion of compaction shall be closed with a final plain pass of the roller. Once the compaction is achieved, the bed is tested for levels and compared with the design level with tolerance of +10mm to -20mm.The bed is also checked for surface regularity with a 3m long straight edge where the allowable difference between the road surface and the underside of straight edge when placed parallel with or perpendicular to the center line of the road is 8mm.

Spreading and compaction contd…

Wet Mixed Macadam- Base Course Material: Coarse aggregates shall be crushed stone . The aggregates shall conform to the physical requirements as given below: Loss angeles abrasion value – Maximum 40% orAggregate Impact value - Maximum 30%Combined flakiness and Elongation Indices(Total) - Maximum 30% Water absorption - < 2%Plasticity Index for material finer than 425 micron - Not exceeding 6 Layer thickness: Minimum 75mm & maximum 200mm Minimum % compaction : 98% of MDD

Grading of Aggregates for WMM

IS Sieve % passing by weight

53mm 10045mm 95-10022.4mm 60-8011.2mm 40-604.75mm 25-402.36mm 15-30600 micron 8 – 22 75 micron 0-8

Production of Mix - WMM Wet mix macadam is produced in a pugmil of suitable capacity having provision for controlled application of water and forced positive mixing arrangement.Optimum moisture content for mixing shall be determined in accordance with IS:2720 after replacing the aggregate fraction retained on 22.4 mm sieve with material of 4,75mm to 22.4mm size.While adding water in preparation of mix in a pugmil , due allowance is made for evaporation losses considering the time lag between laying of mix and the production of mix.The proportioning of the mix in the pugmil is achieved by calibrating the gate openings of the feeder bin which is then maintained throughout for the production, excepting little adjustments for variation in gradation of individual constituents.

Transportation, Laying & Compaction The mix produced in the pugmil is directly discharged to a dumper placed directly below the discharge hopper which then carries the mix to the place of laying.

The mix is then discharged by the dumper gradually onto the receiving hopper of the paver. The conveyor then discharges the mix to the auger box which is then mixed and spread by the auger over the entire width by the spiral rotary action of the auger.

The screed of the paver which follows the augers gives initial compaction to the mix through vibration of the screed and then pushes forward any extra quantities to achieve the required pre compaction level.

The movement of the paver in the longitudinal direction is controlled by the operator .while the profile and cross fall of the paver screed is controlled by the sensor arms of the paver on either side which moves over the string line fixed to the desired levels taking into account the design level and camber/super elevation.

The level of the laid surface is immediately checked from time to time as the paving continues and deficiencies if any are immediately rectified either by trimming the high spots or adding fresh mix at low spots.

Once the mix has been laid to required thickness, grade and cross fall, the same is compacted with a vibratory roller of 80 to 100KN static weight.

Laying & Compaction Contd…

WMM Laying & Compaction

Laying & Compaction Contd..

Rolling is commenced from the lower edge and progressed gradually towards the upper edge with uniform over lapping each preceding track by at least one third width until the entire surface is rolled with the pre determined no of passes as established during trial section to ensure that desired compaction of 98% is achieved.

After final compaction of wet mix macadam course, the surface is checked for density, levels and surface regularity whose permissible values are:

Percentage compaction: Minimum 98% of maximum dry density. Tolerance in surface level: +10mm to -10mm.Surface regularity with 3m straight edge: Maximum 8mm.

Prime CoatMaterial: The choice of bituminous primer is dependent on the porosity of the surface to be primed as detailed below.

Type of surface Viscosity in Rate of applicationcentistoke @ 60ºC in Kg per 10 sqm.

Low porosity 30 – 60 6 to 9

Medium Porosity 70 – 140 9 to 12

High Porosity 250 – 500 12 to 15

Cleaning of WMM surface for Priming

Prime Coat The primer shall be bitumen emulsion, complying with IS 8887 of a type and grade as specified in the contract.The use of medium curing cutback as per IS 217 shall be restricted only for sites at sub-zero temperatures.The surface to be primed shall be swept clean of dust and loose particles , care being taken not to disturb the inter locked aggregates. This is best achieved when the surface layer is slightly moist . For this purpose a mechanical bromer is used to loosen the dust from the surface which is then blown out using a compressor mounted on a tractor. A bitumen sprayer (calibrated earlier for application pressure and speed of movement) carrying the required primer is then used to prime the surface. The rate of actual spray is then found out by the tray test . The primed surface is allowed to cure for at least 24 hours

Tack CoatMaterial: The binder used for tack coat shall be a bitumen emulsion complying with IS 8887 of a type and grade as specified in the contract.The use of cutback bitumen as per IS 217 shall be restricted only for sites at sub-zero temperatures.The surface on which tack coat is to be applied shall be clean and free from dust, dirt, and any extraneous material.Immediately before the application of tack coat, the surface shall be swept clean with a mechanical broom, and high pressure air jet.The application of tack coat shall be at the rate specified in the contract.The rate of application of tack coat for various types of surface shall be as per the table 500-2.

Table 500-2(MORTH specification)

Type of surface Rate of application in Kg/m²

Normal bituminous surface 0.2 to 0.25

Hungry bituminous surface 0.25 to 0.30

Granular surface treated with primer 0.25 to 0.30

Granular base not primed 0.35 to 0.40

Cement concrete pavement 0.30 to 0.35

Tack Coat The normal range of spraying temperature for bituminous emulsion shall be 20°C to 70°C and for a cutback 50°C to 80°C if rapid curing/setting material is used.Where the surface to receive an overlay is a freshly laid bituminous surface which has not been subjected to traffic, or contaminated by dust, a tack coat is not mandatory where the overlay is completed within two days.The tack coat is applied by the help of a bitumen sprayer calibrated to required application pressure and speed of movement for the desired rate of application.The tack coat is left to cure until all the volatiles have evaporated before any subsequent construction is started.

Base courseDense Bituminous Macadam

Scope: Construction of dense graded bituminous macadam(DBM) for use in base course in single or multiple layers on a previously prepared base or sub base.

The thickness of a single layer shall be between 50mm to 100mm as stipulated in the contract.

The minimum compaction requirement shall be 98% of marshal density.

Material

Bitumen: The bitumen shall be paving bitumen of penetration grade 60/70(S65) or 80/100(S90) as specified in the contract and shall conform with the Indian standard IS:73. Modified bitumen such as CRMB or PMB may be used if specified in the contract.

Coarse aggregate: The coarse aggregate shall consists of crushed rock retained on 2.36mm sieve . The aggregates shall be clean, hard ,durable, of cubical shape, free from dust organic or other deleterious substance . If the aggregates have poor affinity for bitumen, then the bitumen shall be treated with an approved anti striping agent .

Material for DBMFine aggregate: Fine aggregate shall consists of crushed or naturally occurring mineral material passing the 2.36mm sieve and retained on 75 micron sieve.

Fine aggregates shall be clean, hard,durable and free from soft or friable material, organic or other deleterious matter.

The fine aggregates shall have a sand equivalent value of not less than 50 when tested in accordance with the requirement of IS:2720 part 37.

The plasticity index of the fraction passing 425 micron sieve shall not exceed 4 when tested as per IS:2720 part 5.

Coarse aggregate for DBM The coarse aggregate shall satisfy the physical requirements as specified below.

Aggregate Impact value: Maximum 27%Los Angeles abrasion value: Maximum 35%Combined flakiness & elongation Index: Maximum 30%Water absorption : Maximum 2%Stripping value: Minimum retained coating 95%Water sensitivity: Retained tensile strength of minimum 80%Grain size analysis: Maximum 5% passing 75 micron sieve. Soundness: With sodium sulphate- Maximum 12% With Magnesium sulphate- Maximum 18%

Filler Filler shall consists if finely divided mineral matter such as rock dust, hydrated lime or cement as stipulated in the contract.The filler shall be free from organic impurities and have a plasticity index not greater than 4. The plasticity index limit shall not apply if the filler is lime or cement.Cement or hydrated lime is not required when the lime stone aggregate is used.Where the aggregate fail to meet the requirements of water sensitivity test , then 2% by total weight of aggregate, of hydrated lime shall be used.The filler shall conform to the grading requirements as per table 500-9 of MORTH specification.

Table 500-9 of MORTH specification

IS Sieve Cumulative % passing by weight of total aggregate.

600 micron 100

300 micron 95-100

75 micron 85-100

Aggregate grading & binder content.

The combined grading of coarse and fine aggregates and added filler for the particular mixture when tested in accordance with IS:2386 part I shall fall within the limits of table 500-10 for dense bituminous macadam grading I or II as specified in the contract.

The type and quantity of bitumen, and appropriate layer thickness is also indicated for each mixture type.

Grading I IINominal aggregate 40 mm 25 mmLayer thickness 80-100mm 50-75mmIS Sieve Cumulative % by weight of total aggregate passing45 10037.5 95-100 10026.5 63-93 90-10019 71-9513.2 55-75 56-804.75 38-54 38-542.36 28-42 28-420.30 07-21 07-210.075 02-08 02-08Bitumen content % Minimum 4% Minimum 4.5%By mass of total mixBitumen grade S65 or S90 S65 or S90

Table 500-10

Design criterion-DBM

The mixture shall meet the following requirements:

TABLE: 500-11 ———————Marshall stability at 60°C: Minimum 9 KNFlow : Between 2mm to 4mmCompaction level:75 blows on each face of the specimenPercent air voids: 3 to 6Percent air voids filled with bitumen(VFB): 65 to 75Percent voids in mineral aggregate: As per table 500-12

Table 500-12 Nominal maximum Minimum VMA % related to designParticle size in mm air voids in %———————————————————————————

3% 4% 5%———————————————————————————9.5 14 15 1612.5 13 14 1519 12 13 1425 11 12 1337.5 10 11 12

Binder ContentThe binder content shall be optimized to achieve the requirement of the mixture as stated earlier.

The marshal method for determining the optimum binder content is adopted as described in asphalt institute manual MS2 by replacing the aggregate retained on 26.5mm sieve by the aggregates passing the 26.5mm sieve and retained on 22.4mm sieve.

Where the maximum nominal size of aggregate used is 40mm, the modified marshal method described in MS2 is used.In this case the minimum stability value as stated earlier is multiplied by 2.25 and the minimum flow shall be 3 mm.

The job mix formula submitted to the Engineer for approval shall contain the following details.Source & Location of materialsProportion of all materials expressed as follows.•Binder type and % by weight of total mixture.•Coarse aggregate/Fine aggregate/Mineral filler as % by weight of total aggregate including mineral filler.A single definite % passing each sieve for the mixed aggregate.The individual grading of individual aggregate fractions and the proportion of each in the combined grading.The marshal test results as given in table 500-11When the mixing plant is a batch type plant, the individual weight of each type of aggregate, and binder per batch.The results of physical characteristics of the aggregate to be used.Mixing temperature and compacting temperature.

Job Mix Formula

Plant trials Once the laboratory job mix formula is approved, plant trials is carried out at the mixer to establish that the plant can be set up to produce a uniform mix conforming to the approved job mix formula.

The permissible variations in the individual percentages of the various ingredients in the actual mix from the job mix formula to be used shall be within the limits specified in Table 500-13.

Once the plant trials have demonstrated the capability of the plant and the trials are approved, the laying operation can commence.

Table 500-13(Permissible variation from JMF)

Aggregate passing Permissible variation from JMF sieve size Base course wearing course

19mm or larger ±8% ±7%13.2mm & 9.5mm ±7% ±6%4.75mm ±6% ±5%2.36,1.18 & 0.6mm ±5% ±4%300& 150 micron ±4% ±3%75 micron ±2% ±1.5%Binder content ±0.3% ±0.3%Mixing temperature ±10°C ±10°C

Laying trialsOnce the plant trials are approved, the laying trials are carried out to demonstrate that the proposed mix can be successfully laid and compacted.The laying trials are carried out in an area not forming part of the permanent works.The area of laying trial shall be minimum 100 sqm of construction similar to that of the project road, and it shall be in all respect, particularly compaction, the same as the project construction on which the bituminous material is to be laid.The density of finished paving layer shall be determined by taking cores after 24 hours.Once the laying trial is approved, the same plant & methodology is then followed in laying of the material on the project.

Construction OperationThe mix produced from the Hot mix plant is discharged on to the dumper standing below the hopper in batches till the dumper is filled to its capacity. The mix is then covered with a tarpaulin to avoid contamination with dust during the travel. This also prevents the mix to retain its temperature during transit.The area to be paved are primed in advance and are swept clean from dust and other extraneous material by using a mechanical broom , and a jet of compressed air before the commencement of paving.Level stakes are fixed on either side of the carriageway to be paved to required level . The top of the stakes are tied with a string line which is stretched to avoid sagging between the stakes using winches fixed to the ground at both ends.

Preparation for DBM Laying

Construction Operation The Paver is then set to the required screed level using packing below the screed. The heating of the screed is then started to bring the screed temperature above 100ºC prior to paving to avoid sticking of mix to the screed.The sensors attached to the paver are then set on the string line on both sides for level and cross fall control.Once the paver setting is complete, the dumper carrying the mix is brought inside the paving area after checking the mix temperature for its acceptance.The dumper then gradually unloads the mix into the paver hopper wherefrom the mix is conveyed to the auger box through the belt conveyor.While paving the paver pushes the dumper forward while the dumper unloads the mix gradually into the paver hopper.

Laying & Compaction of DBM

Compaction of DBM

DBM Rolling by PTR

The mix in the auger box is then spread over the entire width of the bed through the rotary spiral action of the augers.As the paver moves forward, the screed moves over the mix imparting initial compaction through vibration of the screed and the tamping bar attached to the screed. The screed gives a smooth finish to the surface of the mix and pushes forward the extra material .While the paving continues, grades and cross fall are checked with respect to a string tied across the level stakes facing each other on either side by taking dip measurements. The actual levels of the paved surface is then compared with the corresponding design level by allowing for the compaction factor.Points found beyond tolerance are then immediately corrected by trimming high points and adding hot mix at low points.

Construction Operation

Construction operation Tandem smooth wheeled vibratory roller having controlled water sprinkling arrangement over drums shall be used for rolling the hot mix.

The rolling shall immediately start with a initial plain pass of the tandem roller to give a set to the mix in its position.

Rolling shall start in the longitudinal direction from the lower edge and proceed towards the upper edge with at least one third overlap of each adjacent track . The speed of the roller shall not exceed 5Km/hr.

After the initial plain pass is complete, subsequent rolling is continued with vibratory passes until no roller mark is visible on the surface.

Construction operation

A pneumatic tyre roller(PTR) is also used for rolling along with tandem roller . The PTR imparts kneading action to the surface of the mix and thus gives a close finish to the surface.

The rolling is terminated with a final plain pass of the tandem roller once no roller mark is visible on the surface.

All the rolling operation shall be completed before the temperature of the mix falls below 100ºC.After 24 hours of laying , cores are taken from the surface for determination of density and the laid surface is checked for surface levels and surface regularities . The permitted tolerance in surface level being ±6mm and 6mm for surface regularity.

Extraction of DBM Core

Wearing course-Bituminous Concrete

Scope: Construction of bituminous concrete for use in wearing course in single/multiple layers over a previously prepared bituminous bound surface.

The thickness of a single layer shall be between 25mm to 100mm in thickness as stipulated in the contract.

The minimum percent compaction shall be 98% of marshal density.

Materials for Bituminous ConcreteBitumen: The bitumen shall be paving bitumen of penetration grade S65 or as stipulated in the contract complying with IS:73.Modified bitumen such as crumbed rubber modified bitumen(CRMB) or polymer modified bitumen(PMB) may be used if specified in the contract.

Coarse aggregate: The coarse aggregate shall be crushed stone aggregates satisfying the physical requirements as detailed in table 500-17.

Table 500-17Physical requirements of C.A. for BC

Grain size : Maximum 5% passing 75 micron sieve.Combined flakiness & elongation index: Maximum 30%Los angeles abrasion value: Maximum 30%Aggregate impact value: Maximum 24%Polished stone value: Minimum 55Soundness: with sodium sulphate : Maximum 12% with magnesium sulphate : Maximum 18%Water absorption: Maximum 2%Stripping: Minimum retained coating 95%Water sensitivity: Retained tensile strength of Minimum 80%

Fine aggregate: The fine aggregates shall be crushed or naturally occurring mineral material passing 2.36mm sieve and retained on 75 micron sieve.

The fine aggregates shall be free from organic, soft and friable matter or any other deleterious matter.

The fine aggregate shall have a sand equivalent value of not less than 50 when tested as per IS 2720 part 37.

The plasticity index of the fraction passing 425 micron sieve shall not exceed 4 when tested as per IS:2720 par 5.

Material for Bituminous concrete

FillerFiller shall be finely divided mineral matter such as rock dust, hydrated lime or cement as specifically specified in the contract.

The filler shall have the grading as indicated in table 500-9 stated under DBM earlier.

The filler shall have plasticity index not greater than 4 if the filler is rock dust. This limit of PI shall not apply to foreign fillers such as hydrated lime or cement if used.

If the coarse aggregate is gravel, 2% by weight of total aggregate, shall be portland cement or hydrated lime and the % of fine aggregate reduced accordingly.

Aggregate grading & binder content

When tested in accordance with IS:2386 part I, the combined grading of the coarse and fine aggregate and added filler shall fall within the limits shown in table 500-18 for grading 1 or 2 as specified in the contract.

Grading 1 2Nominal aggregate size 19mm 13mmLayer thickness 50-65mm 30-45mmIS Sieve(mm) Cumulative % passing by wt. of total aggregate————————————————————————————————26.50 10019.00 79-100 10013.20 59-79 79-1009.50 52-72 70-884.75 35-55 53-712.36 28-44 42-581.18 20-34 34-480.60 15-27 26-380.30 10-20 18-280.15 5-13 12-200.075 2-8 4-10Bitumen content % by 5.0 to 6.0 5.0 to 7.0Mass of total mixBitumen grade S65 S65

Table: 500-18

Design MixRequirement for the mixture: Apart from the combined grading and quality requirements for individual ingredients , the mixture shall meet the following requirement.

TABLE: 500-19———————

Minimum stability at 60°C: 9.0KNFlow: 2 to 4mmCompaction level: 75 blows on each of the two faces of specimenPer cent air voids: 3 to 6% void in mineral aggregate(VMA): As per table 500-12% voids filled with bitumen(VFB): 65-75Loss of stability on immersion in water at 60°C:Min. 75% retained strength

The binder content shall be optimized to achieve the requirement of the mixture as stated under design mix.

The marshal method for determining the optimum binder content shall be adopted as described in asphalt institute manual MS2 by replacing the aggregate retained on 26.5mm sieve by the aggregates passing the 26.5mm sieve and retained on 22.4mm sieve.

Binder Content

Job Mix Formula(JMF)The job mix formula submitted to the Engineer for approval shall contain the following details.Source & Location of materialsProportion of all materials expressed as follows.•Binder type and % by weight of total mixture.•Coarse aggregate/Fine aggregate/Mineral filler as % by weight of total aggregate including mineral filler.A single definite % passing each sieve for the mixed aggregate.The individual grading of individual aggregate fractions and the proportion of each in the combined grading.The marshal test results as given in table 500-19When the mixing plant is a batch type plant, the individual weight of each type of aggregate, and binder per batch.The results of physical characteristics of the aggregate to be used.Mixing temperature and compacting temperature

Plant trials & Laying trials After the job mix formula is ready, plant trials shall be carried out to establish capability of the plant for satisfactory production of mix conforming to the job mix formula with permissible variations as indicated in Table 500-13.

Following the satisfactory production of mix from the hot mix plant, laying trials are carried out in a place not forming the part of the project road to demonstrate satisfactory laying and compaction of the mix in a fashion similar to DBM trial.

Construction OperationsPreparation of base: The surface on which bituminous concrete is to be laid is thoroughly swept clean by mechanical broom and dust removed by compressed air.Tack Coat: If specified in the contract, a tack coat shall be applied at rates as specified. Level stakes at suitable intervals i.e. 10m c/c for straight alignment and 5m c/c for curves are fixed on each side of the carriageway to be paved and their top adjusted to desired level .A string line is then tightly tied connecting each of the stakes with the help of winches.The mix produced in the plant is brought to site, laid using sensor paver and compacted to desired density as described earlier for dense bituminous macadam.

Opening to traffic

After 24 hours of laying, cores are taken from the surface for ascertaining % compaction. The surface is then checked for levels and surface regularity. The acceptable limit of tolerance for surface level being ±6mm and 3mm for surface regularity when checked with a 3m straight edge.

The newly laid surface is then opened to traffic.

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