Highway Construction | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Chapter 6  

 HIGHWAY CONSTRUCTION

TYPES OF HIGHWAY CONSTRUCTION
The highway types are classified as below : (i) Earth road and gravel roads (ii) Soil stabilized roads (iii) Water Bound Macadam (WBM) raod (iv) Bituminous or black-top roads (v) Cement concrete roads PAVEMENT TYPES
The selection of base course and the surface course depends upon the following factors : (i) Type and intensity of traffic, (ii) Funds available for the construction project and for the subsequent maintenance. (iii) Subgrade soil and drainage conditions. (iv) Availability of construction materials at site. (v) Climatic condition. (vi) Plants and equipment available. (vii) Time available for completing the project. (viii) Altitude at which construction has to be done.  The low cost roads can successfully function with the traffic intensity of 30 tonnes to 200 tonnes per day.  WBM roads may for over 500 tonnes per day.  The bituminous pavements and cement concrete roads are considered suitable for heavier traffic in density.  In highw ay cons tr u ct ion us u ally stage construction technique is adopted.
Earthwork : Equipment 1. Bull dozer : may be used for clearing site, opening up pilot roads, moving earth for short haul distances of about 100 m and also in several other jobs. 2. Scraper : is considered as one of the useful earth moving equipment as it self operating-it can dig, haul and discharge the material in uniformly thick layers. However scrapers are not capable of digging very stiff material.
3. Power shovel : is used primarily to excavate earth of all classes except rock and to load it into wagons. Power shovels may be mounted on crawler tracks and so they can move at low speeds.
Do you know ?
The power shovel can effectively operate to excavate earth from a lower level where it stands and when the depth of the face to be excavated is not too shallow. 4. Dragline : is used to excavate soft earth and to deposit in nearly banks or to load into wagons. It can operate on natural ground while excavating from a pit with the bucket; thus it is not necessary for the dragline to go into the pit in order to excavate. 5. Clam shell : is useful for excavation of soft to medium materials and loose material at or below existing ground surface. 6. Hoe : is meant to excavate below the natural surface where the machine is stationed and is capable of having precise control of depth of excavation at close range work. Hoe can excavate stiff material which normally can not be excavated by dragline.
EMBAKMENT
 When it is required to raise the grade line of a highway above the existing ground level it becomes necessary to construct embankments.
The grade line may be raised due to any of the following reasons: 1. to keep the subgrade above the high ground water table. 2. to prevent damage to pavement due to surface water and capillary water. 3. to maintain the design standards of the highway with respect to the vertical alignment.

TYPES OF HIGHWAY CONSTRUCTION
The highway types are classified as below : (i) Earth road and gravel roads (ii) Soil stabilized roads (iii) Water Bound Macadam (WBM) raod (iv) Bituminous or black-top roads (v) Cement concrete roads PAVEMENT TYPES
The selection of base course and the surface course depends upon the following factors : (i) Type and intensity of traffic, (ii) Funds available for the construction project and for the subsequent maintenance. (iii) Subgrade soil and drainage conditions. (iv) Availability of construction materials at site. (v) Climatic condition. (vi) Plants and equipment available. (vii) Time available for completing the project. (viii) Altitude at which construction has to be done.  The low cost roads can successfully function with the traffic intensity of 30 tonnes to 200 tonnes per day.  WBM roads may for over 500 tonnes per day.  The bituminous pavements and cement concrete roads are considered suitable for heavier traffic in density.  In highw ay cons tr u ct ion us u ally stage construction technique is adopted.
Earthwork : Equipment 1. Bull dozer : may be used for clearing site, opening up pilot roads, moving earth for short haul distances of about 100 m and also in several other jobs. 2. Scraper : is considered as one of the useful earth moving equipment as it self operating-it can dig, haul and discharge the material in uniformly thick layers. However scrapers are not capable of digging very stiff material.
3. Power shovel : is used primarily to excavate earth of all classes except rock and to load it into wagons. Power shovels may be mounted on crawler tracks and so they can move at low speeds.
Do you know ?
The power shovel can effectively operate to excavate earth from a lower level where it stands and when the depth of the face to be excavated is not too shallow. 4. Dragline : is used to excavate soft earth and to deposit in nearly banks or to load into wagons. It can operate on natural ground while excavating from a pit with the bucket; thus it is not necessary for the dragline to go into the pit in order to excavate. 5. Clam shell : is useful for excavation of soft to medium materials and loose material at or below existing ground surface. 6. Hoe : is meant to excavate below the natural surface where the machine is stationed and is capable of having precise control of depth of excavation at close range work. Hoe can excavate stiff material which normally can not be excavated by dragline.
EMBAKMENT
 When it is required to raise the grade line of a highway above the existing ground level it becomes necessary to construct embankments.
The grade line may be raised due to any of the following reasons: 1. to keep the subgrade above the high ground water table. 2. to prevent damage to pavement due to surface water and capillary water. 3. to maintain the design standards of the highway with respect to the vertical alignment.

 The design elements in highway embankments are.
(i) Height
(ii) Fill material
(iii) Settlement
(iv) Stability of foundation, and
(v) Stability of slopes

• Height : The height of the embankment depends on the desired grade line of the highway and the soil profile or topography. Also the height of the fill is some times governed by stability of foundation, particularly when the foundation soil is weak.
•  Fill material :Granular soil is generally preferred as highway embankment material. Silts, and clays are considered less desirable. Organic soils, particularly peat are unsuitable. The best of the soils available locally is often selected with a view to keep the lead and lift as low as possible. At times light-weight fill material like cinder may be used to reduce the weight when foundation soil is weak.
   
• Settlement : To accelerate the rate of consolidation of saturated foundation clay, vertical sand drains are sometimes constructed. These are vertical columns of sand installed in the compressible foundation like marshy soils in order to decrease drainage path and thus accelerate the rate of consolidation. The vertical sand columns may be of 30 to 60 diameter and 2.5 to 6 meter spacing, arranged in a hexagonal pattern. A horizontal sand blanket, 40 to 60 cm thick is placed at the top of the drains extending across the entire width of embankment at its bottom.

•  Stability of foundation : When the embankment  foundation consists of weak soil just beneath or at a certain depth below in the form of a weak stratum, it is essential to consider the stability of the foundation against a failure. This is all the more essential in the case of high embankments.
•  Stability of slopes : The embankment slopes should be stable enough to eliminate the possibility of a failure under adverse moisture and other conditions. Hence the stability of the slope should be checked or the slope should be designed providing minimum factor of safety of 1.5.

CONSTRUCTION OF EMBANKMENTS

•  The embankment may be constructed either by rolling in relatively thin layers or by hydraulic fills. The former is called rolled earth method and is preferred in highway embankments.
•  Compaction is carried out at optimum moisture content so as to take advantages of maximum dry density using a specified compacting effort and equipment. Preparation of Subgrade
•  The preparation of subgrade includes all operations before the pavement structure could be laid over compacted. Thus the preparation of subgrade would include site clearance, grading (embankment or cut section) and compaction.

COMPACTING EQUIPMENT
Soil compaction is achieved in the field either by rolling, ramming or by vibration. Hence the compacting equipment may also be classified as rollers, rammers and vibrators. Compaction of sands are also chieved by watering ponding and jetting.
ROLLERS

•  The principle of rollers is the application of pressure, which is slowly increased and then decreased. The various type of rollers which are used for compaction are :

1. Smooth Wheeled Rollers

•  These are suitable to roll a wide range of soils, preferably granular soils. These are particularly found to be useful in compacting soils and other materials where a crushing action is advantageous.

2. Pneumatic Tyred Roller 

• Suitable to compact nonplastic silts and fine sands. In addition to direct pressure due to rolling, there is also a slight kneading action.
  

3. Sheepsfoot Roller 

• Suitable to compact clayed soils. 
• The efficiency of the sheepsfoot rollers depends on the weight of the roller and the number of feet in contact with the ground at a time.

4. Rammers 

• Rammers are useful to compact relatively small areas and where the rollers can not operate such as compaction of trenches, foundation and slopes.

Do you know ?
The output of rammer is much lower than that of roller

5. Vibrators 

• Vibrators are most suited for compacting dry cohesionless granular material. 
• These are also vibrator mounted roller to give the combined effects of rolling and vibration.

6. Watering 

• Watering (Jetting and ponding) is considered to be an efficient method of compacting cohesionless sands.

Field Control for Compaction 

• For adequate quality control in construction, it is essential to have proper field control in compaction. The two field control tests needed are :
  (i) Measurement of moisture content
  (ii) Measurement of dry density 
• The moisture content of the soil may be found before compaction by any one of the rapid methods suitable at the site. If the moisture is controlled at the OMC, then the next control needed is the dry density, the desired value of which may be achieved by increasing the number of passes for the selected equipment and the thickness of each layer. Dry density may be found by any suitable method; the sand replacement method is considered quite satisfactory.

CONSTRUCTION EARTH ROADS

• The max imum cr oss s lopes of 1 in 20 is recommended to avoid erosion due to rain waters and formation of cross ruts. 
• The steep cross slope (ranges between 1 in 20 to 1 in 33) helps to keep the pavement surface free of standing water, otherwise the soil being pervious the water would damage the pavement section by softening it. 
• The construction of earth road may be divided into following steps :

1. Material 

• Soils of the following properties are considered satisfactory for earthen roads : 

•  The pavement section is totally made out of the soil available at site and at near-by borrow pits.

2. Location 

• The centre line and road edges are marked on the ground along the alignment, by driving wooden pegs.

3. Preparation of Subgrade

The various operations involved are as follows :

(a) Clearing site

(b) Excavating and construction of fills to bring the road to a desired grade

(c) Shaping of subgrade

4. Pavement Construction 

• The soil is mixed, spread and rolled in layers such that the compacted thickness of each layer does not exceed 10 cm. At least 95% of dry density of I.S. light compaction is considered desirable.

5. Opening to Traffic 

• The compacted earth road is allowed to dry out for a few days before opening to traffic.

CONSTRUCTION OF GRAVEL ROADS

•  This type of road can cater for about 100 tonnes of pneumatic tyred vehicle or 60 tonnes of iron tyred vehicles per day per lane.

Two types of construction methods are followed : 1. Feather edge type 2. Trench type

•  The feather edge type is constructed over the subgrade with varying thickness, so as to obtain the desired cross slope for the pavement surface. 
• In the trench type, the subgrade is prepared by excavating a shallow trench. 
• Since there is better confinement for the gravel, the trench type is preferred.

CONSTRUCTION STEPS
1. Material 

• Hard variety of crushed stone or gravel of specified gradation is used. 
• Material Gravel to be used for the construction is stacked along the sides of the proposed road.

2. Location 

3. Preparation of Subgrade 

• The width of the trench is made equal to that of the carriageway.

4. Pavement Construction 

• The layer is rolled using smooth wheel rollers starting from the edges and proceeding towards the centre with an overlap of at least half the width of roller in the longitudial direction.

5. Opening of Traffic A few days after the final rolling and drying out, the road is opened to traffic.

CONSTRUCTION OF WATER BOUND MACADAM ROADS

•  The WBM may be used as a sub base, base course or surfacing course. 
• The term macadam generally used for pavement base course made of crushed or broken aggregate mechanically interlocked by rolling and the voids filled with screening and binding material with the assistance of water.

SPECIFICATIONS OF MATERIALS FOR WBM PAVEMENT

•  Type of Coarse Aggregate : Hard varieties of crushed aggregates or broken stones are used.

However, soft aggregates like over burnt bricks, Kankar or laterite may be used. Blast furnace may also be used. 

Size and Grading Requirements of Coarse Aggregates

1. Grading No. 1 consists of coarse aggregates of size range 90 to 40 mm and is more suitable for sub base course. Thickness of compacted layer is usually 100 mm.

2. Grading No. 2 consists of aggregates of size range 63 to 40 mm.

3. Grading No. 3 consists of range 50 to 20 mm and compacted thickness of each layer is normally 75 mm. 

• Screening :
• The screenings are used to fill up the voids in the compacted layer of coarse aggregates. 
• The IRC has suggested that from economic considerations, predominantly non-plastic materials such as Kankar nodules, morum or gravel may be utilized as screening material provided the wL < 20%, IP < 6% and proportion of fines passing 0.075 mm sieve is less than 10%. 
• B inding Material : Binding material consisting of fine grained material is used to prevent ravelling of the stones. Kankar nodules or lime stone dust may also be utilized. 
• I_P range between 4 to 9% for WBM surface course and < 6% for WBM base or sub base course binding material with bituminous surfacing

Do you know ?

Binding material is not required if crushable type screening is used, unless the plasticity index value is low.

CONSTRUCTION STEP
1. Preparation of foundation for Receiving the WBM Course 

• The foundation layer i.e. subgrade, subbase or base course is prepared to required grade and camber and the dust and either loose materials are cleaned. 
• On existing road surfaces, the depressions and pot holes are filled and corrugations are removed by scarifying and reshaping the surface to the required grade and camber.

2. Provision of Lateral Confinement 

• Provision of Lateral Confinement may be done by constructing the shoulders to advance, to a thickness equal to that of the compacted WBM layer.

3. Spreading of Coarse Aggregates

4. Rolling  

• Rolling is started from the edges and then gradually shifted towards the centre line of the road.

5. Application of Screenings 

• After the coars e agg reg ates are rolled adequately, the dry screenings are applied gradually over the surface to fill the interstices in three or more applications. Dry rolling is continued as the screenings are being spread and brooming carried out.

6. Sprinkling and Grouting 

• After the application of screenings, the surface is sprinkled with water, swept and rolled. Wet screenings are swept into the voids using hand brooms.

7. Application of Binding Material 

• After the application of screening and rolling, binding material is applied at a uniform and slow rate at two or more successive thin layers. After each application of binding material, the surface is copiously sprinkled with water and wet slurry swept with brooms to fill the voids.

Do you know ?
When crushable type screenings like moorum or gravel are used, there is no need to apply binding materials, except in the surfacing course.

8. Setting and Drying 

• After final compaction, the WBM course is allowed to set over-night. On the next day the ‘hungry’ spots are located and are filled with screenings or binding material, lightly sprinkled with water if necessary and rolled. No traffic is allowed till the WBM layer sets and dries out.

CHECKING OF SURFACE EVENNESS AND RECTIFICATION OF DEFECTS

• The surface evennes of longitudinal direction is checked by 3.0 m straight edge. 
• The number of undulations exceeding 12 mm in the case of WBM layer of grading No. 1 and 10 mm in the case of grading nos. 2 and 3 are recorded in each completed length of 300 m; the maximum number of undulations permitted in each case is 30. 
• The spots with 15 mm undulations are marked for rectification of defects. 
• The cross profile is checked using camber template. 
• The maximum variation from special profile should not exceed 12 mm in the case of aggregate grading no. 1 and 8 mm in the case of gradings 2 and 3.

CONSTRUCTION OF BITUMINOUS PAVEMENTS
The following construction techniques are in use :

1. Interface treatments (a) prime coat (b) tack coat

2. Surface dressing and seal coat

3. Grouted or penetration type constructions (a) Penetration Macadam (b) Built-up Spary Grout

4. Premix which may be any of the following : (a) Bituminous bound macadam (b) Carpet (c) Bituminous concrete (d) Sheet asphalt or rolled asphalt (e) Mastic asphalt

1. Interface Treatment 

• The surface of the existing pavement layer is to be cleaned to remove dust and dirt and a thin layer of bituminous binder is to be sprayed before the construction of any type of bituminous layer over this surface. This treatment is necessary to provide the necessary bond between the old and the new layers.

(a) Prime Coat 

• Bituminous prime coat is the first application of a low viscosity liquid bituminous material over an existing porous or absorbent pavement surface like the WBM base course. 
• The main object of priming is to plug in the capillary voids of the prous surface and to bond the loose mineral particles on the existing surface using a binder of low viscosity which can penetrate into the voids. Usually MC or SC cutback of suitable grade or viscosity is chosen depending on the porosity of the surface to be treated. 
• The primed surface is allowed to cure for at least 24 hours, during which period no traffic is allowed.

Do you know :

The bituminous primer is sprayed uniformly using a mechanical sprayer at a rate of 7.3 to 14.6 kg per 10 m2 area, depending on the porosity of the surface.

(b) Tack Coat 

• Bituminous tack coat is the application of bituminous material over an existing pavement surface which is relatively impervious like an existing bituminous surface or cement concrete pavement or a pervious surface like the WBM which has already been treated by a prime coat. 
• Bituminous material of higher viscosity like hot bitumen is used and in cold state, bituminous emulsion may also be applied.

Do you know ?
Tack coat is usually applied by spraying bituminous material of higher viscosity like the hot bitumen at the rate of 4.9 to 9.8 kg per 10 m2 area depending in the type of the surface.

2. Bituminous Surface Dressing (BSD) 

• Bituminous surface dressing is provided over an existing pavement to serve as thin wearing coat. 
• The single coat surface dressing consists of a single application of bituminous binder material following by spreading of aggregate cover and rolling.

The main function of BSD are :

(a) to serve as a thin wearing course of pavement and to protect the base course

(b) to water proof the pavement surface and to prevent infiltration of surface water

(c) to provide dust-free pavement surface in dry weather and mud-free pavement in wet weather.

Seal Coat  

• Seal coat is usually recoomended as a top coat over certain bituminous pavements which are not impervious, such as open graded bituminous constructions like premixed carpet and grouted Macadam. 
• Seal coat is also provided over an existing bituminous pavements which is worn out. 
• A premixed sand bitumen seal coat is also commonly used over the premixed carpet.

The main functions of seal coat are :

(a) to seal the surfacing against the ingress of water

(b) to develop skid resistant texture

(c) to enliven an existing dry or weathered bituminous surface.

3. Grouted or Penetration Type Constructions

(a) Penetration Macadam 

• Bituminous Penetration Macadam or Grouted Macadam is used as a base or binder course. 
• The coarse aggregates are first spread and compacted well in dry state and after that hot bituminous binder of relatively high viscosity is sprayed in fairly large quantity at the top.
• The bitumen penetrates into the voids from the surface of the compacted aggregates, thus filling up a part of the voids and binding some stone aggregates together.

• Full grout is adopted in regions of heavy rainfall and semigrout is adopted in regions of moderate rainfall and traffic.

(b) Built-up Spray Grout (BSG) 

• Built-up Spray Grout (BSG) consists of two-layer composite construction of compacted crushed aggregates with application of bituminous binder after each layer for bonding and finished with key aggregates at the top to provide a total compacted thickness of 75 mm. 
• This method is commonly used for strengthening of existing bituminous pavements. A suitable wearing course is invariably provided over this layer before opening to traffic.

4. Premix Methods 

• In this group of methods the aggregates and the bituminous binder are mixed thoroughly before spreading and compacting. The common types of premixed bituminous constructions are bituminous macadam, bituminous carpet, and bituminous concrete. Other types of premixed constructions include sheet asphalt and mastic asphalt.

(a) Bituminous Macadam (BM) or Bituminous bond Macadam 

• Is a base course or binder course, should be covered by a suitable surface course before exposing to traffic. 
• B.M. base course is considered to be much superior than other types of base course materials such as WBM w.r.t. load dispersion characteristics and durability.

(b) Bituminous Premixed Carpet 

• Premixed Carpet (PC) consists of coarse aggregates of 12.5 and 10.0 mm sizes, premixed with bitumen or tar binder are compacted to a thickness of 20 mm to serve as a surface course of the pavement. 
• Open graded, should be covered by suitable seal coat. 
• The PC consists of all aggregates passing 20 mm and retained 6.3 mm sieve.

(c) Bituminous Concrete or Asphalt Concrete (AC)

• Is a dense graded premixed bituminous mix which is well compacted to form a high quality pavement surface course. 
• The AC consists of a carefully proprotioned mixture of course aggregates, fine Aggregates, mineral filler and bitumen and mix is designed by an appropriate method (Marshal Method). 
• The IRC has provided specification for 40 mm thick AC surface course for highway pavements. 
• The thickness of bituminous concrete surface course layer usually ranges from 40 to 75 mm.

(d) Sheet Asphalt 

• Sheet asphalt or rolled asphalt is a dense sandbitumen premix of compacted thickness 25 mm, used as wearing course.  This is usually laid over cement concrete pavement to provide an excellent riding surface.

Do you know ?
The sheet asphalt also protects the joints in cement concrete pavements and could cause a reduction in warping stresses due to a decrease in the temperature variations between top and bottom of the concrete slab.

(e) mastic Asphalt 

• Mastic asphalt is a mixture of bitumen, fine aggregates and filler in suitabhle proportions which yields a voidless and impermeable mass.
• Mastic asphalt can absorb vibrations and has a property of self-healing of cracks without bleeding. 
• It is a suitable surfacing material for bridge deck slabs. 
• The mastic asphalt should be spread at a temperature of about 200°C to a thickness between 2.5 to 5.0 cm. No Rolling is required.

BITUMINOUS CONSTRUCTION PROCEDURES
Following types of bituminous construction have been presented.

1. Surface Dressing 

• The surface dressing work is done only in dry and clear weather when the atmospheric temperature is above 16°C.

Specification of Materials 

• Bitumen grades between 80/100 and 180/ 200 are generally used. Tar or cutback may also be used. 
• Ag greg ate type should conform to the following requirements :
  Los Angles abrasion value 35% max.

Aggregate impact value 30% max.
Flakiness index 25% max.
Water absorption 1% max.
Stripping at 40°C after 24 hrs immersion (CRRI test) 25% max.

Construction Steps

1. Preparation of existing surface
2. Application of binder
3. Application of stone chippings
4. Rolling of first or final coat
5. Application of binder and stone chippings for second coat
6. Rolling of second coat
7. Finishing and opening to traffic

2. Penetration (Grouted) Macadam 

• The construction of penetration macadam is recommended for thickness of 50 and 75 mm.

Specification of Materials 

• Bitumen 80/100, 60/70 and 30/40 are generally used. Road tars RT-4 and RT-5 could also be used. 
• The physical requirements of stone aggregates are specified by the following test values : Los Angles abrasion value 40% max.

Aggregate impact value 30% max.
Flakiness index 25% max.
Stripping at 40°C after 24 hrs. Immersion (CRRI test) 25% max.
Loss with sodium sulphate, 5 cycles 12% max.
Do you know ?
The coarse aggregates required for 50 mm compacted thickness is 0.06 m³ per 10 m² area and for 75 mm compacted thickness is 0.90 m³ per 10 m².

Construction Steps 

1. Preparation of existing surface
2. Spreading the coarse aggregates
3. Rolling
4. Bitumen application
5. Spreading of  Flakey aggregates
6. Seal coat
7. Finishing
8. Opening to traffic; after a minimum period of 24 hours

3. Construction of Bituminous Macadam 

• The Bituminous Macadam is a premix laid immediately after mixing and then compacted. 
• It is an open graded construction suitable only as a base or binder course.

Specification of Materials 

• The grades of bitumen used are 30/40, 60/ 70 and 80/100 penetration. Road tar RT-4, cut back and emulsion can also be used in cold mix construction. 
• Aggregates used are of low porosity fulfilling the following requirements for the base course:

Los Angles abrasion value 50% max.
Aggregate impact value 35% max.
Flakiness index 15% max.
Stripping at 40°C after 24 hrs immersion (CRRI test) 25% max.
Loss with sodium sulphate, 5 cycles 12% max. 

• For binder course the specified max. abrasion and impact values are 40 and 30% respectively. 
• The quantity of aggregates required for 10 m2 of bitumen bound macadam are 0.60 to 0.75 m3 and 0.90 to 1.0 m3 respectively for 50 and 75 mm compacted thickness.

Construction Steps

1. Preparation of existing layer
2. Tack coat or prime coat application
3. Premix preparation
4. Placement
5. Rolling and finishing the paving mix

4. Construction of Pre-m ixed Bitum inous Carpet Specification of Materials 

• The bitumen binder of 80/100 grade or road tar of grade RT-3 is used. 
• The quantity of binder required for 2 cm thick carpet is as under.
  (i) Prime coat on a WBM surface 7.3 to 9.8 kg bitumen or 12.20 to 14.64 kg road tar per 10 m2.
  (ii) Tack coat on an existing black top surface 4.9 to 7.3 kg bitumen or 7.3 to 9.8 kg road tar per 10 m2. 
• The aggregates fulfilling the following requirements may be selected.

Los Angeles abrasion value = 35% max.
Aggregate impact value = 30% max.
Flakiness index = 30% max.
Stripping value = 25% max.
Water absorption = 2.0% max.

Construction Steps

1. Preparation of the existing surface
2. Application of fack coat
3. Preparation and placing of premix
4. Rolling and finishing
5. Application of seal coat
6. Surface finish
7. Opening to traffic

5. Construction of Bituminous Concrete Roads 

• The thickness of the bituminous concrete layer depends upon the traffic and quality of base course.

Specification of Materials
 (a) Binder 

• Bitumen of grade 30/40. 60/70 or 80/100 may be chosen depending upon the climatic condition of the locality.

(b) Aggregates and filler maximum% Aggregate impact value 30% Loss Angeles abrasion value 40% Flakiness index 25% Tripping at 40°C after 24 hours, 25% Soundness Loss with sodium sulphate in 5 cycles 12% Loss with magnesium sulphate in 5 cylces 18%

(c) Bituminous Concrete Mix  Marshall Stability Test-number of blows to be applied on either side of Specimen: 50 Marshal Stability value, min. kg: 340 Marshall Flow value; 0.25 mm units : 8 to 16 Voids in mix, percent : 3 to 5 Voids filled with bitumen, percent : 75 to 85

Construction Steps

1. Preparation of the existing base course layer
2. Application of Tack coat
3. Preparation and placing of premix
4. Rolling
5. Quality Control of bituminous Concrete Construction
6. Finished surface

6. Construction of Cem ent Concrete Pavements 

• The cement concrete pavement maintains a very high recognition among the engineer and the road users alike. Due to the excellent riding surface and pleasing appearance, the cement concrete roads are very much preferred.

Further the engineers have inherent confidence in the cement concrete materials for its use in any construction project. It is also true that the life of a cement concrete road is much more than any other type of construction. 

• The cement concrete pavements are constructed with or without the sub-base courses. This decision is made depending upon the soil type, design load and economic consideration.

Purpose of Providing Sub-base Course 

1. to provide a strong supporting layer.

2. to provide a capillary cut-off preventing the damages due to mud pumping.

3. to reduce thickness requirements of cement concrete slab and lower the cost of construction.
WBM is the most popular type of underlying layer generally adopted in India. Soil stabilized layers can also be used with advantage.

4. to increase the service life of the CC pavement.
The construction of cement concrete pavement is dealt under the following groups.

1. Construction of Pavement Slabs 2. Construction of joints.

Construction of Cement Concrete Pavement Slab

Various specifications for construction of cement concrete pavements are listed below : 

• Cement grouted layer 
• Rolled concrete layer 
• Cement concrete slab 
• In cement grouted layer, open graded aggregate mix with minimum size of aggregates as 18 to 25 mm is laid on the prepared subgrade and the aggregates are dry rolled. The loose thickness is compacted to provide 80 percent of rolled thickness. The grout made of coarse sand, cement and water is prepared. The proportion of cement of sand is taken as 1 : 1½ to 1 : 2½.
•  In rolled concrete layer, lean mix concrete is used. Lean mix of aggregate, sand, cement and water is prepared and laid on the prepared, subgrade or sub-base course. This rolling is done similar to WBM construction. 
• Cement grouted and rolled concrete are suitable for base course only. 
• There are two modes of construction of cement concrete slab
  (i) Alternate bay method
  (ii) Continuous bay method 
• Alternate bay method of construction means constructing a bay or one slab in alternate succession leaving the next or intermediate bay to follow up after a gap of one week or so. 
• As in figure, in alternate bay construction the slabs constructed are in sequence of X, Y, Z etc., leaving gaps of bay X', Y', Z', etc. This technique provides additional working convenience for laying of slabs.

Transverse Joint Construction Method of Cement Concrete Road But it has many drawbacks as follows :

1. Large number of transverse joints are to be provided. This increases the construction cost and reduces the smooth riding quality of the surface.

2. During rains, the surface water collects on the subgrade between the finished bays.

3. The construction is spread over the full width of road and the traffic will have to be completely diverted.

In continuous (bay) method all the slabs or bays are laid in sequence i.e., X', Y, Z', etc. or X, Y', Z etc.
Construction joints are nowever provided at the end of the day’s job. In general the later method is preferred mainly because of the advantage that construction of  half the pavement width can be taken at a time while essential traffic could be diverted on the other half of the road.

Plants and Equipment 

1. Wheel Borrow : are used to transport concrete for short distances from the mixer.

2. Float : used for smoothing the concrete

3. Straight Edge : used to check the finished pavement surface in longitudinal direction.

4. Belt : Canvas belts are used for finishing the pavements surface before the concrete hardens.

5. Fibre Brush : is used to make broom marks across the pavement surface and to make it skid resistant.

6. Edging Tool : is used for rounding the transverse edges at expansion joints and the longitudinal edges.
 

Specification of Materials  Aggregates : The max size of coarse aggregates should not exceed one fourth the slab thickness.

• Proportioning of Concrete : The concrete may be proportioned so as to obtain a minimum modulus of rupture of 40 kg/cm² on held specimens after 28 days curing or to develop a minimum compressive strength of 280 kg/cm² at 28 days, or higher value as desired in the design.

Construction Steps 1. Preparation of Subgrade and Sub base 

• The minimum modulus of subgrade reaction obtained with a plate bearing test should be 5.54 kg/cm². 
• The subgrade is prepared and checked at least two days in advance of concreting. 
• The subgrade or sub base should be saturated with water for 6 to 20 hours in advance of placing concrete.

 Placing of Forms

3. Batching of Material and Mixing 

• The coarse aggregates and fine aggregates are proportioned by weight in a weigh-batching plant and placed into hopper along with necessary quantity of cement. Cement is measured by the bag.

Do you know ?
All batching of material is done on the basis of one or more whole bags of cement, the weight of one bag is taken as 50 kg or the unit weight of cement is taken as 1440 kg/m3.

4. Transporting and Placing of concrete

5. Compaction and finishing

6. Floating and straight edging

7. Belting, Brooming and Edging 

• Just before the concrete becomes hard, the surface is belted with a two ply canvas belt. 
• After belting, the pavement is given a broom finish with fibre broom brush.

8. Curing of cement concrete
(i) Initial Curing : The surface of the pavement is entirely covered with cotton or jute mats.  The mats are thoroughly saturated with water.

(ii) Final Curing

1. Curing with west soil : The soil is thoroughly kept saturated with water for 14 days.

2. Impervious membrane Method : Liquid is applied under pressure with spray nozzle to cover entire surface with a uniform film. It hardens with 30 min after its application.
When the concrete attains the required strength or after 28 days of curing the concrete road is opened to the traffic.

CONSTRUCTION OF JOINTS IN CEMENT CONCRETE PAVEMENTS

•  Joints are provided in cement concrete roads for expansion, contraction and warping of the slabs due to the variation in the temperature of slabs. 
• During the mid-day the top of the pavement slab has higher temperature than the bottom of the slab.

This causes the top fibres of the slab to expand more than the bottom fibres, and the slab curls at the edges as shown in fig. (b). This phenomenon is known as warping down of the slab. 

• By about the mid night the temperature of the bottom of the slab is higher than the temperature of the slab top. The slab warps up during this time as shown in fig. (c).

Do you know ?
In reality, the weight of the pavement slab prevents the slab to take a warped shape thereby developing stress in the slab which are known as warping stresses. The magnitude of warping stresses are maximum at the interior region and are minimum at the corner region.
JOINTS
These are further classified as :
(a) Expansion joint
(b) Contraction joint
(c) Warping joint
(d) Construction joint 

Tranverse Joints Expansion Joints

• Expasion joints in India are provided at interval of 50 to 60 metre for smooth interface laid in winter and 90 to 120 metre for smooth interface laid in summer. 
• For rough interface the spacing between expansion joints may be 140 m. 
• The approximate gap width for this type of joints is from 20 to 25 mm. 
• The stresses induced due to the wheel loads at such joints are of very high order at the edge and corner regions. 
• The load transference across the transverse joint is carried out through a system of reinforcement provided at suitable intervals projecting in the concrete in longitudinal direction upto 60 cm length. Such a device is named as dowel bar. 
• In the des ign, 40 percent of wheel load is expected to be taken up by the group of dowel bars and transferred to the adjoining slab. It ranges between 20 to 30 mm. The total length of dowel bar varies between 40 cm to 73 cm depending upon the dowel diameter. 
• Spacing between the dowel bars is generally adopted as 30 cm.

Contraction Joints

•  These joints are spaced closer than expansion joints. Load transference at the joints is provided through the physical interlocking by the aggregates projecting out at the joint faces. 
• The maximum spacing of contraction joints in unreinforced CC slabs is 4.5 m and in reinforced slab of thickness 20 cm is 14 m. 
• There seems to be no need to providing any load transference, as mainly this will be done by the aggregate interlocking. For added safety, some agencies recommended the use of dowel bar which are fully bounded in the concrete.

Warping Joints 

• The warping joints are provided to relieve stresses included due to warping. These are known as hinged joints. 
• Longitudinal joints with tie bars fall in this class of joint. These joints are rarely needed if the suitably designed expansion and contraction joints are provided to prevent cracking.

Longitudinal Joints

•  Longitudinal joints are provided in cement concrete roads which have width over 4.5 m. On soil subgrade of clay, such joints are provided to allow differential shrinkage and swelling due to rapid changes in subgrade moisture under the edges than under the centre of road. 
• The longitudinal joints are provided to prevent longitudinal cracking in the cement concrete pavements. 
• This type of joint acts as a hinge and helps to maintain the two slabs together, at the same level. 
• IRC recommends to use plain butt with tie bar type of joints. 
• In cement concrete slab of thickness 20 cm, (a) 10 mm diameter deformed tie bars of length 35 cm or plain bars of length 45 cm are placed at 45 cm spacing, or (b) 12 mm diamter deformed bars of length 40 cm or plain bars of length 55 cm are placed at 64 cm spacing. In slabs of thickness 25 mm, tie bars of diamter 10, 12 or 14 mm and length 35 top 46 cm are placed at 30 to 62 cm spacing. 
• Sympathetic cracks are often formed in line with the transverse joints. It is therefore recommended to provided joints across the longitudinal joint in same transverse alignment as shown in figure (b)

Joint Filler and Sealer 

• The infiltration of water damages the soil subgrade and gives rise to the phenomenon known as mud pumping espcially if the subgrade is of clayey soil.

If stone grit enters into the joint space, the effective joint width gets reduced and faults like spalling of joint edges take place. 

• Soft wood, impregnated fibre board, cork or cork bound with bitumen are generally used as joint filler. 
• Bitumen, Rubber-bitumen used as a sealing compound. Air blown bitumens may be used with advantage, as they are less susceptible to the temperature changes.

Reinforced Concrete Pavements 

• The greater quantity of reinforcement should be placed in the longitudinal direction. 
• The reinforcement should either be placed in the mid depth or towards the top of the pavement for better functioning.

CONSTRUCTION OF SOIL STABILIZED ROADS

Methods The methods of soil stabilization which are in common use are :

1. Mechanical soil stabilization

2. Soil-cement stabilization

3. Soil-lime stabilization

4. Soil-bitument stabilization

Mechanical Soil Stabilization  

• Suitable for granular soil 
• Proportioning of aggregate and soils is done to get a mechanically stable layer. 
• Stability or a soil-aggregate mix cou ld be increased by increasing its dry density. Hence proportioning of the mixed is done to attain maximum dry density. 
• Proportioning of Materials may be done by Triangular chart or Rothfutch’s Method.

Mix Design in Mechanical Stabilization 

• The factors to be considered in the design of mix are gradation, density, index properties and stability. Of these, the graduation is the most important factor 
• Gradation : The particle size distribution that gives maximum density is generally aimed at. The theoretical gradation for maximum density is given by

Here, P = percent fine than diameter ‘d’ (mm) in the material D = diameter of largest particle, mm n = gradation index, which have values ranging from 0.5 to 0.3 depending upon the shape.
Soil-Cement Stabilization 

• Soil-cement is an intimate mix of soil, cement and water which is well compacted to form a strong base course. 
• In granular soil, the mechanism of stabilization is due to the development of bond between the hydrated cement and the compacted soil particles at the points of contact. 
• In fine grained soil, the stabilization is due to reduction in plasticity and formation of matrix enclosing small clay lumps. 
• As the material has poor resistance to abrasion and impact, this can not be used as a surface course. A bituminous wearing course is placed over the base course.

Soil Lime Stabilization 

• Soil-lime is widely used either as a modifier for clayey soil or as a binder. 
• When clayey soils with high plasticity are treated with lime, the plasticity index is decreased and the soil becomes friable and easy to be pulverized, having less affinity with water. 
• Lime imparts some binding action in granular soils. 
• Soil-lime is quite suitable as sub-base course for high types of pavements and base course for pavements with low traffic.  Soil-lime cannot be used as surface course due to poor resistance to abrasion and impact.

Stabilization of Black Cotton Soils 

• Black cotton soils are highly clayey soils, greyish to blackish in colour. 
• The black cotton soils are found to contain montmorillonite clay mineral which has high expansive characteristics. 
• The most effective method to stabilize BC soils are by using lime along with suitable additives. 
• The cement r equireme nt for satisfactory stabilization of BC soil is so high that it is not advisable to use portland cement for stabilization.

Stabilization of Desert Sand 

• Soil-bitmen stabilization is generally used for Desert sand. 
• The most promising bituminous material in desert region seems to be the emulsion.