Unit:4  Road Construction Technology Note (Highway Engineering -II)

4.1 INTRODUCTION 

• Planning, geometric and structural design of highways are studied in the previous chapters.

• How the highways are built i.e. highway construction. 

• In fact road construction project can be divided into two phases. 

¢  E/W and preparation of subgrade  

¢  Laying of pavement 

¢  Protection of work like retaining wall, toe wall, parapet wall, etc.construction of drainage facilities. 

 Road Construction Technology is the branch of highway/Road engineering which deals with all kinds of activities and technology or operations for changing existing ground to the designed shape, slope and to provide all necessary facilities for smooth, safe and efficient traffic operation. It also includes the reconstruction of existing roads. Any technology for civil construction depends upon the available resources which are available equipment, plant and human resources. The construction technology differs as per the adoption of the resources. 

4.2  Activities and Techniques Used in Road  Construction 

As per the nature, type of works and elements of road to be constructed various activities can be broadly divided into several works as follows. 

4.2.1) Earthwork 

-  Site Clearance 

- Earthwork in filling for embankment 

-  Excavation for cutting 

-   Excavation for borrow pit 

-   Excavation for structural foundation 

-  Disposal of surplus earth 

 4.2.2) Drainage Works 

- Side drains 

-   Culverts 

-   Sub-surface drain 

-   Causeways 

-   Minor bridges 

- Other water management structures .

4.2.3) Pavement Works 

- Sub-grade preparation 

-  Sub-base course 

-   Base course 

-   Wearing course 

4.2.4) Protection Works 

- Earth retaining structures 

-  Gully control works 

-  Landslide stabilization works 

-  River training works 

-   Bridge protection works 

4.2.5) Miscellaneous Works 

-  Road furniture 

-  Traffic sign/signal/marking etc. 

-    Bio-engineering works. 

 

4.3 EARTHWORK  

  4.3.1 INTRODUCTION

 It includes all construction operations required to convert the road land from its natural condition and configuration to the sections and grades prescribed in the plans.

The eventual aim of the earthworks phase of the construction is to position the subgrade underlying the pavement layers in the right location and at the correct level, and to provide drainage. The operations to be performed are: 

•  formation of cuttings by excavating through high ground, 

•  formation of embankments by filling over low ground, • shaping the finished surface to design levels, and 

•   Excavating for drainage works.

The earthworks is often the largest task in the road building process and therefore careful planning and organization are essential. Speed and efficiency depend very much upon the quantity and types of earthmoving plant available. 

* Sequence of Operations

The normal sequence of operations in cut and fill work is: 

In Cut 

•   excavate to the depth necessary to reach formation level

 •  transport away from the site undesirable material such as organic soils, • haul suitable materials from cuts to fill areas, and 

 •  Suitably dispose of any excess cut material. 

In Fill 

• drain water from depressions and dispose of any unsuitable underlying material,

• spread fill material in horizontal layers not more than 250 mm thick, and 

• thoroughly compact these layers to required density. 

Mass haul diagram 

A graphical representation of the cumulative amount of earthwork moved along the centerline and distances over which the earth and materials are to be transported. Mass curves determine the average haul, free haul, and overhaul on a given segment of roadway. Mass diagrams tell the contractors and inspectors the quantity of material moved and how far it can be economically moved.  

4.3.3  Earthwork in embankment/excavation 

 E/W in excavation

Excavation is the cutting or loosening and removing earth including rock from it’s original position, transporting and dumping to the site as a fill or spoil bank. E/W is done when the natural ground level is higher the designed grade line level. The depth of cutting depends up on the height of grade line below natural ground level and can be calculated from L-Section and Cross section of the road. The slope to be provided for excavation depends upon the nature and type of soil and depth of cutting construction of side drains also requires excavation along road side. 

The excavation equipments are bulldozer, drag line, scrappers, clam shell hoe etc. The selection of particular type of equipment depends upon the type of soil, availability of equipment and cost benefit analysis of project.    

E/W in Embankment 

If the filling of earth or soil to achieve the desired grade line with the consideration of vertical alignment. It is necessary when natural ground level is below the grade line level or formation level. The grade line may be raised due to any of the following reasons 

-  To keep the subgrade above the high ground water table. 

-  To prevent damage to pavement due to surface water and capillary water. 

-  To maintain the design standards of the highway with respect to the vertical alignment.  

¢ The Design element of Highway Embankment are:-  

1.  Height of fill

 Depends on the formation level and location of natural ground. In case of weak soil, its bearing capacity and stability control the height of embankment. 

2.  Fill materials 

Generally, granular soil is preferred as highway embankment material. As far as possible organic soils, silts should be avoided. If the foundation is very weak then high soil as cinder may also be used as fill material. 

3.  Settlement of embankment 

The settlement of fill material i.e. embankment may be caused due to   

- Settlement of fill material itself 

- Settlement of foundation  

- Both of the above 

To reduce the settlement of foundation at high moisture content sometimes following remedy is taken into account:- 

The use of vertical drains and sand blanket will reduce the path of flow so that there is no danger from settlement point of view. Sand blanker is extended beyond the bottom width.

 Whatever is the type of settlement it is desirable that the settlement is almost complete before the construction of pavement. 

4.  Stability of foundation 

The foundation stability is evaluated and the factor of safety is estimated by any of the following approaches. 

- Assuming a certain failure surface such as a circular arc or any other composite shape and analyzing it with Swedish circular arc analysis or method of wedges as the case may be. 

-  Estimating the average shear stress and strength at the foundation layers by approximate method and estimating the factor of safety. 

-  Using theoretical analysis based on elastic theory. 

5.  Stability of Slope  

Embankment slopes should be stable enough to eliminate the possibility of a failure under adverse moisture and other conditions. The stability of the slope should be checked by providing minimum factor of safety of 1.5. Flatter slopes are preferred than in cutting. 

¢ CONSTRUCTION OF EMBANKMENT   

The embankment may be constructed either by rolling in relatively thin layers called rolled earth method or by hydraulic fills. In rolled earth method each layer is compacted by rolling to a satisfactory degree or to a desired density before next layer is placed while rolling the layers of soils are maintained at optimum moisture content. Compaction at optimum moisture content provides the maximum dry density. The thickness of the layers may vary between 10-30 cm depending on various factors such as soil type, equipment, specification etc.

The practice of dumping the earth without compacting properly and allowing the fill to get consolidated under weather during few subsequent seasons should be avoided as the settlement will continue for a very long period if the pavement is constructed before the settlement of the fill is almost complete. The pavement is likely to become uneven and also fail later.  

  4.3.4 Relation of optimum moisture content and maximum dry density

Mr. R. R. Proctor deeply studied the soil with respect to moisture in 1933 A.D. He found that density of soil increases in water content upto certain limit. This limit is known as optimum moisture content (OMC). As the water content is further increases beyond this limit the density of soil starts decreases. Hence it is desirable to compact the soil at the OMC in highway construction. 

Mr. R. R Proctor also plotted a graph between the moisture content & density of the soil. The graph thus obtained is called compaction curve which is shown in figure below:

fig- Moisture density relationship 

 The theoretical relationship between moisture content & dry density is given below: 

𝜸

             𝜸_𝒅 =                    _{…………..} (i)  

𝟏+𝒘

                          𝜸_d - unit weight of soil soil

                        𝜸 - unit weight of soil      

                           w- moisturecontent. 

   4.3.5 Field control of compaction and test required for field control  

Compaction of soil is the process by which the soil particles are constrained to pack more closely together through a reduction in air voids generally by mechanical means. The object of compacting soil is to improve its properties and to increase its strength and bearing capacity reduce its compressibility and decrease its ability to absorb water due to reduction to volume of voids. The various factors influencing soil compaction include moisture content, amount and type of compaction, soil type and stone content. There is optimum moisture content for a soil which would give maximum dry density for a particular type and amount of compaction. Hence it is desirable to compact the soil at the OMC after deciding the compacting equipment. The moisture content during compaction must also be specified and carefully controlled during construction to achieve the maximum density by the selected method of compaction. 

* FIELD CONTROL OF COMPACTION 

For adequate quality control in construction, it is necessary to have proper field control in construction. The two field control tests needed are; 

-  Measurement of moisture content 

-  Measurement of dry density 

Before compaction of earthwork is undertaken, it is always preferred to know the optimum moisture content for the soil, which can be determined by Proctor's field control method. If the moisture content of the soil during compaction is controlled at 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 (sand replacement method is widely used.)

 In field it is not possible to achieve 100% results in comparison to standard results obtained in the laboratory. However, by field checks it is possible to control the compaction to achieve the best possible results. 

 ¢ LOW COST ROAD  

A low cost road is a road constructed at a low cost and capable of being maintained at a low cost. In villages and undeveloped areas the immediate need is not of good roads which may serve the traffic needs. As the traffic increases on the road, as a result of the development existing roads may be improved upon to serve the needs of the increased traffic. This enables economical use of the funds. Construction of low cost roads is very preferred in developing countries like Nepal where large length of roads are to be constructed in the rural area with the limited available funds. Earth roads, Gravel roads and soil stabilized roads are the examples of such roads. 

  4.4 Construction of earthen road:Introduction, materials required, equipment   required, construction procedure 

¢  INTRODUCTION  

An earth road is the cheapest type of road in which the pavement structure is made totally from the available soil at the site. This is the lowest form of the surface used and is the first stage in the development of a road which is to be further developed as increasing traffic requires. The performance of these roads mainly depends upon the effective maintenance and drainage. 

¢  Materials Required:

The road structure may have a base course and a wearing course (sand provides stability and clay imparts bindings property but excessively clay content is harmful as it tends to swell when wet and shrink when dry.). Soil having following properties is considered satisfactory for constructing earthwork. 

Soil Type	Base Course	Wearing Course
Clay content	≤5%	10-18%
Silt content	9-32%	5-15%
Sand content	60-80%	65-80%
Liquid limit	≤35%	≤35%
Plasticity index	≤6%	4-10%

¢  CONSTRUCTION PROCEDURE:-  

’  MATERIAL: 

The soil survey is carried out and suitable borrow pits are located within the economical haulage distance. The borrow pit is the place from which the soil is collected.

’  LOCATION: 

The center line and road edges are marked on the ground along the alignment by driving wooden pegs. To control the vertical profile of the road reference benchmark pegs may also be driven which also help to check level of the road during construction. 

’  PREPARATION OF SUBGRADE: 

Earthwork may be done manually or mechanically depending upon the available resources. The various operations involved in the preparation of the sub grade are as follows; 

-  Site clearing 

-   Excavation and construction of fills to bring the road to a desired grade. 

- Sloping of subgrade 

The subgrade should be graded to the desired camber and profile. If needed, certain amount of water is added to bring the soil up to OMC and compacted to impact strength and stability to the subgrade. 

’  PAVEMENT CONSTRUCTION: 

Soil to be used in the construction of the road pavement is suitably proportioned and spread on the prepared subgrade. The field moisture content is checked and additional water is added if necessary to bring it up to OMC. The soil layer so prepared is compacted in such a way that the compacted thickness does not exceed 10cm, the type of roller for compaction is decided based on the soil type, desired amount of compaction and availability of equipment. The camber of the finished pavement surface is checked and corrected if necessary. 

’  OPENING TO TRAFFIC:- 

The compacted earth road is allowed to dry out for a few days before opening to traffic so that the road pavement may acquire sufficient strength and stability. 

Suitable side drains on both edges of the road should be provided slightly away from the bottom edges off the formation to drain off rainwater from road surface and adjoining land. 

  4.5 Construction of gravel roads:Introduction,materials required, equipment   required, construction procedure 

Gravel roads are considered superior to earth roads.  Compacting a mixture of gravel and earth makes the carriageway of these roads. This type of road can cater for about 100ton of pneumatic tyred vehicle or 60ton of iron tyred vehicle per day per lane. These gravel roads can have camber ranging between 1 in 25 and 1 in 30. Gravel roads may be constructed in the form of trench type or featheredge type patterns.

 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 lateral confinement for the gravel pavement this method of construction is preferred. 

’ Materials :  

-   Hard variety of crushed stone or gravel of specified gradation is used. 

-    Rounded gravel & pit run gravel (obtained from the river beds) are not preferred because of their poor interlocking.

They may be hard, durable, acceptable, & free from rounded & river gravel as there is poor interlocking. 

’ Construction Procedure :  

•   Location and Preparation of sub grade:- Compacted subgrade is prepared as in the case of earth roads. 

•  Usually trench type construction technology is followed. Trench is formed to the desired depth of construction; the width of the trench is made equal to that of the carriageway. Compaction of trench base is done to obtain hard-consolidated base for the gravel layer. 

•   Crushed gravel aggregates are placed carefully in the trench so as to avoid segregation. Aggregates are spread with greater thickness at the center and less towards the edges and obtained the desired camber. 

•  The layer is rolled using smooth wheel rollers. Rolling is done from edges to the center with an overlap of at least half the width of the roller in the longitudinal direction. 

•   Some quantity of water may be sprayed to get effective compaction. 

•   The camber is checked and corrected using a template or camber board. 

•    Opening of traffic:- Few days after the initial rolling and drying out the road is opened to the traffic. Efficient drainage facilities should be provided.   

 4.6 Construction of soil stabilized roads: Introduction to soil stabilization, types of   soil stabilization, mechanical stabilization  of soil (materials, equipment, construction   procedure) 

¢  Introduction to Soil Stabilization 

Soil stabilization is a process which brings about improvement in the performance of soil subgrade or soil base. In other words, its brings about improvement in the stability & bearing capacity of the soil. These changes are brought about through controlled proportioning, compaction &/or the addition of suitable admixtures or binders. In the process of soil stabilization, emphasis is given for maximum utilization of local materials so that cost of construction may be minimized to the maximum extent. The type and degree of stabilization required in any given case is largely a function of the availability and cost of required materials as well as the use, which is to be made of the stabilized soil mixture.

¢  Mechanics of Stabilization 

The basic principal in soil stabilization is as follows: 

a. Evaluating the properties of soil 

b.  Deciding the admixtures which will make or provide the lacking requirements to the original soils. 

c.   Evaluating the properties of soil admixture mix with respect to the design criteria. 

Soil stabilization may result in any one or more of the following changes. 

a.  Change in physical properties like – density, stability, shrinkage, expansion etc  

b.  Change in chemical properties  

c.   Retention of some minimum strength properties by waters proofing.  

¢ Techniques of soil Stabilization  

1.    Proper Proportioning: Locally available soils and aggregates are mixed in suitable proportion and compacted to get the desired objective. 

2.    Cementing agent: Strength can be increased using cementing agents like Portland cement, lime or lime fly ash. Bituminous material impacts binding property to non-cohesive soils like pure sand. 

3.    Modifying agents: When small amount of modifier is added to the soil to be stabilized, it brings a lot of improvements in the performance of the soil (like cement) 

4.    Water proofing and repelling agent: to prevent loss of strength properties of soil while it comes in contact with water. Bituminous material – water proofing, Vinyl resin - water repelling. 

5.    Water retaining agents: Some non-cohesive soils maintain sufficient stability and strength when their layers posses certain minimum amount of moisture. Calcium chloride if added to the stabilized soil will continue to absorb moisture from atmosphere and thus retain its stability. 

6.    Heat treatment: Thermal stabilization is useful for clayey soils. 

7.    Chemical stabilization: There are several chemicals which when added single or in combination even in trace quantities (less than 0.5% by weight of soil) may impart useful changes in certain type of soils.  

Before any soil stabilization, methods are adopted soil survey along the route is carried out. Soil samples are collected and field identification tests carried out to establish the general quality of the soil along the road alignment. Laboratory tests (liquid limit, plastic limit and sieve analysis etc.) may be carried out to assess their strength characteristics.

¢ METHOD OF SOIL STABILIZATION :- 

Following are the various types of soil stabilization used in highway construction; 

1. Mechanical Stabilization  

2. Soil-cement stabilization 

3.Soil lime stabilization  

4.Soil bitumen stabilization 

Except mechanical stabilization other three methods are generally used for high cost roads. However mechanical stabilization is used for low cost roads and generally used for village roads and may also serve as pavement surface on such roads. Other methods are used for base or sub-base source in high quality pavement surface. 

¢ MECHANICAL STABILIZATION:- 

 Mechanical stabilization is also referred as granular stabilization or as soil aggregate roads. It is a process in which gradation is so chosen that the resulting construction depend upon the soil aggregate mixture alone for its stability. 

The two basic principles involved in mechanical stabilization are proportioning and compaction. Properties of granular soils containing negligible fines if mixed with a certain portion of binder soil like clay its stability & strength both are improved. Similarly, the stability of a fine graded soils (clayey soils) can be improved by mixing a suitable proportion of granular materials to get a suitable gradation.

Two different approaches have to be followed in obtaining the maximum density:

1.  Ideal gradation concept 

2. Aggregate binder concept  

* PROPERTIES OF SOIL AGGREGATE MIX  

The desirable properties of soil aggregate mixture are as follows:- 

a)   It should be hard, durable and can bear wheel loads. 

b)   Incompressible 

c)    Less change in volume and stability with variation in moisture content. 

d)  Good drainage less frost action  

e)   Ease in compaction  

Three typical states in which compacted soil aggregate mixes can be formed are as follows. 

a) When the aggregate is without fines (or very little fines)  

-  Stable only under confined conditions (no cohesion). Highly permeable, no frost action and practically no variation in volume and stability with moisture variations, difficult to compact. 

b) Voids in the compacted aggregate are just filled with compacted binder  

-    The compacted mass such as a mixture will exhibit good stability even when unconfined due to reason that full fractional resistance is available due to grain to grain contact of the aggregate. 

-   Less permeable and will be affected by the frost action 

-    Is likely to undergo change in volume and stability due to change in moisture content. 

c) When the aggregate is mixed with excess fines and compacted  

-   Aggregate grains float in the binder soil, the angle of friction and stability are decreased. 

-    Less desirable because poor drainage, more variation in stability and volume with moisture variation  

-    Ease in compaction  

* FACTOR AFFECTING MECHANICAL STABILITY   

The following are the factors which affect stability; 

-   mechanical strength of aggregate 

-    proportioning of the mix i.e. gradation  

-  properties of soil  

-   presence of harmful ingredients like salt, mica etc. in the soils 

-  amount of compaction 

When the soil is used in small proportion just to fill up the voids in the aggregate, the strength of the mix is controlled by the crushing strength of the aggregate. Properties of soil such as plasticity characteristics would affect the performance of the mix. Presence of salts like sulphates and presence of mica is harmful but presence of salts like calcium chloride are beneficial. 

¢ CONSTRUCTION PROSEDURE; 

’  MATERIALS:- 

The required construction materials are collected from the selected borrow-pits and are attacked along the sides of the roads.  

’  EQUIPMENT:- 

 Equipment needed are for excavation, haulage for short distance and for compaction. 

’  CONSTRUCTION STEPS :-     

-   Prepare the sub-grade to correct camber & profile.  

-  The materials are mixed to the desired proportions as per design ( in volume basis) 

-   Moisture content of the mix is checked and if required additional water is spread and the material is remixed. 

-  The prepared mix spread along and across the road to the suitable grade and compacted by rollers. 

-   Rolling is started from the edges and with adequate longitudinal overlap, it is continued up to the center. Rolling is continued till adequate compaction is achieved. 

-   Two field control tests are necessary  

i. Determination of moisture content of the mix before compaction 

ii.Determination of density during and just after compaction

Based on these two tests, modification if needed in moisture content or compaction may be decided. 

-  The stabilized road is opened to the traffic after the compacted layers hardens by drying.

  4.7 Constructions of Water Bound Macadam (WBM) roads: Introduction, materials required, equipment required, construction procedure 

    Introduction:- 

The water bound macadam road construction technique was given by John Macadam. Macadam means the pavement base course made of crushed or broken aggregate mechanically interlocked by rolling & the voids filled with screening & binding material with the assistance of water. Water bound macadam is constructed in thickness ranging from 10-15 cm. The number of layers and total thickness depends on the design details of the pavement. When used as surfacing course WBM gets deteriorated rapidly under adverse conditions of traffic and weather, therefore it is desirable to provide a bituminous surfacing course over the WBM layer in order to prolong its life, so their principal use is as base for flexible pavements. 

1. Materials required   

•   Coarse aggregates should be hard durable and of acceptable shape, free from flaky and elongated particles. 

The coarse aggregates for each layer of construction should as far as possible confirm to any one of the three grading specified on the recommendation.

Grading	Size (mm)	Remarks
1	90-40	Suitable for base course thickness of compacted layer is 100mm
2	63-40	Thickness of each layer is normally 75mm
3	50-20	"

 

•  Screening ( Filler materials):- 

Screenings consists of aggregates of smaller size and are used to fill voids in coarse aggregates, should generally consist of the same materials as the course aggregate. From economic considerations, non-plastic materials such as kankar or gravel ( other than river rounded aggregate ) and if used for this purpose should have liquid limit and plasticity index below 20% and 6% respectively and fine passing 0.075mm sieve is less than 10% . 

Grade Classification	Size of aggregate
A	12.5mm
B	10mm

As far as possible, the screening should confirm to the grading. Screening of type A should be used with coarse aggregates of grading 1. With grading type 2. type A and type B may be used but with coarse aggregates of grading 3, only type B screening should be used. 

•  Binding Materials :- 

-   Fine grained material to prevent raveling of stones ; lime stone dust may be utilized if locally available  

-    Plasticity index    

   4-9% surface course

        ≤ 6% base and sub-base course 

Construction procedure:-  

•    Preparation of subgrade   

1.Careful preparation of the subgrade is necessary 
2. Macadam roads are flexible in nature 
3. Weak spots in the subgrade must be corrected before the base or surface is placed and the subgrade soil brought to a high degree of uniformity, density and stability. The subgrade is brought to the desired elevation and cross section before any stone is laid. It is also important that this underlying layer should be well drained. 

•   Provision of lateral confinement :-  

Lateral confinement is to be provided before starting WBM construction. If trench is not made, this is done by constructing the shoulders to advance to a thickness equal to that of the compacted WBM layer and by trimming the inner sides vertically. 

• Spreading the coarse aggregates: 

 The coarse aggregate should be spread uniformly & evenly on prepared subbase. Before rolling, all the surface irregularities are eliminated and liberal use of templates and long straight edges is made in checking the uniformly of the spread material (thickness, camber etc) 

•      Rolling : 

After spreading the course aggregates properly, rolling is done with 6 to 10 tones three wheeled power rollers or vibratory rollers. Rolling is usually started from the edges and progresses towards the center except on super elevated curves where the rolling begins on the low side and progresses towards the high sides. Passes of roller are made parallel to the centerline and are overlapped each preceding rear wheel track by one half width. Slight sprinkling of water may be done during rolling if necessary. The process is repeated by rolling from the edges towards the centerline until adequate compaction is achieved.

•  Application of Screening : 

After the coarse aggregate have been dry rolled, the screening shall be applied gradually & uniformly in successive thin layers at a slow rate in three or more. Dry rolling shall be done when the screening are being spread so that vibration of roller cause screening to settle into the voids of coarse aggregate. This shall be accompanied by rolling & brooming. 

After the application of screening, the surface shall be sprinkled with water, swept & rolled. The sprinkling, sweeping & rolling operations shall be continued & additional screenings applied when necessary. 

•  Application of Binding materials (filler)

A suitable filler material is then applied at a uniform and slow rate in two or more successive layers. After each application of filler material, the surface shall be sprinkled with water the resulting slurry swept into fill the voids properly and surface rolled by a roller. 

•   Setting and Drying   

After final compaction of course, the layer shall be allowed to dry overnight. Next morning, hungry spots shall be filled with screening or binder material, lightly sprinkle with water & rolled. No traffic shall be allowed till the macadam sets. The compacted WBM course shall be allowed completely dry & set. 

4.8 Construction of bituminous roads: Introduction,types   of bituminous surfacing, interface treatment   (prime coat, tack coat), seal coat, functions of each coat

Road in which bitumen is used as binder are termed as bituminous roads. There are a wide range of construction materials (type, size and grading of aggregates and type and grade of bituminous binder) and bituminous pavement construction techniques in use.Bitumen content over an optimum value is actually harmful and reduces strength and stability values of the designed mix. Therefore, based on the surface area of the aggregates and the technique of the construction, the optimum binder content may be determined. Another problem associated with the construction of bituminous pavements is the control of the proper viscosity of the bituminous aggregate mixtures during mixing and compaction operation. 

- Stage development of the roads can be adopted but not in the cement concrete road. 

* CONSTRUCTION OF BITUMINOUS ROADS:-  

Bituminous roads can be classified in the following ways; 

i.    Classification based on construction technique  

-    Interface treatments like prime coat and tack coat 

-   Surface dressing and seal coat  

-    Grouted or penetration macadam  

-     Premix construction  

Bituminous bound macadam, bituminous carpet, bituminous concrete, rolled or sheet asphalt, mastic asphalt. 

ii. Classification based on temperature of the binder 

-   Hot mix  

-   Cold mix 

iii. Classification based on the method of mixing  

-   Road mix method  

-    Control plant mix method

 iv. Classification based on texture exhibited by compacted layers  

-   Open graded  

-    Dense graded  

Bituminous binders may be road tar, cutback, emulsions, etc. The choice of particular depends upon the type of construction, availability of materials and equipment, climate conditions etc. 

Bituminous concrete:- mixture of controlled graded coarse aggregate and bitumen  

Mastic asphalt:- mixture of bitumen fine aggregate and fillers 

Rolled or sheet asphalt:- mixture of sand and bitumen without any coarse aggregate. 

Hot mix:- binder and aggregate both are heated before use ( bitumen and tar)  Cold mix:- binder and aggregates are not heated cutbacks and emulsions are used. 

Road mix:- materials are mixed at the road side. 

Center plant mix:- hot mix plant is situated centrally to the length of the road  

Open graded:- premix carpet- rough surface  

Dense graded:- bituminous concrete  

* 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.

When a bituminous wearing surface is to be placed upon a previously untreated compacted foundation layer such as earth, gravel, stabilized soil, kanker or water bound macadam, it is necessary that the newly laid bituminous surfacing posses a bond with the existing base at the interface which can be developed with the help of bituminous material and thus treatment is known as interface treatment. There are two types of interface treatment depending upon the condition of the existing base. 

a.PRIME COAT:-   

 It is the first application of low viscosity (RC-0, MC-1, SC-1) liquid bituminous material and is applied to the existing bases of the pervious surface like WBM road before the application of any bituminous treatment to the surface of the road. 

The function of prime coat are; 

-   Develops adhesion or bond between the base and the wearing surface. -  Seals the pores and capillary voids thus making it water proofing -  Binds together any loose aggregates on the existing surface. 

- Stabilize or binds the surface fines together & promotes bond hot mix asphalt layer. 

Construction steps:   

i    Materials MC or SC outback of suitable grade of viscosity
ii The existing surface pavement is repaired and cleaned.  
iii The bituminous primer is sprayed uniformly using a mechanical sprayer at a rate of 7.3 to 14.6 kg per 10 m area depending on the porosity of the surface.   
iv The primed surface is allowed to cure for at least 24 hours. During which period
no traffic is allowed.

 b.TACK COAT:-

 It is a single initial application of bituminous material on surfaces which have previously been treated or prepared such as existing bituminous, cement concrete or a pervious surface like WBM which has already been treated by a prime coat. Tack coat is usually applied by spraying bituminous material of higher viscosity like the hot bitumen at the rate of 4.9-9.8 kg/m2 depending upon the type of the surface. 

The function of tack coat are;  

-  Ensures adhesion between the existing surface and the new bituminous surface. 

* Construction steps: 

 i The surface of the existing pavement layer is to be cleaned to remove dust and dirt. 

 

 ii  Tack coat is applied by spraying bituminous material of higher viscosity likehot bitumen a

the rate of 4.9 to 9.8 kg per 10 m' depending on the type of surface. 

  

  c.  SEAL COAT:- 

Seal coat may be defined as a very thin surface treatment which is either applied as a final step

in the construction of certain bituminous surfaces or to existing surfaces which have cracked or

worn out. 

The main purpose of providing seal coat are:-  

-  To seal the surfacing against the ingress of water. 

-   to increase the resistance to skidding. 

-     to increase the strength and bearing capacity of the existing surfaces. 

-    A filler for existing cracks or raveled surface 

-     An anti-glane surface during wet weather & an increased reflecting surface for night driving. 

 
Construction steps:   

i    Surface preparation: Surface defects, such as potholes are repaired and the existing surfaces cleaned.

 ii. Asphalt material application: Typically, an asphalt emulsion is applied from a spray truck or sprayer to the surface existing pavement.  

iii. Aggregate application: A thin aggregate cover is sprayed on the asphalt material it has set. The aggregate is usually uniform gradation. 

 iv. Aggregate embedding: A roller is used to push the age into the asphalt material and seal it firmly against the underlying pavement. Generally about 50% of each aggregate particle should be embedded in the asphalt material after final rolling.  

  4.9 Surface dressing: types (single, double), materials required, equipment required, construction procedure 

The tern surface dressing refers to thin surface coverings of bituminous layer and mineral aggregate. The stability of WBM road depends upon the gradation, shape of the aggregate and binders which keeps the stone pieces together. Since the binder in WBM is the soil slurry, it is easily removed by the fast moving vehicles and cause the failure of WBM road. Bituminous surface dressing prevents the removal of binding materials from WBM road surface. Surface dressing may be  

1.  Single coat surface dressing   

2.  Double coat surface dressing 

 1. Single coat surface dressing  

It includes the application of thin layer of bitumen followed by the cover materials of specified size stone aggregate which is then compacted by rollers. 

 2. Double coat surface dressing

In this type of surface treatment, immediately after laying the first coat, second application of binder is applied and followed by uniform spreading of the cover materials of smaller size aggregate and then rolled.  

The main function of surface dressing are:- 

-  to provide a dust free pavement surface over a base course. 

-  to provide a water proof layer to prevent infiltration of surface water. 

-   to prevent the base course (from raveling and abrasive action of traffic.) 

Materials required for bituminous surface dressing; 

-  depends on whether the surface dressing is done in a single coat or two coats,   on black top pavement or on a WBM pavement. 

-  The chipping should exhibit 

 Los Angeles Abrasion value maximum 35% 

  Aggregate impact value 30% 

 Flakiness index of the chipping 25% 

  Water absorption of the chipping 1%

   Percentage loss with Na2SO4 for 5 cycles 12%  

   Stripping at 40⁰ C after 24 hours immersion (CCRI) test 25%  

- Selection of different grade of binder to be used in surface dressing depends upon the climate conditions. Mostly between 80/100 and 180/200 grades are used. 

Plants and Equipment for the surface dressing:- 

-   Equipment for heating of bitumen,  

-   Mechanical sprayer,  

-    Mechanical blower or hand brushes, 

-   Roller   

* Construction steps; 

i.  Preparation of existing surface:- The surface defects such as pot holes, ruts, depression etc are repaired & surface is cleaned. If the existing base course is pervious then prime coat is applied.   

ii. Application of binder:- The bituminous binder is applied at specified rate using mechanical sprayer on prepared surface. 

iii. Application of stone chipping:- Stone chipping as per requirement is spread after the application of binder, to cover surface uniformly. 

iv.   Rolling of first or final coat:- Rolling is done with tandem roller of 6 to 8 tonnes weight starting from edges preceding towards the center longitudinally with  overlapping not less than one third of the roller tread. 

v.  Application of binder and stone chipping for second coat:- The binder is again applied to the prepared surface as per requirements. Immediately ston chipping is spread as before & rolling is done as before. 

vi.  Rolling of second coat:- Rolling is done with tandem roller of 6 to 8 tonnes  weight starting from edges preceding towards the center longitudinally with  overlapping not less than one third of the roller tread. 

vii. Finishing and opening to traffic:- The surface is checked for longitudinal & cross profiles. Then the road surface is opened to traffic after 24 hours.  

4.10 Grouted macadam: types (full, semi), materials required, equipment required, construction procedure 

Grouted or penetration macadam is a wearing course composed of broken stone bound together with a bituminous binder which is applied to the surface & permitted to penetrate into the layer of stone. In penetration macadam construction, the bitumen is sprayed after the aggregates are spread and compacted. The bitumen penetrates into the voids from the surface of the compacted aggregates thus filling up a part of the voids and binding the stone aggregates together. 

Depending upon the quality of bitumen sprayed and the extent of penetration i.e. when bitumen penetrates to the full depth of compacted aggregate, it is called full grout and when it penetrates up to about half depth, it is called semi grout. Full grout is adopted in regions of heavy rainfall and semi grout is adopted in regions of moderate rainfall and traffic. 

MATERIALS:-  

-   Bitumen of grade 80/100, 60/70, 30/40 or tar RT-4 and RT-5 may be used. 

-  The quantity of bitumen required depends upon the depth of penetration.

- The physical properties of stone aggregates should be as follows. 

Los Angeles Abrasion value  < 40% 

Aggregate impact value  < 30% 

Flakiness index     < 25% 

Stripping at 40 degree C ( 24 hours, immersion) <25% 

Loss with Na2SO4 for 5 cycles  < 12% 

*  Two types of aggregates:- coarse aggregate and key aggregate 

The compacted macadam layer over which the binder is sprayed is made from coarse aggregate. After this, comparatively small sized aggregate is again sprayed on the sprayed surface before rolling is done. The aggregate which is applied after spraying process is known as key aggregate

* Plants and Equipment:

- Equipment for heating of bitumen,  

- Aggregate spreader,  

- Mechanical blower or hand brushes, 

-  Bitumen distributor, 

-  Roller 

* Construction procedure:  

i.  Preparation of existing surface: 

The surface defects of underlying course are repaired and cleaned. Prime coat may be applied if

required on porous surfaces.  

ii.   Spreading the coarse aggregate: 

The coarse aggregate is sprayed by mechanical spreader or by hand with proper edge protection.

  

iii.  Rolling: 

The aggregate are dry rolled with 10 tonnes roller until the aggregates are compacted and

interlocked with starting from edge and processing towards Centre and overlap of 30cm.   

iv.  Bitumen application: 

The binder is applied uniformly with bitumen distribution or mechanical hand sprayer over the

dry and compacted course aggregate. 

v. Spreading of key aggregates: 

After the application of bitumen binder the key aggregates are spread and rolled and cross profile

is checked.   

vi. Seal coat: 

If another surfacing course is not constructed immediately the seal coat is applied and rolled.   

vii. Opening to traffic: 

The finished surface is opened to traffic after a minimum period of 24 hours.  

* BITUMEN BOUND MACADAM:-  

It consists of one or more course of compacted crushed aggregate premixed with a bituminous binder and laid immediately after mixing and then compacted. This is used primarily for use as a base course because the load transmitted by it to the sub grade and the surface deflections of the pavements are substantially lower than WBM or gravel types. When this layer is exposed as a surface course at least a seal coat is necessary. 

* MATERIALS:-  

Bitumen of grade 30/40, 60/70, 80/100 penetration and tar RT-4, cutbacks and emulsions may be used in cold mix construction technique. The coarse aggregate should fulfill the following requirements.

 

Properties	Base Coarse	Binder Coarse
Los Angeles abrasion value	<50 td="">	<40 td="">
Aggregate impact value	<35 td="">	<30 td="">
Flakiness indext	<15 td="">	<15 td="">
Stripping at40 Degree c (24 HRS)	<25 td="">	<25 td="">
Loss with Na2So4 for 5 cycles	<12 td="">	<12 td="">

 * Plants and Equipments; 

¢  Sprayer  

¢  Mechanical mixer or improvised hand mixer  

¢  Spreader – mechanical paver or finisher grader or manual method 

¢ Roller

 

* CONSTRUCTION PROCEDURE 

-  Preparation of underlying layer  

-   Application of tack coat or prime coat( 4.75kg/10m2 for back top layer and 7.5- 10kg/10m2 for untreated WBM layer)  

-   Premix preparation ( separately heated and then mixed )  

-  Placement ( minimum time should be spent between the placement of the mix and the rolling because compacting temperature also influences the strength characteristics of the resulting pavement structure. 

-  Rolling and finishing ( 8-10t tandem roller ) .