5.1 Construction of Subgrade | unit 5 highway construction

Unit-5

Highway Construction

The foundation of the pavement structure is known as subgrade. Preparation of subgrade consists of all operations before the pavement structure could be placed over it and compacted. The subgrade may be situated on embankment or excavation or at the existing ground surface. In all the above cases, Site Clearance -Clearing Grubbing Operation should be done before starting the pavement structure construction. After that, the grading operation is started as per the design and drawing of the highway plan and profile.

5.1.1 Subgrade Construction Equipment

The following sets of equipment are necessary for the planned progress of road subgrade construction work.

Excavator

Air Compressor & Pavement Breaker /Jack Hammer

Tippers /Dumpers

Motor Graders

Vibratory Compactor

Tractors Dozer with Disc Harrows/Spreading Blade/Ploughs

Water Bowser with Sprinklers etc.

Crawler Dozers

5.1.2 Steps for Preparation of Subgrade

The road subgrade shall be prepared as per the MORTH specifications if it is not mentioned in contract technical specifications.

The limits of filling shall be marked by fixing batter pegs at regular intervals on both sides of the layer and working line with a white the help of lime powder.

The layer shall be built 300mm wider than the designed and drawing dimensions so that after proper compaction is achieved up to the toe, the surplus materials shall be trimmed to get the properly compacted slopes of subgrade. Where the fill is to be deposited against an existing subgrade, continuous horizontal benches of 300 mm wide shall be cut into the old slope.

In cutting section, where cutting is to be done up to subgrade top its top layer, shall be loosened and re-compacted as per technical specification.

The topsoil in the borrow area shall be removed by grubbing or stripping, so that earth without vegetation is excavated and loaded.

Soil from approved borrow areas shall be excavated with Excavators and loaded onto tippers or dumpers for transportation to the stretch ready to receive fill Material.

The material shall be dumped between the limiting lines marked with lime powder.

The material shall be spread in layers of a uniform thickness not exceeding 250mm of compacted thickness. Grader or a combination of dozer and grader can be used for this activity.

The grader will initially spread the heap of earth dumped over a stretch maintaining an approximate line and level.

At this stage, the material should have an Optimum Moisture Content (OMC), ranging from +1% to –2%. The following mixing or drying process should be adopted if it is not found within the permissible limit.

Key Takeaways:

Optimum Moisture Content: - The water content at which a soil can be compacted to the maximum dry unit weight by a given compactive effort. Also called optimum water content.

5.2 Water Bound Macadam (WBM)

Water bound macadam road is a road in which the wearing course consists of clean crushed aggregates which are mechanically interlocked by rolling. These aggregates bound together with filler material and water laid on a well-compacted base course.

Water bound macadam road is the most commonly used road construction procedure for over more than 190 years and in the first phase in most of the road projects, water-bound macadam road is constructed, and the surfacing is done with the premix carpet bituminous macadam or cement concrete.

5.2.1 Materials for water bound macadam road

Coarse Aggregate

Screenings (Filler Material)

Binding Material

5.2.1.1 Coarse aggregate

Coarse aggregates consist of a mixture of hard and durable crushed aggregates and broken stones, and aggregate should be properly graded for each layer of the WBM road construction.

 It should be hard and durable.

 The coarse aggregate should be free from elongated particles and flaky particles.

5.2.1.2 Screenings: (filler material)

Screenings are the material that is used up to fill up the excess voids present in the compacted layer of course aggregate and these materials are the aggregates of a smaller size than the coarse aggregates.

5.2.1.3 Binding material

Binding material should be properly approved by engineer and it should have a plasticity index value less than 6 for the construction of water bound macadam road.

5.2.2 Construction procedure of water bound macadam road

There are the following steps in the construction procedure of WBM road as given below;

5.2.2.1 Preparation of foundation for WBM road

For the required grade and camber of WBM road, the subgrade or base course is properly prepared and the potholes and the depressions on the surface of the road is properly filled up & compacted.

5.2.2.2 Provision for lateral confinement

The shoulders having a thickness as that of compacted WBM layer should be constructed before laying of aggregates and with the proper quality of earth, they should be constructed.

The purpose of constructing shoulders is that the road surface to be constructed retains in between them and for the further laying of course aggregates, it becomes easy.

5.2.2.3 Spreading of coarse aggregates

After the construction of the shoulders, the coarse aggregates are uniformly spread on the prepared base and the total number of layers and thickness of water bound macadam road depends upon the details of pavement design.

Single-layer of compacted thickness 75 mm may be sufficient for ordinary roads and 2 layers of 150 mm each compacted thickness may be provided for special roads.

5.2.2.4 Rolling operation

For compacting the coarse aggregates, the rolling operation is carried out and it is done with the help of vibratory rollers or with the help of 3-wheeled power rollers weighing 6 to 10 tones.

For driving the rollers, skilled operators should be used because the fault rolling operations cause the formation of corrugations, the unequal finish of road surface, etc.

5.2.2.5 Application of screenings

The screenings are applied to properly fill the voids remained after the rolling operation is properly finished and in 3 or more layers as per the site conditions screening may be applied.

For each layer of screenings, compaction is carried out with the help of dry rollers, and to remove the uncompacted screening material, brooming of each layer should be properly done after compaction.

5.2.2.6 The sprinkling of water and grouting

The road surface is properly sprinkled with plenty of water after the application of screening and brooming is done to sweep the wet screening properly into the voids after the water is sprinkled.

5.2.2.7 Application of binding material

As screenings, the same procedure is used in this step and rolling operation is carried out after each layer of water is sprinkled.

To washout, the binding material that gets stuck to the wheels of the rollers, the wheels of the roller should be constantly watered at the time of rolling operation.

5.2.2.8 Setting and drying of surface

The road is allowed to cure or set over-night after the final rolling operation and the next day again sufficient amount of screenings or binding materials can be used and compaction is done if the depressions or voids are visible.

5.2.2.9 Preparation of shoulders

The shoulders are constructed alongside by filling earth to specified cross slope at the time of the curing of the road and compacted properly.

5.2.2.10 Open for traffic

The road is then made open for traffic after proper drying and without any depressions. By placing obstacles longitudinally in the form of drums, barricades, etc the traffic should be well distributed over the full width of the road.

5.2.3 Maintenance of WBM road

By the period of time when the potholes and ruts occur on the road, they should be filled with adequate materials and properly compacted.

By means of dragging, the corrugations that occurred on the roads should be removed and by fresh materials, broken materials of the roads should be properly restored.

In 2-5 years or based according to the traffic volume, the surface of the road should be renewed.

The loose aggregates start coming on the top of the surface of the road so they should be removed and leveled surface should be added by fresh binding material.

5.2.5 Advantages of water bound macadam road

There are the following advantages of water bound macadam road such as;

The construction cost is comparatively low in WBM road.

No skilled laborers are required in the construction of WBM road.

From locally available materials they are constructed.

It can resist a load of traffic of about 900 tons per lane per day if these roads are maintained properly and from time to time.

5.2.6 Disadvantages of water bound macadam road

There are also some disadvantages of water bound macadam road such as;

There is a high cost for maintenance on this road.

There is a very less overall life span of these roads.

The WBM roads can cause inconvenience and danger to traffic if they are not properly maintained.

It leads to softening and yielding of subsoil because WBM roads are permeable to rainwater.

Key Takeaways:

Hardness: - Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion.

Durability: - Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal.

5.3 Wet mix macadam (WMM)

Wet Mix Macadam consist of laying spreading and compacting of clean, crushed, well-graded granular materials on a prepared and approved Granular sub-Base. The material is well mixed with water and rolled to a dense mass. ... The thickness of single compacted Wet Mixed Macadam (WMM) Base shall not be less than 75 mm.

While constructing a road, there needs to lay down the base course material right before hot mix asphalt is laid, and that process done through the Wet Mix Macadam. The materials used for the sub-base, base or existing pavement are prepared in a plant as per the specifications provided. Once it is prepared, it is brought to the site for overlaying and rolling under the guidelines of engineers.

The WMM process is a newly developed concept. Earlier it was done through the WBM (water bound macadam) process. Though WBM process is cheaper than the WMM, the later gives plenty of benefits. In short, the outputs of WMM pay you of the debt. So, let’s find out the benefits we get from the WMM process.

The roads constructed through WMM process are durable

The WMM roads get dry sooner as compared to that of WBM

The WMM process allows constructing road faster than other processes

Roads can be ready in no time and can be applied or topped with Bituminous layers soon after the laying process done

WMM process saves plenty of water

Apart from that, the WMM machines are easily operated and give a bunch of benefits.

Similarly, there are so many additional benefits which can compensate you well if you use this wet mix plant to construct the road.

Here’s the list of components which set the Wet Mix Macadam plant apart from others.

Feeders

Vibrating Screen

Conveyor

Pug Mill mixer

Conveyor

Silo or Hopper

Water tank

Control Panel

Aggregate Feeders: It’s a four-bin aggregate with a manually adjustable gate in each to store four types of aggregates. It helps to discharge the materials and smoothens wet mix process.

Vibrating Screen: It works to filter the oversized aggregates and deliver it to the charging conveyor belt.

Conveyor: Conveyor or charging conveyor delivers the material to pug mill mixer after that they are filtered through a vibrating screen. Once the panel receives the material, the water and cement flow get synchronized and ease the mixing process.

The pug mill mixer consists liner plates, arms, and tips and spreads materials when comes in contact with water.And at last, the load out conveyor transfers the mixer to the storage which later transmitted to other truck for further process.

All components used during the process to simplify the mixing process and help wet mix macadam plants achieve the highest goal.The wet mix plant is very much in trend for its friendly features including ease of operation, providing high-quality services and giving a satisfactory result to the builder.

Key Takeaways:

WMM (Wet Mix Macadam): - (40 mm down aggregates, mix design required)

5.4 Granular Sub Base (GSB)

Granular Sub Base (GSB) is natural or designed construction material, used for road construction as a sub-base layer. Granular Sub Base is a layer in road foundation just above the compacted sub-grade layer. GSB or granular sub-base prevents capillary water from rising; its particle size is so designed that the capillary action stops and can’t go beyond GSB layer. Secondly, it works like a drainage layer where water can pass without damaging other road layers.

5.4.1 Equipment and Machinery

The following sets of equipment are necessary for the for GSB construction work.

Grader /Loader/Excavator

Vibratory Rollers (80 -100 KN static weight)

Water Tanker (10,000 Litres)

Tippers / Dumpers -10T /20T (as per site requirement)

5.4.2 Material

The material shall be crushed stone aggregate free from organic and other deleterious constituents or natural river bed material having proper gradation. It shall conform to grading-I of Table 400-1 of MORTH specification, with the percentage passing 0.075mm size restricted to 5%. The material shall have four days soaked CBR of a minimum of 30%.

5.4.3 Physical properties of Granular Sub Base Material

The portion of the total aggregate passing through 4.75mm sieve shall have a sand equivalent value of not less than fifty when tested in accordance with the requirement of IS 2720 (Part-37).

IS Sieve Designation (mm)	Percent by weight passing the IS sieve Grading-I
75	100
53	80 – 100
26.5	55 – 90
9.5	35 – 65
4.75	25-55
2.36	20 – 40
0.425	10- 25
0.075	3 – 10

A mix of different sizes of Crushed aggregates from approved sources shall be so proportioned to achieve the grading specified.

The Proportioning shall be done by ascertaining the proper gradation of the individual ingredients and the blend determined by trial and error method to achieve the gradation specified. MDD & OMC shall be established for the material so blended. It will be ensured before the actual execution that material used in GSB has a CBR value of 30% or more when compacted and finished.

In case of variation of gradation in the course of work, the proportion shall be suitably modified, and the entire required test shall be carried out in accordance with relevant specification.

The material shall be blended at source/crusher to achieve the specified gradation. This shall be jointly checked at the site for conformance to gradation and other tests as defined in section 900 of MORTH.

5.4.4 Granular Sub Base Construction Method

5.4.4.1 Spreading:

Mark the lines of GSB with lime powder or pegs on the completed subgrade.

Requisite sub-surface drain and drain outlet at the level of sub-grade/sub-base shall be completed before the commencement of GSB construction.

Now prior to the laying of sub-base, the subgrade already finished shall be made by removing all vegetation and other Sub-extraneous matter, lightly sprinkled with water if necessary and rolled with two passes of 80 -100 KN smooth wheeled roller.

The material shall be loaded with the help of loader on the dumpers for transporting to the GSB location.

The required material shall be dumped between the limiting lines marked with the help of lime powder.

The material of the GSB layer confirming Table 400.1 shall be spread in layers of uniform thickness of 200 mm compacted thickness for new flexible pavement,150 mm compacted thickness for Service Roads and rigid pavement at Toll Plazas. Grader shall be used for this activity.

The moisture content of the material should have of OMC +1.0% or –2.0%, if not, either mixing or drying process must be adopted. Water can be added to raise the moisture content up to the required limits. After spraying water with the help of the browser, the material shall be properly mixed to obtain a homogeneous mix. The mixing can continue with the help of grader. The grader shall then carry out final well-defined grading.

5.4.4.2 Compaction

Compaction of GSB shall start immediately after achieving the required moisture content. The compaction shall be done with a vibratory compactor. The compaction pattern, including no of passes required, shall be finalized after the full-scale experiment at the site to achieve 98% of MDD determined as per IS: 2720 (Part -8). The general pattern shall be as follows:

a) Initial rolling -2 static passes with Vibratory roller

b) Subsequent rolling – 4 vibratory pass

Note: One roller pass shall include both forward and reverse movement of the roller). The speed of the roller shall not top 5.0 Km /Hour.

The compaction shall commence from the lower edge and shall move to upper edge width by width. Quality control tests shall be carried out prior to the commencement of the next layer.

The rain cuts shall be repaired before placing the drainage layer.

The compaction behind structure shall be accomplished with the use of vibratory roller or plate compactors to achieve 98% if MDD.

The surface of GSB shall have a suitable cross fall to enable efficient surface drainage.

The finished level GSB shall be within the tolerance limits specified in Table 900.1, i.e. +10.0mm to – 20.0mm.

Key Takeaways:

GSB (Granular Sub Base): - (63 - 40 mm aggregate or 70mm down aggregates).

5.5 Tack Coat

Tack coat (also known as bond coat) is a light application of asphalt emulsion between hot mix asphalt layers designed to create a strong adhesive bond without slippage. Heavier applications may be used under porous layers or around patches where it also functions as a seal coat.

A tack coat is thin bituminous liquid asphalt, emulsion or cutback layer applied between HMA pavement lifts to promote bonding. Adequate bonding between constructions lifts and especially between the existing road surface and an overlay is critical in order for the completed pavement structure to behave as a single unit and provide adequate strength. If adjacent layers do not bond to one another they essentially behave as multiple independent thin layers – none of which are designed to accommodate the anticipated traffic-imposed bending stresses. Inadequate bonding between layers can result in delaminating (deboning) followed by longitudinal wheel path cracking, fatigue cracking, potholes, and other distresses such as rutting that greatly reduce pavement life.

5.5.1 Application

Tack coats should be applied uniformly across the entire pavement surface and result in about 90 percent surface coverage. In order for this uniformity to be consistently achieved, all aspects of the application must be considered and carefully controlled. Specific aspects are:

The condition of the pavement surface receiving the tack coat

The application rate

Tack coat dilution

5.5.2 Condition of the Pavement Surface Receiving the Tack Coat

The pavement surface receiving the tack coat should be clean and dry to promote maximum bonding. Emulsified tack coat materials may be applied to cool and/or damp pavement; however, the length of time needed for the set to occur may increase. Since existing and milled pavements can be quite dirty and dusty, their surfaces should be cleaned off by sweeping or washing before any tack coat is placed, otherwise the tack coat material may bond to the dirt and dust rather than the adjacent pavement layers.

Key Takeaways:

Tack Coat in Paving: - Tack coat is a sprayed application of an asphalt binder upon an existing asphalt or Portland cement concrete pavement prior to an overlay, or between layers of new asphalt concrete.

5.6 Prime Coat

A prime coat is a single application of either a specially formulated asphalt emulsion or a low-viscosity asphalt cutback. Their primary functions are to penetrate quickly into the granular surface and bind the material together, to partially waterproof the granular surface to prevent water erosion, to provide a temporary riding surface prior to overlay or seal coating and to provide a bond between the existing surface and the new wearing surface.

The asphalt emulsions used as prime coats typically contain a combination of asphalt and specially engineered agents to aid in penetrating the granular surface, while binding the aggregate particles to achieve stabilization.

5.6.1 Design Criteria

In order to satisfy the prime coat function some emulsion or cutback must penetrate into the base. The tightness of the granular surface can influence the degree of emulsion dilution needed. The type and the gradation of aggregate will require adjustments to the quantity of prime coat needed. If the surface is very hard it may be necessary to loosen the surface by scarifying prior to spraying. Damping the surface will help to allow for penetration. If ponding develops a blotter sand may be required to absorb the free standing emulsion or asphalt cutback.

5.6.2 Materials

5.6.2.1 Asphalt Emulsions

The asphalt emulsions used as prime coats typically contain a combination of asphalt and special penetrating agents to aid in penetrating the granular surface, while binding the aggregate particles to achieve stabilization. Products such as Enviro-Prem or EDL can be used for this purpose.

5.6.2.2 Cutback Asphalt

The cutback asphalt used as a prime coat is low viscosity asphalt containing a high quantity of solvent. The solvent allows the asphalt to penetrate into the granular and then evaporates out to leave the asphalt residue. RC 30 or Primer is used as prime coats.

5.6.2.3 Performance Guidelines

In order to construct a proper well designed prime coat the following guidelines should be followed:

Determine the type of prime coat to be used.

Evaluate the road surface to be primed.

Determine the dilution rate of the emulsion.

Determine if scarification is needed.

Calibrate and inspect all equipment.

Follow proper construction techniques.

More than one application of dilute emulsion may be needed.

A sand blotter may be needed.

Use traffic control to protect seal.

Work only in weather suitable for type and grade of emulsion being used.

Key Takeaways:

Pavement Priming: - The use of a prime or a primer seal is common practice in pavement construction where unbound or lightly bound base material is present. In a similar nature to painting, the use of a prime promotes cohesion between the underlying base/sub base material and the overlying layers.

5.7 Seal Coat

A surface treatment is placed on a crushed stone base to provide a roadway with the least expensive permanent type of bituminous surface. It seals and protects the base and provides strength at the road surface so that the base can resist the abrasive and disruptive forces of traffic. It also provides many of the functions that a seal coat provides.

When applied to a bituminous pavement surface, a seal coat provides a durable all-weather surfacing that:

Seals an existing bituminous surface against the intrusion of air and water

Enriches an existing dry or raveled surface

Arrests the deterioration of a surface showing signs of distress

Provides a skid-resistant surface

Provides the desired surface texture

Improves light-reflecting characteristics where these are required (by use of light-colored stone)

Enables paved shoulders or other geometric features to be demarcated by providing a different texture or color

The functions of the asphalt binder are to bind the aggregate particles to the underlying surface and to provide a waterproof seal. The functions of the aggregate are to resist traffic abrasion, to transmit wheel loads and to provide skid-resistance and the desired surface texture.

A seal coat or surface treatment has little or no structural strength itself but by preventing the ingress of water it enables the inherent strength of the pavement and the subgrade to be preserved. If a pavement shows evidence of traffic load associated cracking (alligator, longitudinal, transverse), a seal coat is only a temporary solution. Areas that show load-associated cracking may require base repair prior to a seal coat or overlay. A thick asphalt concrete overlay or reconstruction is normally required to correct these problems. Seal coats applied to pavements showing signs of non-traffic load associated longitudinal and transverse cracks have proved somewhat effective. Seal coats usually bridge these cracks in a more satisfactory manner than thin asphalt concrete overlays.

Ride quality of a pavement cannot be improved significantly by the application of a seal coat. Overlays of various thickness, spot level-up maintenance patches, or reconstruction are normally required to restore pavement ride quality.

Pavements demonstrating flushing or bleeding are difficult to repair with seal coats. The binder normally migrates through an added seal coat unless the asphalt quantity applied to the roadway can be altered at these spot locations. Seal coats utilizing a large maximum size aggregate are suggested if seal coats are used on flushed surfaces.

Key Takeaways:

Grouted Macadam: - Grouted macadam, a material that consists of a porous asphalt skeleton with 25–35% voids, filled with a cementitious grout, has traditionally been used as a specialist surfacing with excellent resistance to both deformation and fuel spillage.

5.8 Surface Dressing

Surface Dressing is a mixture of polymer modified bitumen emulsion and a layer of chippings. It will seal the surface, improve surface texture and prolong the life of the road by many years.

It is a speedy, efficient and economic method of preventative maintenance and carrying out minor re-profiling of carriageway surfaces.

5.8.1 The Surface Dressing Process

The surface dressing process is very weather dependent and cannot be applied to wet or cold roads or during high air humidity levels or very high road temperatures.

The road is swept and cleared of any built up detritus and all ironwork masked over.

The Sprayer machine is driven slowly along the road spreading the bitumen evenly across the width of the box.

This is followed closely by the chipping spreader and lorries. There is an excess of chips spread on the road to prevent traffic or pedestrians from picking up wet tar.

Depending on the type of surface dressing there may be two coats of tar and chippings applied or two layers of chippings.

The material is given an initial embedment with a pneumatic tyred roller. The action of further trafficking helps the material to embed and achieve its optimum strength.

Following completion of the work there will be a degree of stone loss. This will be removed by a suction sweeper in stages. Traffic signs will be left in place advising of the risk of loose chippings and showing an advisory speed limit.

The first sweep will normally take place 24 to 48 hours after the initial process. Sweeping earlier than this may damage the surface in its early life.

The masking is then removed from the ironwork in the road.

Trafficking will continue to help with the embedment of chippings. However this must be at low speeds.

It is advisable to remove outdoor shoes prior to entering your house and to check pets for any sticking tar. This is important in the early life of the process but should quickly settle down.

When there is no further loss of aggregate, all temporary signing will be removed.

5.8.2 During the Work

Generally, work will be carried out between 8.00 a.m. and 6.00 p.m.

On street parking will not be possible during these times other than for very short durations.

Vehicle access to your properties will be possible most of the time during the works but you may experience delays immediately after the material has been laid.

Please drive slowly whilst travelling through the site. This is to protect our employees and your vehicle. Be prepared to be guided by one of our operatives if necessary.

Pedestrian access to your property will be maintained throughout the works.

Please do not hesitate to enlist the help of one of our operatives if you are in need of any assistance.

The material will remain wet for anything from 15 minutes to 40 minutes after it has been laid, depending on the temperature and humidity.

5.8.3 After Completion

The new surface can be driven on generally within 15 minutes after completion.

An excess of loose aggregate will build up on the road for a period of time after completion. This is normal. Please drive slowly and observe the temporary signs that will have been left in place.

The road will be swept again after a few days to remove the surplus aggregate.

Further sweeping may be necessary for a while if there is continued stone loss.

Depending on traffic flows it can be up to six months before the surface achieves its final visual and structural characteristics.

Key Takeaways:

Double Surface Dressing: - A simple surface dressing consisting in the application of a bituminous binder to a surface, followed by spreading and tamping the aggregate layer.

5.9 Bituminous Macadam (BM)

Bituminous Macadam (BM) or Bitumen Bound Macadam is a premixed construction method of one or more courses of compacted crushed aggregates premixed with bituminous binder, laid immediately after mixing. The BM is lad in compacted thickness of 75 Mm or 50 mm and three different gradations of aggregates have been suggested for each thickness to provide open graded and semi-dense constructions.

The BM is essential a base course or binder course and hence should be covered by a suitable surfacing course exposing to traffic.

BM base course is considered to be much superior to other types of base course materials as WBM with respect to load dispersion characteristics and durability.

Key Takeaways:

Asphalt Surfacing: - Bitumen is a thick and sticky by-product of the fractional distillation of crude oil which is used to bind aggregate and minerals in both Asphalt and Macadam’s or Tarmac.

5.10 Semi dense bituminous concrete (SDBC) and Bituminous concrete

5.10.1 Semi dense bituminous concrete

Semi Dense Bituminous Concrete (SDBC) is similar to BC, but with a higher void content (5-10 per cent, as against 3-5 per cent in the case of BC). The thickness of the layer is 20-25 mm. The binder percentage is 4.5-6.0. The mixing and rolling operations are similar to BC. The mix design is done by the Marshall test, and a stability value of 3.4 kN (340 Kg) is aimed at.

5.10.2 Bituminous concrete

Bituminous concrete or Asphaltic 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 proportioned mixture of coarse aggregates, fine aggregate, mineral filler and bitumen and the mix is designed by an appropriate method such as the Marshall method to fulfill the requirements of stability, density, flexibility and voids. The thickness of bituminous concrete surface course layer usually ranges from 40 to 75 mm. The IRC has provided specifications for 40 mm thick AC surface course for highway pavements.

Key Takeaways:

Dense Bituminous Macadam: - Dense Bituminous Macadam (DBM) is a binder course used for roads with more number of heavy commercial vehicles and a close-graded premix material having a voids content of 5-10 per cent.

5.11 Dry lean concrete (DLC)

DLC acts as a sub-base for the cement concrete pavements or pavement quality concrete (PQC). DLC material has no slump i.e. zero slump.

5.11.1 Material

1. Sources of Material

The contractor shall indicate to the engineer the source of all material with relevant test data to be used in the dry lean concrete (DLC) work sufficiently in advance and approval of the engineer for same shall be obtained at least 45 days for the scheduled commencement of the work in the trial length. Contractor can take approval of different source of material with relevant test data at least 45 days in advance to use the material by the engineer during the execution of main work.

2. Cement

If the subgrade soil contains soluble sulphates in a concentration more than 0.5%, sulphate resistant cement conforming to IS:6909 shall be used.

3. Fly-ash

Fly-ash up-to 20% by weight of cementitious material (cement + fly-ash) may be used along with 43/53 grade cement may be used to replace Ordinary Portland cement (OPC) cement grade 43 up-to 30% by weight of cement.

4.Aggregate
Aggregate for lean concrete shall be natural material complying with IS:383. Aggregate shall not be alkali reactive. In case engineer consider that the aggregates are not free form dirt, the same may be washed and drained for at least 72 hours before batching, or as directed by the engineer.

5. Water

Water used for mixing and curing shall be clean and free from injurious amounts of oils, acids, alkalis, salts, sugar, organic materials or other substances that may be deleterious to concrete or steel.

Key Takeaways:

DLC Layer: - DLC is a no slump plain concrete with a large ratio of aggregate to cement. It contains less amount of cement paste as compared to conventional concrete. DLC layer is an important part of modern rigid pavements.

5.12 Cement Concrete (CC) road construction

Road having their wearing surface consisting of cement concrete slab are called as cement concrete road. Cement concrete roads are considered as most serviceable and rigid pavements.

5.12.1 Structural components of concrete pavement

Sub-grade

Sub-base

Concrete Slab

5.12.2 Advantages of Cement Concrete Road

Life span of such road is more.

Such roads provide an impervious layer.

Cement concrete roads are strong and durable and are unaffected much by weathering agencies.

They give good visibility at night.

Cement concrete roads provide dustless and sanitary surface.

Cement concrete roads does not develop corrugations and hence it grants noiseless surface.

It can be designed more accurately for load distribution.

Cement concrete roads is practically unaffected by weather and temperature.

It is possible to make use of old concrete road as foundation for new concrete road or for bituminous road.

5.12.3 Disadvantages of Cement Concrete Road

Initial cost of construction is high especially when suitable aggregate is not locally available.

It is liable to crack, warp and twist.

Skilled supervision as well as skilled workmanship is required for their construction.

It becomes noisy under iron tired traffic.

Cement concrete roads cannot be opened to traffic earlier as it require long time curing.

In case of such pavements, it is very difficult to locate and repair sewers and water mains, which are lying under it.

They are less resilient than W.B.M road or bituminous road.

5.12.4 Method of Construction of Cement Concrete Roads

Alternate Bay Method

Continuous Bay Method

Expansion Joint and Strip Method

5.12.4.1 Alternate Bay Method

The construction work is carried out in old bays of one lane and even bays of the other lane as shown in fig. The construction of the next bay is commenced after the concrete laid earlier bays dries out.

5.12.4.1.1 Advantages of Alternate Bay Method

Joints can be constructed easily and its width can be kept as desired.

The bays which have been cured can serve as additional working platform.

5.12.4.1.2 Disadvantages of Alternate Bay Method

During rains, the water collects on the surface of the bays which are not constructed.

More time is required to complete the work.

It requires large number of transverse joints.

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

5.12.4.2 Continuous Bay Method

This is also known as strip method or full width method. In this method of construction, all the slabs or bay of a strip are constructed continuously without any break from one end to other.

In continuous bay method construction joints are provided when the day’s work is not ended at the specified joint. Moreover, in addition to construction joints dummy joints are also provided at 5 m intervals in transverse direction. They are provided to check the planes of weakness and to control cracking.

5.12.5 Construction of Cement Concrete Road

The construction of cement concrete road involves following operations:

5.12.5.1 Preparation of Sub-grade

Sub-grade is natural soil on which concrete slab is laid.

It is cleaned, shaped and leveled.

After cleaning, it is prepared to the required grade and profile.

It should be seen that sub-grade has uniform strength over its entire width.

If any local weak spots are found, they should be removed and strengthened by placing new material which is compacted.

When concrete is to be directly placed on sub-grade, the surface should be saturated with water for 6 to 20 hrs in advance of placing the concrete.

This is done to ensure that sub-grade does not absorb water from the concrete.

5.12.5.2 Provision of Sub-base

When natural sub-grade is not very firm, a sub-base over the sub-grade is provided.

Depending upon the type of soil, design load, intensity of traffic and economic consideration, the decision for providing the sub-base is taken.

5.12.5.2.1 The sub-base serves the following three purposes

It provides a capillary cut-off and the damage caused by mud pumping is prevented.

Provides strong supporting layers.

It reduces the thickness of concrete slab and thus leads to lower cost of construction.

5.12.5.2.2 The sub-base may consists of any one of the following layers

A layer of well graded soil-gravel mixture of maximum thickness 15 cm.

Brick soling with one layer of W.B.M. of maximum total thickness 10 cm.

Two layers of W.B.M. of maximum total thickness 15 cm.

A layer of lean cement concrete of maximum thickness 10 cm.

5.12.5.3 Placing the Forms in Cement Concrete Road

The forms may be made up of steel or timber.

The steel forms are of mild steel channel sections and their depth is equal to the thickness of the pavement.

Forms are properly braced and fixed to the ground by means of stakes.

Forms are fixed in position by 3 stakes at back of each 3 m length.

When the forms are fixed, they must be checked for their trueness.

The maximum deviations permissible in the vertical plane are 3 mm and in horizontal plane 5 mm in 3 m length of the form.

The forms are oiled before placing concrete in them.

5.12.5.4 Watering the Prepared Sub-grade or Sub-base

After the forms are fixed, the prepared surface to receive concrete is made moist.

If the sub-grade is dry, it should be sprinkled with as much quantity of water as it can absorb.

It is advisable to wet the surface at least 12 hrs in advance of placing the concrete.

When insulating layer of water-poof paper is provided, the moistening of the surface prior to placing the concrete is not required.

5.12.5.5 Batching of Materials and Mixing

After determining the proportions of ingredients for the Concrete mix, the fine and course aggregates are properly proportioned by weight in weight-batching plant.

They are then fed into the hopper along with necessary quantity of cement which is also measured by weight.

The ingredients of concrete are mixed in proper proportions in dry state. The mixing should preferably be done in a concrete mixer.

5.12.5.6 Transporting and Placing of Concrete

After mixing, the concrete is transported to the site in wheel burrows or in pans which are manually carried.

The mixed concrete is deposited rapidly on the sub-grade in layer of thickness not more than 50 mm to 80 mm or about two or three times the size of aggregates.

The concrete should be placed over the entire width of bay in successive batches as a continuous operation and topmost layer is laid about 10 mm higher than the actual profile for further tamping.

The top layer should also be laid to the required camber and gradient, while placing the concrete it is roded with suitable tool to eliminate voids.

Segregation of concrete is avoided during transportation and placing. When reinforcement has been specified in road slab, concrete is placed in two stages.

In first stage, concrete is placed and compacted to the depth corresponding to the level of reinforcement shown on the drawings.

Reinforcement is then placed on top of compacted concrete and remaining thickness of slab is then completed in second stage.

5.12.5.7 Compaction of Cement Concrete Road

After the concrete is placed in its position, it should brought in its proper position by heavy screed or tamper fitted with suitable handles.

The wooden tamper is at least 75 mm wide and its underside is shaped to the finished cross-section of the slab.

Its weight is about 10 kg/m.

It should have sufficient strength to retain its shape under all the working conditions

Its length is equal to length of bay plus 60 mm.

The underside of tamper is provided by a metal plate of 5 mm thickness as shown in Fig. 4.14.4.

The tamper is placed on the side form and its handles are griped by the men who use the tamper.

Concrete is also compacted by means of a power driven finishing machine or by vibrating hand screed.

Up to 12.5 cm thickness of slabs screed vibrators alone can be used for compaction.

For greater thickness, immersion vibrator is used.

5.12.5.8 Floating

After compaction, the entire slab surface is floated longitudinally with a wooden float board.

The purpose of floating is to provide an even surface free from corrugations.

5.12.5.9 Belting

After floating, the surface is further finished by belting just before the concrete become hard.

The purpose of belting is to make the road surface non-slippery and skid resistant. This operation is sometimes omitted.

5.12.5.10 Brooming

After belting, brooming is done by drawing brushes at right angles to the center line of road from edge to edge.

Brooming is done just before the concrete becomes non- plastic.

This operation is also sometimes omitted.

5.12.5.11 Edging

After brooming, the edges of the slab are carefully finished with an edging tool before the concrete is finally set.

5.12.12 Curing Process of Cement Concrete Road

Curing consists of checking the loss of water from the concrete slab, and keeping the fresh concrete slab moist during hardening period.

Initial curing is done for 24 hrs.

By this time, the concrete becomes hard enough to walk upon and then wet mats are removed and final curing done for 2 to 3 weeks.

Final curing is done by any one of the following methods:

Ponding Method.

By covering the slab with 4 to 8 cm thick layer of wet sand or earth.

Spraying a suitable chemical such as sodium or calcium chloride on concrete surface.

5.12.13 Fillings of Joints and Edging

After curing, the surface is cleaned and washed.

The joints are then properly filled-in attains with a suitable sealing compound.

5.12.14 Opening to Traffic

Concrete road is opened to traffic when it attains the required strength or after 28 days of curing.

5.12.5 Types of Defects in Cement Concrete Road

Spalling of Concrete.

Deformation.

Surface Texture Defects.

Joint Sealant Defects.

Cracking.

When the concrete dries and hardens, the excess water from the concrete evaporates and the concrete shrinks. As a result, cracks are developed in cement concrete road. Due to too dry Weather, after the wet concrete mix is spread it takes few weeks to complete the whole hydration process. The hydration process givesstrength to the concrete and needs sufficient water for hydration it tends to develop cracks.

Key Takeaways:

Ordinary Portland cement: - Ordinary Portland cement is the most widely used type of cement, which is suitable for all general concrete construction. It is the most commonly produced and used type of cement around the world, with annual global production of around 3.8 million cubic meters per year.

References:

1. L.R. Kadiyali, Transportation Engineering, Khanna Publishing House

2. Saxena, Subhash C, A Textbook of Highway and Traffic Engineering, CBS Publishers &

Distributers, New Delhi

3. Kumar, R Srinivasa, “A Text book of Highway Engineering”, Universities Press,

Hyderabad.

4. Kumar, R Srinivasa, “Pavement Design”, Universities Press, Hyderabad.

5. Chakraborty Partha& Das Animesh., “Principles of Transportation Engineering”,

Prentice Hall (India), New Delhi,

6. IRC : 37- Latest revision, “Tentative Guidelines for the design of Flexible Pavements”

Indian Roads Congress, New Delhi

7. IRC:58-2015 Guidelines for the Design of Plain Jointed Rigid Pavements for Highways

(Fourth Revision) (with CD)

8. IRC:65-2017 Guidelines for Planning and Design of Roundabouts (First Revision)

9. IRC:73-1980 Geometric Design Standards for Rural (Non-Urban) Highways

10. IRC:106-1990 Guidelines for Capacity of Urban Roads in Plain Areas

11. IRC:93-1985 Guidelines on Design and Installation of Road Traffic Signals.

12. IRC:92-2017 Guidelines for Design of Interchanges in Urban Areas (First Revision)

13. IRC: SP: 68-2005, “Guidelines for Construction of Roller Compacted Concrete Pavements”,

Indian Roads Congress, New Delhi.

14. IRC: 15-2002, “Standard Specifications and Code of Practice for construction of Concrete

Roads” Indian Roads Congress, New Delhi.

15. MORTH, “Specifications for Road and Bridge Works”, Ministry of Shipping, Road

Transport & Highways, Published by Indian Roads Congress, New Delhi.

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