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.
5.1.2 Steps for Preparation of Subgrade
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 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.184.108.40.206 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.220.127.116.11 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;18.104.22.168 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.22.214.171.124 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.126.96.36.199 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.188.8.131.52 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.184.108.40.206 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.220.127.116.11 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.18.104.22.168 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.22.214.171.124 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.126.96.36.199 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.188.8.131.52 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.
5.2.5 Advantages of water bound macadam road
There are the following advantages of water bound macadam road such as;
5.2.6 Disadvantages of water bound macadam road
There are also some disadvantages of water bound macadam road such as;
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.
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.
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.
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.
The following sets of equipment are necessary for the for GSB construction work.
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
80 – 100
55 – 90
35 – 65
20 – 40
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
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.
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.
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:
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.
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.
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.184.108.40.206 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.220.127.116.11 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.
18.104.22.168 Performance Guidelines
In order to construct a proper well designed prime coat the following guidelines should be followed:
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:
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.
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
5.8.2 During the Work
5.8.3 After Completion
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.
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.
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.
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.
If the subgrade soil contains soluble sulphates in a concentration more than 0.5%, sulphate resistant cement conforming to IS:6909 shall be used.
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.
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.
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.
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
5.12.2 Advantages of Cement Concrete Road
5.12.3 Disadvantages of Cement Concrete Road
5.12.4 Method of Construction of Cement Concrete Roads
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.
22.214.171.124.1 Advantages of Alternate Bay Method
126.96.36.199.2 Disadvantages of Alternate Bay Method
188.8.131.52 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:
184.108.40.206 Preparation of Sub-grade
220.127.116.11 Provision of Sub-base
18.104.22.168.1 The sub-base serves the following three purposes
22.214.171.124.2 The sub-base may consists of any one of the following layers
126.96.36.199 Placing the Forms in Cement Concrete Road
188.8.131.52 Watering the Prepared Sub-grade or Sub-base
184.108.40.206 Batching of Materials and Mixing
220.127.116.11 Transporting and Placing of Concrete
18.104.22.168 Compaction of Cement Concrete Road
5.12.12 Curing Process of Cement Concrete Road
5.12.13 Fillings of Joints and Edging
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
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.
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