Unit-5

Highway Construction

Q1) Write down the construction steps for water-bound macadam road.

A1)

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

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.

The shoulders having a thickness as that of the 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.

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.

A 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.

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.

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.

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.

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.

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.

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

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.

Q2) What are the advantages and drawbacks of cement concrete roads?

A2)

Advantages of Cement Concrete Road

Drawbacks of Cement Concrete Road

Q3) Enumerate the steps for the preparation of subgrade.

A3)

Steps for Preparation of Subgrade:

Q4) Briefly list the method of construction of granular sub-base.

A4)

Granular Sub Base Construction Method

Spreading

Compaction

            a) Initial rolling -2 static passes with Vibratory roller

            b) Subsequent rolling – 4 vibratory pass

Q5) What are the steps that are involved in the construction of surface dressing?

A5)

The Surface Dressing Process

Q6) Briefly explain semi dense bituminous concrete and bituminous concrete.

A6)

Semi dense bituminous concrete

Semi Dense Bituminous Concrete (SDBC) is similar to BC, but with a higher void content (5-10 percent, as against 3-5 percent 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.

Bituminous concrete

Bituminous concrete or Asphaltic concrete (AC) is a dense-graded premixed bituminous mix that 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 the bituminous concrete surface course layer usually ranges from 40 to 75 mm. The IRC has provided specifications for a 40 mm thick AC surface course for highway pavements. 

Q7) Write down the construction steps for wet mix macadam road.

A7)

Wet Mix Macadam consists 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 is 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 the WBM process is cheaper than the WMM, the latter 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.

Q8) Discuss the uses and limitations of the roller-compacted concrete road.

A8)

Roller-compacted concrete (RCC), which takes its name from the construction method used to build it, is concrete of zero-slump consistency in its unhardened state. It is defined as concrete compacted by a roller.

Materials, Production, and Placement

Roller compacted concrete consists of the same basic ingredient as conventional concrete: cement, water, and aggregates, such as gravel or crushed stone. However, unlike traditional concrete, the mixture is drier and adequately stiff to be compacted by a vibratory roller.

The mix design of RCC usually consists of a high amount of stone, Portland cement with a replacement of fly ash, a minimal amount of sand, and of course, water to hydrate the mix.

Advantages

The main advantage is reduced cost and time for construction (cost-effective and fast construction). So, it can achieve high quality in terms of strength, durability, and surface finish at the relatively low device and personnel costs.

Disadvantages

Q9) Briefly explaintack coat.

A9)

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 construction lifts and especially between the existing road surface and an overlay is critical 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 like 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 reduces pavement life.

Application

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

Q10) Briefly explain prime coat.

A10)

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 before 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.
Materials

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.

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 the granular and then evaporates out to leave the asphalt residue.