Construction Steps for Cement Concrete Pavement

Hi,
Concrete pavements are rigid pavements having very high flexural strength as compared to flexible pavements.





 Concrete pavements can be constructed using two different methods:

1. Alternate Bay method
2. Continuous bay method

• In alternate bay method, concrete pavement slab are laid on whole width of pavement in alternate bays.
• In continuous bay method, concrete pavement slabs are laid continuously only on one bay and another bay is open for the traffic. 

Generally the second method of continuous bay, is preferred over alternate bay method because, traffic movement is allowed while it is restricted in the first. Also, the alternate empty spaces invite the rainwater collection and create inconvenience to the construction work.

Various steps for the construction of concrete pavements:

1. Preparation of Sub-grade and Sub-base
2. Placing of forms
3. Batching of material and Mixing
4. Transporting and Placing of Concrete
5. Compaction and Finishing
6. Floating and Straight Edging
7. Belting, Brooming and Edging
8. Curing of Cement concrete

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Road user Characteristics (Traffic Characteristics)

Hi,

According to S.K.Khanna and S.E.G.Justo from book Highway Engineering, Traffic Engineering is the branch of engineering which deals with the improvement of the traffic performance of road networks and terminals.
 For achieving that we have to perform systematic traffic studies, analysis and then its engineering application.

First of the most important scientific study is the study of the traffic characteristics. Traffic can be classified into two classes:

1. Road Users.
2. Vehicular traffic.

In this post I will discuss the road user characteristics that effect the traffic performance.

• Road user characteristics

Human beings performing different roles in the traffic are most important elements of the traffic and so we have to study their characteristics and behavior. Various roles of human are such as driver, pedestrians, cyclists etc. The physical, mental and emotional characteristics of human beings affect their ability to operate motor vehicle safely or to service as a pedestrian. Hence it is important for a traffic engineer to study the characteristics and limitations of the road users.

The various factors which affect road user characteristics may broadly be classified under four heads:

1. Physical
2. Mental
3. Psychological
4. Environmental

• Physical characteristics: The permanent physical characteristics of the driver are vision, hearing, strength and the general reaction to the traffic situations. 

1. Vision include the acuity of vision, peripheral vision and eye movement; glare vision, glare recovery and depth judgement.  Field of accurate, clear vision is about a 3 degrees cone however the vision is fairly satisfactory up to 10 degrees in general and 20 degrees in horizontal plane.In vertical plane the vision may be limited to 2/3 of that in horizontal plane.
2. Hearing is helpful for drivers but of more important for the pedestrians and cyclists. 



Road users (Google images)

3. Strength is not an important factor in general , lack of strength may make parking maneuvers difficult, particularly for heavy vehicles.

• Mental Characteristics:  Knowledge, skill, intelligence, experience and literacy can affect the road user characteristics. Knowledge of vehicle characteristics, traffic behavior, driving practice, rules of roads and psychology of road users will be quite useful for safe traffic operations. 
• Psychological factors: These effect reaction to traffic situations of road users to a great extent. Attentiveness, anger, fear, anxiety, phobias, superstition, and impatience may effect the traffic performance to great extent. 
• Environment factors: The various environmental conditions affecting the behavior of road user are traffic stream characteristics, facilities to the traffic, atmospheric conditions and locality. The traffic stream may consist of mixed traffic or heavy traffic whereas facilities to overtake to the faster vehicles may be limited. The behavior of the driver varies from one traffic stream to another. 

Similarly the facilities of the traffic separators, multi-lanes etc will effect the performance. Surrounding environment effect the performance of the traffic because one will get slower at the market places and will be faster at the open places. 

Length of Summit Curves (Vertical curve)

Hi,
 Today, I am going to put here the details and engineering behind the length of the Summit Curves required for the vertical alignment of the highways.

• Design criteria for the length of Summit Curves:      Summit curves are the vertical curves having convexity upwards. These curves are introduced for the driving comfort and aesthetic purposes. In summit curves we do not need any transition curves because centrifugal acceleration is not acting laterally outward or inward but, its acting upward. 

So, rate of introduction of providing the superelevation is not the criteria. 
If we provide the circular curve without the transition curve and since centrifugal force is upward, driver will not face the sudden jerk.  The main criteria for the design of the length of the summit curves is the sight distance.

Length of the road is governed by the stopping sight distance and the overtaking sight distance.

• On the basis of the Stopping sight distance(SSD): Suppose length of the transition curve is given as Ls and SSD is denoted by S. Stopping sight distance criteria has two cases:

1. L> S - When length of the curve is less than the stopping sight distance.
2. L<S - When length of the curve is more than the stopping sight distance.

For both the cases IS has given the empirical formulas and you have to check both of them

• L>S :  

In this case check this formula    L=   (NS^2)/ [ (2H)^(1/2) + (2h)^(1/2)]
Here, 
N = Change in grade
S = Stopping sight distance
H = Height of the eye of the driver, taken as 1.2 m
h = Height of the object, taken as 0.15 m.
 putting these values    L =  (NS^2)/ 4.4

• L<S : 

In this case check this formula   L = 2S - [ (2H)^(1/2) + (2h)^(1/2)] / N

Here, 

all terms denote the same as above so,

  L = 2S -4.4/N.

• On the basis of overtaking sight distance(OSD):

For this criteria use the same formulas given above for the SSD but, use the object height as that of the eye level of driver means, take H= 1.2 m and h= 1.2 m also.

• L> S:      then,    L=   (NS^2)/ [ (2H)^(1/2) + (2H)^(1/2)]  = NS^2/ 9.6
• L<S:     then,   L = 2S - [ (2H)^(1/2) + (2h)^(1/2)] / N   = 2S - 9.6/N

you have to use trial method to check both the cases.

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Grade Compensation for the Horizontal Curves on Highways

Hi,

Grade or Gradient is  the ratio of vertical rise or fall of the road center line with respect to its length. It has to be provided for the vertical alignment of the road. 

When we provide gradient to the road, vehicle has to apply more power to overcome the component of the gravity force. Higher the grade/slope,  larger is the effort required.


In a horizontal curve the effective tractive effort of the vehicle is reduced because of the turning angle. 

Suppose 'T' is the tractive effort of the vehicle, and the turning angle of the front wheel is 'theta' then the tractive effort in that direction will be less.

There will be a loss of tractive effort equal to (T-T.cos.theta). This loss in the tractive effort has to be compensated at the horizontal curves which also have the vertical grade. It is done by reducing the grade, this is known as the grade compensation.

According to IRC, a grade compensation  of,  (30+R/R), maximum 75/R  has to be provided for the grades more than 4%.
No grade compensation is required for the lower values of the the grade.
Here, R is the radius of the horizontal curve in meters.

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Steps for Construction of Earth roads

Hi, 
today I am going to share with you the steps which are necessary for the construction of the earth roads. 

• General :

First of all what is an earth road? Earth road is a type of road whose whole pavement section is constructed with the locally available earth material preferably. Borrow pits are located at the nearby sites preferably outside the land width  where, the required earth is available.

Sub-grade and the surface of the earth roads are given larger camber of 1in 33 to 1 in 20 because they need faster drainage to be safe from the moisture. A maximum value of camber of 1 in 20 is the limit because higher camber will result in the formation of cross ruts and corrosion of pavement soils.

• Specifications of Materials:

The earth material used for the construction of earth roads are termed as satisfactory if they possess the following properties:
                                               Base Course                         Wearing Course

1. Clay content                       <5%                                      10 to 18%
2. Silt content                       9 to 32%                                  5 to 15%
3. Sand content                     60 to 80%                               65 to 80%
4. Liquid limit                          <35%                                      <35%
5. Plasticity Index                     <6%                                      4 to 10%

• Construction steps(Procedure):

Material: 
Suitable borrow pits are located by doing the survey of the adjacent land which are easy to reach and at economical haulage distance. The various organic materials like trees, shrubs and grass roots are removed before the excavation of the earth.

Location of the centerline: 
The centerline and the road boundaries are marked on the ground by driving the wooden pegs. 

To follow the desired vertical profile of the road, reference pegs are also driven at a certain spacing which depends upon the estimated length of the road construction per day.

Preparation of the subgrade: Following steps are necessary for the preparation of the sub-grade:  (a) Clearing site 

(b) Excavation and construction of fills

(c) Shaping of sub-grade.

The site clearance may be done manually using appliances like spade, pick and hand shovel or using the mechanical equipment like Bulldozer and scraper etc.

Excavation and construction of fills may also be done manually or using the excavation, hauling and compaction equipment.  Dozers are considered very useful for haulage of short distances. 

If the compaction is done manually it will not be sufficient and proper, it should be left to get consolidated under atmospheric conditions. 

Various equipment used by manual labor are shovel, spade, pick-axe, baskets, rammers and hand rollers.                                                        The subgrade should be compacted to the desired grade, camber and longitudinal profile.

Pavement construction: 
The soil is dumped on the prepared sub-grade and pulverized. The soil may be a mixture of more than one soil to get the desired properties. The moisture content is checked and if extra moisture is needed, is added to bring it to OMC.
The soil is mixed, spread and rolled in layers such that the compaction thickness of each layer does not exceed 10 cm. 

The type of roller for compaction is decided based on soil type, desired amount of compaction and availability of equipment. At Least 95% of dry density of I.S. light compaction is considered desirable. The camber of the finished surface is checked and corrected when necessary.

Opening to traffic: The compacted earth surface is allowed to dry out for few days and then is opened to traffic.

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Highway Compaction Equipment

Hi,
 There are various types of compaction equipment:

1. Rollers
2. Rammer
3. Vibrators

a. Rollers: Rollers are vastly used in the highway construction for compacting the subgrade and other layers. They are again of three types:

Smooth wheeled roller (Image source:Google images)

1. Smooth wheeled rollers:  These are used for compacting granular soils and specially useful where crushing is required. Smooth wheeled rollers generally vary from 4 to 22 tonnes in their weight. 

b. Pneumatic Tyre rollers: These are used for compacting the silty and sandy soils and not preferred for clayey and granular soils.

Pneumatic Tyre roller ( Image source: Google images)

 c. Sheep's foot rollers: These rollers are having hollow cylindrical wheels having sheep's foot shaped projections on their surfaces. 

They are suitable for compacting the clayey soils, specifically where needing action is required and along with them smooth wheeled rollers may be used for giving a smooth surfacing to top surface.

2. Rammers: Rammers are used where rollers are not suitable due to the area constraints. They are mechanically operated using the electric energy.

3.Vibrators: Vibrators are suitable for compaction of the sandy soils and specifically cohesion-less soils. They can also be used along the rollers in their dual combination.

Vibrators (Image source: Google Images)

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Highway Excavation Equipment

Hi, 
how you doing? Today I am going to post here what I learnt about the highway construction equipment.

As we know that highway construction at field can be broadly classified into two phases:

1.  Excavation or preparation of sub-grade.
2. Pavement layers construction.

In the phase of the excavation or preparation of the sub-grade we need two types of equipment:

1. Excavation equipment
2. Compaction equipment

In this post I will discuss the excavation equipment. Types of excavation equipment that is to be used depends upon the type of the soil and availability of the equipment. Various equipment used in general for the excavation of the sub-grade are

1. Bull Dosser
2. Scraper
3. Power Shovel
4. Drag-line
5. Clam-shells
6. Hoe

• Bulldozer 

Bulldozer is a crawler equipment used to push and haul the earth material up a distance of 100 m using its front blade and a ripper attached at its rear.

Bulldozer (source: wikipedia)

 Bulldozer is a versatile equipment because it can be used in undulating ground or level ground, can be used for various building construction projects including pilot road constructions or even in farms.

• Scraper: Scraper is having a blade in its rear part which is used to scrap the earth and then to store it in the bowl. Once the bowl is full the loose earth can be hauled to the deposition site. There is an elevator arrangement for the blade which can elevate or lower the blade according to the need.



Scrapper (Source: Google images)

• Power Shovel: Power shovel is a power equipment used to dig the earth material at lower depths than the standing elevation. The parts of a power shovel are crawler mounting, cab, boom, shovel and hoist cable.

  

  Power Shovel (image Source: wikipedia)

  Power shovel can be used to excavate the loose earth material effectively and can  be used in undulating grounds because it has a crawler mounting.

• Drag-Line: Drag line has a bucket suspended from the book which can be dragged with a drag cable and hoist cable to collect the loose material inside the bucket. 

  

  Drag Line (Image source: wikipedia)

  Image shown above shows a drag-line with all its major parts. Drag line can be used to collect the loose material from a much larger depths.

• Clam-shell :

Clam-shell bucket is combination of two shells which are joined together with hinges at top and when these half are closed form a bucket. It is attached to a cable and a boom arrangement and to lift the material it is made to fall on it and then lifted up. In the lifting operation clam-shells close them up and will lift the material filled inside.

Clam-shell

• Hoe: Hoe can be used to excavate relatively stiffer earth from a depth below the standing level of the equipment. This material can excavate all types of earth materials except hard rocks.

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