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.

Thanks for your kind visit!

Design of Dowel bars - Rigid Pavements.

Hi,
How you doing? Let's discuss the design of the dowel bars. So, what are dowel bars first?

• Dowel bars

 Dowel bars are steel bars, generally round in shape,  and are used in the expansion joints which, are provided in the concrete pavements(rigid pavements) to allow the expansion of the concrete slab.
The main function of the dowel bars is to transfer the wheel load from one slab to the adjacent slab through the shear and bending moment.

Dowel bars are embedded on both the slabs, and on one slab there is a dowel cap which provides space for the bar to occupy it when there is expansion in both slabs.
This empty space is filled with some easy to contract material so, that it may adjust its volume according the need of the dowel bar.

Design of Dowel bars:
In order to design the length and diameter of the dowel bar we have to check its failure under the action of wheel load on the pavement slab.
Failure may occur in three ways:

1. Failure of the dowel in shear
2. Failure of dowel bar in flexure
3. Failure of concrete in bearing.

Steps:

1. According to Indian roads congress(IRC) the following formulas(given in image) are used to find the load capacity of the dowel under shear, flexure and that of concrete in bearing. Three formula are given in the image below.

   

   Dowel bars- design formula (Courtesy: Highway Engg. By S.K. Khanna and C.E.G.Justo)

2. Generally the thickness of the expansion joint and diameter of the dowel bars are fixed.  We first find out the length of the bar by equating the second formula with the third formula assuming that the load capacity of the concrete in bearing is equal to the flexure capacity of the dowel bar. The formula is given below in image, length has to be found by trial and error method.

   

   Length of Dowel bar ( Source: Highway Engg. by S.K.Khanna & C.E.G. Justo)

3. Then use that length to find out the three load capacities corresponding the these three formulas. The lesser of the three will be the capacity of the dowel bar because, failure at any case is the failure of the pavement system.

It is assumed that the wheel load is transferred to a distance of 1.8.l from the application of the load. The capacity factor of the dowel system is defined as the ratio of the load capacity of the dowel system to the load capacity of the single dowel bar.

We must check the dowel system to be sufficient to provide the corresponding load factor and that will decide the spacing of the dowel bars and therefor the numbers of dowels required.

Thanks for your visit!

Bitumen Penetration Test to Find out its Grading.

Hi,

We know that penetration test is one of the important tests, which is performed to test the grade of the Bitumen. You might be looking this article to jot down the theory and procedure of the test into your laboratory/practical notebook or might be checking just to refresh your knowledge. If you have one of these reasons, reading this article surely is going to help you in fulfilling the purpose. So, here we go:

AIM: 

To find out the consistency/hardness of the Bitumen and thereby to find out its grade using the Bitumen penetration test.

APPARATUS: 

1. Standard Penetration test apparatus with all the accessories such as, needle, weight and timer.
2. Heater
3. water bath with temperature control, maintained at 25° ± 0.1 °C, containing not less than 10 litres of water.

Accessories to the Standard penetration test are:

• Container: flat bottomed cylindrical metallic dish having 55 mm diameter and 35 mm depth.
• Penetration Needle: A straight, highly polished cylindrical hard steel rod having a diameter of about 1 mm.
• Time measuring device with an accuracy of 1 second.

THEORY:

 This test is used to check the grade of the bitumen. In other words, it is used to check the consistency or the hardness of the bitumen which is useful to find out the suitability of the bitumen to use under different climatic conditions.

Penetration value is a measure of hardness or consistency of bituminous material. It is the vertical distance traversed or penetrated by the point of a standard needle in to the bituminous material under specific conditions of load, time and temperature. 

This distance is measured in one tenths of a millimeter so, if the reading is 50 mm, that denotes a penetration of 5 mm. This test is used for evaluating consistency of bitumen. It is not regarded as suitable for use in connection with the testing of road tar because of the high surface tension exhibited by these materials.

PROCEDURE:

1. Preparation of sample: Soften the material to a pouring consistency at a temperature not more than 60°C for tars and 90°C for bitumen above the approximate softening point and stir it thoroughly until it is homogeneous and is free from air bubbles and water. Pour the melt into the container to a depth at least 10mm in excess of the expected penetration. Protect the sample from dust and allow it to cool in an atmosphere at a temperature between 15° to 30° C for one hour. Then place it along with the transfer dish in the water bath at 25° ± 0.1 °C, unless otherwise stated.
2. Put the test cup upon the stand of the penetration apparatus.
3. Clean the needle with benzene, dry it and load with the weight. The total moving load as per standards, must be 100 ± 0.25 gms, including the weight of the needle, carrier and super-imposed weights.
4. Adjust the needle to make contact with the surface of the sample. This may be done by placing the needle point in contact with its image reflected by the surface of the bituminous material.
5. Make the pointer of the dial to read zero or note the initial dial reading.
6. Release the needle for exactly five seconds for which you can use the timer or stopwatch if timer is not provided with the apparatus.
7. Adjust the penetration machine to measure the distance penetrated.
8. Make at least 3 readings at points on the surface of the sample not less than 10 mm apart and not less than 10 mm from the side of the dish. 
9. After each test return the sample and transfer dish to the water bath and wash the needle clean with benzene and dry it. In case of material of penetration greater than 225, three determinations on each of the two identical test specimens using a separate needle for each determination should be made, leaving the needle in the sample on completion of each determination to avoid disturbance of the specimen.

PRECAUTIONS:

• Remember that sample must be tested at 25 degrees Celsius so, sample must be prepared with the standard procedure listed above.
• There should be no movement of the container while needle penetrates into sample.
• The needle should be cleaned with benzene and dried before penetration.

OBSERVATIONS

Actual test temperature = °C

Penetration dial reading	Test 1	Test 2	Test 3
(i)Initial
(ii)Final
Penetration Value=

Mean = 

RESULT:

Mean Penetration Value =

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Critical Stress Combination for Rigid Pavements

Hi, How you doing?
As we know that the design of a pavement is governed by the analysis so, this is very important to know the various combinations of stresses that will be induced in the slab. There are two kinds of loading:

1. Traffic Loading
2. Temperature variation.

Traffic loading will induce the compressive or tensile stresses in the pavement and temperature variation results in to kinds of stresses. One is known as warping stress and another is frictional stress.

That makes total of three kinds of stresses that will be induced in the pavement slab. Now, we have to find out the critical combination of these three.

• In Summer Season:

In summer season the average temperature is higher and it results in the overall expansion of the slab. This induces the frictional stresses which will be compressive so as to restrict the expansion. Remember friction will always act in the opposite direction of the movement of the bottom of the slab. That means when it tries to expand, friction will try to contract it or restrict the expansion. That makes it to induce the compressive stress.

In the after mid-noon hours in summer season, the temperature at the top layer will be higher as compare to the bottom layer. That will make the slab to tend to warp in downward direction. Under the self weight of the slab warping will be restricted and therefore will result in the tensile stress in the bottom layer.

 Downward warping results in contraction of the bottom layer which is restricted through the induction of the tensile warping stress. So, warping stress of tensile nature is induced.
Now, at the edge region the traffic load stress of tensile nature is also induced and that is where the critical load combination is formed.

Critical stress combination(At the edge)  = Traffic stress + Warping stress - frictional stress.

• In Winter Season:

In the after mid-noon hours in winter season again, traffic load stress and the warping stresses will be tensile in the bottom layer. Also, the frictional stresses will be tensile because, there is overall contraction of the pavement due to lower average temperature and therefore friction will act along the tensile direction.
So, the Critical stress combination(At the edge)  = Traffic stress + Warping stress + frictional stress.

At the corners the critical load combination is that of the traffic load and the warping stress at the top layer at the mid-night hours.
See, in the night the temperature at the bottom layers will be higher as compare to the temperature at the top layers so, there will be upward warping. That will induce tensile warping stress at the top layer.
 So, Critical stress combination(At the corner)  = Traffic stress + Warping stress 

Thank you for your time!

Sanjay.

Flash(Open cup) and Fire point Test - Highway Engineering Lab

Flash and fire point (open) test apparatus( courtesy: Aimil Ltd.)

Hi, I am going to explain flash(open) and Fire point test.

• AIM: To find out the flash and fire point of a given bitumen sample using open cup.
• APPARATUS: 

1. Flash and Fire point apparatus (Open Cup). This apparatus consist of the a test cup, heating plate, heater, test flame applicator and supports.
2. Thermometer (range -6 to 400 degrees Celsius).

• THEORY: Bitumen is a volatile material and when temperature is increased, vapors are formed.

In order to work safely with the bitumen at the site, one must know the temperature at which bitumen may become hazardous to work with.

Flash and Fire points are the lowest temperature at which, sufficient amount of vapors are formed to catch the flash and fire respectively when a flame is brought near to it. At flash point sufficient amount of vapors are formed so as to catch a momentarily flash.

Flash and fire point vary for different variety of Bitumen. 

• PROCEDURE:

1. Heat up the bitumen sample to a pouring temperature and then pour it down into the cup up to the marking. Stir the bitumen properly to remove any foreign gases present.
2. Put the thermometer in place and start raising the temperature and 5 to 6 degrees per minute.
3. When the temperature is about 25 degrees Celsius near to the expected flash point, start the flame and bring it near to the vapors.
4. Note down the temperature at which the vapors catch a momentarily flash.
5. Now keep raising the temperature and note down the temperature at which fire is started.

• RESULT: 

1. Flash point of the given bitumen sample = ......
2. Fire point of the given bitumen sample = .....

NOTE: For Indian standard specifications for flash and fire point of Bitumen, please refer to IS:1209-1978.

Factors affecting the Stopping sight distance

Hi,

Factors effecting the Stopping sight distance

As per IRC(Indian Roads Congress), Stopping sight distance is such a distance along the center line of the road that if there is an obstruction of 0.15 m at the other end, it must become visible to the eye of a driver at a height of 1.2 m from the road surface .

In order to analyse the stopping sight distance which is required to design a given highway we have to consider the following factors which, effect the sight distance.

1. The total reaction time of the driver
2. Speed of the vehicle
3. Friction between the Tyre and the pavement surface
4. Break efficiency
5. Gradient of the road 

• The total reaction time of the driver: Total reaction time of a driver is the time from the instance the obstruction is visible to the driver to the instance when he effectively applies the break. 

 In the total reaction time vehicle moves at the speed at which the driver is moving or taken as the design speed. So, if the total reaction time of the driver is more, more will be the distance traveled and more will be the stopping sight distance.

Stopping sight distance can be phased into two parts: 1) Perception time 2) Brake reaction time

Perception time is the time taken by the driver to realize that there is an obstruction on the road.
Brake reaction time is the time taken by the driver to effectively apply the brakes. This depends upon the skills of the driver, friction of the tyre and road surface and the brake efficiency.

PIEVE theory is used to analyse the total reaction time. According to this theory total reaction time of a drive is phased into four classes:

1. Perception time
2. Intellect time
3. Emotion time
4. Volition time

• Speed of the vehicle: Speed of the vehicle affects the distance traveled by the driver in the total reaction time, more the speed more will be the distance.  This is known as lag distance.

Similarly,the distance traveled by the driver after the application of the brakes. More the speed more will the braking distance.

• Friction between the Tyre and the road surface: The friction between the Tyre and the road surface depends upon the type of road surface and the condition of the tyre. 

Also, it depends upon the speed of the vehicle. More the friction, less will the stopping sight distance required but, if less is the friction, more it will be.

• Brake Efficiency: 100% brake efficiency means the rotation of the Tyre is completely locked, but it will surely result in the skidding of the vehicle. Efficiency of the brakes are considered by reducing the original value of the friction in a range of 0.35 to 0.40.

• Gradient of the road: Gradient may be positive or negative and accordingly the required stopping sight distance will be less and more respectively. In case of upward(positive) gradient, a component of the gravity force will help in stopping the vehicle.

In case of the downward gradient the component of the gravity will be in opposite direction to the direction of the friction force so this will increase the stopping sight distance.

Thanks for your visit!

Tri-axial method of flexible pavement design

Hello,

• In this method of pavement design we use the Tri-axial shear test to determine the Elastic modulus of the sub-grade material or those of the other layers. Boussinesq's equation is used to determine the thickness of the pavement.
• In order to include the factors, amount of traffic and rain-fall 'X' and 'Y' are multiplied to the equation which gives us the pavement thickness. More the traffic and amount of rainfall higher will be the values of 'X' and 'Y'.
• A stiffness factor is introduced when the Elastic modulii are different for the sub-grade and pavement layers. This factor (Es/Ep)^(1/3) is multiplied to the former value of thickness in which, the moduli are considered equal.
• In order to find out the thickness for the different layers we simply use the stiffness factor according to which, thickness is inversely proportional to the Elastic modulus raised to a power of 1/3.

Thanks!