Thursday, May 15, 2014

Failures in Rigid Pavements


With the reference of the book titled as 'Highway Engineering,'  by S.K.Khanna and C.E.G.Justo, I created this post, which covers the failures in rigid pavements. If you have the book, please consult the book, because that would be a better option. Failures in cement concrete pavements are recognized mainly by the formation of the structural cracking.

Failures occur due to two factors:
(a) Deficiency of the pavement materials.
(b) Structural in-adequacy of the pavement system.

  • Deficiency of the Pavement Materials:
  1. Soft aggregates
  2. Poor workmanship in joint construction
  3. Poor joint filler or sealer material
  4. Poor surface finish
  5. Improper and insufficient curing.
Here are the various defects that creep in due to the causes above:

  1. Dis-integration of the cement concrete
  2. Formation of cracking
  3. Spalling of joints.
  4. Poor riding surface
  5. Slippery surface
  6. Formation of shrinkage cracks
  7. Ingress of surface water and further progressive failures.
  • Structural in-adequacy of Pavement System:
Inadequate sub-grade support, less pavement thickness would be major cause of developing the structural cracking in pavements. Following are the causes and types of failure which develop:

  1. Inadequate pavement thickness
  2. In-adequate sub-grade support and poor sub-grade soil
  3. Incorrect spacing of joints.
Above would give rise to the failures of the following types:

  1. Cracking of the slab corners
  2. Cracking of the pavement longitudinally
  3. Settlement of slabs
  4. Widening of joints
  5. Mud Pumping.
Now here are the typical pavement failures which occur in the rigid pavements and will be discussed here is details:

  1. Scaling of the cement concrete
  2. Shrinkage cracks
  3. Spalling of joints
  4. Warping cracks
  5. Mud Pumping
  6. Structural cracking

  • Scaling of Cement Concrete:

Whenever there is deficiencies in the concrete mix or presence of some chemical impurities may result into the scaling of the cement concrete. Also when excessive vibrations are given to the mix, cement mortar will come on top during the construction and thus with use the cement mortar gets abraded exposing the aggregates of the mix. This makes the pavement surface rough and shabby in appearance.
  • Shrinkage Cracks: During the curing operation of cement concrete pavement immediately after the construction, the shrinkage cracks normally develop. The placement of cracks are in longitudinal as well in transverse direction.
  • Spalling of Joints:
Sometimes when pre-formed filler materials are placed during the casting of pavement slabs, the placement is somehow dislocated and filler is thus placed at an angle. The concreting is completed without noticing this faulty alignment of the filler material. Thus this forms an overhang of a concrete layer on top side and the joint later on shows excessive cracking and subsidence.

  • Warping Cracks:
If the joints are not well designed to accommodate the warping of slabs at edges, this results in development of excessive stresses due to warping and the slab develops cracking at the edges in an irregular pattern. Hinge joints are generally provided for relieving the slabs of warping stresses. There is no structural defect due to warping cracks if proper reinforcement is provided at the longitudinal and transverse joints as it takes care of the structural in-adequacy.



Tuesday, May 13, 2014

Traffic Capacity Studies (Highway Engineering)

Hi there,
How you doing?
As we know, traffic volume is the numbers of vehicles that pass a point on a highway on a particular lane on particular direction in unit time, generally in per unit hour, there is a term which represents the capacity of a road to accommodate the traffic volume.

Traffic capacity is expressed as the maximum number of vehicle in a lane or a road that can pass a given point in unit time, usually an hour, i.e., vehicles per hour per lane or roadway.

Traffic capacity and traffic volume has same units, difference between the two is that traffic volume represents the actual rate of flow of the traffic and responds to the variation in the traffic demand, while capacity indicates a capability or maximum rate of flow with a certain level of service characteristics that can be carried by the road.

Traffic capacity of a roadway depends upon a number of prevailing roadway and traffic conditions.

  • Basic capacity is the maximum number of vehicles(PCU) that can pass a given point on a lane or roadway during one hour under the most nearly ideal roadway and traffic conditions which can possibly be attained. Two roads have same physical features will have same basic capacities irrespective of the traffic conditions.

  • Possible Capacity is the maximum number of vehicles which can pass a given point on a lane or highway during one hour under the prevailing roadway and traffic conditions. 
  • This means that the possible capacity of a highway will always be lower than the basic capacity unless the prevailing conditions of the traffic, approach the ideal conditions. Therefore the possible capacity may vary from 0 to the maximum, i.e., Basic capacity.
  • Practical Capacity is the maximum number of vehicle that can pass a given point on a lane or roadway during one hour, without traffic density being so great as to cause unreasonable delay, hazard or restriction to the driver's freedom to man-oeuvre under the prevailing roadway and traffic conditions.According to the book "Highway Engineering" by S.K.Khanna;  
 It(Practical Capacity) is the practical capacity which is of primary interest to the designers who strive to provide adequate highway facilities and hence this is also called the design capacity.
Center to Center spacing of Vehicle (Traffic Capacity Studies)

  • Determination of Theoretical Maximum Capacity
                        Using the relation:      C = 1000.V/S 
one can easily determine the theoretical Maximum Capacity; Here,
                      C = Capacity of a single lane, vehicle per hour
                      V = Speed, kmph
                     S = Average center to center spacing of vehicles, when they follow one behind the other as a queue or space headway, m.
Thus capacity depends upon the Speed and Spacing. Spacing is governed by the safe stopping distance required be the rear vehicle in case the vehicle ahead stops suddenly.

Numerically spacing is given by,
                         S = Sg + L
Where Sg is the space gap(Head to rear) between the vehicles and L is the average length of the vehicle, both combined makes the center to center spacing of the vehicles.
   Here,                     Sg = 0.278 V.t  , where V is in Kmph and Sg in m.
                          t is the total reaction time of the driver, generally assumed to be equal to 0.70 to 0.75 sec.
Assume t = 0.70;
  S = (0.7v + L) = (0.2V + L), m
Thus knowing the design speed, the spacing S can be found and thus the theoretical capacity of the lane can be found.

Thanks for you kind visit


Thursday, May 8, 2014

Failures of Flexible Pavements

Hi there,

Failures of flexible pavements can occur due to the failure of one of the four layers. Failure of Sub-grade will result in the failure of the pavement, because it will get reflected in the top surface. 

Similarly failure of the sub-base or base layer will also result in the failure of the pavement. There are various causes of the failure of the flexible pavement, I will post another article for this. 

Here we will talk about the general failures that occur in the flexible pavements.

  1. Alligator(Map) cracking
  2. Shear Failure
  3. Frost Heaving
  4. Longitudinal cracking
  5. Consolidation failure
  6. Wearing of the Surface
  7. Reflection cracks

  • Alligator Cracking: 
Alligator or map cracking occurs on the surface of the flexible pavement due to the relative movement of the material or failure of the materials of the pavement layers. This may be caused by the repeated application of the heavy wheel loads resulting in fatigue failure due to the moisture variations resulting in swelling and shrinkage of sub-grade and other pavement materials. A localized weakness of the under-lying base course would also cause a cracking of the surface course in this pattern.
  • Shear Failure: 
Shear failure of the flexible pavement occurs due to the weakness of the pavement mixtures, the shearing resistance being low due to in-adequate stability of excessive heavy loading. The shear failure causes upheaval of pavement material by forming a fracture or cracking. 
  • Frost Heaving: 
Areas having cold climates, are prone to frost heaving. When the water present in the pores of the layers turns into ice, it causes swelling of the ice, and therefore results in the upheaval of the area affected by the frost. 

It must be remembered that the difference between the frost heaving and the shear failure is that in shear failure the depression is followed by the upheaval of the adjacent area but in case of the frost heaving, there is no depression.
  • Longitudinal Cracking: 
Longitudinal cracking may occur due the differential settlement of the pavement due to the differential volume change. The area of the pavement near to the pavement edge is more prone to moisture and therefore it may swell more as compare to the interior region of the pavement sub-grade. 
This will cause a differential volume change of the pavement and therefore may lead to the longitudinal cracking of the pavement. Generally, these longitudinal cracks traverse through the full pavement thickness.
  • Consolidation Failure: 
Consolidation of the subgrade due to the continuous action of the wheel load along the wheel path results in the formation of the ruts along the wheel path. 
  • Wearing of the Surface: 
Generally wearing of the surface may be caused due to the use of inferior material or due to the lack of the inter-locking of the surface layer with the bottom layers. 
Lack of the interlocking may be a result of the non-use of the prime and tack coat. Specifically in case of the overlays over the existing cement concrete pavements or the soil cement roads have poor inter-locking.
  • Reflection cracks: 
Reflection cracks are formed in the overlays laid over the existing cement concrete pavements. In such overlays, if any cracks are there in the existing cement concrete pavements, will get reflected in the surface layer also. These cracks are known as the reflection cracks.

Thanks for your kind visit!

Tuesday, May 6, 2014

Accident Studies- Causes and Prevention (Highway Engg.)

Hi there,
In the syllabus for Highway Engineering(HPU) , this topic is under the section C of Traffic Studies. I have discussed the parking studies in my previous article and in this post I am going to post about the Causes and Prevention of the accidents.

  • Causes

Under the mixed traffic conditions and different types of man-oeuvres avoiding the accidents to 100% is not possible, but by studying the various causes and characteristics of the accidents we can provide some preventive measures or improvements to reduce the rates of the accidents.
In any accident there are four elements which take part in it:

  1. Road User
  2. Vehicle
  3. Road
  4. Environment.
Road user may be a driver or a pedestrian. Generally driver becomes the cause of the accidents when he is in drunken state, tired, sleepy or in some temporary state of anger or grief. Pedestrians becomes the cause of accident when they are careless while crossing the roads.
Vehicle may becomes the cause of accident when its dynamic characteristics such as braking, accelerator and steering are not working properly. A sudden tyre burst may also become a cause of accident.
Various Road geometric characteristics such as in-sufficient sight distance, excess gradient, less width and surface characteristics such as less friction or presence of pot holes or another failures may lead to accidents.
Environment factors also play sufficient role in the accidents. Heavy rain fall, presence of fog and very high temperature are some of the factors which may result in the accidents by causing in-convenience to the drivers.

  • Prevention:
Prevention of the accidents can be done with the help of 3 E's:

  1. Engineering
  2. Enforcement
  3. Education
By using the various engineering measures to construct the best road for the required traffic performance, it is possible to prevent the accidents to some extent.
Enforcement of the motor vehicle act to each and ever traffic may reduce the accidents. Various traffic laws must be enforced with proper guidelines.
Educating the road users about the traffic laws and the safe practices can play a big role in effective traffic performance. This can be done with the help of various medias such as TV and radio and also traffic safety weeks may be organised to educate the traffic.

This was a brief introduction to the Causes and prevention of the Accidents.
Thanks for your kind visit!

Friday, May 2, 2014

Parking Studies (Highway Engineering)

After teaching this topic of highway engineering to 1st batch of Civil Engineering at JNGEC Sunder Nagar, I thought to write a post about it.
Certainly, I have not lived in a big city such as Chandigarh,Mumbai or Delhi and more than that I have spent first 10 years at a mountain village of Himachal Pradesh and next years at its nearby district towns.
I love short distance travels and I feel good when I travel. I have not taken more than 2 days train journey but, that doesn't make me less curious to know more about Highway Engineering. Completing a four years degree in Civil Engineering was a totally different thing than to teach it.

In order to teach it I have to have a deep understanding of the subject matter. Now, let's come to the topic at the instant. Parking Studies are done to find out the present capacity and the demand of the parking in the area under study and thereby to suggest any kind of improvement to it.
I have seen that parking studies may not be important in the places where I live at because parking demands are less, except at few cities. At the places such as Shimla, even if we have parking demand in peak seasons, not much can be done to fulfill it.

At the metropolitan cities and at industrial, commercial and residential places where parking demand is high, it becomes very necessary to perform the parking studies.
According to S.K.Khanna and C.E.G. Justo, in their book "Highway Engineering", parking studies can be phased into three phases or parts:

  1. Parking Demand
  2. Parking characteristics 
  3. Parking Space Inventory
  • Parking demand can be found using few methods such as cordoning of the area, noting down the number of the parked vehicle or by interview method.  In the first method of cordoning the area, the area under study is cordoned at all he entrances at the exits, and difference between the numbers of vehicles entering and the outgoing will give us the parking demand of the area. 
In the second method, one has to count the number of the vehicles which are parked in the area under study. If we note down the number plate of each vehicle, then we can also note down the total time of parking of each vehicle. 
In the interview method, one can find out the parking demand by interviewing the drivers of the parked vehicles. This method is specifically more useful when the parking demand in the study area is higher than the parking space available.
  • Parking Characteristics: This study is useful to find out the characteristics of the parking practices of the area. In the kerb parking, it is necessary to study the parking pattern, interference to smooth flow of traffic and the accidents involved during parking and un-parking operations.
  • Parking space inventory: Area under study is fully surveyed and a map is prepared showing all places where kerb parking and off-street parking facilities can be provided to meet the parking demands. The traffic engineer has to strike a balance between the capacity and parking demands and to design proper facilities for parking. 
At last A special thanks to the book I mentioned above (Highway Engineering by S.K.Khanna and C.E.G. Justo )
and thanks for your kind visit!

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