Comparison between flexible pavement and rigid pavement in highway engineering

Flexible Pavement

Rigid Pavement

1. Flexible pavement differ from rigid pavement in terms of load distribution. In flexible pavements load distribution is primarily based on layered system.  2. Structural capacity of flexible pavement depends on the characteristics of every single layer. 3. In flexible pavements, load intensity decreases with the increase in depth. Because of the spreading of loading in each single layer. 4. In flexible pavement deflection basin is very deep, because of its dependency on the underlying layers. 5. Flexible pavement has very low modulus of elasticity (less strength). 6. In flexible pavements, underlying layers play very important role. Therefore, more role are playing only underlying layers.

1. While, in case of rigid pavements most of the load carries by slab itself and slight load goes to the underlying strata. 2. While, the structural capacity of rigid pavements is only dependent on the characteristics of concrete slab. This is so, because of low bearing soil capacity of underlying soil. 3. While, in case of rigid pavement maximum intensity of load carries by concrete slab itself, because of the weak underlying layer. 4. While in case of rigid pavement, deflection basin is shallow, this is because of in dependency of rigid pavement on the underlying layers. 5.While Modulus of elasticity of rigid pavement is very high, because of high strength concrete and more load bearing capacity of the pavement itself. Than compared to flexible pavements. 6. While In case of rigid pavements, slight function of underlying layers. Maximum role is playing by the top layer (that is slab) by itself. Therefore, minute part is taking by sub layers.

What is semi rigid pavement in highway engineering

 ➤ Semi Rigid Pavement: These pavements are constructed with the materials like the pozzolana  concrete, the lime-fly ash-aggregate mix, or lean cement concrete and thus have sufficient flexure strength. They possess the flexural strength in between those of the rigid pavement and that of the flexible pavements. They are designed as the flexible pavements but with some correction factors applied to it, or they are designed by some other methods. These materials don't have the good resistant against the impact and the abrasion loads so the surface course is constructed with the flexible pavement materials.

What is rigid pavement in highway engineering

➤ Rigid Pavements: These kind of pavement possess a high flexural strength and can transfer the loads coming over it to the lower layers with the slab action of the various layers. The reason behind the flexural strength is the kind of material used in its construction. The various materials like plain cement concrete, reinforced cement concrete or the pre-stressed concrete are the good materials to be used as the rigid pavement materials. There is no need to construct the sub-base course or the base course but for the precautionary purpose the base is generally provided. A good base course of course aggregates makes it more stable, and is more economical in the long run. The base course is more important when the sub grade is formed of the finer material. The rigid pavements are analysed and designed by using the elastic theory, in which the surface layer is considered as the elastic layer, which is supported by another elastic plate called the subgrade. The rigid pavement will not deform like the flexible pavements if the undulating surface is present at the base.

What is flexible pavement in highway engineering

➤ Flexible pavements: These are the pavements which have very less flexural strength. They transfer the load coming over them directly to the lower layers through grain to grain contact. So the load transfer largely depends upon the type of the grains/aggregates that are used in the construction of the pavement.  If the aggregates are well compact/ well graded then their grain to grain contact will be good and they can transfer the load to a wider area. The flexible pavement has the following layers:

(a) Wearing course

(b) Sub base course

(c) Base course

(d) Sub grade

The stress intensity is maximum at the top layer of the pavement and it decreases with depth. Due to ability of the pavement to transfer the loads to lower layers in the truncated form, the layer concept has been introduced for its construction. The pavements is made of layers of different materials. The top layer is made of superior material, because it has to face the largest stresses, and the lower layers are constructed with with inferior materials.  So the wearing course is made of the superior quality material and sub-grade can be constructed of the locally available earth too. The materials like bituminous concrete, is best to use as the surface course and the others like water bound macadam, and other crushed aggregates, gravels can be used for the base and sub base courses. The flexible pavements are designed with the help of the empirical design charts or some equations taking into account the various design factors. Some semi-empirical and theory methods are also available for the design of the flexible pavements.

Types of pavement in highway engineering

The pavement is the structure which takes up all the traffic loads coming on the road. According to their behavior they can classified in these major categories :-

 

➤ Flexible pavements: These are the pavements which have very less flexural strength. They transfer the load coming over them directly to the lower layers through grain to grain contact. So the load transfer largely depends upon the type of the grains/aggregates that are used in the construction of the pavement.  If the aggregates are well compact/ well graded then their grain to grain contact will be good and they can transfer the load to a wider area. The flexible pavement has the following layers:

(a) Wearing course

(b) Sub base course

(c) Base course

(d) Sub grade

The stress intensity is maximum at the top layer of the pavement and it decreases with depth. Due to ability of the pavement to transfer the loads to lower layers in the truncated form, the layer concept has been introduced for its construction. The pavements is made of layers of different materials. The top layer is made of superior material, because it has to face the largest stresses, and the lower layers are constructed with with inferior materials.  So the wearing course is made of the superior quality material and sub-grade can be constructed of the locally available earth too. The materials like bituminous concrete, is best to use as the surface course and the others like water bound macadam, and other crushed aggregates, gravels can be used for the base and sub base courses. The flexible pavements are designed with the help of the empirical design charts or some equations taking into account the various design factors. Some semi-empirical and theory methods are also available for the design of the flexible pavements.

➤ Rigid Pavements: These kind of pavement possess a high flexural strength and can transfer the loads coming over it to the lower layers with the slab action of the various layers. The reason behind the flexural strength is the kind of material used in its construction. The various materials like plain cement concrete, reinforced cement concrete or the pre-stressed concrete are the good materials to be used as the rigid pavement materials. There is no need to construct the sub-base course or the base course but for the precautionary purpose the base is generally provided. A good base course of course aggregates makes it more stable, and is more economical in the long run. The base course is more important when the sub grade is formed of the finer material. The rigid pavements are analysed and designed by using the elastic theory, in which the surface layer is considered as the elastic layer, which is supported by another elastic plate called the subgrade. The rigid pavement will not deform like the flexible pavements if the undulating surface is present at the base.

 ➤ Semi Rigid Pavement: These pavements are constructed with the materials like the pozzolana  concrete, the lime-fly ash-aggregate mix, or lean cement concrete and thus have sufficient flexure strength. They possess the flexural strength in between those of the rigid pavement and that of the flexible pavements. They are designed as the flexible pavements but with some correction factors applied to it, or they are designed by some other methods. These materials don't have the good resistant against the impact and the abrasion loads so the surface course is constructed with the flexible pavement materials.

What is Porous Pavements andd design of porous pavements

What is Porous Pavements

Porous pavements, both asphalt and concrete have been around for years.  In most areas they haven’t really caught on.  Now, with the large focus on environmental issues and green building, are they worth looking at again

Pavement design

 
Traditional pavement design
Typically when pavement mixes are designed, they include different sizes of aggregate.  They use a wide range from fine sand to coarse stone.  The largest size depends on the expected use of the material.  Then it is all bound together with binder or cement.  With asphalt pavement that top layer will go on a water proof layer then a base.  Concrete pavement may go on a base or directly on the ground.
This results in an impenetrable surface that blocks rain water from getting into ground water systems and increases runoff.

Porous pavement design

 
Porous pavement or pervious pavement is designed using medium and large sized aggregate without any smaller fines such as sand.  It is then held together using with cement or binder.  The lack of fines in the mix creates relatively large pore space in the pavement.  This large pore space allows water to pass through.
The top layer is placed either directly on the ground or on other porous base layers to allow water to drain completely through the system into the ground.

Pros and Cons

 
Here are some pros and cons as well as a few notes on them.

Pros

 
Increased water quality – Oils, heavy metals and other contaminates on the pavements are not carried downstream and into stormwater drainage systems.  Also, water is filtered as it passes through the pavement.
Lower initial construction costs – Construction costs may be lower because porous pavements lower the amount of stormwater drainage facilities that a site will need.  Fewer and smaller inlets, detention ponds and storm drain pipes means lower construction costs.
Lower long term costs – less maintenance needed for storm drain and filtration systems.
Fewer fees – Storm water impact fees may be lower since porous pavements are proven to reduce runoff.
Less runoff – Less runoff means less potential flooding and lower peak flows.
Increased safety – Since water drains through the pavement there is a lower chance of hydroplaning and an increase in traction.
LEED Points – It can indirectly help gain LEED Points.  It can contribute in the areas of Stormwater Design, Heat Island Effect, Water Efficient Landscaping, Recycled Content, and Regional Materials.  There may be other ways that using it can help LEED certification.

Cons

 
Higher initial construction cost – Yes, I know I listed construction cost as a pro also.  The cost of constructing the pavement itself tends to be higher than regular pavement.
Soil restrictions – The soil below the pavement must drain at least as well as the pavement.
Clogging – The pores in the pavement may clog.  Suppliers and other proponents say that regular cleaning and maintenance will nearly eliminate clogging.
Pavement strength – Porous pavements are structurally weaker than standard pavements.   That generally results in them being used only for low traffic roads and parking lots.  Extra care must be taken when designing a pavement for high traffic or heavy traffic.
New/Untested technology – That’s not entirely accurate.  The technology has been tested since at least 1971.  However, most contractors don’t have experience with it.  Proper training, clear instructions, material testing, and site investigations should be done to ensure that the pavement meets all applicable standards during construction.
Contamination – Pavement surfaces usually have a lot of contaminates on them.  Porous pavements can filter contaminants, but no system is 100%.  Since water drains directly into ground soil it is possible that it will take the contaminants with it.

Conclusions

 
There is a lot of potential for porous pavements in future projects.  Each project would have to be investigated independently to determine any cost or environmental savings that might be gained by using porous pavements.  However, the potential positives do seem to outweigh the potential negatives.  It would certainly be worth your time to investigate it and present your findings to your client.
What are your thoughts on Porous and Pervious Pavements