Calculation of Bearing capacity of Pavement

[enlightenme]

Dear Freinds,

I am a mechanical engg. (pipeline) have been asked to calculate the safe loads or maximum bearing loads (for different vehicle axle loads) on the pavements (Rigid pavings and Flexible Pavings) inside the process areas of a refinery. The pavements has been classified into light duty (100mm Thk concrete) & heavy duty pavings (150mm Thk.) The concrete strength is 30.0 MPa. I am intending to classify this into 2 groups ie. rigid pavements (light duty & heavy Duty) & Flexible pavements(Roads and access roads). Can anyone please advice me on how to proceed or the formula to be used for the calculations? Awaiting valuable advices and suggestions

Enlightenme

 

[Ron]

It isn't based on a single formulaic solution. Numerous factors should be considered. I would not recommend a 100mm pavement for any application....whether it is considered "light duty" or otherwise. Your "heavy duty" pavement section would typically suffice for "light duty" applications, but would be insufficient for heavier axle loads such as deliveries of piping, equipment, etc.

Rigid pavements and flexible pavements are approached differently in their designs. For rigid pavements, the primary criterion is the tension in the concrete at the bottom of the pavement surface section (from flexure). For flexible pavements, the primary criteria are the tensile strain at the bottom of the asphalt layer AND the vertical strain at the top of the subgrade.

Not sure where you are located, but in the US, the authoritative pavement design resources are AASHTO and the PCA (for rigid pavements)...and to some degree, the Asphalt Institute for flexible pavement. Several quality texts are available...Yoder and Witczak, Corp of Engineers, etc.

 

[oldestguy]

Hello enlightenme:

Ron has some good advice. I understand there are some British standards that also might be looked at.

One point to remember with what you are trying to do, is that most of the pavement design situations relate axle applications per unit time, along with the weight of that axle in the system. But, these are rolling loads, not standing loads as you probably have. Standing and slow turning loads are much worse than rolling loads.

Another load that those design systems to do account for are punching loads, as one would have from the dollies of long trailers not attached to the "tractors".

I think for the concrete pavements, I would refer also to concrete design methods, slab on grade, with point loads, and a supporting medium that is elastic. In that case you need data on what is under these pavements. Actually you need that for any pavement design. A local geotechnical engineer might be a good contact for you in that regard for proper subgrade classification.

 

[enlightenme]

Hello,
This is a refinery area and the main traffic here will be cranes( ranging from 60 to 200 Tonne, maintanence vehicles, forklifts etc doin works inside the process areas). The details of the pavement I am attaching below.

According to their refinery Civil specification on the lightduty paving they have limited the axial load to 10kN.( Light Movable Equipment)
and for the Heavy duty paving is limited for HS20-44 of AASHTO or equivalent vehicle.

Am i to somehow stick with their refinery spec. Guys can anyone pls tell what method should i choose? Can anyone suggest a sample calculation or something. Or where to start? And pls see the attachments too!

 

[Ron]

Here are the steps you need to follow:

1. The 10kN axle load is very light and almost all the equipment you have listed will exceed that.

2. Since you are designing for an industrial pavement application, you have to take the linear highway pavement approach to a higher level, accommodating point loads, moving and turning, potentially high loads at or near pavement edges, joints and joint load transfer.

3. 30mPa will likely suffer durability issues. I would suggest using 40mPa concrete for durability, using the largest coarse aggregate practicable for the placement.

4. Constructability is a considerable issue. With a 100mm concrete section, the chances of getting 100mm are quite low. Increasing the thickness will allow a bit of "tolerance" for the construction...otherwise require full time quality assurance/quality control and all thicknesses are minimum with no undertolerance.

5. Use the guidelines in AASHTO and PCA to develop a thickness requirement, considering that each of these methods depends on accurate prediction of traffic frequency and loading, both of which you will not have with any reasonable certainty.

6. Analyze the pavement section using elastic layer analysis. Check the lateral stresses at the bottom of the concrete layer, and if they exceed 50% of the design flexural strength (not compressive strength as specified), then increase the thickness until the result is achieved.

7. Good luck.

 

[enlightenme]

Dear Ron,

The problem is that this is an existing facility. The scope is to determine the maximum allowed axial load on the flexible and rigid pavements. Even though we can suggest them modifications to existing facilities, the basic aim of this project is to determine maximum allowable axial load on the pavement types.

Regards

Enlightenme

 

[Ron]

enlightenme....in that case, I would get an accurate determination of the thickness of the pavements, then do an elastic layer analysis of the sections to determine probable capacity.

If you want it more accurate, then I would suggest falling weight deflectometer testing, or at the least, Benkelman beam deflection testing. Depending on where you are located, those devices might not be available, so the theoretical capacity will be your better approach.

For limiting criteria and development of capacity estimates, I would suggest that you consult Yoder and Witczak, particularly the section on elastic layer analysis.

The Washington State Department of Transportation has free downloadable software for elastic layer analysis. See EverSTRESS.

 

[BigH]

Benkelman beam on the flexible pavements - not heard of it used on rigid (?)

 

[cvg]

recommend that you obtain some samples and test them for flexural strength

http://www.pavement.com/Concrete_Pavement/Technical/FATQ/Construction/Strength_Tests.asp

 

[Ron]

BigH...can be used for either...more commonly used for flexible as you note.

 

[enlightenme]

Dear all

Thank you guys for helping me out, now i ve got a direction and thank you all for sharing it with me, And for your info have you seen the drawing i have uploaded among this post. I think that will give you a more clear cut idea abt the pavement details and its thickness. Please feel free to refer that drawings too. I have also fumbled upon certain documents. So hopefully i can solve this problem soon.


regards

enlightenme

 

[Ron]

enlightenme...I had previously looked at the drawing. It doesn't change any of my comments. I'm not sure why there is a polyethylene sheet beneath the slab for an exterior slab.

 

[hotgas]

When I took a pavement engineering class 30 years ago, the professor noted that if we didn't do the design correctly, it did not have very far to fall.

 

[enlightenme]

Dear Ron,

As far I know the polyethelene sheet is provided inorder to provide a vapour barrier.

Rgrds

Enlightenme

 

[Ron]

Enlightenme....I know the use of the polyethylene, I'm just questioning why you need it for an exterior slab.

 

[TerryScan]

This sure sounds like the appropriate case to hire a geotechnical engineer to analyze and evaluate the existing paving system.

 

[enlightenme]

Dear All,

Thank you very much for sharing your ideas, and it certainly has enlightened me. I am extending my sincere gratitude and thanks to all.

We have finally arrived in a satisfactory solution. I am sure you guys have heard abt Westergaards equation. We reversed the calculation to find the maximum wheel load for particular wheel contact area. Since this is an already existing structure, all the input data was available n we were able to find the maximum bearing load of the concrete pavement. And the clients found it satisfactory.

But friends I have a question. There is a layer of welded wire fabric(WWF) of 150*150*6 mm in between the pavement of 100 mm thick and 2 layer of WWF in 150 mm thick pavement. Will this add to the tensile strength of the concrete? If so how much increase in percentage of tensile strenght of concrete will happen? Will there be any significant increase?


Regards & gratitude to all

Enlightenme