Base Course for Highway Pavement 1

[jenofstructures]

Hello everyone

I am designing a certain structure that needs highway design in front of it
i have no experience on design of pavement

I already found a good reference on rigid pavement design
but our client also want to see a calculation for the base course

my question is, how can I design a base course under a pavement? Is it like deflection-way of computing the slab?
can you give me good reference on this one?

thank you in advance and have a happy new year!

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[dik]

Is the pavement rigid (concrete) or flexible (asphalt)? What is the intended loading and frequency of loading?

AASHTO has a couple of spreadsheets that can aid with the design.

Dik

 

[jenofstructures]

the pavement is rigid pavement

what do you mean about frequency?

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[Ron]

A rigid pavement generally has no base course...and when it does, it is called a subbase. You are essentially designing a concrete slab on grade for various point loads, keeping the bending stress at the bottom of the unreinforced section to something below about 50 percent of its modulus of rupture. If you keep the stress level below 50 percent, the frequency (number of load repetitions) is irrelevant.

There are various approaches to rigid pavement design. In the US, the two most popular are the AASHTO method and the PCA method. Both will yield acceptable results.

 

[a2mfk]

I think I'd give the same advice that we'd give if someone asked a structural engineering question but wasn't a structural engineer- hire a competent, licensed civil engineer with roadway surface design. You did say highway and not just a small concrete parking lot, which a structural engineer may get involved with.

Why would this fall into your lap? Its outside the scope of any structural engineering project, unless you work at a full service office. In which case, your civil engineer should be doing this...

Even a little parking lot in most jurisdictions has very specific drainage, runoff, landscaping, curb, etc. etc. design standards they just adhere to, and the agencies you will deal with are usually very strict especially with water runoff and drainage design. In some areas, you may have to coordinate your design with several different government agencies. I have worked in an office with civil engineers and its not always the most technical, but knowing how to weave through the process is very complicated.

 

[Ron]

a2mfk...while I agree with you that Civil Engineers generally handle the drainage aspects of a pavement, they are often ill equipped to handle the structural design of a pavement. Yes, it can be done "cookbook" fashion, but most of them do not go through any analytical approach at all to pavement section design...they will generally pick the last one that was done off the shelf and repeat it.

I commonly do pavement structural analysis and design as well as other structural functions.

Matt Witczak (co-author of Yoder and Witczak, "Principles of Pavement Design") once told me that pavement section analysis is just another form of structural engineering.

 

[FixedEarth]

Everything is ultimately carried by the subgrade. So why not get a soils firm and a pavement engineer to colloborate on this design? From their respective reports you can pick up some of pavement design fundamentals.

 

[Jalthi]

It is worth to note that the roadway design is not necessarily applicable to parking lot design. Parking lots are generally “designed” based on prior experience. Pavement sections for roadways are, on the other hand, designed based on ESALs.

However, if you choose to go with AASHTO, the design procedure does not require sub-base for rigid pavements. Sub-base is required:
- If the soil conditions are so weak that that heavy construction machineries cannot work on it.
- If the soil conditions vary so much that the pressure on the pavement slab is quite non-uniform.

A minimum thickness of 5 inch is also recommended.

 

[a2mfk]

Ron- I gothcya, its not common in Florida to do concrete parking lots or roadways other than residential driveways. I did do a parking lot and limited roadway design of a concrete pavement for a warehouse project many years ago, something about subgrade modulus and wheel point loads

But the civil engineer in our office laid it all out with slopes, elevations, and all of the drainage design. I think I just specified the thickness and concrete type, some subgrade notes, and laid out the joints...

 

[Ron]

a2...agree. We don't usually see much concrete pavement except for heavy commercial, industrial or some roadways....mostly asphalt on limerock base.

 

[hokie66]

For you Florida guys, what's the difference between limerock and limestone? Limerock seems to be a Florida specific term.

 

[a2mfk]

Ron- not sure what part of the state you are in but they are doing a large section of reinforced concrete roadway on I-95 in the Cocoa-Melbourne corridor. I just read it cost $70M more than asphalt, have not read why they chose it over asphalt (ie soil conditions or just a long term investment).

Also in October I was up in Wisconsin and saw that they are doing reinforced slab with asphalt on top. I assume this gives them a very stable base in the concrete and then they do asphalt as a wear surface that can be milled and repaved every so often? I imagine that in that type of climate with the frost heave, snow clearing equipment and chemicals, etc. that they have tried just about everything. Sounds like a big up front investment that would pay dividends in the long run, which is smart thinking for a DOT.

 

[jenofstructures]

thanks guys for the advice

I am doing an assumed thickness for the design details. I believe our senior engineer who specialized in this field will check on it

But of course if you are assuming, it must be nearer to reality
I used the graph of Elwyn Seelye book on the pavement thickness design. UFC just gave a little information about the subbase for rigid pavement design and it is all about particle sizing (sieving analysis), but I found a graph in AASHTO 1993

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[Ron]

hokie66...our "limestone" is so crappy, it's called limerock. Not even close to the limestones that you would find in Indiana, Ohio, Canada, etc. It has some strata that are very strong and good, but generally its "ratty" rock. When crushed, it breaks into a variety of particles from nice aggregate to calcareous silt. Some areas make good concrete aggregate.

 

[a2mfk]

Ron- don't you think that is more a red neck thing to call it lime rock? I work with geologists who would cringe at that term, its limestone. And I think they would prefer that you say our limestone is just younger than the northern limestone, not that its crappy!

Makes me think of cement vs concrete or masonary vs masonry. And, "What's CMU mean" (asked that one today)?

I skim read a FDOT research paper on compressive strength of limestone and it basically said that if graded right, most Florida limestone would be fine for use in concrete because its comp strength will be much larger than most standard concrete mixes, ie, its not the controlling factor in a concrete mix.

And its used almost exclusively for road bed material under flexible pavement.

And Florida is number one exporter of phosphate (any Borat fans?), all other states have inferior phosphate.

 

[hokie66]

So I can just call it "cracker rock"?

 

[Ron]

hokie66...you might be on to something!! Cracker Rock...I like it.

a2...that's probably more accurate than anything else, considering the history of our illustrious DOT.

Actually, much of the limestone in Florida is younger and will never be subjected to the metamorphic transition that many of the more dense limestones are starting (in geologic time though, so won't happen tomorrow).

The limestone in Florida is the most widely used concrete aggregate in the state and is actually a good aggregate when the source is controlled. The Brooksville area has "harder" rock, but it is interspersed with dolomite and also has some reactive seams of chalcedony chert. Much of the deeper rock is used for concrete aggregate and the near surface rock is used for base material. The Pennsuco area produces very good concrete aggregate with no reactive seams, but is actually a slightly weaker rock (more oolitic formation). Both will consistently produced concretes in the 8ksi and higher range.

Interesting that the most prodigious phosphate areas are almost side-by-side (again, geologicly speaking) with good aggregate areas (just south of the Brooksville area and just north of the Ocala formation).

Also...a2...I'm in the northeastern part of the state, but from Central Florida.

 

[a2mfk]

Ron we must be neighbors, I currently live in Gainesville...

 

[Ron]

a2mfk...pretty close. I'm in Jax.