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Municipal Pavement Design

Municipal Pavement Design

(OP)
I recently ran across a City street standard requiring 5000 psi concrete strength in 28 days.  My recollection of the AASHTO Road Studies indicated that the purpose of pavement is to shed water and transfer wheel loads to the subgrade.  I seems to recall that there isn't much difference in load transfer between 3000 psi pavement and 5000 psi pavement for a given depth of pavement.  Wouldn't this City be better off requiring additional depth of concrete rather than the high strength?  Or better yet, putting the additional cost of cement in the improvement of the sub-grade?

RE: Municipal Pavement Design

The use of 5000 psi concrete is not necessary to produce a good pavement, though my guess is they are doing it more so for durability and overkill than any other reasons.  3000 psi is too low for good concrete pavement, primarily because of the reduced durability in the exposed conditions of a pavement.  

The function of a concrete pavement is not simply to tranfer load to the subgrade, but to mitigate that load as well.

As with many other applications of engineering, it is better to specifically tailor the pavement to the needs of the roadway, not meet some nebulous, generalized criteria.

RE: Municipal Pavement Design

(OP)
Here in the south, our pavement structures are not subject to the durability issues common in the northern climes.  The 3000 psi figure is the most common standard.  Surface deterioration is rarely a problem.  Load cracking is all too common, however, especially with sheetrock trucks, brick trucks, and concrete transit mixers.  Garbage trucks are getting heavier all the time. Pavement thickness and standards in new sub-divisions are dictated all too often by politics rather than by engineering design.  My impression is than pavement depth mitigates sub-grade loads more than psi rating.

RE: Municipal Pavement Design

Higher strength concrete will allow you to have reduced thickness.  PCC pavement fails in flexural mode (tension), so many times it will be flexural strength that is specified as opposed to compressive.  In any event, the flexural strength for 5000 psi concrete will be higher than that for 3000 psi concrete.  In PCC pavement design, both strength and thickness are important.

RE: Municipal Pavement Design

(OP)
Thanks for your input.  My experience is that sub-grade is what fails and that the pavement merely reflects that failure.  I agree that 5000psi concrete will bridge better than 3000 psi, but if the sub-grade stress can be be reduced by additional depth, the concrete pavement won't have to bridge a failed sub-grade.

RE: Municipal Pavement Design

Jim6758...subgrade is indeed a key, but a plain sand subgrade, well compacted, works fine for a concrete pavement.  The subgrade response is one influencing factor on deciding the required thickness. Pavements work like beams, bending in all directions from the point load of wheels.  They are subject to fatigue and the performance is dependent upon initial strength, concrete mix properties, and durability.

Even though you are in the south, durability is still an issue. (I practice in the south, as well).  Not necessarily freeze-thaw durability, but other durability such as paste integrity, abrasion resistance, shrinkage, etc.

For all exterior pavements and interior pavements subject to forklift traffic, a minimum 4000 psi concrete should be used.  The aggregate should be as large as practicable and proper jointing should be done to reduce shrinkage crack potential.  Thickness tolerances should also be controlled by careful attention to subgrade flatness and finish of the concrete.

RE: Municipal Pavement Design

Standard pavement concrete will usually be in the 550-650 psi flexural range - most city, county, & state specifications require concrete in this range.  This translates to about 3700-5200 psi compressive [flex = 8to10 * sqrt(compressive)].  So for example if you order standard pavement concrete from your ready mix supplier, chances are you'll be getting at least 600 psi flexural at 28 days.

RE: Municipal Pavement Design

To you all,

Don't forget that what ever strenght or thickness you use, municipal pavement will always endure threnches to repair the underground infrastructures. This is true for all type of municipal pavement, and considering the life span of concrete this even more true. The main problem with concrete pavement is that the backfilling and repairing procedure is very expensive and many municipalities are cutting costs, so the infrastructure maintenance suffers. So, always consider that when designing; Extra cost at the beginning will not pay off if not maintain and repair properly during all of the life cycle.

Neville Parker, CCNY-CUNY, is very knowledgeable on the matter.

Please be tolerant with my english writing.

I work for the Center for expertise and Research on Infrastructures in urban areas.

www.ceriu.qc.ca

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