A QUESTION FOR ACI 302

[hparissil]

Regarding the subject of the ACI 302, I have the following question. I´ve designed a pavement with 10 cm of thickness for a refinery´s non traffic pavement area. But the owner is not agree with this thickness, he has mentioned to me that the minimum thickness for a pavement is 13 cm according to the ACI 302-80, Reference 2.1.1.1, but this reference does not especified a concrete strenght. In addition to this, I have checked the latest edition of the ACI and I have not found any similar reference like the mentioned on ACI 302-80. So, my question is, It is established a minimum thickness for a pavement, even if the calculation memory said that 10 cm is enough?

 

[Ron]

I would agree with the owner. 10cm is too thin for a pavement, traffic or no traffic. Reasonable constructability has to be considered for slabs on grade. As an example, if you constructed a 10cm pavement section, the undertolerance would allow a thickness of about 9.4cm. Since contractors seem to have tremendous problems keeping a subgrade very flat, it is likely that you will see thicknesses ranging from about 7 to 12 cm, with the AVERAGE volume justifiable by the contractor. THIS WILL PROMOTE "RANDOM" CRACKING due to the inability to control the thickness to tolerances reaching the required control joint depth.

Further, unless you erect barriers to prevent any traffic, and all the concrete is likely at the same level on the top, the drivers don't know which areas are thinner...they just drive where it is easier for them at the time.

For constructability, durability and reasonable control, the 13cm thickness is more appropriate.

 

[Qshake]

While I agree with Ron in siding with the owner I disagree with the logic. If I remember my conversions right, 10cm is roughly 4". And here in the midwest, 4" is our standard concrete sidewalk thickness. Of course one must take into account the footprint of the pavement, i.e., I wouldn't place a 100' by 100' piece of unreinforced pavement.

I am not familar with that particular reference though.

 

[swaalkes]

ACI 330 has recommendations on minimum pavement thickness. Their charts go all the way down to 3.5 inches (= 8.9 cm), depending on the amount of truck traffic the pavement will receive and the subrade strength.

So if you went through your design and came out with 10 cm, I say go ahead with that. I generally say people should use 4 inches minimum pavement thickness (this usually only applies to sidewalks, driveways, and light traffic parking areas - design programs such as AASHTO and PCA will generally give 8-10 inches or more for medium to heavy traffic pavements such as highways, freeways, arterials, etc.).

 

[jheidt2543]

Generally speaking, you can't go too far wrong giving the Owner what he wants, especially if he wants MORE thickness. Sometimes practical experiance means more than engineering calculations (although I still compute it!).

In the midwest of the U.S. typical concrete parking lot thicknesses are 4", 5" and 6". Truck traffic areas are 8". A lot also depends on the sub-grade soil and the base course under the slab.

In an industrial plant for a steel forging operation, the floor slab was designed as 12" thick unreinforced concrete and has held up very well. In the work areas in front of the heat treating and forges, they used a 1/4" thick shake on metal hardener for floor surface protection.

 

I have noticed, in my time dealing with concrete, that spec's period, are ambiguous at best.

To specify a thickness, without regards to mix design is illogical.

I regularly practice placing 4" thick slabs for bi-pedal traffic and would not place less than 5" for anything else.

WWF throughout with 2" coverage and #3 or #4 steel doubled 6" apart around the perimeter, with 5" horizontal coverage from the outside bar to the edge.

Sub-grade must be screed off, just as in placement of the concrete. Along with being consolidated properly.

Concrete is a very reliable hard surface, if common sense, once the basic education of the composition, placement and curing procedures are acknowledged, and has no respectable amount of tensile strength worth mentioning.

Again, to specify that a concrete pavement installation must be a certain thickness, whether it is comprised of pea gravel or 3/4" clean; regardless of the sub-grade or its respective preparation; nor the type and placement of any re-enforcment; etc. is not accurate in the real world.

Spec books on job sites, regarding the concrete aspect, are mostly like architects drawings, in regards to commercial work. A plethora of stock protocol pulled from, in the case of concrete, ACI and ASTM standards for placement, finishing and curing, which are bereft of genuine specifics and rarely enacted.

These spec books are simply a security blanket for the client to feel as if the guidelines are being followed verbatim.

Finally, if the client wants thicker, give them thicker. Concrete is inexpensive and more cannot hurt per se. Although if you are removing the excavated maerial to make room for the thicker concrete and base material, there is this obvious additional cost.

BRIAN