Fire Station Pavement 3

[bank]

Does anyone have any tips for designing concrete pavement for a fire station driveway? I've been told to design for an 80 kip truck with 70% of the load on the rear axle. The City wants a structural slab with two mats of rebar. I'm having problems finding information on designing that type of slab.

 

[castiglian]

HI.. you cna design your slkab in one direction using the ACI method 3 is very simple

 

[KRSServices]

Just a question of my own, being that I own a very heavy truck, why would the specification read 70% of the rear axle load? The maximum rear axle load is either prescribed by the tires or axle rating. Fire truck chassis design primarily utilize the full weight of the unit and the tires/axles are specified accordingly. Usually, these two numbers are quite close, so if the design of the support structure is for 70% of the axle load, are you not under designing the slab? The trucks are always fully loaded and ready to go. KRS Services

http://www.krs-services.com

 

[CarlB]

If the truck can enter the building, see what slab was used there - the outside driveway should be similar. I've seen a few slabs designed for heavy trucks, and usually 6" concrete and a single mat is sufficient. Concrete thickness depends on your subgrade; if it is poor then possibly more than 6" may be needed. Thickest concrete I've seen for heavy aircraft loads is 15", but still a single mat was used. The Military design methodology used doesn't rely on the steel for strength.

I understood that 70% of the total truck load is transferred to the rear axle, not that you would design for less that the rated axle load.

Try

http://www.pcase.com

for Government freeware for rigid pavement design.

HTH,
Carl

 

[dirtsqueezer]

Just a note about subgrade if you're also writing the specs. I recently inspected a slab for a fire sation in Seattle with some lowest-bidder contractors. I accepted their rebar placement satisfactorily and concrete was completely in spec, but experienced extensive and some pretty ugly cracking afterward. The contractor's subgrade, though firm, was highly irregular. Even though they achieved minimum clearances, somehow that difference in mass caused problems. Luckly, there was a section in the specs that grade was to be even and flat, and the contractor ended up paying for the epoxy injection repair. I knew the dangers of soft subgrade, but wasn't aware that irregular grade could cause cracking under high loads. Just a note, if you hadn't heard that before. before.
Best of luck.

 

[KRSServices]

Assuming the GVWR (gross vehicle weight) is 80 kip or 80,000 lbs, that means there are multiple axles (tri-axle, dual front) on that unit because that is one heavy rating. Most tractor units have a rating of 80,000 lbs on the trailors, but that is carried over 8 axles (or 16 tires). Most tandem axles rate from 40,000 to 46,000 and tires will be the limiting factor (each rear axle is between 20,000 to 23,000). In your structural design, your loading is either going to be the summation of the rear axles (assumed heaviest loading). If the load is 70% transferred, then the axle rating is about 56,000 lbs. That's heavy for a tandem group and I doubt it is a correct number. I suggest to check with the Department and get them to look at rating (usually a stamp on the drivers door) to get the rear axle rating. I can only come up with about a max of about 64,000 lbs (46,000 rears + 18,000 front). Unless this unit has three rear axles, or dual fronts.

Just a little tidbit on loading parameters. KRS Services

http://www.krs-services.com

 

[djeep]

You can obtain a design procedure for concrete pavements from the Portland Cement Association (PCA). I think you can download a pdf from their website for free. This procedure has been used succesfully to design high load high traffic highways.

Regardless of the design procedure adopted pay careful attention to pavement structure drainage and differential support conditions (as noted by dirtsqueezer above). I find the root of most pavement distress problems for flexible and rigid pavements alike stem from either(or a combination) of these factors.

Dont skimp on granular material (it is reducing the applied stress in the subgrade)and filter fabric if necessary . Regardless of what a computer program or design algorythm indicates I like to see a total structural pavement thickness of not less than 24 inches for heavy trucks.

 

[dicksewerrat]

We put a few driveways for Fire Stations in Minnesota. They wanted 8" 4,000 psi concrete, #6 bars 12"o.c. each way, each face 2" cover on bars for the slab. Base was 6" crushed concrete, 3/4" minus compacted 95% proctor. Most municipal owners want the best, strongest that money can buy for Fire stations. And they only want to pay once every 100 years. And of course they want it done in 48 hours.

 

[stadry]

the truck weighs the most when parked,,, rolling loads weigh less - newark airport's o'nite parking ramp was 18" 4,000 psi @ 28d - load transfer dowels transverse & longitudinal as i recall,,, seems to me 10"'ll work fine,,, recyc conc for base over fabric - either keyed or doweled joints w/bars 1/2way vertically,,, squeezer's right w/his post, too - best

 

[GeoGuy]

I am a firm believer that the subgrade and base are more important for slab performance than the slab itself. No slab will last without a good and even support. The key to good slab performance is ensuring the base is uniformly stiff at the time of placement. Measure, measure, measure in-place with suitable instruments such as GeoGauge and DCPs. Density or Proctors does not cut it as indicators of uniform stiffness.

 

[Focht3]

DCPs are pretty useless in my book - an inappropriate technique for evaluating pavement support. The GeoGauge is too new and yet an unproven predictor of long-term performance.

Uniformity prior to construction is all well and good, but what assurance is there that it will stay that way? And you claim that, "No slab will last without a good and even support." I know of one heavily travelled artery in Houston - Westheimer Road - that was rebuilt in about 1984. As of six weeks ago, it remains in great shape. The soaked CBR values for the expansive clays beneath the road are quite low, with some as low as 1 or 2. But the double matted steel has provided good reinforcement for the 10 inch thick pavement.

A heavily reinforced slab can reduce some of the problems associated with a poor subgrade.