Heavy Machinery loads on Slab on Grade design 6

[jbuening]

Does anyone have any insight or tips on a slab on grade design for heavy mining trucks? For example, a mining truck manufacturer needs a parking lot designed for their finished heavy machinery and one of their trucks has a single wheel load of 62k but is spread in 13.2 ft^2 contact patch (large tires). That results in about 4.7 ksf pressure. I have other ones as well as a tandem trailer with 150k load to analyze. That company typically uses 12" thick slab with #5 bars in each direction and on 6" of agg subbase, but I'd like to determine if this section will work.

I have the ACI Slabs on Grade manual but it doesn't address that kind of contact area size. Those are designed for forklift or standard truck traffic it seems. I also have the Army Manual TM 5-809-12 and it does a little better, but they base the loadings on Design Index, with 10 being the highest (120k track laying vehicles). Nothing really seems to fit the bill with the Army Manual, but a DI of 10, Subgrade modulus of 150psi, flexural strength of 530psi results in a slab thickness of about 8.5". I don't exactly have a track vehicle though.

Slabs on Grade have always seemed like black magic to me, as there are tons of charts in the ACI/Army publications but no real equations to show how those numbers were determined. When you are outside the chart limits, no direction is provided. Thoughts?

 

[ishvaaag]

"... a study of the concrete pavements constructed from 1922 to 1931, inclusive, was made. The data collected were analyzed and the committee submitted its findings and recommendations to the Chief Highway Engineer on February 1,1932. The investigation revealed that during the period mentioned, 4,633 miles of pavement had been constructed without expansion joints, that 3,940 joints had been cut in these pavements, and that up to the end of 1931, 3,400 blowups had occurred on the 4,633 miles. The study showed that the rate at which blowups occurred increased with the age of the pavement; for example, pavements built in 1922 averaged two blowups per mile in 1931. During the same period (1922-1931, inclusive), 3,417 miles of pavement were constructed with 4-in. open joints, of which only 269 miles, all in one district, had been constructed prior to 1928. In the pavements built prior to 1928, the ends of the slabs adjacent to the joints were not strengthened by edge thickening, a procedure which was followed in the pavements built during and after that year. The data assembled by the committee showed that the 4-in. joints closed at the rate of approximately one inch per year, and that blowups could be expected about the fifth year after construction, although scattered blowups might occur earlier. This conclusion was based not only on measurements of the width of the joints in pavements of various ages, but also on data as to the occurrence of blowups and the necessity of widening existing joints and cutting new joints to keep blowups to a minimum. That blowups may occur early in the life of a pavement, in spite of 4-in. joints, was demonstrated by the fact that five blowups occurred in one district during the year the pavement was built. The committee agreed that expansion space of some kind should be provided, but did not feel that the data available justified definite recommendations as to particular types. While the 4-in. joints were not considered satisfactory, it was recommended, in the absence of amore suitable type, that their use be continued during 1932. In the meantime, it was thought advisable to construct a number of experimental sections in which joints with smaller openings spaced at shorter intervals would be provided. It was suggested that both 1-in. joints spaced 200 ft. apart and 2-in. joints spaced 400 ft. apart be used, and also that 4-in. joints, preferably of the mechanical type, be provided every 1,000 ft. It was felt that the strengthening of the slab ends adjacent to joints should be accomplished by some means other than the edge thickening then in use."

UNIVERSITY 0F ILLINOIS BULLETIN
Vol. 45 December 3, 1947 No. 23
ENGINEERING EXPERIMENT STATION BULLETIN SERIES No. 365
EXPERIENCE IN ILLINOIS WITH JOINTS IN CONCRETE PAVEMENTS

Surely more current info in

ACI 224.3R-95
Reapproved 2001
Joints in Concrete Construction
Reported by ACI Committee 224
Chapter 6

"6.3—Expansion or isolation joints
Expansion or isolation joints are constructed with a clean break throughout the depth of the slab to permit movement (Fig. 6.1d). Expansion joints are no longer used in mainline pavements,* except that expansion joints with dowels for load transfer are used at bridges. Isolation joints are used at fixed structures like manholes and drainage inlets, and at T or other nonsymmetric intersections. The clear distance across the joint is often maintained at about 3/4 in. (20 mm), although openings of 1/2 in. (12.5 mm) and 1 in. (25 mm) are also used. Since the joint has no aggregate interlock, it is necessary to provide some type of load transfer. Thickened edges [Fig. 6.1(h)] have been used at expansion joints to reduce or eliminate the need for dowel bars. When thickened edges are specified, consider cost, constructibility, and the restraint it may provide to slab contraction.
The structural adequacy of an expansion joint is determined to a large extent by its load transfer device. If adequate load transfer is provided, deflection of the slabs is minimized, and pumping action is reduced. It is necessary to maintain the joints, periodically, and in some cases to replace the filler material in the joint. Common types of fillers include fibrous and bituminous materials and cork. It is essential to seal the joints periodically to prevent infiltration of surface water. Resealing of joints is best accomplished during a cool period, when the joint has opened, thus permitting placement of a sealant. Expansion joints also may gradually close up in pavements that have unsealed contraction joints that can fill with incompressible material. This is a very undesirable condition that should be avoided by proper design, construction, and maintenance. Nearly all states have discontinued the use of expansion joints, except at fixed structures, because they appear to be unnecessary and are difficult to construct in a slip-form paving train. However, one state successfully uses expansion joints in lieu of contraction joints. A few states use contraction joints with every third or fourth joint being an expansion joint; this system causes the adjacent joints to open and fail.
For airport pavements, isolation joints should be placed between new and old concrete slabs and between different pavement features, such as ramp-to-taxiway and taxiway-torunway.

*At one time, blowups were a major consideration for joints in highway pavements. These typically occurred when incompressible materials entered unsealed joints, often in the winter when joint widths were greatest. In summer, the pavement expanded in response to daily and seasonal temperature changes. For a joint containing incompressible material, compressive stresses developed that lead to failure in some cases. Properly designed pavements with sealed and maintained joints are not susceptible to
blow-ups. True expansion joints in pavements are needed only in very unusual conditions of construction or with unusual materials. See PCA (1992b)."

So it seems that if you decide to place expansion joints they may need to be mechanical, sealed and properly maintained for proper operation, indluding the proper specifications for maintenance.

 

[MiketheEngineer]

What did they use last time??

 

[racookpe1978]

You've got a couple of advantages = You "know" your (current) heaviest loads, and you're on dirt, so you can control the compaction. Doesn't sound like you're needing fill or partially excavated, partially filled un-compacted dirt.


Make you're FEW program NOT the small 15x15 (foot ?) single slba: Take you're current heaviest truck and size your load for at one, if not two, increases in capacity. (If these were pickup trucks, and you know you have Ford F150's, size the load for an F-250 pulling a trailer, as an example. Don't go overboard, but you've got to assume the loads are never going to decrease over the life of the slab.

Next, figure out the true footprint and tire loads. Some of these earth movers/rock haulers have six tires, some 4. All will have heaviest loads in back under the back axle. So calculate based on two trucks parked next to each other. NOT one truck with the one evenly divided between all six tires, and only one tire on a 15x15 foot square.

Edge loads: Assume you have "parking stripes" prohibiting trucks from parking next to each right on the edge. (Otherwise, you're sure to have some time in the future when four drivers all pull right out to the edge of the lot in a nice row, with their trucks tire-to-tire-to-tire. )

If they're not parking near the edge, then the drive-on forces will be greatest right when those loaded double-tire rear axles pull on-to the edge of the concrete as the truck moves from dirt side to concrete pad. Maybe make the outside 18 to 36 inches deeper with more rebar around the edges?