Design of Slabs on Ground (ACI 360R, Army TM 5-809-12, etc.)

[fel3]

I would like some guidance on this topic because it's been about 20 years since I last did this. As part of an industrial site project, I am designing two outdoor traffic-rated slabs. The bigger one (about 20'x60') must support a large, rubber tired fork lift weighing about 13 tons plus loads up to about 4.5 tons. The smaller one (about 12'x40') must support a medium-capacity pallet jack to move around pallets under 4000 lbs. The smaller slab will be inaccessible to the fork lift, so I can design it for the lighter loads.

I am using the method in Chapter 7 of ACI 360R (Guide to Design of Slabs on Ground), which is for unreinforced slabs. I also have PCA's Slab Thickness Design for Industrial Concrete Floors on Grade and Army Technical Manual 5-809-12 for reference. When I run through all the numbers, I get required unreinforced slab thickness of 10" and 6", respectively.

I would like to add some steel for crack control as discussed in ACI 360R, Chapter 8, and here is where the confusion sets in and where I need some guidance:
[1] ACI 360R, §8.3, recommends a minimum steel ratio of 0.5%, which results in As = 0.60 sqin/ft and 0.36 sqin/ft, respectively. This is "lots'o'rebar" territory.
[2] TM 5-809-12, §5.5, recommends a minimum steel ratio of 0.06% for odd-shaped slabs (this also seems reasonable for normal shaped slabs), which results in As = 0.072 sqin/ft and 0.043 sqin/ft, respectively. This is welded wire fabric territory and seems far more reasonable to me.
[3] I haven't found anything in the PCA manual regarding reinforcement for crack control.

Since the design method in ACI 360R is for unreinforced slabs and any added rebar is for crack control and not flexure or shear, I wonder if ACI 360R meant to say 0.05% instead of 0.5%. What do the experts think?

BTW, I plan to divide each of the slabs into three panels and use diamond plate dowels at the joints. I will also thicken the edges for warp control.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[Ron]

Your slab thicknesses are about right. As for crack control, that is better done with proper jointing than with reinforcing. Use minimal reinforcing and space the control joints at close as practicable. Timing of the control joints is extremely important. Sawcut the joints the same day as placement.

 

[hokie66]

My opinion and experience are different from Ron's. In industrial applications, the joints are more often than not the main problem. Vertical differential displacement of the joints by heavy loading often leads to pumping of the underlying material, causing voids at the joints. 0.5% Ag reinforcement will hold restraint shrinkage cracks tightly together, while lesser amounts won't. More and more in industrial and heavy commercial work, "jointless" slabs are being used.

 

[DaveAtkins]

I believe 0.5%Ag reinforcement is for large (essentially infinite) spacing of slab joints ("jointless" slabs as mentioned by hokie66).

If you put in contraction joints, you can use far less reinforcing. If you put in contraction joints at close enough spacing, you can eliminate reinforcing altogether.

As I recall, the PCA Slabs guide has a formula for calculating reinforcing required as a function of joint spacing.

DaveAtkins

 

[WARose]

In the version of ACI 360 I have (i.e. ACI 360R-92; Reapproved in 1997), Chapter 6 (section 6.3) calculates the reinforcement needed based on the “subgrade drag equation”. A variable in this equation is the distance between joints/ends. As the introductory paragraph states: this is the amount of temperature/shrinkage reinforcement needed (and to control crack widths).

Keep in mind: that reinforcement is not intended to serve as Flexural reinforcement…..that’s what your thickness is based on. Most of the time, I come out of this equation with a level of reinforcement that a wire mesh can satisfy.

 

[hawkaz]

The VooDoo of slab design- All good responses above, none are wrong, all are subjective.
My witch doctor prefers diamond dowels and dowel baskets at all joints (cold or sawcut) and an un-reinforced slab. As was stated above, the joints are the problem areas.