Slab on grade

[daru]

Can a plain cement slab (3500 psi)on grade 6 inch thick resist a 60 ton crane loaded with concrete panels(41 ton) lifting them and placing them on the wall?. Can this floor slab resist this much load (41 ton +self load of crane)without any structural defect like cracking in the slab on grade?

 

[Ron]

First of all, it is a plain concrete slab, not plain cement. Secondly, you have not provided enough info to solve the problem analytically.

60 tons is likely the capacity of the crane, not its weight.

 

[daru]

I am sorry. It is plain concrete slab with 3500 compressive strength and 6" thick. Can this slab on grade be used
1) to make 32'x27' panels on it
2) The walls are panel walls, so the panels that were nade on top of slab have to be lifted to put them in place on the wall. The ceiling height of the hall is 90'. If we calculate weight of each panel that comes out be around 41 ton.
3) Can we safely use the crane to come inside this hall and lift these panels one by one and put them in place on the wall without any crack/damage to the slab on grade.
4) Outside of this hall is wooded area and there is no place for crane to lift these panels from outside. So crane has to come inside the hall.
Please ask me, if you need more information.

 

[daru]

Correction: The panel size is 32'x27'x7.25".

 

[Ron]

The relevant information necessary to solve this is:

1. What type of crane is it? (wheeled or crawler)...most likely wheeled as crawler cranes are usually not used for tilt-wall work because of surface damage to the concrete slab.

2. How many tires on the cranes and what is the footprint of the tires/axles. You will need this to get the wheel loads and to compute overlapping stresses from adjacent wheels.

3. Once you have that info, it becomes a pavement stress calculation that you can do using either AASHTO methods or PCA methods.

4. Casting the walls on the existing slabs does not overload the slab. Contact pressure based on the dimensions you gave and the weight of the panel is about 85 psf, though it would also appear that you have overestimated the weight of your panels by several thousand pounds.

It would seem from your description that you are proposing to lift architectural panels inside a roofed building. This is odd. How do you intend to attach the panels to the exterior wall? What is the structure of the exterior wall. If it is the cast panels, what is holding the roof up? Why are you putting up wall panels after a roof is in place? If that's the case, you would be better off to get a larger crawler crane and work from the outside and use a casting bed for the panels.

 

[daru]

Thanks a lot Ron:
The construction is already done. I mean everything is in place. I have just entered in this project just now. Actually the slab on grade is 130'x110'x6" and divided into panels of 15'x10' each panel. Walls are erected by tilting up panels. The ceiling height is around 90'. The reason I entered in this project is to find out the cause of cracks that appeared in slab on grade. The first thing I did was to get the cores exactly on top of cracked portion of slab and came to know that the crack went up to full depth of the slab.There is no reinforcement provided in the slab. Almost 50% slab panels have cracks in it. I heard from the owner that crane came inside to lift up the concrete panels made for wall.So nobody knows about the type of crane. When the crane came inside the hall, there was only slab on grade constructed and wall panels were on the floor. There was no roof constructed at that time. Crane came in and started lifting up panels and putting them in wall place.The panel size is 32'x27'x7.25". I am trying to figure out what caused the slab to be cracked like this.I really appreciate your help.

 

[Ron]

What is the crack pattern and what is the spacing of the control joints? You can get a good insight as to the cause of cracking by the pattern of the cracks and the crack edges. For instance, cracks caused by excessive flexural loading tend to be wider at the bottom than the top. The top of the crack may also exhibit slight spalling from compression. These are rare.

Check to see if there is any "faulting" at the cracks. Check to see the consistency of the thickness (take cores and see how close the thickness is to the design thickness. Variations in thickness cause variations in curing (shrinkage) stress distribution, thus contributing to "random" cracking.

You say the slab is divided into "panels" of 15x10 feet. This is a rectangular ratio of 1.5:1, which can influence cracking (my rule of thumb is no more than 1.2:1). You will need to know when the sawed joints were placed. Look into the sawcut joints and see if they have cracks in them. If not, look for a "random" crack nearby. See how deep the sawcut joints are. All of these contribute to cracking.

 

[daru]

Ron

I got four cores from the floor area. Two cores from top of crack and one exactly from contraction joint. Last one, a good one for compressive strength.Two cores from crack area show a full depth crack and third one from joint area show a crack exactly below joint to full depth and joint itself is only 1/2 " instead of 2" proposed by structural engineer.Also the concrete report shows a 4" slump during concrete testing for this slab on grade and cylinders strength was much higher than designed.It looks like a drying shrinkage crack instead of moving load cracks. But I need more response from you.Do you think I need to take more cores from top of joints in the affected pannels?Do you also think that the slump should have been even lower than this 4"? The crack pattern seems to be drying shrinkage.The crack width is same throuhout the slab depth.Thickness is almost same troughout.But they used #57 stones below concrete slab. The slab itself is unreinforced.I will check panel sizes again.There is no record that when joints were saw cut after hardening.
Thanks again for both of you.

 

[daru]

I forgot to mention one thing in my previous mail. The contractor used steel fiber concrete (4000 PSI)in place of regular 3500 PSI proposed in the drawings.Did this change also contribute to more shrinkage/expansion/cracking?

 

[Ron]

daru...your concrete parameters are fine. A 4-inch slump is OK for the application and you have described common shrinkage cracks. These likely resulted from poor planning of the finish/sawing schedule and/or poor thickness control of the concrete placement. Check the core lengths for consistency.

Placing the concrete on a bed of #57 stone can lead to more "random" shrinkage cracks because of the friction restraint, the inability to check consistent grading, the tendency for displacement during concrete placement, and other factors. Even of a vapor barrier were placed onto the #57 stone, some of these issues remain.

I'm surprised that you got a full depth crack in the 1/2" deep joint. That implies some significant variation in the concrete thickness, as placed.

If you have steel fiber in the concrete, it will tend to hold the concrete together nicely; however, it will usually cause the cracking to be a bit farther apart with slightly wider cracks.

 

[daru]

Ron,

The slab is 6" thick and joint depth is 1/2" that means the crack at the joint starts at 1/2" from top and goes to full depth.Do you also think that these cracks may be due combination of moving load and shrinkage problem?

 

[Ron]

daru...my point is that the slab is likely NOT 6" thick, uniformly. ACI provides a tolerance of -1/4", +3/8" for a 6-inch thick slab. If you are beyond these tolerances, your likelihood of "random" cracking increases without regard to the control joints that were sawn to a depth of only 1/2".

Unless you have faulting at the cracks, it is likely that they are not vertical load induced, but more likely just drying shrinkage.

Measure the length of each of the cores you have taken. Measure at least 4 points on each core and then report your results back here.

 

[daru]

Ron,

I measured the cores and depths are as under

Core # 1 Core # 2 Core # 3

5.969" 6.200" 6.181"
6.000" 6.262" 6.400"
6.009" 6.232" 6.369"
5.932" 6.182" 6.224"

I feel that bottom surface of these cores are not leveled may because of #57 stone layer. I did not see any faulting at the cracks.

 

[Ron]

Lengths are consistent and within acceptable tolerances, assuming these represent the slab accurately. Indications are that you have shrinkage cracks, likely resulting from late and/or insufficient sawing of the control joints.

 

[daru]

Ron,
Thanks a lot for your help. See you again.

Daru

 

[daru]

Ron,

I have one more question. What do you think if slab is little bit raised a joints and center of panels is depressed? Then what should be the cause behind this? I have a little feeling that this is also a case for slab on grade.

 

[Ron]

daru...what you've described is called "curling". This is caused by differential drying shrinkage between the top and bottom surfaces. Top dries faster than bottom, thus shrinks and "pull" edges upward.

 

[daru]

RON,
What are the other factors which accelerate top drying other than atmosphere temperature? The slab was poured in month of May and is steel fiber.When can we say that slab is CURLED?I mean how much slab joint is raised then condition can be termed as curling.This curling condition is very prominent or only an engineer's eye can judge it.Do you also think that delayed joint installation with depth less than required are also a cause of curling and cracks?

 

[Ron]

It is moreso the humidity difference than the temperature difference, though both affect the condition.

Delayed joint installation leads to cracking more than curling. Curling is a materials/curing phenomenon.

You can check the extent of curling by doing floor profiling across the joints. Use either an appropriate straightedge (10 feet long) or a profiling device such as the Face "Dipstick".