Warehouse Slab on Grade Supporting Rack Post Loads 5

[ajk1]

Anyone really experienced in design of slab on grade supporting rack post loads? If so, can you help with the following:

Is it more economical to use reinforced or unreinforced slab on grade?

Any good reference papers specifically addressing this?

If you use reinforced slab, how far apart do you place the constucution joints?
Do you use intermediate saw cuts?
What might be the typical reinforcement used?
Do you use SAFE to model it
In an unreinforced slab, where do you place the sawcuts - do you do one down the middle of the aisle where stresses are amximum?
Do you thicken the slab at edge conditions?

The post loads are about 18,000 pounds unfactored, the modulus of subgrade reaction is 100 pci. I was thinking of 30 MPa concrete (4350 psi)with a specified 28 day shrinkage not exceeding 0.035% as per CSA A23.1.

Very preliminary calculations indicate a 10" thick unreinforced slab would work. I suppose that an 8" slab with rebar with equivalent resisting moment to the plain concrete might also work.

 

[Ron]

I would hesitate to specify a 90-day strength for horizontal/flat work, simply because one of the reasons for specifying higher strength concrete at 28 days is to achieve durability. 30MPa at 28 days has better durability than 30MPa at 90 days. For this application, abrasion resistance and surface durability are important.

Specifying 90-day strengths is fine for vertical work.

I still consider that the forklift loading on the slab will likely be more critical than the rack loading.

Agree with dik's comments, except I do not specify joint spacing greater than 15 feet unless absolutely necessary. Keep the L/W ratio at 1.2 or less.

 

[dik]

I usually go the 28 day route, except for high fly-ash... may not have come out in the post... I could have noted that the shorter spacing is more preferred, but I've gone longer with higher amounts of rebar to more distribute cracking.

Dik

 

[ajk1]

While I appreciate the responses, I must say that having spent so much of my 47 year engineering career trying to reduce shrinkage cracking, I strongly disagree with the advice to "specify any concrete strength you want". The most common problem with slab on grade is shrinkage. The greater the concrete strength, the more cement and water; shrinkage varies as the amount of water. Therefore it is counterproductive to use concrete of greater strength than required to carry the load. ACI 360 explains all this very well -- although I had no hand in writing ACI 360, I am in complete agreement with it. ACI 360 also suggests specifying 90 day strength for slab on grade for this reason and I can certainly see the logic.

Hard to see how 30 MPa at 28 days is going to be so much better than 30 MPa at 90 days with respect to durability. 30 MPa is 30 MPa. The strength gain is about 10% and that is negligible effect on durability. If you are thinking that there may be an abrasion issue, that goes more to the the quality of the aggregate. If it needs a hardener, then that is what should be applied, in my opinion, and not try to fenegle it through the concrete strength.

Although 15 foot centre joints is appropraite for the standard school or garage slab proportioned with 20 mm aggregate, I cannot agree with the advice to place the joints at 15 foot centres for a warehouse slab where the concrete has 40 mm aggregate and has been proportioned for maximum bulk density and minimum shrinkage starin as per CSA A23.1. We have successfully used 40 times slab thickness for many years. ACI 360 suggests 36 times and we succeeded in extending this by paying attention to the mix design and shrinkage, as noted above.

Hard to see how forklift can be more critical for the slab thickness design for the particular case I described, but thanks for the alert and we will check it out. I do agree that forklift effect on the joint is an issue.

 

[dik]

Even as a kid, I had a tough time deferring to my elders...

You forgot to include, '...as long as you're happy with what you get'! Don't let my flippant response suggest that I would recommend specifying a 90 day strength... I have often spec'd a 56 day strength with 25% fly-ash, but never 90 days and never with only Portland cement. You are simply spec'ing an inferior 28 day strength...

With loaded, working, slabs, it is important to have a high strength slab. as I noted, for added shear and flexural strength and as Ron noted... for durability.

30 MPa concrete is more durable than 25 MPa at any time in the future history... I know that 30 MPa @ 28 days is more durable than 30 MPa @ 90 days because at 90 days, the 28 day strength concrete will have gained an additional 5 MPa... It will have an added 15% flexural and shear strength... These are important for loaded working slabs.

You can accommodate the shrinkage from the higher cement content by several means... reducing strength should not be considered. I generally spec a very stiff mix to start; contractors hate it. I spec a maximum slump... not a stipulated slump which gives the contractor an added inch or so latitude.

Regarding joint spacing, the bay spacing often determines the frequency... as noted, however, sawcutting time is critical. 20' is pushing a 6" slab, but it can work if the job is done properly.

I've included a copy of my normal notes for SOG construction:

SLAB ON GRADE

THE MODULUS OF SUB-GRADE REACTION IS ASSUMED TO BE 200 PCI. THE CONTRACTOR, THROUGH THE CLIENT IS RESPONSIBLE FOR OBTAINING A GEOTECHNICAL REPORT TO CONFIRM THIS DESIGN ASSUMPTIONS. THE [CONSULTANT | ENGINEER] ASSUMES NO LIABILITY FOR THESE DESIGN ASSUMPTIONS OR FOR ANY FOUNDATION REDESIGN NECESSITATED BY DIFFERING SOIL CONDITIONS

TEMPERATURE DIFFERENTIAL BETWEEN THE GRANULAR BASE AND THE TOP OF THE CONCRETE SLAB SHALL BE MINIMIZED

SLAB FINISHING SHALL BE DELAYED AS LONG AS POSSIBLE

THE SLAB IS DESIGNED FOR [LIGHT | MEDIUM | HEAVY] DUTY USE ONLY

THE EXPOSED SUB-GRADE FOR SLAB ON GRADE SHALL BE PROOF ROLLED. SOFT SPOTS SHALL BE EXCAVATED, BACK-FILLED WITH GRANULAR FILL AND COMPACTED TO 95% STANDARD PROCTOR. GRANULAR FILL AND COMPACTION METHODS TO BE APPROVED BY THE [CONSULTANT | ENGINEER]

THE COMPACTED GRANULAR FILL SHALL PROVIDE A UNIFORM SURFACE WITH NO ABRUPT CHANGES IN LEVEL. MAX DEVIATION IN LEVEL SHALL NOT EXCEED 0.5 INCHES

PROVIDE MIN [6 | 150] CMPT GRAN 'A' BASE COURSE BENEATH SLAB-ON-GRADE U/N. CMPT TO 95% [SPDD | MPDD] U/N

INSTALL 6 MIL PEVB MEMBRANE CONFORMING TO CAN3-51.33-M80 BTN SLAB AND FILL WITHIN THE AREA OF THE BUILDING

LAP PEVB JOINTS [12 | 300] MIN AND SEAL WITH MASTIC CEMENT

U/N REINF FOR SLAB-ON-GRADE SHALL BE:
DEPTH REINF
5 10M@16 TEW

LAP WELDED WIRE FABRIC, ONE FULL MESH AT SPLICES

REINF STEEL TO BE LOCATED [2 IN | 50 MM ] BELOW TOP OF FINISHED FLR SLAB

SUPPORT SLAB TOP REINF ON HIGH CHAIRS AND/OR 15M SUPPORT BARS WHERE REQUIRED. ALL BARS AND CHAIRS MUST BE SECURELY TIED TOGETHER

PROVIDE SUFFICIENT CHAIRS AND SUPPORT BARS TO MAINTAIN CONC PROTECTION AS SPECIFIED

CUTTING OF CONTROL JOINTS SHALL BEGIN AS SOON AS THE CONC SURFACE HAS HARDENED SUFFICIENTLY TO RESIST RAVELLING AS THE CUT IS MADE [.SAWCUTTING SHOULD COMMENCE APPROX 6 TO 8 HOURS AFTER FINISHING SLAB]

SAWCUT SLABS INTO PANELS NOT EXCEEDING 20' IN ANY DIRECTION. MAX CONTROL JOINT SPACING SHALL PREFERABLY NOT EXCEED 30 TIMES THE SLAB THICKNESS AND SHALL NOT EXCEED 36 TIMES THE SLAB THICKNESS. THE LENGTH TO WIDTH RATIO OF THE CONTROL JOINT PANELS SHALL NOT EXCEED 1.5. IF A SAWCUT INTERSECTS ANOTHER SAWCUT, THE SMALLER ANGLE OF INTERSECTION SHALL NOT BE LESS THAN 50 DEGREES

CONTRACTOR SHALL PROVIDE SHOPDRAWING SHOWING THE PROPOSED CUTTING PATTERN AND THE PROPOSED SAWCUTTING PROCEDURE WELL IN ADVANCE OF PLACING THE CONCRTE FOR THE SLAB ON GRADE

CONC SLAB-ON-GRADE HAS BEEN DESIGNED FOR THE FOLLOWING:
WHEEL LOAD XXXX POUNDS
INFLATION PRESSURE XXX PSI

DIM A XXX
DIM B XXX
DIM C XXX

LOAD REPETITIONS XXXXX THOUSAND
SAFETY FACTOR XXX

IF DIM A=0 THEN VEHICLE AXLE HAS SINGLE WHEELS
IF DIM B=0 THEN VEHICLE HAS SINGLE AXLE ONLY

These are edited as required depending on the project. The typical sawcut and filler material is specified on a typical detail.

Dik

 

[Ron]

ajk1...your comments indicate that you don't fully understand concrete technology. 30 MPa is not 30 MPa if specified at different dates to achieve the required strength. As an example, a 30 MPa mix at 90 days might be as low as 20 to 22 MPa at 28 days....that concrete will clearly not have the same durability characteristics as one that achieves 30 MPa in 28 days.

While abrasion resistance is slightly related to the abrasion resistance of the coarse aggregateit is primarily a function of the cement paste integrity. The quality of the cement paste significantly affects abrasion resistance and that is directly related to w:c ratio and strength.

You state that you are greatly concerned about shrinkage cracking yet you take the spacing to an upper limit by anyone's stretch of the concept. Those are incompatible. Your experience might very well be anectdotal as compared to other applications. I see many problems with improperly spaced and untimely control joints, so 15 feet is the maximum for me.

ACI 360 only states that you can consider 60 or 90 day strengths to reduce slab thickness...it does not carry that consideration to durability. Further, it states that durability is directly related to strength and w:c ratio....which are contrary to using higher design-day requirements.

 

[Ron]

Forgot to add one other thing....if you want to reduce shrinkage, increase the coarse aggregate size...it will reduce the amount of cement paste required for the same coverage of the aggregate.

 

[ajk1]

Very interesting. Thanks for taking the time to respond. I am always willing to learn and appreciate being told when I don't understand something. I find that it is when I think I know all the answers that I am most likely to be wrong!

Just wondering how much concrete you guys have done lately that does not have SCM's in it (for which it is pretty common practice to specify 56 day strength or even 90 day strength), or are you recommending that SCM's not be used in warehouse slabs? The best slab I ever did had 50% slag cement.

Is it your standard practice to do warehouse slabs with joints at 15 foot centres? How many warehouse slabs have you done with joints at 15 fet centres and how are they performing with respect to curling and cracking when the forklift goes over them? Have you gone back to look after a year or 2?

Differential drying shrinkage causes the concrete at the joints to curl. For this reason, the less water in the concrete, the less curling there should be. That is why I am thinking to specify only the concrete strength needed to support the load and space the joints as far apart as possible, so there are fewer of them to cause problems. However, if you can point me to a reference publication that recommends for warehouse slabs that closer spacing of joints is better than greater spacing of joints, or that they should be spaced at 15 feet centres, then that will be interesting,

My slab is 8" thick warehouse slab. I have clearly said that we use 40 mm aggregate and do other things to reduce shrinkage. Ron - did you miss that or are you recommending larger than 40 mm aggregate?

Thanks again for the info.

 

[dik]

Largest pour was about 6,000 cu.m. (continuous tremie cut-off) and largest cargo terminal building was about 400,000 sq.ft.

Latest work has been in Sask. where concrete can be exposed to both chloride and sulphate attack; this requires a higher fly-ash content (some people use as high as 40% or more flyash, but I haven't gotten that far). Type 5, 50, HS cement is contraindicated for chloride attach, hence the use of fly-ash with Type 10, GU cement). 30 MPa @ 28 days is a good start.

You're on your way using 40 mm aggregate and also minimising the cement, hence the use of SCM's and not trying to achieve a extended term strength. I typically use a 3" maximum slump for flatwork and try to space rebar 12" (300) o/c min. so that they can walk through it, else they have to use foot boards. SCM's also have a role if reactive aggregate is available.

With an 8" slab, I would typically be looking at 20' sawcuts; 15' is better, but not always possible. The tricky part is arranging the sawcuts so that they work with isles and racking. I also provide the client with a 'Maintenance Manual' for slab upkeep. (I started doing that with parking garages nearly 40 years back.) You can extend the sawcut spacing a bit by increasing the amount of rebar. Timing of sawcutting and curing is absolutely critical. The use of a sofcut saw is really advised.

I use poly directly under the slab and not under the granular levelling course (this is contrary to some practices) and I've not had problems with curling. I suspect this is because of the sawcutting and curing. It may also be that I've not had a project done under the hot sun.

At pour joints (aka shrinkage/expansion joints), I typically use 20M or 25M smooth dowels (latter preferred) in a proper 'rack' and discontinue reinf at pour joints. Make sure that the dowels are parallel with no 'burrs' from shearing. I keep dowels 9" away from a pour joint at right angles (I don't want to get too close).

At sawcut joints in the floor I typically run rebar through sawcut locations, using 2" cover and 1-1/2" deep cuts. The cuts are 1/4" wide to permit filling. This provides support for the edge of the cut for wheel loads. Depending on the use, I use three different fillers. I generally show a typical sawcut pattern (this can be a challenge for irregular column locations and for architectural floor finishes), but, generally have the contractor provide a sawcut pattern for review. Timing for sawcutting is critical. I notice that for most highway work, with sawcutting, the rebar is discontinued and dowels are used.

There's probably a bunch of other stuff... but this is a start for discussion...

Dik

 

[dik]

Should have added that I've not used 'diamond' plates for dowels. I don't know how they work in the long term. There is a lot more metal edge and the difference in hardness between concrete and steel may generate tensile stresses that cause a tensile fatigue failure in the concrete. I did some work a long time ago on teeth where the tensile stresses from steel pins was causing a tensile fatigue failure of the amalgam.

I'm not recommending against their use, I just haven't had time to look into this.

Dik

 

[ajk1]

Hi Dik - This is excellent information. Thanks very much. Wow, that is certainly a big area (about 4 times the size of our current project) and it would be great to learn how it performs.

How old is it?

Have you gone back to see how it is performign after a year, or are you planning to? Be interesting to see if there are any any lessons to be learned.

Your comments are excellent and I agree fairly well with what you say, except I am a little concerned about poly directly under the floor because it promotes water rising to the surface which has undesirable effects on finishing and durability. The floor finishers in the Toronto area have told us not to do it and that seems to be supported by the literature. I assume that you have not had this problem. Generally we have stopped doing it for regular slabs on grade such as for schools, hospitals, etc. unless the geotechnical engineers says that it is required for moisture vapour permeation (or radon gas) reasons.

Regarding control joint spacing, about 20 years ago we were commissioned by a Canadian Governemnt Department to do a study to arrive at recommendations for their warehouse slabs. The people we assembled for that included one of the foremost experts in the field of concrete technology in Canada at the time and a foremr chiarman of CSA A23.1. I believe that one of the principle recommendations of that study was to space the control joints as far apart as justifiable having due regard to shrinkage and cracking, to minimize the number of joints, as they were one of the principle problem areas.

To achieve that, without causing undue cracking issues, the concrete technolgy expert came up with specific recommendations, not only about the size of course aggregate but also with respect to the proportioning for maximum bulk density and other aspects of the design of the mix, curing, etc. If you are not doing those rather special things that go well beyond just using the well known advice to use 40 mm coarse aggregate, then I can perhaps understand why you favour 15 or 20 foor joint spacings for an 8" slab.

Although I was not part of that study, I highly regard the people who researched and authored it. Now in their 80's they are still alive and I suppose I could consider going back to them and get their comments.

In my opinion, some of these things are trade-offs of one aspect of performance for another, so engineers' opinions can differ. It is interesting and useful information that, on balance, you have had better overall success with warehouse floors with rack storage and forklifts when you used control joints at closer spacing than greater spacing. Have you ever tried greater control joint spacing?

Regarding sawcuts and reinforcing, I suppose the reinforcing diminishes the effectiveness of the sawcuts, but overall I agree that it is not practical to stop the rebar at the sawcut control joints. In any event the reinfoecemnt to control crack width is in accordance with ACI.

Incidentally, in the Toronto area, we are quite near to a good source of high quality ground granulated blast furnace slag and far from fly ash source. With coal-generated electricity being phased out in Ontario, fly ash sources will have to be from the U.S. or Quebec, when it is no longer available from northern Ontario. Mostly it is slag as the SCM in the Toronto area, and I believe that little concrete is produced here that does not include slag. We do not generally have the sulphate issues that are prevalent on the prairies. Deicing chemicals (chlorides and other reactive ions) are a very big issue here for parking structures and the like, but this warehouse floor would not be subject to that and chloride in the soil has not been an issue here generally.

Some other questions, if you do not mind:

1. By "pour joints" do you mean the "constrcution joint" that would occur at the end of the day's pour?

2. Do you have any maximum limit on the length of slab placed in any one day? My concern would be opening of the construction joint to too wide a width if the length of pour is too long, particularly since the rebar thru the sawcut control joints may limit how much of the shrikage movement actually occurs at the sawcut joints and it may accumulate at the construction joint at the end of the pour. We had this issue with a topping placed on top of insulation (it was not a warehouse)

3. Do you provide "additional" local rebar in the slab at the construction joints to control cracking when the forklift vehicle goes over it when it is curled upward?

4. Do you normally use any hardener such a as a dry shake or a liquid hardener (such as Ashford formula) on the floors?

Best Regards

 

[ajk1]

...also, do you use fibres in the concrete?

 

[a2mfk]

ACI's latest view (ACI 302, 5.9.3) on polypropolene fibers seems to be that they are of little use in crack control, except for plastic-shrinkage and settlement cracks. They may also contribute to durability. But they have little impact on the concrete after it hardens, ie, crack width control...

My experience and research affirms this, though you may get luck on 4-5" SOG with lots of control joints...

 

[a2mfk]

ajk- you had a question on diamond dowels, see my first response..

 

[dik]

My largest slab is for Vista Cargo Terminal in Mississauga... it was done about 2 decades ago and I was by it approx 8 years ago... still standing and in good shape. Nothing to be gained from it... some interesting construction features... 40' clear height... roof beams supported at mid span on col tied back to roof with diagonal rods... picked up idea from Eaton's Centre facade...

I haven't had any problems with the PEVB directly under the floor and I suspect this is due to curing and low slump. No problem with bleed water or finishing. Have always done it for moisture transmission.

I typically max my joints out as noted. Ron's spacing is a little less and it doesn't hurt... both of us are comfortable with what we do and if there are problems, it's likely due to other things. With pre-eng buildings and frames at 25' centres, with anything less than an 8" slab, I will put them in at 12.5'. For an 8" slab, they will go in at 25' and I'd likely use 0.25 rebar just to tighten up the cracking a tad.

I've not tried greater joint spacing because it works... for the same reason, likely, that Ron uses his spacing.

The increased aggregate size is good... often use 40mm... problem with concrete placement and pumping... Good supplier, good contractor, stiff mix, timely sawcutting, and proper curing are essential for good flatwork. In addition, it helps that there are a couple of excellent concrete finishers in your neighbourhood... and that helps a lot.

I've often found that forklift traffic is more critical than rack loading... and have done numerous reports for lawyers and insurance companies as a consequence.

Sawcutting weakens the slab and provides a location to crack, but it has to be done at the right time... else, you may as well wait for it to crack and route out the cracks and fill...

There are additional problems with the removal of mercury from flyash using a carbon process... this is causing problems with fly ash and may stop the useage of it.

Parkades and durability are a whole other area of discussion. You have a problem with the use of salt de-icers as well as numerous freeze-thaw cycles... here, in Winnipeg, it freezes in the winter and thaws in the summer <G> one cycle...

Pour joints are construction or cold joints...

I used to restrict the area of slab in one day, but this is a 'means and methods' for the Contractor... When I first started practicing, it was not uncommon for checkerboard pattern of slab placement. I have, however, had the Contractor sawcutting at 3:00 AM... and the first time this happens, they are really unhappy... Most projects are not of sufficient size and those that are big, are generally limited by what the Contractor can do.

Proper dowel design and placement accommodates forklift locations. Where dowels are not possible, the slab edge is thickened by 1/3.

I've not encountered curling with any of my work, but, I've seen it... Repair is pressure grouting under the slab and grinding flush.

I have used metallic hardeners on the occasional industrial slab, but for the most part don't use them... they and traprock can be problematic. I've not used a liquid hardener.

Dik

 

[ajk1]

Thanks. If you found what you do works, I can't argue with success. I like your making them sawcut at 3 in the morning. Good stuff. That is what is necessary for it to work right. Yes I know there are some excellent specialist floor finishers around Toronto whom I usually run special floor finishing projects like this by and I will coontact them tomorrow. Thanks for your time and knowledge. Much appreciated. Going to watch the Victoria day fireworks now!

 

[Ron]

ajk1...I re-read my last post and I owe you an apology. I did not mean to be disrespectful nor condescending. I apologize.

I did not notice your aggregate size in your post. That's critical to reducing shrinkage and your approach is good. I also noticed that you have an 8-inch thick slab. For that thickness I would go up to 18 to 20 feet between joints.

I'm a believer that fewer joints simply means wider joints, assuming no cracking in between. This is particularly true if you are using synthetic fibers.

Steel fibers are excellent for warehouse floor slabs. They solve a lot of problems...but they're expensive.

 

[ajk1]

Ron - not a problem. I appreciate and value your advice. Thanks.

 

[dik]

Ron... have you ever tried to put a shovel into a steel fibre reinforced mix?

Dik

 

[Ron]

dik...yep...not easy. If you can get the stuff placed though, it is some tough concrete.

There is a slab in Central Florida that I designed almost 25 years ago. It is a pavement in an office park for a loading dock and dumpster traffic. Went by a while back and found good performance, no uncontrolled cracks, with only an occasional small rust stain from the fibers. On the same project, I designed an asphalt pavement with steel fiber in the asphalt. Over 20 years later, no overlay and not potholes.

I like steel fiber...just can't get clients to pay for it!

 

[ajk1]

I just spoke to one of the specialist contractors here in Toronto who has done millions of square feet of warehouse floors in the last 20 years. He says he always uses steel fibres. He says he has not seen reinforcement in these slabs in the last 10 years unless there are real soil problems.