rebar instead of contraction joint for slab on grade 3

[delagina]

i have a big slab on grade with 4 pedestals inside the slab supported by isolated footing.

i will provide isolation joint for the pedestals but can i not provide contraction joint instead put say 1 ft rebar close to the opening to intercept potential crack?

i will provide contraction joint for the slab just not exactly where the columns are located.

 

[BAretired]

According to PCA Bulletin "Concrete Floors on Ground":

Large areas can be cast without control joints when the amount of distributed steel is about one-half of 1% of the cross sectional area of the slab. Special effort should also be made to reduce subgrade friction in floors without control joints.

Usually, sawcut control joints are a more economical alternative but there are times when joints are not desired.

BA

 

[Ron]

Even though it can be done, you will still get cracking. The joints are there to control the location of the cracking that inevitably occurs, even with reinforcement. Depending on the reinforcement and its location, your cracks will occur with regular spacing and will be tight cracks. Variations in the rebar placement, slab thickness, mix characteristics, finishing and curing will cause the cracks to become irregular and unsightly, even though tightly closed.

Put the joints in and forget the rebar.

 

[delagina]

@Ron,

the big slab has already too many openings from equipment foundations which i didnt design. they already had set contraction joints on slab.

i was just involved with adding 4 more pedestals.

if i add more contraction joints i will have too many or irregular spaced contraction joints.

@BA, thanks..

 

[delagina]

this is what i plan to use. corner bars instead of contraction joint.

 

[Ron]

I'm confused by your descriptions. At first you discuss adding reinforcing or joints, then you say that the joints are already in. Your sketch shows two columns with proper isolation and jointing.

Bottom line is this...reinforcement will not stop cracks due to concrete shrinkage. It will change their location and spacing, but it will not eliminate them. Use control and isolation joints.

 

[delagina]

@ron,

the drawing already has joints but it hasnt been issued yet for construction. i can still change it, i'd rather not though because i am just adding 4 small pedestals.

 

[Ron]

The jointing and isolation shown on the sketch are proper.

 

[delagina]

@ron, the sketch was taken from ACI not my actual drawings.

anyway, thanks.

 

[hokie66]

delagina,
I think BA and Ron may have misunderstood your question, as I believe you are only asking about the reentrant corner bars. And yes, the corner bars will tend to control the stress riser cracks which are inevitable in those locations. Make them about a meter long, and put them as close to the corner as possible.

As to the other issue of random drying shrinkage cracking, the older I get, the more I tend to lean toward the approach given by BA. For many applications, joints present more problems than cracks. I was in an IKEA showroom last week...no joints...lots of cracks, but nobody except me noticed, and the floor is completely serviceable.

 

[Ron]

hokie66...I rarely disagree with you, but respectfully do so at this point. In the configuration of the OP's last attachment, the corner bars will help, but the re-entrant condition will ultimately prevail and a crack will develop...

My clientele are generally fanatic about cracks and want them prevented. While I agree they have little or no impact on the performance of the floor, they are unsightly and have an impact on the ultimate real estate transaction that will likely take place. In more specialized industrial applications that is less of an issue. In commercial construction, it is something to be considered.

 

[hokie66]

Ron,
I don't think we are really disagreeing. I agree that the corner bars will only control the reentrant corner cracks, not prevent them. There are many cases in slabs on ground, e.g. drainage boxes, where providing contraction joints intersecting all corners is just not practical, so reinforcing the corners is the next best option.

Jointed vs. jointless has to be approached on a case by case basis. I have had the same experience as you with clients not liking cracks, but also a lot of experience with clients, mostly of the industrial type, who don't like joints because of the maintenance problems that result.

 

[TXStructural]

Rebar in slabs is intended to control propagation and width of cracks, not prevent them. You will have fewer cracks with a better concrete (control w/c ratio and aggregate grading) and concreting practices (curing.) Use 2-#4 x 4'-0" at each reentrant corner (all four corners).

You want full development beyond the expected crack, and too short will simply move the crack location (it will propagate around the end of the bars).