Effects of incorrectly place rebar in slab 1

[brandonl]

Situation - 8" (min.) thick continuously reinforced concrete slab used to pave a drainage channel. Channel section is trapezoidal with 12' wide bottom and 7ft. deep. The reinforcing steel (#5 @8" longitudinal, #4@16" transverse) was placed too low in the slab. Bottom cover is thus around 1". Slab thicknesse vary from the 8" minimum up to as much as 11 inches thick. The result is that the concrete cover over the rebar to the top surface is on the order of 6 to 8 inches.

I know intuitively that the rebar is not going to provide the proper crack control to the top surface of the slab, but have been unable to find any literature or publications that have examined this effect (or lack of it). Typically the steel should be placed at 1/3 to 1/2 the depth of the slab. Does anyone know at what depth in a slab on grade such as this that the reinforcing steel becomes worthless as a crack control agent?

 

[PT999]

Go to

http://www.wirereinforcementinstitute.org

Tech Facts - Innovative Ways to Reinforce Slabs on Ground.

While it doesn't answer your question directly, it provides design criteria and sample problems for slab on grade design with steel.

An additional problem you now have, is that bottom cover is insufficient, thus the steel will corrode way before its time. 3" cover is desired for concrete in contact with earth.

 

[SlideRuleEra]

To add to PT999's good advice - when steel corrodes, the corrosion products occupy a larger volume & can exert considerable pressure. Therefore you not only "loose" the steel, you also have concrete that is being "exploded" from inside.

 

[dbuzz]

The 1" cover under the reinforcement may not be adequate for its protection. Where I work the minimum cover to reinforcement in the base of a slab, cast against non-aggressive soil, is:
(i) 30mm if the slab is protected by a damp-proof membrane; or
(ii) 40-50mm, if the slab is cast directly against ground
40mm for concrete strength 40MPa and over
45mm for 32MPa
50mm for 25MPa

The 6-8" cover over the reinforcement does sound excessive. Unfortunately I don't have any literature on "maximum" cover to back that up.

 

[UcfSE]

ACI 318 lists cover for concrete against earth as 3 inches at the bottom.

 

[whyun]

Is there an exception to slabs on grade ragarding 3" cover requirement?

For example, it is common to see 4" slab on grade with #3@18"oc each way at mid-depth of slab. 2" minus 3/8" for #3 bar gives only 1-5/8" cover at the slab bottom in contact with soil.

 

[brandonl]

While I appreciate all the replies indicating what proper steel clearance is (I am familiar with ACI code requirements on this), what I'm really looking for is thoughts/opinions, or info on any literature that address excessive cover. This is a continuously reinforced slab - i.e no joints other than what was required for construction. The steel is present to provide control of cracks due to concrete shrinkage and temperature movements. As the steel cover increases as measured from the top surface of the slab, the crack widths apparent at the surface will increase, which leads to a more rapid rate of moisture/chemical intrusion into the slab which will speed up the deterioration (not to mention the effects of only 1" cover at the bottom of the slab). Obviously temperature and shrinkage reinforcement should be placed (reasonably) close to the surface to provide proper control of cracks and crack widths. But I would imagine that there is some critical depth of cover that, if exceeded, the steel effectively no longer performs its job.

Thoughts/opinions/discussion?

 

[MotorCity]

brandonl,
Other than adding unneccesary dead load, there is no harm in having too much cover. The capacity of the slab will be determined by the effective depth of the rebar, not the depth of the concrete cover. Concrete cover does not increase the slab strength, its sole purpose is to protect the rebar. The point at which the rebar is no longer effective is when the allowable load is exceeded. This allowable load should include the self weight (i.e. cover).
Good luck.

 

[brandonl]

MotorCity:

You are correct, and I understand what you said, however, this is a slab on grade used as a drainage channel lining. There will be no traffic loads imposed other than an occassional skid-steer loader to remove sediment, etc. The reinforcing steel, in this case, is for the purpose of restraining cracks that will occur due to temperature and shrinkage stresses - that is all. It is not there to provide flexural strength as in a traditional elevated slab. There are no joints in this slab - it is continuously reinforced. The further from the surface the steel is, the less effect it will have on keeping cracks tight at the surface, and at some point, I would think that the steel could be so far from the surface as to provide little to no influence on the crack width. I just have not been able to finsd any information or discussion on this specific topic.

Any and all thoughts or opinions are still welcomed!

 

[blake989]

Try the PCA Handbook of Circular Tanks Without Prestressing. I know you are not designing a tank, however, if I remeber correctly there is a great explanation of shrinkage stresses and steel behavior in the first few pages of that handbook.

 

[SlideRuleEra]

brandonl - Here is an old (circa. 1965) article from Concrete Construction Magazine that discusses continuously reinforced pavement (see page 1)
ftp://imgs.ebuild.com/woc/C650259.pdf
The recommendation then, as now, is for the rebar to be in the upper half of the slab. Cracking can be expected above the rebar (no matter where it is located).
I have designed & constructed continously reinforced slabs and understand that the rebar/concrete bond strength is of highest importance to get the advantages of continuous reinforcement. Another problem with the 1" cover on the bottom (besides corrosion) is the likelihood that during placement, concrete did not addequately encase the rebar - probably causing a poor bond.
From what you have described, it sounds like the rebar is not going to be effective.

 

[apsix]

Even though crack width at top surface is likely to be excessive, the reinforcement should still have adequate protection from the top as the crack width will reduce as it approaches the reinforcement level.
However, as stated by others, lack of adequate cover to the bottom surface does compromise durability and therefore reduce its life.
The reduced life may be acceptable if the Contractor accepts reduced or no payment for the affected concrete. Replacement can then be undertaken when required.

John

 

[apetr26542]

You may want to check out ACI 302.1 R - 96 Guide for Concrete floor and Slab Construciton. A generally helpfull publication.