SOG concrete work failing!
SOG concrete work failing!
(OP)
Background: The site is an industrial site in Houston, TX used to store containers, large compressors, i.e. heavy items for offshore piping services. Semi's & a huge RT fork lift are the main loads in concerned. There is a main drain for water.
Problem: I did not see the previous installment nor do I have access to drawings. All I can tell is that the concrete is in about 20'x20' sections (between expansion joints). Where four sections meet (in several locations about 1300 sq ft) the concrete has cracked and settled in some cases close to 4 inches. Not all the concrete was poured together..this is why only some concrete has failed.
I think this is do to ponding water seeping into the backfill through expansion joints, then the truck drives over and causes a build up of pore pressures, which in turn cracks the concrete.
To prevent this I have a couple ideas:
Repour concrete so that no ponding water occurs.
Backfill possibly was done wrong? 2' DGA compacted and compaction test to confirm.
Slab Design - not enough rebar, strength, dowels
Put in more drains
Finally what about caulking joints with a waterproof caulk so that water can't get below concrete. Or am I missing the point, Does the water come from below. Hard to say without knowing depth to water table.
I think the soil i s Beaumont clay: I obtained this from websoilsurvey.
Problem: I did not see the previous installment nor do I have access to drawings. All I can tell is that the concrete is in about 20'x20' sections (between expansion joints). Where four sections meet (in several locations about 1300 sq ft) the concrete has cracked and settled in some cases close to 4 inches. Not all the concrete was poured together..this is why only some concrete has failed.
I think this is do to ponding water seeping into the backfill through expansion joints, then the truck drives over and causes a build up of pore pressures, which in turn cracks the concrete.
To prevent this I have a couple ideas:
Repour concrete so that no ponding water occurs.
Backfill possibly was done wrong? 2' DGA compacted and compaction test to confirm.
Slab Design - not enough rebar, strength, dowels
Put in more drains
Finally what about caulking joints with a waterproof caulk so that water can't get below concrete. Or am I missing the point, Does the water come from below. Hard to say without knowing depth to water table.
I think the soil i s Beaumont clay: I obtained this from websoilsurvey.





RE: SOG concrete work failing!
Jointing is a problem with water; if left open you can get something called 'pumping' in these locales. The saturated soil has a reduced shear capacity, hence, a reduced bearing capacity.
How thick is the slab; it might be underdesigned for the applied loadings.
Dik
RE: SOG concrete work failing!
The slab looks to be about 8 inches?
Between expansion joints, shouldn't there be dowels or a key way. It seems the concrete wasn't dowled or the dowels weren't epoxied into exsiting concrete.
RE: SOG concrete work failing!
One would think that this is a common problem in Houston where the ground surface has been dropping for years due to ground water extraction.
You need to do a geotechnical investigation.
RE: SOG concrete work failing!
Look to see if material from the subgrade is being pumped up through the joint. If it is not, then you might simply have a poor subgrade.
You might be putting heavier loads and higher repetitions of load than the slab was designed for. It's a concrete pavement, not just a slab on grade.
RE: SOG concrete work failing!
It looks as if the subgrade is being pumped up...it hasn't rained in 10 days and all the spots where this is occuring is muddy and wet. It appears that shrinkage has occured.
Yeah that is my guess. The loads are extremely large with the heavy duty forklift.
I have pictures to post later as well.
As far as the geotech report goes..personally I think a test pit using DCP would be sufficient as oposed to borings. Basically trying to find the bearing capacity.
RE: SOG concrete work failing!
Now serious... not the concrete, but sometimes with large flat concrete surfaces, the water doesn't run into the soil and it drys underneath the slab... and shrinks if highly plastic. The uneven shrinkage (or at least the interior parts) can cause settlement of the interior portion.
Depending on the loading and concrete strength, 8" may be a tad shy... Heavy wheel loads can generate a fair tension and it depends on the location of the wheel loads and what treatment was made at joints. In some cases, without properly restrained joints, an 8" slab can behave like a 6" one.
Any reinforcing?
Dik
RE: SOG concrete work failing!
Any particular manufacturers for PEVB. I'm not familiar with this.
RE: SOG concrete work failing!
RE: SOG concrete work failing!
Dik
RE: SOG concrete work failing!
Also thanks for everyone's help. It is greatly appreciated.
RE: SOG concrete work failing!
RE: SOG concrete work failing!
agreed and by doweling the edges and sealing the cracks which should have been done in the first place, no water intrusion, no pumping, no loss of material and no cracking will occur - all assuming that the slab and base are sufficiently strong for the loads
RE: SOG concrete work failing!
RE: SOG concrete work failing!
Joint design is the most important consideration for the slab itself. I know that US practice is to use closely spaced joints, but in my experience, joints are the problem, so the fewer the better. 20' spacings don't bother me.
The corners tend to initially crack due to differential shrinkage, especially if the dowels are placed too close to the corners. Then wheel loading completely separates the corner, and depressions occur as the OP described. In Australia, we use square dowels with a compressible material on the sides to prevent the shrinkage cracking from starting. Diamond shaped dowels are another option. The dowels have to be well supported normal to the joints.
Sealing of joints is rarely done well if left to the contractor to supervise. Joint shape, cleanliness, and priming are the steps which are often deficient.
RE: SOG concrete work failing!
Mike McCann
MMC Engineering
RE: SOG concrete work failing!
RE: SOG concrete work failing!
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it
RE: SOG concrete work failing!
RE: SOG concrete work failing!
I'm not convinced that the subgrade is the problem. I do know that if you don't design and construct the pavement section properly, you'll have problems like this. A lot of attention must be paid to thickness control (assuming you have the right design thickness for the loading), joint spacing (I disagree with hokie66 on joint spacing...they should not be 20 feet apart unless the slab is 8 inches or more in thickness), joint load transfer (to dowel or not), joint sealant design, quality and strength of the concrete (largest aggregate possible, w/c at or below 0.50, and strength above 4000 psi), and proper maintenance.
RE: SOG concrete work failing!
Time to core the slab and take some profile/soil samples to determine the construction.
nudder $.02...
Dik
RE: SOG concrete work failing!
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RE: SOG concrete work failing!
RE: SOG concrete work failing!
Dik
RE: SOG concrete work failing!
My thought is to tie in at least one drain if not two. I don't think that the slope was done correctly.
RE: SOG concrete work failing!
RE: SOG concrete work failing!
There are two design guides that are used often for concrete pavement design:
1. AASHTO Pavement Design Manual
2. PCA Concrete Pavement Design
You can use an elastic layer design procedure if you are familiar with that approach, though the other two are a bit easier to follow.
I have attached a paper on Industrial Pavement Design considerations that might be helpful...
RE: SOG concrete work failing!
Dik
RE: SOG concrete work failing!
RE: SOG concrete work failing!
Dik
RE: SOG concrete work failing!
RE: SOG concrete work failing!
RE: SOG concrete work failing!
RE: SOG concrete work failing!
RE: SOG concrete work failing!
Dik
RE: SOG concrete work failing!
The company isn't sure they want to pay for a Geotech report. Not sure how to design without knowing subsurface condition.
Also wanted to thank everyone for the generous help!
RE: SOG concrete work failing!
1) All concrete cracks.
2) Soaked CBR values correlate to subgrade modulus, which is often the basis for concrete pavement design. THE DESIGN VALUE IS A SOAKED VALUE!
3) Control joints are used to force concrete cracks at regular intervals and keep cracks from becoming too wide. This in turn preserves aggregate interlock. Dowels provide even better protection. Your slab does not have stress transfer across these joints. As a result, some caulk/foam/stuff will not help. There is a product called the 10-minute crack mender, but it won't give you the load transfer of aggregate interlock or dowels.
4) Your subgrade has failed. Excessive cracking and loss of aggregate interlock has allowed movement during times of saturation, which has rendered the subgrade below these joints to mud. There's no recovery without concrete removal.
5) There's a possibility that the original subgrade preparation was flawed. It's possible that the earthwork testing referenced an incorrect proctor or allowed a soil type that was not simliar to the soil assumed by the pavement design. There is also a possibilty that the concrete design was done by guesswork.
These can be interesting problems and the company that I work for has quite a little cottage business looking after failed concrete slabs.
f-d
¡papá gordo ain't no madre flaca!
RE: SOG concrete work failing!
For Failing Concrete
7" single matt, 4000 PSI 5.5 Sack, #4 12" OCEW, #6 dowels 18" OC with epoxy
RE: SOG concrete work failing!
RE: SOG concrete work failing!
You note #6 dowels with epoxy??? That's not the way dowels should be done. First, they should be smooth and hardened steel. Second, they should only be bonded to one side of the slab, not both or you'll get cracks at the end of the dowels. They should be 18 to 24 inches long and should be spaced at 12 to 18 inches. Grease one side of the dowel and put a small piece of 1/2" thick foam on the end of the greased section to allow for expansion.
Lastly, a 117 kip axle load is going to require more than a 7-inch slab thickness.
RE: SOG concrete work failing!
RE: SOG concrete work failing!
RE: SOG concrete work failing!
Ron: First of all yes 7" seems way to small, that is just what one of the contractors prepared their bid with.
It seems that all the contractors are using rebar and dowels. But from what everyone on here have said, ditch the rebar and use dowels only? I think the epoxy was for the existing concrete, but dowels should only be retained in the new concrete? and left "floating" in the existing concrete.
This is my newest quote:
I would, probably, recommend saw cutting through the existing concrete ( as long as the surrounding concrete is in good shape, otherwise saw cut 3" and save the money), remove 18" of the sub-grade, place and compact 8" of a granular base ( limestone, crushed concrete, or calcium sulfate), 10" 6 sack (4000 psi), 2 mats - 1/2" rebar @ 12" ocew, Drill/Set 1/2" x 8" rebar dowels @ 12" cc around perimeter (or, if present, use existing rebar as dowels), Match the existing grades, trowel w/ broom finish.
RE: SOG concrete work failing!
Assuming clay subgrade with a k value of 100 and your very high axle loads, you probably need a pavement thickness closer to 12 inches thick
RE: SOG concrete work failing!
RE: SOG concrete work failing!
Dik
RE: SOG concrete work failing!
What's that saying about the definition of the word "insanity?" Doing the same thing over and over and expecting different results... or something like that.
RE: SOG concrete work failing!
1/2" dia.x8" dowels are insufficient for the noted loads. Rebar should not be used for dowels. The dowels are for load transfer across the joint...they are not for slab reinforcement continuity.
RE: SOG concrete work failing!
Dik
RE: SOG concrete work failing!
A: Too much tensile strength and not enough re-bar/reinforcement. Bingo. We can all pick this apart and disect it into a billion parts...i.e soil,joints,pumping etc. The problem is the cracking. The fix...replace concrete and redesign with a thicker slab, heavilly reinforced. Use a good high flex concrete mix design similar to bridge deck quality. DONE!
RE: SOG concrete work failing!
diggerman - if the concrete is too thick, cracking will occur during curing.
RE: SOG concrete work failing!
1) Concrete does not crack because of too much tensile strength. It cracks because it is restrained.
2) Concrete does not crack during curing because it is too thick. See 1)
RE: SOG concrete work failing!
8.5" concrete reinforced, 6" crushed limestone with little or no fines, 6" stabilized lime or cement stabilized sand.
RE: SOG concrete work failing!
I'm not likin' the sounds coming from this geotechnical survey. . .
f-d
¡papá gordo ain't no madre flaca!
RE: SOG concrete work failing!
K = Effective Modulus of subgrade reaction for base = 600 pci
The soil is coletche.
Another alternative for 6% lime is cement stabilized sand.
RE: SOG concrete work failing!
RE: SOG concrete work failing!
http://en.wikipedia.org/wiki/Caliche_(mineral)
The term is not a particularly good descriptor of the engineering properties of the soil. Caliche is a form of soil that is "cemented" together by water and mineral action. Degree of cementation is related to the soil strength. Highly cemented it may be treated as rock. Lightly cemented is just strong soil. Thickness and variability of the cementation affects engineering properties.
RE: SOG concrete work failing!
The contractor is now placing rebar. Again the slab is 8.5" - and the rebar is single mat #4 12" ocew. I have calculated the depth to rebar (From top of concrete down) and got 3.125". I used the article "Concrete Floor Slabs on Grade Subjected to Heavy Loads". Does this sound right?
I know I will be lucky if the rebar is not at the bottom of the concrete after the contractor walks all over it.
Any suggestions?
RE: SOG concrete work failing!
1)If my statements are not true...then why does concrete require rebar?
2) Concrete does crack while curing...its called SHRINKAGE Cracking. Actually, alot of the cracking will ocur during the curing process due to stresses and strains. It is very important that these characteristics are known. It's concrete 101.
Remember, concrete can handle high compressive loads, not very good with tensile/flexural stresses hence the need for heavy reinforcement. See 2.
RE: SOG concrete work failing!
Surely concrete cracks during curing due to stress on the concrete. But with no load, there is no stress. What loading exists? The load comes from the restraint. If you could instantaneously cast a concrete slab of infinite size on a frictionless surface, no cracking would occur, as the volume would change without stress.
RE: SOG concrete work failing!
I agree. I just want to highlight that stresses (although not much) do exist within a freshly poured slab/structure. :)
DiggDug
"Things change but nothing changes"
RE: SOG concrete work failing!
Most likely comes with a high water table. Its impossible stuff to work with. Hardly ever dries out, but if it does it cracks into coffee saucer size plates and sometimes curls up almost into a coffee cup. If 4 wheels hit it at once, you'll all of them spin forever.
Good luck.
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