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SOG concrete work failing!
6

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.

RE: SOG concrete work failing!

I'm not familiar with the soil, but if it is highly plastic, then the slab could have dessicated and caused shrinkage.

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!

(OP)
Are you refering to the soil taking moisture from the concrete?  Wouldn't this only happen if the concrete was placed directly on this clay.

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!

2
First, the joints are control joints, not expansion joints.  Secondly, they are too far apart. This causes joints to open more and causes poor load transfer capability, thus the "pumping" you described is more prominent.

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!

(OP)
Yes sorry control joints with foam type product.

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!

Nope... that's why I use PEVB between the slab and the soil <G>

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!

(OP)
Yeah I don't know the previous reinforcing but I plan to design with reinforcing.  

Any particular manufacturers for PEVB.  I'm not familiar with this.   

RE: SOG concrete work failing!

water entering at the joints will cause expansion of the clay subgrade resulting in lifting at the corners. the interior areas of the slab will be dryer and will tend to contract resulting in lowering. So your pavement depressions at the corners seem to defy logic. However, wet soil has lower strength so I would surmise that you have bearing failure concentrated at the wet areas - corners and edges. This is exacerbated by the differential lifting and lowering creating tensile stresses in the slab, resulting in cracking. The slabs should be doweled and the joints sealed. Thick concrete pavements are not usually reinforced except for the joints.  

RE: SOG concrete work failing!

I can provide a Canadian spec for it... but, generally 6 mil PEVB as used for most construction. If exposed to the sunlight for any length of time make sure it has UV blockers (different spec in Canada). Tape or mastic the joints...

Dik

RE: SOG concrete work failing!

(OP)
Can this be used when the water table is high?  Would this cause more problems if the joints weren't properly sealed?

Also thanks for everyone's help.  It is greatly appreciated.

RE: SOG concrete work failing!

cvg..my take from his description is that pumping has caused material loss at the joint intersections, thus losing slab support.  High loads crack those corners back a couple of feet, then because of loss of material and even higher unit loads on the small pieces of concrete, you get deformation.

RE: SOG concrete work failing!

Ron
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!

cvg...exactly!

RE: SOG concrete work failing!

Some good advice has been given.  On clay soils, keeping water out of the clay by adequate surface drainage is critical.  If water gets in, pumping will occur under heavy wheel loads, especially if sand is used under the slab.  The layer directly under the slab needs to be large enough not to come up through the joints with the water.

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!

But to repair it?  How about slab jacking or a low pressure grout to replace any worked material lost at the joint and level the slab edges relative to one another?

Mike McCann
MMC Engineering

RE: SOG concrete work failing!

I tried that once, and there was no joy.  It all looked fine for a while, but the problem reoccurred.  In my opinion, demolition and reconstruction is the answer.

RE: SOG concrete work failing!

Agree with Hokie on this one however I have always wanted to try saw cutting about 1'-2' each side of the joint and in fill with non-shrink concrete and a heap of dowels.  

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!

I think the problem requires a geotechnical assessment and probably some subsurface exploratory work to get a handle on what is going on beneath the slab.

RE: SOG concrete work failing!

I also agree with hokie66.  Slab jacking on a dynamically loaded slab, with heavy, recurring loads will only cause the same problem somewhere else.

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!

s0hanc01 noted that it appeared the slab was 8" thick... In these environs, it is common to spec concrete with a w/c ration of 0.4 max... Settlement of 4" would indicate goetekkie issues (s0hance01 is one of these guys). The type of loading indicates concrete thickness of something in excess of 8" thick for many soils. I've used slab jacking with success, but if the overall system is underdesigned, jacking will be of no long term benefit. If it is repairable, then 8" is a little thin for post installing dowels. Roadworks contractors have a 'dowel drilling' machine that they often use for concrete replacement.

Time to core the slab and take some profile/soil samples to determine the construction.

nudder $.02...

Dik
 

RE: SOG concrete work failing!

Some of those photos indicate that there have been previous attempts to repair the pavement, and those repairs have failed.  Pavements don't last forever.

RE: SOG concrete work failing!

Do you have a footprint/outline/layout of the slab with grade shots?

Dik

RE: SOG concrete work failing!

(OP)
I don't, but working on it, and right now that is the main concern I have...I'm not sure even if the concrete is level with the surrounding concrete it will drain properly.  

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!

(OP)
Lastly does anyone have any good design references for designing the concrete.  I'm a bit lost for reinforcing/non-reinforced, slab thickness, dowel placement.

RE: SOG concrete work failing!

Unless you have a specific subgrade problem, the concrete generally does not need to be reinforced for pavement loads.  

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..can you post the spreadsheets?  I have two spreadsheets as well.  Would like to compare.

RE: SOG concrete work failing!

Yes... later today... will post a third one that deals with rigid airport pavements...

Dik

RE: SOG concrete work failing!

Thanks, dik...I'll see how they compare.

RE: SOG concrete work failing!

Let me know how you fare... I picked them up... just in case and haven't had a chance to use or vet them. Too often, I've encountered stuff on the net and figured I'd get them later only to find that they had disappeared... bit of a packrat.

Dik

RE: SOG concrete work failing!

(OP)
Well after a few days I figured out my critical load - Hyster 550 forklift.  The critical axle load is roughly 117 Kips.  This is why pumping is occuring.  

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!

(Sorry, I haven't read everything, but so far cvg and ron are on the right track.)

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!

(OP)
Sorry I got pulled off this project at work for a while, but this is what the contractor came up with.

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!

what about the subgrade / base?  Your original problem was probably directly related to a poor base and only partially to lack of caulking and dowels. You need to address that first. That will require investigation of the subgrade sufficient to estimate the CBR and then design the necessary thickness of good aggregate base to be placed under your new slab repair areas.

RE: SOG concrete work failing!

s0hanc01...If you have a competent subgrade, you don't need the rebar, just dowels.  If you have a non-competent subgrade, the rebar won't save you for the section you've noted.

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!

Might want to check underground utilities...a slow water lead..or a sewer leak , may be adding the water that is affecting the base. And check to see where the roof water drains are going.

RE: SOG concrete work failing!

Is this site subjected to flood? The mud seems spreaded in quite large area, or just carried around by wheel traffic.

RE: SOG concrete work failing!

(OP)
cvg: I plan to excavate at least 18" and backfill compacting 6" lifts.  Silly enough the company doesn't seem to want to pay for a new geotech survey.  Basically they just want a free recommendation from a geotech.  I would think a geotech survey, however, would benefit in the long run.  

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!

Pictures show that the wet area and failed sections do seem to be under what appears to be a repaired section possibly over a trench. Is there a water or sewer line in that trench? If so, was the water or sewer constructed after the original paving?  Could the pipe be leaking? could the backfill over the pipe have been poorly compacted?

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!

(OP)
yeah the area holds water due to the concrete that has settled, but we are also looking to put another drain in to help this problem.  There is only one drain for about a football sized lot.

RE: SOG concrete work failing!

Make sure the client is aware that no geotechnical work is 'his' call.  With the loads involved, a serious design for this slab has to be undertaken, else, you will end up with the same result as your photo.

Dik

RE: SOG concrete work failing!

With the existing issues, and the high loads, your client needs a little hand-holding in money management.  His initial interest in saving money by not hiring a gootechnical engineer the first time is why he is in this predicament.  Encourage him to spend an extra few thousand dollars (on geotechnical engineers) to ensure his significant investment (of at least $100k?) performs correctly for the long-term.

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!

Quote:

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)

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!

I would not use 1/2" dia bars for dowels of any kind... prefer 1" dia and never less than 3/4" dia. Smooth bars and if they are sheared, make sure the sheared end in the sliding part does not have a 'burr' from shearing.

Dik

RE: SOG concrete work failing!

OK...No brainer I think you nailed it from the begging.  We're getting out of the point.  Why does concrete crack?
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!

(OP)
Sorry been away for a while dealing with different problems.  Update - a geotech firm is hopefully coming out tomorrow to take a look.  I want this work guaranteed!

diggerman - if the concrete is too thick, cracking will occur during curing.   

RE: SOG concrete work failing!

A couple of misstatements in the last two posts:

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!

(OP)
Alright - Geotech Survey has been completed.  The soil was in good shape, k = 600 pci.

8.5" concrete reinforced, 6" crushed limestone with little or no fines, 6" stabilized lime or cement stabilized sand.

RE: SOG concrete work failing!

What field worok did the geotech perform and  how did the geotech determine k=600 pci (that's a high value!)?  Just for reference, k=600 pci would correlate to a CBR of 60! Now that may be appropriate for the dense- or open-graded aggregate, but that is awefully high for natural soils.

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!

(OP)
Two borings with shelby tube;

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!

So are you proposing a total pavement thickness of 20.5" over the natural soil?  What is coletche?  What is proposed to be stabilized by lime (this is normally for stabilizing clay soils).

RE: SOG concrete work failing!

caliche not coletche.

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!

(OP)
Alright - Cement stabilized sand was used and passed the compaction and compressive strength test.  The sub-base also passed compaction.  

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!

Hoakie-

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!

diggerman,

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!

hokie66-

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!

"Beaumont Clay", known locally as slick grey dog-s*** taffy.
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.

**********************
"Being GREEN isn't easy" ..Kermit
http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

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