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SOG concrete work failing!
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Source：Internet Author：Unknow Pubdate：2010-02-19
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s0hanc01 (Geotechnical)
13 Nov 09 9:13
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. 字串3
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.
dik (Structural)
13 Nov 09 9:17
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. 字串5
How thick is the slab; it might be underdesigned for the applied loadings.
Dik
s0hanc01 (Geotechnical)
13 Nov 09 9:41
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.
bimr (Civil/Environme)
13 Nov 09 10:57
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.
Ron (Structural)
13 Nov 09 11:07
字串4
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.
s0hanc01 (Geotechnical)
13 Nov 09 11:57
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. 字串6
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.
dik (Structural)
13 Nov 09 12:02
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. 字串7
Any reinforcing?
Dik
s0hanc01 (Geotechnical)
13 Nov 09 12:12
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.
cvg (Civil/Environme)
13 Nov 09 12:16
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.
字串3
dik (Structural)
13 Nov 09 13:22
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
s0hanc01 (Geotechnical)
13 Nov 09 14:13
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.
Ron (Structural)
13 Nov 09 16:22
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.
字串8
cvg (Civil/Environme)
13 Nov 09 16:32
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
Ron (Structural)
13 Nov 09 17:52
cvg...exactly!
hokie66 (Structural)
13 Nov 09 18:38
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. 字串4
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.
msquared48 (Structural)
15 Nov 09 1:52
字串6
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
hokie66 (Structural)
15 Nov 09 1:59
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.
rowingengineer (Structural)
15 Nov 09 2:51
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
SirAl (Geotechnical)
15 Nov 09 3:01
字串7
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.
Ron (Structural)
15 Nov 09 7:27
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.
字串3
dik (Structural)
15 Nov 09 10:29
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
s0hanc01 (Geotechnical)
15 Nov 09 18:00
[IMG]http://i10.photobucket.com/albums/a101/skeetman2000/DSCN0029.jpg[/IMG] 字串6
[IMG]http://i10.photobucket.com/albums/a101/skeetman2000/DSCN0027.jpg[/IMG]
[IMG]http://i10.photobucket.com/albums/a101/skeetman2000/DSCN0028.jpg[/IMG]
[IMG]http://i10.photobucket.com/albums/a101/skeetman2000/DSCN0026.jpg[/IMG]
[IMG]http://i10.photobucket.com/albums/a101/skeetman2000/DSCN0025.jpg[/IMG]
hokie66 (Structural)
16 Nov 09 3:52
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.
dik (Structural)
16 Nov 09 9:08
Do you have a footprint/outline/layout of the slab with grade shots?
Dik
字串8
s0hanc01 (Geotechnical)
16 Nov 09 9:16
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.
s0hanc01 (Geotechnical)
16 Nov 09 16:58
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.
Ron (Structural)
16 Nov 09 18:29
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: 字串8
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...
- http://files.engineering.com/getfile.aspx?folder=50e3d559-b6d1-4445-8d29-3a
dik (Structural)
16 Nov 09 20:23
Attached is a pretty good *.pdf file... don't know where I gleaned it from. I also have the AASHTO excel spreadsheets.
Dik - http://files.engineering.com/getfile.aspx?folder=e7ed1009-c5ed-42ea-8487-12
Ron (Structural)
17 Nov 09 5:48
dik..can you post the spreadsheets? I have two spreadsheets as well. Would like to compare.
字串2
dik (Structural)
17 Nov 09 9:26
Yes... later today... will post a third one that deals with rigid airport pavements...
Dik
dik (Structural)
17 Nov 09 21:07
SS#1 - http://files.engineering.com/getfile.aspx?folder=a0cf6cc7-2566-4980-82ab-69
dik (Structural)
17 Nov 09 21:08
SS#2 - http://files.engineering.com/getfile.aspx?folder=4e8bd93f-bf38-43dc-809c-cb
dik (Structural)
17 Nov 09 21:09
SS#3 - http://files.engineering.com/getfile.aspx?folder=07941167-dec1-4626-8d1a-67
Ron (Structural)
18 Nov 09 5:53
字串6
Thanks, dik...I'll see how they compare.
dik (Structural)
18 Nov 09 13:37
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
s0hanc01 (Geotechnical)
18 Nov 09 15:03
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!
fattdad (Geotechnical)
23 Nov 09 11:01
字串3
(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. 字串3
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!
s0hanc01 (Geotechnical)
16 Dec 09 9:00
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
cvg (Civil/Environme)
16 Dec 09 9:38
字串8
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.
Ron (Structural)
16 Dec 09 12:03
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. 字串4
Lastly, a 117 kip axle load is going to require more than a 7-inch slab thickness.
roncity (Civil/Environme)
16 Dec 09 12:59
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.
cntw1953 (Civil/Environme)
16 Dec 09 13:22
Is this site subjected to flood? The mud seems spreaded in quite large area, or just carried around by wheel traffic.
s0hanc01 (Geotechnical)
16 Dec 09 13:31
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. 字串3
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.
字串7
cvg (Civil/Environme)
16 Dec 09 13:38
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
s0hanc01 (Geotechnical)
16 Dec 09 13:39
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.
dik (Structural)
16 Dec 09 13:49
字串1
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
civilman72 (Civil/Environme)
16 Dec 09 17:37
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.
字串1
Ron (Structural)
16 Dec 09 18:00
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.
dik (Structural)
16 Dec 09 22:07
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
diggerman (Civil/Environme)
28 Dec 09 13:57
OK...No brainer I think you nailed it from the begging. We're getting out of the point. Why does concrete crack? 字串2
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!
s0hanc01 (Geotechnical)
25 Jan 10 15:42
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.
hokie66 (Structural)
25 Jan 10 16:10
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. 字串3
2) Concrete does not crack during curing because it is too thick. See 1)
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