SOG concrete work failing! 6

[s0hanc01]

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.

 

[dik]

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]

 

[hokie66]

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]

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

Dik

 

[s0hanc01]

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]

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]

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...

 

[dik]

Attached is a pretty good *.pdf file... don't know where I gleaned it from. I also have the AASHTO excel spreadsheets.

Dik

 

[Ron]

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

 

[dik]

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

Dik

 

[dik]

SS#1

 

[dik]

SS#2

 

[dik]

SS#3

 

[Ron]

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

 

[dik]

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]

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]

(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!

 

[s0hanc01]

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]

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]

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.