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.

 

[roncity]

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]

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

 

[s0hanc01]

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.

 

[cvg]

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]

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]

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]

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.

 

[Ron]

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]

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]

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!

 

[s0hanc01]

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]

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)

 

[s0hanc01]

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.

 

[fattdad]

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!

 

[s0hanc01]

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.

 

[hokie66]

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).

 

[cvg]

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.

 

[s0hanc01]

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?

 

[diggerman]

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.

 

[hokie66]

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.