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]

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

 

[s0hanc01]

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]

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]

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]

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.

 

[dik]

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

 

[s0hanc01]

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]

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.

 

[dik]

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]

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]

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.

 

[cvg]

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]

cvg...exactly!

 

[hokie66]

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.

 

[msquared48]

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]

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]

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]

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]

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.