Depth of Excavation ?? 1

[Nawabfarhan]

Dear All,

Background :

Around 10000 sqm area is built 30 years ago to transfer/carry ship-lift loads from the piers to the dry docks for ship repairs. The cross section of the base is as below,

from bottom: 1. Compacted Subgrade to 95% of Max Dry Density (MDD).
2. Compacted to 100% MDD sub base layers (2-3 layers min of 150mm thickness each)
3. Lean concrete 20 MPa compressive strength (50mm thick)
4. Insitu reinforced concrete 40-50 MPa compressive strength laid in panels (5mx10m) with construction joints.
5. There are U type carbon steel bolts embedded into the 300mm concrete with the top protruding threaded portion of about 100mm.
6. Above the concrete surface, grout will be laid 30-40mm thick to provide base to install 136 RE Tee Rails. These rails will carry the trolley above which the ships will be rested. The rails will be fastened to the grout-concrete base by the embedded U bolts & rail clips.

Problem :

After due course of its service life, the area is now subject to repair/restoration. The rails are to be removed and refurbished. The concrete is to be demolished in two ways, 1) only 150mm thick from top to be demolished and re-skinned with new concrete. & if the deterioration is high then 2) the entire depth of 300mm is to be replaced with new concrete panels.

Now in case 2, we proposed our Clients to also replace the lean concrete and 300mm subbase in 2 layers compacted to 100% MDD to enable sound base for the anticipated heavy loads. Our Clients are proposing only 100mm subbase replacement which is very impractical and may cause settlements due to lesser compacted subgrade thickness.

Please put your comments and suggest the min depth of excavation for a safe concrete base to carry heavy ship-lift loads?

Thanks

NawabFarhan

 

[BigH]

Do you have any evidence that the lean concrete and 300 mm subbase is damaged? These are such thin layers - they aren't really adding anything to the structural capacity of your mat and I don't think that raising 95% to 100% will do anything "significant". Why not concentrate on the mat? BTW - have you not done any cores to determine the anticipated condition of the slab?

 

[SlideRuleEra]

Too add to BigH's comments, the 50mm lean concrete is almost certainly a "mud mat" that was used only as an aid for construction, not strength. A mud mat protects the compacted base from weather and provides a firm working surface to support "chairs" that hold rebar in place until concrete placement.

Assuming that the cores BigH has recommended prove the existing slab must be replaced, suggest that you discard Option 1 of removing the upper 150mm of the slab. Doing so is a recipe for trouble and expense for a number of reasons (we can get into why this is so, if needed).

IMHO, Option 2, complete removal of the existing slab and the lean concrete under it is much better.

Perhaps I'm missing something... why would any of the base material arbitrarily need to be replaced? Unless contaminated, the existing material can most likely be recompacted even after being disturbed during slab removal.

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[Nawabfarhan]

The problem is not much the concrete base than the embedded U bolts. These bolts are ASTM A307 Grade A quality under use for the last 30 years. During this time most of these U bolts under static & dynamic lateral loads have sheared from the top. So we cant use them any more leading us to ripping-off the whole slab of 300mm for new bolts installation.

We are breaking the concrete with pockline / heavy concrete breakers, so how is it possible not to disturb the compacted subbase/subgrade?? It will definitely be disturbed hence requiring replacement of a min certain depth which is what we want to know.?

Regarding the repair of the top 150mm thickness we want to know what are the vulnerabilities? We are being proposed to use water soaking for bonding between old and new concretes..

Thanks in advnace

 

[SlideRuleEra]

Nawabfarhan - Ok, here are some reasons that I consider taking off the the upper 150mm of a 300mm thick slab to be a bad idea:

1. There is no good way to do this. As you have discovered heavy pavement breakers is about the only timely process. Pavement breakers are not precision instruments. While the upper part of the slab is being removed, it is VERY likely that the lower part of the slab will be cracked. Once cracked, what to do then? Replace that complete section? Ignore the cracks and get reflective cracking in the new concrete?

2. A 300mm heavy duty slab may (or perhaps should) have doweled joints. If the upper 150mm is removed how are the joints addressed?

3. You mention that the slab is reinforced, probably has (or should have) two rebar mats, and upper and a lower. If there is an existing upper mat, then removing the upper 150mm of concrete with pavement beakers gets more tedious.

4. For the slab to perform as intended, the bond between the existing lower 150mm of concrete and the upper 150mm of replacement concrete needs to be very good... everywhere. Doable, but will take much care an plenty of inspection.

5. If the anchor bolts are the problem, having them embedded in only the upper 150mm (replacement) concrete is not going to help. Without adequate embeddment, the high strength bolts may not shear... they will just pull out of the concrete.

Those are a few of my concerns, think about it, there are probably more. I'm not making this up, we did this (removed the upper portion of some small slabs) in 1985. Over time, it was a disaster.

As for the subbase... sure it will be disturbed. As stated above, unless contaminated with mud, etc., I don't see a problem. Regrade and recompact. Why would new material be any better than existing material? Frankly the existing subbase may be ideal - it has been under load, undisturbed, for 30 years - IMHO, leave it alone, as much as possible.

Even if you arbitrarily choose to remove a certain amount of base material, the subgrade UNDER the specified removal amount will then be disturbed during the removal process. Then what? Remove more?

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[TDAA]

If you can come up in elevation a bit, what about crack and seat the existing slab, base to level, as needed, and build a new slab on top?

That would lessen the disturbance and reduce the workload and waste.