Concrete Overlay Under Rail Track 1

[farhanjnawab]

Dear All,

Around 10,000 sqm of tansfer pit area contains 9000 meters of rails laid to carry ships lifted over trolley wheels in a Naval Dock Yard. After 30 years of service life without any maintenance the area is left deteriorated and subject to refurbishment. The specs call for chipping 150mm out of a 300mm concrete base slab, dowelling, replacing the top rebars and filling self levelling concrete. The rails will be removed during these works and the embedded U bolts (depth = 250mm inside concrete) to be kept undisturbed. Upon relaying the new concrete the rails will be re-installed and ships to move again.
Now, is it structurally ok to chip half concrete and replace with a more good quality concrete than the base concrete. sounds like we are building a new storey above a weak foundation. Your expert comments please...

 

[cntw1953]

Take core samples to determine the depth of deterioration, and lamination, if any. Otherwise, keep an eye on repair details and good worksmanship. Make sure the exposed concrete are sound, cracks properly sealed, surface roughened, clean of debris, proper bond preparation, watering....Suggest to make a check list and follow through.

 

[Ron]

Agree with cntw1953. You might also try to determine the cause of present failure. If mostly abrasion and surface deterioration, then repair method is appropriate. If flexural failure of the concrete, then you might want to think again.

cntw's suggestion for a checklist is a good one.

 

[cntw1953]

Ron:

Thanks for your kind comments. I guess it is more likely casused by long term exposure to moist/humid environment. One area he might want to pay close attention to is the levelness of the rail tracks and concrete surface, looking for indication of area with potentially weakened subgrade due to water penetration.

 

[farhanjnawab]

Meaning we need to dig the whole slab (300mm), recompact the subgrade (if possible replace a single layer of base course, compact to 100% MDD and lay new concrete to required levels?? Is this a more durable solution than the repair as specified in our Contract?? some light please...

 

[Ron]

Full depth reconstruction is almost always a better solution than a partial repair. This would be no exception. The interface between old and new concrete will always be susceptible to delamination/debonding.

You probably don't need a base course (actually called a subbase under concrete), unless you have a subgrade drainage issue that needs correction. In that case I would use a free draining coarse sand or combination of sand/gravel.

 

[beton1]

Costs vs. benefit. If you are already removing 1/2 the concrete, what is the cost for full replacement? There seem to be no restrictions, if rails are removed anyhow. Ron's point about potential for delaminating/debonding of the new over old is a good and valid point. Cntw1953 is pointing you in the right direction, if you decide with partial replacement. Perhaps the core samples will make your decision easier.

 

[cvg]

your "core" samples should probably extend some distance into the subgrade so you can evaluate the suitability of this foundation layer to support your loads. It is probably also advisable to do a structural analysis of the rail / slab / foundation to determine if the existing design was adequate or if it needs to be improved. In addition you may want to check to make sure you have good drainage as that might also be contributing to the poor condition of the structure. Beton has a good point, the extra cost to do full remove and replace may be minimal and don't forget about the surface preparation and potential need for bonding agent that may be necessary for an overlay.

 

[concretemasonry]

Since this is a Naval yard, it is probably in an area with a high and variable water table.

Check the water level (and variations) and make sure it is similar to what was assumed in the original plans. Coastal areas have experienced settlement.

After 30 years of service with no maintenance, the project is probably near the end of its life and anything near the surface is just a temporary repair.

 

[dicksewerrat]

A complete remove and replace will probably be cheaper and faster than chipping away the 6 inches of concrete, shoveling into wheel barrows and removing. Making sure to not disturb all the rail connectors left in place. I'd look real hard at that scenario along with the specified repair.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com