Concrete Sidewalk

[jenn454]

I am trying to repair a concrete sidewalk which is approximantly 400' long. Currently various sections of the sidewalk are breaking apart and becoming uneven. My orginal plan was to remove exiting sidewalk and soil beneath. Then refill excavated area with compacted sand (95% dry density) and repour a new sidewalk. New Sidewalk would be 4" thick with #4 rebar at 12" O.C. in both directions. New sidewalk would have control joints every 5'.

It has down been proposed that we should instead pour a new 4" thick sidewalk with rebar on top of the existing. I am concerned that this will result in future probelms. Any comments or suggestions.

 

[GeoPaveTraffic]

Do you know why the existing sidewalk is breaking apart and becoming uneven? If you don't know why it failed, then you can't "design" a replacement.

Based on our City's experience possible reasons include: tree roots, sewer lines, other utilities, and freeze/thaw action.

 

[Ron]

Agree with GPT....figure out why it failed first. I WOULD NOT place a new slab over the existing cracked one. Cracks will reflect through and the new one will look like the old one in short order.

Generally no need for rebar. Plain concrete is fine for this application. Most common cause of cracking in sidewalks is the items GPT mentioned plus poor thickness control/poor quality control during construction.

For a thin slab such as this, it is critical that the subgrade be flat and that very little thickness variation occur. Sawcut the joints the same day as placement. Spacing is right for 4-foot width.

 

[jenn454]

Thank you for your input. I believe the existing sidewalk is cracking sue to sinkage and settling of the soil beneath. Location is Southern Louisiana where majority of constructionis built over swamps.

Ron you basicly confirmed my intial concerns that the cracks would resurface in the new concrete pour.

 

[Ron]

jenn454...be careful with your assessment that the cracks are caused by subsidence or settlement. Remember, the slabs have no traffic and the dead load is not that high, so unless there is significant settlement, that shouldn't be the cause. It is likely that you had shrinkage cracks from the first construction, and if you lose the support through subsidence, then you will get what you described.

Again, be sure to specify quality placement processes and controls...that will alleviate many of the problems.

 

[GeoPaveTraffic]

jenn454, you indicate that the area may have settled over time. Is there other evidence of this other than the sidewalk? If not I would look elsewhere for the problem. As Ron pointed out it could have simply been poor intitial construction. How long was the sidewalk in place before the problems started? If it was more than a year, I would consider it unlikely that it was poor construction.

Another thing that came to mind, is there any chance that the damage was caused by someone driving on this section of the sidewalk? I know you don't normally drive on sidewalks, however, I have seen it many times. People are just in too much of a hurry.

 

[jenn454]

Sidewalk cross a large expanse of grass there is no other construction near by to compare settlment to. The sidewalk has been in place for several years now. I am not sure when the problems intially occured, it was just recently brought to my attention although I believe the sidewalk has been in this state for sometime.

The sidewalk is in fact driven over by large tractors when they cut the grass field surronding the sidewalk. This is why the rebar will be installed in the new sidewalk.

A second question: If contractors where to install expansion joints instead of control joints what is the typical spacing for a 4' wide sidewalk.

 

[Ron]

jenn454...don't confuse expansion joints and control joints. They serve two distinctly different purposes. You can get by without expansion joints, but you can't get by without control joints. Put the control joints at 4-5 feet on center.

Just because the landscape tractors drive on the concrete doesn't mean it needs to be reinforced. If the subgrade is appropriately stable and compacted, those loads shouldn't be much of a problem for a 4-inch thick slab as the contact stresses are low and the total load is not that high either.

 

[sc]

From an Aussie perspective, what diameter is #4 rebar?

If it is being driven over by a "tractor" a minimum thickness of 200mm with F92 top and bottom would be approppriate.

F92 = deformed bar mesh with 9mm diameter bars at 200mm spacing.

Also soils are not just soils, they are sand (fairly stable) silt (slightly reactive) and clays (will react to moisture swell shirnk, etc). This will effect the design.

Also always dowel joints where movement is expected as it keep the footpath (sidewalk) together when it moves.

regards

sc

 

[GeoPaveTraffic]

sc,

A #4 rebar in the US is 4/8 of an inch or 0.5 inches in diameter.

 

[fortmac]

Agree not to pour on top of existing sidewalk. This will only become a short term cosmetic repair and you will still be left with voids from existing sidewalk that will eventually transfer to your topping.
Remove damaged sidewalk, investigate reason for heaving or depression. Remove reason, fill material, and recompact with new granular fill.

 

[chollan2]

Something you should check out is water dranange.

I see this as a significant cause of damage.
In time, the water softens the soil causing erosion and the softenting of concrete leading to cracks.

Other than that, it is pipes or roots beneath the soil.

Regards,

 

[roncity]

another cause of cracking could be the tractors ( or other vehicles ) driving over the sidewalk when it was new. These cracks could lead to water infiltration and settlelement . This type of damage could take years to show up.
When you place the new sidewalk, be sure to rope it off and keep traffic off of it for at least 7 days...