Curb cracking next to Slot Drain 1

[jharman]

We designed a slot drain system adjacent to a curb and gutter along an urban street in front of a high school that was installed 3 summers ago. After the first winter we saw substantial cracking in the curb and gutter next to the slot drain. There are many cracks near the pipe joints as well as cracks at other intervals. Some cracks occur within inches of control joints. There are regular contol joints installed at 10' intervals. The City is now requiring that we remove all of the curb and gutter and re-pour it. We want to be sure to address the cause of the cracking so the problem does not occur again.

Here's some facts: The manufacturer recommended that the curb be poured monolithically with concrete around the pipe for support. The City did not agree and required that we pour the pipe support first, then pour the curb and gutter separately on top. We suspect that there is differential expansion and contraction between the concrete encased slot drain and the curb and gutter. The contractor tried to install a stone dust bond break during construction, but the City inspector would not allow it and it had to be removed.

We think that the bond break was a good idea and should be used for the repair. Altough, we are thinking that masonry sand might be better than stone dust for drainage reasons. Has anyone had a similar experience weith a successful solution or can offer any advice? The construciton detail is shown for reference. Thanks for your help.

 

[JedClampett]

Dumb question: Are the curb and gutter reinforced? There's a reference to a standard detail, but I'm not sure that it includes reinforcing.

 

[jharman]

No, there is no reinforcing in the curb. The City's stanards don't call for it, and typically we do not have a problem with them cracking without it.

 

[cvg]

your curb and gutter has partial support on concrete and partial on earth. Add water / ice and that could lead to differential settlement. In addition, you should have anchor bolts to tie the slot into the gutter slab. With adequate subgrade support, additional reinforcement of the curb should not be necessary.

 

[hokie66]

No doubt the cracking is due to restraint by the previously cast concrete. Recasting the curb without effectively breaking the bond will just lead to reoccurence of the cracking. Why is the cracking a problem? Is it just aesthetics? If you were directed to do it this way in spite of advice to the City that their way would lead to cracking, the City should bear the expense of replacement. Have you talked to your lawyer?

 

[jharman]

Hokie66, The City is concerned about concrete spalling and further deterioration over time. Unfortuantely they require a construction bond with a warranty, so they will not release the contractor's bond until they are satisfied. The contractor needs their bonding capacity back and does not want their bonding credit damaaged. There is also salt damage due to a heavy winter follwing the installation and they are blaming that on the contractor too. That is not the contractor's fault, but the City want's to blame the quality of the concrete. The curb is in rough shape for a new installation. There are a lot of other mitigating political issues too that make litigation a very unattractive alternative. The conctractor is willing to replace it, they just want to ensure that it doesn't happen again as the city's warranty period restarts after each repair. It's five year warranty!

Would you recommend a 1" sand layer between the curb and concrete encased pipe as an effective bond breaker?

 

[hokie66]

No, I would use a slip sheet, say two layers of polyethylene. I would make sure the joints are effective...may even cast in hit and miss fashion. Add some reinforcing to control any cracking that still occurs. Cure well. May seem like overkill to a civil contractor, but he doesn't want to have to do it again...does he?

 

[Drumchaser]

Slotted drain assemblies are a pain in the arse for sure. Our prob.s have mostly been associated with alignment.

I would be interested in the QC of the C & G poured. Generally speaking, the curb and gutter prep and placement is usually not on anyones radar until there are problems. While it is routinely the grade line or % slope of gutter, spalling C & G is not that uncommon when constructed incorrectly or allowing loads on it before conc. has attained strength.
Foot traffic around a high school even when school is out might be the culprit. Folks tend to make pours late in the afternoon on hot days....then all go home with no curing or limiting ped. access to freshly poured area.....bicycle tracks in conc. paving/sidewalks comes to mind.

 

[hokie66]

No, Drumchaser, foot traffic doesn't cause concrete to crack once it is hard enough to walk on.

 

[eea]

What was the design strength of lean backfill material? I agree with the manufacturer of slot drain here, it should have been poured monolithically. The City overruled the manufacturer’s guidelines and now wants to nail the Contractor? Most CMP culverts need at least a minimum of 12" to help distribute the load to prevent failure. Sounds like the pipe is flexing from a result of too little cover and yielding backfill to me. I'd almost argue, based on the PDF detail, that the curb becomes a point load on the top of the CMP. It would be worth it to run some numbers on it if you don't want it to happen a second time around.

 

[cieg22]

What was the depth of the pour directly above the pipe? Assuming there was a bond-break there due to the split pour, some of the cracking could be due to the shallow depth of that section. I've always understood that you don't want your concrete chunks to be less than 4".