Construction Joints in Conc Pedestrian Tunnel

[lrhg]

I'm working on a reinforced concrete pedestrian/service tunnel located under a street to connect the basements of two exisitng buildings. The tunnel is 350 long, 15' wide, 15' tall. The floor of the tunnel is 18" thick, T & B reinforcement, primary load is hydrostatic uplift, as the tunnel is below the water table.

I would appreciate input on recommendations for construction joint spacing and control joint details in the tunnel floor. Floor is subjected to wheel loads from forklifts.

Thanks.

 

[LittleWheels]

Pneumatic-tyred or solid-tyred? Large or small diameter wheels?

 

[lrhg]

There are serveral different types of wheels. The smallest is a 3.25" diameter solid poly wheel.

I have currently proposed 40' o.c. construction joints, installed everyother. Contractor has proposed saw cuts 1" deep at every 20' caulked.

Thank you for your input.

 

[jike]

I agree with the contractor. I assume that the entire tunnel will be waterproofed from the outside so no need for waterstops in the joints.

In 350 feet, will you have an expansion joint (which will really work to absorb shrinkage)?

 

[dik]

I'm just working on one for electrical services, but I've put expansion joints at 120' and control joints at 20' with proper joints at 120' and crack control joints at 20'. The crack control joints have 1/2 rebar terminated and a 'V' notch cast into the conc.

Dik

 

[ron9876]

I would try to limit joints since each is an opportunity for a leak. Maybe at thirds. Add extra reinforcement to resist shrinkage. Contractor should be able to pour each segment in one pour (floor/walls/roof). Should a flexible waterproofing system.

 

[civilperson]

No joints will work best since the temperature is nearly constant below grade. Close spacing of the top layer reinforcement, (less than 8"), will keep cracks to a non- visible size. I recommend a patterned pour, sections 1, 3, 5, etc. and then 2, 4, 6, and remaining to limit shrinkage effects.

 

[jike]

I think that it is unlikely that the tunnel will be poured in alternating segments. Temperature is not the issue, it is shrinkage.

If the sections are small enough, you might have the contractor entertain geting a price for precast. This works well if you have to close off part of a busy road and need to get it done in quickly.

 

[ron9876]

Have designed a few hyrostatic slabs near the beach where the slab is under nearly constant water pressure. Didn't allow construction joints and placed control joints (really shrinkage relief joints) at about 100' c/c and so far no problems. Thr problem with conctruction joints is you have to waterproof them like a control joint and try as you might if you do this enough you will have a problem sooner or later. Waterproofing details are critical. Spending extra money on reinforcement is minor compared to the cost of futue repairs.

 

[civilperson]

Construction joints can be keyed or can incorporate ribbed waterstop or bentonite expansive tape at the centerline. Monolithic pours demanded of the contractor cause increase in costs greater than the use of construction joints.

 

[ron9876]

I agree. But the joints aren't free and it is a trade off between contractor concerns and risk of a leak after construction. 18" thick-15 foot wide x 100 foot long is about an 85 yard pour. Where I live this wouldn't be considered a problem. If I could convince myself that there wouldn't be shrinkage problems (which I think there would be) I would pour the whole bottom in one pour. The wall and roof pours are where the worst cracking risk exists in my opinion due to restraint. Depending on the depth of the water I might consider a different solution above the water table.

 

[lrhg]

Thanks all for the ideas and thoughts.

To address some of the responses:

Precast was considered and ruled out due to length of time to permit + fabricate (this municipality requires precast shop drawings submitted at permit time.) R/C was faster + cheaper.

Contractor suggested the 40' construction joint alternate pours in the floor - no cost impact from one continuous pour. Each joint will have a waterstop.

We have used small bars, tight spacing in the entire tunnel.

Exterior Waterproofing details are by the Architect and no expense is being spared. But since WP can fail or get punctured during construction, all joints have waterstops.

I am somewhat concerned about the performance of saw cut joints with the wheel loads.

 

[civilperson]

Construction joints are not sawn, rather smooth floated on both sides of the joint.

 

[lrhg]

They proposed saw cut control joints (caulked) between the construction joints at 20' on center.

 

[jike]

Sawcut joints should perform well under wheel loads if you use a joint filler that properly supports the concrete edges and it is maintained properly.

 

[civilperson]

Forklift wheels are often solid with subsequent damage to joints. Less joints means less maintenance. Any cracks that do appear may be routed out and sealed similar to joints but may never appear. A high strength floor or added hardener for the surface will give the longest traffic life.

 

[miecz]

I would not saw or form joints. Once you have a joint, you can expect the joint to open as the concrete shrinks. This shrinkage will continue for years, so checkerboarding the pours has little effect. A flexible joint filler cannot support the concrete edges. A rigid joint filler cannot expand to fill the joint as it expands, and so ultimately doesn't support the concrete edge either. Best to heavily (0.6%) reinforce the slab and distribute the shrinkage to many small cracks.

 

[JAE]

But since WP can fail or get punctured during construction, all joints have waterstops.

Yes....agree.