Shrinkage and Thermal Expansion vs Monolithic Concrete Dock 1

[GrahamParkinson]

The Situation:

As part of a island home dock project (designed more with sweat than brains), we poured seven 24" dock pillars from 6' to 18' high, anchored into rock. Twenty foot lengths of 5/8" epoxy rebar run up through the pillars from rebar sockets set in fractured rock. As the stability of the steeply sloped rock footings is questionable, we wanted to tie the tops of the pillars together with as monolithic a set of beams as possible, especially on the outer two pillars. The beam lengths and width of the dock vary from 8 feet at the outside to 18 feet at the shore. The longest side of the dock is about 45 feet (spanned by three beam sections) where it is restrained at the rock of the ocean shoreline.

The Problem:

We have built the floor of the forms and are now trying to decide on how to accomodate shrinkage and thermal stresses. This is complicated by the heavy content of continous 5/8" steel we will be placing in the beams (some already tied to the bent over rebar rising from the pillars).

The Solutions?

We thought that we would pour the center sections of the 16" by 24" beams seperately from their joints (from a pumper on barge) so that they could shrink while curing. The pour would end about 2 feet from the beam intersections in the cap region of the pillars. The pour bulkheads would be sloped at about 45 degrees. With this slope the temporary beam joints would result in the ends of the beams overlapping the pillar caps. After curing we would then pour the pillar caps and remaining beam regions by hand with our mixer. This may deal with the curing shrinkage, but the question is do we need to incorporate a thermal expansion joint over the pillar at the center of the longest beam (45 feet) or would the freestanding structure accomodate movements? Daily temperature swings in the Pacific Northwest are about 30 degrees F maximum.

We thought that the only solution would be to debond the bases of the beams from the pillar and cast in a y-shaped, elasomer filled vertical thermal expansion joint in the final pour where the three beams meet over a pillar.

Thanks for any suggestions to help us deal with the thermal expansion and curing shrinkage issues that we didn't realize are so important.

Other solutions - limestone aggregate for low thermal expansion, cure with forms wrapped in poly, low water content with superplasticizer, nylon fibers to limit cracking, a welded mesh wrap around rebar to further control corrosion inducing cracking???

- from a Mining guy - Not yet a Concrete Whiz, (let me know if you need advice about any backyard mining geophysics questions.)

Thanks, Graham and Laurie

 

[JAE]

I guess I don't see the issue with shrinkage on a raised, structural floor. In almost all concrete construction, the formwork is placed around the piers/columns, the rebar tied, and the whole floor placed at once...or at least placed in sections due to limitations of time and quantity of concrete.

I do not totally have a good vision of what you mean when you start talking about expansion joints, but for a free-standing concrete structure, there are no subgrade drag forces and thus, shrinkage cracking is limited.

The concrete "floor" or deck should simply be reinforced with adequate beam and slab steel to keep your cracks closed. By adding some kind of expansion joint around the piers it sounds like you are creating shear planes of failure.

Have you consulted with a local engineer on this? You might want to get one to at least look over what you are doing so that you don't create a potential for an abrupt failure.

 

[GrahamParkinson]

JAE, Thanks for the reassurances. I work in a large engineering firm and have had structural assistance with the beam design. The dock pillars are connected with upside down cast in place "T" and "L" section beams using both tension and compression steel and alternating square stirrups. Timber joists will sit on the flanges of the "T"'s supporting plank decking.

The concern is cracking due to differential shrinkage between the continous rebar and the concrete in the up to 45 foot long run of beams (14 foot each).

I've heard that the maximum run of a reinforced concrete beam (without post tensioning) is about 20 feet before shrinkage cracks become a problem.

Maybe my question should be: Is length of the beam a factor in the amount of cracking, or is the onset of cracking only related to the ratio of concrete to steel area?

What we are now thinking of as the most practical solution, given your comments about shear planes, is to cast expansion joint gaps over the pillars to allow shrinkage of the concrete in the beams without a network of cracks. The rebar would run through the expansion gaps and a bead elastomer would be used to seal the gap from the salt corrosion.

The real concern is to limit cracking as much as possible to prevent corrosion since the area is in a heavy wash and salt spray zone (thus the solid concrete construction)

- Graham

 

[JAE]

If the beams shrink, won't they simply pull the structure together - slightly bending the piers? If you keep your water content down (low water/cement ratio < 0.4) and use aggregates up to 1 1/2" you really shouldn't have all that much shrinkage over 45 feet.

 

[AlohaBob]

Shrinkage should be on the order of a half inch overall. You can plan where the stress from this will occur. The key over the columns sounds like a good plan. You would see maybe an eighth inch shrinkage at the ends of the beams. I don't think this would be significant considering the isolation you are planning. You have a good strategy as opposed to a single pour.

Sea salt is a killer. There's doping admixtures to slow down the salt migration into the concrete.