Low permeability, Low Shrinkage Concrete Mix

[VAStrEngr]

What items can I specify in a concrete mix design (contract document stage) to ensure I get a low-permeability and low-shrinkage concrete mix? The objective is to replace concrete paving in an exterior sidewalk/plaza application in an effort to minimize water seeping into a room below.

I am already caulking all joints and providing waterstops at the expansion joints. I am also putting backer rods with sealant at the perimeter of the site where the new paving backs up to existing paving. I am also specifying fiber reinforced concrete in an effort to minimize intitial cracking while still including a wire mesh for temperature/shrinkage reinforcment.

I am thinking that I need a low air entrainment mixture but don't know what the "acceptable" limits are...I currently use 6% in most applications. I also want to include a provision that the contractor should wait a specified time period before putting the caulking in the joints...I was thinking 28 days for simplicity but didn't know if there was a more accepted number.

Please advise regarding the air entrainment and shrinkage time period. Also, if anyone has done similar work, do my actions seem overkill, adequate, or lacking?

Thanks

 

[tolchijb]

I would replace your waterproof membrane (or provide) below the exterior flat work and above the room below. I would specify a w/c ratio of no more than 0.38. The earlier you cut your control joints the better to minimize shrinkage cracks. Definitely wet cure! I would not think the timing of the sealant would have any affect on the concrete. I would probably keep the air entrainment the same.

 

[VAStrEngr]

I didn't mention that in the description but we are installing a membrane on top of the room below beneath this paving.

Great advice on the w/c ratio and joint cutting. Thanks.

 

[VTPE]

If you have the money, you could add silica fume to the mix. Very fine pozzaloans tend to make the concrte less permeable. Also, consider using a blended (steel and poly) fiber and no WWR. This tends to make the concrete tougher.

 

[VAStrEngr]

With a steel/poly fiber mix, how do you detail the control vs. expansion joints? Wouldn't they essentially be the same? Typically I run half of my reinforcement through a control joint where I stop all of it at an expansion joint.

Do you instead dowel the control joints and not the expansion joints?

 

[TXStructural]

If you are putting in a waterproofing membrane system below the concrete, there is no reason to worry about the permeability of concrete. The membrane will be wet, regardless, and it is designed for that.
The effort in the exposed concrete should be to improve aesthetics and durability, since having to repair it will likely damage the waterproofing.

For control joints, you are forcing cracks to occur at the planned location by creating weakness. Cut the concrete early but keep the reinforcing consistent unless you know exactly where the cuts will be. (Reducing reinforcing across control joints encourages crack development since elongation of steel under tension is proportionate to area of reinforcing.) A small mis-location will result in a crack away from the joint.

Smooth dowels, placed perpendicular to the joint, should be used across expansion joints.

I disagree about fiber. Until I spec'd it on an exterior placement just like you describe, I was a fan of macro fiber. It makes concrete difficult to finish and may reduce durability as fiber near the surface deteriorates. Steel fiber should not be used where the surfaces are exposed to moisture, especially if deicing agents will be applied. Deterioration of the near-surface fiber and spalling of concrete around the fiber encourages further deterioration.

Maintain sufficient cover on all reinforcement to provide the protection required in the code.

 

[cvg]

sometimes the most effective way to reduce leakage is to reduce the water. So I would not concentrate solely on waterproofing and crack proofing the concrete (which just can't be done), but maybe put some if not most of my effort in moving the water away from the problem location. Proper grading and sloping of pavement, proper site drainage, roof drainage, downspouts etc. may help.

 

[JAE]

Air entrainment actually helps make concrete more impermeable.

Also, agree with VTPE above - silica fume, fly ash, etc. are good materials to decrease permeability.

Larger coarse aggregates reduce shrinkage.
Lower cement ratios, and lower cement, also reduce shrinkage.

But.....concrete still will crack.

 

[cvg]

I'm not sure about the permeability, but air entrainment does make concrete more durable, especially where freeze thaw occurs. The result is perhaps better long term resistance to cracking and water penetration. Sure, you can reduce permeability, but your water seepage occurs when the concrete is inundated with water which seeps through cracks, not because of "high permeability". Concrete naturally has low permeability and fly ash and other methods just make it lower.

 

[Ron]

Saw cuts should be that same, with or without fiber. Don't buy the crap that the producers try to sell you about doubling your joint spacing or doing away with joints altogether.

Agree with TXstructural about the permeability of the concrete...should be irrelevant, since you don't want to depend on the concrete for the waterproofing.