Low permeability, Low Shrinkage Concrete Mix
Low permeability, Low Shrinkage Concrete Mix
(OP)
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
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






RE: Low permeability, Low Shrinkage Concrete Mix
RE: Low permeability, Low Shrinkage Concrete Mix
Great advice on the w/c ratio and joint cutting. Thanks.
RE: Low permeability, Low Shrinkage Concrete Mix
RE: Low permeability, Low Shrinkage Concrete Mix
Do you instead dowel the control joints and not the expansion joints?
RE: Low permeability, Low Shrinkage Concrete Mix
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.
RE: Low permeability, Low Shrinkage Concrete Mix
RE: Low permeability, Low Shrinkage Concrete Mix
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.
RE: Low permeability, Low Shrinkage Concrete Mix
RE: Low permeability, Low Shrinkage Concrete Mix
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.