Type of Cement to use in brackish water?

[coatsandrew]

I have a project where we are placing a concrete slab (10'x3'x2') within brackish water (Chesapeake Bay area) to act as an anchor point for a Water Treatment Plant's outlet piping diffusers. Originally, it was specified to use Type V cement in the concrete mix. The contractor states that they cannot find Type V cement in the area, and have proposed to use 50% Type I/II cement and 50% Grade 120 Slag cement. Anyone have experience with placing concrete in brackish waters, and can recommend which type of cement to use?

thank you,

 

[Ron]

Provided the sulphate content of the water is not high, the type of cement won't be much of an issue. Brackish water is not deleterious to the concrete, but obviously can affect reinforcing steel. Make sure the concrete is high quality, dense, low water cement ratio and well consolidated above all. Increase the cover for rebar.

As for the contractor's proposed mix, be aware that 50% slag cement will seriously slow the strength gain of the concrete. The Type I/II gains strength almost as fast as straight Type I, but will a slight retarding effect. If you need to have the strength up in line with typical Type I portland cement mixes, the proposed mix will not get you there. Further, if you are placing in water in that area, the water temperature is probably 60 degrees or less, so that will also retard the strength gain.

 

[concretemasonry]

coatsandrew

What is your definition of "brackish" water? Where on the Chesapeake Bay - Hampton/Norfolk or up at D.C or even Aberdeen?

That is large area and the water is fairly constant except where the tides have a great influence. I worked in that area and there was never much of a problem obtaining different types of cement and even non-specification specialty cements. I also fished that area and any fisherman can give an anecdotal opinion of how "brackish" the water is.

Why is the contractor proposing the mix design? Has he already selected a concrete supplier and that concrete supplier is feeding him the mix that is the easiest to provide (site cement storage)? That happens often with the smaller concrete suppliers especially on small local projects.

The strength may not be a problem unless there is a "canned" spec that was adopted for convenience and it was bid. You may want to look at whether the 28 day cylinder compressive strength is really needed.

That mix design may be adequate for your project. Type V is fairly common near the mouth of the bay but it may be bagged instead of bulk.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[coatsandrew]

It's near the Baltimore area, and is up a tidal river a bit. In the spec that was provided for Bidding, Type V was specified. Now the contractor is submitting a mix design to meet the spec criteria and is saying he cannot get Type V cement. I'm hearing that they may cast this off site and lower into water as a "precast" unit. So it sounds like type I/II is adequate since brackish water won't affect concrete, just possibly the rebar. The rebar is epoxy coated. thanks for info.

 

[dicksewerrat]

I would call a few cement suppliers to see if what they are telling I accurate. And ask people you call about the difference in cost.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com