Wednesday, October 19, 2011

RE: cantilver conc balcony

The SCC concretes are very dense and very durable.  These days when you get precast concrete, most of it is SCC.  Contact a local readymix supplier for what they like for a mix for SCC. 
I am a bit dubious when it comes to admixtures.  Xypex has been around for quite a while.  But I would ask for the evaluations that the Canadians do periodically relative to the NCHRP 244.  Alberta DOT has done some research regarding admixtures and sealers. 
The reason for my caution is from an admixture used many years ago called Sarabond.  Google Sarabond and you will be very cautious of any admixtures.  If I can get the performance that I want with just water, water reducers, cement, and aggregate, I am happy to keep it simple.  If you want to move forward with Xypex, acquire independent research like Canada's National Research Council's Institute for Research in Construction.  Paul Carter, Gerry Litvan, and Ed Kottke did some fantastic work on hundreds of compounds years ago.  You can also contact the NRMCA.  They are very helpful.  Also contact the cement supplier chemists in your area.  Ash Grove has some very good people.  You need to talk to the researchers who do not have a dog in the fight. 
Assess the performance that you want.  Measure the desired performance with conventional concrete and cautiously weigh any admixture relative to the risk.  I came close to specifying a bad product many years ago and was headed off at the pass by a cement chemist with Ash Grove.  If it hadn't been for some independent sage advice by experts in the past, I would be selling shoes right now. 

Regards, Harold Sprague

Date: Tue, 18 Oct 2011 15:49:58 -0400
Subject: cantilver conc balcony

Thank you all for your input regarding my question on a long cantilever on exterior concrete deck.
Deck is competently exterior, no door threshold to consider.  Deck is relatively small, 25'long x 15' cantilever and 10' back span.  It is on a bluff above the ocean in Malibu, so limited rainy season but [per one to morning fog.  Architect wants concrete surface exposed to view - possibly acid washed or sand blasted to expose aggregate, so there will be no hiding cracks. 
I have a concern about the concrete acting like a sponge and soaking up moisture during rains.  I am looking into bonded pt, epoxy rebar for temp bars (or possibly main steel if job is too small for the pt guys)  and w/c of 0.45 was suggested as a couple means of protecting rebar.  Also thinking about adding Xypex to mix.  Any other ideas to protect rebar if concrete is the final finish?
Thank you,
Paul Franceschi