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+--- Thread: Diluting concrete with water to make it workable (/showthread.php?tid=13185)
RE: Diluting concrete with water to make it workable - Rob Tucker - 02-20-2014
It may be a matter of opinion. Vibration is not often used on residential concrete. Vibration can also cause problems. Vibrating moves the concrete within the form. Too much movement and it may break the forms. I've seen it happen.
In conditions where there is a lot of bunched rebar vibration helps make sure the consolidation around the rebar is good. Bridges and abutments have lots of rebar. When there are large finished faces of concrete vibration will help ensure a smooth face without pocketing or voids. Rarely does residential work exceed 12" in depth. Commercial and industrial work does.
My advice for residential is do not rent or use a vibrator and do pay attention to the proper slump. Light tamping and tapping the forms will usually provide all the vibration needed on a residential pour.
RE: Diluting concrete with water to make it workable - unknownjulie - 02-20-2014
WOW, A big mahalo! I thought I was supposed to put the water on the top for a few days, and that's how the conversation started. He said, "no need, I'll add the water myself". I guess I will order the concrete myself (high psi) and have to supervise closely.
RE: Diluting concrete with water to make it workable - Haaheo okole puka - 02-20-2014
This is a perfect time to bring up this issue.
The aggregates used in Hawaii present an issue with regard to concrete. Most aggregates and fines in Hawaii are derived from Basalt rock and are of a crushed source. This presents a particular oddity not typically found elsewhere across the mainland. The very nature of Basalt makes it's porosity and density variable. It therefor may have a tendency to absorb some of the hydration put into the mix and it may even absorb that hydration more greatly over an extended period while submerged in the mix. The other factor being the crushed aspect of this aggregate, this alone changes slumping factors compared to that of a say a smooth oblong round granite stone. Thus the inherent workability of the aggregates and fines used on the island have a less working friendly characteristic, right off the bat. Unfortunately this results in a tendency to over hydrate a mix here in Hawaii and especially when the mix is not properly vibrated prior to working the mix. This is why I frown on additional hydration here. Instead I would advocate the proper use of a vibrator is of utmost importance and depending on the application there may be a need to specify additives that will assist in the finishing process.
RE: Diluting concrete with water to make it workable - Haaheo okole puka - 02-20-2014
Rob,
If encountering the breaking of forms (blow outs) from the vibrator then either the operator of the vibrator is not vibrating properly or your form builder is not building a proper concrete form. I have been working with concrete for over 35 years, including specialty concrete applications, the few forms I have seen blow out were due to improper form building techniques. The necessity in vibrating concrete and especially structural concrete is common amongst all concrete contractors. Regardless of it being a residential application or up to industrial applications, it is a necessary step in concrete work unless under special applications and specified specialty mixes. Vibrating is not just an air pocket issue about rebar and mesh, it is a permeability issue as well.
RE: Diluting concrete with water to make it workable - Mtviewdude - 02-20-2014
General rule of thumb, you can add 1-2 gallons per cubic yard to increase the slump without it being too much of an issue. If I had to choose between 1-2 gallons -vs- poor slump, I would add the water, any more I would reject the batch.
However if you are curious, from the studies I have read.
Adding ~2.5 gallons per cubic yard reduces the strength ~25%
Adding ~5.0 gallons per cubic yard reduces the strength ~50%
Adding ~7.5 gallons per cubic yard reduces the strength ~75%
Hawaii concrete mixes (due to their aggregates) tend to be stronger then specified. On the 4k psi mix we ordered the cylinders were breaking at ~5.2k.
Usually on the batch form it will specify "max add water".
RE: Diluting concrete with water to make it workable - Haaheo okole puka - 02-20-2014
Air pockets = a place for water and air. Places for water and air equal several not so home friendly things. One - it reduces the structural integrity of the product. Two - it promotes premature erosion of the product. Three - it promotes premature steel failure by virtue of rust. Four - it enhances a concrete product ability to harbor mildews not only externally but also internally.
RE: Diluting concrete with water to make it workable - Haaheo okole puka - 02-20-2014
http://www.concreteconstruction.net/images/Concrete%20Vibration_tcm45-341602.pdf
RE: Diluting concrete with water to make it workable - Haaheo okole puka - 02-20-2014
For every percent of air within the mixture there is a 3%-5% reduction in the compression value of the concrete.
Concrete averages 20% air by volume without vibration/consolidation. This means a 2k mix can lose between 60% and 100% of its compression value rendering a 0k - .8k mix without vibration/consolidation.
I initially screwed up that math
it's far worse than what I originally wrote... I knew it was a big no-no not to vibrate but not even I realized it was potentially that bad. Vibration/consolidation is a must, no and's if's or but's about it.
RE: Diluting concrete with water to make it workable - Rob Tucker - 02-20-2014
Then by all mean vibrate. I think the percentage of air in a decent mix is nearly zero unless the mix is very dry (low slump).
RE: Diluting concrete with water to make it workable - Haaheo okole puka - 02-20-2014
Even a higher hydrated mix is averaged at 5% air. Causing a 3k mix to become a 2.25k - 2.55k product without vibration/consolidation. If an engineer specifies a given test, failure to consolidate will result in undermining the engineers specs if the specified test is ordered. With a 1.25 - 1.5 redundancy factor for most residential engineering specs these days, there would be a high possibility of compromise/failure during a seismic event.