Excessive Bleed Water in Concrete 6

[sponcyv]

In concrete pours, is there any information out there that gives guidelines to acceptable amounts of bleed water? I am wondering if there are any guidelines that limit the amount of bleed water in large concrete pours requiring several lifts. If a contractor pours the first lift and a noticable amount of bleed water is present on top of the concrete, should the contractor try to vibrate this concrete to get a better mix? I have researched this, but can not find any information.

 

[LobstaEata]

Over vibrating concrete in the forms (this is a wall right?) will force aggregate downward leaving mostly concrete paste at the top. This will also force more water to the top of the placement and what will be left is a weak cold joint and poor bond for a subsequent lift.

Suggest starting with a low water-cement ratio (0.40 - 0.45) and introducing a superplasticizer if better workability is desired. This will greatly reduce bleed water, increase strength and improve durability. To acheive a superior bond with subsequent lifts, try using "Rugasol" on top surface of green concrete to retard the surface paste set. The paste may then be hosed away exposing aggregate that will interlock with the next lift and result in an excellent bond with much improved water tightness.

Hope this is helpful.

 

[sponcyv]

LobstaEata,

This was a roughly 40' tall bridge pier. Large amounts of bleed water was observed during the pours. The forms gave and the pier collapsed. I am looking for guidelines that give specific direction when bleed water or excessive segregation is encountered. What would be ideal is a guideline that limited the amount of bleed water during sequential pours.

Thanks

 

[hokie66]

I doubt the amount of bleed water had anything to do with the formwork failure.

Nor do I think that overconsolidation would have contributed to the excessive bleed water. In fact, I have never experienced overvibration, but lots of times have seen undervibration.

Excessive bleed water is caused by a combination of poor gradation and too much water. Keeping the w/c ratio low as suggested by LobstaEata is the key.

Guideline for acceptable amount of bleed water? I don't know of any.

 

[msquared48]

The forms may have been poured too quickly, not allowing the concrete at the bottom to set up sufficiently. There is a maximum design rate of pour for any form system.

Mike McCann
MMC Engineering

 

[sponcyv]

hokie66 and msquared48,

To put it in perspective, we are talking about bleed water that would be greater than 6" deep due to the large pours. The concrete was transferred from a truck on shore to a bucket and then transported to the pier for discharge. There was probably a 45 minute window from the time the concrete left the mixing truck to the time it was poured. I believe this allowed the solid materials a chance to settle to the bottom because the bleed water was observed in the bucket.

You are right about the pour rate. That was an obvious problem, but I am trying to investigate all components.

Thanks

 

[LobstaEata]

This is a caption from the Portland Cement Association under the concrete technology section of their website relating to problems relating to overconsolidation and resulting bleed water.

"Consolidation, usually through vibration, sets the air and water bubbles into motion. A proper amount of vibration sends both entrapped air and excess water to the free surface of the concrete – either vertically winding through the matrix or laterally in a direct route to the form wall. Improper vibration will either insufficiently liberate the voids or over-consolidate the concrete resulting in segregation and bleeding. (See ACI 309 or PCA’s Design and Control of Concrete Mixtures for a full description of consolidation using vibration.)"

Many moons ago, I had designed forms and shoring for nuclear plant construction and had to watch for just this type of thing when out in the field observing the pours. We also limited the liquid head of wall and pier placements to a maximum of 10 feet to avoid blowing out formwork.

I hope this is helpful.

 

[cedent]

Weird day. I was just looking at this in (Concrete Microstructure, Properties, and Materials by Mehta and Monteiro):

"There is... an ASTM standard test for the measurement of the rate of bleeding and the total bleeding capacity of a concrete mixture. According to ASTM C-232, a sample of concrete is placed and consolidated in a cylindrical container.... Bleeding is expressed in terms of the amount of accumulated water as a percentage of the net mixing water in the concrete sample."

If this test could be done... you could find out if you did indeed have a rate of placement issue as msquared48 suggests.

 

[cvg]

it's possible that vibration of the bucket while transferring to the pier caused some segregation of the concrete mixture, however 6" of water in the bucket should have sent up a big red flag that something was wrong with the mix or the placement procedure. I have never seen any significant amount of bleed water prior to placing in forms - only after placement and vibration within the forms.

 

[sponcyv]

cvg,

Actually, there wasn't 6" in one bucket. There was 6" after several buckets, but each bucket had noticeable bleed water.

 

[conceng]

What admixtures were being used? I've seen napthalene based high-range water reducers (superplaticizers) cause excessive bleed in certain circumstances. This condition may also retard the set, increasing formwork pressures.

I'd also run a gradation of the fine aggregate being used. It may have gotten too coarse and cause the excessive bleed.

Are we sure all the cementitious materials they are supposed to have in there is getting in there. Lean mixes will tend to bleed more.

Gregory A. Johnson, P.E.

http://www.midwesttestinglabs.com

 

[sponcyv]

conceng,

The mix contained both retarders and superplasticizers. They added the retarders because the previous mix started to set up before placement on the long ride to the pier. They didn't adjust their pour rate for the longer setting time and higher slump and instead increased the pour rate because they were nearing completion.

I will inquire about the possibility of running the gradation test on the samples.

Thanks

 

[conceng]

You might want to contact the admixture tech rep to look into possible admixture issues with using the retarder and superplasticizer. Most retarders are also water reducers and the combined effect of the two together could be the cause for the excessive bleed and possible excessive retardation. I've been able to leave thumb impressions in 24-hour old concrete due to this type of issue.

Gregory A. Johnson, P.E.

http://www.midwesttestinglabs.com

 

[Dinosaur]

I have also noted superplasticizers retard the mixes. I would also look carefully at the batch proportions to insure there wasn't a bust in the mix design. I would also look carefully at the truck operator to make sure he wasn't adding water to keep the mix "wet" while it was being transported on the barge.