Unstable mix after addition of HRWDA 6

[LKPRECAST]

Goal is to not have to vibrate our precast forms. Max w/c is .44 for our vaults. After the addition of Adva 575 HRWRA at 5oz per 100wt of cement the mix is unstable and segregates. A grace rep said get the batch to a 1" to 3" slump before adding the HRWR, however to achieving a 2" slump exceeds our max .44 W/C. Is it acceptable to use a WRA and a hwrda together? As in reach a higher slump with the WRA before adding a HRWRA?

Current mix:

1yd

Cem 650 lbs
3/4 chip 788 lbs
3/8 chip 788 lbs
Sand 1400 lbs
H2o 285 lbs
Fiber mesh 1lb

Additives:
26oz per yard Grace 575 Advacast HRWRA
2oz per yard Air for 5 to 6 percent air entrainment.

Pulling my hair out getting this stuff stable and eliminating voids

We also have access to Grace WRA 27 and VMAR 3.

Thanks

 

[concretemasonry]

Vibration can be magical when it comes to manufacturing concrete products in a plant environment. Usually, with a decent batching system it is just a matter of fine tuning the mix design, moisture and the vibration frequency. - different aggregate gradations and shapes can be critical changes that should be able to use. Angular aggregates can be a problem because you really want high densities and rounded fines are very beneficial.

The vibration frequencies could alter the air entrainment admix if you want to maintain the 5-6%.

Look to other admix suppliers that may have more experience (with similar products)using the local aggregates.

My experience is manly in the area of zero slump or low slump mixes used for extruded zero slump or pre-stressed mixes that have high quality accelerated curing controls and rapid turn-over - for what is worth, since your mix seems a little wet.

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

 

[hokie66]

Resistance to the need to compact concrete seems to be a recurring theme lately, with lots of folks wanting their concrete to "self consolidate". I don't believe in this philosophy, and don't have much sympathy when such schemes fail.

 

[Ron]

Continuing with hokie66's theme (with which I agree), the various admixtures and processes available to enhance concrete should not be used to circumvent or supplant good concrete practice. Admixtures have their limitations. You will likely find that you have to bend your procedures to better match the capability of the concrete, not the reverse.

 

[LKPRECAST]

Thanks for the responses. Unfortunately round agg around here is rare. (Idaho) Everything is crushed. A void free finish is possible with the HRWRA as we have had it working for a while, but then recently something has changed. Aggregate gradiations or Im wondering about excessive fines in our sand spoiling our Cement to agg ratio.

Going back to vibrating could be beneficial, though with an SCC mix it can segregate the large aggregates.

 

[LKPRECAST]

Anyone see any issues with my batch weights? Thinking about taking out some of the sand to reduce some of the surface area.

Can I use a WRA to meet the w/c ratio and then use the HRWR to get it self concolidate or is that redundant? My grace specs say the two admixtures are compatible.

 

[LKPRECAST]

Did a trail batch today. Used 3oz per 100wt of WRDA 27 to reach a 2.5 inch slump maintaining a .44 w/cm , then mixed in the HRWRA 575 at 5oz per 100wt. Im pleased with the results. The mix was stable and flowed well, but I think I can push it further, either a higher dose or cut some of the sand. Some small air voids are still present, but not excessive for an under ground vault, but this may be due to the form oil, and smooth steal form finish.

 

[Hoaokapohaku]

It is not only acceptable to use those two types of admixtures together, it is virtually a standard practice. I don't think I've ever seen a mix design that uses a high range water reducer that didn't also use a Type A water reducer such as WRDA-27, and can't think of any reason why anyone would want to. All designs for self consolidating concrete will include both types of water reducers.

A LOT can be done to "push it further" with this mix, but in what direction do you want to push? Higher strength (using same cement content)? Lower cost? More slump? Less segregation?

I notice you don't mention any air entrainment. That seems a bit odd for freeze/thaw conditions in Idaho. Did you just not mention it? Unless your specifications say you may not use it, there are a bunch of reasons why you should - including better workability and lower cost.

 

[LKPRECAST]

Most of our product line only requires 4000psi. Though with the current design over 6000 is common

I am using a small amount of air. 2 oz per yard (650 lbs cem) was giving me 5%, however this was with my old mix design and it had far more sand. Almost 1600 lbs. I think I need to increase it but I wont know with out doing an air test.

Ultimately the direction I would like to go is slighlty more slump, and stability, along with form finish. As of now the mix seems to be border line stable. After the addition of the ADVA 575 mixing requirments seem a bit long just over 10min (mixing in a truck). The slump is good and is flowing down into the forms though Im still getting an "airy" finish. Small voids on most of the horizontal finsh.

Getting the cost down would be good for the company provided I still maintaned an scc mix with good form finish, and 4000psi min.

Updated Disign"

1yd

Cem 650 lbs
3/4 chip 788 lbs
3/8 chip 788 lbs
Sand 1400 lbs
H2o 285 lbs
Fiber mesh 1 lb

Aditives:
WRDA 27 3.5 oz per 100wt
ADVA 575 4 oz per 100wt
Air .5 oz per 100wt

Thanks for all the input. Having a lot of fun, learning a lot!

 

[Hoaokapohaku]

For starters, use the maximum recommended dose for your Type A water reducer, 6 oz/ctw. This will reduce the water demand of the mix probably by about 10 to 15 pounds. I would trial batch with the increased WRDA-27 at the 6 oz and probably reduce the water to 270 or 275 pounds. Also reduce the cement content in the trial batch by the amount required to maintain your present .44 w/c ratio. You don't have to worry about strength loss with the lower cement content. As long as you maintain the same w/c ratio and reduce the amount of both water and cement you will gain strength. The point of this trial batch is to find the amount of water and cement required to hit your desired slump while using the maximum water reducer dosage. If you don't hit your target first time around, make adjustments and trial batch again. Repeat as necessary until you get the target slump - 2" I think you said. Except for workability, virtually everything about concrete gets better with less water (up to a point anyway, and you aren’t anywhere near that point). In this case, you are going to get the workability you need by your high range water reducer (HRWR) so the difficult workability inherent in a 2” slump mix isn’t a factor. Cement is expensive; WRDA-27 is comparatively not expensive so this modification will have significant cost advantages.

As long as you are at the same slump before adding the Adva as you were in your previous design, the same dose rate will get you the same slump after dosing as before, though the total amount of admixture added will be less because of your lower cement content.

By saying you want better “stability.” I’m assuming that you mean you want a mix that is less prone to segregation. If so, what you will have already done at this point will have already helped because of the reduced water content. If trial batching that includes Adva indicates that this is so, you may be able to increase your Adva dose rate to gain more flow. You will gain more stability yet by using a viscosity modifying admixture (VMA). Since it appears you use all Grace admixtures, you should look into their V-MAR-3. Personally, I use BASF products for this use so I can’t vouch for the Grace products, but I expect one would get similar results. VMA admixtures help hold all the mix together and therefore will help get your needed stability. In addition, these admixtures make great “band-aids” for problems with aggregate gradation and particle shape. Based on your problem with voids in the finished surface and the fact that your previous design used to work but now doesn’t when no other variables were changed, you very likely do have either an aggregate gradation or particle shape (flat and/or elongated particles) problem. You may not have control of this and VMA can help overcome aggregate problems.

Your problems with surface voids is probably a fine aggregate gradation problem. This happens a lot with crushed sand. It is caused by over-packing the mix with more of a certain size aggregate than will fit in the overall aggregate void structure, usually the size passing a #4 and retained on #8 sieve (4.75 mm x 2.36 mm). If the particles are flat or elongated so that they sit in a horizontal orientation on the surface of the mix, the problem is much worse. When the mason trowels the surface, the over abundant particles pop out, creating a void and angry cursing masons. The only way I know to fix this without blending a natural sand is to produce a crushed sand with less of the offending particle size. If you don’t have control of your aggregate source, lot’s of luck with that! You’ll probably have to live with angry cursing masons.

Regarding your desire for a mix time under 10 minutes, that may be an unrealistic expectation. You can verify that the mixer drum fins are not excessively worn or have excessive concrete build up. Also make sure the aggregates' moisture content exceeds the aggregates' moisture content at SSD condition for at least 12 hours before mixing. A full explanation is longer than I want to type, but in my own operation I do not allow concrete discharge until a minimum of 20 minutes have passed since water and cement mixed. The reason for the needed time is related to the chemical composition of the cement, so the timing can be expected to vary with different cements, but 10 minutes is very fast.

Also, increase your entrained air to 7%. Normally, it will improve your mix in virtually every aspect without strength loss. More than 7% will start to cause strength loss in proportion to the added air.

I'm curious, from what kind of rock are your aggregates produced?

 

[LKPRECAST]

First off...I wish I could reach through this screen and shake your hand sir. This is like a dream, finally talking to someone else who runs a plant and actually knows what's going on. Thank you.

I did a batch today, increased the air to .53 oz per 100wt, increased the Type A to 4oz/100wt, and decreased the ADVA 575 to 3.4oz/100wt. Stability was great, though the flow was down. Definately not a true scc mix, but very workable. Form finish is unknown, will see tomorrow. I still need an air test done. Back when the company had me at 1500+lbs of sand per yard, only 2oz of air per yard gave me 5 to 6 percent. Now that the sand is way down I would assume it needs more. Hopefully I havent exceeded 7% with just .53oz per 100wt.

I like the idea of maxing out the WRDA 27 and reducing cement and water. Though Im wondering two things. One, Im reducing the batch weights so I need to make up for it in some aggregate no? If so what size is beneficial. Two, if I reduce the cement, wont that cut my aggregate to paste ratio and make the HRWR less stable? A 2" slump is what I have been keeping the mix at before the addition of the HRWR because when I dosed a 3" slump it was unstable. I have no slump target per say. Would a higher slump be better before adding the HRWR? I just kept it at 1 to 3 because thats what a grace rep told me. His exact words were "get the mix to a 1 to 3 inch slump by adjusting with water"!!!!! Guess they don't know that practice is not acceptable. He then said add the ADVA 575 and mix for 6 minutes. So there again the only reason I was mixing it for 10 minutes is because I thought over mixing was bad and you only have about 30 minutes with the HRWR.

Now that im using the Type A, I do a moisture test on the sand, minus 1% for what it probably absorbs and figure my water weight per yard from there. Daravair 1000 is going in with the water at the begining, then the wrda 27 in torwards the end with the sand after all cement is discharged, then about a minute later I add the HRWR with the last of the sand and my washdown. I hold out about 20 gallons for washdown. Before I was transfering the load to the production bay and checking the slump, and then adding the HRWR by hand into the truck.

Before I was concerned about stability, but now with the increased air and WRDA 27 it seems very stable for the amount im using thus far. Though it seems I have a lot of kinks to work out. Ideally I want a good mix that I can just adjust the HRWR for the amount of placement I need.

On the VMA. I do have VMA-3. I thougt it reduced workability, however did save a load when it was unstable. This was all back when we were using no wrda 27 and a different HRWR. It was adva 555, and I guess it is already mixed with a VMA, though not 100% on that. I see better results with the 575 and for the amount im using it is about 1/3 the cost. Though I would be curious to see how the 555 performs with the WRD27 maxed out.

Our sand definately has flat particles. I have noticed them popping up here and there. But I don't think it is excessive, especially since I cut the sand down.

I beleive the crushed agg here is granite, seems to have quite a bit of quartz in it.

The SSD condition I have always wondered about. The process to find the aggregates absorption value seems very complicated. When I asked for the absorption values from the agg suppliers they don't seem to know what im talking about. I thought they were supposed to know, maybe not. So aggregate below SSD is bad? We do have sprinklers over the agg piles but never knew they should soak for 12 hours.

If 20 minutes of mixing the cement and water is good practice, when are you adding the water reducers? Both the WRDA and HRWR? I was mixing for 15 min, then adding the HRWR and mixing for 10 min. Yesterday and today I just added them both torwards the end, then it probly mixed for 15min total.

Thanks again. You really know your stuff! I owe you big time, Feel like we should send you a check!

 

[Hoaokapohaku]

Thanks…I’ll consider my hand shaken!

About whether your design is a “SCC” mix or not, actually it doesn’t look like one to me, but I didn’t want to say that because I really don’t know since I don’t know your materials. It looks like a standard mix with HRWR to make it free flowing. It doesn’t really matter if it is SCC or not, as long as it performs as needed.

About your form finish, I may have some comments about that but let’s see how your latest test comes out first.
Re wondering item #1 – Yes, you must adjust your batch weights to replace the reduced volume of cement and water. Normally, you used fixed amounts of cement, water and course aggregate, air and the amount of fine aggregate is whatever quantity needed to make the total volume of all ingredients equal 27 cubic feet (1 CY). In my own designs, I also account for the water content of all my admixtures and include it as part of my water. According to ACI, you must do this when the total admix is more than ½ gallon/CY, but it’s a pain to sometimes do it, sometimes not. Though it is sometimes required but always permitted, I always do it. Then I also know no matter how I may adjust my admixtures, I will not inadvertently over water the mix by way of admixture water.

Re wondering item #2 – Probably not, though I can’t say for sure because again I don’t know your materials. I’ve got about the harshest aggregate you can imagine, crushed Hawaii basalt and I know I could produce a mix suitable with your needs with 500 to 550 pounds of cement. You probably can too, but that’s probably a bigger jump than either you or your company want to take all at once. In the case of the current adjustment, you are taking out cement and water, but you are replacing it with air and fine aggregate, which are both in the mortar fraction of the mix, as are cement and water. If you can’t produce a mix like this, it is very likely a problem with the aggregate. Either it is gap graded, or there is an excessive amount of flats and/or elongated in the course. If this is the case, and if your specifications allow, you would be better off with only one course aggregate, probably the 3/8”. If you do have an aggregate problem and you can’t fix it, for sure use the VMA. If the Grace product doesn’t do it for you, by all means try another. We use a BASF VMA because we found it worked better in our situation, though we use mostly Grace for other admixtures. Your situation may differ.

About the slump before dosing – unless the mix is a very low w/c ratio mix with a strength requirement significantly higher than 4000 psi, this is primarily an economic question. The lower the initial slump, the more HRWR it takes to gain a given amount of slump. I’ve produced very low w/c ratio mixes where it took up to 16 oz/ctw of a particular very expensive HRWR just to get the first quarter inch of slump, another 6 to get to 2”, another 3 to get to 4, and 1 oz to get every 1-1/2” of slump needed after that. At a certain point, the HRWR gets too expensive and it’s cheaper to just use more cement, or other cementitious materials. However, the mix needs a certain amount of fluidity to effectively get the HRWR into the mix for it to be effective. In these cases, you can add the HRWR, or a portion of it, with the head water to facilitate mixing. There are issues with that, but they can all be easily overcome with adjustments in the batch cycle.
Your Grace rep is not wrong. It is easier to work with a HRWR if you attain a slump in that range first – or even 4”, but it certainly is not a requirement. And the 6 minutes is correct too, providing that at the end of the 6 minutes it has also been at least 20 minutes since the load was batched.

To find your own local minimum time from batch to pour, batch a load at about 3” design slump. Mix it for 5 minutes or so and then have about 3 competent technicians with wheelbarrows and slump cones sample and slump one after another as fast as they can. Record and plot the results on a chart. You will see rapid slump loss for a period of time, probably about 12 to 15 minutes, then a short slowing of slump loss, then a slight slump gain, then a long period of relative slump stability for probably about 3 hours with a slump loss of about 1” per hour. Your minimum time from batch to pour is that amount of time that it takes to get to the period of slump stability.

I didn’t answer all your questions this time, but for now I’m out of time. I’ll try to add more later.

 

[dik]

Hoaokapohaku:
Excellent information, thanks.

Is there much difference in what you've noted if the mix design has say 25% flyash?

Dik

 

[Hoaokapohaku]

Dik:

No, not much difference with fly ash. Admixture rates must be based on total cementitious materials so in that regard a pound of fly ash is the same as a pound of cement. However, use of fly ash usually requires less water to attain a given slump because the spherical particle shape of fly acts like ball bearings in the mix, allowing it to flow more easily. This is one reason it is helpful in pump mixes.

There is a lot of variability from one to another fly ash product, so there is little that can be said about fly ash that is universally applicable. Generally, mixes with fly ash require less water, develop less early strength, and overcome ASR problems with reactive aggregates, but there are many exceptions.

 

[dik]

and with a 25% or so flyash substitution, there may be a bit of a cost savings, too.

Thanks very much... one of the more informative postings.

Dik

 

[concretemasonry]

LKPRECAST -

Just out of curiosity, what are the required properties (texture, appearance, absorption, strength, tolerance, etc.) of the products produced in addition to the requirement of no vibration of the precast molds? I assume the products are cured in some manner within the molds.

I have been through hundreds of concrete plants internationally in 40+ countries, and usually the products properties (strength 4000-7000 psi) absorption, freeze-thaw resistance, density, etc., are the main requirements. These ranged from 400' beds for 6" high prestressed floor filler beams to be cut to length, to 3 sandwich layers of different concrete (air entrained, lightweight and non entrained with a smooth finish) wet poured simultaneously in 8'x20' panels 10" thick, thousands of 3" thick 2 room bath modules with embedded piping and many others, in addition to the mundane flat slabs and beams.

They all had a common thread in that the product properties control, are critical and the products must be made on a schedule in controlled automated plants with large aggregate storage, multiple cements/fly ashes and accelerated curing. Local aggregate were within 1000 miles because of the economical rail shipment option. They were all in the broad range of concrete from zero slump to normal weight and lightweight wet concrete.

I ask this because it is fascinating what can be done with different raw materials and processes to achieve a necessary result. I know of a company in Europe that makes both plain or pigmented and architectural concrete block (24/7) in a totally automated plant and also makes and sells antiqued wet cast copies of classic painted in 2" thicknesses of concrete that are pigmented and sold as wall hangings for $2,000 - $10,000 each, so anything is possible.

Dick


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

 

[LKPRECAST]

Sorry for the delay in response. I stopped getting emails for some reason. Anywho. Initially I mis-spoke. Horizontal form finishes are fine, not to hard to work, and can get smooth or stamped if needed. It is the vertical finish from the form where i am still seeing air voids. Have tried a few more batches and am still not where I want to be.

Our products vary quite a bit, but mostly we do vaults such as septic tanks, sand and grease traps,man holes, sometimes water storage. Minimum strength requirment is 4000psi, however I still need to be able to strip and roll most items the next day.


As advised, I did max out the WRDA 27, and reduced the cement to 610 at a .44 w/c. It was good the first go around, but the second time i tried it i got some agg blocking. From reading the grace specs sheets on SCC, the cement to agg ratio needs to be increased so I upped it to 630. Perhaps like you said it's not really SCC but just a standard mix with high flow.

One mistake I made when I cut the cem down to 610 is I made up the weight in course agg and not in sand, but what I was always told is that sand will slow down the flow???

Our 3/4 agg is very irregular with lots of elongated pieces. Additionaly it seems to vary quite a bit from load to load. This crushed agg is just about the only available source here and is what others use as well. As for batching with only the 3/8 agg, I have never tried it, and not sure how much to start with.

Our form finishes dont have to be spotless, but customers like a tank that looks pretty, even if it is going in the ground. They look really bad if segregation occurs, and leaves sandy trails on the sides.

As for vibration, with this mix the tanks or manholes I vibrate look worse than the ones that were not vibrated! I have tried vibrating them as they fill as well and the one I didnt vibrate still looks better.

Todays batch:

Cem 630
H20 277 (.44 w/c) Adjusted for agg moisture

3/4 798
3/8 798
Sand 1400

WRDA 27 @ 6oz/100wt
ADVA 575 4oz/100wt
AIR 3oz per yard
VMA 15oz per yard
Mixed for 20 mins, then added 575 and mixed another 10min.
Batch was stable, flow was ok, could be better.

From the sound of it, our batch weights need some serious adjusting and cut out a lot of the 3/4

At this point....my brain is smoking

Thanks again for all the info.

 

[LKPRECAST]

From what I see, our mix time needs to be a bit over 20 minutes. At 20 minutes the slump seems lower, and stability with the HRWR is border line. After mixing it another 10, the flow is increased and the mix is stable. Still getting air voids.

 

[concretemasonry]

LKPRECAST -

Your approach is different than most precasters since it is relying on mix design and admixtures only and rather the a combination of mixes and vibration to consistently produce a product that meets the appearance and property requirements economically. The need for an excessively long mixing cycle is not realistic unless you have extra mixing capacity that is not needed.

There are many different admixture types and vibration methods (uni-directional, bi-directional and omni-directional) that can be located and used in a selective combinations, especially with harsh angular aggregates and a concern for appearance, since the mix design and curing provide the strength. Your 24 hour or next day form use is very common since I assume you will be using specialized form for stripping and handling.

Just an opinion.

Dick

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

 

[Hoaokapohaku]

LK...

Ok...in your last post I think you touched on the the root of your problem. You have a lot of flat and/or elongated particles, and the amount of those particles varies widely. The same production variables that usually cause flat/elong particles also cause variations in gradation. These are critical varibles because they change the void structure in the course aggregate portion of the mix. The amount of mortar needed in a mix is dependent upon the volume of voids that must be filled in the course aggregate structure. You need enough mortar to fill all the voids and push apart the course aggregate particles just enough to allow the course aggregate particles to be easily manipulated. If you have just enough mortar to fill the voids, you will have a mix that is very unworkable because the course aggregate does not have room to move and rotate in the mix. If your volume of mortar is less than the volume of voids in the course aggregate, you WILL have segregation and/or surface voids because you simply don't have enough mortar to fill the voids. If you have more mortar than you need, you will have reduced flow. This is because of greater friction in the mix. Friction is a function of resistance per unit of surface area. Fine aggregate has much higher friction than course aggregate because there is a much higher ratio of surface area to unit of mass in the fine aggregate compared to the course aggregate. Also, mixes with higher internal surface area present more surface area that must be coated with cement paste to bond the mortar to the stone. Because of this, mixes with excessive fine aggregate require higher amounts of cement and water to get a given result.

Your problem is likely that because of the wide variability in your course aggregate particle shape and gradation, there is also wide variability in its void structure. Because its void structure is varying, the amounts of needed sand, cement, and water are also varying. This explains why your mix designs seem to work fine and appear "stable" on one day , but unstable when the same design is used on other days.

A couple approaches come to mind to fix or work around the problemk, listed below. Only the first is a true fix, the second is a work around.

1. Get your aggregate producer to produce course aggregate of consistent gradation and particle shape, or buy better rock elsewhere. Look for 5% or less flat and/or elongated particles when tested IAW ASTM D4741 with a 3:1 ratio. Many specs call for =<8%, and you can live with that, but flats/longs are the devil and the lower you can get the better. My guess is you are too small a fish to apply a lot of pressure on your supplier, but sometimes all they need is to understand your needs and they can do better.

2. Proportion your design for worst case (highest voids) course aggregate void content when tested IAW ASTM C29 in dry rodded condition. This will probably be the aggregate that has the most flat/longs. I don't know if you are familiar with C29 or if you have access to the standard, but if not a little quality time with Google and I'm sure you'll get it. It's a simple test that you can probably perform yourself. There is a video on youtube but it's all in spanish. Using this work around, you will always have enough mortar for your mix and you can counteract the negative effects of higher fine aggregate and possibly cement by adjusting using a little higher dose of HRWR as needed. I would test the worst case 3/4" first, then blend the 3/4" with varying amounts of the 3/8" and find the blend with the lowest voids. Then, test the 3/8" alone and determine the voids, if the 3/8" alone has lower voids than your best 3/4"-3/8" blend (which it probably will), and if your specs allow, I would just use the 3/8" and leave out the 3/4". I would probably still use it even if the voids are a bit higher than your best blend assuming the 3/8" is more consistent. Then the amount of voids in the selected course aggregate blend and the gradation of the fine aggregate will determine the required amount of course aggregate (See ACI 211, Table 6.3.6). Performance requirements will determine the amounts of water, cement, air, admixtures, etc., and the balance will be sand. You probably won't be far off from where you are now so you can just leave all of your current design intact except for the ratio between course and fine aggregates and then adjust as needed from there.

Question - Who does your testing? Do you have your own lab? It's pretty tough to do the kind of work you're trying to do well without a lab unless you want to spend lot's of money for independent testing. On the other hand, most of what you would need is cheap and the tests are easy - a worthwhile investment.

About ConcreteMasonry's comments about vibration - I've been assuming that for whatever reasons you might have, efficient means of vibration are not an option. If that is not the case, I agree with CM is that your best option is a good design and vibration. All of what I have said in my posts applies even if you use vibration.