Estimating concrete workability

[Maxwell]

Is there a quicker way to judge the flowability of redi-mix concrete than a slump test?

I have been pouring some of my own concrete with little experience, but it seems the hardest part is judging what the redi-mix company has sent me. Seems no mater what slump I order, there is never any consistency in what a receive, other than it is always much too stiff.

Also, when you have to add water to a truck load of 3 or 4 slump, how long must the mixer run to insure a uniform mixture of something like a 6 slump?

I am told about 10 gallons per yard will raise (or lower) the slump 1". Is this good info.

Any and all help appreciated, as well as where to buy a cone or something for on-site testing.

[RicodJour]

Let the concrete company have their driver bring one of their slump cones.

R

[marson]

[Bobk207]

Maxwell-

Take a look at these references...they go a long towards answering your questions.

Here is a mix design calculator (but doens't estimate strnghts)

Talk with someone knowledgeable at your concrete supplier, let them know what you're building.....slab, wall, large foundation, etc.

Have them work with you / give you guidance about a mix design that meets your project needs; concrete strength, durability, workability, etc. Agree upon a water / cement (W/C) ratio and withhold some water at the plant. Have them give you a batch ticket that specifies the mix maximum W/C ratio & the amount of water in the mix...from this you'll know how much water you can add to increase slump without exceeding allowable W/C ratio.

The plant sends it out with minimal water so that the slump can be adjusted at the jobsite. Unless you have a specific mix design (we ordered with a mix number that had all the pertinent information included) you will have batch to batch variation.

Approx 1 gallon per yd will increase slump ~1". Adding 10 gallons per yard of concrete will make soup since a yd of concrete with a proper W/C ratio will only have about 30 gallons of water.

After adding water it is only necessary to mix for a few minutes.

A commercial building supply should have slump cones, rods & pans you can get them online as well.

Oh, considered the use of a vibrator to ease placement & consolidation of the concrete. A decent sized vibrator can even get low slump concrete to flow like water......just don't over vibrate!

cheers Bob

[DanG]

Maxwell,

You do want to keep the water/cement ratio as dry as possible to maintain strength.

Now that I've said that, let's talk practical. A good 4" slump concrete will just barely come down the chute, it may actually need a little help with a shovel unless the chute is very steep. This is about where you want it, a 5 or 6 is easier to work, but you are at the threshold of weakening the concrete.

Standing at the back of the truck with the driver trying to guess at what to add is always that - a guess. Age and experience makes it a better guess. I gallon for each yard of concrete in the drum adds about 1 " of slump. If you ordered 3 yards, and it's close, add 3 gallons. Rev up the truck and try it. You can always add more, once it is too wet you can't go back. A full truck takes longer to blend than a short truck.

You do get to where you can tell by the sound of the concrete in the drum as they pull in. A good driver does this every day, all day, tell him you want a pretty honest 5 and let him help . Be aware, they are prone to getting things a bit wet. I suspect there are lots of non architectural pours that go on where the finishers get to wet it down to make life easy.

[Maxwell]

Thanks so much for all the input guys, I need all the help I can get. Two things I forgot to mention originally is, I'm pouring 4" patio slabs on a literally a river sand base, and the I am getting "zero" cooperation from the concrete companies.

I have tried three different vendors for four different pours now, and got something different every time. The last order would hardly come down three sections of side, lowered all the way to the ground, and the driver swore it was a 6 when it left the plant, about 5 miles away by expressway.

So I really seem to be on my own. I have been buying 5 to 8 yards at a time, and adding at least 2 gallons per yard to each batch. Even though I always specify a 6 slump. So it would appear I will have to guess at the first add ( from 1 to 2 gal per yard), run the mixer 30 turns or so, and then run my own slump test before attempting to use anything I buy.

I realize the mix is costing me some ultimate strength, but that hasn't been a problem yet. Except when the driver of the third pour wound up mixing soup. I did get some hairline cracking out of that one. Fortunately not a real killed on this particular job, but I do want to avoid it in the future.

Again, thanks to all that have helped.

Max

[Bobk207]

Max-

Per my previous post you need a real mix design (like the mix calculator I linked to)

that lists the weight of the components & the amount of water (at design's specs).

You also need a batch ticket that provides this info PLUS how much water went into the mix.

Hold back some water at the initial mix & add the remainder at the jobsite (up to the limit of the mix design)

Without knowing how much water the mix design needs, how much went in at the plant....you'll just be guessing at the jobsite & risk adding too much water....or alternatively too little water that will make working with the mix a pain.

Too much water can reduce the strength, create durability & finishing issues & give you shrinkage cracks.

There are admixtures that you can use to improve workability without losing strength.

A good mix design with proper admixtures will make your job easier AND give you better concrete.

btw where are your jobs located? I'm speaking about practices used in major metro areas in which I have experience. (SoCal)

cheers Bob

[RicodJour]

Max, Bob is talking about plasticizers or superplasticizers (same thing).

the way to go if you need greater than normal workability and don't want to compromise the concrete strength.

R

[Big_Jake]

Absolutely. I have done several concrete jobs over the last 10 years, but I still consider myself a rookie. I nearly always order it with a mid-range water reducer to improve the workability without losing strength. Sure makes things easier, but these things have a pretty short life, so you need to get the concrete placed pretty quickly or the additive will "go away", leaving you with a really stiff batch of concrete.

JK

[Maxwell]

Again, thanks for all the input.

The job site is Tulsa Oklahoma, and the level of cooperation from at least the three largest suppliers is nil. Like I was saying, they won't even be honest about delivering either the slump I have ordered, or what they intend to supply regardless of my request. I tried to talk with one of them, and got only the "1 gal, per yrd, per inch" rule. Other than that, no help. I'm beginning to seriously wonder if I'm really getting 4000 psi mix.

Before my next pour, I will make it a point to call all the vendors in my area and try discuss mixes and additives. But my experience so far has not been good. They act as though demand in this area is high enough they could care less, and aren't offering any options. Seems that if I can't evaluate what they have sent me, and hope they will truly add as much water as I request - I just have to do the best I can with what they leave me. I poured a 20 x 30 last Friday that was cured beyond our ability to trowel it within

60 minutes of starting the pour. And we were spraying water on it within 45 minutes just trying to save it. It truly looks like crap.

[DanG]

Maxwell,

If you don't know about this site, use it. No relationship, etc

I'm sorry you're having problems. It really helps to have a relationship with your concrete company, especially when you need a call back or a short yardage pour. I'm in OKC and get 1/2 yd when I really need to. I try really hard to not abuse the situation. On something like that I remain very flexible about times, etc. This kind of relationship comes over years, good times and bad, good weather and bad. You probably are a pretty small customer compared to a paving company, stem wall/footing contractor, and most general contractors. Be aware of it and accept the consequences that it creates. These guys are working some really long hours. Housing may have slowed a very small amount, but commercial work is booming. I talk to drivers that are working 12 hour days, six days a week or more. All that rain in June put many projects behind. If it makes you feel any better, there are DIY types that they won't even talk to.

Ask if they have an outside salesman. Ask if he could come out to your next site at least a day before the pour, preferably more. Tell them you need some technical advice on some special problems. Be prompt, be courteous, but most of all - be there. He will probably bring a box of donuts or some Cokes - maybe not the first trip. How do you suppose we all get those gimmee hats, tablets, and pencils? Tell him your problems. Ask for his advice on design mix, additives, slump. If you don't know - admit it, don't B.S. him. You probably aren't big enough to have your own design mix, ask him if there already is one for your type of work that you can ask for. Be very aware, there are many things to affect slump. Time of day, cleanliness of trucks, water content (from rain or sprinklers) in the sand and the gravel (it keeps changing throughout the day and throughout the pile), high PSI concrete with lots of Portland, etc. It's not bad as long as it is too stiff and you can make slump with 20 or 30 gallons. You really don't want to pour curbs or ramps with that pea soup that comes out early in the mornings sometimes. Everyone knows once the concrete is too wet, no one can use it, it very rarely comes out exactly at any one slump. Some big technical pours, Corps work, etc can count the revolutions on the drum, not allow any water to be added, make required slump at the truck, not at the end of the pump. This is a bit different caliber of work and they reject a healthy percentage of trucks.

Sounds like you had a hot load if it blew up in an hour even in the heat. This can happen if the load is old, they topped off an old load, hot stone, hot water, and other reasons. This should not happen with good companies that pour for architects and engineers.

[RicodJour]

For anybody.

R

[Maxwell]

Again, thanks for you help guys, I really appreciate it.

Although I am a business, I am indeed a DIY guy in that I am expanding my facility, concrete is not what we do. So I do understand I get the lowest priority on the time of day, and have been accepting without comment. But on the other hand, I have been willing to accept any circumstances, including time windows as wide as two hours, and paying in advance too. So you would think they would at least be diligent enough to send me what they promise. But that hasn't been the case. So lacking the experience to visually look at the mix, and know how much water to add, I was looking for an easy way to measure it. If a slump test is all that's available, I guess it will have to do. Because seems certain I will have to judge it myself for the many reasons you guys have mentioned.

I do think I will give calling in one of their salesmen a shot, it certainly couldn't hurt to get a little better acquainted.

Thanks again for all the help, Max