I recently read the tickets for a couple of recent pours and note the following:

I specifically requested 6" slump each time.  The first pour of 3.5cy of 4000psi with 3/8" aggregate arrived at a slump of about 2" so we added water to finally get about 4" for the sidewalk, but had to add more water as we moved to the footings and part of a 4" ICF stemwall.  It ended up pretty soupy.  My only consolation is that we used Fastfoot and the concrete was allowed optimum curing conditions so we hopefully didn't lose as much strength as we could have.

The second pour was for the balance of the 4" ICF walls.  2.5 cy of 3000psi, 3/8" gravel mix was delivered again with a stiff 1 to 2" slump.  The ticket said "3 gals. water per witheld".   We added   30 gals of water to get a 6" slump which we verified with cone. 

Do you pros expect the concrete to be delivered at the desired slump?

Does it arrive close to what  you expect?  Or do you usually have to add water?

I understand some additives to increase flowability may be added on site. 

Farmboy, Yes concrete should be delivered at the correct slump and yes it arrives close to what i expect. Consider the following -- if you read the small print on the ticket you'll find that adding water as you did will likely reduce or void the concrete guys liability for his product. Thus, (at least in my experience) they always bring the mix dryer rather than wetter. Here's how I solve this problem. BEFORE the truck backs up to the pump i check the concrete and if the slump is not correct I call the plant and ask them (and/or driver) to correct it before i accept it (or allow the truck to back up to the pump). Thus the concrete supplier adds water to deliver the correct slump. The key here is that the batch plant is adding water to deliver concrete as promised and they are still responsible for the design mix properties -- e.g. 3000 psi, 4000 psi etc. After you've completed several jobs with a given batch plant they know to provide you with the appropriate slump.

Consider ordering 3" slump - delivered to the jobsite.

@ the site add in superplasticizer to bring the slump to 5.5".... you get stronger concrete & less formwork presusre than you would with a regualr 5.5" slump. Works like a charm.

how would superplasticizer @ 5.5" be less formwork pressure than regular 5.5" slump?

Texas ICF has it right.  Get what you specify!

A batch plan should consider the moisture content of the aggregate (which should be checked daily and controlled) when they calculate how much water to add to the mix.  In hot weather, sprinklers are usually needed to keep aggregate moisture levels where they belong.

Dry aggregate will definitely cause a mix to stiffen.  When the slump of concrete changes when pumped, the cause is usually the pressure of the pump forcing water into dry aggregate.  This is especially problematic with some open cell lightweight aggregates.

Bruce

Bruce, Interesting info. The weather had been a cold, sunny, windy and no rain for a week or more before the second pour, which I suspect could have dried the aggregate some. But with no mix recipe noted on the ticket, I can't confirm what the moisture content was.

I talked to the batch plant mgr after my second pour and asked why the mix was so stiff when it arrived. In essence he said achieving desired slump at batch plant can be problematic due to varying moisture content of aggregate and humidity. Since you can't dry out a wet mix, it's easier to wet a dry mix.

I plan to continue learning more before we stack and pour our dream home walls later this year.   I'll be helping a contractor stack an ICF detached garage and home in a few weeks, so I'll be keenly interested in how his concrete supplier operates.

Not ruling out some additives or superplasticizers, but I'd want some experience onsite during that kind of a pour.

Thanks for all inputs.  Dave

The point is that the ready mix companies SHOULD pay attention to what they are doing.  Many get sloppy unless they know it is a big commercial job with good QC. 

It is certainly not unusual for water to be added at the site, but you want that to be the responsibility of the ready mix company. 

Bruce

Farmboy is right - superplasticizers can be tricky. However if managed properly they work miracles. Imagine regular 5.5" concrete and SCC concrete @ 5.5" side by side. The slump is the same so that tells you that the pressure has to be at least the same. That's just logical thinking. But they will not pour the same. SCC behaves like toothpaste - it is liquid but once it stops moving the pressure is gone "...once placing is finished, thixotropic gelling can be an advantage as leakage at joints and formwork pressure are both reduced soon after movement stops." The first one will always be the slump problem maker whereas with the second one you have reduced water (rember your slump is at 3" brought with superplasticizer to 5.5 withc means low w/c ratio which means higher strength) and you can always add more superplasticizer to increase its slump. You just have to see that they behave differently provided they are both exactly 5.5". Most people see SCC as if it's pure liquid. That's something we do with slabs on grade or precast girders but not with ICFS...