How come some concrete is pitted from winter salt and some isn't?

[/pine]

We discovered nothing but rock pockets on the underside as the forms were stripped. many areas of completely exposed rebar.

Was the pour not vibrated?

 

[orezok]

It was vibrated as the State Inspector watched. Actually, the vibration exacerbated the situation by causing the heavier rock to settle out and slide downhill in front of the pour. The significant amount of rebar in a stadium seating area trapped the rock and allowed the fines to segregate further. The 2 subsequent pours were made uphill as the first should have been and had no issues.

 

[Gary Fowler]

Poured concrete all my life.....and you are textbook junkies that'll believe east is west and west is east. Just print it out in a manual and you guys'll gobble it up,with no experience in the real world.
FLYASH has ALWAYS been used as a STRETCHER in the real world.....that's fact,fellows! Also,air entrainment has been used as a stretcher. ANYTHING that they can rob the lil' guy that doesn't know any better,they WILL do. But on my jobs,they NEVER got to rob the householder.....concrete with flyash and PLENTY of cement is okay,but you don't NEED it,you just need the cement.
And as for pouring uphill....you are crazy if you do that and you're wearing out your men for nothing. don-ohio ^I

I sure wouldn't want you pouring any concrete for me as you have made so many false statements that it is obvious that you know nothing about concrete design or placement as in doing it correctly. Concrete is always poured up the slope not down, air entraining is not for stretching the amount of concrete , 2-4% air entrainment is normal and makes for easier placement and stronger concrete and it costs a few bucks in chemicals to make the air so it isn't saving money. Addition of fly ash also makes for better freeze protection and protection from sulfates plus a stronger concrete. I haven't seen a mix design in 20 years that didn't have fly ash in it.

IIRC type 5 cement is used for sulfate and salt resistance along with 20-25% flyash as you get stronger mixes that don't heat up as much upon hydration. Pock marking sometime is the result of the type of aggregate used. Any aggregate that has a high iron content will pock mark up as the aggregate basically rusts up and pops out of the exposed pieces. We had a lot of problems with the local aggregate in Northern Alberta Canada on a project that I was Quality Manager on as it all had high iron content and we had to have aggregate shipping in to prevent the pockmarking of exposed concrete surfaces.

 

[/pine]

One of the greatest advances in concrete technology was the development of air-entrained concrete in the late 1930s. Today, air entrainment is recommended for nearly all concretes, principally to improve resistance to freezing when exposed to water and deicing chemicals. However, there are other important benefits of entrained air in both freshly mixed and hardened concrete. Air-entrained concrete contains billions of microscopic air cells. These relieve internal pressure on the concrete by providing tiny chambers for the expansion of water when it freezes.

Cement & Concrete Basics: Air-Entrained Concrete | Portland Cement Association (PCA)

 

[Rock knocker]

Fly Ash

 

[crazyal]

Like I said, I have lots to learn before doing it. I'll be pouring uphill, no choice about it. But the slope isn't too much on most of it. Frost will be an issue so it'll need expansion or control joints of some sort. Also I would like to have plenty of rebar in it, maybe two rows in a box shape so I'm guessing that it'll need to be thicker than a couple of inches.

 

[ghenges]

I don't understand why my neighbor's garage pad is all pitted and flaking from winter salt corrosion yet the concrete pads at the gas station look as good as the day they were poured. Is it merely the PSI strength of the mix or some dark masonary secret?

There is very little concrete placed that meets the specifications of the mix when it left the plant.

Concrete begins to set the moment water hits it, the time it takes to get to the jobsite and then the time it takes to get placed are prime factors in determining how much of the potential PSI to deduct. Add temperature to the equation for more PSI losses.

Then take the amount of revolutions of the mixing drum. Look for a revolution counter on the drum.
Then consider the slump.
Then consider consolidation.
Then consider finishing technique.

TIME is the big factor, along with too much water. The concrete truck drivers that delivered to me said another contractor and I were the only ones they delivered to that kept the water to the spec. They said we were gluttons for punishment because proper slump concrete is very hard to place.

Worked on the subway job in Wash DC. hot summer 1980.
Many, many trucks got rejected because of time - temperature of the mix even though the plant was batching with ice in the mix. They just couldn't get the mix right within the revolutions permitted.

Your neighbor's concrete's strength is probably less than a third its potential PSI strength, plus the finisher most likely sprayed water on the surface of an already set concrete to get enough sap to get it finished and broomed.. Ain't practically nobody in the residential concrete business that does a proper job.

The best concrete trucks were the old flat drum type that carried the water in a tank above the cab.
The water never touched the cement until at the jobsite, couldn't get fresher concrete.

Today on the farm I'll buy the gravel, sand and cement and mix it in an ancient 2 bag mortar mixer. Pick a cool day and it's nothing to mix 1-2-3 or even 4 yards at a crack. Don't save any money, but have total control of the process, plus it is good exercise and the best heart attack test I know of.

 

[vtsnowedin]

Like I said, I have lots to learn before doing it. I'll be pouring uphill, no choice about it. But the slope isn't too much on most of it. Frost will be an issue so it'll need expansion or control joints of some sort. Also I would like to have plenty of rebar in it, maybe two rows in a box shape so I'm guessing that it'll need to be thicker than a couple of inches.

What is this pad for and what types of load or traffic will it bear? If it is more then a pedesrtian sidewalk it should be much thicker then two inches in Vermont and on a base of clean gravel or crushed stone.

 

[Rock knocker]

Poured concrete all my life.....and you are textbook junkies that'll believe east is west and west is east. Just print it out in a manual and you guys'll gobble it up,with no experience in the real world

Your strident attitude where anyone who disagree's with you is a "text-book junkie" with no real world experience is tiresome. Almost everyone here who has disagreed with you probably has experience with concrete that would dwarf yours. You showed the same lack of respect regarding those that disagreed with your erroneous understanding of the physics of automobile tires.

While fly ash can be used as a means to lessen the amount of cement in budget basin concrete, almost everyone here has been clear in their statements that fly ash used in properly designed mixes can result a superior finished product.

You are somewhat knowledgeable in several subjects, but your needlessly argumentative and patronizing attitude paints you as sort of a jerk.

Read Gary's post and learn:

I sure wouldn't want you pouring any concrete for me as you have made so many false statements that it is obvious that you know nothing about concrete design or placement as in doing it correctly. Concrete is always poured up the slope not down, air entraining is not for stretching the amount of concrete , 2-4% air entrainment is normal and makes for easier placement and stronger concrete and it costs a few bucks in chemicals to make the air so it isn't saving money. Addition of fly ash also makes for better freeze protection and protection from sulfates plus a stronger concrete. I haven't seen a mix design in 20 years that didn't have fly ash in it.

IIRC type 5 cement is used for sulfate and salt resistance along with 20-25% fly ash as you get stronger mixes that don't heat up as much upon hydration. Pock marking sometime is the result of the type of aggregate used. Any aggregate that has a high iron content will pock mark up as the aggregate basically rusts up and pops out of the exposed pieces. We had a lot of problems with the local aggregate in Northern Alberta Canada on a project that I was Quality Manager on as it all had high iron content and we had to have aggregate shipping in to prevent the pockmarking of exposed concrete surfaces.

 

[zonta223]

That's a lot of cylinders...I hope for your arm's sake you're vibrating those rather than having to rod them...

We hand rod them