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Bad Concrete

Posted by scott2006 (My Page) on
Sun, Nov 4, 07 at 6:29

There are some companies in my area useing a 5 sack mix with a bag of regrind to make it what they call a 6 sax mix. Home owners are having problems with the concrete spalding big time. One very large gas station had to take all of the new concrete out and have all new put in. Please beware of what the mix is. The concrete installers would call for a 6 sax mix and the company wouldn't tell them that one bag is regrind. Oh yes... and they are using it for the highways also. ( and you can't figure out why they have to repair the roads so much)

Scott


Follow-Up Postings:

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RE: Bad Concrete

Walgreens built a new store here about 4 years ago. they ground up the old parking lot and mixed it in for the new concrete. it looks as good as the day it was poured. if it is done correctly, you won't have problems. if it is done by soemone who does not mix it properly, then yes you get the problems you describe.


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RE: Bad Concrete

"...spalding..."

Spalling? As in the surface layers separating?

It is usually caused by working the material to soon after screeding it off level or working it during screeding too much.
The aggregate is pushed out of the top layer leaving just sand and cement.
This layer easily breaks loose from the rest from age and freeze-thaw cycles.

Never time to do it right, always time to do it over...


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RE: Bad Concrete

It's nice to try and protect people, but you misunderstand the basic ingredients of concrete. Reground material is used as aggregate, and not in quantities you'd measure in bags. The sacks referred to in a "five (or six) sack" mix are sacks of portland cement per cubic yard, and a five-sack mix isn't bad concrete.


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RE: Bad Concrete

I doubt they are using it for highways. Most state and federal funded projects have strict quality control measures. In most cases the contractor must arrange to have the material tested to prove it meets the specifications (they are very detailed) and will often send inspectors to the plants to be sure there is no cutting corners.


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RE: Bad Concrete

Jason,
You are right. the problem the regrind mix meets the states standerds..slump and what ever....but it still spalds and is just junk.

Scott


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RE: Bad Concrete

scott,
its not the cement it sounds like a problem with the guys installing it. However where I am (detroit mi) a bunch of roads and bridges were built in the 70's using special lightweight concrete. After problems with delamination on the underside of bridge decks recently (resulting in a massive bridge inspection (sounding & scaling) program) they found out the stuff was made with slag. Modern testing has found that it is a very poor material.

I am in Michigan so I can tell you all about concrete and road building (winter is often a brutal freeze thaw cycle).


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RE: Bad Concrete

Spalling is caused by how the concrete is worked.
If you drive the aggregate out of the upper layer by overworking it WILL separate after a few freeze-thaw cycles.


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RE: Bad Concrete

My daughter in law just had a indoor pool built. The cement was died a iron oxide color. It is spalding in about a dozen places so far around the pool.The freeze -thaw cycle does not apply nor did I see any small chunks of aggregate It is a constantly heated area. Any thoughts on the cause of something like this?


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RE: Bad Concrete

The term is SPALLING, not Spalding. Spalding is a sporting goods company.


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RE: Bad Concrete

Recycled/crushed/ground concrete is a mixture of rock, sand and hydrated cement and cannot legally be called portland cement or used as such in a "6 bag" concrete mix. I have yet to hear of concrete being recycled for anything other than as aggregate in concrete paving, or road base and I suspect it could only be used as fill in a building.

The term "spalling" is usually used when a piece of concrete breaks away due to a deep weaknesses in the concrete and the terms "scaling" and "flaking" are usually used for surface failure although they are sometimes used inter-changeably.

Both failures are often due to a high-water content concrete mix, and/or soft or porous aggregate and scaling is most often due to finishing before bleed water has evaporated or after the addition of water to the surface. Air entrainment can reduce the likelihood of spalling and/or scaling from freeze-thaw cycles but should not be used for interior steel troweled finishes. Curing can be a factor too especially in hot dry climates. To reduce cracking and increase surface hardness for industrial slabs it is not only advantageous to reduce the water to cement ratio but the water & cement ratio to aggregate.

Because of the many different materials and admixtures used by today's concrete suppliers, it is best to avoid specifying concrete by the number of 94 lb. bags of cement per cubic yard of concrete and specify a minimum concrete strength and explain the conditions and uses of the concrete to the plant mix designer.

The most common residential mixes are:
3,000 psi or 5 3/4 bag mix
3,500 psi or 6 1/2 bag mix
4,000 psi or 7 bag mix
A rough rule of thumb for a cubic yard of residential concrete is a minimum of 6 bags of cement, a maximum of 6 gallons of water per bag of cement, a curing period of 6 days, and an air content ratio of 6%.


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