Concrete driveway spalling

I had a concrete driveway installed in July of 08. In March of 09 I noticed spalling on the entire surface which exposed what I thought were stones close to the surface. After pressure washing, more aggregate was uncovered and I noticed the aggregate I was seeing was black in color and not stone. This black "aggregate" turns to powder when dug up and some of the spots are rusting as well. Is this fly ash and why is the spalling occurring?

 

Where are you located?

Is the concrete air entrained? Around here you cannot get concrete for a driveway unless it is 4000 psi and air entrained, although some small suppliers will sell anything for a price.

The concrete could be failing and exposing the aggregate below.

If they are black, it is not fly ash. Fly ash is about the same color as cement and does not form into "stone-like" pieces.

Did you call the contractor or the ready-mix concrete supplier?

Dick

 

I live in the Pittsburgh area. I have had the contractor look at it . I am currently in contact with the concrete supplier and will get more info next week. The contractor is not giving me sufficient answers to the cause and effects and is not returning my calls. I am now trying to get as much info as possible so I can recover my installation costs in court. The concrete was poured on a hot July day and may not have been worked properly and also possibly dried too fast. Whatever the cause I have never seen a coal like material in cement nor an iron content in the mix before this.

 

Gravel mixes typically contain a fair amount of "chert", which is a very porous, sandstone-like material. I've seen it almost black, but also sandy colored or reddish. When water absorbs into the concrete, it also absorbs into this soft material and when it freezes it expands and pops the surface. If when you dig the soft material out of the popout the resulting hole is rounded like a pebble, then you got a gravel mix, not stone. Stone is more angular. As to the rust, gravel mixes also contain a lot of iron ore which rusts and bleeds on the surface when exposed to weather. That's why a lot of times you see rust stains on exposed aggregate concrete.
Being in Pittsburg, it's also possible that they are using slag in the mix. It is fairly common. As to flyash, the color of it around here is sort of a tannish gray color.

 

1st question, is the entire surface scaling, or are you seeing popouts across the entire surface. If it is scaling, you most likely have 2 issues. The first being what may be poorly placed, cured or manufactured concrete. (or a combination of the above) The only clear way to figure this out is to core and have a petrographic analysis done. (about $1000) . Then typically they are inconclusive as to who owns the issues. If the air entrainment is found to be below the standard ( 6% +/- 1.5% for that region assuming 1" nom max aggregate was used) then that would most likely be the culprit of the failure of the surface.

For the soft black "aggregate" Pecos hit it on the head it is most likley chert or lignite.

Here are some questions you should ask your contractor.

1) was the concrete air entrained? if so was it tested and what was the air content?
2) what was the strength of the concrete ordered. The American concrete institute recommends that 4000 psi AE concrete be used in for exterior concrete that may be exposed to deicing agents.
3)What was the slump of the concrete? was it greater than 4"? if so, was a mid range or hrwra added to increase slump?
Slump is the measure of consistency of concrete, in laymans terms, how wet. Typical concrete is designed at about 3"-5" slump. Depending on many factors. Higher slump (more flowable) concretes should utilize a High Range Water Reducing admixture to make it more workable. Just adding water to make it easier to work, can reduce the durability of concrete upwards of 50% depending on how much was added and the design of the mix.

4) How was the concrete cured? If they say air cured, that means there was no curing. ACI states that all concrete should be cured properly. Typical methods are wet curing (covered with wet burlap or curing paper) or spray on curing compound. Concrete that is not cured properly is less durable than the same concrete that is cured properly.
5) What was the water to cement ratio of the concrete placed? ACI also states that the w/c ratio for concrete exposed to weathering (freezing and thawing in a sever weathering region which you are in) be a maximum of 0.45.
6) Were motorized steel trowels utilized during finishing. It is very common for finishers to utilize steel trowelling and then broom the surface. This is a no-no for air entrained concrete. If this occurred, it would have densified the surface disrupting the purposfully entrained air in the surface and prematurely seel moisture under the surface of the concrete causing surface scaling and delamination. If hand steel trowels were used, it can also do the same thing.
7) you said the concrete was placed during a hot dry day. Was the temperature of the concrete taken? What was it? How old was the concrete? The typical standards are a maximum of 90 degrees F, and total of 90 minutes from the time the concrete is batched. When these limits are exceded, the concrete begins to degrade and ultimatley will lose some strength and durability.

These are the basics to get you started. Get yourself a copy of ACI 318 to get all the industry standards in writing. If you have a pending suit, it will benefit you to know the standards and if the product you bought met them.

Good Luck

 

The "pop offs" are only on top of the black porous material . Since my original post , the driveway has been inspected by the cement supplier at the request of the concrete supplier. He did a surface strength test and found the surface to be on average 3700 PSI. He described the different aggregates which are used and how they affect the mix. He stated that it was obvious the lighter aggregates came to the surface absorbed moisture ,then popped when they froze. He didn't commit to saying the contractor didn't work the surface properly, probably for legal reasons. When the cocncrete was delivered I didn't see a slump test taken nor did I see a temperature measurement take place. Steel trowels were used and it was broomed afterwards. The surface was not covered after finishing. The concrete supplier is 15 min from the job site location. So, unless he stopped somewhere before hand the concrete was fresh. Thanks for your help

 

I doubt it was a finishing problem. It sounds more like the aggregate in the concrete was to blame. There's no way that the lighter aggregate "floated up". A rock won't come to the surface of concrete. It will only sink or stay where it is.
To me, it sounds like the ready mix supplier is buying crappy aggregate with a lot of chert.