ran out of concrete and ended up needing a second pouring for about 1/4 and there he put in driveway. two circles closest to the sides the distances are 13", 18", 34", and 35". between
Cambridge, MA
Was that an "exposed aggregate" concrete? Why are you mentioning acid? How was it finished?
s there a difference between the two sections? What is "white scaling"? Was the stone done before or after the concrete?
f I got the geometry correctly, you have a 25x18 section that was poured without joints, and round areas of stone in the middle. A section this big should have joints unless significant reinforcement is used to control cracking. At a minimum I would have put a joint at the center of the 18 ft side, and three joints in the other direction. Actually, I would have expected more than hairline cracks - but the stone circles may help by reducing the concrete area (and the potential shrinking) significantly.
You say that the wiremesh "overlaid" the clay. That wiremesh should be centered in the concrete in order to arrest cracks. Lying below it does no good.
In general, stones should not break - so something was done wrong. I would suspect that the finishing was done incorrectly but I need more information.Again, the dimensions of the entire drive is 33' x 18'.
The section that was poured later abuts the street sidewalk. One reinforcement joint connects the large circle to the sidewalk. If you draw a line from the center of the circle perpendicular to the sidewalk that would overlay the joint. The second joint runs from the big circle to the right side, again if you scribed a line from the center of the circle perpendicular to the right edge of the drive it would overlay the joint.
There really isn't appreciable difference between the first day and second day sections.
The pavers were formed into the circles before the concrete was poured.
I don't know what "exposed aggregate" is, but the intent was to have a pebbled drive. When the concrete was poured and, while still wet, workers threw the small stones on top and troweled them in.
I mentioned acid because in other websites talking about this stuff they mention some sort of acid treatments that cause the stones to rise but I am very ignorant here.
As far as finish, there wasn't any finishing. They just troweled in the stones, and let it dry. Is that what you mean?
By "white scaling" I mean crumbling, the stones loosen leaving a whitish, crumbly concrete. Also in other areas no stones rose to or stayed on the surface giving a splotchy appearance of bare concrete.
My wife tells me the wire mesh was handled correctly as you pointed out. I wasn't clear in my description. She also added that the stones are sharp edged, they haven't been set with any treatment so you really can't walk on the drive barefoot.
My son attends U of Wisconsin and when we visited him last fall and I saw the excellent condition and glazed treatment of the aggregate concrete sidewalks on State Street, the main commercial street on campus, I was puzzled at why our new drive was so bad. Also, the City used aggregate concrete in their handicap access sections at corners as well as for trash barrel holders, all totally without cracks, very good looking. I couldn't understand why our drive could not be as durable and good looking (granted they had a mix of the pebbles and larger stones but I don't know if that would make a difference). I felt we had a problem. Our contractor says, "All concrete cracks and I bet there are cracks in the sidewalks in Madison." I don't remember any.
What about the psi? I read something about 3000 psi or 3500 psi. Does that make a different in this case?
The contractor is going to rip out the concrete and do it over in the spring. I'm wanting to get up to speed on the process so we can improve our chances at getting a wonderful driveway. Thanks for your thoughtful response.
For starters - what you describe is called "exposed aggregate concrete". If you search the Internet for this expression you will find more information.
On my Q&A area you can find a few prior questions as well .
Now specifically.
1. From your description it sounds like my type of joints will interfere with the architectural appearance of the slab. There will be a tradeoff between joints and some hairline cracks. If the appearance of the existing joints is acceptable to you then I would suggest providing joints as I described before.
2. From your description sounds like this is not the "best" contractor around...
Anyone who runs out of concrete in the middle of a job is missing something. Have you seen prior work that he did for similar applications? Does not sounds like he knew what he was doing. By the way, I would hesitate to take anyone's word that he will come back next year and redo the job at his own expense. Did he make this a written commitment? Is there any way to enforce that?
3. The procedure you describe is only part of the process. After sprinkling the finish aggregate on the surface they are supposed to tamp them into the concrete until they are flush with the top, and then finish the surface normally. After the concrete hardens, they are supposed to come back and sweep/wash away the top 1/16 to 1/8 inch of cement and expose the aggregates. This is a tricky process. If they do it too early, or with too much force, they will dislodge the aggregates (as you described). If they wait too long or do not use retarders they may not be able to expose the aggregates at all (again, as you described).
4. An alternative way of exposing the aggregates is by using acid that will cause the top cement to decompose without harming the aggregates (that are much stronger and harder). This is still an alternative, but should be done by someone experienced (I somehow doubt that your contractor qualifies...). This process does not cause the aggregates to "rise", but instead removes the cement matrix around them.
5. Exposed aggregate applications are usually done with river gravel that is rounded and smooth. I do not know why he used sharp stones.
6. As for the strength. Driveway application is usually done with 2000-2500 psi concrete. Stronger concrete (3000-3500 psi) will be better but more expensive. This kind of concrete is usually used for structural elements such as walls, columns and beams. I suspect that he used concrete with sufficient strength since no ready-mixed concrete supplier will supply anything weaker than that.
7. I hear the claim that "all concrete claims" every time there is a problem. In a way it is true, but this is why we use wire mesh and joints. The joints are actually cracks we put in the slab where they are acceptable in order to prevent uncontrolled cracking. If we fail to provide the joints, nature will do it for us in the form of cracks.
8. Since the city is using the same material successfully, I would call the building department and ask for a copy of the specifications they use. You may have to apply some pressure, but under the "freedom of information act" I believe you are entitled to a copy. The specifications should detail the accepted materials and procedures for the concrete.
Articles that describes the process very well can be found at: click here here or here.
I had a question about the glaze I remember atop the stones in Madison. Is that the sealant or is there some special coating like gives it that sealed in look? Is it possible to send you actual photos of the drive for you to look at? Then you can see in detail what I'm talking about. Also, the
number of cracks have multiplied, there're about a dozen now. Our general contractor did considerable renovation on our house and subcontracted the drive. The general contractor has committed to having the drive redone and I believe there is no question that will happen. The tougher question is whether or not the subcontractor who poured it originally can meet a higher standard. This is why I want to get as clear as I can on what the specs look like for a superior result. As far as the stones, I don't think they were unusual for a job of this sort, they just seemed to stick out...maybe because that sealant wasn't applied? Thanks again for you help. What kind of work do you do?
You are welcome.