Dr. Mor & Associates
13036 Mindanao Way, #6
Marina del Rey, CA 90292 
Phone: 310.574-0080

Concrete for Special Applications:

Lightweight Concrete

I ran across your Web page while looking up information on "lightweight concrete." I am building a home outside Healy, Alaska near Denali National Park. I would like to pour a self-leveling concrete type product approximately 1.5" deep in my home for in-floor hydronic heating coils. Wirsbo cross linked poly with an oxygen diffusion barrier was what I have planned to run as the tubing. My problem is I may be not have enough strength in the current joists to support the dead load of concrete. I am interested in any products available that will be suitable for this application and the correct formula for figuring the load on the floor joists. If you would point me in the right direction, I would appreciate it. By the way, I thought your Web page was very forward thinking.
It sounds like you need to get a structural engineer to look at the original design of these joists and calculate the extra capacity required for the concrete. This is a relatively simple calculation when you have the plans - but not something you can do by e-mail. Good luck with your project.


I am planning to order a concrete slab for a 13,000 pound motor home. It will be approximately 10 x 20 feet, perhaps a bit larger. One potential contractor is touting fibermesh concrete and says a pad just 4 inches thick would be adequate. Another says he would prefer six inches with wire reinforcement. Four inches seems a bit thin to me for a pad or a driveway which I might also consider.

Could you advise me as to the advantages and disadvantages of fiber reinforced concrete and what you think would be the best route for me to take in planning this slab (and driveway) bearing in mind the load it would be handling.

I agree with your observation that 4" fibermesh slab may not be enough for load carrying purpose. In my experience, even residential slabs that carry no load failed to perform with fibermesh when not jointed properly.

The main problems were excessive cracks that opened up to 1/2" wide due to shrinkage alone.

The key here, however, is the fact that neither fibermesh nor wiremesh are considered reinforcement for concrete. Both serve only to control and minimize shrinkage cracking of the fresh concrete.

Concrete with both will require contraction joints spaced about 10' apart in order to perform. The joints are actually more important than the mesh.

Another critical factor is the type of soil and base preparation since the slab is fully supported on the ground. When the base is compacted properly it will carry most of the load without stressing the slab. In that case a 4" slab may be adequate. In every case, however, a 6" slab will outperform a 4" slab

The last critical issue is curing of the concrete. 7 days wet curing will almost always result in crack free slab if the joints are provided and the design strength is achieved.

My recommendation:

Ask your contractor to show you a similar slab that he placed at least 6 months ago under similar conditions. If it looks good, and he will back his work, then you may go with it. If not, then I would stay with the traditional wire-mesh or rebars.

Always ensure that base compaction, joints, and curing are done correctly.

I got your name from a a section of an internet site called "Ask the Doctor." I had entered "concrete" in a search engine, came up with 260 entries, scrolled through them and found your site.

My thanks for your detailed answer. I was only able to find one other reference that touched on my information, a short explanation of fiber mesh concrete and what it should and should not be used for.

I think I am going to go with the 6" slab. I also asked a friend of mine who is a contractor but cannot do my job and he says that he has had some problems with fiber reinforced concrete. He says if it cracks there is nothing to hold it together and the pieces can often come loose and separate if there is any movement or shifting of the ground for any reason. He also says it is "hairy" because of the fibers. He was hesitant to recommend its use to me.

I live in South Florida, quite close to Jupiter and West Palm Beach in an unincorporated section of Palm Beach County. My ground is pure sandy soil.

Fibermesh Concrete

We had concrete poured for the garage apron of our new house.  No rebar was used.  About 8 inches of compacted aggregate (I don't know if I am using the correct terminology) was used for the base.  The concrete consisted of fibermesh.  Joints were cut the same day that the concrete was poured by using a cutting device on a long pole.  Within two days, a major crack off of the corner of the garage appeared.  A week later, another crack appeared on an inside curve.  Several other cracks have appeared since with spalling and pealing where the snow run off from the garage hit the ground and concrete pealing on the edges of the joints.  There is fibrous debris on the surface of the concrete.  From what have I read about concrete and fibermesh, if you overwork the concrete containing the fibermesh, more of the fibers will appear at the surface.

I appreciate any information you can give me.

Thank you, CL

Sorry it took me longer than usual.

I understand that we are talking about the apron/driveway leading to the garage.
What you describe sounds wrong.  There should not be any cracks in new concrete like that.

Some possibilities:

1. The concrete is of inferior quality and failed to get the needed strength.

2. The joints were not cut properly.  This is the most likely explanation since the concrete started cracking after 2 days.

  • Joints must be at least 1 third of the depth of the slab.  The deeper the better.
  • The distance between joints is critical.  8-10 feet is probably the maximum for such application.

3. Was it snowing/freezing at the time it was placed?  Concrete needs warmth to harden properly.  At very low temperatures it may not gain strength fast enough to prevent drying cracking.

4.  I do not think that the surface fibers have anything to do with the cracks.  Some are unavoidable.

5.  Concrete spalling and peeling (what does that mean?) sounds bad.  New concrete should not do that.  It may be a result of early freezing.

 I suggest you try to get the builder to replace the slab.  You may want to contact Fibermesh and tell them the story.  They are very sensitive to problems and may help resolve it.

If all else fails you may want to consult an attorney - but I would do anything to avoid that including replacing the slab and trying to get the builder to pay part of the cost.  Litigation is a lose-lose situation. 

Hope that helped a little.  If you want me to look at additional information you are welcome to send it to me. 

Exposed Aggregate Patio

I saw your website and thought i would see if you can help me with a project.  I have poured a patio with exposed aggregate and would like to put a high gloss finish on it.  I have seen this type of finish but have not been able to find out what it is. Someone told me it was a mixture of linseed oil and kerosene but I don't know for sure. If this is correct, what ratio do I use.  Do you know of any other high gloss finishes.

I am not familiar with this method.  However, it does not seem to have a potential to harm the concrete.  I would suggest that you try on a small section first to see if it works as expected.

Why don't you ask the contractor who poured the concrete?

Exposed Aggregate Repair

I have a 15 year old exposed aggregate driveway that I need to cut a section out to lay a 3 inch pipe under the driveway. Can the section cut out be re-poured to look like the weathered remainder of the driveway?


Probably not.  It is hard enough to match appearance of concrete poured on two days. Another problem to consider is that the new concrete will shrink when it dries, and will not bond securely to the old one.

Would it be possible to insert the pipe under the driveway?  There are companies that can "jack" it through, or use water blast, or dig...

The alternative which I would prefer is to use that as an architectural feature.  Use a concrete cutting saw to create a neat straight line and fill it with a different material.  A good option will be red brick set in a concrete bed.

I have checked with local contractors about boring under the driveway, but the minimum cost is $1000 because we live in an area that is mainly rock.
You did give me another idea. Is there a problem with making the cut at the expansion joint? I was thinking if I cut the expansion joint and take a 1.5-2 inch section out that I could dig out enough to get the pipe under and fill back with another material as you suggested. In doing this would I create a problem with the driveway heaving or something else?

Thanks for the information.


There should not be a heave problem unless you undermine the adjacent section.  If it did not heave in 15 years it must be well compacted and stable.  Cutting at the joint is the preferred location since only one cut is required.

I would probably go with a 4" cut and a brick strip.  Using concrete to fill is possible, but you must realize that it will not bond securely to the cut edges.  Besides, you need to keep a joint.

Exposed Aggregate Finish

I would like to pour a 14 X 21 foot slab for a patio surface and wish the finish to be exposed aggregate. Can you offer any instruction on this process? thank you, LC

There are four basic methods for doing that, and experience is important in order to get the desired uniform result.

1. Use surface retarders. These admixtures are sprayed on the finished surface and penetrate only the very top layer of cement. They will prevent this layer from hardening. A few hours later - after the rest of the concrete hardened enough - you can come back with a broom and/or hose to expose the aggregates.

2. Abrasive materials. Use steel brushes (or similar) to remove the cement cover before it hardens too much, but after it gained enough strength so that the work will not dislodge the aggregates. Choosing the right time can be tricky.

3. Acid wash. Use muriatic or phosphoric acid to dissolve the cement on the surface. Use diluted acid and practice on scrapes. This can be done after a few days. Acid wash may also be used to finish concrete done in one of the previous methods. Be careful not to overdo it and damage the deeper concrete and/or the aggregates.

4. Water blast. Get high pressure water blasting equipment. Use it after the concrete is strong enough (12 to 24 hours) so the water does not blast the aggregate out. Depending on the type of concrete and temperature, you may have to wait a few days.

Note: If you want to use fancy (expensive) aggregate, it is not necessary to use it for everything. Most contractors will use regular concrete, and spray the finish aggregate on the surface. They will tap it into the concrete surface to achieve a flat finish, and use one of the above methods to expose it later.

Be aware that if you never did that before it is possible (likely) that your first try will be less than perfect. Get professional help if you can

Good luck

Pool Deck - Tamped Concrete

Hamilton, USA

We've just had a pool installation completed which included a stamped concrete deck and patio surrounding the pool. We chose a pattern known as ashler slate. There are a number of what I would call surface defects in the concrete. Mostly on the edge of the individual slates there are spots in which the concrete is not smooth. It appears grainy and slightly depressed below the finished surface. I asked the mason if the forms were kept clean as I thought the problem might have been caused by concrete on the form pushing into the surface. The mason said no. It is caused by tension on the surface of the concrete when they tamp down on it. He indicates this is normal for stamped concrete and won't affect the integrity of the concrete. I don't know much about concrete, and I also don't want to be unreasonable, but his explanation sounds a little too convenient. I would appreciate any advice you could give. 

Thank You, C.L.


It is not easy to visualize the problem from your description, so do not expect too much from my answer.

It is not clear to me what size and depth are these depressions. Does it look like something broke off (spalled) or what you see is the surface as left by the mason?

His explanation does not make much scientific sense, but that may be because he does not know the correct terms. It sounds to me like some concrete stuck to the forms when they lifted them. The stress he is referring to may be the stress when the 'skin' of the concrete stretches and breaks. This would leave a rough surface inside a depression.

I tend to agree that there is no danger to the integrity. If the concrete is over 14 days old you should take a pocket knife or sharp screw driver and try to scratch the surface of the concrete (do not push hard). It is OK to see a lighter color scratch, but no material should come off. Surface dusting is a sign of finishing problem.

Is the concrete colored? If yes, is there a difference in color inside the depression?

Is that a visual problem? Seems to me that a slight irregularity will fit ok within the stamped texture (whatever ashler slate is...).

If you need more to make you feel secure go look at some of his other stamped concrete (you did that before you hired him, right?). If this is a common thing then you may accept the explanation and enjoy the pool...

Concrete Pool

San Jose, US

I have an outdoor swimming pool and would like to get rid of the pool and fill it with concrete so it would look the same as the yard. Any suggestions on how I can go about doing this?

Thanks and Have a Great Day


Are you sure you want to fill it with concrete? Why not fill it with soil and plant some nice plants/grass/trees?

Filling a swimming pool with concrete will take a LOT of concrete, be expensive, and you will end-up with a huge piece of very hard rock in the middle of your yard. Consider the resale value problem you will have later...

My recommendation would be to break the pool up and fill the hole with good soil.

If that sounds too expensive you may elect to leave the pool alone and just fill it.

If you want to leave the pool deck and extend it over the pool you should do it very carefully.  I still suggest you fill it with soil, but this soil must be compacted very well before you place any concrete. That means that you add the soil in thin layers (a few inches at a time) and spend a lot of time and energy compacting it. Once you are done, leave it standing for a year (through a winter and summer) to give it a chance to settle completely.

If you don't do that, the weight of the concrete will compress the soil and the deck will shift and crack.

If I were to fill it with soil, are there any forms that I have to fill out for the city to review or can I just do it without any prior approval from the city?

each city has its own rules. Simply call the building department and ask.

I don't see any reason why there should be any forms/permits required for landscaping.

Recycled Aggregate

I am doing a science fair project that uses recycled materials as the aggregate for a concrete mix. The materials are crushed glass, stripped plastic, and bottom ash from a coal fired power plant. I have mixed concrete and poured 6" dia. by 12" tall test cylinders. They were taken to a test facility and broken after 7 days. A question I have is what should a mix break at on 7 days. I know that they reach the rated strength at 28 days, but I don't know what they should test at on the 7 day break.

Any charts or graphs would be most grateful. Any information you may have on the use of recycled materials in concrete will be of great help. Thank you for your time and considerations.

As a rough rule of thumb, normal Portland cement concrete reaches about 70% of 28 day strength after 7 days. However, this ratio depends a lot on the type of cement used (some react faster), on the curing conditions (hot and humid will be faster), on the amount and quality of the ash (some fly ash will react but take longer to gain strength), etc.

I have seen ratios of between 50% to almost 90% in 7 days tests.

As for sources for graphs and charts - if you have access to an engineering library (at some college) I would look for the "Concrete Manual" by the U.S. Bureau of Reclamation, or for any text book on concrete.

I don't have anything ready to upload into the Web at this time. If I get some I will send you a copy.

A good source for information on recycled concrete would be the ACI special publications. Again, a college library is the best source unless you live next to ACI headquarters. ACI have their own Web Site which may help you locate these publications. For its address check links from my page.

Where are you located? What sources for information do you have nearby? If you have the time, I can mail you copies of some graphs.

I am currently a senior Civil Engineering student at the [ ] School of Engineering in New York City. Our Senior Experimental Project this semester involves the design, mixing, and testing of common household glass-concrete samples. While we are aware that such testing has been performed in the past, we are currently finding difficulty in obtaining information regarding this topic.

If you have any information which you would deem useful to our project I would greatly appreciate if you could let us know. My email address here at the [ ] is [ ]. Thank you.

I am sorry, but I don't have anything on the subject available now. A good source may be the magazine "Concrete Construction". They used to have articles on any popular subject and they may be reached through the Internet. You may also try the ACI web site.

Since this seems to be a popular subject in schools (you are the second one this week) I will try to find out more in the next few weeks.

post-tension for new home

My husband and I are about to begin a custom home in Oklahoma (south of Tulsa). We found, after the initial excavation, that our soil was "pumping". A soil sample was taken and piering down to the more stable shale layer at 14.5' was suggested. I would like to know if a post-tension slab and standard footing could be used in lieu of this piering option or if it should be used in combination. We have already brought in 30" of fill (minimum) to the site per the soils company. The fill has a plasticity rating of 9. At this point, we have received so many opinions, we are not sure what is the right course of action. I realize, this is a big question. If you cannot help, do you have any suggestions for sources of information to assist us in this decision?
This is a design issue that should be addressed by your structural/Geotechnical engineer.
In my experience, using post tension slab and standard footings on "highly expansive" soils is not a good solution because of the way these slabs are designed. They are flexible and you may see large deflections and movements. Also, because of the relatively low tension in these residential slabs they are not very effective for structural loads or cracking. In Southern California there is currently a move away from these slabs (California design) because of poor performance and very expensive lawsuits.

Good luck

Basement refinishing

My husband and I are considering putting in a carpet in our basement in order to finish off the room as a playroom for our kids. I don't know whether or not we should put in a small subfloor first before the pad and carpet or whether we can put the pad and carpet directly onto the concrete. It is fairly dry down there and in the 3 years we have owned it we only had 1 small spot of moisture near the bulkhead after days of driving rain. We do run a dehumidifier down there all summer. I am concerned about mold growth in a basement though, and since this is to be used by kids, I don't know what the current trend is to prevent that. Are there vapor barriers that are used now for this purpose? Any advice you can offer is kindly appreciated.

Framingham, USA

I would suggest you start by performing a very simple test.

Take a sheet of plastic (shopping bag without holes will be fine) about 1x1ft (or larger). Clear an area of the slab from all debris and paint/stucco/etc. Using a "duct tape" secure the plastic to the floor around its edges. The idea is to seal it in a way that water vapor will not be able to escape from under the plastic.
Leave the plastic for at least 2-3 days and watch for condensation. A small amount is expected, but if you can actually see drops of water you will know that there is a significant amount of moisture coming through. Make sure that the temperature and humidity in the room during the test are similar to what will be the condition when in use. More moisture will come through when the air is drier and warmer.
Based on the wet spot you found I would expect to see significant condensation. Make sure you test a few locations including the area where the wet spot was.
If that is the case then you have a problem. Carpets and vinyl floors should not be installed when the "vapor transmission" exceeds a standard value (3 lbs for 1000 sq.ft. in 68 hrs) in the standard "moisture dome" test (which is a more complex and accurate version of the test above). Any installer that does not test is failing to meet the standard requirements by the manufacturer and there is a good chance that the glue will fail and mold will develop.
If you have excessive amount of moisture coming through then the best option will be to leave the concrete uncovered or use concrete/ceramic tiles without a moisture barrier. These tiles "breath" and will allow the moisture to escape into the air where the humidifier will take care of it. Carpets and vinyl will trap the water and create a mold hazard.
There are some products that claim to seal the concrete from the inside. A few (very few) actually seal most of it if applied professionally, but the moisture pressure from outside is high enough to drive it through most sealants. It will be expensive to apply and I doubt that it will be covered by any warranty. I suggest you look for professional installers and ask for their opinion. Search the Internet for a product called "Xypex" and I am sure you can find installers in your area.
Hope this helps. Feel free to ask again if something is unclear. Let me know what happened with the plastic test.

Good luck

Follow up question:
Attached is your suggestions to my wife for testing our basement moisture. Given most of the basement - in the finished side - is painted, I executed the test you suggested on the unfinished side of the basement. The plastic sheet (dome test) was run for 5 days - rain did occur during this time. No moisture was present under the plastic sheet!
The water by the basement door my wife spoke of is due to a rusted out drip edge on the bulkhead door that has leaked if snow thaws on the door frame (soon to be fixed too).

Based on our results - we will pursue a carpet company to install a carpet in the basement. We will continue to run our de-humidifier during the humid months. Are some carpets and/or pads better suited for basement use?

Another question:
We are looking to install a stamped concrete front walk to simulate a gray cobble stone appetence. Do you have knowledge in this area to help us select the best techniques when searching for an installer. We have questions re: fading?, snow-melt/salt?, maintenance, durability. Any insight and suggestions would be appreciated. We live in Framingham, MA - is it too late in the season to start this project? So far we have called 3 installers and only one returned our calls - who still has not dropped by as was planned.
Nice to get the follow-up.
I am glad the test was negative. Makes life so much simpler...

As for the carpets - the local suppliers would know better than me; and the same goes for the local conditions for the concrete work.
Weather becomes a problem when you have heavy rain on external concrete, when fresh concrete freezes, and when the base is muddy. Proper planning and precautions will probably allow concrete work at this time IF the contractor knows what he is doing.

Concrete can be colored in two basic ways:
1. Add the pigment to the mix during batching so that the whole load has a uniform color.
2. Sprinkle the pigment on the surface so that it only penetrates the top. You probably save some money this way, but the color will not be as uniform or fade-resistant as the first option.
In a thin slab application I would prefer the first method.

You may also choose to leave the natural gray concrete color and avoid this complication.

Snow melting and salt should not be a problem for un-reinforced concrete. Only when you have rebars inside you need to worry about corrosion caused by salt. Make sure that the surface drains away from the slab so that water do not pond inside the grooves of the texture.
Wow! That is good information! I will follow-up with the website info you included and with the City of Madison.
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?
The glaze you saw in Madison is probably some coating sprayed-on. There must be many brands and your contractor should know. The additional cracking is to be expected under these conditions.

You are welcome.

Radiation Protection

I'm looking for standard concrete mixture for radiation shielding purposes. Will u please guide/give me some information regarding;
a) Concrete standard.
b) Its' composition/mixture ratio.
c) Aggregate size.
d) Density.
Radiation shielding concrete is heavy weight concrete made with aggregates and additives that help absorb radiation and can resist its destructive effect better than regular aggregates.
There are 2 sources I would recommend for some basic information. Both are from the ACI (American Concrete Institute) Manual of Concrete Practice.
The first is from Standard Practice 211 (appendix 4) which provides guidelines for a step-by-step design of heavy concrete.
The second is Chapter 11 of Standard Practice 304R that provides general information about aggregate selections and properties of additives. Table 11.2.2 will provide you with a list of typical aggregates and their properties. Table 11.3.2 includes typical proportions for nuclear shielding concrete of different sources and different functions. These proportions and aggregates should be used in the design procedures of practice 211.

The manual should be available in any university engineering library that has a Civil Engineering program. You can also get it directly from ACI by following the links on my web site.

Where are you and why do u need that information?

Exposed aggregate driveway

We had a new driveway put in a year ago.  It is 33' x 18' edged with 4" granite slabs.  Within the drive are three circles filled with circular patterns of brick pavers.  The circles are 5', 7', and 12' in diameter.  The space between circles is 4'.  The 5' circle is 13" from the back edge and 18" from the side.  The 12' circle at the other end is 34" and 35" from the sides.  The 7' circle in the middle is well inside the drive.  The rest of the drive is poured concrete aggregate on a bed of packed sand/clay (I'm not sure of this material) overlaid with wire mesh.  The plan was to have no expansion joints.  However, when poured, there wasn't enough concrete and about 1/4 of the area had to be poured the next day.  This section was separated by joints.  The aggregate is a variety of 1/2" stones.  The drive looks terrible.  Stones started breaking free almost immediately.  There is white scaling, the surface is uneven and collects water.  Hairline cracks have developed between the circles and from one circle to the side.  To my knowledge there was no acid or sealant used.  We are very disappointed in the result.  We realized it was a tricky job but still expected a very sharp result.  What went wrong?  

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

Some questions:

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.
Hi, Thanks for your speedy response. Let me try to describe the drive a little better. Closest to sidewalk (which you would cross going from the road into the drive) is the first circle. It is 12' in diameter and centered about 45" from the street edge and 35" from either side of the driveway. Next comes the second largest circle, 7' in diameter. It is 4' from the first circle and off center to the left if you are looking at the drive from the street. Finally, the smallest circle, 4' from the middle circle, is very off center to the right, way back in the corner. The effect of the three circles is a crescent shape. The cracks appear between the big circle and the middle circle, between the middle circle and the little circle, and from the little one to the right side edge.

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.
That helps a little.
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.
Wow! That is good information! I will follow-up with the website info you included and with the City of Madison.

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?
The glaze you saw in Madison is probably some coating sprayed-on. There must be many brands and your contractor should know. The additional cracking is to be expected under these conditions.

You are welcome.




drmor.com Web
go back