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

Concrete for Special Applications:

Concrete Topping for Fire Damage

We had a garage fire the garage was 32X48 We have been told we can lay a new 3" concrete on top of the old after the fire. Then we were also, told that we could build the building on the old concert and pour the new concert inside the build on top of the old concert. we live in northern MN so we also have a weather issue. Could you please help ?
Thanks you

Grand Forks


Both options seem reasonable.
I am assuming that the fire was not intense enough to cause the concrete to spall and/or develop large cracks.
Placing a 3 inch topping on the old concrete sounds right, but make sure they use a 'bond-breaker' (slip sheet) between the old and new concrete. Otherwise, you will see old cracks transfer to the new concrete. Also follow all the normal procedures for new concrete as far as placing, curing and joints. A garage of that size should be divided into no less than 6 sections (15x15) using joints.
I assume that contractors in your area know how to deal with freezing temperatures. Placing the concrete after the structure is enclosed will help keep the concrete from damage by freezing.
By the way, is it really necessary to add concrete topping? Can't you clean the old concrete (acid etch or sand blast or bead blast...)to restore its appearance? Adding 3" of topping could cause problems with elevations at the doors.

Good luck.

Concrete strength gain

Question: I just poured footings for a retaining/stem wall. The footings are 1' deep, 3' wide, and 36' long. How long should I leave the footing forms in place, and when can I put up the forms for the stem wall and pour it??

Astoria, OR

In most cases, concrete is strong enough to maintain its shape after 24 hours. That means that an element supported by ground like the footings can be stripped after 24-48 hours. However, most people use low strength concrete for footings (2000 psi) and that may require a longer period to gain the strength you want. As a rule of thumb, concrete gains about 70% of design strength in 7 days (1400 psi for a 2000 psi concrete). That is strong enough to carry its own weight and more than enough for stripping.
Bottom line: 48 hours should be ok. 7 days is long enough for any application.

Post-tension Slab

I am concerned about the post-tension slab method of slab construction for our new church. I am the pastor and several folks, I don't know why, came to me with concerns about this type of slab. The slab is hexagonal with a diameter of 79 ft. But the architect has only called for 4 inches of concrete. Is this adequate? The concerns are about cracking once tension is applied since the thickness is only 4 inches. Do we need to be concerned?

I think most of the problem lies with us since none of us have any experience with this type of concrete slab construction. Any information you might be able to send our way would be greatly appreciated.

Thanks in advance for your help,

Panam City, FL

My first impression is that it is a very thin slab.
You are talking about an Architect, but I really hope that there is a structural engineer involved in the design of the slab. The code requires such plans be signed by a licensed engineer.
In my recent experience in California I have not seen post-tebsion slabs-on-grade designed for less than 5", and recently these are usually 6" and up. The reason here is the nature of the soil that is very expansive. A flexible slab will deform with soil movement and crack. Even the 6" slabs are stiffened with "grade beams" under the slab.
If you do not have expansive soil issues, they may have specified post-tension so they can pour the slab as one large section instead of creating joints every 10-15 feet. Again, based on my experience, I doubt a 4" slab will guarantee crack free surface. By the way, the cracking is not the result of the tension. Concrete cracks because it shrinks when it dries. The cables are actually supposed to minimize the cracking.
Another problem with such a thin slab is the location of the cables within the concrete. In order to be effective the steel needs to be located near the center of the section. It is going to be difficult to do with such a thin one.
I suggest you contact the engineer who signed the plans (his stamp should be on them) and raise your concern. Get a second opinion from a structural engineer who specializes in post-tension construction.

The cost difference between 4 or 5 inch slabs should be minimal.

1920's concrete

I grew up in and my parents still live in a 17 storey reinforced concrete building in Manhattan which was built in the 1920's.

A few years when we were doing some work on a radiator, the plaster was chipped off the wall, exposing the concrete below it. I touched the concrete, and it flaked off, as if it were VERY SOFT sandstone.

This made me nervous at the time, and now that I live in San Francisco, I feel even more nervous.

Is it possible that this building could collapse on its own (there are dozens of other such buildings along West End Avenue in NYC and elsewhere)?

What about if there is a small earthquake there?

Should it be inspected somehow? And what if it is found to be unsafe? Would there be any repairs possible, or would it have to be torn down?

Thanks a lot!


New York City

The situation you describe could mean different things, depending on the location and extent of the "flaking".

If the flaking and softening was of a structural element (beam or column) this could be a very serious problem - even without earthquakes.  If, on the other hand, this is some type of filler material (between structural elements) this could be harmless.

I suggest you contact the building department and describe the problem to one of their engineers.  Since this is a "life and safety" issue they should attend to it.

A problem with these old houses is that there are usually no plans or records left - making the analysis more complicated and less reliable.  The building department should know better than anybody else how to deal with these houses. 

If they do not help, you may wish to consult a local structural engineer. 

Another resource you may be able to use is a local University with a Civil Engineering program.  From my experience, any chance to investigate such old concrete in service is a welcome opportunity to research engineers who specialize in concrete.

As for repairs, I doubt that it will be possible.  This house was built long before existing building codes were in effect and will have to be brought up to current codes as part of extensive repairs.  Unless it is one of the "historic" buildings that are protected from demolition - the cost may be much more than the cost of tearing it down and building a new modern structure.

Regardless, I believe you should do something.  It is possible that old concrete deteriorated to the level you describe, creating a serious hazard.

Cracked Driveway

I am a Gen. Contractor and hired a licensed grading and paving contractor to do a very large drive (14'x65' + 14'x45'). There are two cracks that are the width 14' of the drive and go right through to the soil. Pour is on native soil, compacted and he did use wire in pour. He says "cracks happen". Well they did happen two weeks after the pour. No one drove on them. Either I need a little education on concrete, or he does. So, whats the basic reason for these cracks?
Crack 'happen' when people do not follow the proper procedures...br>
The reason concrete cracks is usually "drying shrinkage".
Concrete is poured wet. As it dries and loses water, its volume decreases (it shrinks). Small amount of shrinkage results in "microcracks" that are not visible to the naked eye and do not create a problem. Larger amounts (which is normal) can result in cracks.

To avoid cracks we try to use concrete with less water (less shrinkage potential). This is done by reducing the water/cement ratio through the use of admixtures, or increasing the cement content (both increase the cost). Higher strength is also achieved by these changes, further reducing the cracking (the concrete is strong enough to resist the stresses).
Try to imagine the concrete is a very stiff rubber band. When it shrinks, the band is pulled and stretched. If it is not strong enough, it will break. If it is stronger, it will stretch without breaking. Keep stretching (drying) and it will eventually break.

Another contributing factor will be the curing of the concrete. Curing is a process of keeping the concrete moist for as long as possible during its early days. As you probably understand now, keeping it moist will reduce the drying part at the early age when concrete is weakest. Drying that occurs later, after it gained more strength, is less likely to cause cracks. A rule of thumb (another one) is that concrete should be kept moist and protected for 7 days or more.

Since cracking may not be avoidable, we must use joints in larger concrete elements. Joints are basically pre-created cracks in the concrete. We place those in straight lines at locations that are acceptable to us (sometimes as decorations).
A rule of thumb for joint locations on a driveway such as yours would be every 10 to 14 feet, depending on the thickness of the slab (the thicker it is - the larger allowed spacing).

When shrinkage is the cause, the cracks will develop between existing joints. If a crack develops near the joint it may indicate that the joint was not done properly and does not relieve the stresses in the concrete.

Wire mesh has very limited ability to reduce cracking, even when it is placed in the center of the slab. When located near the bottom of the slab (as we see often) it can be practically useless.

That's it in a nutshell...

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.

Garage slab

I'm planning on pouring a 24 by 24 garage floor. should I pour it in 12 by 12 sections, or one pour as my local concrete plant says I should? how long should I wait before pouring each section and should I separate them with anything? If I do it in sections should I pour the footing first and how should I tie the footing to the slab? Can I use concrete blocks for the footing and should they be filled? The ground slopes about 8 inches in the 24 feet of the floor.  you have a great site. thanks for the help.

Lots of questions - some I can answer , but others require local engineering advise.

  • 24x24 is too large.  It will crack.
  • 12x12 should be OK if the slab is at least 4" thick and reinforced with wire-mesh (centered) or #3 rebars.
  • If you pour the sections on different days you probably do not need any separation - they will not stick together.  If you pour the whole thing in one day you will have to either cut joints (with diamond blade saw) the next day, or use "zip strips" to create the joints.  Your local Home Depot should have what you need.
  • Cut joints should be at least 1/3 of the slab thickness (1/2 is better)
  • The normal practice is to leave the garage slab as a 'floating' slab.  That means that it is poured after the foundations are done and is separated from the foundations with 'felt'.  You do not tie the slab to the footing.
  • I cannot answer the question about the concrete blocks.  That depends on the soil conditions, the weight of the garage, etc.  Normally I would use concrete footings poured in trenches in the ground and reinforced with #4 bar at top and bottom.
  • The garage slab needs to slope towards the door (for drainage).  You probably need at least 1-2 inches difference between the front and the back.
  • Some points:
    • Get the best concrete you can afford.  I would go with at least 3500 psi.
    • Check if the soil in your area is corrosive (includes sulfates).  You will need special sulfate resistant mix for that.
    • Wet curing is critical for good looking functional slab.  Start 'misting' the concrete soon after it hardens on the surface.  Cover it with plastic.  Keep watering it for at least a week - the longer the better.
I hope that helps.

Make sure you get local advise regarding the footing...

Efflorescence in Hawaii

I'm contemplating purchasing beach adjacent cottages in Hawaii that have noticeable efflorescence (it is powdery and thick, in some places actually "standing" 1/2" or so high, like cotton fibre). The cottages have bare concrete floors. The floors show some sign of water staining along outside facing walls. The buildings are not new but were substantially remodeled approx. 2 years ago. How do I know if this is "typical" or more significant which may lead to more problems in the future? I'm a bit perplexed as all the online info. I read say it's not unusual and nothing seems to say that it can cause damage. The current owners say they simply vacuum up the powder and are not aware that it is causing any problems.

Normally, Efflorescence by itself does not mean that the concrete is damaged.
Normal efflorescence is made of dissolved salts from the concrete reacting at the surface and crystallizing there.
Efflorescence is considered a problem when:

1. It contains sulfates. This is an indication that the concrete is exposed to potential attack. Unless the concrete was designed to withstand sulfates, it could be a long term problem leading to deterioration and failure. It is simple enough to collect a sample of the powder and have it analyzed for sulfates by a testing laboratory.

2. It is excessive. When the quantities of 'leached' salts is very large the concrete may actually lose some mass and strength, while becoming more permeable. This is highly unusual, but your description does sound like a 'lot' of crystals. If you clean this buildup (sweep it) and it comes back within a week - I would consider that a potential problem.

3. The concrete transmits excessive amounts of moisture. This would be a concern in residential construction where interior flooring or belongings can be damaged. If you are dealing with beach cottages that are not fully enclosed in Hawaii, this is probably not an issue.

I would not be too concerned about 'staining' of bare concrete. If the surface starts spalling or dusting it is an indication that the concrete is undergoing significant internal changes which may get worse over time.

To put these things in perspective - even if you have one of these problems, the concrete slab will probably outlast the rest of the structure by many years. It may not look so pretty, but it will be structurally sound and perform its role as slab.

You need to understand that you will not be able to install any type of flooring without eliminating the efflorescence (or the moisture transmission) first.

If you want to minimize the buildup of efflorescence you will need to apply some type of sealant to the surface of the slab. Since it is a bare concrete floor the cost should be minimal. If you decide to do that make sure that the product you choose is recommended by the manufacturer for such application to wet concrete and follow all the instruction. It is very important to NOT leave untreated areas in the slab because the moisture will concentrate there.

Hope that helped.

Fibermesh vs. rebar in Residential Slab

i am getting quotes from contractors on cement work to build new  home and attached garage.  One contractor uses fiberglass mess mixed in the cement and another recommends re-bar.  Which is the best?  Or should we have both?

Barton City, Michigan


If I understand you correctly, you are trying to decide between using Fibermesh (not fiberglass mess) or rebar in a residential floor slab. When all good practice rules are followed, either method will result in a good, functional slab.

In my experience, rebar centered properly in the slab will perform better.
It can handle higher stresses and keep cracks to a minimum.  Fibermesh will also work well in minimizing cracks as long as the concrete is allowed to cure properly and is not exposed to large stresses soon after placing. These stresses can be the result of rapid drying and shrinking under conditions of high temperature and strong winds.

The advantage of Fibermesh is that it is premixed with the concrete and does not require the labor of placing rebar and maintaining it centered in the slab.  The downside is that it makes placing and finishing a little more "challenging".  However, experienced contractors should be able to do it.
It can also lead to larger cracks (instead of multiple smaller cracks) when good practice is not followed.

The rebar system is the 'old and tried' way of doing it that will normally give good performance.

Mixing the two is not required since it will be a duplication with no significant additional benefits.

You may make your decision based on cost.

I would recommend - as in any construction project - to visit a few earlier projects by the same contractor. If he produced satisfactory slabs with the
same process before he can probably repeat the quality.

I would like to hear about your choice and final result.

Good luck.





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