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

Concrete for Patios and Decks:

Concrete Deck


I need some advice. We poured a concrete deck. in 1997.The concrete was dyed red and was sprinkled with a black substance and then stamped to look like cobblestone. Our steps and back deck were poured on one day.
The big deck that surrounds our steps was poured a couple days later. The big deck immediately started chipping. We were told it is because of the shale in the concrete.
They called it shale pop.
They told us it would not chip anymore and would be O.K.
Today our deck is getting more red by the day. The steps and our back deck are fine, and the big deck has a lot of chipping.
We have had them look at it, and they try to make up excuses. What should we do?

Please help me.

SORRY to hear that.

If the thing they call "shale pop" is what we call "aggregate Silica Reaction - ASR" then you have a problem.

The good news is that it is very simple to verify whether the problem is really ASR. (and it sounds like it). A concrete Petrographer can identify it easily under a microscope.

The bad news is that they are wrong. It will not stop chipping until all the reactive particles in the concrete finish reacting. In many cases it may result in unusable concrete.

The normal standards of practice require the Ready Mixed Concrete supplier to test the aggregates and never use reactive aggregates (such as shale).  When they happen to use some, they should take full responsibility and replace the concrete. In my experience, once the presence of ASR was established, the supplier would replace it at their expense.

The contractor may not have been able to prevent it, but as the one who sold it to you is contractually responsible to you, and should get the supplier to replace the deck.

You may wish to consult the building department at your locality for their experience with similar problems.

By the way - where are you located?

To learn more you may search the Web on "ASR Concrete Aggregate.  "Compare the photos and information you find to your situation.

Good luck,

Thank you so very much in responding to my problem. I am from South of FD, Iowa in the country.  Where are you from? 

Please forgive me, but I forgot what your title was? I think I found your address in concrete doctor?  Is that correct? Where would I find a person to test our deck for ASR?  I am not sure where to go from here.

I REALLY APPRECIATE any help that you can give me.

Dear E.,

Find out who does the concrete testing in your area. I could not find in the Yellow Pages, but check with the Ready Mixed concrete supplier (Is there only one? Did they sell you the concrete?) and the building department.  In Fort Dodge that would probably be at 515 573-2323.

The code requires that concrete be tested - so someone must do it.

If you cannot find one nearby, try calling "Pella" at 515-628-1357 and ask who they use.

Or call any of these laboratories (I don't know if they test concrete):
Geotechnical - 515-270-6542
American - 319-326-4847
AMI - 515-274-3996

Your best bet is probably to call Kevin Macdonald at 612-942-4826.He is the VP of the Iowa-Minnesota chapter of the American Concrete Institute.

Good luck

I just want to thank you so much for responding to me.  I have not had a chance to call the numbers yet, as my son is home this week for Easter vacation. However, I will do so, and get back with you.

It was suggested to us to build this type of deck from an employee of JB.  We thought it was a good idea, because we wanted something that was different, with low maintenance.  We went to DJ, and he told us that yes indeed his company could build one. Mr. J quoted us the price and then all materials needed Mr. J purchased.  I am not sure but I think that there is only one concrete supplier. I am going to check that out further.

Concrete Deck

I'm in the process of building a new home.  A 12'x50' deck will run across  the back of the house.  Rather than use pressure-treated lumber (with all the maintenance it requires), I'd like to use a light weight concrete for  the floor.

Can this be done?

Are there any special requirements for the deck support system if a concrete floor is installed?

How thick should the floor be?  Should it contain any joints? Specifically, what kind of concrete should I use? Can I "score" the surface of the product you're recommending to produce a colored surface?  Though the deck will be covered (second floor roof overhang), it will still be exposed to the elements - can I make the concrete floor non-slip?

Austin, TX, USA


I am assuming you are talking about a raised deck - not supported by the ground.  Is that correct?

Yes, it will be a raised deck.  It has not yet been built and likely will not be until after the house is closed in (builder is about 2/3 of the way through rough carpentry).

I am afraid it is not very doable.

If you are asking about building a structural slab with lightweight concrete, it does not make sense for this application.  Most ready-mixed concrete plants do not know how to make 3000-4000 psi lightweight concrete, and you will need a structural engineer to design it for you.

If you are asking about a "topping" application where the concrete is placed over a supporting floor and only serves as a "walking surface" - again it does not make sense for a deck.  The lightweight material they usually use for such decks is not strong enough to take the traffic.  Usually they cover those with tiles or carpets.

 If you decide to go with a lightweight topping deck on plywood you will have to provide reinforcement and joints since the flexibility of the wood supports will cause the concrete to crack.

A simpler solution may be to use tiles on the plywood base.  These are rather lightweight, easy to install, come in all shapes and colors and may not be more expensive.

Structural Concrete Deck

I am looking for info on building a above ground concrete deck.  I have seen several on homes in the south but not here in the northwest.  I under stand there is a ribbed steel sheeting that can be used for the lower form and left in place.  My plan at this time is to place 6 inch X 6 inch steel reinforced concrete post on 8 foot centers.  Part of the deck will be supported by a concrete wall to give it lateral span.  Any info on span's, reinforcing and supporting the deck or where I can get info on self supporting concrete would be most helpful.

I strongly suggest that you get a structural engineer to design the deck for you.  This is not as simple as it may look and local building codes may require it as well.
If you do decide to try it yourself - the UBC (Uniform Building Code) gives the parameters you are asking for.  ACI-318 is a publication of the American Concrete Institute that defines the methods and formulas for these calculations.  Both should be available at the library in a college that teaches Civil Engineering.

Good luck, and be careful,

Patio Topping

We currently have a concrete patio that is 32'x11'.We want to expand the patio to 32'x16'.The old section is cracked in 3 or 4 places and is slightly uneven as the result of settling over the years.  I want to add new concrete on top of the old when we expand the patio. I've asked 5 different contractors about this and have gotten 5 different opinions on the proper way to do it. What methods would you use to best insure a good bond and less cracking of the new surface? It is under roof and the new section will be also.  Also what depth of concrete can I get by with?  I would like to go with 3 or 4 inches...is this okay?


Adding concrete topping to old concrete is tricky.

The safest way would be to assume that the old concrete is like soil - and add a minimum of 4 inches on top. Under these conditions it does not matter that they do not bond (a bond breaker will be best) or if the bottom slab is cracked. But this is more expensive and will change the elevation of the patio by much.

If the bottom is cracked, it may not be a good idea to bond them together.  The best way will be to place a uniform (4") slab as replacement. An alternative will be to leave the old slab alone and just add a new one next to it. They will never look the same, but you may add some architectural treatment that will make that acceptable.

Adding a topping that is bonded to the old slab is asking for trouble since the new material will shrink relative to the old, and result in new cracks. Even joints (which are a must in any slab - at 6-8 feet spacing) will not prevent cracks in bonded topping.

Hope that helped.

Concrete Tennis Court


Miami Beach, FL U.S.A


Concrete tennis court is basically a concrete slab-on-grade with some special requirements.
The main limitation is the need for very flat and uniform surface.  Usually, concrete slabs require joints (cuts in the concrete) every 8-10 feet in order to prevent uncontrolled cracking.  they also allow some variation in surface flatness (which is fine for walking or applying carpets...)  Tennis courts cannot have that, so a higher quality concrete with reinforcing steel are required.
I would find a contractor who already did many tennis courts and can show a track record.  Go see his past work and ask the owners about him and the finished court.

Proper preparation of the "base" is also critical.  It must be compacted well so it does not start moving and/or deforming later.

As for the concrete - High quality concrete with local materials should be fine.  Normally slabs use 2000-2500 psi concrete.  It has the strength you need, but is likely to crack.  The 3000-3500 psi mix will allow you to avoid joints altogether.  Reinforce the slab with steel bars that are placed at the center of the slab.  This is very important because too often the steel ends up at the bottom where it is useless.  The bar spacing depends on other parameters but should probably be between 10-12 inches apart.  The contractor should be able to use his experience for that.

Another critical thing is the curing method.  If not cured properly, even the best concrete will fail.

In your case I would recommend sealing the surface with curing compound, AND misting the concrete for as long as possible.  The purpose is to keep it from drying before it gained enough strength to withstand the cracking stress.  Three days of moist curing is nice; 7 days is good; more than that will only result in marginal improvement.  Use a misting nozzle at first (start 3-4 hours after they finish work) that will not harm the surface.  The next day you can simply "flood" it with water.  Cover with plastic between watering and protect it from excessive heat.  If it is placed during a hot dry day you may have to accelerate the curing start since heat will also accelerate the concrete hardening and drying.  The contractor should be able to tell when it is safe to start misting.

I am not familiar with contractors in your area.  Check the yellow pages; some of the construction related sites; or walk around the area and ask people who built their courts...

Get the contractor's license number and verify that it is current.  Make sure he is insured (performance bond and workman's comp.).  Check the BBB for complaints.  Get references and check them out!  Make sure he obtains the needed permits.  Retain a testing laboratory to sample and test the concrete (this is a little more than most people do and you may not wish to spend the extra couple hundred dollars...).  And do not pay in advance!  In California, by law, a contractor cannot ask for more than 10%.  Most of them will do anyway, but you should not pay them.  If he cannot wait for the payment, there may be something fishy in his operation.

Hold back at least 25% for a week after completion.  At that time go over the concrete very carefully and look for any cracks or deformations.  Make him repair or replace before final payments. 

I could go on forever about precautions with contractors - but maybe you will find the "good" one and have a pleasant experience.

Good luck.

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?


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