Dr. Mor & Associates

I wish to extend my driveway and create a patio behind our house.
1.  How do I get a rating of 3000 psi on the concrete
2. The area adjacent to the existing driveway is not level (slants away @ +3% grade, what precautions do I need to take?
3. Sand is the content of my soil (90-99%) do I need to create pillars in the sand at each corner, when pouring the concrete.
4. My father-in-law has a cement mixer can we continue to make up new batches of concrete or do we make just enough for each form, then move on?

 1.  I assume you are asking for a recipe to mix it yourself.  The exact mix will depend on your local materials, but assuming good quality aggregates and cement, you may try: (for 1 cu.yd. of concrete)

Cement - 550 lb.
Sand - 1275 lb.
Stone - 1875 lb.
Water - between 250 and 350 lb.

The more water you use, the weaker the concrete.
At 250 lb you will get moderately workable mix with potential strength of over 4000 psi
At 350 you will be closer to the 3000 psi without much of a safety factor.

To convert these numbers to cubic feet divide them by 27 (I assume your mixer is smaller than a cubic yard)

2.  I am not sure what the problem is.  Just make sure that water drains away from the finished concrete.  If you are concerned about placing the concrete - there should not be a problem unless you add so much water that it starts flowing (and then your strength will go away)

 3. I assume you are asking about foundations.  Sand (clean) is the best base you can find.  It is good practice to make the edges of the slab thicker than the rest (up to double the 4-5" you will be placing).  Also make sure the sand is well compacted and moist before you place concrete (so it will not suck water out of your mix)

 4.  Concrete may be useable for up to one hour after you mix it (depending on the cement) but I would not recommend mixing more than you can place in 15-20 minutes.  Remember, it takes time to spread and finish it.

Other important rules to follow:
Separate the slab into sections of no more than 15 feet each side.  10 feet is much better.  These joints can be created by cutting the fresh concrete, inserting plastic joints, forming, or any other creative way.  This will keep it crack free.

Keep the concrete wet for as long as you can.  Start by covering it with plastic, wet burlap, or plain sand - and keep watering it for at least 3 days.  7 days would be even better.  Start spraying it as soon as it sets - 3-4 hours, but no hard spray.  Fogging is best.

Good luck.

Thanks for the information it will really help!
I'm realizing how much of a novice I really am!
I found your site by doing a search through metacrawler.com on the key word cement! I live in Colorado Springs, Colorado on the north end of town, number 5 questions was coloring of cement I want to make it look like giant tiles with a burnt orange color (the front yard has decorative rock the traditional black/gold and I wanted to maintain some continuity in the backyard with the "patio"! I do appreciate the info. I was wondering your site mentioned using wire mesh to increase the strength to 3000 psi how or what is the process? Pour the cement then add the mesh or ???
Thanks for all you help!

If you are such a novice you should seriously consider getting help from someone who did that a few times before.

 5.  You can purchase Concrete pigments from specialized suppliers.  However, getting the right color and making sure it is uniform may be tricky.  Good luck...

<I was  wondering your site mentioned using wire mesh to increase the strength to 3000 psi how or what is the process? Pour the cement then add the mesh or ???>
You probably misunderstood what was said there.  Wire mesh does not increase the strength.  What it does is reduce the potential cracking (which is very important by itself).  The strength can only be increased by reducing the water.
The best way to use mesh is by placing it in the forms on small pieces of concrete that will keep it about 1-2 inches above the sand.  You can also buy plastic "chairs" for the same purpose at concrete supply location.  Some people simply lay the wire mesh on the sand, place the wet concrete on top of it, and then try to pull it up into the correct position (which is about half of the concrete thickness).  As you may guess, this is a very inaccurate method that can lead to problems.

Good luck,

Your web page is terrific!

I am considering constructing a small cottage in a high-desert area of north-central Arizona. I like what I'm reading on the web about concrete, for its environmental benefits, low health impacts (I have MCS, Multiple Chemical Sensitivities, and am especially reactive to petrochemical products that out gas over many years), fire and other hazard safety, longevity, and low maintenance requirements.

Question #1.I haven't yet found much info on relative expense for concrete construction. How do building costs compare with standard stick-built wood construction, straw bale, rammed earth?

#2.I love the old Southwestern adobe-and-plaster houses that have soft, rounded corners inside and out, free-form seating benches, fireplaces, niches, and so on. I assume this is possible with concrete and would love to have some information on this.

#3.Insulation: What insulating methods work well for a concrete residence and which ones do not?  Any advice on insulating only the exterior wall vs. insulating the interior wall? (Here again I am concerned about my exposure to petrochemically based products.)

#4.How do I go about finding really knowledgeable, skilled, experienced contractors for this kind of work?

Thank you so much for any and all advice and guidance you can provide.

Thanks for the compliment. I am happy you like it.

#1 - Relative expense. When building with concrete frame and blocks, the cost should be comparable to stick construction or a little higher because you do not have as many competing contractors.  If you go for 100% poured-in-place concrete you will find the costs are higher.  The other methods you mentioned are usually associated with self-builders and I don't have any realistic cost estimates.

#2 The soft rounded style is a function of the finishing process. It is usually achieved by the application of stucco.  To achieve the same effect with cast-in-place concrete will be expensive because of the forms involved, and the extra labor. But, again, when using block in-fill with concrete frame you can achieve similar finishes if you hire good contractor...

#3 Concrete itself will provide pretty good insulation because of its mass. It is especially evident in the desert. One method involves using insulating boards as part of the formwork, and leaving it in place. In Europe they use special blocks made from lightweight concrete.  We do not have similar products here.

#4 Finding a good contractor is always a challenge. Contact a few; ask for references for similar work, and actually go and talk to these people. See and touch their work. That is the only way. Of course you should also check with your state's contractor licensing board and the BBB.

Good luck.

Thanks for your quick reply and good information. I'm mulling over possibilities...if I go with concrete I will probably be back in touch with you!

Where can I find articles on the effect of sulfates on concrete? Thank you.

There are hundreds of papers on the subject.

The best compilations are from the American Concrete Institute (ACI), and you should be able to find those in any University Library with a respectable Civil Engineering department.

Also check the current textbooks on the subject of concrete.

We would like to paint or stain our concrete carport and porch.
Could you please tell me what procedures to take in preparing the concrete. The concrete has several long cracks in it. What to fill it with? Is a stain or paint best to use? Can we use a floor polyurethane paint? Is polyurethane a good product for sealing the paint? Please let me know what you advise. Thank you.

Let me address your questions one by one.
- To repair the cracks may require more effort, depending on width.  If they are open more than 1/8 inch and are starting to branch you may have to clean them and remove debris before sealing.
- In slab-on-grade it may be best to use a product designed for cracks which can be found at most large hardware warehouses.  Look for flexible materials.
- Stains are difficult to apply uniformly because of different absorption in concrete.  They may last longer, however, since they are absorbed into the concrete.
- Paints are a problem when applied over concrete that is not dry.  Where moisture exists under the surface you may see bubbles and peeling later.
Oil stains and other dirt may also cause the paint to fail soon.  It may be necessary to "acid wash' the concrete first to remove the surface.  This too is a tricky operation that might cause more damage if done with strong acid for too long.
- At the same hardware store ask for paint that is specifically designed for concrete.  They will usually be able to recommend methods of preparation and application.  Some, like our local "HOME DEPOT" have free literature.
- As a rule "never try to seal external slab-on-grade".  It needs to 'breath' and allow moisture in and out.  The moisture will find its way out of the concrete - usually by lifting the paint/sealer off the surface.

Good luck with your project,

HI,
I JUST HAD A CONCRETE SLAB ADDED OUTSIDE MY GARAGE. IT IS 4" THICK AND "6 BAG" MIX.  IT ALSO HAS WIRE MESH AND FIBER ADDED. IT WAS SEALED AS SOON AS IT COULD BE WALKED ON AND COVERED WITH A WATER PROOF TARP FOR 7 DAYS.  MY QUESTION IS:  HOW LONG BEFORE I CAN DRIVE ON IT AND HOW LONG BEFORE I CAN PARK A 3000 LB CAR ON IT??
THANKS FOR YOUR HELP AND TIME 

Phil,

First, sounds like you did all the right things as far as curing is concerned.  Did you also put expansion joints every 8-10 feet (or so)?  This is critical for crack control...

Concrete will reach about 70% of its design strength in 7 days, reach or exceed design strength at 28 days, and keep gaining anywhere from 10% to 100% during the next couple of years.  These are "rules of thumb" and every concrete is different.

I assume the slab was not 'designed' so there is no 'design strength' to reach.  Normally, it would be safe to drive and park after the 28 days.  A lot depends on the base under the slab.  It is carrying all the load and if it "gives' even a little the slab can crack.  If it is well compacted then you can probably drive on it after 7 days.

One thing to look for is 'edge curling'.  When concrete dries faster on the surface, the corners 'curl' up.  This may not be noticeable to the naked eye, but as you drive over these corners that are no longer supported by the ground they will break.  I doubt that this could be the case with your curing procedures, but I would stay off the corners for as long as possible.

Hope this helped...

I wish to extend my driveway and create a patio behind our house.
1.  How do I get a rating of 3000 psi on the concrete
2. The area adjacent to the existing driveway is not level (slants away @ +3% grade, what precautions do I need to take?
3. Sand is the content of my soil (90-99%) do I need to create pillars in the sand at each corner, when pouring the concrete.
4. My father-in-law has a cement mixer can we continue to make up new batches of concrete or do we make just enough for each form, then move on?

 1.  I assume you are asking for a recipe to mix it yourself.  The exact mix will depend on your local materials, but assuming good quality aggregates and cement, you may try: (for 1 cu.yd. of concrete)

Cement - 550 lb.
Sand - 1275 lb.
Stone - 1875 lb.
Water - between 250 and 350 lb.

The more water you use, the weaker the concrete.
At 250 lb you will get moderately workable mix with potential strength of over 4000 psi
At 350 you will be closer to the 3000 psi without much of a safety factor.

To convert these numbers to cubic feet divide them by 27 (I assume your mixer is smaller than a cubic yard)

2.  I am not sure what the problem is.  Just make sure that water drains away from the finished concrete.  If you are concerned about placing the concrete - there should not be a problem unless you add so much water that it starts flowing (and then your strength will go away)

 3. I assume you are asking about foundations.  Sand (clean) is the best base you can find.  It is good practice to make the edges of the slab thicker than the rest (up to double the 4-5" you will be placing).  Also make sure the sand is well compacted and moist before you place concrete (so it will not suck water out of your mix)

 4.  Concrete may be useable for up to one hour after you mix it (depending on the cement) but I would not recommend mixing more than you can place in 15-20 minutes.  Remember, it takes time to spread and finish it.

Other important rules to follow:
Separate the slab into sections of no more than 15 feet each side.  10 feet is much better.  These joints can be created by cutting the fresh concrete, inserting plastic joints, forming, or any other creative way.  This will keep it crack free.

Keep the concrete wet for as long as you can.  Start by covering it with plastic, wet burlap, or plain sand - and keep watering it for at least 3 days.  7 days would be even better.  Start spraying it as soon as it sets - 3-4 hours, but no hard spray.  Fogging is best.

Good luck.

Thanks for the information it will really help!
I'm realizing how much of a novice I really am!
I found your site by doing a search through metacrawler.com on the key word cement! I live in Colorado Springs, Colorado on the north end of town, number 5 questions was coloring of cement I want to make it look like giant tiles with a burnt orange color (the front yard has decorative rock the traditional black/gold and I wanted to maintain some continuity in the backyard with the "patio"! I do appreciate the info. I was wondering your site mentioned using wire mesh to increase the strength to 3000 psi how or what is the process? Pour the cement then add the mesh or ???
Thanks for all you help!

If you are such a novice you should seriously consider getting help from someone who did that a few times before.

 5.  You can purchase Concrete pigments from specialized suppliers.  However, getting the right color and making sure it is uniform may be tricky.  Good luck...

<I was  wondering your site mentioned using wire mesh to increase the strength to 3000 psi how or what is the process? Pour the cement then add the mesh or ???>
You probably misunderstood what was said there.  Wire mesh does not increase the strength.  What it does is reduce the potential cracking (which is very important by itself).  The strength can only be increased by reducing the water.
The best way to use mesh is by placing it in the forms on small pieces of concrete that will keep it about 1-2 inches above the sand.  You can also buy plastic "chairs" for the same purpose at concrete supply location.  Some people simply lay the wire mesh on the sand, place the wet concrete on top of it, and then try to pull it up into the correct position (which is about half of the concrete thickness).  As you may guess, this is a very inaccurate method that can lead to problems.

Good luck,

Given two types of batching plant i.e.. complete batch and mix or dry mix (where materials are weighed off and finally mixed within the transit trucks) which plant would be the preferred machine to produce consistent concrete to provide constant slump concrete at the concrete pump.

Hong Kong,  Hong Kong

Assuming everything works as designed, I would expect the dry mix trucks to produce a more consistent slump at the pump.  Mainly because there is no element of travel time and mixing time that will cause the most variation.

Additional advantage is the ability to batch exactly what you need when you need it - reducing waste and scheduling problems.

Other considerations include better control of aggregate moisture in the dry mix truck - if the equipment works properly...  moisture meters in the batch plant are notoriously inaccurate for many reasons.  It is much easier to maintain uniform moisture conditions in the relatively small bin on the dry mix truck.

I do not know what type of equipment is used in either batch plant in Hong Kong, but in our area the variation between loads of wet-mixed concrete at the pump can be significant (+- 1 or 2 inches).

I am a graduate student studying Architecture and am in a Seismic Design Course.  I want to know definitively why concrete is weak in tension. Given the fact that it is, I am having a hard time finding out WHY?  I am looking for as complex an answer as can be provided.  Related Formulas, thresholds, etc are all acceptable and greatly appreciated. 

thank you

You are asking for a lot...

Basically, concrete is strong in tension - it is much stronger in compression (ten times).

The reason is in the structure. Concrete is made of a collection of materials (several aggregate types, cement, pozzolans, water, air...), which are glued together with a cement paste.  The "interface" zone is the weakest link in the structure. When compressing, that interface only serves to transfer compressive stresses from one aggregate to the next. That does not require exceptional strength.

Under tension, the aggregates are trying to pull away from each other, and the glue is what holds the whole system together.  Since it is significantly weaker than the aggregates, it is where the failure starts at much lower stresses.

Steel, on the other hand, is uniform and therefore has similar strength in tension and compression.

Hope that helps.

 ["The following is a follow-up on a question regarding white/yellow stains on concrete columns in Canada"]

As you suggest, I obtained samples for analysis and tested for salts and sulfates.

The following results were given.

Ca 22.6%; Si 0.46%; Sr 0.12%; P 0.19%; S 0.02%

The material appears to be primarily CaCO3 in composition. This was confirmed by performing a acid test on a sample of material.

Sincerely Yours S.J.M 

These results are classic efflorescence, where soft rain water leach out the calcium hydroxide Ca(OH)2 from the concrete. Then it gets to the surface and is exposed to the CO2 in the air. The result is precipitation of white crusts of calcium carbonate on the surface when it dries (not freezes).
As I mentioned, in the long run, and in severe cases, the concrete will lose strength and become porous.
In your case, with non-structural concrete, it may be enough to clean the surface from the deposits, and provide some protection from rain water. You may even consider painting it with cement-colored or transparent paint AFTER THE CONCRETE IS TOTALLY DRY!!! But careful, a bad paint job may peel, and/or look worse than the original.

Hello, Doctor Mor I am a C-54 tile contractor with a re-occurring efflorescence problem in my garage. My house sets below another lot where water migrates towards my foundation .  Is there anything next to r/r [Remove & Replace] slab and using moisture barrier that will send minerals back down.
It is starting to deteriorate surface.

1. You cannot send the minerals down, but the limited loss so far cannot be significant.

2. Preventing the problem can be done in three main ways:

- Prevent the water from getting under the slab in the first place by installing cut-off walls and French drains around the house. This can be quite expensive. Its success depends also on the natural water table and your ability to dry out the area under the house.

- Apply sealant to the top of the slab. This is a tricky operation and usually requires a dry slab before application. Many of the advertised sealants do not really work so well. The good ones may cost a lot (epoxy based membrane)

- R&R [Remove & Replace] the slab. This way you know exactly what is under that slab and you can make sure it is done right. Just make sure you take the Visqueen [brand name of plastic moisture barrier used under slabs] up and seal it to the foundation walls since this may be the weak point of the system.

By the way, did you try to talk to your neighbor about reducing his water bill? This could really help. Maybe use low flow sprinklers instead of flooding the whole yard.

I would like to know if you have ever heard of or have any info on concrete houses. I am considering buying a house that the realtor says was designed by Thomas Edison, except for the roof, the house is made of poured concrete. I had never seen or heard of this construction before and have been trying to find out some information on it. So far I have had no luck, but I came across your Home Page during my search and thought you may know where I can look.

In some parts of the world this is practically the only way houses are constructed - including the roofs. In the US tradition tends towards wood construction and even in areas where concrete would be cheaper and better there is no demand.

If constructed properly these houses will:

• Resist most disasters - termites, dryrot, weather, storms, bullets, cars...
• Require very little maintenance - no need to repaint.
• Will not weaken or change with time. Concrete gets stronger with time and exposure to water.
• Will completely resist fire (except for the roof in your case)

However,

• They are difficult to insulate unless someone planned for it.
• They are difficult to modify because they are so strong and solid. It may be tricky to find people who know how to adjust windows and doors for use with concrete. But most good workmen can do it.
• You cannot nail anything to the walls. They are too strong.

By the way, where is that house and how old is it?

For articles on this subject try the "Concrete Construction" magazine which can be found on the Web. They have article index by subject. They will also give you such information by phone or e-mail.

Personally, I lived in concrete houses for many years, but with the way the market is in the US you may want to consider the resale value.

First, thank you for the information and the quick reply, I really didn't know if you would even have time to answer, it looks like you stay pretty busy. This house is located in [...], Ohio, which is [...] east of Columbus, the stat sheet from the Realtor says the house is 76 years old. It looks like it may have some minor water leaks or condensation and the exterior appears to have stucco in need of some repair. The home is 2 story with walk up attic and full basement.

By the way, I work as a firefighter/paramedic for the [...] Fire Dept. What surprised me about this house is that in all the training we have for firefighting and classes on building construction, I don't ever remember seeing this type of construction in residential buildings.

Once again, thank you for your time and the information, it is greatly appreciated.

I am a construction management student at the University of [ ]. For a project in one of my classes we are to research something in construction. My group has chosen to research "Fire Retardant Homes". If you have any information or know where we could find some help please notify me.

Your time is greatly appreciated. MF

I am not sure what exactly you expect, but the ultimate fire retardant (actually fire resistant) home is one built of concrete. There are no questions of degrees of retarding - it just does not burn. In the Q&A section of my web site there is a question from someone who was buying a concrete house and wanted to know about advantages. He happened to be a firefighter who was not aware of the fire resistant properties of concrete.

We have recently concreted a driveway, the concrete was delivered to the site in two parts on two different days and under similar weather conditions.  Although the mix was of the same consistency a number 124 50 slump, it would appear that when it has set it has set in two different colours.

Please can you advise us of the possible reasons for this occurring, and is there any way of toning the colours in so they match, as at present one area is a different colour to the other.

Thank You.

This is common, even though I would not expect a large difference in two loads on one day. The reasons may be any of the following:

More cement -- darker
More water -- lighter
Different aggregate - depending on its color

Small variations are common.

One thing that may have happened is that one of the loads was not what you ordered. Usually, the builder orders concrete with minimum strength properties.  In a driveway, that would be the most basic mix.  If, for some reason, the ready-mixed concrete supplier had an extra load of concrete sitting around (someone canceled) he might send it to you instead of dumping it. You may be getting concrete that is much better than what you ordered, but it will look different. As far as building codes are concerned this may be fine, but it should be easy to find out from the delivery tickets.

Another possibility is a different finishing process.  If the finish is done late, the earlier concrete had time to dry a little and the finish may appear lighter.

The best thing to do is probably to wait a while and let the concrete mature. The color difference may disappear. You may also call it "concrete with character" and live with it.  Or, you can ask the builder/supplier to fix it - which may be more trouble than you need.

Good luck