Dr. Mor & Associates

We are experiencing problems with some large machine tools we have moving, is this type of work you do ? The machines are moving approximately .001" in 24 hrs.

If I understand correctly, your problem is that the whole concrete floor is moving 0.001" per day. If that is the case it may be necessary to reinforce the foundations after investigating the soils.  This should be done by a local soils/structural engineer.

I assume that the machines are not just moving over the floor because that would probably be fixed by bolting !?

By the way, how do you measure such movement? Relative to what?

Interesting problem but I believe you need to get a local expert. If you need help locating one I may be able to help with that.

Laingsburg,  U.S.A.

I am constructing a garage floor 7 foot below my original floor --my local building permit requires --how much weight will the upper floor support ?(Autos).

The floor is 21 x 25 foot concrete slab which is poured over a metal sheeting. this sheeting is placed across the 25 ft. span --overtop (3)  4 x 6 inch I-beams running the 21 ft. span, with an opening of 6 ft between each I beam... the concrete will be 4.5 inches thick with 1/4 inch re-bar in it.   I have a drawing of the idea if you need more info.  Please let me know as soon as possible-- so as I can get my permit approved and can get started on the construction.    Thank you!  

What you are asking is a structural design analysis.  By law I am forbidden to do that in a state where I am not licensed to practice.  Besides, an engineer will need more information regarding the footings, beams, additional loads, etc.  I  strongly recommend you consult a local structural engineer.  This type of analysis is straight forward.

Thanks Doc!

I was hoping to once again skirt the long arm of the building inspector-- but your right-- I need to do this right the FIRST time. Thanks for the speedy info! Gary

I'm running out of time trying to find a company to provide a sub-bid for some very flat concrete work (finished product FF50) that has done some superflat (2-FF75's) work in the last 3 years. Can you do this? If not, do you know a company that can? Job is only 18,900 sf (2 slabs) and is near Sacramento, CA.  It bids 15 October 1996. We can do good flatwork but we specialize in treatment plants which only requires the use of a mechanical screed instead of a laser screed as required for this work.

The only local company that is quoting this work was involved in writing the spec's and is quoting to all general contractors on this project.

Bidding has been extremely tight lately and I need an edge to better my chances of getting this project. Can you help? Please call me at (???) ///-//// .... Thank-you for your time and help.

I am not familiar with companies in your area. sorry.
The companies I know in the Los Angeles area that have been doing "superflat" work did not use laser screed and did not get FF50 or over consistently.
With a little more time I could have checked with the local ACI chapter and/or the superflat concrete committee members who should know these people.
You may want to try the Sacramento or Bay Area ACI chapters. Another source of local information would be the testing laboratories that provide quality control. They usually know who performs good work.

Adana, Turkey

Dear sir, madam
we have poured the slab it was 35 centigrade degree. After pouring I face with cracks although I use burlaps to cover it (cracks nearly 1/8 inches). How can I avoid these cracks and after pouring how frequently do I have to water it?  Can I water it in the afternoon? (here traditionally they say to water only in the morning and in the evening. Not in hot afternoon.  Also there are no cracks within beams and columns - only on the slab and there is leakage through the sub floor and salt color tracks of the leakage. Although this problems I face with, I can't even hammer a steel nail on the slab it is so hard. Does it show that no structural problem?

Well it was surprise for me to find something like that web site.  It is amazing. I would like to express my gratitude to you.

Dear M.A.,

This is a very common problem, even though yours sounds a little on the severe side.

In general, these are shrinkage cracks, caused by rapid loss of water from the fresh concrete while it hydrates and dries.  Higher quality (stronger) concrete uses less water and will experience less shrinkage.  Reinforced concrete will experience the shrinkage, but the steel will hold the cracks together and you will not have those wide cracks.

As to your specific situation.

1.  Pouring concrete at high temperatures requires special care.  35 degrees Celsius (95 deg F) is very hot and will cause the concrete to dry faster with larger cracks.  To minimize that problem you should use stronger concrete (less water) and start misting the surface as soon as it appears dry.  Do it very carefully since you do not want to disturb the "skin" before it hardened, or add water to the fresh concrete.  At these temperatures, normal concrete will probably be hard enough after 3-4 hours, but that depends on admixtures used in your area.

2.  Covering with burlap is good practice if you keep it moist at all times, and apply it before the concrete lost too much water.

3.  The reason for watering in early morning and evening is to save water.  When you water around noon time, the heat will cause the water to evaporate almost instantly instead of penetrating and soaking the concrete.  However, if you water the burlap and cover with plastic sheets you may overcome that problem.

4.  I assume that your slab is placed directly on the ground without any steel.  These are not structural slabs and will not "collapse."  They will, however, remain cracked and cause many problems inside the house.  If you plan to use floor tiles, the cracks might show through the tiles and cause them to crack too.  The salt deposits you observed (efflorescence) are an indication that moisture is coming through.  These will not stop and might get worse.

5.  One thing you can do to minimize the cracks is to reinforce the slab.  In the USA we use welded wire-mesh (steel wires welded to form a "net" with square holes) to arrest the cracks.  In more severe situations we will also use steel rebars centered in the slab.

6.  The most basic method of dealing with this problem is to provide "joints" at regular intervals.  These joints are pre-cut "cracks" that force the cracking to happen where you want it and in straight lines.  The joints can be cut by a concrete saw as soon as the concrete is strong enough to allow the equipment on top (but not later because it will start cracking...), or placed in the concrete during pouring by scoring the fresh concrete to about 1/3 of depth.  Joints should be placed every 10-15 feet (3-4.5 meters) and divide the slab into square sections.

Hope that helps.  If you have access to publications of the American Concrete Institute (ACI) you can find many sources that will help you further.

My name is <  > and I am in the process of having a home built.
We are early in the construction phase with the decking and walls up.

Today we received approximately 1 inch of rain, thoroughly soaking the rough-framed structure. While I believe the 3/4 inch TIG (tongue in groove) OSB material will withstand the soaking, I am concerned about the 3/4 inch TIG plywood which was used as decking material in the main room.
Is there any way to know if separation will occur before I install the oak hardwood flooring over it. The plywood is not of an exterior nature but I have been told by my general contractor that it does contain exterior-type glue. Any comments you could provide would be appreciated.

If the manufacturer did not certify his material for external use then I would hesitate to accept your contractor's word.  You may want to contact the supplier/manufacturer for their opinion.  You will usually find stamps on the plywood that include all the information you need.
I suspect, however, that a single soaking is not the end of the world.  Allow it to dry completely and check it again.  If it did not separate, then it most likely will not unless repeatedly going through wetting and drying cycles.

Will your contractor put his money where his mouth is?  Will he post a bond to guarantee his work?
I assume that simply replacing a few sheets of plywood at this time may be worthwhile considering the costs involved with any other option if failure occurs later.

I hope that helped.

Thanks for the help .. There is some slight separation on one sheet, but I think it can be sanded down and filled with a suitable material before the hardwood flooring is laid. FYI: I have spoken with others who have had similar and more serious water penetration and none report after-the-fact concerns.

We live in an area of Los Angeles where the water table is very high. During the recent rains, the garage flooded. Clean water slowly seeped in on all sides of the garage, where the floor and cement meet. After the rain, all that water found it's way back out. The whole thing is a mystery to me. Where is this water coming from? What can we do to keep the water out? Are you able to come see it and give me some advice? If so, I would greatly appreciate it. We live in the mid-Wilshire area (near the La Brea Tar Pits).
Thank you so much.

Very informative web site!

Los Angeles

The first thing I would be looking at is the landscaping around the garage. Make sure that its level is below the floor level and that the drainage system works.

If that is not the problem, then you may have ground water accumulating under the garage during rain, and developing pressure because of runoff from higher neighbors or landscaping. Since the garage slab is not sealed to the side walls (by design) the gap may be enough for water to come through under pressure.

Solving this problem is a two step process:
1. Prevent water buildup under the garage. This is done by providing drainage around the perimeter using "French Drains" installed at the footings level. However, this may also be expensive, depending on your landscaping and yard. 2. An alternative, or second measure, would be to seal/caulk the gap around the perimeter of the slab. For proper application, you will have to clean the gap very thoroughly, route it to at least a 1/8" wide by 1/2" deep groove, and use an epoxy-based sealant/caulk to fill it. Preparations and applications MUST follow the manufacturer's instructions to the letter!

I have a concern regarding the interior. I would not expect the same flooding since the slab is usually sealed to the walls in the living area, and a special water-proofing membrane (Visqueen) is applied under it. However, if the water level and pressure are that high you may have some seepage under the carpets/linoleum, or hardwood flooring. Given time, you may develop some nasty mold or just ruin the flooring. You may want to try lifting the edge of carpets at the corners and look for white salty deposits or black moldy areas.

Regardless of what you find, you should do something to improve the drainage from the yard including maintenance type clearing of drain pipes. You should also check with your neighbors to see if the problem is widespread, in which case you may be able to share the cost of investigating.

Good Luck

---

  The linked sketch shows what a proper installations should look like.  There are other variations, but that will give you an idea as to what you should find under the garage 

I am putting a hot water heated concrete floor in my garage and I am wondering if it is a good idea to put insulation under it? I have been getting mixed opinions about this. I would also like to know if it is worth putting fiber in it if I am also using rods?

P.S. We live in northern WI.

I can understand why you are getting mixed opinions.  The right choice depends on additional information I need.

1.  Is that a slab-on-grade?  If so then it depends on the soil for support.  If you use some sort of insulation between the concrete and the soil it can weaken the system.
However, if when you said you are using "rods" (reinforcing bars?) you meant that it is a structural slab which can stand on its own then the support is not critical.  Was it designed by a structural engineer?

2.  If you can find insulation that is strong enough to support the weight of the concrete and your cars without crushing then I see no reason to worry.  But if that insulation is going to deform in any way it will damage the concrete.

3.  As for fibers - depends...
If you are adding fibers in order to gain some strength then forget it.  They don't.
If the purpose is to reduce the cracking in the slab then there are better ways to do it, especially when you have reinforcing bars (or is it wire mesh?)
Fibers add complications in the placing of the concrete and I see no benefit in your case.  You should however consider cutting joints that will split the slab into sections that are no bigger than 15x15 ft.  After the concrete matures (3-6 months) you can seal the joints and forget about them.

What were the concerns others had about the insulation?  Let me know if there is additional information about the slab and what you decided to do.

The slab I am about to pour is on-grade. My main concern is the amount of heat loss going down into the  ground without insulation. I plan on insulating around the outside footings but not under the slab. Some say the loss is minimal others say it will be a great amount. I would like to hear your opinion on this matter.
 Thank you for your advice on fiber reinforcement. It seams everyone in this area is putting it in. Not having poured concrete with this type of reinforcement I wanted to get some input before I start.

I am afraid I am not too familiar with the thermal properties of concrete in your area, or the temperature differences between inside and soil under the slab.
Another option you may want to consider is using Light Weight Aggregate Concrete.  This material can provide considerable insulation on its own but is not as strong as regular concrete.  Check with local Ready Mix plants to see if they can produce the strength you need (it is very possible if they know the material) at a price you can afford.  Since that is not standard material there may be a premium.
With appropriate reinforcement LWA concrete can be used for any structural purpose.  Personally I produced LWA concrete with strength of over 5000 psi without a problem.

Hello, Can you tell me what code says about cracks in a new construction home slab? We are having a house build and the slab started showing cracks (some 12 foot long) after just a few days. We have been told not to worry about them, but are afraid of them getting worst.

Orlando, FL

It all depends on the size and extent of the cracks.

The code - Uniform Building Code (UBC) - does not address slab cracks directly since these do not constitute a "life and safety" issue. However, the appearance of such long cracks so soon indicates that there is a real problem that should be addressed immediately before they build the structure and make it too expensive to fix. For starters, the appearance of these cracks shows a serious mistake by the contractor. There is no excuse for that and you should not accept them as inevitable.

Some of the basic guidelines they should have followed:
1. Use quality concrete - in your case at least 2500 psi, or whatever the plans call for. DO NOT add water before placing it - water weaken the fresh concrete and make it more likely to crack.

2. Create control (Contraction) joints as recommended by the ACI (American Concrete Institute). These are either formed during construction, or saw-cut soon after to a depth of 1/4 thickness of slab. A basic rule of thumb suggests to space control joints (in feet) about 2.5 times the slab depth (in inches). For example, a 5 inch slab will require joints every 12.5 feet. The resultant panels should be as close to square as possible.

3. Use reinforcement that is placed in the center of the slab. Most people still use wire-mesh, but rebars are a much better solution. If the reinforcement is not near the center, it may be useless in limiting cracks.

4. Provide wet curing. Concrete needs to be kept wet for as long as possible. Common recommendations is at least 7 days. When placed in hot weather areas such as Florida, the concrete may dry faster and start cracking within hours after placing. Curing can be done by covering the surface with wet burlap, plastic sheets, wet sand, or more commonly by spraying a curing membrane on the hardening concrete.

Missing any of these items can cause cracking. A contractor who fails to follow these guidelines is not following the best construction practice as recommended by the ACI. There are a few publications available from the ACI (through the online store on their website) - "The Contractor's Guide to Quality Concrete Construction" and "Slabs on Grade" are two of the best - which provide this information with the authority of the ACI behind it.

As for the "not to worry..." part. It usually means they either do not know better, or are trying to avoid fixing their mistakes. You should worry because these cracks will grow for at least another year or so and affect any kind of flooring you try to install. It may or may not get worse, but do you want to wait and find out after the house is completed and any repair will be very expensive and disruptive? Unfortunately, at this point when the concrete is still young, the only guaranteed repair is R&R (Remove & Replace) of the slab. Trying to patch the cracks is not a good approach since the concrete will keep shrinking and the cracks will reopen.

By the way, the plans should have specified the joints, reinforcement, and concrete quality. If the contractor simply followed the plans then he is only partly to blame. If the plans failed to specify that, he should have asked for instructions, or used the best practice as recommended by the ACI.

I would expect serious resistance from the contractor to any R&R demand. You may need to bring in an attorney at his time and prepare for a lengthy process. You will hear that "concrete always cracks" and that "every slab has cracks". The answer to the first is that this is why we have the four requirements above, and the second is wrong. One possible compromise would be to cut expansion joints in the slab today (using diamond-blade saw) and epoxy seal the cracks. That may minimize future cracking and fix the existing ones. But there is no guarantee.

Good luck.

Aurora, USA

I need to build a 7'x7' 6 inch thick concrete slab for a spa/hot tub. The grade from an existing slab to the outside edge (7 feet) of the proposed slab drops 19.5 inches. I assume I need to build a retaining wall. Is my assumption correct? If so, can I use 6"x6"x8' landscape timbers? also, are there any other considerations I should be aware of?

R., 

The answer to that depends on the soil conditions, but most likely you will need some sort of retaining system.. 

My first choice would be to build the slab with "thickened edges" that serve as footings.  On the sloped side you would make these edges go deeper into a trench in the soil all the way down to "solid base."  If you make the "footings" 8" wide by 7' long by 30" deep (guesstimate) you may need an extra 1/2 yard of concrete, but will end up with a much stronger and stable system.  It would be better to use at least 2 rebars at the top and bottom of the footings (length wise) and additional rebars to tie the slab to the footings directly.  The slab itself will also benefit from rebars in both directions placed about half depth of the concrete.   Common spacing is 1 foot on-center.  If you extend those bars into the footings you will have a very strong and stable system.

A spa can be significant load on the base and may require recommendations from an engineer who is familiar with your soil conditions.

I am assuming you are planning to add fill material in order to create a level base for the concrete.  Make sure this fill is very well compacted and stable before placing any concrete on it.  Otherwise it could deform under the load of a full spa and cause the concrete to crack.

I am building a 1500 sf garage and want to install a heated floor system in it. My concrete contractor wants to use a two foot square re-bar grid and my heated floor contractor wants to use sheets of wire mesh. They both agree either system would work but they want me to be the deciding factor. It's my understanding re-bar would be stronger but wire mesh would work. Can you help me with this decision? I live in the mid-west, so mother nature may be a factor, it can be 100 in the summer and -40 in the winter.

Glenwood, USA

This is a pretty big garage.
The key here is to create joints every 12-15 feet. Otherwise, it is practically guaranteed that you will have cracks. It is also important to pour the slab separate from the stem wall / footings so that it can move and shrink freely - what is called a "floating" slab.
Once you got that, either option is acceptable. You need to understand that wire-mesh does NOT contribute anything to the strength of the slab. If the soil is expansive, or you have soil settlements, the rebar will help the slab survive some movements. The wire-mesh will not. Both will work to minimize cracking in the slab, as long as the joints are installed properly.
One problem with rebar is that it can interfere with the joints function by bridging the joints and actually allowing cracks to develop outside the joints. However, as long as the rebars are uniformly placed and properly over-lapped it should by fine. A big problem with wire-mesh is that it rarely stays where it is supposed
to be (centered in the slab) because laborers walk over it during pouring and
it might end-up in the bottom of the slab where it is useless. Rebars are placed on "chairs" that support them in the right position, and the 2 feet spacing allows people to walk in the work area without stepping on the rebars.

Bottom line - rebar requires a little more work from the contractor, but is likely to result in a better final product.

Thank You for this information, I think I will go with re-bar. I do have another small problem I hope you will be able to help me with. My heating contractor says I need to install some sort of insulation between the concrete floor and the outside walls to prevent heat loss. He suggested using high density foam insulation. But my concrete contractor has never done this before and is concerned the foam will be exposed into the working area of the garage and will be susceptible to damage and water. Is there a good solution to this problem?

High density foam would be an acceptable choice as far as the concrete is concerned. It would be best if you keep it to no more than 1/2" thickness.
However, this 1/2" around the garage will be damaged and collect dirt, water, etc. A solution will be to replace the top 1/4-1/8" of the board with epoxy based caulking material - after the concrete hardens for at least 28 days (the concrete will shrink somewhat during that time). Choose caulking that is designed to harden, but have some flexibility to stretch as the concrete shrinks and expands over time. Apply the caulking carefully so that it slopes slightly up towards the wall. It will create a strong, flexible protection to the board.
A good way to create the needed gap on top of the board will be to use wood or plastic inserts during concrete placing, and remove those as soon as the concrete is hard enough. If left too long the bond with concrete may make removing the inserts a problem. You may also use grease or oil as a bond breaker between the inserts and the new concrete.

Make sure the inserts are not too thick. Good quality caulking material can be expensive if you need to fill a large gap around the slab.

Hope that helps.

HI
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

M,

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.

 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,
D.

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.

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

F,

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.