Dr. Mor & Associates

 Let me first of all congratulate you and your colleagues for an excellent web site and the information provided. This is the second time that I'm am requesting guidance in this area and very much appreciate your time. I have browsed through the frequently asked questions and have a new problem.

My situation is as follows: I have a poured concrete slab ceiling directly over a transformer vault. There are numerous hairline cracks visible to the eye. Water migrates through these cracks on a rainy day and consequently leak on the transformers and switchgears. The concrete slab doubles as a handicapped walkway entrance (and roof to the transformer vault) to our business building. The transformer room is naturally below grade. The handicapped walkway/roof finish is coated with a non-slip epoxy coating.
This appears to be in good condition and the only waterproof membrane indicated on the drawings. I am somewhat surprised that this passed inspections. Wear and tear on the walkway finish naturally would accelerated the life of the membrane.  My question, what alternatives are available to seal these hair- line cracks?.. Painting the underside of the ceiling?,...
hydraulic injection of a sealant/caulking into the cracks?.. routing out the cracks and installing a rudder strip and caulking? refinishing a epoxy coating, or some type of roofing membrane that can double as a walkway.

It is really nice to hear from you again. Thanks for the compliments.  Check again in a couple of weeks after I switch to a "frames" site with additional content.

This time it sounds like you have a serious problem that must be fixed.  I understand that:
1.  This is a structural slab with two-way reinforcement (or post tensioned?)
2.  There are cracks that appear closed (hairline) but which let water through.
3.  There is a waterproof membrane over the concrete; epoxy based; in good condition.  Does that mean that you cannot see the concrete or cracks from the top?

Questions:
1.  From below, do you see any deposits?  Yellow/white like last time, or rusty?
2.  What kind of traffic are we talking about?  foot, carts, wheels, light, heavy?
3.  How old is that slab?  Is it done curing, cracking, and developing its strength?
4.  Is there a contractor who built it and may be willing to fix it?

Solutions:
1.  The best would have been through original design of two part slab.  After the structural slab is in place you apply roofing membrane and pour another 1.5-2 inch plain slab on top.  The top one serves as wear surface that protects the membrane.  Usually you provide drains at the edge of the slab to collect the water that pass the top and runs over the membrane.  It is more expensive to install but there is no maintenance later.  If you have the money, and the clearance to add 2" of concrete - go with that.  But I doubt it.
2.  Sealing the underside is practically impossible.  There are some products that claim to do it but I hear so many complaints that unless someone guarantees it and shows prior experience I would not trust it.
3.  A readily available solution will be to remove the existing membrane to bare concrete; possibly sand blast the surface; and then apply one of the many industrial type epoxy finishes.  This expensive solution will give you a very strong wearing surface and you don't need to deal with the cracks themselves.  Concrete surface preparation is vital to ensure that the epoxy penetrates the concrete and creates a good bonded surface.
4.  All the options of crack repairs you mention are not perfect and might reopen when the concrete 'works' under thermal  and moisture variations.  They also require maintenance.
5.  Some type of roofing membrane that can double as a walkway - if it is designed for the kind of use you anticipate and is guaranteed.
 
Good luck and let me know what you decide,

I have crack in my cellar wall how do I fix it to stop the water from seeping in when it rains?

 thank you

R.,

This is not simple to fix.  A lot depends on the size of the crack, the condition of the concrete, and your abilities.

The best way to do it is by "epoxy injection."  In this process you inject fluid epoxy into the crack and let it harden inside.  If all works correctly it will seal the crack.

For products and procedures go to my search page at  search.htm and search the 'buildnet' engine for "epoxy inject".

Good luck

We are building a new house and had our concrete post-tension slab poured just 3 weeks ago. It was about 60 degrees when they poured and windy out, sunny day.  Well now, after the framers started framing, I noticed places all over the slab that are 'flaking' severely. I took a nail and was able to dig a 1/2 inch deep six inch across hole with little effort. The concrete came apart like layers and seemed to be powdery below.  I guess the framers are knocking the weak spots in the concrete and that is why they are showing up now.  I did watch them finish the slab.  One guy used a power trowel with another guy following him with a bottle of water and a hand trowel to finish. 
Can you please tell me what is going on with my slab? Is it serious? Can it be fixed? How do I know that the interior concrete is not weak? Thanks?

From your description it sounds serious.

When you say "hole", do you mean crack or actually a hole?

Some cracking is normal, but holes are not.  Was the slab "tensioned" already?

The purpose of post-tension is mostly to minimize cracking.

If there is actual flaking - large pieces of concrete spalling and leaving holes behind - I would stop the construction immediately until the concrete is tested and approved by an independent laboratory.  Doing it later will be terribly costly.

I assume you are in Texas. Check with your building department for information about local testing facilities. Have a qualified engineer review the concrete test results so far.  At 3 weeks, the strength should be at least 3000 psi for post-tension slab. (Unless the specific design calls for something else)

The finishing process you describe sounds normal.  However, if the trowling began while there was still free water on the surface of the concrete, it could "lock" water just under the surface - resulting in a weak layer near the surface. This by itself is not dangerous - just ugly and bad finish. It can be easily repaired at this time by using one of the "self leveling" epoxy-based toppings. After the frame is done it will be very difficult to do.

You are lucky to have caught it early.  You may want to hire your own "building inspector" to do regular inspections for you. This can save a lot of trouble down the line.  Also, document everything and keep good records.

 Good luck.

block wall was hit with car next to door opening. can you provide me with complete instructions and plans (if req'd) by next Friday 10-11-96?

Your letter does not provide any details but I assume you are talking about a building wall (as opposed to garden wall) and that it is part of a structure. If the damage is small and the wall does not support any structural elements then any contractor can just repair it. In many localities - and depending on the cost of the repair - you may need a permit.  Sometimes you can simply have the damaged section replaced with the same.

You did not say where you are, but I suggest you talk to a local contractor and find out if anything is needed and if he can provide it as part of the repair cost.

I cannot create plans for a project I have not seen and which may be in an area I am not licensed to practice at.

Lake Worth, Fl. USA

I am president of a small, 2 story, 35 year old condo on the Intracoastal in South Florida. We are dealing with cracks in some of our balconies closest to the water -- and a structural engineer who evaluated the problem for us is saying that we should remove the concrete balconies entirely and rebuild from the re-bar out.

Because of the heavy assessment costs this would involve, most of our owners would prefer going with the band-aid approach and patch the cement for the next five years until we have enough in our reserve account to pay for the balcony replacements.

The question is: Would this damage our structural support to postpone major repairs for 5 more years? Have you spoken to others facing this similar problem.

All the best and I hope to hear for you in time for our mid-August meeting.

P. T.

Dear PT,

You are aware that I do not have enough information regarding the current condition of your balconies - so I have to defer to the structural engineer who says that they must be removed and replaced.

Usually, cracks like that are the result of steel corrosion inside the concrete.  From his solution I understand that the rebar is still salvageable.  Most likely, it will no longer be this way after 5 more years of exposure.  Patching will not stop the corrosion.  Ironically, it might actually accelerate the rate of corrosion because of the extreme variations in alkalinity between old concrete and patches.  (note: that high alkalinity is what protects the rebar from corrosion in the first place.  Sea salt and water cause reduction in concrete alkalinity over time.  Most likely, your concrete is close to pH 10 by now compared to pH 12 for new concrete.)

That leaves the option of leaving the balconies alone for a while longer and hope things do not get much worse, or replace immediately before the rebar is ruined.  Patching is not an option in your case and be wary of anyone who tries to sell you the magic patching material.  I have seen it too often when patched concrete corrodes much faster.

If you decide to wait, the rebar corrosion may not be a total disaster.  It is always possible to add new rebar later for a small premium over reuse of the old rebar.  Your concern should be accidents where concrete chunks or whole balconies drop down without warning.

I suggest you get estimates from a few contractors and ask for both options - reuse or replace rebar.  You should probably consider using corrosion resistant rebar (epoxy coated) which is VERY expensive...

Good luck

Dear Concrete Doc,

Thanks for your prompt and informative response.  I was hoping that your answer would buy us more time -- but it doesn't appear to be the case.

You have raised one question, however, when you discussed options. We do have the option -- through the licensed engineer we hired -- of actually going for a  patch. But the price he quoted us for the patch was so similar (nine thousand as opposed to ten thousand per balcony) that we thought it best to go for the total replacement. That's because the equipment mobilization, removal and subsequent replacement of all the screening etc. really boosts the price when dealing with large concrete construction companies as I understand it.

But if we initially go the single concrete mason route -- without the engineer --  we avoid having the 'extra' construction costs and screen replacements added on.

We are still looking at total balcony replacement down the road, but does this added bit of info help you in arriving at your decision?  Or does your original decision still stand?

All the best, P.T.

Not really.  My concern about patching is that it hurts in the long run without a real financial savings.  Besides, I doubt that the "single mason" is capable of doing it right.  Preparation and proper execution are critical.
Regardless of your decision, get a least 3 bids and pick the best one (not the cheapest!).

By the way, I did not get the "screening" thing.  Does not sound like a "job expense" item.

Thanks again,'

Best,

 P.T.

I am working on a construction project where we are using steel tunnel forms (5" wide) that are used to form the finish concrete surface for prison cell walls.  We are seeing a large amount of surface defects (bug holes) that will have to be patched.  We need to eliminate these holes, but have yet to come up with a good solution. We have adjusted our mix design several times with changing the quantities and/or types of sand, aggregate, admixtures, etc....To date, we have yet to find a working solution.  We have also experimented with various types of vibrators. Any suggestions?

THIS is a tough one. Sounds like you investigated all the potential remedies.

Have you tried "form vibrators" attached to the steel forms? These will apply the vibration directly to the area that needs it more.  Also investigate the effect of increasing the proportion of fine materials in the concrete.

Another approach may be to reduce the size of lifts.  If you place the concrete in smaller lifts - and vibrate each - you may allow the air to escape upwards.

The best example of bug-hole free (almost) concrete around is founds in CalTrans projects. I have never checked for their secret, but we know it can be done.

My company name is *** Services and we do a lot of special projects contracts with a large university here in ***, Florida.  The problem they have, all over the sidewalks, are stains from the golf carts. The batteries in the golf carts are charged every night and batteries produce sulfuric acid, when they drive the golf carts in the rain, the rain water runs into the battery compartment and then on the metal parts of the golf carts. This produces a rusty, bloody looking stains all over the concrete, especially when they park the carts in front of the buildings.  If it is raining, they park that much closer to the door, the stains are horrible looking. Will acid stains cover this stain??

This is a very large project; there are miles and miles of sidewalks.  Can you help?? Get back with me ASAP

Sounds like a tough problem. The stuff you describe sounds nasty, and any kind of acid will eat the concrete.  Rust stains are very difficult to remove, and probably impossible to prevent on sidewalks.

Is that a common problem at golf courses?  I am not familiar with that as a "golf" problem.

The one sure way to prevent that would be to "impregnate" the concrete with epoxy based sealers. But that will probably be substantially more than the original cost of the concrete.

If this is a problem of appearance (not structural integrity) you may want to consider staining the concrete with dark stain that will make the problem less evident.

Sorry, but I don't have the magic solution...

my driveway has a very large area that is covered with a stain that looks like someone's radiator boiled over. it appeared after snow and ice melted. the driveway was about 3 or 4 months old. the builder came and cleaned it but it came back. there are many other spots on the driveway that are not near as big but the same color. the builder said that they are from the yard being fertilized.  could you please give me your opinion as to what these spots are from and if there is a remedy for them? thank you

I am afraid there is not enough information in the email.

How did they clean it? Did they manage to completely remove it? What does "came back" mean?

The "fertilized" excuse does not make sense.

Is that a dark or light stain?

Dark stains are usually an indication of moisture coming through the concrete.

By itself it is no threat to the concrete.  It may mean that the affected area is more porous than the rest, but that may still be structurally fine.

If you are very concerned you may want to call a local concrete-testing laboratory and have them sample the concrete.  This may be expensive and not too helpful.

Sounds like the builder was very responsive.  Maybe you can work with them to test it.  Most likely it is not a major problem, but you will have to do some more investigation.

Recently, I clipped my neighbor's concrete retaining wall with my 18 wheeler.  The woman is very understanding, and suggested that I fix it myself.  The portion I hit is the corner post, about 2' square and 3' high.  I knocked the corner off the upright and the beveled decorative lip at the top.  I worked in construction for several years and am comfortable working with concrete.  I know I should make forms and pack the concrete, but I have only done "new" work and am not sure how the repair will "stick".  Will any concrete bond adhesive work or is there a special type I should use? 

Reading the Q & A, I am not sure if this is the type question you are soliciting.  If not, I appreciate your time and any tips or references you can provide.  Thank you.,

 Enhaut, PA,  USA

R.,

 I am not very clear about the extent of the damage, but seems like you knocked off part of the corner post.

In general:

Remove all damaged concrete (chip it) until you have strong, rough surface.

Drill into the existing concrete for dowels.  I suggest drill 4 holes, at least 4" away from the edge (you don't want to knock-off the edges).  Get some #4 deformed bars and epoxy them into the holes.  That means that the holes should be 1/2" or slightly more, and 4-5" deep.  Have the dowels extend almost to the top of the post (1-2" below the top of the new concrete).

Use good, fluid epoxy.  Pour some epoxy (don't fill it) into the holes and then push bars in until epoxy overflows.  Let cure as recommended.

Any concrete bonding agent should be OK when you place the concrete.  Follow the instructions on the package. Use concrete mix with high cement content and better strength.  It will be more workable and stick better.

Make sure she understands that the new concrete will look different from the old...

My aluminum flagpole broke near the base.  It is in the middle of a flower bed, anchored by concrete. In order to replace the flagpole, I have to remove the old concrete, so that I can set the new flagpole and pour new concrete. 

I don't want to have to chisel the old concrete. Is there a product that will dissolve the old concrete without endangering the surrounding flowers?

Thank you for your assistance.

Interesting problem.

The answer is no.  There is nothing that will dissolve concrete without harming other things.

The only thing that dissolves concrete is acid - and that is a very dangerous substance to use.

Theoretically, you could use it very slowly and carefully so that it only worked on the concrete.  Once it reacts with the concrete it becomes harmless.  If you check my website, there is a photo of a pocket knife stuck in concrete that was softened by acids. But acid requires safety clothing, goggles, etc.  Scary.

I have never heard of anyone using it this way, but you might be able to drill a hole and fill it with acid and let it work... but I strongly recommend against it.  Acid can be a very nasty substance.

One way people have been demolishing large concrete blocks without explosives is to drill holes, fill them with materials that expand, and seal the holes.  Since concrete has low strength in tension, the internal expansion causes it to break into pieces.  I am not sure what you can do at home to duplicate that.

The only readily available material like that is Gypsum.  You probably need to drill a hole that is at least 3/4 to 1 inch diameter, fill it with fluid gypsum mix, and hope it has enough expansion potential to do the job.

I have never done that so I cannot give you any exact quantities or chances of success.

Good luck.  Let me know if any of that works.

My house was built in the early 70's.
The front porch is concrete, had existing color on it. Cleaned the porch w/tsp and bleach, waited 48 hours to stain w/waterbased stain, waited 24 hours to put 2nd coat. One week later went to hose off and stain started coming off. I'm now using a stripper to remove both the old and new. What is my next step. Thank you so much for your help.

Richmond

That is not much information, but...

Possible reasons:
- Some type of sealer was applied to the concrete before, and now prevented the stain from penetrating.
- Concrete needs to be completely dry before application (see manufacturer's instructions) to allow penetration.

A stain is not supposed to sit on the surface - it should penetrate the concrete. Your best bet is to read the instructions VERY carefully and make sure they are followed to the letter. Contact the manufacturer for additional ideas and/or assistance with the specific product.

In any case, applying paint or stain to old concrete is very tricky and may not be possible if the concrete was sealed/stained/oiled/etc before. Professionals will usually prepare the concrete by pressure washing, sand blasting, or even "bead blasting"

Good Luck

Thank you so much for your reply, I do appreciate it.
The stripper seems to be removing both color down to the concrete. From what you are telling me, it seems that even with this being done, it's a possibility that I still may not be able to apply any color to the porch. Is this correct.

Not necessarily. It can be done if you are willing to go with a professional who will "blast" the surface of the concrete in order to reach clean material, and use the more expensive "epoxy-based" paints.

Again, I suggest you check with the manufacturer regarding their recommendations and alternative products they may have.

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, USA

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.

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!

RL
 

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.

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.

J.,J.,

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.