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

Concrete for Miscellaneous Applications:

Concrete Tilt-up / post tensioning

I work with [ ] Tensioning, which is based in the [ ] area. We are a company that supplies and installs PT [ post tensioning] in mainly commercial building and bridge construction. We are looking for some engineering information on PT slab on grade and PT tilt-up wall panels. Any leads you could pass on would be greatly appreciated.

The first source I would check is the ACI (American Concrete Institute) publications on PT concrete.

American Concrete Institute
PO Box 19150
Redford Station
Detroit, Michigan 48219


Another good source that publishes guides is:

Precast/Prestressed Concrete Institute
175 W. Jackson Blvd.
Chicago, Illinois 60604


For Tilt-ups check:

Tilt-Up Concrete Association
P.O. Box 204
107 First Street West
Mount Vernon, Iowa 52314

(319) 895 - 6911

All these organizations also have Web sites. You can find links in my Home Page

A good place to start would be at the engineering library at the nearest university.

Selecting a Contractor

My wife and I are looking for a contractor to do some work for us. We need to redo our driveway, front walkway, entryway and patio. I'm not sure if you give recommendations but I figured it wouldn't do any harm to ask. At least you might be able to tell us who the "bad" guys are so that we can avoid using your other services later on.

We live in Los Angeles, not far from the Beverly Center and would obviously like to use a local company or at least someone who is familiar with the area.

Thank you in advance for any help you can give.

You are right, I don't dare give recommendations. However, some good ways to find one include walking the area and discreetly checking recently performed work.

When it looks good you can talk to the owners and find out what they think about the contractor. Usually people like to share these experiences.

You may also wish to check with the Better Business B. for complaints.

The "Blue Book Building and Construction" (which can be found at the Beverly Hills Library and most engineers/architects/contractors, among others) lists many contractors. One trick I use is going to an older version (1992 in my case) and call from there. If they are still in business they may be doing something right.

But regardless of who you choose, ask for a list of recent jobs and take the time to see the work and talk to the owners before you close the deal.

Sealants / Caulking

Can you provide a list of suppliers who may sell / make caulking for use in outdoor cistern / collectors for rain water runoff? My customer wants to buy something that can be pumped through his pneumatic grease type system and would like bulk rather than tube type containers to work from.(Big project.)

I am assuming you are in Los Angeles.

There are two major manufacturers with whom I am familiar:

SIKA at 800-222-7452

Euclid at Ohio 800-321-7628 or locally (714)665-7969 (Bob DeWert)

Local suppliers:

Lowery and Associates - 800-439-4546 (Anaheim)

KDM - 800-585-9123

Brick & Weatherguard 213-772-2813

Building Codes

I was going research in the plumbing area when I saw your Website. If you can answer my questions, I would appreciate it. If you can not answer, I would still be grateful if you could point in the right research direction.

Question 1

If a home owner is acting as his or her own General Contractor, does that mean the Sub Contractor S/he hires is not legally responsible for damage caused by the Sub Contractor?

Question 2

Does a Sub Contractor hired by the above Home Owner Contractor have the legal obligation to perform all plumbing work to City/State Codes?

I live in Detroit, Michigan.

As you may have guessed, I cannot give legal advise. The first question should be answered by an attorney who specializes in construction issues. You may also try to check with the local "Contractor's association" but I doubt that they will be willing to give legal advise as well. It is not allowed and very risky. The "Building department" of the city should be able to provide that information and answer the 2nd question.

Your second question is easy. All work, regardless on who does it, must conform to the codes. If you are using a licensed contractor you have the right to expect performance. However, I believe that legally, you the home owner are the one responsible to the city. Depending on the work involved, you may also have to apply to the city for permit and inspection by their inspector. If the inspector rejects the work then you shouldn't pay until it is fixed.

Just make sure you are using a licensed contractor who has good credentials. It may be a little more expensive, but worth it. They have every reason to do a good work.

Building Codes - Sheetrock

We're being told by a builder that they only have to put one coat of mud on the tape in corners of closets.  The tape peels or there are ridges so the tape is very visible.  Where can we find the Missouri codes to get this person to provide what we asked for - a quality custom built home?
I don't know that there is a local building code for that issue.  Building codes address mostly 'life and safety' issues.  The only reference to joint treatment in the Uniform Building Code is as it relates to fire safety in Gypsum Wallboards.  You may want to look into recommended practices published by the drywall association.  Another good source is usually the manufacturer of the tape and/or the mud used (or any tape or mud - check the label for phone or address).  They have their recommendations on the proper use of their product.  Find their technical department and get either a specification sheet or recommended practice.
If you have a problem locating any of those let me know.  I would also like to know what you find.

High pH in Concrete Lake

We own a seaside resort in Chile with a 16 acres man made lake (it can be seen in www.uva.cl/snalfmar )  The water is obtained from wells beside the sea . It has HDPE liner on the bottom and concrete walls. The water of the lake is having a severe increase in pH . This is causing turbidity, chemical and biological problems. Please let us know if these problems could be caused by the concrete.

We also have high sulfate levels. Could this aggravate the problems. We keep waiting your valuable reply.

Thank you for your mail. It was nice to see that we reach around the world as well.  I was unable to see your lake - Web site could not be found - but I will try again later.

I seriously doubt that the concrete is causing the high alkalinity of the water. It would require a very large surface of concrete and very small volume of water to cause any such effect, if at all. I suspect that it has to do with the wells (and soil). Did you have the well water tested?

Besides, alkalinity in concrete comes from Ca(OH)2. In contact with sulfates it reacts to create Gypsum - removing both from the system.

- The high sulfate level in the water (I assume it is in the water...) is not good. Sulfates will cause deterioration of concrete that was not designed to withstand that attack. It takes concrete made with Type V Cement and very low water/cement ratio to achieve protection. This attack will be evident above water level, especially in the area close to that level where the concrete goes through cycles of wetting and drying.

The early signs of attack will be surface scaling, pitting, and spalling of the concrete. If sever enough, the dusty material removed from concrete surface into the water could cause some cloudiness of the water (but not increase the pH).

I suggest sending samples of water from the well and from the lake for complete analysis. This will give you a better understanding of the mechanism involved.

Let me know if this helps, and if you have additional information. Good luck

Many thanks for you reply.

We have done complete analysis of water from the wells and the lake.  The pH from wells is at an average 7.6 , but when we drain the lake and fill it again , after a couple of weeks the pH starts increasing (over 8.8) and the water gets turbid.

Thin Wall Panels

Dr. Concrete,  

We are building wall panels from concrete for a custom job. I am making 2' x4' forms 7/8" thick to pour cement in. I would like to keep it lite and also am concerned about cracking because we have to ship it to the job when finished. I was thinking about using "Dens Shield" from Georgia Pacific or "Durock" to set in the cement to keep it stiff and lite (used in tile setting), that will leave only about 3/8" of cement on top or the face side.

Can I pour it that thin and keep it from cracking? Can you suggest a mixture.

Can you send answer to this address instead of one I mailed from?
Thanks for any help!   JM

 Interesting problem.  I hope you do not have a short deadline.

3/8" thick concrete topping?  The problem will be to get the concrete to bond securely to the panels.  I can see a big problem with the concrete below the board since a lot of air and water will be trapped at the interface.  It might work if you could find a board that has holes in at least half its surface.  This way there will room for the air/water to escape and the two sides will bond.

Even 7/8" is very thin for concrete.  You will not be using any aggregates larger than 3/8", and will need relatively high strength (above 5000 psi).  The mix design depends on the materials available to you in your area and your best bet will be to get your concrete supplier to pull out of his archives a proven mix.  There is no way I could do that without all the local info and extensive experimenting.

If you do not have a proven mix design, you will have to spend a lot of time and money getting the mix to exactly what you need.  I can imagine that the penalty for cracked panels can be expensive.

You may want to consider using fibers (such as Fibermesh) to stiffen the concrete for transport.  A layer or two of "chicken wire" centered in the panel will also help.  I would have recommended using actual rebars, but this panel is too thin.  Even if you use #3 bar (3/8") you will have less than 1/4" cover of concrete on the steel.  This will cause corrosion and cracking problems.

An option to consider will be to use light-weight aggregate.  You can reduce the weight of the concrete panel from 1000 lbs to 750 with commonly available aggregates.  Further reduction can be achieved by entraining air.

Ground Water

I'm in the process of having a new house built with slab on grade foundation. The lot has a gentle slope from the main sewer to the foundation.

When the plumbers made the trench from the slab to the main sewer line it filled up with water. They believe that ground water was moving along the outside of the main sewer pipe and that when they disturbed the soil, it gave the water an easier path to follow.

My question is how often does this situation occur and is the normal fix to fill and compact the fill dirt in the trench and would this force the water to continue down the O/S of the main sewer line and not under the slab.

No water was found in the footings trenches or the natural gas service trench.

Thank you for your assistance. TL


It does sound suspicious.

All the other excavations were at the same time and weather conditions? (did it rain? landscape irrigation?...)

Usually sewer trench will be deeper than the gas, but not necessarily the footings.  If it was not deeper and the area is the same then there is no reason for this trench to attract water.

I have two concerns:

1. Where did the water come from?  Is it possible you have a leak under the slab somewhere?

2. Did they fill and compact the trench properly?  Trenches are a big problem because it is difficult to compact even under ideal conditions.  Usually, there is an inspector that verifies that the compaction was proper (90 to 95%).  Improperly compacted trenches can cause the soil above to subside, concrete to fail, and cause cracking of flatwork.

I am not really concerned about the water traveling backwards to the footings - unless I am missing something.

 A geotechnical engineer is the person who should be designing and supervising compaction work.  You will find them on larger projects because it is difficult to justify the cost for a single house.  However, it may be worthwhile to consult a local one at this time.

 Hope this helped.

Concrete Finishing

I am writing to request your assistance in answering a question on concrete finishing. I am a ... in the US Navy stationed at & & &., CA. I am attempting to answer the following question:

What is the correct process for concrete finishing? Is this documented in ACI or ASTM?

  • consolidating (jitterbug)
  • screeding
  • floating
  • finishing

I would appreciate any information you could provide.

Yes, ACI and PCA (Portland Cement Association) both have excellent sources for finishing instructions. From your list I assume you are talking about finishing slabs on grade and not other deep or massive elements.
A PCA publication called "Concrete Floors on Ground" is the best one I know. It covers all aspects of forming, placing, and finishing concrete slabs.
ACI has in its "Manual of Concrete Practice - Floor and Slab Construction - 302.1R" another good source
You may prefer their "Concrete Craftsman Series" which provides a more direct "how-to" information geared towards the contractor/craftsman level. Call them at 313-532-2600 and they will be happy to provide you with lists and publications.
And, if you want the ultimate publication by the US Government regarding anything you wanted to know about concrete.  try the "Concrete Manual" by the Bureau of Reclamation (renamed "the Water and Power Resources Services in the US Department of the Interior"). They cover any type of concrete construction with extensive input from the Corp of Engineers. Buy it from the US Government printing office, Washington, DC, 20402.


I manage shopping center construction. my current problem is with shopping center sidewalks in general; converting brick side walks to concrete; etc. I am also involved in new building construction; renovation etc. could I submit questions to you as they arise via email? please provide more info on rates; training programs; and services. thanks

As you probably know, this e-mail consulting is not meant to solve complex problems and I use it mainly as a way to help with small issues or provide directions for further investigations.  Anything that can be resolved by e-mail is complimentary. I like to do it as a way to make new contacts and keep in touch with what is happening.  If the problem requires more work, and/or a site visit (and you are in the greater Los Angeles area) we can figure it out for each case.

I currently do not have a training program, but depending on your area of interest I can direct you to established sources.

As far as services are concerned, most of my work is with remedial and defective construction. Unfortunately, most people look for advise after they make the mistakes - so I really appreciate your approach. Depending on your needs, I can help with the selection of the best method, the specifications, inspection and testing for QC, and follow-up with problems that arise.

Feel free to send e-mail questions and I will try to help.

Examination of Concrete Structures

What factors need to be taken into account before any decision is made about remedial action that may be needed when cracks are present?

You need to be more specific. Are you repairing the cracks, or are they present in an element that is being repaired for other reasons?

What kind of structure/element is involved?

A structural beam will require one approach while a plain slab-on-grade will require another.

By the way, where in the world are you, and how did you get to my web site?

Mortar for Stone Wall

I'm building a stone wall from field stone.

I was wondering , what type of cement and what mixture do I use for putting it up??   Can you please reply by,5/26/00?

Thank you for your time .


I am assuming that the wall is solid stone with cement mortar between them - not concrete wall with stone facing.

The best choice for you would be to try the nearest Building Materials supplier, such as Home Depot.  They have sacks of pre-blended mortars for every purpose, and these come with mixing instructions as well.

If you want to do it yourself, you can buy any type of cement - the cheapest will do fine.  Most likely it will be type II.
Buy a supply of mortar sand - they will know what size.
Mix the cement and sand between 2 to 3 parts sand to 1 part cement.  The more cement you use will make it easier to work and stronger - and more expensive.  The important part is to minimize the water.  Use as little water as possible.  Add the water slowly and mix well before adding more.  Start with a pound of water for every 2 pounds cement (1:2) mix, and keep trying until you find your best fit. 

Remember that more water means weaker concrete.

Again, my choice would be to buy the pre-blended sacks.  It may be a little more expensive to buy, but there is no waste and it is very consistent.

Good luck.

Calcium Chloride

Concrete finishers like to use calcium as an accelerator when the temperature is cool to speed up setting. Word is that calcium weaken concrete. Is this true and if so by what extent. The use of calcium at times just can not be avoided. I would appreciate an answer.


First, a correction - the material is Calcium Chloride. Calcium alone cannot be found as a mineral admixture.

Calcium Chloride does not weaken the concrete when used as an additive to cement in quantities of 1-2% by weight. (1 to 2 pounds for 100 pounds of cement). It actually will result in significantly higher strength in the first 7 days or so. By 28 days, the strength is comparable to the same concrete without the admixture. The concrete's resistance to abrasion and erosion will improve long term, while its tendencies to shrink or creep will increase.

The big problem with Calcium Chloride is its effect on steel rebars in concrete. Usually, concrete protects these bars (or cables, bolts, ...) from rusting because of its alkalinity (high pH). The admixture reduces the alkalinity and may accelerate corrosion. It does not cause corrosion - only reduces the protective properties of the concrete.

I am not sure about the Canadian Building Code requirements, but in Britain, for example, it is not allowed to use CaCl2 in structural concrete with reinforcement because of the potential corrosion.



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