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,
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 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
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
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
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
I am a Gen.
Contractor and hired a licensed grading and paving contractor to do a
very large drive (14'x65' + 14'x45'). There are two cracks that are the
width 14' of the drive and go right through to the soil. Pour is on
native soil, compacted and he did use wire in pour. He says "cracks
happen". Well they did happen two weeks after the pour. No one drove on
them. Either I need a little education on concrete, or he does. So,
whats the basic reason for these cracks?
Crack 'happen' when people do not follow the proper procedures...br>
The reason concrete cracks is usually "drying shrinkage".
Concrete is poured wet. As it dries and loses water, its volume
decreases (it shrinks). Small amount of shrinkage results in "microcracks"
that are not visible to the naked eye and do not create a problem.
Larger amounts (which is normal) can result in cracks.
To avoid cracks we try to use concrete with less water (less shrinkage
potential). This is done by reducing the water/cement ratio through the
use of admixtures, or increasing the cement content (both increase the
cost). Higher strength is also achieved by these changes, further
reducing the cracking (the concrete is strong enough to resist the
Try to imagine the concrete is a very stiff rubber band. When it
shrinks, the band is pulled and stretched. If it is not strong enough,
it will break. If it is stronger, it will stretch without breaking. Keep
stretching (drying) and it will eventually break.
Another contributing factor will be the curing of the concrete. Curing
is a process of keeping the concrete moist for as long as possible
during its early days. As you probably understand now, keeping it moist
will reduce the drying part at the early age when concrete is weakest.
Drying that occurs later, after it gained more strength, is less likely
to cause cracks. A rule of thumb (another one) is that concrete should
be kept moist and protected for 7 days or more.
Since cracking may not be avoidable, we must use joints in larger
concrete elements. Joints are basically pre-created cracks in the
concrete. We place those in straight lines at locations that are
acceptable to us (sometimes as decorations).
A rule of thumb for joint locations on a driveway such as yours would be
every 10 to 14 feet, depending on the thickness of the slab (the thicker
it is - the larger allowed spacing).
When shrinkage is the cause, the cracks will develop between existing
joints. If a crack develops near the joint it may indicate that the
joint was not done properly and does not relieve the stresses in the
Wire mesh has very limited ability to reduce cracking, even when it is
placed in the center of the slab. When located near the bottom of the
slab (as we see often) it can be practically useless.
That's it in a nutshell...
We had a new driveway put in a year ago. It is 33' x 18' edged with 4"
granite slabs. Within the drive are three circles filled with circular patterns
of brick pavers. The circles are 5', 7', and 12' in diameter. The space
between circles is 4'. The 5' circle is 13" from the back edge and 18" from the
side. The 12' circle at the other end is 34" and 35" from the sides. The 7'
circle in the middle is well inside the drive. The rest of the drive is poured
concrete aggregate on a bed of packed sand/clay (I'm not sure of this material)
overlaid with wire mesh. The plan was to have no expansion joints. However,
when poured, there wasn't enough concrete and about 1/4 of the area had to be
poured the next day. This section was separated by joints. The aggregate is a
variety of 1/2" stones. The drive looks terrible. Stones started breaking free
almost immediately. There is white scaling, the surface is uneven and collects
water. Hairline cracks have developed between the circles and from one circle
to the side. To my knowledge there was no acid or sealant used. We are very
disappointed in the result. We realized it was a tricky job but still expected
a very sharp result. What went wrong?
ran out of concrete and ended up needing a second pouring for about 1/4 and
there he put in driveway. two circles closest to the sides the distances are
13", 18", 34", and 35". between
Was that an "exposed aggregate" concrete? Why are you mentioning acid? How
was it finished?
s there a difference between the two sections? What is "white scaling"? Was
the stone done before or after the concrete?
f I got the geometry correctly, you have a 25x18 section that was poured
without joints, and round areas of stone in the middle. A section this big
should have joints unless significant reinforcement is used to control
cracking. At a minimum I would have put a joint at the center of the 18 ft
side, and three joints in the other direction. Actually, I would have expected
more than hairline cracks - but the stone circles may help by reducing the
concrete area (and the potential shrinking) significantly.
You say that the wiremesh "overlaid" the clay. That wiremesh should be
centered in the concrete in order to arrest cracks. Lying below it does no
In general, stones should not break - so something was done wrong. I would
suspect that the finishing was done incorrectly but I need more information.
for your speedy response. Let me try to describe the drive a little
better. Closest to sidewalk (which you would cross going from the road
into the drive) is the first circle. It is 12' in diameter and centered
about 45" from the street edge and 35" from either side of the driveway.
Next comes the second largest circle, 7' in diameter. It is 4' from the
first circle and off center to the left if you are looking at the drive
from the street. Finally, the smallest circle, 4' from the middle
circle, is very off center to the right, way back in the corner. The
effect of the three circles is a crescent shape. The cracks appear
between the big circle and the middle circle, between the middle circle
and the little circle, and from the little one to the right side edge.
Again, the dimensions of the entire drive is 33' x 18'.
The section that was poured later abuts the street sidewalk. One
reinforcement joint connects the large circle to the sidewalk. If you
draw a line from the center of the circle perpendicular to the sidewalk
that would overlay the joint. The second joint runs from the big circle
to the right side, again if you scribed a line from the center of the
circle perpendicular to the right edge of the drive it would overlay the
There really isn't appreciable difference between the first day and
second day sections.
The pavers were formed into the circles before the concrete was poured.
I don't know what "exposed aggregate" is, but the intent was to have a
pebbled drive. When the concrete was poured and, while still wet,
workers threw the small stones on top and troweled them in.
I mentioned acid because in other websites talking about this stuff they
mention some sort of acid treatments that cause the stones to rise but I
am very ignorant here.
As far as finish, there wasn't any finishing. They just troweled in the
stones, and let it dry. Is that what you mean?
By "white scaling" I mean crumbling, the stones loosen leaving a
whitish, crumbly concrete. Also in other areas no stones rose to or
stayed on the surface giving a splotchy appearance of bare concrete.
My wife tells me the wire mesh was handled correctly as you pointed out.
I wasn't clear in my description. She also added that the stones are
sharp edged, they haven't been set with any treatment so you really
can't walk on the drive barefoot.
My son attends U of Wisconsin and when we visited him last fall and I
saw the excellent condition and glazed treatment of the aggregate
concrete sidewalks on State Street, the main commercial street on
campus, I was puzzled at why our new drive was so bad. Also, the City
used aggregate concrete in their handicap access sections at corners as
well as for trash barrel holders, all totally without cracks, very good
looking. I couldn't understand why our drive could not be as durable and
good looking (granted they had a mix of the pebbles and larger stones
but I don't know if that would make a difference). I felt we had a
problem. Our contractor says, "All concrete cracks and I bet there are
cracks in the sidewalks in Madison." I don't remember any.
What about the psi? I read something about 3000 psi or 3500 psi. Does
that make a different in this case?
The contractor is going to rip out the concrete and do it over in the
spring. I'm wanting to get up to speed on the process so we can improve
our chances at getting a wonderful driveway. Thanks for your thoughtful
That helps a little.
For starters - what you describe is called "exposed aggregate concrete".
If you search the Internet for this expression you will find more
On my Q&A area you can find a few prior questions as well .
1. From your description it sounds like my type of joints will interfere
with the architectural appearance of the slab. There will be a tradeoff
between joints and some hairline cracks. If the appearance of the
existing joints is acceptable to you then I would suggest providing
joints as I described before.
2. From your description sounds like this is not the "best" contractor
Anyone who runs out of concrete in the middle of a job is missing
something. Have you seen prior work that he did for similar
applications? Does not sounds like he knew what he was doing. By the
way, I would hesitate to take anyone's word that he will come back next
year and redo the job at his own expense. Did he make this a written
commitment? Is there any way to enforce that?
3. The procedure you describe is only part of the process. After
sprinkling the finish aggregate on the surface they are supposed to tamp
them into the concrete until they are flush with the top, and then
finish the surface normally. After the concrete hardens, they are
supposed to come back and sweep/wash away the top 1/16 to 1/8 inch of
cement and expose the aggregates. This is a tricky process. If they do
it too early, or with too much force, they will dislodge the aggregates
(as you described). If they wait too long or do not use retarders they
may not be able to expose the aggregates at all (again, as you
4. An alternative way of exposing the aggregates is by using acid that
will cause the top cement to decompose without harming the aggregates
(that are much stronger and harder). This is still an alternative, but
should be done by someone experienced (I somehow doubt that your
contractor qualifies...). This process does not cause the aggregates to
"rise", but instead removes the cement matrix around them.
5. Exposed aggregate applications are usually done with river gravel
that is rounded and smooth. I do not know why he used sharp stones.
6. As for the strength. Driveway application is usually done with
2000-2500 psi concrete. Stronger concrete (3000-3500 psi) will be better
but more expensive. This kind of concrete is usually used for structural
elements such as walls, columns and beams. I suspect that he used
concrete with sufficient strength since no ready-mixed concrete supplier
will supply anything weaker than that.
7. I hear the claim that "all concrete claims" every time there is a
problem. In a way it is true, but this is why we use wire mesh and
joints. The joints are actually cracks we put in the slab where they are
acceptable in order to prevent uncontrolled cracking. If we fail to
provide the joints, nature will do it for us in the form of cracks.
8. Since the city is using the same material successfully, I would call
the building department and ask for a copy of the specifications they
use. You may have to apply some pressure, but under the "freedom of
information act" I believe you are entitled to a copy. The
specifications should detail the accepted materials and procedures for
Articles that describes the process very well can be found at: click
Wow! That is good information! I will follow-up with the website info
you included and with the City of Madison.
I had a question about the glaze I remember atop the stones in Madison.
Is that the sealant or is there some special coating like gives it that
sealed in look? Is it possible to send you actual photos of the drive
for you to look at? Then you can see in detail what I'm talking about.
number of cracks have multiplied, there're about a dozen now. Our
general contractor did considerable renovation on our house and
subcontracted the drive. The general contractor has committed to having
the drive redone and I believe there is no question that will happen.
The tougher question is whether or not the subcontractor who poured it
originally can meet a higher standard. This is why I want to get as
clear as I can on what the specs look like for a superior result. As far
as the stones, I don't think they were unusual for a job of this sort,
they just seemed to stick out...maybe because that sealant wasn't
applied? Thanks again for you help. What kind of work do you do?
The glaze you saw in Madison is probably some coating sprayed-on. There
must be many brands and your contractor should know. The additional
cracking is to be expected under these conditions.
You are welcome.
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.
Normally, Efflorescence by itself does not mean that the concrete is
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
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
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.
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
Barton City, Michigan
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
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
not require the labor of placing rebar and maintaining it centered in
slab. The downside is that it makes placing and finishing a little more
"challenging". However, experienced contractors should be able to do
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
same process before he can probably repeat the quality.
I would like to hear about your choice and final result.