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From: John De Armond
Newsgroups: misc.rural
Subject: Re: Hairline cracks in new cement slab
Date: Tue, 25 Jul 2006 00:26:26 -0400
Message-ID: <>

On 24 Jul 2006 20:11:00 -0700, "John" <> wrote:

>I had a slab poured (550 s.f.) for a garage.  Several hairline cracks
>have appeared.  The slab has 12 inch by 12 inch beams built into the
>perimeter and across the middle in both directions.  Also, it has a
>lattice of rebar on 16 inch centers plus thicker rebar, 1/2 inch, in
>the beams.  Also, in between the beams the concrete is about four
>inches thick.
>Are the cracks normal?  At what point, should they continue to develop,
>should I be alarmed?  What probably causes this?

Is the slab mirror finished and are the cracks truly hairline eg., as
if someone drew them on the concrete with a very fine pencil lead?  If
so, that's fairly normal, especially considering the temperature.

>It is summer here.  When the slab was poured the temperature was about
>103 degrees farenheit.  For the next seven days it was mostly in the
>upper 90s with lows in the middle 70s.  During the day I wetted the
>slab with a hose every hour or so for a week since the contractor said
>that would help it cure properly.

I would have tried not to pour under those conditions.  I'm not a
concrete expert but I did operate a large batch plant at a nuclear
plant construction site for about 6 months as one of my college jobs.
Any time the ambient temperature rose over 80 degrees we added ice to
the batch to keep the temperature of the pour down in the 60-70 deg
range.  This WAS nuclear-grade concrete :-) and I do know that there
is more temperature tolerance in less critical jobs but I'd still have
tried to avoid pouring in that kind of heat.

Awhile back I stopped and chatted with the batch plant operator at a
freeway construction site.  The weather was in the low 90s.  They were
batching but they were adding most of the water as ice.

Your keeping the concrete wet probably saved the job.  Still, I'd seal
the slab as soon as recommended by your concrete guy.  That will
prevent those tiny cracks from turning into spalling.


From: John De Armond
Newsgroups: misc.rural
Subject: Re: Hairline cracks in new cement slab
Date: Tue, 25 Jul 2006 13:24:22 -0400
Message-ID: <>

If you can see the reflection of objects - trees, etc - on the surface
then it's a mirror finish.  Sounds like you probably have that.

When the finishers "float" the concrete to make the finish, they work
it so that the very finest cement and sand particles float to the top
and the coarser ones sink.  This is what enables the mirror finish. It
also results in a layer anywhere from about an eight to a half inch
thick that is significantly weaker than the main body.  That thin weak
layer is what gets chipped if you, say, drop a large wrench.  The chip
doesn't go deep; it merely spalls off a thin chip.

If you were to take a concrete saw and saw out a piece, you'd see that
the bulk of the piece is regular concrete - cement, sand, gravel - but
the very top has a thin whitish grey layer with no texture at all.
Unless the curing conditions were optimum, that layer can be easily
chipped loose with a screwdriver or similar object.

Sealing helps harden the top layer and seals it against oil and other
stain penetration.  The simplest seal is some sort of clear liquid
that cures to a hard finish.  It ranges all the way up to an epoxy

You've seen the clear finish, it's what they use at big box stores
like Home Depot and Lowe's on the "bare" concrete floors.  I think
I've seen the finish for sale in those places but I'm not sure because
I always just let the concrete man do it.  He'll know how long to let
the cement cure before sealing and what sealer to use.

If the slab is exposed to bad weather, particularly cold and wet
conditions, the water can get in those microfine cracks, freeze and
wedge loose a chip or spall of concrete.  The crack only penetrates
the soft top layer and then spreads horizontally until the chip
loosens.  Kinda like acne :-(  The sealing chemical penetrates the
microfine cracks, stops their propagation and keeps water out.

I'm concerned that unless you kept the surface of the concrete
floating-with-liquid wet during those severe conditions, the top layer
will be softer than usual because of moisture starvation.  That's even
more reason to seal it.


On 25 Jul 2006 06:35:36 -0700, "John" <> wrote:

>Neon and Leif,
>Thanks.  Your input is appreciated.
>It is very smooth, but I don't know how to define a mirror finish.  It
>is the typical finish you would see in a garage in a subdivision,
>though I am about as far as you can get from one of those, at least in
>these parts.
>I didn't know I needed to "finish" the surface.  What does that entail?
>John P.

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