An interesting
thing happened the other day. When my field staff was pouring a new concrete foundation,
they were given a competitor’s flatwork concrete delivery ticket by mistake. The ticket has shed some light on how a
contractor can meet specifications, while still short changing the customer. In the
end, the customer technically got what was likely specified, but not the long-term
durability I am sure was desired.
The ticket shown does need some deciphering to those
unfamiliar with concrete jargon. Under quantity it shows the number “seven”. Therefore,
seven yards were delivered. The item code states “6 bag AE” and “Mid-Range
8oz”. Let’s break down the “6 bag AE” first.
This refers to the amount of bags of cement per yard in the mix
design. A straight six bag concrete mix
with the correct proportion of aggregates and water should yield about 4,000
pounds per square inch for compressive strength. This is an excellent strength
for most concrete slab applications. The issue, however, is that adding more
cement causes more concrete shrinkage due to the higher amount paste. This leads to more concrete cracking. Also, more cement in the mix will also speed
the setting time or as we say in the concrete world, increase the rate of hydration. Speeding up this process will give off more
heat and lead to decreased strength of the end product.
The other component in the item
code is “AE” which stands for air entrainment. The admixture adds tiny air
pockets to the paste. This air is desirable in exterior concrete poured in
cooler climates because it gives water a place to go if it freezes. When water
freezes below the surface of the concrete it expands and can fracture the
concrete. In a basement, you do not need
to add air for performance.
If you continue reading the concrete
ticket, you will see it noted that the air was doubled. The drawback of adding air is that it
interferes with the cement paste matrix (bond) and weakens the concrete. A standard air entrainment proportion range
is 4 to 6 percent of the cement paste per the American Concrete Institute
(ACI). Higher amounts significantly reduce the strength of the concrete. A
double air dosage can reduce the strength by as much as 600 psi.
The next
item code or ingredient shown on the ticket is mid-range water reducer.
This is added to the concrete to increase the workability of
the concrete without adding water to the mix.
(Adding water will directly decrease the concrete’s strength.) The admixture will, however, accelerate the
speed at which the concrete sets.
You can
also see two hand written notes on the delivery ticket. One is “1%CC” and the other
is “1% HE”. Both of these are accelerating admixtures to reduce labor time
during the finishing process. These will
increase the heat of hydration and may then also increase the chance of
shrinking before the concrete is strong enough to resist the tensile and
flexural forces this will create. If the air temperature is cold, this
combination works. If the air temperature is higher than fifty degrees, the
risks of adding these admixtures start to outweigh the benefits.
So, now you should have a good
understanding of what was in this concrete, which brings me the question of why
was the mix designed this way?
To answer this, I
now must bring up the challenges associated with pouring concrete directly on a
vapor barriers Vapor barriers are necessary to keep water vapor and other
gases, such as radon, below the concrete.
When you pour on the vapor barrier, the bleed water (water that is not
needed for the bonding chemical reaction in the cement paste) can only go up
and out the surface. This is a problem for many reasons.
The biggest
problem is the additional cost of labor. You cannot begin the finishing process
until the bleed water is gone. The vapor
barrier causes more bleed water and we must wait longer before we can start
finishing the surface.
As slabs
harden, they will curl towards the air side of the slab. More bleed water means an increased chance of
curling which ultimately can cause an uneven floor surface. Both of these issues can be controlled
through good concrete practices.
My assessment
of the situation is that rather than use good concrete practices to control
bleed water, this contractor chose to add extra accelerants to the mix and double
the air content to alleviate the problems I just mentioned. The heightened air content will result in a
slab with many voids for the bleed water to hide in as it is making its way to
the surface. Therefore, more times than
not the finishing process will begin before the bleed water is all out. Trapped bleed water will cause numerous
durability issues as time goes by. In fact, I think it is probably the number
one cause of concrete slab call back issues.
The
accelerants added speed up the setting process and decrease the labor time on
the job, but will lower the strength curve over time. The concrete will gain strength quickly, but if
used when air temperatures are above 60 degrees, the ultimate strength of the
concrete will really suffer.
Therefore,
even though the concrete contractor ordered and installed a six bag mix (4,000
psi), the owner received a product that has a slim chance of performing like a
six bag mix should. The corners cut gave the contractor labor savings and gave
his customer a potentially defective concrete floor.
This brings me to the point of this entry. We specify our standard slabs to be a minimum
3000 psi concrete mix and know that when tested, the 28 day strength exceeds
this specified number. I am absolutely
sure that we are putting out a much better product using our concrete mix than
this competitor who may state they are using a higher strength concrete. Unfortunately, just specifying the strength of
concrete does not give you more as the customer. You
need to choose a contractor with the knowledge to properly install the concrete AND the moral integrity to not take short cuts that reduce the structural integrity of your
concrete.
