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12/10/2009 |
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Vacuum vs. Pressure |
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How Vacuum Out Performs Pressure Injection |
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Tecvac, Inc. |
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Authored by: Jeremy S. & Joshua A. Boyd, James B. Michel |
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Vacuum vs. Pressure
How Vacuum Out Performs Pressure Injection
Jeremy S. & Joshua A. Boyd, James B. Michel
Vacuum technology
has been proving its
merit in the repair of concrete, masonry and stone since the processes
were
patented the early 1970's. We at Tecvac,
Inc. have been performing a variety of vacuum processes since 1991. Many years prior to our conversion to vacuum,
I would guess that we pressure injected as much epoxy as any of the
other
injection contractors in the area. We
know the difference vacuum makes in the permanence of repairs and there
is no
question about its unprecedented qualities, especially when coupled
with the
right repair resin. We have vacuum
applied epoxies, esters, silanes and urethanes, but when performing
structural
repairs, methyl methacrylate is our product of choice; and for good
reasons. Oh, I don’t suppose I blame the
pressure advocates, especially those pump manufacturer's who's lively
hood
could be jeopardized with any notion that vacuum could un-do the
prevalence of
conventional pressure injection superiority.
Howbeit, it sort of reminds me of the buggy builder though,
talking
about Henry Ford. No doubt, the guy had
been building buggy's for 100 years and knew a thing or two about
buggy's…but
he knew little about automobiles or Ford Motor Company.
Lets
start
with the basics and the propagation of vacuum's total superiority to
conventional pressure injection. As you
might know, everything is porous. A
typical crack has fissures, voids and interconnecting cracks located in
the
side walls of the fracture. These
fissures, voids and interconnecting cracks continue deeper on a
microscopic
level. Imagine sealing and pressurizing
one of these little voids with air…like a concrete balloon. Is it hard to see how air and/or moisture can
be trapped in the tiny fissures of the matrix?
Then, imagine filling the concrete balloon with a pressurized
liquid. How full do you think you would
get it? Yet, using only a modest 10in of
vacuum would reduce the pressure inside the cavity by half. But, lets just say we can only lower the
pressure in the fracture area to 14psi from 14.7psi…and apply Boyle's
Law. One cubic inch, one cubic foot or 1
cubic
meter of air will evacuate! But vacuum,
stand alone or assisted, employs the application of the most basic
physics
law…You cant put two things in the same place at once.
Despite assertions otherwise, this is a very
important law to be mindful of when considering vacuum vs. pressure.
With
that
said, it has long been touted within the realm of pressure injection,
and
continues to be touted with numerous methods and apparatus to
accommodate the
purpose, that low pressure is better than high pressure.
Now, if vacuum will eliminate ALL pressure,
how can it be said that vacuum cannot reasonably be considered useful? How could anyone state that vacuum will
contribute
little, or be rarely useful, to the pressure injection processes? Doesn’t it make sense that this lower
pressure zone, induced by vacuum, would be more receptive to resin
installation? Try the Boyle's Law thing.
The
installation and penetration of the repair resin is hence, two
different
animals. While pressure injection
continuously labors against atmospheric pressure, vacuum injection flows
because of atmospheric pressure. Think about it…injecting at a very modest
25psi is only serving its purpose by overcoming 14.7psi atmospheric
(1bar). Why would it take 100, 200 or
1000psi to move material in a fracture zone? Porosity?
Fluid resistance? Why would a
vacuum applied to the same
set-up, the same fracture zone, pull the material right out of an open
bucket?...well, it does. Vacuum
injection is starkly adverse to pressure injection; the vacuum process
actually de-pressurizes the repair resin and, naturally
de-pressurizes the
entire fracture zone.
Yes,
all concrete is porous.
The example of splashing a cup of water on a wall is evidence of
that
fact. However, while this porosity is remarkably useful when installing
low-viscosity repair resins, it is not very useful when attempting to
pressurize an enclosed fracture and then, expecting the porosity of the
concrete to dissipate entrapped air.
What is air entrained concrete?
More to the point, its not that concrete is good or bad at
trapping air;
it will tenaciously resist air moving through it. Pick
up a piece of the roughest open-surface
concrete you can find, so thin its fragile to the touch, and try to
blow air
through it. How much pressure do you
think it would take to force air through a piece of solid concrete? Our vacuum processes can seal a sheet of
plastic to a concrete surface so tight that it is impossible to remove
with out
destroying it. How do we do that if air is moving through the concrete? There is a profound difference in air and
moisture vapor moving through a concrete matrix and the dissipation of
pressurized air during an injection process.
I would pose this question to a competent and conscientious
pressure
technician…or pump manufacturer; how does the air dissipate from an
enclosed
concrete fracture during pressure injection?
Lets
think about the little concrete balloon again; and lets say
you have a respectable pressurized fill level of 85% at the point of
refusal. Good luck trying to get that
concrete balloon 85% full without some exhaust.
Anyway, how would one propose to achieve 90% fill level? Stay on the crack longer?
Lower the pressure? No! It would
require you to jack up the
pressure. Only makes sense doesn’t it? Is it so hard for the pressure tout's to
understand that increasing the pressure could, and will with enough
force
applied, extend the crack? Why? Because the entrapped air, the unvented void,
will pressurize and exceed the strength of the matrix.
At what pressure the extension would
transpire is directly related to the design strength and condition of
the
matrix. Apply 3000psi pressure to a crack
in a 2500psi matrix and it will give. If
the greatest advantage of high pressures is production, what is the
greatest
disadvantage? And why do pressure
injection specifications restrict the amount of pressure that may be
applied
during the injection process?
We
have used
vacuum injection on just about all configurations of concrete, masonry
and
stone. In addition to individual crack
repairs, vacuum can repair spider cracking by the square foot instead
of the
conventional lineal foot basis. Our vacuum
processes can have an entire area repaired and back in service before a
pressurized epoxy application can be set up; and at a fraction of the
cost. Unlike pressure injection, vacuum
injection can be safely used for the repair of an unrestrained element. Topping slabs, mosaic tiles, delaminations,
etc., can actually be simultaneously held together using the same
vacuum forces
used to perform the repair. This notion
that high pressure cannot "blow out" a crack is nonsense.
It is generally believed that any pressure in
excess of 40psi has the potential to further damage concrete. REMR Technical Note CS-MR-3.9 states,
"The pressure used for injection must be carefully selected. Increased pressure often does little to
accelerate the rate of injection. In
fact, the use of excessive pressure can propagate the existing cracks,
causing
additional damage." And what of ACI
301-05? "…pressure can propagate the existing cracks, causing
additional
damage." Despite hollow preaching
claims otherwise, the perils are real and well noted.
The chance of further damage, propagation of
the crack condition, is significantly greater by following conventional
pressure injection methods.
Moreover,
the
matrix contains, at minimum, very nearly the same amount of moisture as
the
relative humidity of the atmosphere. Oh
I know about the water insensitive epoxies used for pressure injection.
But,
its back to that basic physics law when it comes to material
penetrations, you
cant put two things in the same place at once.
If there is moisture in the pore structure, there is moisture in
the
pore structure. Nothing will get in unless the moisture is displaced. Pressure applied to the crack will not do
it. The flow and roll of the repair
resin along the interior walls of the fracture may pull some of it out
but what
happens to the moisture? We have found
that because a manufacturer says so, does not necessarily make it so.
Will water
insensitive epoxies bond when mixed with water?
The failure to bond issue that this causes has been explained by
the
pressure injection folks…“It doesn’t need to bond”, its
been said.
The pressure folks go on to explain the mere fact that the
member has
cracked indicates it has relieved the pressures exerted in the first
place. So, all one needs to do is fill
the dormant void. But what if the
pressure is not relieved, and even if it is, what is the need then to
get the
material so deep into the fracture? Is
90% fill better than 70%? Sure it is.
Consider a crack only filled 80% full.
At the leading edge of the repair resin is a weakness. Even if well bonded the whole 80%, if you
apply stress forces, the crack will radiate from this leading edge line
and
form a new fracture. That is, if the
fracture is totally bonded the entire 80%. Of course, if its not
bonded, the
fracture will just open up again when/if it comes out of compression. Many times my pressure injected crack looked
fine, but six inches over, a new crack developed. Coring
of the existing repaired crack could
reveal the new crack emanates from, and because of, the partially
filled
fracture at the leading edge of the epoxy.
Can anyone argue, “The more complete the fill, the more
permanent the
repair”? …and good luck with trying to
rectify an incomplete crack fill. Even
if the pressure advocates had a method, epoxy will not stick to epoxy.
Tecvac,
Inc. routinely fills cracks as narrow as .001". Moreover,
there are petrographic reports from
CTL Laboratories that indicate cracks as narrow as 5 microns wide being
filled
using vacuum technology; that’s the size of a single cement particle. Oh, we sometimes use pressure assist, but our
success is directly attributable to our combination of vacuum and
ultra-low
viscosity repair materials, not pressure.
The technology and the repair resins we use are a bit more
complicated,
but its not brain surgery. And, for the
injection of typical in-depth cracks in concrete members, vacuum
injection prices
are quite competitive.
Our
repair resin of choice is low molecular weight methyl methacrylate
(MMA). This material shares all of the
physical
properties of epoxy, yet is just about as thin as water.
It has the ability to migrate into places
epoxies will never see. MMA has some
drawbacks of course, but this resin material will outperform epoxy in
every
instance when it comes to injection. It
smells nasty, tends to bubble when put under pressure and is not easily
metered. So, there is obviously not a
lot of enthusiasm from epoxy manufacturers or epoxy pump manufacturers. But lets take the splash of water example
again; splash a cup of water on a concrete wall or sidewalk. See how the capillary action sucks the water
deep into the matrix? Now splash a cup
of honey right next to it. Very thin
epoxies are claimed to be 40 centipoise, but most used for pressure
injection
approach way over a 100cps or better.
MMA is 5-7 centipoise. See the
difference? Imagine the same two
materials trying to get into that .001" wide fracture.
Pressure honey or vacuum water…duh. Oh,
you have a crack that is 6"
wide? No matter, the MMA can be thickened
into a paste, if need be, and applied at 20 degrees below zero. We can drill into your pressure injected
crack, where the material only partially penetrated 1"-2", pull our
MMA up against and bond to the shallow epoxy filler.
Vacuum allows us to remove our set up without
grinding or marring the surface of the concrete; we can actually mask
in the
crack to match the existing surfaces. The
benefits and superiority of vacuum injection compared to pressure
injection go
on and on.
Witnessing these benefits over and over again, it’s not hard to understand why we, those of us in the vacuum injection industry, marvel at the natural wonder and the overwhelming power of vacuum. We don’t expect pressure injection folks and those with beneficial interest in that part of the concrete repair industry to understand, much less embrace, this technology. However, we on the other hand, do understand pressure injection. Tecvac technicians have performed 100's of gallons of pressure injection and know what it has to offer. We are ready for any challenge and hopeful that the results would bring the pressure injection folks into the present. The days of wooden wheels and buggy whips are a thing of the past. But hey, if you think our "new fangled contraption" will get you killed, then ride on in your buggy…yah, giddiup.
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