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Every spring, after the winter snow and ice have
disappeared from the tops of concrete driveways,
sidewalks and other exterior flatwork, homeowners
are uncovering what first appear to be surface
blemishes that were not apparent in the fall or in
the case of freshly installed concrete, when it was
first placed.
The concrete slab looked so good, so hard and
durable and yet now it appears to have a number of
localized areas of blemishes, pock marks or surface
blemishes. These blemishes appear to be affecting
the top surface of the concrete as the paste can be
easily picked off and yet the aggregate bond may
appear quite sound and intact.
This condition is not so uncommon, especially to
those of us that live in the colder climates of the
Northeast. This is a topical condition known as
scaling. It can be easily be avoided by adhering to
a few basic principals when ordering and placing
concrete for an exterior flatwork project.
Unlike most manufactured products where the
manufacturer has sole responsibility for the final
quality of its production, concrete is sold to a
project before it is a finished product.
Responsibility for a high quality, durable finished
product is equally shared by the concrete contractor
placing the concrete slab.
Selecting an appropriate mix for the given service
application is critical to getting a durable
concrete product but equally important is the
finishing and curing of the concrete after it is
placed. In order for concrete to have maximum
durability the plastic concrete must not be
overworked.
It must be screeded, floated, saw cut and cured.
Never use a jitterbug or vibrating screed and never
finish when bleed water is still visible. Under no
circumstance is it permissible to add water to the
surface during finishing. The concrete must be
protected from the elements when concrete is placed
late fall or winter. It must be thoroughly cured
before it is put into service. This means that the
concrete must be kept at temperatures above freezing
until it reaches design strength usually at 28
days. Durability of concrete slab is dependent on
how well the concrete is cured.
Under no circumstances should a deicer be used on
concrete that has not fully cured. While deicers
are efficient in melting snow, they can play havoc
with freshly placed concrete. We generally
recommend that deicers not be used in the first year
of the pavement’s life. Typical deicer chemicals are
as follows:
• Sodium chloride (Table Salt)
• Calcium Chloride
• Magnesium Chloride
• Potassium Chloride
• Calcium Magnesium Acetate, Urea, Ammonium
Sulfate, Nitrogen Salts
While there are many types of deicers on the market,
they all work on lowering the freezing temperature
of water. There is a common misconception that
concrete is relatively inert. Concrete, in fact,
continues to cure and change properties as it
matures. The use of these types of chemicals can
significantly affect the concrete durability. If
you have any doubt of their destructive nature,
place a small amount of calcium chloride on a tin
pan and observe what effect it has on the tin
pan. Deicers tend to amplify freeze-thaw
deterioration which can lead to surface defects and
possible structural failure.
There are long term performance studies published by
the Portland Cement Association which quantify the
impact of various known deicers. These same studies
also evaluate the impact of w/c ratio and curing
membranes or sealers. Sealers will seal in moisture
in the concrete which will allow the slab to gain
strength as it cures. A sealer will also prevent
chloride-.laden moisture from migrating into the
concrete slab. Never use ammonium sulfate or
ammonium nitrate as a deicer as these chemicals are
too aggressive and will easily destroy a concrete
surface.
Deicing chemicals can also impact vegetation and
hardscape. They damage plants, change the
properties of soil, corrode steel or iron railings
and can make a Pet ill through ingestion or simply
from continued and prolonged contact.
Note ACI 332-04 Requirements for Residential
Concrete Construction recommend for a climate or
exposure such as found in the Northeast, a maximum
water to cement ratio of 0.45 for exterior
pavements, curb and sidewalk. This concrete should
also have an appropriate amount of air entrainment
as detailed in table 4.2 of that document.
In conclusion, it is not enough to simply purchase
the correct mix design for your specific region. It
must also be placed, finished and cured correctly.
Avoid the use of deicing agents during the critical
first year after placement. For more information on
this subject or any other aspect of concrete
construction please contact the Pennsylvania
Aggregates and Concrete Association.
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Figure 1—Typical concrete pavement
surface that is severely effected by the application
of deicers and the effects of freeze-thaw in a moist
environment. This concrete may not have an adequate
design strength or w/c ratio and may not have
receive proper initial curing.
REFERENCES RE: SALT SCALING LITERATURE
Design & Control of Concrete Materials, Portland
Cement Association, Skokie, IL 14th. Edition, 2002
Scale Resistant Concrete Pavements, IS-117.02P,
Portland Cement Association, Skokie, IL
Verbeck, G, and Klieger, P, Jan. 1956, Studies of
“Salt” Scaling of Concrete, Portland Cement
Association, Skokie, IL Bulletin 83
Guide To Durable Concrete, ACI-201-2R, American
Concrete Institute, Farmington Hills, MI
Concrete International, Volume 16, Issue 8, Deicer
Salt Scaling Resistance, Colin D. Johnson, 1994
Concrete In Practice – Bulletin #2, 2007, National
Ready Mixed Concrete Association, Silver Springs,
MD.
Concrete Deterioration by Deicing Salts, Cody, R.D.,
Iowa State University, May 1996
Mechanisms for Salt Scaling, John Valenza II-George
W. Scherer, 2006, Princeton Institute for Science &
Technology, Dept. of Civil & Environmental
Engineering, Princeton, NJ,
Environmental Stewardship Practices, Procedures and
Policies, 2004, Chapter 7, American Association of
State Highway Officials
Disclaimer:
Pennsylvania Aggregates and Concrete
Association/Pennsylvania Concrete Promotion Council
is a trade association (“Association”) organized
under the Pennsylvania Nonprofit Corporation Law.
The information provided here is intended solely for
the continuing education of Qualified Professionals
who are competent to evaluate the significance and
limitations of the information provided and who
accept total responsibility and liability for the
accuracy of any application of the information
contained in this publication. Others should obtain
assistance from a Qualified Professional before
proceeding. The Association and its members make no
express or implied warranty with respect to the
information contained herein or the accuracy
thereof. They disclaim any product liability in
connection with the publication or any information
contained herein. The information is garnered from
various publications reflecting current industry
recommendations. |
