Investing in Concrete
Nan Harvey, A.I.A.
You wouldn't consider investing in a stock without checking the P/E ratio or
buying a car without looking under the hood but many people investing in a
building, from a house to a high rise office building, will make a big money
commitment without knowing a thing about what the building is founded on. Of
course if you're buying a building that's already built, your view of the
foundation system, which is generally made of concrete, could be limited to
what's exposed in the basement or the below-building parking garage but if
you're investing in a construction project there are things you can look for
during the design and construction phases that can minimize potential future
Concrete is made up of cement, water, aggregate (sand and gravel) and
sometimes admixtures. Factors that affect the finished product primarily are
related to these ingredients and their ratios and, in the case of cast-in-place
especially, the placement and finishing of the concrete.
Hydration, the setting up of concrete, is a chemical reaction, although only
2/3 to 3/4 of cement in a concrete mix reacts as water migrates into it. The
easy way to get more cement to hydrate is add water but as the water increases,
the strength decreases, dry shrinkage increases, the durability is lowered and
the water tightness is reduced. If your finished concrete has a pasty color,
significant color changes from gray to white, or your hand is white from cement
fines after rubbing across the concrete, you have water content problems now
which can turn into serious problems later. Too much water in the mix lowers the
effectiveness of the aggregate/paste bond (the aggregate is where all the
strength is) but too little water increases cracking.
The bottom line? The amount of water in the mix must be controlled. Water is
the easiest "admixture" available at the site and it can be overused to make
setting up concrete easier to work. Water should not be permitted to be added
later. There should be less than 20 to 30 gallons of water per cubic yard of
concrete. A sand layer can be placed over the vapor barrier and under the
concrete to prevent excess water from migrating into the slab from the ground or
from previously placed concrete.
Another way of increasing the concrete slump, the workability of the mix,
without increasing the amount of water, is to increase the amount of air mixed
in, i.e. air entrainment. More air in the concrete can greatly increase the
freeze/thaw durability at the same time allowing earlier finishing times. Too
much air entrainment, however, affects the strength of the finished product.
Unfortunately, air entrainment is controlled significantly at the site with the
chute handler and the worker on the vibrator: huge drops from the chute to the
form can beat air out of mix and the vibrator can churn too much air in.
Once the concrete is placed, there's still a lot going on that can affect the
finished product. Hydration creates heat and 50% of the shrinkage in concrete is
due to thermal differences between the inside and outside of the concrete body.
On hot, dry, windy days the surface of the concrete should be misted prior to
finishing. Cracks perpendicular to the wind indicate the surface was not
protected and drying was too rapid. On cold days the concrete should be
protected and allowed to keep itself warm to prevent thermal shock or freezing.
Finished concrete with surface ice crystals will eventually spall and the
surface aggregate will be exposed. All concrete placed should be tested by a
reputable testing agency with concrete cylinders allowed to set in the same
conditions in which the building concrete is setting. Your architect and
structural engineer should review the concrete test reports.
Once the concrete is set and finished, cracks may become visible. The causes
of cracks are varied and can include thermal shock, early and late age drying
and shrinkage, vibration or shock, corrosion of the reinforcing steel or mesh,
freeze-thaw expansion, uneven settlement, or expansive chemical reaction.
Plastic shrinkage cracks are generally parallel to each other. Some hairline
cracks are the result of the higher tensile strength of the surface and can
allow acids and/or water into the concrete. Cracking under the rebar is a sign
of settlement of the concrete.
have found cracks or other signs of problems in new or existing concrete, you
are not necessarily held hostage as indicated by a speaker at the recent C.S.I.
Convention in Baltimore. Detailed examination of the dynamic forces at work and
analysis of the full extent of existing damage can determine the causes of the
problems and many times a fix can be developed, hopefully in time to save your
project and your investment.
Nan Harvey attended Agnes Scott College in Decatur, GA
(BA in Art), and graduate school at Va. Tech. in architecture. She moved to
Washington, DC then Baltimore, where she renovated a rowhouse and then an old
farmhouse while working for various architectural firms in Baltimore.
Nan started Building Consultants, Inc. construction monitoring and condition
survey services in April 1990, and added environmental services in 2000. As a
dedicated volunteer for Habitat for Humanity International, Nan has built houses
for deserving families in Nepal and Brazil.