Concrete is one of the main building products used in the construction of a house.
Depending on where the concrete is used in the construction process, it can become a
very critical factor in the overall structural strength of the house. Just because the
concrete comes premixed in the concrete truck does not mean that the concrete is
In normal residential concrete, the first conversations might be with the concrete
company's outside sales person. The size of the foundation and how much concrete
will be required are usually the first questions to be answered. The quantity of
concrete is expressed in "yards". One cubic yard of concrete is equal to twenty seven
cubic feet of concrete. An example would be if there was a hole three feet wide, three
feet long, and three feet deep it would require one yard of concrete to fill the hole.
Another value that is used when pouring sidewalks and driveways is that a yard of
concrete will cover eighty one square feet of area (driveway or sidewalk) when the
concrete is four inches thick.
The next issue discussed might be the strength of the concrete. Years ago, the only
strength concrete used in house foundations was 2000 psi (pounds per square inch)
concrete. It's not unusual today to see builders pouring 3000 psi and 4000 psi
concrete, depending on the size and complexity of the project. Some of the larger
homes are designed by an architect and might require a structural engineer. The
structural engineer might require mix designs from the concrete company for his
The sales person might ask next what kind of "slump" is required. This is sometimes
where the concrete subcontractor and the builder might try to reach a compromise.
The lower the slump the greater the strength but the harder the concrete is to work.
The higher the slump the lesser strength but the easier the concrete is to work. When
a structural engineer is involved, he might specify that the concrete is to be 3000 psi
with a three to five inch slump. If this is specified, it is very important to stay within
these limits do not allow anyone to add water to the mix on site without permission
from the individual doing the testing. If the concrete is being pumped, there are
additives that can be added to the mix in order to allow a higher slump but not
jeopardize the strength of the concrete. These additives can also be used in dry and
windy climates to allow a wetter mix and hopefully prevent surface cracking due to
the fast dehydration of the concrete. If the concrete is tested, the slump might be
checked every fifty yards and the cylinders might be taken at the same interval.
Concrete cylinders are taken and tested to verify the compressive strength of the
cured concrete. The cylinders are compressed and broke at specified intervals. A
cylinder broke after 28 days should break at the designed strength or greater.
Also, usually the temperature of the concrete will be checked at this time. The
temperature of the concrete might become more of a factor in the summer rather than
the winter. A rule of thumb might be that if the concrete temperature is above 95
degrees it is not acceptable to pour in the foundation or footings. Sometimes a good
starting point to monitor the temperature of the concrete in a truck is to check the
ticket and see when the truck was loaded at the plant. In warmer climates, the longer
the concrete is in the truck the hotter the mix. In cooler climates the temperature
might not be out of range but the time in the truck might become a factor. Usually a
rule of thumb might be that, if it has been forty five minutes to an hour since the truck
was loaded, the load may have started setting up and might not be acceptable to use.
As you can see there are a few more things to consider in foundation concrete other
than just pouring the concrete out of the truck. As in most any other construction
process, a little pre-planning and basic knowledge might prevent a major mistake that
could cost a lost of time and money.
Category Construction Glossary