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 always good.
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 approval.
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.
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 always good.
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 approval.
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.
Construction Jargon