Concrete Finishing

Concrete Slump

Concrete Slump
 
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.
One factor to be considered when pouring concrete is the slump.
The individual from the testing company will take a sample of
concrete in order to determine the slump. The technician will place
a plate or board on level ground. He then will place a metal cone on
top of the plate and fill the cone with concrete. He would take a rod
and move it up and down in the concrete in order to settle the
concrete throughout the cone.  After doing this,  he will screed the
top of the cone off with the rod. He will then grab the cone on the
both sides and slowly remove the cone upwards. This will allow
the concrete to slump.  The rod is then placed horizontally on top
of the cone and a tape is used to measure the distance from the rod
to the top of the concrete. This distance is considered the slump of
the concrete
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.
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