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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