Chapter 6 Concrete 119
Compression tests measure concrete’s
ability to withstand loads placed on it. Several
cylinders are carefully filled with the concrete
that is being placed. The cylinders are allowed
to sit for at least 24 hours, but most testing typi-
cally takes place after seven days of curing.
compression test: Measures the compression
strength (resistance to crushing) of concrete.
Manufacturing: Portland Cement
The dry process for manufacturing Portland cement involves the following steps.
•
Step 1—Various raw materials, such as limestone, shale, clay, and sand, are crushed.
Each raw material is then stored separately.
•
Step 2—These raw materials are ground into powder and then blended in the required
proportions. Dust collectors are used during this step to reduce air pollution.
•
Step 3—Raw materials are fed into a rotating kiln. The kiln heats the raw materials,
changing them into cement clinker. Heat escaping from the rotating kiln is used to preheat
the raw materials.
•
Step 4—Clinker is mixed with gypsum and ground into Portland cement.
In a variation of the manufacturing process, water is added during the grinding process
(Step 2) to make slurry. The slurry is fed into the kiln. Other than this step, the processes are
essentially the same.
A Portland cement manufacturing plant is very large. The rotating kiln is the largest piece
of equipment. It is basically a large (up to 12′ in diameter) nearly horizontal steel pipe that
is as much as 500′ long. The kiln is lined with firebrick to withstand the 2700°F (1482°C)
temperature required to change the raw material into cement clinker.
Tamping rod
4″ diameter
8″
diameter
12″
Figure 6-19. A slump of less than 1″ indicates
stiff concrete that will be difficult to consolidate.
A slump of more than 5″ indicates too much water
in the concrete.
Figure 6-20. A compression test measures the
compression force required for the concrete
sample to fail.
At that time, the concrete is removed from the
cylinder and compression force is applied to
the concrete, Figure 6-20. The force needed to
crush the cylinder is measured. Two more cylin-
ders are tested after 28 days of curing. A fourth
cylinder is held in reserve in case additional
testing is required.
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