110 Section 2 Construction Materials
Concrete Quality
The quality of the materials used to make
concrete directly influences the quality of
concrete. Quality concrete depends on:
•
The materials from which it is made.
•
The way it is mixed.
•
How it is placed and finished.
•
The curing procedures.
Portland cement used in concrete must be
dry and in powder form. Do not use Portland
cement that has been exposed to moisture and
has become hard. The aggregate needs to be
strong, free of foreign materials, varied in size,
and capable of making a strong bond with the
cement paste. Finally, the water needs to be
clean and relatively free of sulfates.
Different proportions of Portland cement,
fine and coarse aggregate, and water create
critical quality differences in concrete. Small
increases in the proportion of Portland cement
increases compression strength. Various sized
aggregate fills space inside the form completely,
requiring less cement paste to bond the mixture
together, Figure 6-6. Extra water makes
concrete easy to work but reduces its strength,
durability, and watertightness.
Concrete needs to be thoroughly mixed.
The mixing process should coat each piece of
aggregate with cement paste. The full range of
different sized aggregate should be uniformly
Figure 6-5. With the help of vibration, workable concrete flows in and around the layers of steel rebar on this
bridge deck. (CMI)
distributed throughout the mixture. Excessive
mixing and delays in placing concrete can
negatively affect quality. This is because once
the Portland cement contacts water, it begins
to cure.
Concrete needs to be placed in the form
so the distribution of aggregate remains
unchanged. Once the concrete is in the form
it must be consolidated. Consolidating fills air
pockets and voids in the finished product.
Large aggregate Small aggregate
Figure 6-6. High-quality concrete requires
aggregate of varying sizes to fill most of the space.
Portland cement paste fills the remainder of the
space and bonds the pieces of aggregate together.