Concrete Strength Study
Megan Logan
Byng Jr. High, Grade 9
ABSTRACT
Small Aggregate is a necessary
ingredient in concrete. In Oklahoma there are
several different types of sands that can be used.
The purpose of this project was to see if varying the types of sands used in
concrete would affect the strength of concrete. In
this project four mixes of concrete were made, each with varying types of sand. One mix was made with Class-A Sand while another
contained Creek Sand. The remaining two
mixtures contained Masonry and Fill Sand. Each
mix was made into five 2” cubes. After 1
day of hardening, one cube was taken from each mixture and tested with a pressure tester
designed for this purpose. This test was
repeated after 3 days, 7 days, and 28 days. The
sands were then screened to determine the particle size of each sand. Based on the data
collected for this project, the following conclusion has been drawn: The sand in the
concrete did have an effect on the strength. The
results from screening the sand show that the difference in sand particle size probably
did not affect the strength of the concrete. The
final test showed the strongest to be Masonry, followed by Class A Sand, Fill Sand, and
Creek Sand.
Concrete is an artificial engineering material made from a mixture of cement,
water,
fine aggregates, and a small amount of air. Depending
on the material used, the
concrete
will support ten thousand pounds or more per square inch.
Since concrete was
used to
build columns in Egypt 3,600 years ago, it has proven its durability.
Concrete has three raw materials. They
are coarse aggregate, sand, and gray
cement,
slightly coarser than talcum powder. Chemically,
the cement is a mixture of
calcium and
silica, plus aluminum and iron. In the
beginning the cement crystals are
round. When water is added, the crystals become irregular
and interlocked.
While the cement is
dry, the particles move freely. After water
is added, the outer
layers
dissolve and, as the cement cures, it becomes solid again.
However, the dissolved
material does not return as it was. A chemical reaction has occurred. Instead, it
reappears as
growths on the crystal’s surface.(3)
If there is no moisture, there
will be no chemical reaction.(1) It is easy
to think of
concrete
as two major components, paste and essentially inert materials. The paste
consists
of Portland cement, water, and some air, either in the form of naturally entrapped
air
voids or minute initially entrained air bubbles.(2) The inert materials are usually
composed
of sand and gravel, crushed stone, and slag. When
Portland cement is mixed
with
water, the compounds of the cement react to form a cementing substance. In
normally
and correctly mixed cement, each particle of sand and coarse aggregate is
completely
surrounded and coated by this paste, and all spaces between the particles are
filled
with it. As the cement sets and hardens, it
binds the aggregates into a solid mass.
Under normal conditions
concrete grows older. The chemical reactions between
cement
and water that cause the paste to harden and bind the aggregates require time.
The
reactions take place very rapidly at first and then more slowly over a long period of
time.(1) If concrete stays moist, it will harden more and
more every year. It will be even
harder
five years after it is laid than it was after only one year.
Concrete mixtures are usually
specified in terms of the dry volume ratios of
cement,
sand, and coarse aggregates used. A 1:2:3
mixture consists of one part of
volume
of cement, two parts of sand, and three parts of coarse aggregate. Depending on
how it
is applied, the proportions of the ingredients in the concrete can be altered to attain
desired
properties, particularly strength and durability. The
rations can vary from 1:2:3
to
1:2:4 to 1:2:5.(3) The amount of water added
to these mixes is about 1 to 1.5 times to
the
volume of the cement. For high-strength
concrete, the water content is kept low, with
just
enough water added to preserve the ductility of the mix.
Sand is a major
ingredient of concrete. When concrete is
mixed with sand, it will
be
harder and will bear a greater weight than bricks composed of clay only. Geologists
apply
the term sand to natural particles of a certain size, as well as to geologic deposits
composed
of such particles. Sand –sized particles
are rounded to angular fragments or
detritus
grains that are smaller than granules and larger than coarse silt-grains. The size
is
determined by sieving grains through a set of standard screens of different mesh sizes.
Several
scales are used to define sand size, such as those set up by the U.S. Department
of
Agriculture (0.05 to 2 mm/0.002 to 0.08 in), the American Society of Testing and
Materials
(0.08 mm/0.003 in to 2 mm), and the U.S. Army corps of Engineers (0.08 to 4
mm/0.16
in).(8)
As a deposit, sand is
defined as a loose aggregate of sand-sized mineral particles.
Most
sand consists of quartz grains derived from the weathering of granite or other
siliceous
igneous rocks. Wind and water transport
quartz sand great distances.
Generally,
the father the particles are transported, the rounder and better sorted they
become.(8)
Sand has many practical
uses. Some deposits, called Placer Deposits, are rich in
precious
metals such as gold. Quartz sand is used to
make chemicals and glass, in molds
and
casting, and as an abrasive. Quartz and
carbonate sands are used to make chemicals
and
glass, in molds and casting, and as an abrasive. Quartz
and carbonate sands are used
for
mortar, concrete, and building stone.(7)
concrete
would affect the strength of concrete. The
research hypothesis was that the type
of
sand would affect the strength of the concrete, and the null hypothesis was that varying
the
type of sand would have no affect on concrete.
Procedure
Safety goggles were put on before any
work was done in the cement plant
laboratory. All cement was mixed according to the Holnam
standards and tested on
Holnam equipment. The first step was measuring 359 ml of water into
a glass beaker.
Next, 740 grams of cement was
measured and poured into a 500 ml bowl. The
water was
then added to the cement.
The mixer was turned on and the
cement and water were mixed until smooth and
all particles were wet. At the same time, while still mixing, 2035 grams
of Fill Sand was
added to the bowl. The mixer was then turned off and the bowl was
covered, allowing
the mixture to set for one
minute.
After one minute the bowl was
uncovered and the mixture was mixed on high for
two minutes. The mixture was poured into eight 2”x2”
molds filling the molds only half
full. The cement mixture was then tamped 32 times. Next, the molds were filled the rest
of the way to the top. The cement was again tamped 32 times.
The cement mixture was made
four more times, but each time the sand was
replaced
with a different type of sand. The sands were
Masonry, Class A, and Creek
Sand. After all cement mix was placed in molds, they
were placed in a storage tank and
kept at 23
degrees Celsius.
After 1 day, one cube from each
separate mix was placed on the pressure tester.
Here,
pressure was applied until the cube broke. The
PSI or pounds per square inch
registered
on a dial. All results were recorded for each
cube. Remaining cubes were
returned
to the storage tank.
After 3 days another cube from each
mixture was tested on the pressure tester.
Results
were recorded. All other cubes were returned
to the storage tank. The same
testing
procedure was performed again after 7 days .
The final testing was done after 28
days. Again, one cube from each mixture was
tested on the pressure tester and results were recorded.
One hundred grams of each of the sands was weighed and placed in circular
trays with screen in the bottom of each. There
were six levels of trays arranged from top to bottom in specific order of screen size. They were stacked and placed on a machine that
shook the trays. This made the sand fall
through the different levels of screening leaving larger particles in the top trays and
smaller particles near the bottom. The
particles of sand left in each tray was weighed and charted.
Results and Discussion
The
Creek Sand’s PSI was 1090, for the one day test, 2925 for the three day test, 4400
for the seven day test and 5200 for the 28 day test.
The Class A sand for one day was 1070, for three day it was 2820, for seven
days it was 4225, and for 28 days it was 5725. The
Fill Sand was 905 for the one day, 2650 for the three day test, 3675 for the seven day
test, and 5425 for the 28 day test. The
Masonry Sand was 1015 for the one day test, 2875 for the three day test, 4800 for the
seven day test, and 6950 for the 28 day test. Creek
Sand was the strongest on the day one test, then followed in order from strongest to
weakest: Class A, Masonry, and Fill Sand. On
the three day test, Creek Sand was still the strongest followed by Masonry, Class A, and
Fill Sand. Masonry led the seven day test
followed by Creek Sand, Class A Sand, and Fill Sand.
The final test showed the strongest to be Masonry, followed by Class A, Fill
Sand, and Creek Sand.(Refer to graph 1.)
(See
graph 2.) Graph 2 shows the particle size of
each sand. The Masonry and Fill Sand had the
same pattern with smaller particles. Since
masonry held under more pressure than Fill Sand, the particle size probably had little to
do with the strength of the cube. The same
happened with the Class A Sand, and the Creek Sand. Both
had about the same pattern, but this time they were dispersed more evenly. This difference in particle size did not appear to
affect concrete strength. The Masonry Sand
was the only sand to meet the Holnam standards. Those
standards were met on day seven and day 28.
Conclusion
Based on the data collected for this project, the following conclusion has been
drawn: The sand in the concrete did have an effect on the strength. The only sand to meet the Holnam standards was the
Masonry Sand. Those standards were met on day
seven and day 28. The results from screening
the sand show that the difference in sand particle size probably did not affect the
strength of the concrete. Some of the reasons
that might have caused the variation in concrete strength could have been different
amounts of quartz, mica, or more types of minerals. Another
cause could have been due to the amount of moisture in the sands. The research hypothesis was accepted and the null
hypothesis was rejected.
Further study might include researching the amount of minerals in the sand and
performing test based on that, then comparing their results.
`REFERENCES
(1) ‘Concrete,’ Compton’s
Interactive Encyclopedia, 1994 ed.
(2) ‘Concrete,’ How It works,
Illustrated Science And Invention Encyclopedia, 1983 ed.,
p.97.
(3) ‘Concrete,’ Physics Today,
World Book Encyclopedia of Science, 1985 ed., p. 636
(4) Neville, Adam M., ‘Cement and
Concrete,’ Academic America Encyclopedia, 1983
ed., p.103
(5) Walker, J.H. ‘Cement,’ Mc
Graaw’Hill Encyclopedia of Science and Technology.
1992 ed., p.77.
(6) Wolkomir, Richard, ‘Inside the lab
and out, concrete is more than it’s cracked up to.
be.’ Smithsonian, Jan. 1994, p. 23-31
(7) ‘Sand,’ Academic American
Encyclopedia, 1983 ed.
(8) Rhodes, Frank H. T., ‘Sand,’ Geology,
1972 ed. p.103
