Concrete in the Classroom
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2
Lesson 2: What are the Parts of Concrete?
Age: Grades 7-12
Subjects: Science
Skills: Classification, identification,
Comparing similarities and
differences, small group work,
observation
Duration: 45 minutes
Setting: Indoors
Key Vocabulary: Cement, concrete, aggregate, sieve
Objectives
Students will learn 1) What materials are used to make concrete,
2) How much energy is used to separate small or large particles
Method
Students will mix components of concrete together, separate them,
and identify them.
Background
Concrete is basically a mixture of two components: Aggregates and
paste. The paste, comprised of Portland cement and water, binds
the aggregate (sand and gravel or crushed stone) into a rocklike
mass as the paste hardens because of the chemical reaction of the
cement and water. Aggregates are generally divided into two groups:
fine and coarse. Fine aggregate consists of natural or manufactured
sand with particle sizes ranging up to 3/8-inch; coarse aggregates
are those with particles retained on the No. 16 sieve and ranging
up to 6 inches. The most commonly used maximum aggregate is ¾
inch or 1 inch.
Typical concrete mixes are proportioned by absolute weight. Some
typical mixes would include:
| |
Cement |
Water |
Air |
Fine agg |
Coarse agg |
| Mix I |
15% |
18% |
8% |
28% |
31% |
| Mix II |
7% |
14% |
4% |
24% |
51% |
| Mix III |
15% |
21% |
3% |
30% |
31% |
| Mix IV |
7% |
16% |
1/2% |
25-1/2% |
51% |
Materials
You will need a class set (2 to 4 students per set of equipment)
of separating sieves (available from Hubbard Scientific, Northbrook,
IL) and a small bag of concrete mix from your local building materials
dealer, a small bag of mortar mix, and a small bag of cement for
other activities. You will also need an overhead projector, ruler,
250 ml beaker, and scale.
Procedure
To approximate the size of the openings of the sieves, place them
on an overhead projector with a transparent ruler. Not only will
you separate out the ingredients that go into making concrete, but
you will learn the important lesson of the time and energy needed
to separate the largest sized stones from the smallest particles
that contain the cement.
Give each group of students a 250 ml beaker filled with dry concrete
mix. Pour the mixture into the sieve with the largest sized openings
with the succeeding smaller sizes below and the collection container
at the bottom.
Using short brisk motions of the sieves, time how long it takes
for each sized particle to be separated. This activity will lead
you to a working definition of concrete.
The percent of each sized particle can be found by weighing each
and comparing that to the total weight of the sample in the 250
ml beaker. Invite the students to identify each group of particles
from the sieve. The finest particles contain the cement that makes
up the bonding material holding the larger particles together when
all of the four sized materials are mixed together with water.
The next group of particles in the succeeding sieves are the various
sized stones that make up the aggregates of the concrete mixture.
Mix each of the four different sized particles with a small amount
of water to see which of the particles actually set.
If sieves are not available, have students pour out some of the
concrete mixture on a sheet of paper and describe the range of the
sizes of the particles that go into making concrete.
If you are using the sieves, the source of each of the particle
can be identified. The small rocks that make up the largest particles
and the next larger particles are called aggregate and can be mined
from a gravel pit. The third set makes up the smallest sand grains
and cement particles and the fourth contains much of the cement
that is ground to a very fine powder from clinker.
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