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3
Lesson 3: A Further Look at the Content of Concrete
Age: Grades 7-12
Subjects: Science
Skills: Proportions, evaluation, listing,
observation, reading, reporting
Duration: 45 minutes with an overnight break
Setting: Laboratory or classroom
Key Vocabulary: Aggregate, paste, volume,
mortar, particles
Objective
Students will learn about proportioning concrete mixes.
Method
Leave a very wet concrete mix and mortar mix in a jar overnight
to see how concrete particles layer from heavy to lightweight.
Background
Paste, or mortar, in concrete is composed of portland cement, water
and entrapped air or purposely entrained air. Cement paste ordinarily
constitutes abut 25% to 40% of the total volume of concrete.
Materials
Jar, dry concrete mix, water
Procedure
Students can see proportioning in action by adding 3 to 5 times
more water than required to cement and concrete mixes. This will
demonstrate how many sizes of particles are needed in a concrete
mix. Students will test two mixes: water mixed with concrete mix
(cement, small and large aggregate), water, and water mixed with
mortar (cement and small aggregate).
Place one part concrete mix to five parts water a jar. Shake the
mixture and let it set for an hour or overnight.
Remove larger sized aggregate from a dry concrete mix (or use a
mortar mix) and pour one part of the finer mixture into a jar and
add five parts water. Shake the contents and let it settle for an
hour or overnight.
Measure each layer to calculate the percentage of aggregate of each
size in both mixes. Compare the two mixes to understand that different
particle sizes play a role in quality concrete and see the difference
between concrete and mortar.
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