Concrete planter with succulent made during engineering mentorship program

Concrete Planters – Engineering Mentorship

Jonathan kicked off the Making STEM engineering mentorship program, by designing and building concrete planters.  Concrete is a fundamental building material used in many different applications in our society. Jonathan is really interested in learning about molding and casting, concrete is a great material to start to learn the fundamentals of molding.

 

Follow along with this guide to build your own concrete planter for fun or in the classroom

 

During the Making STEM engineering mentorship program, projects are specifically tailored to meet students’ interests and academic goals.  Sign up for a session to start your students learning adventure

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Supplies:

To complete this project you will need the following supplies:

  • 50 lb bag of pre-mixed concrete
  • Bucket for mixing
  • Small shovel for mixing
  • Plastic containers for molds
  • Eye protection
  • Gloves (Do not let concrete touch your skin)
  • Water

Safety:

Concrete can be harmful if it contacts the skin or eyes.  Be sure to wear all approiate safety equipment including

  • Long sleeves
  • Gloves
  • Eye protection
  • Mask when pouring or mixing concrete

Casting Concrete:

The first step in creating concrete planters is to design the molds.  For this project we selected plastic containers and cans, since they have a smooth shape and will hold the concrete well.  To make a planter we needed to select two containers.  The first is used to hold the concrete, and the second, is used to create the cavity where the plant will be placed.

Plastic container with soda can inside used for concrete casting during engineering mentorship program
Molds used for concrete casting

Mixing Concrete

After the molds were constructed, the concrete needed to be mixed. We selected a fast setting concrete mix to increase the cure time.  Concrete is made from two primary materials, Portland cement and aggregate.  The aggregate is usually made from sand or rocks, and the size and type of the aggregate will effect the properties of the concrete.

To determine the correct water/concrete mixture, the “Slump Test” is used. A cone of concrete is placed on a flat surface and the amount of deformation is measured.  If the concrete is too dry it will keep its shape, if it is too wet then it will morph into a blob.  The correct amount of slump is for a 12 inch tall tube of concrete to slump down approximately 4 inches. 

  • Be sure to read the instructions on the bag of concrete
  • Properly mixed concrete will have a consistency of thick oatmeal and keep its shape when sqeezed with a gloved hand
  • Concrete Mixing Instructions
Student mixing concrete
Mixing fast-setting concrete mix with water for planters

Filling the molds

Once the concrete was correctly mixed, the timer started and we had about 20 minutes to fill the molds.  

Fill the constructed molds with fresh concrete

An orbital sander with the sand paper removed can be used to eliminate air bubbles.  Use the sand against the side of the container to vibrate. 

Molds filled with fresh concrete for engineering mentorship planter project
Molds filled with freshly mixed concrete
Sander being used to remove air bubbles from concrete mold
An orbital sander (with sandpaper) is used to vibrate the molds to release air bubbles

 

Curing the Concrete:

Let the concrete sit in the molds at least 1.5 hours if using quick-set concrete or 3.0 hours if using regular concrete.  Concrete will take at least 24 hours to become mostly cured.

Removing from the molds: 

Carefully remove the concrete from the molds.  It may be necessary to cut the molds to release the concrete planter.

Learning Opportunity

The Making STEM engineering mentorship program is designed to learn real-world engineering skills.  That sometimes means learning from mistakes. 

We decided we should add a hole to bottom of the planter, so water could drain out. We planned on using a masonry bit with hammer drill.  On our first attempt the concrete broke and ruined the planter.  After analyzing what went wrong we determined we needed to wait longer for the concrete to cure before attempting to drill a hole. For the next molds we inserted a small cylinder to create a hole instead of drilling.

Broken concrete planter with drill after attempting to drill hole
Concrete planter broke after attempting to drill a hole in the bottom.

Success!

The Making STEM engineering mentorship program helps to teach real-world engineering skills while building fun projects.  After several hours of trial and error, we successful made several concrete planters, which will make perfect gifts for the holidays.  We used concrete to learn the fundamentals of molding and how to effectively use molds to create complex shapes.

Student holding concrete planter be built
Jonathan showing off successful concrete planter
Concrete planters made during engineering mentorship program
Concrete planters with succulent

 

 

 

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