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Lightweight Concrete Pins: A project for our industry!

A PDF of this article can be found HERE

By Luke M. Snell. P.E., Concrete Consultant, Edwardsville, IL

Recently, I was in Mongolia speaking at their annual concrete conference, doing batch plant inspections  and working with the university students. On our first trip to Mongolia in 2002, I saw no concrete batch plants (all concrete was sited  batched), no admixtures being used in the concrete (It gets down to – 50 F,  so admixtures are needed). They now have an elected democratic government, enjoy a growing economy with  great resources of copper, gold and silver.   This had resulted in a  building boom with over 100 modern central batch plants, new concrete trucks and chemical admixture commonly used.

This was  our 15th trip to Mongolia and likely to be our last so we wanted to do something special, by making  others aware of progress in  Mongolia and show the versatility of  concrete. Previously, we had worked with students at  Arizona State University  to make pink concrete heart necklaces for a breast cancer awareness program.  Building on the idea of the pink concrete necklaces being a symbol of hope, I  decided to make lightweight  concrete pins to be given out at their annual  concrete conference and to a Mongolian hospice program.  For the people attending the concrete conference, the pins would show the versatility of concrete and that concrete can be beautiful.  For those in the hospice program, the concrete pins would be a symbol that the people that made them are concerned for them and their family as they face one of life’s most difficult moments.

To start the pins project and  with the support of several chapters of the American Concrete Institutes (see acknowledgments),  I  purchased the needed supplies. The first group to make pins was a church and community  group that wanted to support the hospice program in Mongolia.  They  saw this as a meaningful craft project since they had no experience in concrete. After a 10 minute discussion of what we were doing and a brief introduction to concrete,  they started mixing and placing concrete. Of coarse we had pizza to keep everyone motivated. 

The second groups to make pins were  engineering students from 3 universities  (Southern Illinois University Edwardsville, Missouri University of Science and Technology and Mongolian University of Science and Technology - Ulaanbaatar). They enjoyed making the pins  because they saw a unique use of concrete that was quite different than their concrete classes.   They also were pleased  that their developing engineering skills could be used  in a humanitarian way.   The results of these  combined  efforts  created over 400 pins. Some of these pins  are shown in Figure 1.

Figure 1:  Concrete pins taken to Mongolia

Making the pins:  The pins have to be made in a two separate sessions. The first session is to cast the concrete.  The forms are available at most craft stores and are either candy molds or fancy ice cube trays made of  plastic.  The forms that worked  best are stars, hearts, flowers and trees. I  tried other forms such as cactus and bumble bee.  Most samples broke or had voids when removed from the forms. Thus I do not recommend these types of molds. 

The mixture I recommend is to use two parts perlite as the aggregate (available at most garden stores) and one part cement measured by volume. Water should be added to make the concrete look like a brownie mixture (for concrete people, a low slump). If you add too much water, the perlite will float to the surface, so be very careful and add  the water slowly.

Make small batches since the molds require  very  little concrete.  For example, one pint  of this mixture will make about 10  pins.   For  people inexperienced in making concrete, I recommend making the concrete in  the quart or gallon  plastic bags. That way they will have less mess and will limit their  exposure to the concrete as shown in Figure 2

Figure 2: Making concrete in a plastic bag.  Note that  the concrete has a “low slump”

If you  are in a lab and want to make larger batches, you can use a mixing bowl as shown in Figure 3.

Figure 3:  Students at Southern Illinois Edwardsville and Missouri University of Science and Technology making and casting the concrete

Casting the concrete samples requires some compaction effort. This can be accomplished by  using the bottom side of a plastic spoon or the craft stick.  If they use their finger to consolidate the concrete ( as shown in figure 3),  make sure they wash their hands immediately. Final finishing  can be done with a craft stick. The casting of the concrete will take about one hour.

After the concrete has been cast, the samples  should be allowed to cure and dry at room temperatures.  Leave the samples  in the molds for at least 24 hours, then remove them from the molds. Store them for one week at room temperature so they can both cure and dry out. Note: this concrete when dry will actually float. This will surprise most people because  they expected the concrete pins to be heavy.

Making the pin (Second Session) about one week  later:  Once the concrete had cured and  dried, the tops of the concrete samples, (the surface you finished with the craft stick) should be lightly sanded using sand paper so that the surface is flat. The dust should be wiped off or blown off with compressed air.  The samples ares now ready to be ready to be decorated.

Figure 4: Students coloring the samples

I recommend using fingernail polish. Several colors are  available (these can be purchased a a discount store).  Each bottle has its own brush thus paint brushes are not needed.  Encourage the people painting the samples to be creative and allow their artistic abilities to show.                                                         

The final step is to apply the backer pins to the back of the sample (the side you sanded).

Figure 5:The backer pins  are glued onto the back of the pins.

Why should  we make  concrete pins? Many of us are involved in scouting programs and/or youth groups that would like to do a project that has meaning to them and will make a lasting impression about concrete.  In my opinion, the concrete pins accomplishes this. It is relatively low cost and fun to do.  It  also introduces those making or receiving the pins  to the concrete industry.

This project  was successful because of the  support of the following  Chapters of the American Concrete Institute:





                                    Materials needed

  1. Cement - Any cement will be OK,
  2. Perlite - Available in many stores in their garden center
  3. Molds – Michael/ Hobby Lobby /Walmart/ internet. Molds are typically used for making candy or are fancy ice cube trays. I found the star and heart molds at Michaels. flower molds  at Walmart craft section. Odd shaped molds such as bumble bee and cactus are difficult to use since the samples  tend to break when you  take them  from the molds. Heart, star and flower molds  are the easiest to use.
  4. Backer pins are available at most craft or hobby stores. The back pins at Walmart are the ideal size.
  5. Glue - I used “Elmer’s Glue - All” but you can use any glue that can be washed off your hands. Do not use super glues.
  6. Finger Nail Polish – Cheapest is at discount stores such as the Dollar Store. Make sure you get  a variety of colors.  Note the finger nail polish  has its own brush so is easy to apply. Have finger nail remover available for clean up (people will  get finger nail polish on their fingers as they decorate the sample).
  7. File or sandpaper
  8. Newspapers (to protect the work surface), paper or plastic  cups (to hold the water), plastic spoons, craft sticks and  quart or gallon seal-able plastic bags.  A  measuring cup ( I/4 or 1/2 cup) with a handle  is ideal - this allows you to accurately measure the cement and perlite without physically touching it.


                                               Mixture Design

  1. Two (2) parts perlite to one (1) part cement (by volume). Use the measuring cup to put the material into the plastic bag.
  2. Seal the plastic bag and knead and gently shake the bag until the cement and perlite mixture looks uniform.
  3. Add water and mix by kneading the mixture until the concrete looks like a brownie mix (for concrete people, a low slump). Be careful in adding water - if  the concrete gets to sloppy ( high slump), the  perlite will float to the surface and the  samples will be ruined.
  4. Make sure the concrete looks uniform throughout the mixture with no dry or wet spots spots evident.

                                      Placing Procedure

  1. Place concrete mixture into the molds
  2. Consolidate the concrete with the back of the spoon or with  the craft stick (remember it is a low slump concrete so it  will take some effort to make sure  there are few voids in the finished sample). If you use your fingers to consolidate, wash your hands as  soon as you are done.
  3. Make sure to fill the mold completely with concrete. Once the mold is filled, use the craft stick to make the top surface as smooth as possible.
  4. Dispose the newspapers, spoons,craft sticks and the plastic bag with the unused concrete into the trash. Do not dispose  any left over concrete into the sinks.  Make sure everyone washes their hands.
  5. Leave the concrete in the molds for at least  24 hours. Carefully remove the samples from the molds and clean the molds. Do not dispose the wash water for cleaning the molds into the sink.  

                                              Making the Pins

  1. After removing the concrete from the molds, store the samples in a dry warm place for about a week. This will allow the concrete to cure and dry out.
  2. Sand or file the top surface until smooth. Wipe or blow off the dust using  compressed air. Small cans of compressed air are available at most computer stores.
  3. Paint the formed surfaces with fingernail polish – be creative!!!
  4. Let the fingernail polish harden – this takes only a few minutes. You will need finger nail remover if people get the finger nail polish onto their hands.
  5. Glue the backer pin onto the concrete surface that you sanded.
  6. Let the glue dry. Depending on the glue you use, this may take a few  minutes.
  7. Pins are now complete.