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This page includes background information on our methods for using concrete to create our part for the ISE Cooker.
Concrete Basics
Concrete Basics
"Concrete is the combination of four basic components: water, cement, sand (small aggregate) and rock (large aggregate). When mixed together, hydration, or curing, occurs, where the cement paste acts as a glue binding all the surrounding aggregates" (Concrete Vibration Handbook, MULTIQUIP)
Concrete combined with reinforcement is one of the most commonly used resources in the world and is used in a wide array of applications.
Terms to know:
Terms to know:
Cement: a substance which binds other materials together, such as aggregate and water, to form concrete.⁴
Concrete: a composite mixture of cement, aggregate and water.⁴
Curing: the process by which concrete hardens due to the chemical process of hydration. ⁴
Portland cement: a widely-used form of cement which hardens in contact with water to produce a water-resistant product. ⁴
Molds: Using forms to keep concrete in specific shapes as it cures.
Reinforced concrete: Reinforced concrete makes up for concrete low tensile strength by the including reinforcement (often rebar) to increase its strength.
Draft angle: In concrete molding, a taper or draft angle is required on the mold to be able to pull the concrete out of the mold.
Concrete offers a cheap method to forms that meet the design requirements of an ISE Cooker. Concrete is durable, insulating, and can be formed into complex shapes. As seen in the Design Drawings section, our group is working on designs the best methods for creating the inside forms of an ISE Cooker that will benefit from being made out of concrete.
The difficulty of using concrete is that if it is not poured and molded correctly the strength and durability of the concrete will decrease significantly. According to the same handbook above: "28 days is considered the benchmark for ultimate concrete strength and represents 96% to 98% of the total strength over its lifetime." This means that we need to design our concrete methods in a way that it can be properly cured over multiple weeks to insure best strength. Since our lab sections are once a week, we have only been curing our concrete for a week which does not convey the concretes full strength. We would recommend that someone following our methods for pouring concrete should allow their molds to cure for as close to 28 days as possible.
Concrete Recipe:
Concrete Recipe:
The following recipe is what we have been using for our concrete:
Cement: Portland Cement
Aggregate: Sand
Ratio: 3:1 (Sand to cement)
Reinforcement: 0.25" Gap Chicken wire
Concrete Molding
Concrete Molding
Concrete in its various applications is often poured and cured in a mold that create the desired shape for the concrete. The mold is often necessary since concrete is poured as a slurry that can be hard to shape. Once in the mold, the concrete can be worked to the desired surface finish and strength. The mold also has to have a water resistant coating so the concrete doesn't stick to it. Our group has been using duct tape along with motor oil inside the mold to prevent the concrete from sticking. Using duct tape as a release agent is also beneficial as it allows for molds to be made of multiple parts that are taped together.
For our design we are working on creating a mold that creates the desired piece for the ISE Cooker. For the concrete part we are trying to make its possible to make the piece on a single mold, for example; the pot that is going to be used in the cooker. The concrete is then placed over the mold and can be formed to match the shape. An example of this process can be seen while Pete was in Uganda during his sabbatical. Our group is working on creating a multi-piece mold that the concrete can be poured into which will hopefully make it easier to create our concrete part for the ISEC.
The top photo on the right is of a multi-part mold we 3D printed for testing. Below it is a multi-part mold that was created by Salma, an ISEC collaborator in Togo.
Multi-Piece Mold Pros and Cons:
Multi-Piece Mold Pros and Cons:
Pros:
The mold streamlines the manufacturing process if multiple concrete pieces are needed
The mold can be moved easier if needed during the curing process
The mold can hold water while the concrete is being cured or immersed completely if possible
The mold creates the desired shape instead of having to mold the shape by hand
Vibration can be applied to the mold to help settle the concrete and remove air
Cons:
Working the concrete over a one part mold may create a better surface finish then using a multi-part mold
The concrete is hard to get out of a multi-part mold as it has more surface area to stick to
It is difficult to shove the concrete into the small gaps of the mold which may decrease the strength of the concrete
Molds are more expensive and harder to fabricate
Concrete Vibration
Concrete Vibration
Summary
Summary
We found that vibrating the concrete while pouring it into our mold was a great solution to the drawbacks of a multi-part mold. This was discovered after testing a concrete pour combined with the use of a massage gun to vibrate the concrete. Vibrating the concrete as we poured it into the mold caused the concrete to settle into the gaps more efficiently then pushing it in by hand. In large construction projects concrete pours are vibrated to remove air pockets in the concrete which improves its surface finish and strength. We can learn from the methods used to vibrate concrete in large scale construction and apply it to our own project.
Useful Videos
Useful Videos
This video is a great demonstration of air pockets exiting the concrete due to vibration.
In the video you can watch the air bubbles traveling up through the concrete as its vibrated.
IMG_8562.MOVThe video to the left is of our initial testing of using the massage gun to vibrate our mold. In the video we had already poured concrete into the vertical gaps of the mold, but as the mold was vibrated you can watch as Kelly is able to continuously add more concrete as it settles into mold.
About halfway through the video I increase the rpm of the massage gun, which removes even more air from the mold. A gap forms between the two parts of the mold as the concrete settles and more concrete is able to be added.
IMG_1815.MOVHere is another video of us vibrating concrete into a test chamber. In the video you can observe the concrete rapidly settling into the concrete around the metal mesh. This could be done repeatedly to "pour" concrete into the mold and insure that the mold is completely filled with concrete and has a quality surface finish.
Benefits and Methods
Benefits and Methods
Vibrating the concrete while we pour it into a mold would a great solution to make up for the lack of strength and surface finish we worry about with use of the mold.
According to the Concrete Vibration Handbook: " Right after placement, concrete contains up to 20% entrapped air." Vibrating the causes the particles in the concrete to move rapidly which allows the air to escape and the slurry to settle. Large concrete pours in construction projects are vibrated with special equipment that vibrates the concrete more than 10,000 times a minute. Because we are using concrete in smaller context, the 3,000 rpm of a common massage gun will be adequate to vibrate the concrete.
To vibrate our concrete, which can be seen in the Lab Log, we used the Hypervolt Massage Gun by Hyperice. These can be quite expensive, but other much cheaper massage guns should be sufficient. Other methods could be used to vibrate the concrete on small scales like a Sawzall.
Future Vibration Considerations
Future Vibration Considerations
The following subjects will be worth investigating further as we use vibration on our concrete molds for our project. There are also considerations below that are out of our scope with the time we have left and would be useful to keep in the mind for future concrete projects.
Vibration Dynamics:
This is a complicated explanation below. The important part is that different molds will require different vibrating speeds from something like a massage gun to achieve the best result. But, too much vibration will cause the mold itself to shake and will make it hard to apply the vibrations from something like a massage gun.
If we use an external device like a massage gun on the outside of the mold, the vibrating device will apply forced vibrations to the mold. This is is different from when a rod is inserted into concrete, since the rod vibrates just the concrete and not the mold as well. When vibrations are applied to the mold it changes the dynamics of the vibration. There's some complicated physics at play which are worth pursuing further, since the mold and the concrete is the system being vibrated instead of just the concrete. Systems vibrate differently at different frequencies based on their mass and stiffness. Some of these frequencies create resonance in the system, which is when the amplitude of the vibrations increase drastically due to the applied disturbance. A good example is how a washing machine may shake rapidly if there is an imbalance in the load. This is caused by the imbalance disrupting the frequency at which the machine spins to the point it gets out of control. This shaking is the frequency of the rotating load creating resonance in the washing machine, to a drastic effect. For most mechanical systems resonance is a bad thing since it may cause the system to vibrate to the point it damages the system. For vibrating concrete, resonance may be beneficial since we want to apply the frequency that creates the most vibration possible in the concrete and not the mold. For our vibrating device we will have to test what the best RPM is to create this vibration. This RPM will depend on the characteristics of the mold and how vibration is applied and could be higher or lower depending on the certain mold. Too much vibration may also be bad if it vibrates the mold to the extent that we can't hold it or it disrupts the reinforcement we've placed in it.
More information on vibration dynamics can be found on the Wikipedia page: https://en.wikipedia.org/wiki/Vibration#Types
Vibration Time: For our mold we found vibrating it was a good way to settle the concrete into the mold. The drawback of this is applying the vibration for too long may settle the aggregate, which will decrease the strength of the concrete. It would then be necessary to keep this mind while pouring the concrete into the mold and will require further testing. We may have to test other methods of settling the concrete into the mold, before vibrating the mold for a short amount of time at the end to have the desired benefit.
Other Vibration Methods: The Hypervolt massage gun we were using had a max Revolutions Per Minute of about 3,000. This is similar to a Sawzall which has about a 3,000 strokes per minute maximum. But, while using a mold simpler methods may work as well. Something as simple as knocking the mold on the ground to settle the first couple layers may also be beneficial. Taps with a rubber mallet or similar tool could also help settle the concrete into the mold. Electric toothbrushes also vibrate at upwards of 30,000 vibrations-per-minute. A possibility for vibrating concrete in the mold could be to remove the toothbrush head and replace it with a small rod that mimics commercial concrete vibrators. This head could then be inserted into the concrete for a similar effect. There are endless posiibilities, anything that creates vibrations could be productive.
Helpful Links:
Here's a YouTube video of someone using a Sawzall to vibrate forms: https://www.youtube.com/watch?v=IeQV2aO7jr8
A good demonstration of the benefits of vibrating concrete: https://www.youtube.com/watch?v=neySSvNzDpg
A video of using a concrete vibrator on larger forms: https://www.youtube.com/watch?v=S-Puk4RorDo
Concrete Vibration Handbook: https://cessco.us/files/MQ_Concrete_Vibration_Handbook.pdf
A better description of resonance by Steve Mould: https://www.youtube.com/watch?v=dihQuwrf9yQ
How to make a concrete mold: https://blog.certifiedmtp.com/easy-guide-how-to-make-a-concrete-mold-for-your-project/
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