Subsystem Build & Test
Table of Contents
Team Vision for Subsystem Level Build & Test Phase
The team was focused on building and testing the first (X) dimension. Additionally, supplies for the second (Y) dimension was ordered.
Accomplishments during this phase include...
- Stress testing the X dimension for motor heat.
- Testing concrete material for extrudability, penetration, slump.
- Extruding and Moving X-axis simultaneously.
Test Results Summary
Various tests from the test plans were performed. Additionally, some objective experiments were performed to assess the feasibility of the concrete and electro-mechanical systems.
Concrete Material TestingThe main focus of the project was on building the printer rather than developing the ideal mixture. However, not just any mixture of cement, water, aggregate, and admixture can be used. To narrow the design space, we selected Type S Sakrete mortar mix as the base of the mixture, and only varied the water:cement ratio in our development and specification.
From all testing, it was determined that 25.5% is a good concrete to water by mass ratio.
For the set of procedures developed for the following sections, see this link.
For the full document containing data from all of the following sections, see this link.
To determine the mortar weight and water volume necessary to achieve a specific water/mortar (W:M) ratio, we developed a spreadsheet calculator. The basis for the calculator was backwards approximation of the mortar’s density using input-to-output estimations provided by the manufacturer. An estimated density of 2.44 g/cc was adequate for the purpose of making several liters of mixture per batch. Then, water volume can be determined by multiplying the desired W:M ratio with the desired volume of mixture, and mortar weight can be determined by multiplying the difference between desired volume of mixture and water volume required with the mortar density.
The amount of each component included in the mixture is not the only consideration to developing an adequate mixture. The mixing procedure is important for obtaining between-batch consistency. The procedure we developed was based on traditional mortar mixing techniques and adapted to be performed with the tools and equipment available.
The procedure is as follows:
- Use relevant PPE: gloves, filtering mask, safety glasses.
- Determine the desired volume of concrete and the desired W/M ratio. From these, the exact amounts of mortar mix and water required in the batch can be found.
- Weigh out the desired amount of mortar mix.
- Shake or sift the mortar mix into the water.
- Insert electric mixer beaters into the center of the mixing receptacle. Start the mixer at a low speed of ~140rpm for 30 seconds.
- Stop the mixer and change to a medium speed of ~285rpm for 30 seconds.
- Stop the mixer and let the mortar stand for 90 seconds. During the first 15 seconds of this period, use the flat mixing tool or spatula to quickly scrap any mortar that is on the sides or on the bottom of the receptacle. Then, for the remaining 75 seconds of this period, cover the bowl with a lid or seal.
- Start the mixer at a medium speed of ~285rpm for 60 seconds. During this period, use the spatula to remove any mortar adhering to the sides of the receptacle.
Extrusion TestsThis stage included the first example of stacked extruded concrete. The images below show the extrusion samples. Flow rate as a function of motor speed calculations--an idealized Matlab model of the extrusion assembly geometry was used to predict the relationship between auger angular velocity and the outlet flow rate of mortar. It should be noted that this model is rudimentary and includes many unrealistic assumptions regarding the shear behavior of the mortar mix.
Slump TestsTo specify the mixture, the standard slump test [temp1] was modified. The slump cone geometry was reduced by approximately 50%, then multiple iterations of mixtures across the design space were tested. This was to identify over what range of slump values our ideal mixture could be specified with the modified cone. A measured slump of approximately 2.5-4cm was regarded as ideal, achieved with a water:cement ratio of 0.25. Another major criteria for mixture selection is the setting time, or the time required until one bead can support at least another on top of it. A characterization of setting time for three mixtures using a Brainard Kilman S-170 pocket penetrometer was attempted. However, the useful conclusions from this testing were that the mixture does not need to be set to the extent that the pocket penetrometer measures. That is, qualitative observations indicated a less-set mixture than what was measurable with the pocket penetrometer could support layers above it. An alternate test method for characterizing the desired setting time was ideated but not executed.
A set of slump tests were performed. The document Mixture Qualification Procedures details how the concrete mixtures were tested. A number of the tests were omitted based on learnings from other tests.
Concrete penetration testing was performed. Different concrete/water-ratio samples were placed into containers. As the concrete was curing, it was tested using a penetrometer to see how quickly it would cure. The results of the testing can be found in penetration results document.
BOM and Meeting Notes for PhaseThe Bill of Materials for this phase can be found here. A preview png can be found below. For the final Bill of Materials, see this link.
Plans for Next Phase
For the next phase, we plan to construct the X and Y