By: Lani Walker
For the junior year structures class at SIUC, teams of two constructed a concrete structure from a bag of Quickrete and various other materials. The constraints of the project were the following requirements; Using no more than one - 40# bag of Quickrete (4000 psi), groups are to construct a cast concrete rigid frame monolithic structural unit that will conform to the configuration required with the cantilever of 18 inches and clear span of 30 inches and clearances of 4 inches anywhere below the spanning structure. Live Loads will include 8 – 5.5# modular units at end of cantilever and 16-5.5# modular units at center of main span. The structural unit is to be 50 inches minimum overall length. Units weighing more than 46 pounds will be disqualified.
To begin, I sketched out some different trials of structures and calculated their capacity to hold the required loads. After several trials and errors, I found a design that I believed would meet all the requirements and could withstand the concentrated load and the uniform load. To begin making the formwork, I purchased ½” black foamcore and duct tape to line the formwork. It took almost four hours to measure and cut all the pieces for the formwork and glue them together with a hot glue gun. Once the formwork was constructed, I lined it with duct tape so that the concrete mix would not stick to the formwork. Other groups tried lining their formwork with saran wrap or spraying it with pam. I am unsure if those projects were successful, but I had tried the duct tape once before so I was confident it would work. After I made the form, it was time to form the rows of 19 gauge galvanized steel wire (20 pound safe load) which would act like rebar in the concrete structure. I cut strips of wire to stretch the entire length of the concrete structure for both sides, and strips of wire which would be inside of the columns. For each strip of wire, I cut multiple strips of wire at the same length so that I could wrap them together. Using a battery powered drill (and some major help from my father!) I put five strips of equal length wire together and turned on the drill to wrap the wires together tightly. After doing this for each section that needed wire, all of the materials were ready to take to school the next morning (when the project would begin).
The next morning, our class met at 9am to begin mixing and pouring out concrete structures. We found it was so difficult to mix the entire bag of Quickrete with water in the cylindrical shaped bucket we had brought. After we mixed the concrete with water, we did a slump test to insure we had the correct ratio of water to concrete mix. Once we were sure it was correct, we poured the concrete into the foamwork and hurried to place the 19 gauge wire strips and the 6-32 12 inch threaded plated steel rods into the concrete in the locations in which I decided needed the most reinforcements (from the prior calculations I had done). After that, it was time to cover the formwork with a tarp to allow the structure to cure.
After about a week, our class went outside to remove the formwork from our new concrete structures and test them! Each structure was weighed, to insure none of them went over the maximum weight limit of 46 pounds. I was very relieved that my structure passed all the preliminary tests of length measurements (18 inch cantilever and 50 inch minimum overall length) and weight limit. Then, it was time for the load testing. We loaded our structure with the ‘Live Load’ of 8 (qty) 5.5# modular units at end of cantilever and ‘Dead Load’ of 16 (qty) 5.5# modular units at center of main span. Our structure withheld all the weight loads! I was so relieved and so proud of myself for everything that I had learned through the course of this project. I had to do many calculations and trials with the design of the structure and the formwork to make the project successful. I am not sure how many schools of architecture get the opportunity to do a project like this, but I can assure you that it was one of the best learning experiences I had at Southern Illinois University Carbondale.
Concrete Project and Testing, Photo by Author