Concrete Activity
The Impact of Materials on Society
Review and Concrete Activities
Readings and Homework Assignments
Making Concrete
The goal of this exercise is to give everyone the opportunity to fabricate a material and test its toughness. The material is concrete which as you know is a composite. The test is what is called an impact test and is used to measure the energy absorbed upon breaking (also called the fracture toughness or the fracture energy).
We will meet in the building space between the Materials building and the Mechanical engineering building just to the west of the Reitz Union ground floor outdoor colonnade (do not go to the classroom).
Wear clothes that might get dirty. You will work in your groups. Each group will be making 2 concrete bars. The first bar will be made with no reinforcement. You will use whatever ratio (by volume) of cement to sand to gravel you want (feel free to use the internet for help). Please note the ratio when you make your unreinforced bar.
| Group # | Cement | Sand | Gravel |
The second bar will be made again with the same ratio of cement and aggregate and you can reinforce it with anything you want to use to make it stronger (except rebar). In the past people have brought in everything from popsicle sticks to marshmallows (didn’t work very well) to palm fronds to plastic bags etc. The goal is to study how reinforcements affect the fracture energy.
Class Activity
Breaking Concrete
We will break all the bars in class using an impact test. To do this we need to be extremely efficient. Each group will have people who load the bars, hold the measuring stick, drop the hammer, film the process using their phone and record the data. You will be required post your results on the google docs file we will create during class. On Wednesday Oct. 4 you will need to turn in a short report (1 page) which discusses what you used to make each bar, your observations from the breaking process, the height of the sledge hammer before and after it strikes the bar and the calculated energy absorbed (energy absorbed = change in mass time gravity constant times height) (mgh). Use the mass of the sledge hammer in kilograms (10kg), g=9.81m/s2 and the height difference in meters to get the energy absorbed in joules. So for example if the sledgehammer is raised up 1 meter and after it goes through the bar it only goes up 3/4 of a meter then the difference in height due to breaking the bar is ¼ meter. So the energy absorbed is
E=10kg x 9.81m/s2 x 0.25 m = 24.5J
When you divide the Joules absorbed by the cross-sectional area of your bar you will get the work of fracture. So you need to measure the cross-sectional area of your bar. If the bar is 0.2m x 0.1m in cross-sectional area then the work of fracture is
W = E/area = 24.5J/(0.2 x 0.1) = 1225J/m2
The work of fracture for concrete is also called the fracture energy. Find a reported fracture energy for concrete on the web and compare your value with the reported value. How did reinforcing the concrete bar change your fracture energy. Speculate why.
We have 27 groups 54 bars and 50 minutes so ~2 minutes per group or 1 minute per bar. Should be an adventure.
