Introduction:
You may be required to bring a volume of approximately one quart (a cube about 10 cm on each side) of beach sand or fine gravel to lab. (The course instructor will inform you several days before the lab class.) The gravel particulate should not be more than 5 mm in diameter (a screening apparatus will be available) and should be clean of vegetable matter and dirt . Portland cement will be provided. You should have read the Chemistry of Cement from the Chemistry section of this module before the lab session. For additional background on concrete composition and the role aggregates play in the strength of concrete, read Concrete Mixing from Home Hardware.
WARNING: You will be mixing the concrete by hand. Inevitably, some of the mixture will splash onto your clothes. Wear work clothes to the lab, or bring an oversized T-shirt to protect your clothing. Cement is insidiously difficult to remove from clothing.
You will make two sets of four different mixtures of concrete, consisting of cement, fine aggregate, course aggregate, and water. The different mixtures will be tested for flexural strength. One set will be tested next week, after a one week curing time. The second set will be stored and tested at the end of the semester. This will allow us to determine the role curing time plays in the strength of the various mixtures.
Procedure
Using the paper provided, cut (or tear with a straight edge) 10 sections of paper of length 15 cm each. Slide a glue stick across both the inner and outer edges and then roll the paper tightly around the 5/8" dowel provided. Secure the edge in place with masking tape. Also using masking tape, seal one end of the tube. Slide the dowel out of the paper, leaving a rigid tube. These will serve as the "forms" for the concrete. Your instructor will demonstrate the technique.
Using a micrometer or venier caliper measure 10 different "pebbles" of the course aggregate. Find the average diameter for the course aggregate.
You will prepare four different concrete mixtures based upon the ratios of Portland cement to aggregate. You will need a volume of approximately 80 cm3 (cc) to fill two tubes. Every group will use the same fine to course aggregate ratios, but each group will be given a specific cement ratio. For example, the mixture ratios by volume to be used by "Group 1" will be:
| Portland Cement | Fine Aggregate | Course Aggregate |
| 1 : | 2.5 : | 4.5 |
| 1 : | 3.0 : | 4.0 |
| 1 : | 3.5 : | 3.5 |
| 1 : | 4.0 : | 3.0 |
Group 2 will use the same table of ratios, except the cement ratio will be 1.2, Group 3 with 1.4, etc. The assignments will be made at the first of the lab period.
With a small beaker (50 ml), measure a quantity of each dry ingredient in the chosen ratios and place them into the mixing bowls provided. You will need at least 80 cm total, so you will need to estimate the quantity of each. For example, if you are preparing a 1:3:4 mixture, you might think a 10 ml : 30 ml : 40 ml (plus water) would provide a total volume of more than 80 ml. But this is not the case, since the course aggregate takes up more volume when dry. Using a 15 ml : 45 ml : 60 ml combination will provide the same ratio and a sufficient volume for two tubes.
Slowly add water and mix until you have a wet but firm mixture. The concrete mixture should just be able to hold a shape. The amount of water will be approximately 5% to 20% by weight, but it will depend upon the "wetting" characteristics of the aggregate. It is important not to add too much water. Your instructor will help you determine the right consistency. Once you have reached the desired texture, record the amount of water added.
[Note: Your instructor may wish for your group to investigate such issues as the importance of the correct amount of water or the characteristics of a pure cement mixture. If so, the instructor will work with you to determine a suitable procedure.]
With a spatula, put the concrete into the tube, about 1 cc at a time. Tap the bottom of the tube on the table. Follow with a firm tamping with the dowel provided. Add another 1 cc and repeat the procedure above until the tube is nearly full. It is critical to thoroughly tamp the mixture after each addition. Any bubbles or voids will seriously effect the strength of the tube. Using a pen with waterproof ink, clearly label the tube with your group's name, the dry ingredient amounts, and the amount of water added. Place the tube in an upright position.
Repeat the procedure above for the remaining concrete mixtures. The tubes will be stored. During a portion of next week's lab, you will test the tubes for flexural strength. The procedure will be outlined in that lab's manuscript. Note that the groups will share their data and your final lab report will include the results of all groups.
| RETURN |