The following covers only that part of the ASTM designation: C 109 that is required to determine the compressive strength of 50 mm (2 in) of Portland cement mortar cubes.
Tools and Materials
- 0.1 g. Exact 2 kg scale for
- six 50 mm (2 in) cube molds
- Hardened rubber tampered with 13 × 25 mm (1/2 × 1 in) cross section and 12 to 15 cm (5 to 6 in) in height
- rubber gloves
- small steel trowels
- big spoon
- Electrically operated mechanical mixer equipped with paddle and mixing bowl
- 500 grams portland cement
- 1375 grams Ottawa sand (or standard sand)
- 42 cc water
- Place 242 cc of water in a mixing bowl, add 500 g of cement, and mix on slow speed (140 ± 5 rpm) for 30
- Continuing to mix at a slow speed, add 1375 grams of Ottawa sand over a period of 30 seconds.
- Stop mixing, change the mixer setting to medium speed (285 ± 5 rpm), and mix for 30 seconds.
- Turn off the mixer and let the mortar stand for 90 seconds. During the first 15 seconds, scrape up in batch any mortar that has collected on the sides of the bowl. Keep the bowl covered for the remaining gap.
- Finish preparing the mortar by mixing at medium speed for 60 seconds.
- Immediately after mixing, begin molding the samples by placing a 25 mm (ie 1 inch) layer of mortar in all six cube compartments. As in Figure 1, in four rounds, tamp the mortar layer in each cube compartment with a hard rubber tamper 32 times within about 10 seconds. Each round should be at right angles to the other and should consist of eight adjacent strokes on the surface of the specimen. Use sufficient tamping pressure to ensure uniform filling of the molds. Complete the lift in each mold in turn before moving on to the next mold.
- Complete the filling of the molds by adding another layer and mimic the tamping process. At this point the mortar should be slightly above the top of the molds. Carefully cut off excess mortar flush with the edge of the steel trowel.
- Place the finished mortar cubes in a damp closet protected from dripping water for between 20 and 24 hours, after which the cubes are to be separated from the mold.
- Put the mortar cubes in a saturated lime water bath until ready to test. To keep the water clean, lime water should be changed from time to time.
- All samples must be tested within a given time period.
Before testing, the samples must be cleaned. Apply the load only to the actual surfaces of the cubes. Use a straight edge to probe cube surfaces. Any loose grains of sand or other extraneous material should be removed from surfaces in contact with the testing machine. The specimen should be placed under the center of the upper bearing block of the test machine. A light coating of oil should be applied on the upper plate. The rate of load application should produce failure of samples during time intervals of 20 to 80 seconds.
Record the maximum compressive load and calculate compressive strength in pascals or pounds/square inch using the following formula:
compressive strength = load / cross-sectional area
The following data sheets should be used to record the findings.
|load time in seconds||weight in kilos or pounds||Cross-sectional area of the sample in meters2 or inches2||sampling power in pascals or psi|