Compressive Strength Test of Hydraulic Cement Mortar

Purpose

The following covers only that portion of ASTM Designation: C 109 that is required to determine the compressive strength of 50 mm (2 inches) of Portland cement mortar cubes.

Equipment and Materials

• A 2kg scale accurate to 0.1 gram
• Six 50 mm (2 inch) cube molds
• Hard rubber tampers 13 × 25 mm (1/2 × 1 inch) cross section and 12 to 15 cm (5 to 6 inches) in height
• Rubber gloves
• Small steel trowels
• Large spoons
• Electrically driven mechanical mixer equipped with a paddle and mixing bowl
• 500 grams of Portland cement
• 1375 grams of Ottawa Sand (or Standard sand)
• 42 cc of water

Test Procedure

1. Place the 242 cc of water in the mixing bowl, add the 500 grams of cement, and mix at a slow speed (140±5 rpm) for 30
2. Add the 1375 grams of Ottawa Sand over a 30-second period while continuing to mix at a slow speed.
3. Stop the mixing, change the mixer setting to medium speed (285±5 rpm), and mix for 30 seconds.
4. Stop the mixer and let the mortar stand for 90 seconds. During the first 15 seconds, scrape down into the batch any mortar that may have collected on the sides of the bowl. Cover the bowl for the remainder of the interval.
5. Finish preparing the mortar by mixing for 60 seconds at medium speed.
6. Immediately upon completion of mixing, start molding the specimens by placing a 25 mm (i.e. 1 inch) layer of mortar in all of the six cube compartments. Tamp the mortar layer in each cube compartment, with the hard rubber tamper, 32 times within about 10 seconds in accordance with Figure 1 in four rounds. Each round should be at right angles to the other and consist of eight adjacent strokes over 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 one.

   

7. Complete the filling of the molds by adding another layer and duplicate the tamping procedure. At this point the mortar should be slightly above the top of the molds. Carefully cut the excess mortar flush with the edge of a steel trowel.
8. Place the completed mortar cubes in a moist closet, protected from dripping water, for between 20 and 24 hours, after which the cubes are to be stripped from the molds.
9. Insert the mortar cubes in a saturated lime water bath until ready for testing. Periodically the lime water should be changed to keep the water clean.
10. All specimens should be tested within a specified time period.

Prior to testing, the specimens should be wiped clean. Apply the loads only to the true surfaces of the cubes. Use a straight edge to check the cube surfaces. Any loose grains of sand or other extraneous material should be removed from the surfaces in contact with the testing machine. The specimen should be placed under the center of the upper bearing block of the testing machine. A light coating of oil should be applied to the upper platen. The rate of load application should produce failure of the specimens during a time interval of 20 to 80 seconds.

Computations

Record the maximum compressive load and compute the compressive strength in pascals or pounds/square inch using following formula

Compressive strength = Load / Cross-sectional area

The following data sheet should be used to record findings.