Tensile Strength Test of Hydraulic Cement Mortars

Purpose

Determination of the tensile strength of hydraulic cement mortars using briquet specimens.

Equipment and Materials

• 2 kg scale accurate to 0.1 gram
• Briquet molds
• Small steel trowels
• Clips for holding the test specimens
• Testing machine capable of applying the load at a rate of 2.67 ± 11 (600 ± 25 lb/ft²)/minute
• Ottawa Sand with at least 85% passing the 850-μm (No. 20) sieve and not more than 5% passing the 600-μm (No. 30) sieve
• Portland cement

Test Procedure

Normal room temperature shall be assumed for the laboratory, curing facility, and the water used. The specimens will be prepared in briquet gang molds in multiples of three.

1. The proportions of the standard mortar shall be 1 part cement (300 grams Types I, IA, II, or III) to 3 parts of Ottawa Sand (900 grams) for 6 specimens or 400 grams of cement and 1200 grams of Ottawa Sand for 9 specimens. The amount of water required shall be determined by reference to the following table after the normal consistency of a neat cement is obtained in accordance with ASTM-C-187.

2. Weigh and thoroughly mix the dry materials on a smooth nonabsorbent surface and form a crater. Pour the proper amount of clean water into the crater and completely mix for 30 seconds with a steel trowel. Permit the materials to absorb the water for an additional 30 seconds, during which the drier materials on the edges are turned into the mortar mass in order to reduce evaporation and promote absorption. For the next 90 seconds, vigorously mix the mortar with the hands fitted with snug-fitting rubber gloves.
3. Coat the briquets with a thin film of mineral oil and place them on a piece of clean glass or metal.
4. The briquets should now be filled with the mortar. Fill the molds heaping full without compaction. Firmly press the mortar into the molds with the gloved hand, applying the thumb pressure twelve times for each mold. Again, heap additional mortar on each specimen and strike off the excess with a steel trowel. Cover the specimens with a similar piece of glass or metal and turn the assembly over, reversing top and bottom. Repeat the process of heaping, thumping, and trowelling the excess mortar as performed on the opposite surface.
5. Place the specimens in a curing room with the upper surface exposed but not subject to dripping water for between 20 to 24 hours. Then strip the specimens from the molds and place in saturated lime water until ready for testing. The lime water should be changed periodically as required to be kept clean.
6. All specimens should be tested within a specified time period.

Prior to testing, the specimens should be wiped clean and surface dry. 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 centered in the clips and the pressure applied at the rate of 2.67 ± 0.11 kN (600 ± 25 lb)/minute.

Calculations

Record the breaking load for each specimen and compute the tensile stress in kilopascals or pounds/inch² in the following data sheet. The tensile stress of all acceptable specimens, made from the same mortar, should be computed to and averaged to the nearest 34.5 kPa (5 psi) strength.