ASTM C 109 Compressive Strength of Hydraulic Cement Mortars(4)

10.4.3Start molding the specimens within a total elapsed time of not more than 2min and 30s after completion of the original mixing of the mortar batch.Place a layer of mortar about 1in.or [25mm](approximately one half of the depth of the mold)in all of the cube compartments.Tamp the mortar in each cube compartment 32times in about 10s in 4rounds,each round to be at right angles to the other and consisting of eight adjoining strokes over the surface of the specimen,as illustrated in Fig. 1.The tamping pressure shall be just sufficient to ensure uniform filling of the molds.The 4rounds of tamping (32strokes)of the mortar shall be completed in one cube before going to the next.When the tamping of the first layer in all of the cube compartments is completed,fill the compartments with the remaining mortar and then tamp as specified for the first layer.During tamping of the second layer,bring in the mortar forced out onto the tops of the molds after each round of tamping by means of the gloved fingers and the tamper upon completion of each round and before starting the next round of tamping.On completion of the tamping,the tops of all cubes should extend slightly above the tops of the molds.Bring in the mortar that has been forced out onto the tops of the molds with a trowel and smooth off the cubes by drawing the flat side of the trowel (with the leading edge slightly raised)once across the top of each cube at right angles to the length of the mold.Then,for the purpose of leveling the mortar and making the mortar that protrudes above the top of the mold of more uniform thickness,draw the flat side of the trowel (with the leading edge slightly raised)lightly once along the length of the mold.Cut off the mortar to a plane surface flush with the top of the mold by drawing the straight edge of the trowel (held nearly perpendicular to the mold)with a sawing motion over the length of the mold.

10.5Storage of Test Specimens —Immediately upon completion of molding,place the test specimens in the moist closet or moist room.Keep all test specimens,immediately after molding,in the molds on the base plates in the moist closet or moist room from 20to 72h with their upper surfaces exposed to the moist air but protected from dripping water.If the specimens are removed from the molds before 24h,keep them on the shelves of the moist closet or moist room until they are 24-h old,and then immerse the specimens,except those for the 24-h test,in saturated lime water in storage tanks con-structed of noncorroding materials.Keep the storage water clean by changing as required.

10.6Determination of Compressive Strength :

10.6.1Test the specimens immediately after their removal from the moist closet in the case of 24-h specimens,and from storage water in the case of all other specimens.All test

specimens for a given test age shall be broken within

the permissible tolerance prescribed as follows:Test Age Permissible Tolerance 24h 61⁄2h 3days 61h 7days 63h 28days 612h If more than one specimen at a time is removed from the moist closet for the 24-h tests,keep these specimens covered with a damp cloth until time of testing.If more than one specimen at a time is removed from the storage water for testing,keep these specimens in water at a temperature of 73.56 3.5°F or [2362°C]and of sufficient depth to completely immerse each specimen until time of testing.10.6.2Wipe each specimen to a surface-dry condition,and remove any loose sand grains or incrustations from the faces that will be in contact with the bearing blocks of the testing machine.Check these faces by applying a straightedge (Note 7).If there is appreciable curvature,grind the face or faces to plane surfaces or discard the specimen.A periodic check of the cross-sectional area of the specimens should be made.N OTE 7—Specimen Faces—Results much lower than the true strength will be obtained by loading faces of the cube specimen that are not truly plane surfaces.Therefore,it is essential that specimen molds be kept scrupulously clean,as otherwise,large irregularities in the surfaces will occur.Instruments for cleaning molds should always be softer than the metal in the molds to prevent wear.In case grinding specimen faces is necessary,it can be accomplished best by rubbing the specimen on a sheet of fine emery paper or cloth glued to a plane surface,using only a moderate pressure.Such grinding is tedious for more than a few thousandths of an inch (hundredths of a millimetre);where more than this is found necessary,it is recommended that the specimen be discarded.10.6.3Apply the load to specimen faces that were in contact with the true plane surfaces of the mold.Carefully place the specimen in the testing machine below the center of the upper bearing block.Prior to the testing of each cube,it shall be ascertained that the spherically seated block is free to e no cushioning or bedding materials.Bring the spherically seated block into uniform contact with the surface of the specimen.Apply the load rate at a relative rate of movement between the upper and lower platens corresponding to a loading on the specimen with the range of 200to 400lbs/s [900to 1800N/s].Obtain this designated rate of movement of the platen during the first half of the anticipated maximum load and make no adjustment in the rate of movement of the platen in the latter half of the loading especially while the cube is yielding before failure.N OTE 8—It is advisable to apply only a very light coating of a good quality,light mineral oil to the spherical seat of the upper platen.11.Calculation 11.1Record the total maximum load indicated by the testing machine,and calculate the compressive strength as follows:fm 5P /A (1)where:fm =compressive strength in psi or [MPa],FIG.1Order of Tamping in Molding of Test Specimens