COMPRESSIVE STRENGTH OF MOULDED MASONRY MORTAR CUBES (ASTM C780)

IX. COMPRESSIVE STRENGTH OF MOULDED

MASONRY MORTAR CUBES (ASTM C780)

1. Scope:

This method covers the procedure for determining the compressive strength of mortars for masonry

using 50.8 mm (2 in.) three-compartment gang cubes.

2. Apparatus:

Moulds: Cube moulds for 50.8 mm (2-in.) cubic specimens shall be in accordance with Test

Method ASTM C 109 (compressive strength test of cement) and shall be made of non-corrodible

material and shall be sufficiently rigid to prevent spreading during moulding. The moulds shall have

not more than three compartments, and shall be separable into not more than two parts (plus a base

plate). The specimen compartment shall be 50.8+0.25mm (2+0.01 in.). The interior surfaces shall be

smooth, and the top and bottom surfaces shall be plane and parallel. The base-plate shall be made of metal or plate glass not less than 6.35 mm (1/4 in.) thick and having a plane, smooth surface.

3. Summary of Method:

During the actual construction, evaluation of masonry mortars is possible by sampling the mortar at

various stages of construction and performing tests. The test results permit further verification of

batch-to-batch variations introduced during mortar production and use at the construction site.

4. Sampling:

4. 1 Obtain plastic mortar samples at the construction project by taking uniformly distributed

samples, and mixing them to form a bulk sample from which the quantity of mortar required for a

specific test or specimen can be randomly taken.

4. 2 Take the batch mixer samples immediately after mixing, either during the discharge of the

mixer or after the mortar has been discharged into the mortar boat, buggy, or other receptacles. If

samples are taken during the discharge of the mixer, they may be taken at any time except for the

first and last 10% of the batch. Samples of the mortar taken after discharge from the mixer should

be representative of the entire batch.

4. 3 Record sampling procedures to include the date, time, place and method of sampling. When

applicable, note and record the climatic conditions.

5. Test Specimens:

5. 1 For determining hardened mortar properties, prepare three test specimens for each test age and

property.

5.2 During actual construction evaluation of mortars, and when batch-to-batch variations are to be

established, sample three consecutive batches and test them.

Brick

6. Preparation of Cube Specimens

6.1 Number of Specimens: minimum of three specimens shall be made for each period of test

specified.

6.2 Preparation of Cube Moulds: Thinly cover the interior face of the specimen moulds with

mineral oil or light cup grease. Thinly cover the contact surfaces of the halves of each mould with

mineral oil. After assembling the moulds, remove the excess oil or grease from the interior faces

and the top and bottom surfaces of the mould. Set the moulds on a plane, nonabsorptive base plate

that has been thinly coated with mineral oil. Apply a wax similar to micro-crystalline wax at the

outside contact lines of the moulds and base plates, so that watertight joints are affected between the moulds and the base plates.

6.3 Immediately after the mortar has been sampled, mould the test specimens in accordance with

Test Method ASTM C 109 (compressive strength test of cement).

7. Storing the Test Specimens:

7. 1 Immediately on completion of the moulding, as far as possible, job-site storage facilities

should be provided with approximate laboratory storage conditions.

7.2 The storage temperature should not be less than 4°C nor greater than 32°C. If temperatures

exceed these limits, the specimens shall be discarded.

7.3 Remove the specimens from their stored position at the end of 24+4 hours and carefully

transport them to the laboratory. Place the specimens in a moist closet or moist room for

the next 20 to 24 hr with their upper surfaces exposed to the moist air but protected from

dripping water. Next, following the 20 to 24 hr period, remove the specimens from moulds. The

specimens shall be maintained in a moist cabinet with the relative humidity maintained at 90% or

more, up to the testing age for each 7 day or 28-day test.

8. Test Procedure:

8.1 Test the cube specimens immediately after their removal from the moist cabinet. If more

than one specimen at a time is removed from the moist closet for each test, cover these cubes with a

damp cloth until the time of testing. Wipe each specimen to a surface dry condition, and remove

any loose sand grains or particles from the faces that will be in contact with the bearing block of

the compression tester.

8. 2 Prior to the testing of each cube, it shall be ascertained that the spherically-seated block of

compression tester is free to tilt, No cushioning or bedding material shall be used. The diagonal or

diameter of the bearing surface shall be only slightly greater than the diagonal of the face of the 50

mm (2 in.) cube specimen in order to facilitate accurate centring of the specimen.

8. 3 Apply the load to the specimen laces that were in contact with the true plane surfaces of the

mould. Carefully place the specimen in the testing machine below the centre of the upper bearing

block.

8. 4 An initial loading up to one half of the expected maximum load for specimens having expected

maximum loads of more than 13. 34 KN (3000 Ib) may be applied at any convenient rate. Apply no

initial loading to specimens having expected maximum loads of less than 13. 34 KN (3000 Ibf).

Adjust the rate of load application so that the remainder of the load (or the entire load in the

case of expected maximum loads of less than 13.34 KN (3000 Ibf) is applied, without interruption,

to failure at such a rate that the maximum load will be reached in not less than 20 nor more than 80

seconds. Make no adjustments in the controls of the testing machine while a specimen is yielding

rapidly immediately before failure.

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9. Calculation:

Record the total maximum load indicated by the testing machine and calculate the compressive

strength in MPa (psi). If the cross-sectional area of a specimen varies more than 0.39cm^2 (0.06 in^2) from 25.81 cm^2 (4 in^2 )use the actual area for the calculation of the compressive strength. The compressive strength of all acceptable test specimens made from the same sample and tested at the same period shall be averaged and reported to the nearest MPa (psi).

10. Faulty Specimens and Retest:

In determining the compressive strength, do not consider the specimens that are manifestly faulty,

or that give strengths differing by more than 10% from the average value of all test specimens

made from the same sample and tested at the same period. After discarding specimens or strength

values, if less than two strength values remain for determining the compressive strength at any given period, make a new test.

NOTE: Reliable strength results depend upon careful observance of all the specified requirements and procedures Erratic results at a given test period, indicate that some of the requirements and procedures have not been carefully observed. For example, improper centring of specimens resulting in oblique fractures or lateral

movements of one of the heads of the testing machine during loading, will often cause lower strength

results. A specimen so broken shall be considered "manifestly faulty" if its strength differs by more than

10% from the average of all test specimens made from the sample and tested at the same period.

11. Report:

The report shall include the following:

- A description of the specimen by size

- The curing procedure, dimensions, total load, and compressive strength of the individual

specimen for each test age.

- Average result for each test age

12. Specification:

The specified strength of mortar should not be lower than the specified strength of brick for masonry work under any circumstances.