DETERMINATION OF THE COMPRESSIVE STRENGTH OF BRICK

 II. DETERMINATION OF THE COMPRESSIVE STRENGTH OF BRICK

1. Compressive Strength Test as per ASTM C67:

 1.1 Test Specimens: The test specimens shall consist of dry half brick, the full height and width of the unit, with a length equal to one half the full length of the unit +25.4mm (1 in.).Test specimens shall be obtained by any method that will produce, without shattering or cracking, a specimen with approximately plane and parallel ends. Five specimens shall be tested.

 1.2 Capping Test Specimens:

 1.2.1 specimens shall be dried (at 110 to 115°C at least for 24 hours to constant weight and cooled before any portion of the capping procedure is carried out.

 1.2.2 If the surfaces which will become bearing surfaces during the compression test are recessed or panelled fill the depressions with a mortar composed of 1 part by weight of quick hardening cement (Type III cement of Specification ASTM C 150) and 2 parts by weight of sand. Keep the specimens at least 48 h before capping them. Where the recess exceeds 12.7 mm (1/2 in.), use a brick or tile Slab section or metal plate as a core fill. Cap the test specimens using the procedures described below.

 

1.2.3 Gypsum Capping: Coat the two opposite bearing surfaces of each specimen with shellac and allow to dry thoroughly. Bed one of the dry Shellacked surfaces of the specimen in a thin coat of neat paste of Calcined Gypsum (Plaster of Paris) that has been spread on an oiled non-absorbent plate, such as glass or machined metal. The casting surface plate shall be plane within 0.076 mm (0.003 in.) in 406.4 mm (16in.) and sufficiently rigid, and so supported that it will not be measurably deflected during the capping operation. Lightly coat it with oil or other suitable material. Repeat this procedure with the other shellacked surface. Take care that the opposite bearing surfaces so formed will be approximately parallel and perpendicular to the vertical axis of the specimen and the thickness of the caps will be approximately the same and not exceed 3.18 mm (1/8 in.). Age the caps at least 24 h before testing the specimens.

 

NOTE: A rapid-setting industrial type gypsum, such as Hydrocal or Hydro-stone, is frequently used for capping.

 

1.2.4 Sulphur-Filler Capping: Use a mixture containing 40 to 60 weight % sulphur, the remainder being ground fire clay or other suitable inert material passing a No.l00 (150micron) sieve. The casting surface plate requirements shall be as described in 1.2.3. Place four 25.4mm (1 in.) square steel bars on the surface plate to form a rectangular mould approximately 12.7 mm (1/2 in.) greater in either inside dimension than the specimen. Heat the sulphur mixture in a thermostatically controlled heating pot to a temperature sufficient to maintain fluidity for a reasonable period of time after contact with the surface being capped. Take care to prevent overheating, and stir the liquid in the pot just before use. Fill the mould to a depth of 6.35 mm (l/4 in) with molten sulphur material. Place the surface of the unit to be capped quickly in the liquid, and hold the specimen so that its vertical axis is at right angles to the capping surface. The thickness of the caps shall be approximately the same. Allow the unit to remain undisturbed until solidification is complete. Repeat this for the other face. Allow the caps to cool for a minimum of 2 h before testing the specimens.

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1. 3. Procedure:

 

1. 3. 1 Test brick specimens flatwise: Centre the specimens under the spherical upper bearing within 1.59 mm (1/16 in.).

 

1.3.2 The testing machine shall conform to the requirements of Methods E 4 (description given in Concrete chapter). The diameter of the bearing surface of the machine shall be at least 12/mm (5 in.). Use hardened metal bearing block beneath the specimen to minimise wear of the lower platen of the machine. The bearing block surfaces intended for contact with the specimen should have a hardness not less than HRC 60 (HB 620). These surfaces shall not depart from plane surfaces by more than 0.03 mm (0.001 in.). When the bearing area of the spherical bearing block is not sufficient to cover the area of the specimen, place a steel plate with the surface machined to true planes within 0.03 mm (0.001 in. )and with a thickness equal to at least one-third of the distance from the edge of the spherical bearing to the most distant corner between the spherical bearing block and the capped specimen.

 

1. 3. 3 Speed of Testing: Apply the load, up to one half of the expected maximum load, at any convenient rate, after which, adjust the controls of the machine so that the remaining load is applied at a uniform rate in not less than I nor more than 2 min.

 

1.4 Calculation: Calculate the compressive strength of each specimen as follows:

 

Compressive Strength (kg/cm^2 or Mpa or psi) =W/A

where,

W=maximum load (kgf or N or Ibf) indicated by the testing machine

A =average of the gross areas of the upper and lower bearing surfaces of the specimen (cm^2 or in) as per ASTM

 

2. Compressive Strength Test of Brick as per Bangladesh Standard (BDS-208, 1980):

 2. 1 Specimen Preparation: Twelve bricks taken at random from sample shall be halved and one half from each whole brick used for determining the crushing strength. The overall dimension of each bedding face shall be measured to the nearest of 1. 3 mm (1/20 inch.) and the area of the face having smaller area shall be taken as the area of the bricks for testing the crushing strength.

 2. 2 Procedure: The specimens shall be immersed in water at 15°C to 20°C for 24 hours. They shall then be removed and allowed to dry at room temperature for about 5 minutes, their frogs shall be filled with cement-sand mortar of 1:1.5. Sand should be clean and well-graded passing 3.2mm 1/ 8 in. sieve. The mortar shall be trowelled off flush with the surface of the bricks. The bricks shall then be stored under the damp sacks for 24 hours after filling the frogs and shall then be immersed in water for six days before bricks are considered ready for testing. After seven days of filling mortar, specimens will be taken out, Wiped dry With a damp cloth and placed with flat surface horizontally, and the mortar filled, face facing upward between two number 3 plywood sheets each approximately of 3 mm thickness and carefully centred between the plates of the compression testing machine. The compressing plates or the testing machine shall have a ball seating in the form of a portion of sphere, the centre of each coinciding with the centre of the face of the plate. The load shall be applied axially at a unit (2000 Ibs. per square inch) per minute, until failure. The failure shall be deemed to have occurred when no further increase in the load is registered with unchanged rate of moving head travel.

 

2.3 Calculation: Calculate the compressive strength of each specimen as follows:

 

Compressive Strength (kg/cm^2 or Mpa or Ib/in^2)=W/A

 

where,

W = Maximum load (kgf. or N or Ibf.) indicated by the testing machine.

A = The area (cm or in) of the face having smaller area shall be taken as the area of the

bricks for testing the crushing strength as per BDS-208, 1980.

 2. 4 Results: Maximum load in kilogram (Ib.)at failure divided by the minimum area of the bedding surface of the half bricks in square centimetre (square inch.) shall be taken as the crushing strength. The mean crushing strength of twelve half bricks along with minimum of individual bricks shall be compared with the specification.