Specifications for Compressive Strength Test

Generally, masonry unit specimens are tested in the bedface orientation, the same orientation they are used for masonry construction, and in the similar manner cast concrete and mortar are tested, that is using a testing device that can apply a stress through either a force or displacement control manner.

In this study, the compressive strength test was conducted following the guidelines presented in the European standardBS EN 772-1 (2011): Determination of compression strength. The standard specifies that the minimum number of specimens to be tested is six. A testing machine with a capacity to crush all specimens at an appropriate loading rate is required.

The specimen should be prepared by ensuring that the faces through which the loads will be applied are plane within the provided tolerances; this can be done either by capping or grinding the faces. Various conditioning regimes are specified in the standard. Based on the nature of masonry units being investigated in this study, the adopted conditioning methods are conditioning to the air dry condition and conditioning by immersion.

As per the standard, specimens conditioned to the air dry condition should be stored for at least 14 days in the laboratory space where free air circulation is achieved, and a temperature higher than 15°C with a relative humidity less or equal to 65%. Specimens conditioned by immersion should be immersed in water at a temperature of 20°C ± 5°C for a minimum period of 15h after which they are allowed to drain for 15 to 20 minutes before they are tested.

Once the loading face has been prepared as specified, the necessary dimensions (length and width) should be measured and used to determine, in square millimetres, the gross area of the specimen. The testing machine should also be wiped clean and prepared for the test. A pre-determined loading rate based on the rates specified in the standard is used for the loading application. The maximum compressive strength (fu) of the specimen is calculated through the division of the maximum load achieved by the loaded area as shown in Equation (4.3). For a given unit type sample, the compressive strength of the sample is then calculated as the average strength value of the individual specimens. The coefficient of variation (COV) of the sample is also calculated.

fu = F

A (4.3)

Where:

fu Compressive strength of the unit specimen, [MPa] F Maximum load reached, [N]

A Loaded cross-sectional area, [mm2]

In order to use the experimental test results for design application, the compressive strength of masonry units should be converted to the normalised compressive strength. This conversion takes into consideration the confinement of the specimen resulting from the use of steel plates during the test. An informative annex provided inBS EN 772-1 (2011) allows for the conversion, which is based on the converted air-dry compressive strength of the unit and the appropriate shape factor provided in the annex.

4.1.1.2 Mortar

The compressive strength of mortar is determined in the same manner as concrete, by casting cubes of 100mm side dimension in lubricated moulds. This dimension is recommended for concrete of maximum aggregate sizes of 20mm or less. The moulds are accurately created in such a way that opposite faces are smooth and parallel (Domone and Illston,2010).

It has been observed that the cracking patterns resulting from the compressive strength test on cubes have a double pyramid shape after failure, indicating that the stresses within the cube are not uniaxial but the concrete specimen is rather in a triaxial stress state due to the boundary conditions. In the cube-testing machine, the specimen is placed in between two stiff steel platens with similar dimensions as the specimen loading face. Owing to the Poisson effect, the applied compression force induces tensile strain in steel plate as well as the specimen. The difference in stiffness between steel and concrete, and the friction at the steel plate-concrete interface restrain the concrete against outward expansion leading to the triaxial stress state (Domone and Illston, 2010).

Although the restraint in the cube test may result in failure at a higher stress as opposed to the true unrestrained strength, the test is used for its simplicity and enables comparison between different concretes. To overcome the restraint effect in the cube test, various guidelines around the world recommend a cylindrical test, as the restraints around the middle section of the cylinder specimen are reduced, because of the length of the cylinder, resulting in near uniaxial failure. The strength from the cylindrical test is often assumed to be about 20% lower than the cube strength (Domone and Illston,2010).

The compressive strength test of mortar was performed according to the specifications provided in the South African Standards, SANS 5863 (2006): Concrete tests - Compressive strength of hardened concrete. At the time of testing, the specimens were taken from the curing tank. The dimensions of the loading face were recorded, and the specimen was tested immediately while still saturated as prescribed in the code. Once the test is completed, the compressive

strength (fm) of the mortar specimen is calculated, similarly to the unit compressive strength, using Equation (4.3). The mortar compressive strength is determined by taking the average compressive strength of all the specimens.