Masonry Magazine October 1975 Page. 11
The h/t of the specimens shall be determined by dividing the actual measured height of the specimen by the actual measured thickness of the specimen. These specimen dimensions shall be determined in accordance with paragraph 6.2 of ASTM E 447.
FIG. 2
Solid Brick Compressive Prism
Tensile Splitting Failure
The cross-sectional area shall also be determined based upon actual dimensions of the specimen in accordance with paragraph 6.2 of ASTM E 447. The cross-sectional area to be used for determination of ultimate compressive strength shall be the specimen thickness times the specimen length. For masonry units in accordance with ASTM C 62 and C 216, the gross cross-sectional area shall be used (t x I). If units are hollow brick (ASTM C 652), the net cross-sectional area must be used for determination of ultimate compressive strength. The net cross-sectional area shall be determined as follows: the actual gross cross-sectional area (tx 1), using measured dimensions, less the area of voids in the total cross section as measured or determined as outlined in Technical Notes 39.
If the coefficient of variation (v) of the test results on the specimens exceeds 10 percent, the ultimate compressive strength to be used must be modified. This should not be confused with the 12 percent coefficient of variation requirement for the test samples of individual units as covered in Technical Notes 39. If less than 10 percent, the average of the specimen tests should be used for (f'm) ultimate compressive strength. When the coefficient of variation exceeds 10 percent, modify the average compressive strength of the specimens by the following equation to obtain f'm:
f'm=[1-1.5 (100-0.10)]
where:
X
(1)
f'multimate design compressive strength, psi (kgf/
cm²)
V
X
= coefficient of variation of the specimen samples
tested, percent
= average compressive strength of all specimens,
psi (kgf/cm²)
The test report should include the average compressive strength, the standard deviation and the coefficient of variation. If this information is not included, they may be calculated as follows:
FIG. 3
Hollow Brick Compressive Prism
Tensile Splitting Failure
X=
n
(2)
Σ(Χ - Χ)
S
(3)
n-1
v = 100$
where:
(4)
X
X,
= compressive strength of individual specimen, psi
(kgf/cm²)
total of all individual specimen compressive
strengths, psi (kgf/cm²)
n
= number of specimens
S
- standard deviation, psi (kgf/cm²)
V
= coefficient of variation, percent
3