Environmental Engineering Reference
In-Depth Information
Charts for individual strength tests
Required average
strength f cr
Average of two
cylinders
28
(a)
21
Specified strength, f c
Required strength = f c + k σ
14
Moving average for strength
Each point, average strength
of five previous test groups
(b)
28
Required average strength f cr
21
Moving average for range
2.1
(c)
Average range for two cylinders = .0564 f cr
Average range for three cylinders = 0846 fcr
Each point average of
ten previous range
0.7
0 4 8 12 16 20 24 28 32 36 40 44 48
Sample numbers
FIGURE 12.2 Quality control charts for concrete. 6
3.
Chart (c) or “c” shows the moving average range where the range for the
average of the previous ten groups of companion cylinders is plotted each
day or shift. The maximum average range allowable for good laboratory
control is also plotted.
For any particular job, a sufficient number of tests should be made to insure
accurate representation of the variations of the concrete. Concrete tests can be made
either on the basis of time elapsed or cubic yardage placed, and conditions on each
job will determine the most practical method of obtaining the number of tests needed.
A test is defined as the average strength of all specimens of the same age fabricated
from samples taken from single batches of concrete.
ACI Committee document 214-77 6 also gives some recommendations about the
rejection of test results. The practice of arbitrary rejection of test cylinders, which
appear “too far out of line,” is not recommended since the normal pattern of prob-
ability establishes the possibility of such results. Discarding tests indiscriminately
can seriously distort the strength distribution, making analysis of the results less
reliable. Occasionally, the strength of one cylinder from a group made from a sample
deviates so far from the average as to be highly improbable. It is recommended that
a specimen from a test of three or more specimens be discarded if its deviation from
a test mean is greater than 3σ, and should be accepted with suspicion if its deviation
is greater than 2σ. If questionable variations have been observed during fabrication,
curing, or testing of a specimen, the specimen should be rejected. The test average
should be computed from the remaining specimens. A test (average of all specimens
of a sample) should never be rejected unless the specimens are known to be faulty,
since it represents the best available estimate for the sample.
Search WWH ::




Custom Search