Building Materials, Third Edition

QPP f2w—

—

The number of results that are likely to fall below the target strength (mean strength) are
related to standard deviation,
. The relationship is given by the following equation:
Target strength = fck + k

where,
fck = characteristic strength below which certain percentage of test results are expected
to fall.
k = constant depending on the probability of certain number of results likely to fall
below fck

= Standard deviation.
Characteristic strength is defined as that value below which not more than 5 per cent (1 to
20) results are expected to fall, in which case the value of k will be 1.65 and the equation for
target strength reduces to
Target strength = fck +1.65

k varies according to the probability of results falling below the specified characteristic
strength. Table 11.8 gives the value of k for different probabilities.

„—˜"2II    V †—"22
22h2€˜—˜"

ƒ!#2x €˜—˜!225—!

    

—!!2˜!2™

I I2

2S HVR

P I2

2IH IPV

Q I2

2IS ISH

R I2

2PH ITS

S I2

2RH IVT

T I2

2IHH PQQ

For the probability of 1 in 100 the concrete will have to be designed for a much higher
strength than the specified strength. In such a case target strength
= f ck + 2.33

—2g22g™

The importance of quality of concrete is being increasingly realised to derive the optimum
benefit from the materials employed. Quality control does not merely signify testing of concrete
cubes at 28 days; rather it actually permeates all aspects of the choice of materials, design, and
workmanship—it commences much before any concrete is available for testing at 28 days.
Normally, specifications stipulate the 28 days compressive strength requirements. Though
useful in establishing criteria, the limitations of compressive strength data must be considered.
Test specimens indicate potential rather than actual strength of a structure, while poor
workmanship in placing and curing of concrete may cause strength reductions which are not
reflected in the cube strength results. To place too much reliance on too few tests will, therefore,
be erroneous.
Control of concrete introduces many problems basically because strength is not known until
28 days after placing the concrete. Most specifications require minimum strength which must
be exceeded, but statistical analysis indicates that such specification are difficult to comply
with. It would, therefore, be more realistic to take a calculated risk based on the probabilities
arrived at by statistical methods and allow a certain percentage of lows. By permitting a