Civil Engineering Project Management (4th Edition)

240 Civil Engineering Project Management
Reliable values of S cannot be obtained unless enough sample results are
available. BS 5328 requires a minimum of 30 tests to be made before assessing
S; and for n 30 to 100 tests, the value S should be taken where S(0.86
2/n)S, with a minimum value of 6 N/mm 2 . The DoE manual, Design of
normal concrete mixes (1988), recommends that for Grades C20 and above, S
should be taken as not less than 8 N/mm 2 until more than 20 test results are
available, and a minimum value of 4 N/mm 2 should apply however many
tests are taken. For trial mixes, values of 2S would also normally be taken
(rather than 1.64S) to give a further margin of safety. However, a less exacting
approach is also allowable as described in the next section.
19.3 Practical compliance with concrete standards
It can be seen that although designing a trial mix to meet a given grade can be
done within a reasonable time using recommended minimum S values, the
proving of a mix by statistical analysis of cube strengths is a lengthy business.
At least 20 batches of concrete would have to be made up (preferably 30 or
more) and at least two cubes from each batch tested at 28 days. (BS 5328
requires the mean of two cubes to be taken in each case.) The statistical method
is therefore mainly used to monitor concrete quality when large quantities
are being placed, and is not practicable for small sites. However, BS 5328 laid
down simpler criteria for concrete of grades C20 and above, namely:
• the average cube strength of four consecutive samples must exceed the
desired characteristic strength by 3 N/mm2 ;
• no single test shall be more than 3 N/mm2 less than the characteristic
strength.
Thus four batches tested are the minimum requirement for testing of a trial
mix. As an example, a trial mix for Grade C25 characteristic strength would be
satisfactory if samples from each of four batch mixes give an average strength
of not less than 28 N/mm2 with no single cube strength under 22 N/mm2 .
The specification may also set other requirements such as minimum cement
content, minimum density of concrete, and minimum tensile strength under
bending. BS 1881 (1981): Methods of testing concrete describes (inter alia) standard
methods of sampling mixes, making and curing test specimens, density testing,
and tensile testing – the latter labelled ‘determination of flexural strength’.
Cubes are usually tested in an off-site laboratory. A density test can be carried
out on site provided it is defined as on ‘fresh, fully compacted concrete’ (see
Section 19.8). The requirement for the flexural test is shown in Fig. 19.1. A test
apparatus is simple to make on site, and the tensile strength of the concrete is
easily calculated from the value of the central load which causes failure of the
beam. This tensile strength is important in water retaining concrete, and can be
helpful in deciding when support props can be removed from slabs and beams
if the test beam is cured alongside and in the same manner as such slabs and