Testing - Concrete Mix Design, Quality Control and Specification - page 141

organisations. For those to whom it matters, their control system will be
providing a within-sample standard deviation. Density is unlike strength in
that small variations in assessment of the same concrete by different labo-
ratories are probably unimportant. Detection of change in average density
or change in within-sample variation is probably what matters.
To accurately measure density, the authors strongly advocate weighing
the test specimens in water and air, as the principle source of errors is the
calculated volume, especially for cubes.
7.3.5 Cubes versus cylinders
The world is divided as to whether it is better to assess concrete strength
by cube or cylinder specimens. The United Kingdom, much of Europe,
the former USSR, and many ex-British colonies use cubes; the United
States, France, and Australia use cylinders. The advantage of cubes is that
they are smaller and do not require treatment (capping) prior to testing.
The advantage of cylinders is that they are less dependent upon the quality
and condition of the molds and that their density can be more readily and
accurately established by weighing and measuring.
Both proponents naturally feel that the specimen with which they are
familiar is preferable. The debate should be settled on the basis of which
gives the most accurate (i.e., repeatable) result. This is best judged by the
average pair difference achievable or the average range of three. Either of
these can be converted into the within-sample (sometimes called within-
test) standard deviation. In the case of pairs the average pair difference is
divided by 1.13 to obtain the within sample σ. For the average range of sets
of three, the divisor is 1.69.
Day received his initial concrete QC experience in the United Kingdom
on cubes, and has owned and operated testing laboratories in Australia
using mainly cylinders and in Singapore using mainly cubes. Both
specimens are perfectly satisfactory and capable of very low pair differ-
ences if used carefully and cast in well-maintained molds. The problem
is that the test specimens must be prepared in the field by relatively low-
level technicians. The quality of training provided is crucial and is often
inadequate. The really basic fault is often that the people training the
technicians have inadequate knowledge, practical experience, or dedica-
tion to the task.
Capping used to be something of a problem with cylinders, although
more of an initial than a continuing problem. It is still a problem in areas
where cubes are the predominant specimens and the testing laboratories
rarely test cylinders or cores. Once the proper equipment is obtained and
the operator has gained experience, capping was never much of a problem.
The capping referred to is the use of a molten sulfur mixture to achieve a
smooth test surface on the end of the cylinder.