Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 10

sold as being a particular strength grade. Generally higher strength
grades are more expensive but less can be used to meet a strength
specification. The selection of a high strength cement becomes impor-
tant when very high strength concrete is required, since an increase in
cement content will not give a strength increase beyond a certain point.
It is desirable for ready-mix producers in particular to develop a good
working relationship with their cement supplier. A variation-free product
cannot be expected, but honesty in reporting current test results, and help
in interpreting and compensating for their likely effects on concrete, and
cooperation in tracking down any problems is valuable. This kind of coop-
eration is unlikely if all concrete problems are automatically blamed on
the cement, and the concrete producer fails to carry out and keep proper
records of control tests on concrete.
An important, if relatively rare, occurrence is an unfavorable interaction
between the cement and admixtures in use. Examples have been encoun-
tered where a particular cement and admixture, both satisfactory with
other admixtures and cements, have given trouble in combination. In one
example the trouble was a false set. A false set is one that occurs for a
limited time and can be overcome by continued mixing. This may give
no trouble when held in a truck mixer until directly discharged into place
but cause a severe loss of pumpability if discharged into a pump hopper
during or prior to its occurrence. If suspected, such an occurrence can be
investigated using a Proctor needle penetrometer on mortar sieved from the
concrete to construct time versus penetration resistance curves.
A particularly delicate question is that of cement that provides a lower
strength. It is of substantial assistance to a concrete producer if he can
rely upon the cement producer advising him of a strength downturn. This
enables the concrete producer to increase his cement content or make other
modifications and avoid low test results. However, since the cement pro-
ducer is responsible for the need for the additional cement, there is a natu-
ral tendency for the concrete producer to feel that the cement producer
should bear the additional cost. It will obviously not encourage the cement
producer to provide the early warning if the result is a deduction from his
invoice.
The reverse kind of assistance is also valuable. Cement suppliers tend to
receive unjustified complaints from customers who have inadequate control
systems. It is of value to them to find a regular user who has a good control
system so that they can rely on feedback data.
In summary, the development of a good relationship and an effective
early warning system with your supplier can be of considerable benefit,
and your own good control system is a necessary starting point for such a
relationship.