|Frequently Asked Questions
Q: We are planning to build a concrete countertop.
Can you tell me which type of cement to use for this application?
I’ve heard that Type II is best.
A: The recommendation to use a Type
II cement for countertop construction is usually based on the premise
that concrete with a Type II cement will have lower shrinkage potential
and, therefore, lower cracking potential. While there may be slight
variations in shrinkage potential between cements from different
manufacturers and between cement types, it is of little practical
value to limit the cement type based on this fact.
As can be seen in the figure below, the shrinkage potential of
a plain concrete mixture (no reinforcement) typically ranges between
600 millionths to 790 millionths. The spread of shrinkage data for
concrete mixtures is similar across the cement types. For small
dimension projects that have little restraint in service like countertops,
cement type is probably not of critical importance.In general, if
Type II is available locally in bags, use it; if not, a Type I or
a Type III cement should give close to the same results.
To reduce the potential for cracking, it is probably of greater
importance that the concrete be properly cured. Concrete gains strength
when it has adequate moisture, temperature, and time. Maintaining
the moisture content of the concrete for curing has the added benefit
of extending the time at which drying shrinkage takes place. If
the concrete is allowed to begin to dry in the first few days after
it has been cast, it shrinks, producing tensile stresses that can
cause cracking if the concrete still has low strength. However,
if the moisture is maintained in the concrete, the shrinkage takes
place later in the life of the concrete, after it has developed
additional strength. This allows it to better resist the tensile
stresses that cause cracking. With this in mind, it may be more
important to maintain the moisture in the countertop for a period
of 7 days to reduce cracking potential than it is to be overly particular
about cement type.