Advice to specifiers - Concrete Mix Design, Quality Control and Specification - page 3

concrete. Second, an unnecessarily high cementitious content may lead to
increased cracking due to thermal stresses and shrinkage, which could reduce
durability. Unnecessarily wasting cementitious materials also increases the
environmental impact of the concrete.
A difficulty exists in specifying durability performance owing to the
absence of a generally accepted comprehensive test at a reasonably early
age. It also depends on the nature of the deteriorating influences to be
withstood.
An increasing number of specifications require compliance testing of
concrete transport properties during construction in an attempt to improve
the expected durability of reinforced concrete structures. However, unlike
compressive strength, there is little information available on the expected
variation in the results obtained as well as on the relationship between such
compliance tests and in situ properties/performance. Indeed, unlike air
entrainment to enhance freeze-thaw resistance, the required performance
for the different specified parameters to achieve the desired durability has
often not been established.
In the case of chloride-induced corrosion, which is the most common
durability issue, performance requirements may include diffusion, migra-
tion, resistivity or water transport measurements, or combinations of these.
In the United States, the ASTM C1202 coulomb test, has been common.
The leading contenders for adoption are the coulomb test, chloride migra-
tion, and the direct measurement of moisture absorption.
The coulomb test, often called the rapid chloride permeability test even
though it is does not measure chlorides or permeability, is a measurement
of saturated resistivity and is correlated to chloride diffusion. The test result
has quite high variability, so that it should not be specified as a rejection
criterion for the sampled concrete. However, a statistical analysis of results
over the course of a month could be used as a basis for a penalty clause.
An important breakthrough in specification practice is an article on “End
Result Specifications” by the Virginia Department of Transportation (DOT)
in Concrete International for March 2011. This use of performance specifica-
tions, complete with bonus and penalty clauses, by a major U.S. government
department could lead to a rapid transformation in world practice.
Chloride diffusion is perhaps the most relevant test, but it is expen-
sive and time consuming to test and therefore not well suited for compli-
ance testing. Chloride migration is a much faster and cheaper procedure
that still measures chloride penetration. The recently released RMS B80
specification uses both chloride diffusion and migration values for differ-
ent chloride environments. We suggest that the best procedure would be to
measure resistivity frequently and chloride migration occasionally to con-
firm adequate performance based on the concept of a characteristic value.
Where chloride ingress is controlled by limting absorption, absorption
tests are appropriate, i.e., BS1881: P t122 .