Section 2: Concrete Mix Design
2.1 Introduction
The concrete mix design is performed to ensure that the concrete mix formulation meets or exceeds the specification requirements. The mix design is used to establish the proper proportioning of components (hydraulic cement, aggregates, water, pozzolans, and admixtures) in the mixture to achieve the specified properties. Significant concrete properties are strength, air content, slump, and the coefficient of thermal expansion (COTE). The mix design may be developed for the current project or may have been previously developed. TxDOT developed a
document that provides useful information on mix design, production considerations, and testing requirements. Concrete mix design can be facilitated by using the developed by TxDOT. The mix design must be formally approved by an engineer.
2.2 Job Control Testing
In addition to the required mix design strength, the job control
strength shall be established. The job control strength is used
to verify that the concrete being used on the project will perform
similarly to the concrete used to develop the mix design. The default
is to use the 7-day mix strength as the job control strength. This
7-day strength testing may be altered with the approval of the engineer. The
contractor may want to develop job control strengths at an earlier
age, such as four days, to permit the job control specimen to also
be used to open the pavement to traffic at an earlier age. Reduction
of curing time for the job control specimens may reduce the reliability
of the test in ensuring that the specified strengths will be reached.
Testing at four days should still provide a reliable estimate of
the long term strength.
The standard job control testing at seven days was established
many years ago and ensured that the strength test of the job control
specimens occurred on the same day of the week as the concrete paving.
The result is that there would be no testing on Saturday or Sunday,
unless the paving work was performed on those days. This was also
a management tool to eliminate the need for laboratory technicians
to report to work on Sunday to perform a single strength test.
2.3 Opening to Traffic
Many urban highway construction projects have severe traffic
control and congestion issues. To expedite construction and minimize
travel delays for the public, contract restrictions on dates and times
that travel lanes can and can not be closed to travel are imposed.
Project contracts may also impose large bonuses and disincentives
for time of completion. Pavement using Class P concrete may be opened
in as little as two days to contractors' vehicles and as little
as three days to all traffic, if opening strength is achieved. After
curing is complete and when earlier age job control testing is permitted
or required, and the tested strength is greater than the required
opening strength, the pavement may be opened to traffic.
Class HES (high early strength) concrete may be used in small
areas and leave-outs. Class HES has additional strength requirements
beyond Class P concrete to ensure that the high early strengths
for opening to traffic are realized.
2.4 Maturity Method
The maturity method, Tex-426-A, “Estimating Concrete Strength
by the Maturity Method,” may be used to open the pavement to traffic
at an earlier age than seven days with either Class P or Class HES
concrete. It is still necessary to complete the specified curing.
A maturity curve may be developed during the mix design process
to establish the relationship between the concrete maturity and concrete
strength. This may be used to identify what the maturity of the
concrete should be when the opening strength has been reached. This
maturity value may be used to estimate the in situ strength and
open pavement to traffic that has completed the required curing.
A maturity test should not be used in lieu of job control strength
testing to determine the conformance of the mix to the mix design.