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Section 2: Concrete Mix Design

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 properties are strength, air content, slump, and sometimes 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 Concrete Mix Design Guidance document that provides useful information. Concrete mix design can be facilitated by using the Mix Design spreadsheet developed by TxDOT. The mix design must be formally approved by an engineer.

In addition to the required mix design strength, the job control strength needs to 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.

It is recommended that air-entrainment requirements be maintained for concrete pavement at all locations around the state, regardless of the number of freeze-thaw cycles in that area. The benefits provided by entrained air to slip-formed pavement in workability and water demand are substantial. A good air-entrained system also benefits the long-term durability of hardened concrete.

For concrete pavements, the target air content will be between 2.5 and 5.5%, with values as high as 7% acceptable as long as strength requirements are met. Since Item 360 allows occasional and minor (less than 10%) strengths below job-control targets, an entrained-air range of 2.5 to 7% should not be a production challenge, nor should it require excessive cement to ensure minimum strengths are consistently obtained.

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.

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.


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