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Section 8: Hydraulic Cement Concrete

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Primary Ingredients

Hydraulic cement concrete is a composite material that consists essentially of cementitious material (Portland cement and supplementary cementitious materials such as fly ash and ground granulated blast furnace slag), aggregates (coarse and fine), and water and/or chemical admixtures.

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Determining Ingredient Proportions

The properties of fresh and hardened concrete depend on, among other factors, the proportions of the above ingredients and, to a lesser extent, on the characteristics of coarse and fine aggregates. The process of determining the proportions of each ingredient in consideration of the desired concrete properties is called mix design. The American Concrete Institute (ACI) procedures under ACI 211 provide a process to determine the proportions of the ingredients.

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Creating Workability, Durability, and Adequate Strength

In concrete paving operations, good workability and resistance to segregation are important. As for the hardened concrete properties, good durability, adequate strength, and less volume change potential due to temperature variations are characteristics that will provide good performance of concrete pavement. For the concrete to have good workability, durability, and adequate strength, two conditions must be met:

  1. each component material should have desirable properties as required in Item 421 and
  2. the proportions of the component materials should be optimized.

The current coarse aggregate gradations in Item 421 are more or less gap graded, which has less total coarse aggregate volume in a unit volume of concrete compared with well graded aggregate. TxDOT is currently evaluating concrete with optimized coarse aggregate gradation for paving operations. Concrete with more coarse aggregate volume will have a lower coefficient of thermal expansion (COTE) and heat of hydration, and less drying shrinkage, resulting in less potential for cracking and better pavement performance.

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Three Concrete Classes

In the 2004 TxDOT specifications, there are three classes of concrete related to concrete pavement:

  1. Class P
  2. Class K and
  3. Class HES.
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Class

Characteristics

P
  • normal concrete paving

K
  • once used for projects when high early strength was preferred, such as, full-depth or partial-depth repairs
  • classified as structural concrete and subject to ASR mitigation options in Item 421.4.6
  • when used, strength requirements must be shown on the plans

HES
  • developed for 2004 Specifications; intended for projects when high early strength is required.
  • not classified as structural concrete and not subject to ASR mitigation options as long as the cement content does not exceed 520 lb. per cu. yd of concrete
  • when used, default strength requirements are included in Item 360



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