Pavement Manual: Hydraulic Cement Concrete

Section 9: Hydraulic Cement Concrete

9.1 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 slag cement), aggregates (coarse and fine), water, and chemical admixtures.

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9.2 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, 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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9.3 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 shrinkage and 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:

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each component material should meet the minimum properties as required in Item 421, and

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the proportions of the component materials should be designed to minimize paste volume.

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. Concrete with gap graded aggregates tend to require more paste to achieve a desired workability. Concrete with more coarse aggregate volume per unit volume of concrete (optimized) tend to need less paste to fill the voids between the aggregate. Having less paste results in less heat of hydration, less drying shrinkage, and less potential for cracking. All these provide for better long-term pavement performance.

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9.4 Three Concrete Classes

In the 2014 TxDOT specifications, there are three classes of concrete related to concrete pavement:

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Class P,

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Class K, and

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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 when used, strength requirements must be shown on the plans
HES	originally 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 when used, default strength requirements are included in Item 360