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.
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.
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:
- each component material should meet the minimum properties as required in Item 421, and
- 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.
9.4 Three Concrete Classes
In the 2014 TxDOT specifications, there are three classes of concrete related to concrete pavement:
- Class P,
- Class K, and
- Class HES.
Class | Characteristics |
---|---|
P |
|
K |
|
HES |
|