Federal Highway Administration - Research, Development, and Technology

CONCRETE MATERIALS AND MIX DESIGN FOR ASSURING DURABLE PAVEMENTS

INFLUENCE OF MATERIALS AND DESIGN

Let's examine some of the aspects of the concrete and its constitutive materials which influence various aspects of pavement performance.

Aggregates

Aggregates influence the performance of concrete, and the pavement in which it is used, in many ways. The influencing characteristics of aggregates, which are listed in Table 1, are discussed below. As maximum aggregate size increases, the volume percentage of the aggregate in the concrete can also increase (for the same gradation) and the surface area of the aggregate will decrease. This means a more economical mix, since less cement paste is present and therefore less cement will be required for a given volume of concrete. At the same time, when larger aggregate sizes are used, attention must be paid to possible effects on concrete workability, ease of consolidation and susceptibility to segregation. Due the greater volume of aggregate, the hardened concrete will have less shrinkage.

It is often assumed that good concrete can be made with almost any aggregate gradation. Although sometimes not easy, this is usually true, as long as the gradation is consistent from mix design through production. Often, however, the concrete may not be optimal, in terms of either economy or performance. It has been found that a dense (continuous) graded aggregate will often provide the best workability, as well as minimizing the volume percent of cement paste. While maximum density gradation is usually not available for a particular aggregate source, (and may not be practical from a workability standpoint) economical gradations of low void content and high density should be sought.

Another aggregate property important to producing durable concrete is soundness. Often thought of as resistance to freezing and thawing, soundness is more properly the aggregate's resistance to all aspects of weathering. This includes heating and cooling, wetting and drying, and freezing and thawing. Thus in all types of climate, including non-freeze zones, aggregate soundness is important.
The particle shape and angularity of the aggregate, particularly the fine aggregate, will affect the workability of the mixture. This should be assessed during the laboratory design phase, and any necessary adjustments made at that time so that potential problems of placement, consolidation and finishing during construction are avoided.
Good concrete can be made with a smooth gravel or a rough crushed stone. The paste-aggregate bond for these two aggregates will be different, and therefore the flexural and tensile strength of the concrete will also be different. This may require somewhat different mix designs according to aggregate type in order to meet specifications.
The hardness of the fine aggregate (since it is exposed at the surface), in particular, should be considered where pavement surface friction is of concern, as in higher-speed pavements. This is because the fine aggregate, along with any surface texturing applied at the time of placement, is responsible for the friction developed at the pavement/tire interface.

The thermal coefficient of expansion of the aggregate plays a large role in determining the thermal coefficient of expansion of the concrete as a whole. If this influence is not considered during pavement design, it could lead to uncontrolled transverse cracking (in jointed plain concrete pavement(JPCP), or undesirable crack spacing (in jointed reinforced concrete pavement(JRCP) and continuous reinforced concrete pavement (CRCP)). Since the thermal coefficient of expansion of the cement paste is usually 1 ½ to 2 times that of the aggregate (2), the volume percent of each component in the concrete will also have an effect.

Finally, the reactivity of the aggregate with the cement and other components in the mix, in terms of the potential for deleterious expansion due to alki-aggregate reactivity (AAR), can greatly influence the long term durability of the concrete. If in doubt, screening tests should be carried out to evaluate this potential.

Cementitious Components

For paving concrete, the only cementitious component normally used is portland cement, usually Type I or Type I-II. Other types of cement may sometimes be used in those situations where additional sulfate resistance is desired, or when more rapid strength gain is needed, such as with fast track paving. However, even with a given cement type and source, the user should be aware of the potential for changes in the cement actually supplied. One should also consider the fineness, and therefore the rate of hydration, heat generation and strength gain (see Table 1). What is the chemical composition, for instance the alkali content, of the cement and should it be of concern with the other components to be used, including the aggregate?

Additives/Admixtures

There currently is a multitude of mineral and chemical additives/admixtures to improve the characteristics of concrete in both the plastic and hardened state (see Table 1). These may be used to combat alkali-aggregate reactivity (AAR); increase workability; enable the reduction of the water to cementitious plus pozzolanic materials ratio (W/(C+P)), entrain air, etc. The simple advice when using these materials, either singularly or in combination, is to evaluate them during the mix design process, not only for their effect on the concrete, but just as important, their effect on each other, in terms of both effectiveness for the intended purpose, and compatibility with each other. Fly ash is a commonly used pozzolanic admixture in paving concrete. If AAR is a potential problem, the type and composition of the fly ash should be noted, and tests run as appropriate, since some fly ashes appear to increase AAR potential, and others decrease it.

Various composition and types of fibers have been used in concrete for their effect on the plastic and hardened properties. Depending on several factors, including fiber type and volume percent in the concrete, fibers may reduce bleeding, add toughness, reduce cracking and reduce crack opening.

Water/(Cementitious + Pozzolanic Materials) Ratio

With the complexity of present day concretes, often including supplementary cementitious materials and/or pozzolanic admixtures, it is no longer sufficient to consider simply the water/cement ratio, but rather the ratio of water to the sum of all the cementitious and pozzolanic materials (W/(C+P)). Because some pozzolans have low density, volumetric instead of mass equivalencies are sometimes used for these materials. Even with the more complex W/(C+P), however, the principal of keeping the ratio as low as practical within the confines of having a workable mixture capable of being placed, consolidated and finished, remains the same. This is simply because, as this ratio goes down, the strength of the concrete generally goes up, the permeability of the concrete goes down, and therefore the likelihood of a durable concrete pavement improves. Further, if lower water content per unit of concrete also results, the potential for drying shrinkage is also less, decreasing the likelihood of uncontrolled cracking in the pavement.

Introduction | Concrete's Role in Pavement Structural Design/Performance

http://www.tfhrc.gov/pavement/pccp/concr3.htm