Portland Cement Concrete Pavements Team - Paper: Concrete Materials and Mix Design for Assuring Durable Pavements

CONCRETE MATERIALS AND MIX DESIGN FOR ASSURING DURABLE PAVEMENTS

INTRODUCTION

Concern with the durability of concrete pavements is not a new phenomenon. Consider the following quotes from Agg (1) made in 1940.

First, while discussing concrete slabs, he states: "The concrete road slab whether it is to be used as a foundation course or as a wearing course, has the property of an imperfectly elastic solid, and hence on any subgrade its stability under load is related to its structural strength. The use of slabs of this type introduces illusive problems of structural design, since consideration must be given not only to the traffic load to which the slab is subjected but also to temperature and moisture effects on the concrete and the influence of the subgrade support upon the stability of the overlying slab."

On the subject of concrete mixes he says: "Concrete road slabs are designed on the basis of an assumed strength of concrete, and the first consideration in the design of the concrete mixture itself is to make sure that a concrete is produced that will have strength at least equal to that assumed in the design. Although standard specifications frequently mention the proportions of coarse and fine aggregate to be used with a cubic foot of cement, this is generally to be considered only as a guide, and the exact proportions are developed by laboratory tests to establish the most economical combination of the available fine and coarse aggregate that will produce concrete of the required strength. It is recognized that the critical element in a mixture is the water and that the best concrete will be that which is produced with the lowest water-cement ratio consistent with a workable mixture."

Finally, on aggregates: "The aggregate should be sound, by which is meant that the effect of repeated freezing and thawing, together with absorption of water, does not cause the aggregate to soften sufficiently so that it will be unable to resist the wear to traffic. The maximum size should not be greater than about 2 ½ in., ... the minimum size is that which will pass a 50-mesh sieve; of this there should not be more than 5 or 6 percent. Between these two sizes the proportion of the several sizes should be such as to insure that when the particle-size distribution is plotted it will be definitely concave upward, and the more nearly it approaches the curve of maximum density, the better. The ratio of water to cement has so powerful an influence on the strength of the concrete, however, that more attention is paid to water-cement ratio than to the grading of the mineral aggregate. The engineer should, however, take both factors into account and, by laboratory studies on the available materials, determine the proportion of coarse and fine aggregates of the kinds commercially available that will produce concrete of the desired strength at the lowest cost"

These few paragraphs serve to illustrate the thoroughness of understanding of concrete in pavements in the 1940's (and earlier - the first edition of this book was published in 1916). We may quibble with some of the details of these statements, but the basic reasoning still sounds pretty good today. But, perhaps we have learned some things in the intervening 50 plus years, and in this paper I will explore some of the materials-related factors which influence the durability of concrete pavements. [In the context of this paper, durability is considered the resistance of the concrete and pavement to their surroundings, including the environment and traffic.] These factors include: materials and mix design; concrete's role in pavement structural design and performance; the environment; and the construction process.

Abstract | Influences of Materials & Design