title.gif
spacer.gif

Jim Shilstone sent the following articles written in the early part of the century by Duff Abrams on Mix Design/Proportioning. Abrams realized, even at this early time, that concrete mix quality depended on control of water, as he referred to "Water Ratio" as the fore-runner of our Water-Cementitious Ratio today. His comment from the text body says a lot: "Our experimental work has emphasized the importance of the water in concrete mixtures, and shown that the water is, in fact, the most important ingredient, since very small variations in water content produce more important variations in the strength and other properties of concrete than similar changes in the other ingredients." The historical nature of this work by Abrams leads to Jim Shilstone making the observation that "Seems like things don't change." Jim has spent his whole life, it seems, finding ways to "get the water out" for quality mixes with good workability.

AN EXTRACT

DESIGN OF CONCRETE MIXTURES

By Duff A. Abrams

Bulletin 1, Structural Materials Research Laboratory, Lewis Institute

"The design of concrete mixtures is a subject of vital interest to all engineers and constructors who have to do with concrete work. The problem involved may be one of the following:

1) "What mix is necessary to produce concrete of proper strength for a given work?

2) "With given materials what proportions will give the best concrete at minimum cost?

3) "With different lots of materials of different characteristics which is best suited for the purpose?

4) "What is the effect on strength of concrete from changes in mix, consistency or size and grading of aggregate?

"Proportioning concrete frequently involves selection of materials as well as their combination. In general, the question of relative costs is also present.

"The term "Design" is used in the title of this article as distinguished from "proportioning" since it is the intention to imply that each element of the problem is approached with a deliberate purpose in view which is guided by a rational method of accomplishment.

"The design of concrete mixtures, with a view to producing a given result in the most economic manner, involves many complications which have heretofore defied analysis.

"Many different methods of proportioning have been suggested; the most important ones may be characterized as follows:

1) "Arbitrary selection such as 1:2:4 mix, without reference to the size or grading of the fine and coarse aggregate;

2) "Density of aggregates in which the endeavor is made to secure an aggregate of maximum density;

3) "Density of concrete in which the attempt is made to secure concrete of maximum density;

4) "Sieve analysis, in which the grading of the aggregates is made to approximate some predetermined sieve analysis curve which is considered to give the best results;

5) "Surface area of aggregates.

"It is a matter of common experience that the method of arbitrary selection in which fixed quantities of fine and coarse aggregates are mixed without regard to the size and grading of the individual materials, is far from satisfactory. Our experiments have shown that the other methods mentioned above are also subject to serious limitations. We have found that the maximum strength of concrete does not depend on either an aggregate of maximum density or a concrete of maximum density, and that the methods which have been suggested for proportioning concrete by reference to a fixed sieve analysis are based on an erroneous theory. The methods of proportioning concrete which have been proposed in the past have failed to give proper attention to the water content of the mix. Our experimental work has emphasized the importance of the water in concrete mixtures, and shown that the water is, in fact, the most important ingredient, since very small variations in water content produce more important variations in the strength and other properties of concrete than similar changes in the other ingredients.

"New Studies of Concrete Mixtures

"During the past three years a large number of investigations have been under way at the Structural Materials Research Laboratory, Lewis Institute, Chicago, which throw considerable new light on the subject of proportioning concrete. These investigations are being carried out through the cooperation of the Institute and the Portland Cement Association. These studies have covered an investigation of the inter-relation of the following factors:

1) "The consistency (quantity of mixing water).

2) "The size and grading of aggregates.

3) "The mix (proportion of cement).

"Any comprehensive study of proportioning concrete must take into account all of these factors.

"During this period about 50,000 tests have been carried out which have a bearing on this subject. These tests have been largely confined to compression tests of concrete and mortars. These investigations have given us a new insight into the factors which underlie the correct proportioning of concrete mixtures and show the limitations of older methods. Certain phases of these investigations are still under way.

"The following may be mentioned as among the most important principles which have been established with reference to the design of concrete mixtures. In a brief report of this kind it is impracticable to present more than an outline of the methods of applying the principles to practical problems. In only a few instances are experimental data given on which these conclusions are based.

1) "With given concrete materials and conditions of test the quantity of mixing water used determines the strength of the concrete so long as the mix is of a workable plasticity.

2) "The sieve analysis furnishes the only correct basis for proportioning aggregates in concrete mixtures.

3) "A simple method of measuring the effective size and grading of a aggregate has been developed. This gives rise to a function known as the "fineness modulus" of the aggregate.

4) "The fineness modulus of the aggregates furnishes a rational method for combining materials of different size for concrete mixtures.

5) "The sieve analysis curve of the aggregate may be widely different in form without exerting any influence on the concrete strength.

6) "Aggregate of equivalent concrete-making qualities may be produced by an infinite number of different gradings of a given material.

7) "Aggregates of equivalent concrete-making qualities may be produced from materials of widely different size and grading. In general, fine and coarse aggregates of widely different size or grading can be combined in such a manner as to produce similar results in concrete.

8) "The aggregate grading which produces the strongest concrete is not that giving the maximum density (lowest voids). A grading coarser that giving maximum density is necessary for highest concrete strength.

9) "The richer the mix, the coarser the grading should be for an aggregate of given maximum size; hence, the greater the discrepancy between maximum density and best grading.

10) "A complete analysis of the water-requirements of concrete shows that the quantity of mixing water required is governed by the following factors:

"The condition of "plasticity" or "workability" of concrete which must be used--the relative consistency;

"The condition of "plasticity" or "workability" of concrete which must be used the relative consistency,

"The normal consistency of the cement,

"The size and grading of the aggregate,

"The relative volumes of cement and aggregates - the mix,

"The absorption of the aggregate,

"The contained water in the aggregate.

11) "There is an intimate relation between the grading of the aggregate and the quantity of water required to produce a workable concrete.

12) "The water content of a concrete mix is best considered in terms of the volume of the cement -- the water-ratio.

13) "The shape of the particle and the quality of the aggregate have less influence on the concrete strength than has been reported by other experimenters."

Continued - Part II