What Is The Ideal Water To Cement Ratio?

For brittle ceramic materials, including cementitious systems, the strength has been found to be inversely proportional to the porosity. Often, an exponential equation is used to relate strength to porosity; for example,
fc = fcₒe⁻kt

where fc is the strength, fc0 is the intrinsic strength at zero porosity, p is the porosity, and k is a constant that depends on the particular system.

Equations such as this do not consider the pore-size distribution, the pore shape, and whether the pores are empty or filled with water; thus, they are a gross simplification of the true strength vs. porosity relationship.

Nonetheless, for ordinary concretes for the same degree of cement hydration, the strength does indeed depend primarily on the porosity. Because the porosity, in turn, depends mostly on the original w/c ratio, mix proportioning for normal-strength concretes is based, to a large extent, on the w/c ratio law articulated by D.A. Abrams in 1919: “For given materials, the strength depends only on one factor—the ratio of water to cement.” This can be expressed as: fc = K1/ [K2^(w/c)] where K1 and K2 are constants, and w/c is the water/cement ratio by weight.

In fact, of course, given the variability in raw materials from concrete to concrete, the w/c ratio law is really a family of relationships for different mixtures. As stated by Gilkey (1961a):

For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregate, and water (under the same mixing, curing, and, testing conditions) is influenced by the: (a) ratio of cement to mixing water; (b) ratio of cement to aggregate; (c) grading, surface texture, shape, strength, and stiffness of aggregate particles; and (d) maximum size of aggregate.

Thus, in some cases, simple reliance on the w/c ratio law may lead to serious errors. It should be noted that many modern concretes contain one or more mineral admixtures that are, in themselves, cementitious to a greater or lesser degree; therefore, it is becoming more common to use the term water/ cementitious material ratio to reflect this fact rather than the simpler water/cement ratio.

For ordinary concretes, the w/c ratio law works well for a given set of raw materials, because the aggregate strength is generally much greater than the paste strength; however, the w/c ratio law is more problematic for high-strength concretes, in which the strength-limiting factor may be the aggregate strength or the strength of the interfacial zone between the cement and the aggregate.

Although it is, of course, necessary to use very low w/c ratios to achieve very high strengths, the w/c ratio vs. strength relationship is not as straightforward as it is for normal concretes. Figure 1.8 shows a variety of water/ cementitious material vs. strength relationships obtained by a number of different investigators.

A great deal of scatter can be seen in the results. In addition, the range of strengths for a given w/c ratio increases as the w/c ratio decreases, leading to the conclusion that, for these concretes, the w/c ratio is not by itself a very good predictor of strength; a different w/c ratio “law” must be determined for each different set of materials.

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