Abrams' law of concrete strength

Abrams' law (D. A. Abrams, 1918) states that for a given set of materials and age, the strength of fully compacted concrete is inversely related to the water-cement ratio: the lower the w/c ratio, the higher the strength.

$$f_c = \frac{A}{B^{\,w/c}}$$

where $f_c$ is the compressive strength, $w/c$ is the water-cement ratio by mass, and $A$, $B$ are empirical constants depending on the cement, age, curing and testing (Abrams: $A = 96.5$ MPa, $B = 7$ for 28 days).

 f_c
  |\
  | `.
  |   `-.
  |      `--.____
  +----------------> w/c

Explanation: only about 0.23-0.38 of the cement mass of water is needed for hydration; the extra water evaporates leaving capillary pores, which weaken the paste, so strength falls rapidly as w/c increases. The law is valid only for workable mixes that can be fully compacted.