Effect of water cement ratio on strength

The water-cement ratio (w/c) is the ratio of the mass of water to the mass of cement in the mix. It is the most important factor controlling the strength of workable, fully compacted concrete.

Abrams' law

Strength is inversely related to the w/c ratio:

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

where $A$ and $B$ are constants that depend on the cement, age and curing ($A = 96.5$ MPa and $B = 7$ in Abrams' original work).

 Strength
  |\
  | \
  |  `.
  |    `-.
  |        `--.___
  +---------------------> w/c
  0.3  0.4  0.5  0.6  0.7

Why strength falls as w/c rises

• The water needed for hydration is only about 0.23-0.38 of the cement mass; the extra water stays in the paste and leaves capillary pores when it evaporates. Strength depends on the gel/space ratio (Powers), so more pores mean lower strength.
• A higher w/c gives a weaker bond between the paste and the aggregate, a weaker transition zone, and more bleeding.
• A lower w/c increases strength, density, durability and impermeability, but reduces workability. Below about 0.35, full compaction becomes difficult and some cement stays unhydrated, so superplasticisers are used.

Typical values: w/c 0.4 gives about 45-50 MPa, 0.5 about 35 MPa, 0.6 about 28 MPa, and 0.7 about 20 MPa at 28 days (OPC, well compacted).