The water-cement ratio is a convenient measurement whose value is well correlated with PCC strength and durability. In general, lower water-cement ratios produce stronger, more durable PCC. If natural pozzolans are used in the mix (such as fly ash) then the ratio becomes a water-cementitious material ratio (cementitious material = portland cement + pozzolonic material). The ACI method bases the water-cement ratio selection on desired ompressive strength and then calculates the required cement content based on the selected water-cement ratio. Table 5.17 is a general estimate of 28-day compressive strength vs. water-cement ratio (or water-cementitious ratio). Values in this table tend to be conservative (ACI, 2000). Most state DOTs tend to set a maximum water-cement ratio between 0.40 - 0.50 (based on data from ACPA, 2001).
