Water-Cementitious Ratio and Exposure Cap (ACI 318)
The single strongest lever on concrete strength and durability.
Example
You enter
- Total mixing water (lb, incl. aggregate free moisture) 282
- Portland cement (lb) 470
- Supplementary cementitious -- fly ash/slag/etc (lb) 94
- Exposure class (ACI 318 cap) f2_s2
You get
- Water-cementitious ratio 0.500
- Cementitious (lb) 564
Details, formula, and sources
w/cm = mixing water / total cementitious (cement + fly ash + slag + pozzolans). 282 lb water over 470 lb cement + 94 lb fly ash (564 cementitious) is 0.50 -- which EXCEEDS the ACI 318 Table 19.3.2.1 cap of 0.45 for severe freeze-thaw or sulfate exposure. Lower w/cm means lower permeability, so ACI caps it by exposure class (0.55 F1 down to 0.40 F3/C2). Count the aggregate free-moisture and admixture water, not just plant batch water, or the real ratio is higher than the ticket. Checks the durability cap only; strength, minimum cementitious, and air are separate. A durability screen; the spec and mix design govern.
w/cm = water_lb / (cement_lb + scm_lb); checked against the ACI 318 Table 19.3.2.1 exposure-class maximum (F1 0.55, W/S1 0.50, F2/S2-S3 0.45, F3/C2 0.40).
Water-cementitious ratio and exposure caps (ACI 318 Table 19.3.2.1; ACI 211.1), by name; the project specification and the mix design govern.
The w/cm ratio is a simple mass ratio; the exposure-class maxima are the published ACI 318 Table 19.3.2.1 values. The batch masses come from the mix design / ticket.
Estimate. AHJ and licensed professional govern.
Field names used by the API: water_lb, cement_lb, scm_lb, exposure_class, wcm, cementitious_lb
- Total cementitious w/cm uses cement + all supplementary cementitious materials, not cement aloneACI 318 / 211.1
- Exposure caps ACI 318 Table 19.3.2.1: F1 0.55, W/S1 0.50, F2/S2-S3 0.45, F3/C2 0.40ACI 318 Table 19.3.2.1