Sulfate Class S2 Mix Submittal That Exceeds the Maximum w/cm
A ready-mix producer submits a mix design for the strip footings of a cast-in-place retaining wall specified at f'c = 4,500 psi. The geotechnical report lists water-soluble sulfate in the soil as 0.42% by mass (about 4,200 ppm). The submittal shows 564 lb/yd³ of Type II cement, 94 lb/yd³ of Class F fly ash (658 lb/yd³ total cementitious), a water-cementitious materials ratio of 0.47, 5.0% air, and a 5 in. slump. Trial batches averaged 5,380 psi at 28 days, and the producer argues this extra strength more than offsets the w/cm. The batch ticket for the first load shows 37.1 gal of water per cubic yard. On earlier pours the producer's driver routinely added up to 10 gal per 8 yd³ load at the site to bring slump up on hot afternoons. The owner's basis of design states that ACI 318 durability requirements govern and that no substitutions are allowed without written approval by the registered design professional. The first footing pour is scheduled in three days.
1. Determine the ACI 318 sulfate exposure classification for this soil, compare the submittal against it, and state what must change before the mix is approved.
Read the worked answer
0.42% water-soluble sulfate in soil falls in ACI 318 sulfate class S2 (the S2 range is 0.20% to 2.00%). S2 imposes two independent limits: minimum f'c of 4,500 psi and a maximum w/cm of 0.45.
The strength limit is satisfied - the specified 4,500 psi and the 5,380 psi trial results both clear it. The w/cm is not: 0.47 exceeds 0.45. This is a durability issue, not a strength issue. The w/cm governs permeability and the rate at which sulfate-bearing water reaches the hydrated cement, so a mix that is 900 psi stronger but at 0.47 is still a nonconforming mix. Extra cement cannot buy back a w/cm cap, and the producer's argument should be rejected.
The fix: at 658 lb/yd³ cementitious, 0.45 means no more than about 296 lb of water per cubic yard, roughly 35.5 gal - about 1.6 gal/yd³ less than the ticket. That is easily recovered with a mid-range or high-range water reducer while holding the 5 in. slump. Alternatively, raise the cementitious content and re-check heat of hydration, shrinkage, and the S2 minimum. Resubmit the revised mix with a new trial batch and do not batch the footing pour until the design professional has approved it in writing.
2. The producer says the 10 gal site additions are routine and insignificant. Evaluate that practice against the w/cm limit and the procedural rules for adding water at the point of placement.
Read the worked answer
Work the arithmetic first. Ten gallons spread over an 8 yd³ load is 1.25 gal/yd³, or about 10.4 lb of water per cubic yard. On the submitted mix (309 lb of water at 0.47), that pushes the load to about 319 lb and a w/cm near 0.49 - a clear breach of the 0.45 cap. Even on a corrected mix designed exactly at 0.45 (296 lb), one such addition lands near 0.47 and breaches the cap again. So on an S2 mix there is essentially zero water budget left in the truck.
Procedure matters too. Site water is permitted only with the approval of the purchaser's authorized representative before it is added, must be measured, introduced through the mixer with the drum turning, recorded on the delivery ticket, and followed by at least 30 revolutions at mixing speed before the load is sampled or discharged. The final w/cm must still be under the specified maximum. A driver topping up on hot afternoons meets none of these conditions: it is unapproved, unmeasured, unrecorded, and it silently changes the material the durability requirement was written for.
Action: prohibit site water on this mix in writing, require the batch panel and aggregate moisture readings at submittal, and have the inspector verify ticket water against the approved mix on every load. Any load that received unapproved water should be documented and referred to the design professional.