A design mix is a variation or adaptation of one of the more conventional Standard Mixes typically used by contractors. They have been “Designed” for a particular application, be it strength, placement, finish or appearance. For structural requirements, Continuously Approved (CA) design mixes must be approved by the City of Seattle’s Dept. of Planning and Development. The ultimate strength of these mixes can range from 2500 psi to12000 psi. Any mix exceeding 4000 psi must have onsite field inspection and those exceeding 6000 psi must have a Batch Plant inspector, as well. These mixes have been aggressively designed to achieve a particular strength and finishability, through the use of less cement and lesser amounts of water (a low water/cement ratio) with the use of specialized add mixtures. For example; a 4 ½ sack standard mix, which meets the Seattle IBC, will have a 2000 psi rating, where as, a 4 ½ sack CA mix can typically achieve 4000 psi. Other applications for design mixes are: higher earlier strengths, placement in and around congested steel reinforcement, architectural concrete finishes and Flowable Thermal Backfill (FTB)

Salmon Bay Sand and Gravel Co.