ACI 332-08 "Code Requirements for Residential Concrete" currently requires a minimum compressive strength of 4500 psi for exterior concrete exposed to severe environmental conditions. The proposed change is to remove the 4500 psi strength requirement and to replace with a 4000 psi requirement. Reasons for the change are:
- The average strength of a design mixture will be in the range of 15% greater than the specified strength. With a specified strength of 4500 psi, the required average strength for most residential applications is considered excessive and can result in increased potential for cracking. Providing concrete at an average strength of around 4500 psi is considered adequate for most of these non-structural applications. For this reason the f’c for Type 4 concrete is being reduced to 4000 psi.
- Additional requirements for cementitious materials are added to minimize potential for scaling for type 4 concrete.
- Match more closely with International Residential Code. Generally the code eliminates any reference to Exterior flatwork that is not part of the residential building.
- Match more closely to ACI 330 for parking areas (requires 4000 psi for parking areas exposed to weather).
So if we are producing and placing exterior concrete in Nebraska...test results should meet the required minimum compressive strength of 4000 psi in order to assure the owner that the concrete was placed at a maximum w/c of 0.45. In addition, the concrete better be properly cured to ensure "surface" durability.