Across the Midwestern states last December, there was a much greater accumulation of snowfall than usual. Ground temperatures, as a result, tended to remain around freezing. With little snow in January and temperatures frequently fluctuating above and below freezing, exterior concrete slabs experienced more freeze/thaw cycles than usual. By spring of this year, the number of reports of spalled and scaled concrete, especially for residential driveways, was skyrocketing. Concrete installed during the 2000 construction season had the most damage.
Why residential driveways were more susceptible to these problems than other concrete installations raises questions about mix designs, installation methods, and adequate curing. Problems in residential construction sometimes arise because inexperienced contractors are left to install as they wish—with no specification to guide them. Ready-mix producers sometimes let economics dictate the percentage of cement and pozzolans in their performance mixes, and contractors sometimes order 3000-psi mixes for exterior residential use. (ACI's "Guide to Residential Cast-in-Place Concrete Construction," ACI 332R-84 Table 2.2 recommends a minimum of 3500 psi for driveways in severe weather regions but notes that 4000-psi concrete will provide superior durability.) Concrete is sometimes poured with slumps as high as 8 inches, and proper curing is often neglected.