W/CM Code Requirements Inappropriate for Resistance to Deicer Salt Scaling


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Title: W/CM Code Requirements Inappropriate for Resistance to Deicer Salt Scaling

Author(s): C. D. Johnston

Publication: Special Publication

Volume: 145


Appears on pages(s): 85-106

Keywords: compressive strength; deicers; fly ash; freeze-thaw durability; heat of hydration; scaling; silica fume; standards; temperature rise; water-cementitious ratio; water-cement ratio; Materials Research

Date: 5/1/1994

The validity of recent revisions to CAN/CSA A23.1 and ACI 318, which change the criteria for durability under various exposure conditions involving freezing and thawing from limits on water-cement ratio (w/c) to limits on water-cementitious materials ratio (w/cm), is challenged for conditions that involve freezing and thawing with deicing salts. Test results to quantitatively determine resistance to deicer scaling in terms of weight loss per unit area of exposed surface show that concretes with fly ash or silica fume as part of the cementitious material behave very differently from each other and from concretes with only cement as the binder. Fly ash concretes generally scale severely with weight losses in excess of 1 kg/m 2 even when w/cm values are well below the code maximum of 0.45. Performance appears unrelated to w/cm and instead quite closely related to w/c. Silica fume concretes exhibit superior scaling resistance with weight losses less than 0.5 kg/m 2 even when w/cm values approach or exceed the 0.45 limit. Reference concretes exhibit satisfactory scaling resistance at w/c values of 0.45 or less. Clearly, the composition of the cementitious material has a major effect on resistance to scaling in the presence of deicing salts. Replacing the traditional w/c limits with w/cm in CAN/CSA A23.1 and ACI 318 is unjustifiable and may lead to serious scaling problems with concretes proportioned to meet the current w/cm code requirements, particularly those containing fly ash. The traditional w/c limit of 0.45 is associated with satisfactory scaling resistance for properly air-entrained concretes containing cement alone or in combination with silica fume. This limit also appears applicable to the fly ash concretes evaluated according to the trend of available data, but confirmatory tests at w/c values below 0.50 are needed.