Carbonation of Concrete Incorporating High Volumes of Fly Ash


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Title: Carbonation of Concrete Incorporating High Volumes of Fly Ash

Author(s): N. Bouzoubaa, B. Tamtsia, M.H. Zhang, R.L. Chevrier, A. Bilodeau, and V.M. Malhotra

Publication: Special Publication

Volume: 234


Appears on pages(s): 283-304

Keywords: carbonation; fly ash; high-volume fly ash concrete

Date: 3/22/2006

This investigation deals with determining the carbonation resistance of high-volume fly ash concrete. Five air-entrained concrete mixtures were studied consisting of three high-volume fly ash concrete mixtures (HVFA) incorporating 58% of fly ash by mass of the total cementitious materials made with a water-cementitious materials ratio (w/cm) of 0.32, and two control portland cement concrete mixtures, one with a similar w/cm (0.32), and the other with similar 28-d compressive strength as that of the HVFA concrete. Following several curing conditions, some of the specimens were moved to a chamber with 3% CO2 for up to 700 days, and the others were stored in the laboratory environment and/or exposed to the natural environment (without shelter) for more than seven years. The accelerated carbonation test results indicate that for both HVFA and control portland cement concretes, the carbonation depth decreases with an increase in the duration of moist curing from 7 to 28 days. A further increase in the moist curing from 28 to 91 days did not substantially affect the carbonation depth of the control concretes and the HVFA concrete using the high reactive fly ash. The depth of carbonation in concrete was also found to significantly depend on the reactivity of fly ash used; in general, the depth of carbonation deceased with increasing fly ash reactivity. The carbonation results on concrete exposed to natural environment show that the depth of carbonation of HVFA concretes exposed to outdoor environment for seven years ranged from 3.5 to 7.5 mm, and that of the control concrete with a w/c ratio of 0.42 was 2.5 mm. For the control concrete with a w/c of 0.32, the carbonation depth was less than 1 mm. Based on the results obtained, the time required for the carbonation front to reach 40 mm depth in HVFA concrete exposed to the outdoor condition was predicted to be a minimum 240 years. Thus the carbonation is not an issue for HVFA concrete due mainly to its low w/cm and dense structure.