ACI 232.4T-20 LIMITS ON THE PROPORTIONS OF FLY ASH IN CONCRETE

ACI 232.4T-20 LIMITS ON THE PROPORTIONS OF FLY ASH IN CONCRETE. The basis—ASTM C672/C672M has been shown to be an unduly harsh procedure and may overstate the scaling potential of mixtures containing fy ash (Thomas 1997). A more realistic test has shown that concrete with up to 35 percent fy ash, the highest level evaluated in that study, has shown satisfactory performance in scaling resistance (Bouzoubaa et al. 2008). A signifcant factor in concrete surface defects, such as scaling, is related to improper concrete fnishing and curing (NRMCA 1998). Hand-fnished concrete fatwork, such as sidewalks and driveways, are most susceptible to scaling (Thomas 2007). It has been suggested by ACI 201.2R that the Table 1 limits for SCMs be made applicable only for hand-fnished surfaces and not formed and machine- fnished surfaces. Whereas much of the earlier discussion pertains to the physical failure mechanism of deicer salt scaling, deicers such as calcium chloride can react with calcium hydroxide present in hardened concrete and form calcium oxychloride, an expansive product that can cause disruption (Sutter et al. 2008). Pozzolans such as fy ash can consume the calcium hydroxide and thereby reduce the formation of calcium oxy-chloride (Sutter et al. 2008; Suraneni et al. 2016). The use of fy ash generally increases the setting time and decreases the early-age strength of concrete. This may be acceptable in warm weather but can be a concern for construction in cooler weather. Restricting the proportion of fy ash can be an implicit attempt to attain shorter setting times and increased early-age strengths. A research study (Malhotra and Ramezanianpour 1994) using 11 Class F fy ash sources illustrated that setting time and early-age strength of 20 percent fy ash mixtures can vary widely, being similar to or considerably delayed in comparison to control concrete mixtures without fy ash. Concrete temperature also impacts the setting time and strength development of concrete. Restricting the proportion of fy ash does not assure control of setting time and early-age strength. The problem—By limiting the proportion of fy ash: a) There is no assurance that mixtures will have the desired setting times and early-age strengths. b) Workability/pumpability and water demand can be adversely impacted. c) Resistance to later-age durability problems, such as alkali silica reaction (ASR) or sulfate attack, cannot be attained in some cases (Thomas 2007). d) Temperature control in mass concrete members, including maximum temperature and rate of tempera-...