EFFECT OF CRACK-REDUCING TECHNOLOGIES AND SUPPLEMENTARY CEMENTITIOUS MATERIALS ON SETTLEMENT CRACKING OF PLASTIC CONCRETE AND DURABILITY PERFORMANCE OF HARDENED CONCRETE

Abstract

ABSTRACT The effects of crack-reducing technologies and supplementary cementitious materials on plastic settlement cracking and the durability of concrete subjected to freezing and thawing were evaluated. The study of settlement cracking included 86 concrete mixtures containing internal curing (IC), a shrinkage reducing admixture (SRA), optimized and non-optimized aggregate gradations, or the supplementary cementitious materials (SCMs) slag cement and silica fume. Some concrete mixtures contained combinations of these technologies, such as supplementary cementitious materials and internal curing. Both crack length and width were measured. The study of durability included 28 concrete mixtures, divided into three programs. Program 1 involved concrete containing different dosage rates of one of two shrinkage reducing admixtures. Program 2 involved concrete containing different volume replacements of Class F and Class C fly ash and different combinations of a rheology-modifying admixture (RMA) with and without Class C fly. Program 3 involved concrete containing different dosage rates of one of two shrinkage compensating admixtures, one based on MgO that also incorporated a shrinkage reducing admixture and one based on CaO. The study evaluated the effect of the technologies and materials on freeze-thaw durability, based on ASTM C666 Procedure B, scaling resistance, based on a modified version of Canadian Test BNQ NQ 2621-900 Annex B, and characteristics of the air-void system, obtained following ASTM C457. The research also examined the correlation between air-void characteristics, compressive strength, freeze-thaw durability, and scaling resistance for the mixtures. All mixtures experienced increased settlement cracking as slump increased; the increase, however, was very low for the concrete containing both slag cement and silica fume, with or without internal curing. All crack reducing technologies and supplementary cementitious materials tested resulted in a reduction in settlement cracking at all slumps compared to mixtures without these technologies and materials. The use of a non-optimized aggregate gradation increased settlement cracking compared to mixtures with an optimized gradation. The combination of slag cement and silica fume in concrete provided a greater reduction in settlement cracking than slag cement alone. In terms of durability, mixtures with an average air-void spacing factor of 0.007 in. (0.18 mm) or less performed well in the freeze-thaw test. Mixtures with an average air-void spacing factor of 0.007 in. (0.18 mm) or less and a compressive strength greater than 4000 psi (27.6 MPa) performed well in the scaling test. In terms of specific performance, one SRA had no effect on freeze-thaw durability, while the other caused reduced durability. Concrete with either SRA exhibited a reduction in scaling resistance. Mixtures containing Class F fly ash, RMA, or Class C fly ash in conjunction with RMA at all dosages studied performed well in the freeze-thaw test if the air-void spacing factor was 0.007 in. (0.18 mm) or less. Class F or Class C fly ash alone had no effect on scaling resistance when the concrete had an air-void spacing factor of 0.0071 in. (0.18 mm) or less. The RMA without and with Class C fly ash resulted in reduced scaling resistance. This reduction was in all cases associated with a concrete compressive strength below 4000 psi (27.6 MPa). An SCA based on CaO had no effect on the freeze-thaw durability at the dosage used in this study. The SCA based on MgO resulted in lower freeze-thaw durability, but only in mixtures that had increased air-void spacing; the increased air-void spacing may have been due to the shrinkage reducing admixture incorporated in the admixture, which can reduce the stability of the air-void system. With the exception of one mixture with high air-void spacing factor [0.0096 in. (0.24 mm)], the two SCAs had no effect on scaling resistance at all dosages used in this study. All mixtures exhibited a lower air content in the hardened concrete than in the plastic concrete. This reduction in air content was significantly greater for mixtures containing high dosages of SRAs or the RMA.