Title:
Effect of External Loads on the Frost-Resistant Properties of Mortar with and Without Silica Fume
Author(s):
Yixia Zhou, Menashi D. Cohen, and William L. Dolch
Publication:
Materials Journal
Volume:
91
Issue:
6
Appears on pages(s):
595-601
Keywords:
air entrainment; dynamic modulus of elasticity; silica fume; freeze-thaw durability; high-strength concrete; standards; stresses; Materials Research
DOI:
10.14359/1380
Date:
11/1/1995
Abstract:
There is little, if any, information on the effect of external loads on frost resistance of concrete. Most concrete structures in cold regions are subjected to both external loads and freezing-thawing. It is necessary to understand how the resulting stresses influence the resistance of conventional and high-performance concretes to freezing-thawing. Paper presents the results of research on the effect of an external flexural load on the frost resistance of air-entrained and non-air-entrained concrete with and without silica fume. Mortar beam specimens were placed in a specially designed apparatus and subjected to a flexural stress ranging from O to 50 percent of the modulus of rupture. Water-to-cementitious materials ratios [w/(c + m), where c + m = cement + silica fume] studied were 0.25, 0.35, and 0.45. The samples were immersed along with the apparatus in the chambers inside the freezing-thawing machine. Freeze-thaw test cycles followed ASTM C 666 Procedure A, with the modification of applying external loads. There are three deterioration processes that may occur during the freezing-thawing of susceptible concrete: 1) uniform and homogeneous microcracking of the matrix caused by moisture movement and formation of hydraulic pressure; 2) growth of a critical crack caused by the external load-static fatigue-brittle failure mode; and 3) surface scaling at high w/(c + m) ratio. The three processes occur simultaneously in portland cement mortar, but the dominant process determines the failure mode. The failure mode becomes more brittle (Process 2) with increase in preloading level. In silica fume mortar, there is no significant matrix microcracking (Process 1) developed with as little as 10 percent preloading, and the mode of failure is brittle (Process 2). Scaling (Process 3) seems to play a major role for mortar of high w/(c + m) ratio (0.45). Silica fume appears to increase the frost resistance of the mortar of w/(c + m) = 0.35 and sand s/(c + m) = 2.0; however, the resistance drops faster with increasing preload. At 50 percent preload, portland cement and silica fume mortars have nearly the same freeze-thaw resistance.