Title:
Effect of Cracking on Drying Permeability and Diffusivity of Concrete
Author(s):
Zdenek P. Bazant, Siddik Sener, and Jin-Keun Kim
Publication:
Materials Journal
Volume:
84
Issue:
5
Appears on pages(s):
351-357
Keywords:
concretes; cracking (fracturing); crack width and spacing; diffusivity; drying; measurement; moisture; permeability; Materials Research
DOI:
10.14359/1739
Date:
9/1/1987
Abstract:
The increase of overall drying permeability and diffusivity of concrete due to cracking is determined experimentally and formulated mathematically. The test specimens are C-shaped beams deformed by a tie rod and reinforced on the tensile face so that uniformly spaced cracks are produced. The difference in the loss of weight for various drying periods between cracked and uncracked specimens is measured and used to quantify the effect on permeability and diffusivity. The overall drying diffusivity and permeability in the cracking direction, which is theoretically proportional to the crack width cubed and inversely proportional to the crack spacing, is found to increase about 2.25 times for crack width 0.1 mm and spacing 70 mm. Although appreciable, this value is two orders of magnitude less than the theoretical upper bound predicted on the basis of viscous flow calculation if it is assumed that the cracks are of constant thickness, have planar wall, and are continuous. It is concluded that even though the major cracks are seen to be continuous on the specimen surface, they must be discontinuous in the specimen interior, perhaps being interconnected by much narrower necks with a width about 10 times smaller. This fact is of interest for deducing fracture process zone models from visual observations of cracks on the specimen surface. Although approximate, the presently derived formula for the increase of diffusivity and permeability is directly usable in finite element programs for the drying or wetting of concrete.