Title: Capillary suction in concrete with analytical pipe model - part 2: expansion-, contraction- and random sized sections

Author(s): Hung Thanh Nguyen, Frank Melandsø, Stefan Jacobsen

Publication: NCR

Volume: 42

Issue: 6

Appears on pages(s): 89-108

Keywords: Concrete, Capillary absorption, Pore necks, Modeling and Experiments

DOI:

Date: 12/15/2010

Abstract:
The Lucas-Washburn equation (LWE) is based on laminar flow within the simple geometry of a single pipe with a uniform radius. We studied the effect of different pipe geometries oncapillary suction by modifying the LWE to be able to handle pipes with multiple sizes (theanalytical model). To do this a complete second order equation describing the relation betweenthe capillary suction and the suction time was developed. Varying geometries of the pipes ofthe capillary system were investigated including the effects on the flow rate of varyingcombination of lengths, radius and sequence ( expansion  contraction and contraction expansion series). The results showed that largest flow reductions occurred with very narrowsections causing a blocking, reducing the capillary flow rate vs. square root of time several tenfolds compared to uniform pipes without diameter variations. Similar results were also obtained for the random size models. The often observed phenomenon of reduced flow belowthe straight line water uptakevs. square root of time could be simulated with the multiple diameter pipe model, as seen by comparing simulations with experiments with varying concrete qualities and sample thicknesses in simple capillary absorption tests.