Capillary suction model as pipes of different sizes: flow conditions and comparison with experiments

Abstract

ABSTRACT Absorption of water in concrete is often described by the simple linear water uptake vs. square-root-of-time law. However, a deviation from this behaviour is frequently seen depending on factors such as initial water content, water/binder ratio and specimen thickness. The deviation increases with thickness (typically from 25 to 100 mm) and is seen even for very dry specimens with capillary pores. We have applied Laplaces law for suction created under a curved meniscus between air and water to series of pipes with different lengths and radii. The resulting analytical model was first compared with numerical simulations at abrupt reduction or increase of pipe radius showing good agreement. Then a complete second order equation describing the relation between the capillary suction and the suction time was developed. Varying geometries of the pipes of the capillary system were investigated including the effects on the flow rate of varying combination of lengths, radii and sequence. The results showed that largest flow reductions occurred with very narrow sections causing a blocking, reducing the capillary flow rate vs. square root of time in the same manner as in concrete. The often observed phenomenon of reduced flow below the straight line water uptake vs. square root of time could be simulated with the multiple diameter pipe models, as seen by comparing simulations with experiments with varying concrete qualities and sample thicknesses in simple capillary absorption tests. Key-words: capillary suction, pipe geometry, model