W D Hoff

Water movement in porous building materials—II. Hydraulic suction and sorptivity of brick and other masonry materials

Abstract Philips sorptivity parameter (S) is discussed in the context of porous building materials. Experimental sorptivity values are reported for a variety of building stones, bricks, mortars and plasters. Data on the temperature dependence of the sorptivity and absorption of organic liquids are included.

Water movement in porous building materials—VI. Evaporation and drying in brick and block materials

Abstract The processes of evaporation and drying from porous building materials are discussed. Experimental results are reported which confirm the existence of at least two distinct stages of drying, and relative effects of humidity, temperature and farstream air speed on drying rates are described. The cumulative desorption of water during the second stage of drying is discussed in terms of a desorptivity parameter and the thermal effects of the evaporation process are examined.

Dating fired-clay ceramics using long-term power law rehydroxylation kinetics

Fired-clay materials such as brick, tile and ceramic artefacts are found widely in archaeological deposits. The slow progressive chemical recombination of ceramics with environmental moisture (rehydroxylation) provides the basis for archaeological dating. Rehydroxylation rates are described by a (time)1/4 power law. A ceramic sample may be dated by first heating it to determine its lifetime water mass gain, and then exposing it to water vapour to measure its mass gain rate and hence its individual rehydroxylation kinetic constant. The kinetic constant depends on temperature. Mean lifetime temperatures are estimated from historical meteorological data. Calculated ages of samples of established provenance from Roman to modern dates agree excellently with assigned (known) ages. This agreement shows that the power law holds precisely on millennial time scales. The power law exponent is accurately 1 4, consistent with the theory of fractional (anomalous) ‘single-file’ diffusion.

Capillary water migration in rock: process and material properties examined by NMR imaging

We report the application of proton nuclear magnetic resonance (NMR) imaging to the measurement of water content distributions in Lépine limestone, a typical constructional stone. The method is used to observe the kinetics of the absorption of water into this material by capillarity. The water content distributions are consistent with the predictions of unsaturated flow theory. The hydraulic diffusivity of Lépine stone is found to be an approximately exponential function of the water content, in agreement with experimental data on other porous materials. The best estimate of the diffusivity function is D (m2s−1) = 6.3 × 10−9 exp (4.90θr), whereθr is the normalized volumetric water content. The sorptivity estimated from NMR data is in close agreement with the directly measured value (1.00 mm min−1/2). NMR imaging methods appear promising as a non-destructive and rapid laboratory means of determining moisture distributions, especially for the purpose of accurate measurement of the capillary transport properties of porous materials.ResumeOn décrit l’application de l’imagerie de résonance magnétique nucléaire (RMN) à la mesure de distributions de teneur en eau dans le calcaire Lépine, pierre de construction type. On présente les résultats d’une expérience où l’eau est absorbée librement par capillarité de l’extrémité d’une barre rectangulaire. On utilise l’imagerie RMN pour contrôler la vitesse d’absorption de l’eau. On obtient, par l’analyse des images, les distributions à l’intérieur de l’échantillon en fonction du temps.Les distributions de teneur en eau concordent avec l’application de la théorie de l’écoulement en milieu non saturé. La diffusivité hydraulique et le coefficient de sorption se calculent à partir des profiels d’absorption d’eau. Le coefficient de diffusion de la pierre de Lépine est une fonction approximativement exponentielle de la teneur en eau, conforme aux données expérimentales des autres matériaux poreux. La meilleure estimation de la fonction de diffusion est D (m2s−1) = 6.3 × 10−9 exp (4.90θr), oùθr est la teneur en eau volumétrique normalisée.Le coefficient de sorption évaluée à partir des données RMN concorde tout à fait avec la mesure directe (1,00 mm min−1/2). Les méthodes d’imagerie RMN semblent prometteuses en tant que technique de laboratoire non-destructive et rapide pour déterminer les distributions d’humidité, et en particulier, mesurer avec précision les propriétés de mouvement d’eau par capillarité de matériaux poreux.

Rising damp: capillary rise dynamics in walls

We analyse rising damp using the concepts and methods of unsaturated flow theory. A simple first-order Sharp Front model is developed which uses clear physical principles and includes the effects of evaporation and gravity. We find that the simple model captures well the observed features of capillary rise in walls and is supported by the underpinning nonlinear capillary diffusion theory. For most cases, capillary forces are dominant and the effects of gravity can be neglected.

The effects of lime and admixtures on the water-retaining properties of cement mortars

Abstract The water-retaining properties of wet mortar mixes are examined. The desorptivity is defined as a parameter characterising the water-retaining properties of such mixes. A test method based on the American Petroleum Institute filtration cell is described for measuring the desorptivity of wet mixes. Experimental results are reported for a range of wet cement mortars including mixes containing lime and air-entraining and water-retaining admixtures. These show that 1:3 cement:sand and equivalent mixes containing lime all have very similar water-retaining characteristics, but are all much less water-retaining than a 1:3 lime:sand mix. These results therefore demonstrate the strong water-retaining characteristics of lime.

Water movement in porous building materials—XIII. Absorption into a two-layer composite

Abstract The sharp wet front method of analysis is applied to the absorption of water into a composite bar consisting of two dissimilar materials in hydraulic contact. Equations describing both the rate of absorption of water and the cumulative absorbed volume of water per unit area of supply surface are derived. Experimental absorption data can be used to estimate the ratio of the hydraulic conductivities of the constituent materials. Experimental data on composite bars made up of various plaster I sand mixtures are included for the case of absorption through a high sorptivity into a lower sorptivity material. The results show that the absorption of water into the composite is controlled by the properties of the second material.

Moisture expansion and mass gain in fired clay ceramics : a two-stage (time)1/4 process

We report the first measurements of mass gain and expansive strain in clay ceramics immediately following firing. The results show that both mass gain and expansive strain follow a well defined two-stage process, each stage of which is linear with (time)1/4, with a transition from the first to the second stage commencing at t ~ 1.5 h. The first stage mass gain and strain rates are ~3.5 times greater than the second stage rates. The strain is found to show the same linear relationship with mass throughout both stages providing evidence that the underlying physical process is the same in each stage. The early time measurements are unique in providing experimental evidence for t1/4 kinetics for both strain and mass gain over times ranging from minutes to days and are consistent with earlier strain data showing t1/4 kinetics over times ranging from days to centuries.

Moisture dynamics in walls: response to micro-environment and climate change

A coupled sharp-front (SF) liquid transport and evaporation model is used to describe the capillary rise of moisture in monoliths and masonry structures. This provides a basis for the quantitative engineering analysis of moisture dynamics in such structures, with particular application to the conservation of historic buildings and monuments. We show how such a system responds to seasonal variations in the potential evaporation (PE) of the immediate environment, using meteorological data from southern England and Athens, Greece. Results from the SF analytical model are compared with those from finite-element unsaturated-flow simulations. We examine the magnitude and variation of the total flow through a structure as a primary factor in long-term damage caused by leaching, salt crystallization and chemical degradation. We find wide seasonal variation in the height of moisture rise, and this, together with the large estimated water flows, provides a new explanation of the observed position of salt-crystallization damage. The analysis also allows us to estimate the effects of future climate change on the capillary moisture dynamics of monoliths and masonry structures. For example, for southern England, predicted increases in PE for the period 2070–2100 suggest substantial increases in water flux, from which we expect increased damage rates.

Anomalous water transport properties of Portland and blended cement-based materials

The paper presents new data giving further evidence of anomalies in the water transport properties of Portland cement-based solids, and show that the anomalous performance is much less evident when the Portland cement is partially replace by ground granulated blast-furnace slag (ggbs).