Concrete capillary action

The University of California field stations have dealt with dilute pesticide waste disposal on an experimental basis by using lined soil evaporation beds. The beds typically are 20 x 40 x 3 ft pits lined with a butyl rubber membrane and back filled with 12 to 18 Inches of sandy loam soli. Figure 1 Is a cross secton of such a bed. Used containers and spray equipment are washed on an adjacent concrete slab the wastewater drains Into a sedimentation box for trapping particulates, followed by a distribution box In the bed. From the distribution box, the dilute pesticide solutions run underneath the soli surface through leach lines made of 4 Inch perforated PVC pipe. The system Is designed so that water moves up through the soli by capillary action and evaporates off the surface. 

When water comes into contact with a porous material such as concrete, it is absorbed rapidly by the underpressure in the pores caused by what is called capillary action. This action depends on the surface tension, viscosity, and density of the liquid, on the angle of contact between the liquid and the pore walls and on the radius of the pore. In concrete, the contact angle is small due to the presence of molecular attraction between the liquid and the substrate (that is, between water and cement paste). Under these conditions, a drop will spread on a flat surface, while the meniscus of a capillary pore will rise above the level of the surrounding liquid and be concave towards the dry side in Chapter 14 we will see how this aspect can be changed by hydrophobic treatment. 

This relationship, empirically derived from the observation of experimental data, is commonly used to define the parameter S. This is correct only for very porous materials or in the early stage of capillary action. In fact, in concrete with a low w/c ratio, the square root law (Eq. (6)) is changed to a power law with exponent lower than 0.5. 

Like carbonation, the rate of chloride ingress is often approximated to Pick s law of diffusion. There are further complications here. The initial mechanism appears to be suction, especially when the surface is dry, that is, capillary action. Salt water is rapidly absorbed by dry concrete. There is then some capillary movement of the salt laden water throngh the pores followed by true diffusion. There are other opposing mechanisms that slow the chlorides down. These include chemical reaction to form chloroaluminates and absorption onto the pore surfaces. The detailed transport mechanisms of chloride ions into concrete are discussed in Kropp and Hilsdorf (1995). 

A migrating inhibitor with no pretensions of volatility is MFP or monofluorophosphate. This relies on capillary action other transport mechanisms to get the material down to the steel through the concrete cover. A recent paper showed that it could be quite successful in carbonated concrete Raharinaivo and Malric (1998). Calcium nitrite has also been offered in a formulation for patch repairs or overlays with the intention that the nitrite will diffuse out of the patch and protect the steel. 

The method by which the sodium carbonate enters the concrete is contentious with some researchers seeing clear evidence of electro-osmosis Banfill (1994) while others seeing no effect or stating the mechanism to be by diffusion and capillary action Mietz (1998). 

It is important to recognize that diffusion is not the only transport mechanism for chlorides in concrete, particularly in the first few millimetres of cover. There may be several mechanisms moving the chlorides including capillary action and absorption as well as diffusion. Rapid initial absorption occurs when chloride laden water hits very dry concrete. In many circumstances these will only affect the first few millimetres of concrete. If so then the expedient of ignoring the first few millimetres of drillings and then calculating diffusion profiles will work. If the cover is low, the concrete cycles between very dry and wet or the concrete quality is low then the alternative transport mechanisms may overwhelm diffusion, at least to rebar depth. 

Damage of concrete in aggressive media is accelerated by ice formation. There are several factors affecting the destraction of concrete as a result of capillary water freezing. Water does not freeze in the gel pores, because they are too small for stable ice nuclei formation. The monomolecular layer of adsorbed water, bound with the surface forces, does not freeze too. The temperature of water freezing in capillaries varies with capillary diameter, it is assumed as equal of about -15 °C. The permeability of concrete is an important factor, because water is penetrating into it as a result of capillary action. The process occurs significantly more rapidly when concrete is under the unilateral hydrostatic water pressure. 

Transport of liquid from the internal part of concrete to the surface is necessary to initiate efflorescence and hence the capillary phenomena play a decisive role. Diffusion of ions from dissolved compotmds can also occtrr through the absorption of external water, for example from rain. In concrete being in contact with water the so-called capillary action occtrrs it means water penetration to mesopores, tmder the action of water surface tension. This question is discttssed in Chap. 5. 

This wi [ also be a factor in determining hoiv much water there is in the pores to enable the corrosion reaction to be sustained. Chloride induced corrosion is believed to be at a maximum when the RH within the concrete is around 90-95% (Tuutti, 1982). For carbonation there is experimental evidence that the peak is around 95-100% RH. However, it is important to recognize that RH in the pores is not simply related to atmospheric RH ivater splash, run off or capillary action, formation of dew, solar heat gain or other factors may interv ene. 

The type of PCB contamination of concrete that is most frequently encountered and most difficult to treat is just below the surface, resulting either from short-term spill contact or grinding of dry deposits through tracking. PCB has been shown, however, to penetrate concrete to much greater depths than common sense would indicate, through capillary action or other physical mechanisms. Decontamination of floors in particular sometimes shows PCB penetration of 0.5 to 2.0 inches for even small spills of limited contact time. 

They are applied on roofs, slabs on ground, basements, water-retaining structures, concrete blocks, and clay bricks. Waterproofing admixtures reduce the permeability of concrete. The dampproofing admixtures impart water repellency and reduce moisture migration by a capillary action. Examples of these admixtures are soaps and fatty acids which react with cement, conventional water reducers, methyl siliconates, etc. 

Air entrainment generally improves durability by reducing permeability. The resistance of hardened concrete to the action of frost and de-icing salts is considerably improved by the use of air-entraining admixtures. This is-achieved by the entrained-air bubbles acting as expansion chambers to accommodate the ice formed within the capillaries. Because the bubbles break up the continuity of the capillaries they also reduce permeability and water adsorption. 

Wick action is the transport of water (and any species it may contain) through a concrete (porous material) element face in contact with water to a drying face with less than 100% relative humidity of air 129]. The mechanism involves capillary sorption and evaporation. 

During the process of wick action, if there is no evaporation, the solution level can increase through capillary rise in the concrete according to Eq. 5 ... 

---