Damage mechanisms (moisture related)

Moisture related processes

Once the damage found in a building has been identified and the location (area) where it occurs outlined, a consultation of the wiki will help the user to understand the cause of the damage. The system is practice-oriented and is meant to help the user reach a diagnosis in the most direct way.

Moisture related processes, which provide the necessary moisture for most mechanisms to take place, will be handled with, as well as salt and frost damage mechanisms. These are in fact recurrent and can be very dangerous for the materials.

Moisture

Moisture can reach the materials in various ways. The presence of moisture inside or on the surface of the materials is an essential condition for most damaging processes to take place. Besides, moisture itself can cause damage.

Possible sources of moisture

Ground and surface water (rivers, rainwater, snow, leakages, seepage, water in the ground (rising damp), flooding, splash up (30-50 cm high zone), moisture in the ground adjacent to earth retaining wall, air humidity (condensation, RH).

Moisture transport

Capillary transport can take place in the material (rising damp); in this case also dissolved salts can be transported by the moisture. When two materials are joined, the transport takes place from large pores to small pores. The moisture and (possibly) the salts will end up in the material with the smaller pores. The size and distribution of the pores constitute important criteria to base some conclusions and choices upon, like the selection of a suitable plaster or the possibility of desalination.

The height of rising damp depends on the following factors: water supply, pore size distribution, drying speed at the wall surface, and the possible presence of salt in the moisture. The interface between brick/stone and mortar, in masonry, represents an obstacle for moisture transport: as a consequence the height reached by rising damp in practice is often less than the theoretical maximum rising height, and is of ca. 2-3 m above ground level.

In the case of seepage, the moisture moves downwards. Seepage can take place, for instance, as a consequence of a leakage.

Rainwater splash up belongs to the ‘transport’ category and concerns the lower part of a wall.

Aerosols are often transported by rain and wind. The final phase of a transport process is the deposition of transported matter.

An investigation method based on moisture profiles (actual moisture and hygroscopic moisture) is meant to help to find out about quantity and distribution of moisture and (soluble) salts present.

The user of MDCS should ‘build’ the wall or construction under investigation, indicating all the materials used in the masonry and keeping in mind what materials are in contact, like bedding mortar and pointing. This information is important to understand moisture (and salt) transport processes.

Water repellent treatments and moisture

A water repellent treatment can cause damage or increase already present damage[1]. This can happen when a salt loaded wall, in which rising damp is present, is treated with a water repellent product. Water can penetrate into the wall but can not reach the surface in liquid form. The wall will only dry by means of water vapor transport. The salts crystallize at the interface between the treated and the non treated part of the material and can cause spalling of the treated portion.

It is also possible that the treatment becomes (locally) no longer effective in the course of time or is removed together with the materil on wchi it was applied. This is the case when e.g. a section of the pointing is repaired, but not treated again. The moisture can penetrate and spread in the wall, but cannot easily evaporate. Frost or salt damaging processes can take place and cause the treated zone of the material to spall (fig. 1).

 

oudewater

Fig1. Oudewater, waterrepellnt,spalling (frost damge)

Not only water repellent treatments can hinder the drying process, but also coatings and stone consolidants. Consolidants may  also cause problems when their initially water-repellent behavior remains active for too long a time in the presence of salts.

Crust, encrustation and stains

Crust

It should be noted that some damaging mechanisms can be traced back from their effects, as it is in the case of black crust formation (fig. 2,3). A black crust is a gypsum crust, which ieventually becomes dark due to soiling: such a crust can only be present in materials containing lime, like mortar, certain types of natural stone and some types of brick. A black crust is the result of a chemical conversion of the material. During a visual inspection it may be difficult to understand whether the found damage is a crust or a deposit. In order to find out whether a chemical conversion has taken place in the material containing lime, further investigations are necessary, e.g. microscopy investigations. The surface of the material, after conversion into gypsum, can also be washed off by rain water, so that often a black area (not directly exposed to driving rain), may be found near a light area, where the material will be converted and washed out. Crusts will eventually detach and fall off. Each conversion into gypsum, thus, leads to loss of material.

 

Crust crust

Fig. 2 Crust (black area) adjacent to light, area (gypsum washed out)

Fig. 3 Black crust

The study of the damage sources based on the identification of the necessary circumstances for the damage to occur, would imply the measurement of the SO2 in the air in the zone where the object is placed. Thismeasurement is quite complicated and not always possible, within a conservation project, as it costs money and time. Moreover the so-called ‘memory effect’ of the material should also be taken into account: a material could show damage due to a process having taken place many decades before, under different environmental conditions, that is to say within a more polluted environment. In the last decades in most European countries the level of pollution has shown a decreasing trend.

Encrustation

An encrustation, which is different from a crust, can be assessed on the spot carrying out a simple test (fig. 4, 5).

encrust encr

Fig. 4 Encrustation due to leaching of lime contained in the mortar. Moisture source: rainwater

Fig. 5 Encrustation due to leaching of lime contained in the mortar. Moisture source: rainwater

Staining

Test in the laboratory should also be carried out to explain the presence of certain stains on building materials. This is the case of stains due to oxidation of elements like iron, manganese, chromium or vanadium, which give the brick or stone a very typical aspect. Hypotheses can be made on the basis of the colour of the staining (cf. Atlas).

In some cases, cleaning is possible.

Salt damaging mechanisms

Salt crystallization: pure crystallization

Materials: brick, natural stone, mortar, pointing, plaster, (wall) .

Damage types: efflorescence, crypto florescence, delamination, exfoliation, spalling, scaling, chalking, powdering, sanding, crumbling, brick-blistering, bursting, loss of bond, blistering, peeling, push-out, rounded edges (cf. fig. 6-8).

 

crypto

Fig. 6 Crypto-florescence, powdering and sanding in plaster due to salt crystallization. Moisture sources: rising damp and RH of the air. Salt sources: sea flooding, salt in ground

Fig. 7 Powdering of brick due to salt crystallisation. Moisture source: RH of the air. Salt origin unknown (possibly salt present in brick before the construction took place or penetrated when brick surface was polished)

peel

Fig. 8 Peeling of paint due to chlorides (salt crystallization). Moisture source: rain. Salt source: sea salt

Salts can be originally present in the material or can come from an external source (which can also continuously furnish salt). An example of the first case is mortar prepared with brackish water or seawater. External sources of salt can be aerosol, excrements (e.g. cattle in stables, birds), cleaning products, anti frost products, the use (destination) of a building (e.g. salt store), groundwater and surface water, which contains salts.

The presence of salt can be deduced from information of the building and its location (e.g. it is known that a building has been flooded). Still, a scientific basis can be build only on the results of laboratory test and analyses (see also profiles).

Damage mechanism

The process can only take place on condition that:

· at least one of the above mentioned damage types is present;

· a source of salt is present;

· a source of moisture is present;

· the salt is dissolved in the moisture

· a transport process takes place, in the direction of the surface, where the moisture evaporates;

o the evaporation front can also be in the material;

· or

· the salt comes form outside and remains at the surface;

· the salt crystallizes on the surface of the material of just behind it.

Salts dissolve and are transported by the moisture. Certain salts, like chlorides, are very soluble, whereas others, like sulfates, are less soluble. Combinations of salt types are more often present then individual salts. The salts are deposited where the moisture evaporates (evaporation front). They crystallize and, by doing so, they exert crystallization pressure on the walls of the pores. If the pressure is higher than the tensile strength of the material, damage occurs.

If the process finds place on the surface of the masonry, the resulting damage is often limited. If the salts crystallize inside the material, though, the damage caused can be severe. This can happen, for instance, when the liquid moisture transport is hindered (e.g. in the presence of a water repellent layer).

Chemical conversion: formation of swelling salts

Materials: brick (when it contains lime), natural stone, mortar, pointing, plaster, (wall) .

Damage types: crust, exfoliation, sanding, crumbling, bursting, loss of bond, push out, bending, bulging (cf. fig. 9-12).

blist

 

 

 

Fig. 9 Brick blistering due to conversion of lime contained in the brick into gypsum  
Fig. 10 Exfoliation of plaster and brick due to gypsum formation
 

Fig. 11 Cracks in masonry due to swelling of the bedding mortar. Thaumasite found in the mortar (sulfate comes from bricks fired at low temperature)

Fig. 12 Bursting of pointing due to swelling mortar (chloride reacting with tricalciumaluminate from cement). The pointing mortar swells and exerts pressure on the bricks in the masonry eventually causing exfoliation

 

Damage mechanism

The process can take place on the condition that:

· at least one of the above mentioned damage types is present;

· a source of moisture is present.

In the case of brick, stone, or lime mortar:

· the material contains lime;

· a source of sulfate is present (e.g. from the brick or form the air);

· a chemical conversion is possible, which can lead to the formation of gypsum;

· petrografical en chemical analyses (XRD) are necessary to confirm the hypothesis of conversion.

In the case of mortar:

· mortar contains tricalcium-aluminate hydrate;

· chlorides are present;

· a chemical conversion is possible, which can lead to the formation of Friedel’s salt;

· petrographical en chemical analyses (XRD) are necessary to confirm the hypothesis of conversion

· or

· sulfate is present;

· a chemical conversion is possible which can lead to the formation of ettringite or thaumasite;

· petrographical and chemical analyses (XRD) are necessary to confirm the hypothesis of conversion

The process can take place when salts penetrated in the material (wall) can form a swelling compound, together with some component of the mortar. Ettringite and thaumasite can result from the reaction of mortar components with calcium sulphate and water. For the conversion to take place a (very) high moisture content, relatively low temperature (external circumstances) and a very high sulphate content are necessary. The volume of the compound is larger than the volume of the original components. This can lead to swelling and also to exfoliation of the mortar.

Salt damage is often difficult to be distinguished form frost damage during an inspection. The presence of salt is of course a determining factor, but the possibility that the two processes work synergistically is still not excluded. The only way to come to a good diagnosis is to further investigate the damaged materials.

Frost damaging process

Materials: wall, brick, natural stone, mortar, pointing, plaster.

Damage types: layering (delamination, exfoliation, spalling, scaling), sanding, crumbling, loss of bond, peeling, push out, cratering, bending, displacement, bulging (cf. fig. 13-15).

 

Fig. 13 Layering of mortar due to frost. Moisture source: rain
Fig. 14 Bulging of wall due to frost damage to the mortar. Moisture source: rain
  Fig. 15 Exfoliation of brick in lab frost test

 

Damage mechanism:

The process can take place on condition that:

· at least one of the above mentioned damage types is present;

· the material is susceptible to frost damage;

· moisture can penetrate the material;

· the material is soaked with moisture and

· the temperature decreases abruptly under 0º C.

Frost damage is caused by the dilation of water, which is frozen in the material. The fact that even frost-susceptible materials do not show damage after each severe winter can be explained as follows: damage occurs only in ‘typical frost damage winters’, that is to say in cold periods when it heavily rains, the material becomes saturated and then the temperature drops to values under 0º C (fig. 16).

 

 

Fig. 16 Rainfall and temperature change in a typical 'frost damage winter' (over a period of ca. 6 weeks)

The frost susceptibility of masonry can be tested following NEN 2872. If the damage obtained is comparable with the damage found in situ the material is susceptible to frost. If the material is susceptible to frost, damage will appear, under specific circumstances: the material is saturated with moisture and the temperature drops under 0ºC. Much moisture is necessary to saturate the material. The source of moisture should be relevant, like rain on a badly finished (covered) wall, or leakage of water. A retarded evaporation can favor the saturation. The presence of e.g. glazing on brick or a paint (μd >= 0.5m) as well as a water repellent treatment can hinder the evaporation.

In bedding mortars, frost damage may cause exfoliation (i.e. layering) of the mortar in layers parallel to the bedding faces of brick or stone in the masonry; this results in a volume increase (swelling of the bedding joints). In pointing mortars, the pointing may crumble, or can be pushed out either as a result of damage to the bedding mortar or due to the formation of an ice-lens between the bedding and pointing mortar (fig. 17). According to the ice-lens theory, an ice-lens may grow on the boundary between a coarser and a finer porous system. While being formed in the coarser pore or in the void between the two pore systems, i.e. at the interface, the ice may build up a pressure that is determined by the size of the smaller pores: the smaller the pores of the finer system, the higher the ice pressure in the coarser pores or in the void will be. The ice pressure builds up until the ice is able to enter the finer system*) or until the (compressive) tension becomes higher than the local tensile strength of the material: in this case the material breaks at the interface between the coarser and the finer pore system.

Fig. 17 Model ice-lens formation

 

 

NB Fresh lime mortars are especially frost sensitive and need protection when applied in autumn or winter.

The application of a repointing mortar may reduce the drying rate of the bedding mortar, a fact which can increase the risk of frost damage.

In new buildings and in repair mortars the risk of frost damage can be reduced by the use of an air entraining agent in the mortar.

 

*) If compared with capillary behavior, where water sucks up higher in a capillary the finer the diameter, there is a difference: ice does not behave like water, but like mercury. Mercury does not want to enter a capillary unless it is forced into it. One could state that mercury (or ice), contrary to water, shows negative moistening. The pressure needed, again, is higher the finer the capillary.

 

[1] Cf. Report EU project ‘Surface treatments’, Contract EV5V-CT94-0515).