Moisture in building materials

Moisture measurement of materials consists in determining the percentage by weight of water contained within the solid mass of the moist material. The % moisture content is calculated using the following equation:

where M is the percentage moisture, md is the mass of the dry sample and mw is the mass of the wet sample.
The water contained in a medium consists of two portions. One is chemically bound to the substances of which the material is composed (hydration or structural water), the other is simply absorbed into the porosity of the material (physically absorbed water). Structural water cannot migrate from the material and is therefore not an interesting fraction for the purpose of determining moisture, so it must not be involved in the measurement. Absorbed water, on the other hand, is that excess fraction that derives from factors external to the material and can consequently constitute its pathology.
Another very interesting and characteristic parameter of each material is the physiological moisture, PM. Physiological moisture is the % of water found in the material at equilibrium with the environment at 20°C and with 50% RH. PM is therefore the minimum limit spontaneously attainable by the material under those conditions.

The importance of determining this physical quantity is closely linked to the harmfulness that water can exert on the materials or structures in which it is present. In the field of resin flooring and wall coverings, the damage resulting from the presence of water in the substrates on which the coating is laid most often consists of the detachment of the coating. The visible manifestation of detachment is the formation of bubbles of vapour or water between the resin and the substrate. This phenomenon is possible because resin coatings are generally vapour-proof. This causes the vapour that naturally escapes towards the air from the material in which it is contained in the form of moisture to become trapped under the coating and exert a release pressure there. In addition, if the moisture at the interface between the substrate and the coating condenses as a result of a temperature drop, a kind of condensation-dissipating effect is also established on the coating. Ultimately, the possibility and extent of detachment depends both on the moisture content of the substrate and on the extent of the bond between the coating and the substrate (adhesion). There is a limit value for % moisture, specific to each substrate/covering combination, below which laying is to be considered safe and is stated – where relevant – in the Technical Data Sheets of the coating product.

Moisture within materials originates from different physical phenomena that may affect the building envelope (building) of which the material under investigation is a part. The main sources are:

3.1 - rising damp (by capillarity from the soil);
3.2 - condensation moisture (from the air contained in the room volume);
3.3 - seepage moisture (from precipitation infiltrating the building envelope);
3.4 - accidental moisture (resulting from broken plumbing or drainage pipes passing through the material under observation).
3.5 - construction moisture (inherently present as mixing water, in materials based on airborne or hydraulic binders, mortars, plasters, concrete or screeds, or, in the case of aggregates or bricks, as water naturally present or absorbed during extraction, manufacture or storage processes).

The % moisture content is normally given by the combination of all the above factors, although, in general, each of them has a different weight depending on the type and age of the building in which the assessment is made. In newly designed and constructed buildings, moisture normally results from construction activity and tends to decrease over time as the materials dry out. In old buildings, on the other hand, moisture arises from all other factors and is due to design errors or the deterioration of the structures or protective materials installed to limit water absorption. In this case, it may not tend to decrease unless countermeasures are taken to block the ingress of moisture from the environment outside the material.
Whatever the origin of the moisture, it is important to measure its value accurately prior to the laying of resin coatings and to track its development over time, especially when its origin is external. When its value stabilises permanently below the threshold stated in the technical data sheet of the covering material, the latter can be laid.

The drying time of a substrate depends on the structure of its constituent material as well as on the environmental conditions in which it is located. In general, it is observed that very porous materials (dehumidifying plasters, terracotta bricks, air lime mortars) take much less time than compact materials (concrete, cement mortars) to remove the moisture contained within them. One of the practical consequences of this observation is that, for example, the use of cement-based plasters is not recommended for cladding walls founded on damp ground or subject to infiltration.
For the purpose of proper planning of renovation or coating of moisture-impregnated substrates, it may be useful to estimate the drying time. The initial prerequisite is that water absorption is stopped (installation of waterproofing membranes, flashing, repair of leaking pipes, application of waterproof paint, etc.). There are mathematical formulas in the literature (Kettenacker) that allow a rough calculation to be made, at 20°C and 50% RH, of the time to wait for a substrate to dry from the saturation value (complete impregnation) at the PM (physiological moisture) value from the moment when water absorption stops:

where:
t is the drainage time expressed in days
s is the thickness of the masonry expressed in centimetres
p is the drainage coefficient.

a 40 cm-thick structural concrete substrate impregnated with water needs approximately: t = 1.60 x (402) = 2560 days to dry with a residual moisture equal to PM. The same substrate in mortar-laid bricks (where the mortar is a very small part of the masonry) needs approximately: t = 0.28 x (402) = 448 days to dry with a residual moisture equal to PM.

Since the calculated values are purely indicative, it is important to take the actual measurements in the field.

The methods for measuring the moisture in a construction material can be grouped into two macro-categories: 5.1 - direct methods, by which the mass of water contained in the sample is directly quantified by chemical or physical removal of the same from the measurement sample. In direct methods, it is necessary to take a small but important amount of material from the substrate to be examined. By their nature, direct methods are the most accurate in determining the water content of materials; 5.2 - indirect methods, by which the amount of water in the material is estimated by observing related indirect parameters (usually electrical conductivity and the dielectric constant). Although indirect methods generally do not require samples to be taken from the substrate, they are the least accurate because they measure properties that are not always reproducibly related to the moisture content of the material. In general, indirect methods are used as a ‘quick estimate’ of the residual moisture pending its precise determination by the direct method.

consiste nella misura della massa di un campione di materiale prelevato dal supporto di prova prima e dopo l’esecuzione di un trattamento termico di essiccazione ad una certa temperatura per un tempo di circa 24 ore. La scelta della temperatura di essiccazione è fondamentale. Da un lato, infatti, si deve garantire la liberazione dell’acqua assorbita fisicamente nel materiale in un tempo ragionevole e dall’altro si devono evitare l’eliminazione dell’acqua di struttura (quella legata chimicamente ai composti che costituiscono il materiale) e la degradazione chimica del materiale (decomposizione termica). In generale per i materiali edili più comuni, si ritiene che una temperatura di lavoro di 105°C sia al contempo efficace e precauzionale.
Il vantaggio principale del metodo ponderale è che è universale e permette di misurare l’umidità anche a prescindere dalla presenza di sali o inquinanti nel materiale. È inoltre molto economico e permette di effettuare misure su ogni parte del materiale in cui sia possibile prelevare un campione. Gli svantaggi principali derivano da:
1 – necessità di lesionare il materiale per prelevarne una porzione;
2 – necessità di effettuare la prova in laboratorio poiché si impiegano una bilancia analitica (al centesimo di grammo) e una stufa essiccante;

exploits a chemical reaction involving calcium carbide (CaC2) and the water contained in the sample to be analysed. The chemical reaction in question is given below:

Calcium carbide, a solid, reacts with water to produce a gas, acetylene (C2H2), and another solid, calcium hydroxide (Ca(OH)2). If a large excess of calcium carbide was introduced into the reaction, the amount of acetylene gas produced by the reaction only depends on the amount of water in the sample.
The reaction takes place in a closed container (autoclave) in which the wet sample and calcium carbide are intimately mixed. Under these conditions, the gas released by the reaction increases the internal pressure of the container. The pressure value is linearly dependent on the amount of gas produced and thus the amount of water in the sample. By measuring the final pressure at the end of the reaction, it is then possible to determine exactly how much water is initially present in the sample.
For the method to be effective, the apparatus in which the reaction takes place must:

1 - be hermetically sealed to prevent leakage of the gas produced;
2 - be equipped with a pressure gauge for reading the pressure;
3 - contain a system for crushing and pulverising the sample so that the reagents – calcium carbide and water – come into intimate contact.
To carry out the measurement, the sample is taken and prepared according to the following simple procedure:
a - mechanical sampling: using a chisel and mallet, a fragment of the material to be measured is removed directly from the substrate;
b - grinding of the sample: using a plate and hammer, the previously taken fragment is reduced to powder. It is important to conduct the operation in a short time without heating the material, to prevent moisture from being released before the material is introduced into the autoclave;
c - weighing of the pulverised sample: once the sample has been pulverised, a certain amount of powder is accurately weighed. This step is particularly important, because the measured pressure can only be correlated with the water content if it refers to a precise weight of sample introduced.

5.1.b.1 Choosing the sample quantity for analysis:
the mass of the sample introduced into the autoclave determines the amount of water brought to reaction with the calcium carbide and thus the final gas pressure. The pressure gauge has a limited pressure scale (usually between 0 and 2 bar) for reasons of instrument resolution and accuracy. To obtain valid measurements, the gas pressure in the autoclave must be as close as possible to the centre of the gauge scale.

The choice of the quantity of sample to be introduced will therefore necessarily have to take this factor into account. So how do we choose the correct quantity?
Carbide hygrometers are usually equipped with a normogram or graph indicating the optimum amount to weigh depending on the water content of the sample. Some devices are also equipped with multi-scale pressure gauges that directly indicate the water content on different scales, each relating to a precise quantity of the weighed sample.
If you have an idea, even an approximate idea, of the moisture content of the sample, you can choose the sample quantity and the reading scale on the manometer beforehand. If, on the contrary, the sample is totally unknown, it is best to start with the least sensitive scale and, if necessary, repeat the test with the most sensitive scales.

5.1.b.2 Dependence of the pressure reading on the autoclave temperature:
since the determination of moisture is related to the measurement of the pressure of a gas, it is natural that temperature has a great influence on the result. Readings should be taken at 23°C, but environmental conditions often differ greatly from this value. Some instruments are equipped with reversible temperature-sensitive strips that measure the temperature of the autoclave body and provide equations or graphs for correcting the % moisture as a function of working temperature.
Once the quantity to be introduced into the autoclave has been set, the powder is weighed and introduced into the autoclave without spilling, using a brush to clean the plate well. A set of steel balls of different diameters and a glass vial of calcium carbide are introduced. At the end of the addition, the autoclave is hermetically sealed.
The simplicity of the method and the elementary nature of the preparation operations makes it possible to construct portable carbide hygrometers (for field measurements) equipped with simple tools for carrying out the preliminary operations.

Key:
1 - autoclave with pressure gauge;
2 - hammer, mallet and chisel for picking and crushing;
3 - heavy plate for crushing;
4 - set of steel balls;
5 - vials of calcium carbide;
6 - balance for weighing the crushed sample.

Once the autoclave is closed, the measurement starts, which involves the following steps:
d - closing of the autoclave: the autoclave is closed by having the sealing gasket adhere perfectly to the body of the autoclave. It is important that the autoclave is perfectly closed because any gas leaks would compromise the goodness of the measurement;
e - agitation of the autoclave: once closed, the autoclave is shaken vigorously so that the steel balls it contains break up the calcium carbide vial and, at the same time, allow the carbide to mix perfectly and intimately with the sample powder. Stirring will be carried out at intervals and will, on the whole, be prolonged long enough so that a stable pressure value is reached. As already described, reversible temperature-sensitive strips are usually attached to the body of the autoclave to indicate the temperature, the value of which will be used to correct the result;
f - stabilisation of the pressure and reading of the result: given the weight of the sample introduced into the autoclave, the working temperature and the pressure value read on the manometer, once the index has stabilised, the moisture value of the sample is determined using the normogram attached to the instrument. For instruments equipped with special manometers with multiple scales, the % moisture value is read directly on the manometer on the scale relevant to the weight of the sample introduced.

As already described, if the pressure or humidity value falls near the upper or lower end of the gauge, it is advisable to repeat the measurement by changing the weight of sample introduced to increase the sensitivity of the detection.

5.1.b.3 Disadvantages:
1 - the need to take portions of the material to be analysed (destructive method);
2 - delicacy of the autoclave pressure sealing system (to be checked periodically using the calibration kit);
3 - extreme sensitivity to temperature variations;
4 - possibility of underestimation of the moisture value due to insufficient crushing of the sample;

The carbide method is to be regarded as the reference method for determining the moisture content of building materials.

in addition to direct methods, indirect methods are also used to determine the moisture content of construction materials. Although they are not as reliable and accurate as the former, they are used since they are generally non-destructive methods that allow testing directly on the surface without the need to take fragments of material. As far as the above is concerned, indirect methods are essentially intended for preliminary and orientation determinations.
The main indirect humidometric methods are based on the dependence of certain electrostatic quantities (dielectric constant and electrical conductivity) of materials on their moisture content. The devices capable of detecting the variation in these quantities rely on:

a - microwave detectors;
b - conductivity detectors;
c - capacitive detectors.

In this discussion, it is impossible to analyse the merits of each of the indirect methods mentioned so far.
However, there is a common principle on which the measurement is based: moisture in the matrix changes the electrostatic properties.
More specifically, both the electrical conductivity and the dielectric constant of a material increase with water content.
Of course, the dielectric constant and conductivity also depend on factors intrinsic to the material, such as the composition and structure of the solid matrix – factors that are totally unknown in most cases.
If materials were all the same, or their electrostatic properties independent of their specific nature, it would be easy to identify a correlation, however complex, between moisture and the capacity or conductivity of the material.
Unfortunately, this does not occur in reality, so one would theoretically have to work out thousands of equations for different materials and structures. Obviously this operation is impossible, so it is necessary to group the materials present in the building substrates into macro-categories (concrete, wood, sand and cement screed, etc.), which simplifies the models, but at the same time lowers the measurement accuracy.
This is why indirect methods are not to be considered definitive when measuring moisture in materials, but only preparatory to measurement by weight or using the carbide method.