1. Introduction
Architectural heritage has always suffered damage, which is the result of the decay of the masonry materials that occurs over time. Restorers and other professional figures involved in its conservation can slow down this process, but natural hazards are more and more serious and their effects cannot be ignored.
Nowadays, the awareness that architectural heritage is deeply threatened by climate change is becoming ever more apparent [
1,
2,
3]. Changes in average climatic conditions occurring over a long period of time, result in an increased frequency and intensity of extreme weather events, such as the rise in environmental temperature, relative humidity, and freeze-thaw cycles. These phenomena directly affect the physical, chemical, and biological mechanisms, which can cause the deterioration of construction materials and therefore of the cultural assets themselves. Measuring the effects of these phenomena is a necessary condition for the analysis of the heritage assets, and will allow us to undertake a reliable assessment of the risks facing such assets and decide on an appropriate course of action for their protection.
Water is one of the most serious causes of damage to masonry architectural heritage [
4,
5,
6]. It is the main actor in hazards, which lead both to sudden and slow disasters, such as floods on the one hand, and precipitations and water intrusion on the other [
7]. With an increased strain on resources for conservation and protection, these natural events represent a major threat to architectural heritage. Heavy rain falls and flooding recently struck many parts of Europe, causing heavy damages and the collapse of important heritage structures. The city walls of Amelia [
8], Italy, Cockermouth Castle, UK [
9,
10], and the fortress of Magliano (
Figure 1) [
11], also in Italy, are examples of heritage masonry structures recently collapsed or damaged due to water-related issues.
If water penetrates from the ground, the rising damp phenomenon occurs. It can affect any building element which is close to the ground, such as the wall structures.
Figure 2 shows two representative examples in Rome. The first relates to one of the
aulae of the former Bath of Diocletian, now a National Museum, while the second relates to one of the lateral chapels of the
San Pietro in Vincoli church.
The deterioration due to rising damp can be initiated by different factors linked to both the physical properties of the building’s materials, such as porosity, and to errors in design, such as the lack of a water-proof system for the protection of the wall basement.
Rising damp triggers physical, chemical, and biological processes of alteration and deterioration, very often concurrently [
12]. Visible signs (such as color change), environmental discomfort, and worsening of performances are some of the effects of these processes. The latter consists of several factors, including those concerning mechanical behavior. From this point of view, several authors have studied the problem, focusing mainly on the degradation in terms of ultimate strength and stiffness. In 1972, Stang et al. [
13] carried out an extensive experimental campaign of tests on masonry walls made of fired clay bricks and lime mortar, and proved that water reduces the compressive strength of bricks, mortar, and masonry as a whole. Franzoni et al. [
14] used masonry triplets made of fired clay bricks and cement mortar, and proved that water can cause a decrease in compressive and shear strength, as well as a non-relevant change in stiffness. Amde et al. [
15] obtained similar results. They performed experiments on masonry specimens made of bricks and cement-lime mortar, and proved that the ratio between the compressive strength measured in wet and dry specimens is 0.81, while the ratio between the corresponding elastic moduli is 0.88. Graubohm and Brameshuber [
16] studied the problem, also considering the effect of freeze-thaw cycles, concluding that the mechanical properties of the masonry are not significantly affected.
Knowing how these parameters change in a damaged building element compared to a healthy building, allows one to conduct a more reliable structural analysis; this is important to understand their behavior and prevent their ruin. Although these parameters are significant, they are not sufficient to define the mechanical behavior of the wall completely. In fact, in the simplest situations, one also needs to know, at the very least, the unit weight.
This study consists of a basic experimental research, which aims to investigate how the unit weight of masonry varies when it is affected by rising damp. This was done using a set of masonry specimens, which underwent an ageing treatment in the laboratory in order to simulate being exposed to severe environmental conditions.
Unit weight or weight density is defined as the ratio between the weight and the unit volume (usually, measured in kg/m3). In the field of material science, two definitions of density are provided. Real density is the ratio between the weight of the solid particles and their volume. Apparent density is the ratio between the weight and the volume, also including internal voids.
Masonry can be considered a composite material with a heterogeneous and hierarchical structure, because it is an assemblage of units (such as bricks or stones) and mortar which, in turn, is a mixture of a binder and an aggregate. As a result of this structure, defining the density of the masonry is a difficult task, since it is influenced not only by the intrinsic physical properties of the constituent materials, but also by workmanship, method of construction, wall arrangement and deterioration. In a wall affected by rising damp, the voids are filled with water, which contributes to the increased weight of the masonry itself. As a consequence, the apparent density and the weight density increase. Moreover, rising damp usually does not affect the whole wall, but only its lowest part. It is therefore appropriate to divide the wall into different homogeneous parts, and to assign to them the corresponding value of the weight density.
To simulate typical moisture conditions, Smith [
17] proposed conjectural models, validated by data collected by other scholars [
18,
19,
20,
21,
22]. From a macroscopic point of view, it is possible to recognize at least three sections in a wall affected by rising damp: the lowest, which is always characterized by a critical level of water content, the uppermost, where the amount of water is not to be considered pathological, and the intermediate, where the amount of water changes cyclically depending on the environmental conditions. The latter is delimited by the lowest and highest water marks, that is, two borderlines (MinBIh and MaxBIh), within which, the rising front ranges periodically. These borderlines identify iso-humidity (IH) surfaces, so called because each point of masonry belonging to them has the same content of water. Looking at the cross section of the wall, iso-humidity surfaces define, more or less, a Gaussian curve, as shown in
Figure 3.
Water, which rises from the ground and enters the masonry via capillary action, spreads in a way that is closely linked both to the brick bonding pattern and to the thermo-hygrometric conditions to which the two sides of the wall are exposed.
In a wall characterized by a homogeneous material and which has its sides exposed to the same environmental conditions, MinBIh and MaxBIh on both wall sides are approximately horizontal and have the same height from the ground. A schematic representation of this situation is shown in
Figure 3.
In the past, several scholars proposed mathematical formulas to relate the height of the rising front to the other physical parameters that characterize the phenomenon [
23,
24,
25,
26]. The mechanism of moisture transport within a porous material is complex, thus, many parameters influence the height of the rising front. In actual conditions, a direct survey is the most reliable way to check the level of damage. It can be done by means of diagnostic instruments or through a sensitive experience, which is the simplest way. The lowest section of a wall affected by rising damp is easily recognizable thanks to the permanent change of color due to the wetting effect. In the intermediate section, the color changes cyclically, and efflorescences may be also found due to salts carried by the water being layered down before evaporating.
4. Conclusions
In this paper, the results of an experimental study on the effect of water on the unit weight of brickwork masonry have been presented. Experimental tests were carried out on masonry specimens made of fired clay bricks and lime mortar that were exposed to an ageing treatment. During each wetting/drying cycle, the amount of water, absorbed and rejected by masonry specimens, varied over time, causing a change in their weight.
The recorded data allowed us to quantify the increase in the unit weight of the masonry due a water-based treatment. This increase consisted of two rates. The first one is constant, permanent, and is about 19% of the unit weight of the masonry with a no-pathological water content. The second-rate ranged between 19% and 23% depending on the environmental conditions inducted during the ageing treatment. The increase in masonry unit weight was rather fast, and it concentrated in the first two ageing cycles.
The ageing treatment reproduced through the experiment was chosen considering the typical conditions of historic masonry structures affected by rising damp. Since the test results demonstrate that the increase in the unit weight was not negligible, its effect should be considered to assess the structural response of masonry buildings. For this reason, it is important to identify the areas of a building affected by damp.
Tests carried out on brick, mortar, and masonry specimens highlighted their different behavior toward water intrusion and rejection. This means that data obtained through the conducted experiment are specific and linked to the type of brickwork masonry. This could be considered a limitation of this research. However, some interesting conclusions can be drawn. Test results reveal that mortar joints absorb and reject more water than brick, and that they do so more rapidly. For this reason, it is expected that the brick’s height/bed joint ratio is a meaningful parameter to estimate the increase of the masonry unit weight during ageing. In this experiment, the brick’s height/bed joint ratio was 1.4. More tests will be necessary to confirm this, but the emerging trend seems sufficiently clear. Moreover, it could also be interesting to consider other types of construction materials and extend the study to other types of masonry, such as stone masonry, or multi-leaf masonry.
The results of this experiment could be used in conjunction with previous research evidence to demonstrate that the pathological presence of water in brickwork masonry can lead to a significant decline in its mechanical properties. When acting forces are inertial (i.e., earthquake loading), an increase in a masonry’s unit weight may further worsen the structural response of a historic masonry building.