1. Introduction
The basic structural elements of historical masonry buildings are walls and pillars subject to compression. The determination of the masonry mechanical parameters, such as compressive strength and Young’s modulus, is crucial in the analysis of these types of structures. Currently, various testing methods are applied in order to evaluate these parameters.
The best results are provided by in situ tests or by tests carried out on masonry specimens cut from the structure [
1,
2,
3,
4,
5,
6,
7,
8]. The masonry specimens’ dimensions and quantities should be sufficiently large in order to be representative of the structure [
1,
2,
3,
4]. Due to conservation reasons, satisfying this condition in every case is not possible because cutting out masonry specimens is connected with damaging historical structures. Furthermore, the process of cutting specimens out from walls erected on weak lime mortars may result in the splitting of specimens, making them useless in strength tests. Therefore, other methods of evaluating masonry compressive strength are applied as well.
A popular method of in situ masonry testing is a structural investigation with the use of flat-jacks. They are used to estimate the level of compressive stress in the structure and Young’s modulus of the masonry [
5,
6,
7,
8]. Nevertheless, the possibility of determining the masonry compressive strength is limited due to the damage to the fragment of the wall between the flat-jacks that usually appears during the test.
Brick masonry compressive strength is frequently estimated from formulas based on the strength of bricks and mortar [
9,
10]. The compressive strength of bricks may be obtained from laboratory tests conducted on bricks extracted from the structures. Such tests are usually performed on whole or half bricks. In order to minimise damage to historical structures, tests are also conducted on smaller brick specimens [
11,
12,
13]. It is far more difficult to determine mortar strength in the masonry joints. On account of the dimensions of bed joints, it is not possible to cut out mortar specimens of dimensions required by EN 1015-11 [
14], and due to this fact, it is not possible to carry out standard destructive tests on 4 × 4 × 16 cm
3 mortar specimens. For this reason, minor destructive methods are used for the evaluation of mortar strength in historical brick walls. These methods are based on tests carried out on small specimens cut out from masonry bed joints or by in situ tests [
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33]. Most in situ methods are based on the measurement of the depth of steel needle penetration or the amount of energy required to drill a small cavity in the mortar joints. In situ and laboratory tests of mortars are in the process of being developed and tested in order to determine factors affecting the measurement results. Most frequently, the results of minor destructive testing of mortars are conducted on samples made of contemporary materials or materials whose properties are similar to those used in the historical mortars. There are significantly fewer tests performed on original, historical structures.
This article presents the compressive strength tests of mortars in the bed joints of masonry in historical buildings erected at the end of the 19th century and at the beginning of the 20th century in the centre of the royal city of Cracow. The mortar strength in the bed joints of masonry was defined with the use of a penetrometer and double punch tests. The penetrometer tests (PT) were carried out by means of an RSM-15 version 1.0 penetrometer [
34]. The double punch tests (DPT) were performed in accordance with DIN 18555-9 [
35]. A characteristic of the historical masonry was the considerable thickness of bed joints, the presence of large size grains in the mortar (diameters significantly exceeding 2 mm) and degradation of the mortar material in the areas near the external surface of the joints. Due to these factors, the authors of this article proposed modifications to the methods for determining the mortar compressive strength based on the penetrometer tests (PT) and double punch tests (DPT).
3. Discussion of Test Results
Table 3 presents the results of the penetrometer tests (PT), specifying the increases in the penetration depth corresponding to five impacts (
Δdp (5)). The columns present the penetration depth for the impact ranges 1–5, 6–10, 11–15 and 16–20.
When analysing the results provided in
Table 3, it may be stated that the highest penetration value is registered with the first five impacts, then regression and stabilisation follows.
The observed result is caused by the presence of a zone of degraded and weakened mortar material. The zone of degraded and weakened mortar is located at the surface of the masonry walls. For external walls, this is the consequence of the destructive impact of the external environment factors. For internal walls, the zone of degraded mortar arises during plaster removal. Furthermore, the conditions of the contact between the needle and the mortar surface at the beginning of testing, which are individual for each testing point, also exert some influence.
Analysing the results of the conducted penetrometer tests, it may be stated that a zone of degraded mortar is observed in bed joints of all tested masonry buildings (see
Table 3). This zone is further referred to in the article as the surface disturbance zone. In the surface disturbance zone, the penetration depth indications are distorted and should be omitted when the mortar strength is obtained. Therefore, the determination of a range of the surface disturbance zone is crucial in the analysis of the penetrometer test results.
The determination of the range of the surface disturbance zone (
zp,z) should be conducted by means of the procedure presented in
Figure 7.
In the first step, the linear function FA is determined. The function FA is a linear approximation of the test results for nimp = 10, 15 and 20. The next step is to look for an nimp value (marked as nimp,z) for which the difference between the dp, determined on the basis of the FA function () and directly from the tests (), is less than 5%. The value of penetration depth specified for nimp,z is the range of the surface disturbance zone (marked as zp,z).
The ranges of the surface disturbance zones (
zp,z) in the bed joints of the tested historical walls, determined in line with the procedure provided in
Figure 7, are presented in
Table 4. The average values of the surface disturbance zones are from 9.1 to 14.5 mm.
Table 4 also includes the number of impacts corresponding to the boundary of the surface disturbance zones (
nimp,z). After rounding to integer values, the quantities of impacts
nimp,z range from seven to eight.
In [
34], a correlation curve was proposed for the evaluation of the mortar compressive strength based on the penetration depth of the penetrometer steel needle in the masonry bed joint. This correlation curve is presented in
Figure 8. Mortar strength (
fm1) can be determined considering the needle penetration depth after 10 impacts (Δ
dp (10)). The greater the depth of penetration of the penetrometer steel needle after 10 impacts, the lower the mortar strength. Mortar strength, determined based on the conducted penetrometer tests and the mentioned correlation curve, is presented in
Table 5.
Mortar compressive strength (fm1) was determined considering the measurement results registered outside the surface disturbance zones of bed joints. The compressive strength of historical mortars obtained using penetrometer tests were between 1.5 and 2.9 MPa.
The thickness of specimens in the DPT tests performed for four types of mortars was varied. The specimens cut out from the bed joints of building B1 were the thickest (average thickness 20.7 mm); whereas, the samples cut out from the joints of building B2 were the thinnest (average thickness 15.1 mm). The thickness of mortar specimens extracted from tested historical masonry were significantly higher than the thickness of mortar specimens cut out from contemporary masonry performed according to EN 1996-1-1 [
37]. EN 1996-1-1 [
37] recommends a thickness of bed joints in masonry from 6 to 15 mm. Based on this reasoning, the thickness of mortar specimens cut out from contemporary masonry ranged from a few millimetres at a minimum, to a maximum of 15 mm.
The influence of the specimen thickness on the DPT test results and other related significant aspects were analysed in the studies [
21,
24,
26,
27,
38]. As the specimens’ thickness increases, the impact of confinement caused by friction between the surface of the steel punch and the mortar specimens decreases. This affects the reduction in the ultimate compressive force of the specimen. The effect of reducing the ultimate compressive force, and thus the value
fDPT, with the increase in specimen height, is presented in
Figure 9.
The
fDPT values refer to the compressive strength (
fm) of the mortars obtained on standard mortar specimens with height and width values equalling 40 mm. The procedure for determining mortar compressive strength on standard specimens with a height and width of 40 mm is given in EN 1015-11 [
14]. The results presented in
Figure 9 consider the tests conducted on lime mortars and lime-cement mortars presented in [
21,
38]. The total sample thickness (
t2), that is, the mortar sample thickness and the thickness of gypsum caps (see
Figure 5), is considered. The conducted analysis provides a regression function in the following form:
Based on the DPT test results, taking into account the sample height (
t2), mortar compressive strength (
fm2) may be calculated from the following relation:
Table 6 presents mortar compressive strength values (
fm2) determined from Equation (2).
It should be noted, however, that the coefficient of determination (R2) for the proposed function is low. Further research is needed for different mortar types to confirm the proposed relationships.
Figure 10 compares the mortar compressive strength values based on penetrometer tests (
fm1) and DPT tests (
fm2).
The differences ranged from 4% to 27% (on average 14%). The mortar in the manufacturing hall building (B2) had the highest compressive strength. This was also confirmed in PT and DPT tests. In the remaining buildings, the mortars had compressive strength which ranged from 1.4 to 1.9 MPa.
4. Summary
This article presented the strength test results of the mortars used in the erection of four historical brick buildings. The mortars were tested in situ with the use of a penetrometer and in the laboratory by means of the DPT procedure. The laboratory tests were conducted on the specimens collected from the masonry bed joints.
Tests showed that the historical mortars are characterised by specific properties, such as low strength, significant inhomogeneity and the presence of aggregate grains of large sizes considerably exceeding the current requirements. The characteristic properties of bed joints in the brick walls were their significant thickness and the presence of degraded material zones in the areas near the wall surface. The thickness of bed joints in tested masonry far exceeded the recommendations of current standards. Due to the above factors, modification of the applied testing methods and the methods of analysing their results was proposed in this paper.
In regard to the penetrometer tests, the notion of a surface disturbance zone was introduced, and the manner of its range determination was provided.
The DPT test results were analysed, considering the influence of specimen thickness (see Equation (2)). The introduced modifications allowed for more precise determination of the compressive strength of mortar in the historical brick walls.
The compressive strength of the tested historical mortars was minor. Based on the penetrometer tests, the mortar strength values ranged from 1.5 to 2.9 MPa, and the DPT tests ranged from 1.4 to 2.8 MPa.
The penetrometer method is simple and fast in application. Nevertheless, currently, there are not a sufficient number of penetrometer tests carried out on historical buildings in order to verify this method for different types of mortars.
The DPT method requires cutting out samples from wall bed joints and conducting laboratory tests, which is more complicated. Cutting out a significant quantity of samples is sometimes impossible for conservation reasons.
For the reasons given above, the authors of this paper suggest the compilation of both testing methods in order to establish the mortar compressive strength in the bed joints of historical buildings. The penetrometer tests allow limiting the sampling of structures. The results of the DPT should be used for the verification of the mortar strength specified in the penetrometer tests.